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Short title: A H3K36 methyltransferase in nitrate signaling 1
Authors for contact: Dr. Gloria Coruzzi and Dr. Ying Li 2
3
SDG8-mediated histone methylation and RNA processing function in the 4
response to nitrate signaling 5
6
Ying Li1,2,3,*, Matthew Brooks1, Jenny Yeoh-Wang1, Rachel M McCoy2,3, Tara M Rock1, 7
Angelo Pasquino1, Chang In Moon2,3, Ryan M Patrick2,3, Milos Tanurdzic4, Sandrine 8
Ruffel5, Joshua R. Widhalm2,3, W Richard McCombie6, Gloria M Coruzzi1,* 9
10
1. Center for Genomics and Systems Biology, Department of Biology, New York 11
University, New York, NY 10003, USA 12
2. Department of Horticulture and Landscape Architecture, Purdue University, West 13
Lafayette, IN 47907, USA 14
3. Center for Plant Biology, Purdue University, West Lafayette, IN 47907, USA 15
4. School of Biological Sciences, The University of Queensland, St Lucia, QLD 16
4072, AUSTRALIA 17
5. BPMP, INRA, CNRS, Universite de Montpellier, Montpellier SupAgro, 34090 18
Montpellier, France 19
6. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA 20
*Corresponding authors 21
22
ONE-SENTENCE SUMMARY: Nitrogen nutrient can trigger changes in chromatin 23
modifications, thus affecting RNA processing, metabolism and growth in plants. 24
25
AUTHOR CONTRIBUTIONS 26
The work was designed by GMC and YL. YL, MB, JYW, RMM, TR, AP, RMP, and SR 27
performed experiments. YL, MB, JYW, RMM, CIM, MT, SR, JRW, and GMC contributed 28
to the data analysis. WRM performed the sequencing. YL, MB, JYW, RMM, SR, and 29
GMC wrote the manuscript. GMC and MT critically revised the manuscript. All authors 30
edited and approved the manuscript. 31
Plant Physiology Preview. Published on October 22, 2019, as DOI:10.1104/pp.19.00682
Copyright 2019 by the American Society of Plant Biologists
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32
Funding information: This work is supported by Department of Energy (DE-FG02-33
92ER20071 to GMC), National Science Foundation (MCB 1412232 to GMC), USDA 34
National Institute of Food and Agriculture (Hatch project numbers 1013620 to YL, and 35
177845 to JRW), and start-up funds from Purdue University to YL and JRW. 36
37
Email address of Author for Contact: Dr. Gloria Coruzzi, [email protected] and Dr. 38
Ying Li, [email protected]. 39
40
41
ABSTRACT 42
Chromatin modification has gained increased attention for its role in the regulation of 43
plant responses to environmental changes, but the specific mechanisms and molecular 44
players remain elusive. Here, we show that the Arabidopsis (Arabidopsis thaliana) 45
histone methyltransferase SET DOMAIN GROUP 8 (SDG8) mediates genome-wide 46
changes in H3K36 methylation at specific genomic loci functionally relevant to nitrate 47
treatments. Moreover, we show that the specific H3K36 methyltransferase encoded by 48
SDG8 is required for canonical RNA processing, and that RNA isoform switching is 49
more prominent in the sdg8-5 deletion mutant than in the wild type. To demonstrate that 50
SDG8-mediated regulation of RNA isoform expression is functionally relevant, we 51
examined a putative regulatory gene, CONSTANS, CO-like and TOC1 101 (CCT101), 52
whose nitrogen-responsive isoform-specific RNA expression is mediated by SDG8. We 53
show by functional expression in shoot cells that the different RNA isoforms of CCT101 54
encode distinct regulatory proteins with different effects on genome-wide transcription. 55
We conclude that SDG8 is involved in plant responses to environmental nitrogen 56
supply, affecting multiple gene regulatory processes including genome-wide histone 57
modification, transcriptional regulation, and RNA processing, and thereby mediating 58
developmental and metabolic processes related to nitrogen use. 59
60
61
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INTRODUCTION 62
Chromatin modifications have gained increasing attention for their roles in modulating 63
developmental and environmental responses across plant and animal kingdoms (Greer 64
and Shi, 2012; Khan and Zinta, 2016; Sirohi et al., 2016). In plants, environmental 65
signals such as light (Charron et al., 2009), drought (Chinnusamy and Zhu, 2009) and 66
temperature (Lamke et al., 2016) have been shown to affect histone modifications or 67
DNA methylation at functionally-relevant gene loci. While it is well known that nitrogen 68
(N) - especially nitrate - acts as a nutrient signal to trigger global reprogramming of 69
gene expression to optimize N uptake and assimilation (Vidal et al., 2015; Varala et al., 70
2018), little is known about the role of epigenetic regulation in N signaling. One report in 71
Arabidopsis (Arabidopsis thaliana) showed that high levels of nitrogen increased the 72
level of H3K27me3 at the Nitrate Transporter 2.1 (NRT2.1) gene locus, and this 73
increase was dependent on a Pol II complex protein HIGH NITROGEN INSENSTIVE 9 74
(HNI9) (Widiez et al., 2011). However, at the genome scale, our knowledge of the role 75
of chromatin modifications in nitrate or N signaling is limited. Here, we present data 76
which implicates the SET DOMAIN GROUP 8 (SDG8) protein as a specific histone 77
methyltransferase involved in mediating genome-wide responses to a changing nitrate 78
environment. 79
We initially uncovered genes involved in regulation of the N-assimilatory pathway 80
in Arabidopsis using a positive genetic selection for mutants impaired in the regulation 81
of the promoter of Asparagine synthetase 1 (ASN1), a key gene involved in the 82
assimilation of N into asparagine (Thum et al., 2008). This genetic selection uncovered 83
a mutant called sdg8-5 (formerly known as cli186 e.g. in Thum et al 2008), which we 84
later showed was deleted in a gene encoding the histone methyltransferase SDG8 (Li et 85
al., 2015). Despite the existence of 32 SDG proteins encoded in the Arabidopsis 86
genome (Springer et al., 2003), genetic studies show that SDG8 plays a non-redundant 87
role in histone methylation. Specifically, mutant analysis has associated SDG8 with 88
pleiotropic effects on plant growth and development, including early flowering (Soppe et 89
al., 1999; Kim et al., 2005; Zhao et al., 2005; Xu et al., 2008), impaired pigment 90
synthesis (Cazzonelli et al., 2009), enhanced shoot branching (Dong et al., 2008; 91
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Cazzonelli et al., 2009), defects in pathogen defense (Berr et al., 2012; Lee et al., 92
2016), and an impaired touch response (Cazzonelli et al., 2014). Our previous study of 93
the sdg8-5 deletion mutant revealed that SDG8 is also associated with gene responses 94
to carbon and light signals (Li et al., 2015). Our genome-wide transcriptome and 95
epigenome profiling revealed that only a subset of genes lose H3K36me3 modification 96
in the sdg8-5 mutant, and this set of hypomethylated genes are enriched in specific 97
functional categories (photosynthesis, defense, N metabolism, development, etc) (Li et 98
al., 2015). Therefore, SDG8 is likely involved in regulating multiple processes with a 99
certain level of target specificity. Mechanistically, we showed that SDG8 is required for 100
depositing the H3K36me3 histone mark on the gene body, which is required to maintain 101
active expression of its target genes (Li et al., 2015). 102
In addition, several lines of evidence have linked SDG8 with N uptake and 103
assimilation: (i) SDG8 was identified as a regulator of a key N assimilation gene, ASN1, 104
in a mutant screen (aka the cli186 mutant) (Thum et al., 2008); (ii) N metabolism genes 105
are significantly over-represented among the validated genome-wide targets of SDG8 106
(Li et al., 2015); (iii) SDG8 is required to maintain the high expression level of genes in 107
the N uptake and assimilation pathways including the nitrate transporter (NRT1.5) and 108
glutamine synthetase genes (GLN1;1, GLN1;3, GLN1;4) (Li et al., 2015); (iv) SDG8 109
physically binds to the glutamine synthetase GLN1;1 gene locus (Li et al., 2015); (v) 110
SDG8 is required to maintain wild-type H3K36me3 levels at gene loci encoding nitrate 111
and nitrite reductases (NIA1; NIA2; NIR1) (Li et al., 2015); and (vi) the mRNA level of 112
SDG8 is induced by nitrate treatment, specifically in the lateral root cap and pericycle in 113
roots (Gifford et al., 2008). Despite this supportive evidence, it still remains unclear 114
whether SDG8 has a direct role in mediating genome-wide changes in response to 115
fluctuations in environmental N supply. Moreover, how and whether the H3K36me3 116
marks made by SDG8 in the gene body regulate gene expression remain unclear. 117
We previously showed that SDG8 has a preference for H3K36me3 in the gene 118
body, suggesting its role in histone methylation that mediates transcriptional elongation 119
or co-transcriptional RNA processing (Li et al., 2015). This is consistent with the role of 120
SETD2, the Arabidopsis SDG8 homolog in mammals, which prevents transcripts from 121
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starting in the middle of the gene body (Simon et al., 2014). In Arabidopsis, H3K36me3 122
and SDG8 were reported to affect RNA splicing of genes during responses to 123
temperature (Pajoro et al., 2017). Alternative splicing of RNAs promotes the diversity of 124
the transcriptome, which was recently shown to be involved in mineral nutrient signaling 125
such as phosphate signaling (Dong et al., 2018). Therefore, these findings beg the 126
questions whether and how SDG8 mediates RNA processing during nitrate signaling. 127
In this study, we investigated whether the response to nitrate treatment at a 128
genome-wide scale is mediated through chromatin modifications, especially through 129
histone H3K36me3 modification by SDG8, and what impacts this may have on gene 130
expression, especially RNA processing. We hypothesized that SDG8, and the 131
H3K36me3 marks it makes on histones associated with specific N-related genes, play a 132
role in regulating gene responses to N signals, thus affecting physiological responses to 133
N treatment. To test this hypothesis, we compared the N-induced genome-wide 134
H3K36me3 patterns, transcriptome responses including RNA isoform specific gene 135
expression, and physiological responses, between the sdg8-5 deletion mutant (Thum et 136
al., 2008; Li et al., 2015) and its corresponding wild type (WT), in the presence or 137
absence of a N-treatment. Our results show that the sdg8-5 mutant displays impaired 138
H3K36me3 responses at genomic loci functionally relevant to nitrate uptake and 139
assimilation, as well as reduced metabolic and developmental responses to N 140
treatments. We also show that N-responsive transcriptional reprogramming and RNA 141
isoform switching (i.e. differential production of distinct transcript isoforms under 142
different N conditions) are altered in the sdg8-5 mutant. Finally, we investigated the 143
relevance of SDG8-mediated RNA isoform switching of CCT101, a putative CCT motif 144
regulatory gene, which we functionally validated in shoot cells. We also demonstrated 145
physiological relevance of SDG8 to N responses in planta. Overall, our results support 146
the idea that the histone methyltransferase encoded by SDG8 is required for plants to 147
respond to N signals, affecting genome-wide histone modification, transcriptomic 148
response, and RNA isoform processing, as well as modulating root development, nitrate 149
uptake, and primary metabolism. 150
RESULTS 151
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152
SDG8 mediates changes in histone modifications in response to N treatments 153
SDG8, a histone methyltransferase required for deposition of the histone mark 154
H3K36me3 (Li et al., 2015), was initially uncovered in a screen for mutants impared in 155
the regulation of ASN1, a key gene involved in N-assimilation (Thum et al., 2008). We 156
thus tested the hypothesis that SDG8 mediates genome-wide changes of H3K36me3 in 157
genes functionally relevant to response to nitrate treatments. To do this, we exposed 158
the sdg8-5 mutant and its WT control to nitrate treatments, as previously described 159
(Wang et al., 2001; Wang et al., 2003). Briefly, seeds were sown on nitrate-free media 160
(0.5 mM NH4 succinate) for two weeks, followed by nitrogen starvation for 24 hours, 161
before transferring to either 5 mM KNO3 or 5 mM KCl treatments. After 5-hr of nitrate vs. 162
control treatments, the shoots were harvested for histone ChIP-Seq to profile the global 163
pattern of H3K36me3 (Supplemental Fig. S1). Two independent biological replicates 164
were performed for ChIP-Seq. The raw sequencing reads were first trimmed and 165
mapped to the Arabidopsis genome using Bowtie (Langmead et al., 2009). The mapped 166
reads were used to call significant H3K36me3 islands and significantly differentially 167
methylated islands using SICER (Zang et al., 2009). To synthesize the output from the 168
two biological replicates, a nitrate-responsive differentially-methylated gene had to be 169
identified as significantly different (FDR <0.05) in both replicates, and the mean fold 170
change of enrichment of the two replicates had to be greater than 1.2, a cutoff that was 171
chosen based on our previous study (Li et al., 2015) to reflect the dynamic range and 172
stability of the H3K36me3 mark (see methods for details). 173
174
Our results showed that nitrate treatment caused changes in the H3K36me3 175
levels of 196 genes in WT, but only 26 genes in the sdg8-5 mutant (Fig. 1). In detail, in 176
WT, 143 genes show increased H3K36me3 in the N starvation group compared to the N 177
supply group (Fig. 1; Supplemental Table S1). These 143 genes are significantly 178
enriched for GO terms “response to nitrogen starvation”, “defense response”, and 179
“response to stress”, (FDR<0.05; Supplemental Table S2). This group of genes includes 180
WRKY transcription factors and a nuclear transcription factor Y family gene, which were 181
previously reported to control N starvation (Krapp et al., 2011) and stress responses 182
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(Phukan et al., 2016). In WT, we also identified 53 genes with increased H3K36me3 in 183
the N supply group compared to the N starvation group (Fig. 1; Supplemental Table 184
S1). This group includes genes involved in nitrate reduction (G6PD3 and UPM1), 185
nitrogen assimilation (Asparagine synthetase ASN2), regulators of N signaling including 186
HHO2 (Medici et al., 2015) and CIPK3 (Canales et al., 2014), and genes responsive to 187
cytokinin (Arabidopsis Response Regulator ARR7, ARR8, and ARR9) – a downstream 188
signal in the N response (Sakakibara et al., 2006). Significantly enriched GO terms 189
identified among this group of genes include “response to circadian rhythm”, “cytokinin 190
stimulus”, “phosphorelay”, and “sulfate assimilation” (FDR<0.05; Supplemental Table 191
S3). These GO terms have functional relevance to N responses in plants, as cytokinin 192
(Sakakibara et al., 2006) and phosphorelay signaling (Engelsberger and Schulze, 2012) 193
are known to mediate N signaling, and N was previously reported to regulate circadian 194
rhythm (Gutiérrez et al., 2008) and sulfate assimilation (Wang et al., 2003). Overall, 195
these results support that genes relevant to N signaling, reduction, and assimilation are 196
regulated at the chromatin level through H3K36me3 modification in response to 197
changes of N levels in WT plants. In contrast, the N-mediated modification of these N-198
relevant target genes in WT are abrogated in the sdg8-5 mutant. Instead, in the sdg8-5 199
mutant the changes of H3K36me3 occur at a different set of genes in response to N 200
starvation (3 genes) or N supply (23 genes), respectively (Fig. 1; Supplemental Table 201
S4), with no significant enrichment of GO terms. 202
203
In summary, our comparative analyses identified 143 genes whose H3K36me3 204
levels are increased by N starvation in WT, and this response is abrogated in the sdg8-205
5 mutant (Fig. 1). Similarly, we identified 53 genes whose H3K36me3 levels are 206
increased by N supply in WT, but this response is abrogated in the sdg8-5 mutant (Fig. 207
1). We collectively call this group of 196 genes (143 and 53 genes, Fig. 1) “SDG8-208
dependent Differentially Methylated Genes (DMGs)”. 209
210
To determine whether these SDG8-dependent DMGs are directly or indirectly 211
regulated by SDG8, we compared the list of 196 SDG8-dependent DMGs with the set of 212
genes shown to be directly bound targets of SDG8, as previously determined under 213
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highly comparable experimental conditions (Li et al., 2015). This analysis showed that a 214
modest, but statistically significant portion of the 196 SDG8-dependent DMGs were 215
indeed directly bound by SDG8 (Li et al., 2015), (28/196, 14% overlap with an 216
enrichment p-val < 0.001, Supplemental Fig. S2). This significant enrichment provides 217
support that the differential H3K36me3 in response to nitrogen is at least partially 218
attributed to a direct role of SDG8. It is likely that some of the “unbound” DMGs may 219
also be direct targets of SDG8, but they could not be identified by the SDG8 binding 220
assay, which requires stable binding. These false negatives may occur, as the ChIP-221
Seq assay - performed at a given timepoint - identifies only a subset of the bona fide 222
targets, because the interaction of a regulatory protein (in this case an HMT enzyme) to 223
its target genes is a dynamic process (Swift and Coruzzi, 2016). Lastly, a portion of the 224
SDG8-dependent DMGs could be regulated by SDG8 indirectly, i.e. through an 225
intermediate partner. Moreover, we also observed a significant overlap between the 196 226
SDG8-dependent DMGs and a list of 4,058 genes that display significantly less 227
H3K36me3 in the sdg8-5 mutant compared to WT (overlap p <0.001; Fig. 1; 228
Supplemental Results; Supplemental Table S5). In summary, our results suggest that 229
SDG8 controls N-responsive changes in H3K36me3 at a set of functionally relevant 230
genomic loci. 231
232
SDG8 affects transcriptomic responses to N treatments 233
To determine whether and how histone methylation mediated by SDG8 affects the 234
transcription of N-responsive genes, we performed RNA-Seq to compare these 235
responses in the sdg8-5 mutant and WT exposed to nitrate treatments (Supplemental 236
Fig. S1). Specifically, we identified two types of SDG8-mediated regulation of gene 237
transcription: (i) differentially expressed genes (DEGs) that are regulated by SDG8 in 238
the context of nitrate treatment (i.e. genotype x treatment); and (ii) genes whose RNA 239
isoform switching is affected by the deletion of SDG8 in the sdg8-5 mutant. 240
241
Overall, our RNA-Seq analysis supports the notion that SDG8 mediates N-242
triggered reprogramming of gene expression. Specifically, the RNA-Seq reads were first 243
trimmed and mapped to the Arabidopsis TAIR10 genome using the TopHat suite 244
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(Trapnell et al., 2012). The gene counts were generated by HTSeq (Anders et al., 245
2015), and then processed by DESeq2 (Love et al., 2014) with the design 246
(~Genotype+Treatment+Genotype:Treatment) to detect differentially expressed genes. 247
Genes that are regulated by the interaction of nitrogen and genotype were determined 248
based on p-value for Genotype:Treatment interaction, which was adjusted for multi-249
testing error using false discovery rate (FDR) and controlled at 5%. In total, we identified 250
215 DEGs regulated by the interaction of SDG8 genotype and N treatment using the 251
two-factor design by DESeq2 (FDR<0.05; Fig. 2; Supplemental Table S6). This analysis 252
identified genes whose regulation by nitrate is different in WT compared to the sdg8-5 253
deletion mutant. Among this group of misregulated genes are Asparagine Synthase 1 254
(ASN1) – whose promoter was used to isolate the original sdg8-5 mutant (aka cli186) 255
(Thum et al., 2008), as well as genes encoding nitrate transporters (NRT2.1 and 256
NRT1.5) and glutamine synthetase (GLN1;4). Next, using hierarchical clustering, we 257
identified two major clusters of the 215 DEGs. Cluster 1 comprised 126 genes that are 258
highly induced by N starvation in WT, but not in the sdg8-5 mutant (Fig. 2; 259
Supplemental Table S6). The enriched GO terms for this group include “cellular amino 260
acid derivative metabolic process” (FDR<0.0019), “response to nutrient level” 261
(FDR<0.039), and “cellular nitrogen compound metabolic process” (FDR<0.047) 262
(Supplemental Table S7). These Cluster 1 genes whose N-response is misregulated in 263
sdg8-5 share a significant overlap (p<0.001, 6 genes) with the genes that display higher 264
H3K36me3 under the N-starved condition than the N-supplied condition (Fig. 2). This 265
finding for the 126 cluster 1 genes suggests that SDG8 affects the expression of these 266
N-responsive genes through regulating the histone mark H3K36me3, which is known to 267
be associated with actively transcribed genes (Bannister et al, 2005). Cluster 2 contains 268
89 genes induced by N supply in WT, which is abolished or attenuated in the sdg8-5 269
mutant (Fig. 2; Supplemental Table S6). Over-represented GO terms in cluster 2 270
include “cellular amine metabolic process” (FDR<0.0056) and “amino acid transport” 271
(FDR<0.016) (Supplemental Table S8). Similarly, the cluster 2 genes share a significant 272
overlap (p<0.001, 5 genes) with genes that exhibit higher H3K36me3 under N-supplied 273
conditions than N-starved conditions (Fig. 2). Overall, the 215 DEGs have a significant 274
overlap with genes directly bound by SDG8 (Li et al., 2015) (30/215, overlap p < 0.024), 275
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supporting a direct role of SDG8 in regulating this set of SDG8-dependent DEGs. The 276
215 DEGs also have a significant overlap with the 4,058 H3K36 hypomethylated genes 277
(74/215, overlap p < 0.001). Together, these results support that histone methylation by 278
SDG8 regulates transcriptional reprogramming of functionally relevant genes during 279
nitrate treatments. 280
281
SDG8 affects RNA isoform switching in response to N-treatments. 282
Next, because SDG8 mediates H3K36me3 preferentially in the gene body (Li et al., 283
2015), we investigated the role that SDG8 might play in regulating the preferential 284
expression of transcript isoforms in responses to nitrate treatments. To do this, we first 285
tested whether nitrate treatments preferentially regulate expression of specific RNA 286
isoforms in WT plants. We found 26 genes that switched isoform preference between N-287
supplied and N-starved samples (Fig. 3; Supplemental Table S9). Notably, under the N-288
starved condition, the AT3G54830 gene locus is transcribed as a shorter transcript 289
isoform that skips the first four exons, while the longer transcript isoform (with 12 exons) 290
is produced in response to N supply (Fig. 4). This longer isoform encodes a protein that 291
shares high structural similarity with an amino acid/polyamine/organocation 292
transmembrane transporter with ten transmembrane α-helices (Shaffer et al., 2009) 293
(Fig. 4; confidence level 99.9% determined by Phyre2 (Kelley et al., 2015)). Under the 294
N-starved condition, the shorter RNA isoform encodes a truncated putative 295
transmembrane protein that is missing the four N-terminal exons, and the predicted 296
protein structure contains only seven transmembrane α-helices (Fig. 4). Further studies 297
are required to elucidate if there is any functional difference between the putative 298
transmembrane proteins encoded by these two mRNA isoforms of a putative amino acid 299
transporter. 300
301
Interestingly, deletion of SDG8 greatly increases the number of genes with N-302
triggered isoform switches, from 26 in WT to 144 in the sdg8-5 deletion mutant (Fig. 3; 303
Supplemental Table S9). This suggests that SDG8 normally plays a role in maintaining 304
the stability of RNA isoform expression from specific gene loci when plants experience a 305
change in the N environment. In contrast, in the sdg8-5 mutant, the RNA isoform 306
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expression is more likely to alter when plants experience a change in the N 307
environment. Comparison of these two gene lists leads to the identification of 137 genes 308
with RNA isoform switches in the sdg8-5 mutant, but not in WT in response to N supply 309
(Fig. 3; Supplemental Table S9). Overall, we refer to this group of 137 genes as “sdg8-310
dependent isoform-switched genes”. As a group, the sdg8-dependent isoform-switched 311
genes are enriched with functional annotations such as “nucleotide binding” (FDR< 312
0.0011 determined by AgriGO (Tian et al., 2017)) and “membrane/transmembrane” 313
(determined by GeneCloud (Krouk et al., 2015)) (Fig. 3). Transmembrane transport and 314
nucleotide metabolism were also identified as enriched functional groups among 315
alternatively spliced genes involved in maintaining nutrient homeostasis in rice (Dong et 316
al., 2018). In summary, our results indicate that the histone methyltransferase SDG8 317
affects differential RNA isoform production in response to changes in nutrient levels. 318
319
The histone methyltransferase SDG8 affects RNA isoform switching of CCT101 320
which influences its regulatory function 321
We next investigated whether sdg8-dependent RNA isoform switching is related to 322
SDG8-dependent H3K36 methylation. To do this, we compared the gene list of sdg8-323
dependent isoform-switched genes with H3K36 hypomethylated genes in the sdg8-5 324
deletion mutant (Supplemental Table S5). We found a significant overlap between these 325
SDG8 dependent genes (overlap p < 0.002, 32/137 genes), which suggests that SDG8-326
mediated H3K36me3 in the gene body is associated with the RNA isoform switch of its 327
target genes. An example downstream target gene, which encodes a CCT motif protein 328
(CONSTANS, CO-like and TOC1 motif; AT5G53420) (Brambilla and Fornara, 2017), is 329
referred to as CCT101 hereinafter. To study the functional relevance of RNA isoform 330
switches associated with SDG8 targets, we performed functional studies on the 331
CCT101 RNA isoforms as a case study example. 332
333
CCT101 belongs to the ASML2 family of CCT domain proteins (Brambilla and 334
Fornara, 2017). The best studied member of this family is ASML2 (Activator of 335
Spomin::LUC2), which is a regulatory protein that activates expression of sugar-336
responsive genes (Masaki et al., 2005). In the sdg8-5 mutant, the H3K36me3 marks 337
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12
associated with the CCT101 locus are significantly lower, compared to WT (FDR<0.05; 338
Fig. 5). Moreover, the H3K36me3 level associated with the CCT101 gene locus which 339
increases in response to N starvation in the WT, is abrogated in the sdg8-5 mutant 340
(FDR<0.05; Fig. 5). CCT101 has three isoforms based on the TAIR10 annotation 341
(www.arabidopsis.org) (Fig. 5). In our RNA-Seq analysis, we captured all three CCT101 342
RNA isoforms and also found support for an additional isoform that is expressed at very 343
low levels (Fig. 5). CCT101.1, the longest transcript isoform, is the dominantly 344
expressed isoform, while the RNA isoform CCT101.2 is truncated at the 5’ end and is 345
expressed at a lower level (Fig. 5). Interestingly, in the sdg8-5 mutant, the proportions of 346
CCT101.1 and CCT101.2 RNA isoforms change significantly in response to nitrate 347
treatment (FDR=0.02; Fig. 5). In WT, the change in the proportions of CCT101.1 and 348
CCT101.2 RNA isoform due to N treatment is not statistically significant. These data 349
support the notion that SDG8 affects the preferential accumulation of RNA isoforms 350
CCT101.1 and CCT101.2 in response to N-treatment. 351
352
To understand the functional implications of RNA isoform switching of CCT101, 353
we analyzed the regulatory role of the proteins encoded by the two RNA isoforms 354
CCT101.1 and CCT101.2. To do this, we tested their functionality using a plant cell-355
based transient over-expression system called TARGET (Bargmann et al., 2013). In 356
detail, the CCT protein encoded by each of the RNA isoforms was transiently over-357
expressed in Arabidopsis shoot protoplasts, and following controlled nuclear import, the 358
resulting effect of the CCT protein on genome-wide expression was assayed by RNA-359
Seq in three treatment replicates (see Methods). The experiments were performed on 360
the same day using the same batch of protoplasts to ensure that the effect of different 361
CCT isoforms can be directly compared. The overexpression of the protein encoded by 362
the CCT101.1 RNA isoform led to the up-regulation of 341 genes and down-regulation 363
of 122 genes in shoot protoplasts (FDR<0.05, fold-change>2) (Fig. 5; Supplemental 364
Table S10). By contrast, the protein encoded by the CCT101.2 RNA isoform up-365
regulates 414 genes and down-regulates 105 genes (FDR<0.05, fold-change>2) (Fig. 5; 366
Supplemental Table S10). Comparisons of these gene lists indicate that the two 367
CCT101 RNA isoforms encode proteins that regulate overlapping, yet distinct gene 368
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13
targets (Fig. 5; Supplemental Table S10). Moreover, genes regulated by one CCT 369
isoform but not by the other isoform have different enriched GO terms compared to the 370
genes regulated by both isoforms (Fig. 5; Supplemental Table S10), suggesting that the 371
proteins encoded by the two CCT isoforms perform related, yet different, functions. 372
373
Physiological responses to nitrate are abrogated in the histone methyltransferase 374
mutant sdg8-5 375
We next set out to test whether the SDG8-mediated changes in histone methylation and 376
RNA isoforms in response to N-treatment have an impact on nitrogen-related 377
phenotypic responses in planta. To do this, we first examined whether the sdg8-5 378
deletion mutant shows altered physiological responses to nitrate treatments. The sdg8-5 379
mutant and its corresponding WT strain (Li et al., 2015) were grown on ammonium 380
succinate for ten days and then transferred to either low N (0.5 mM KNO3) or high N (5 381
mM KNO3) medium (see Methods). Five to six days after the transfer, we measured 382
chlorophyll content, root architecture, N uptake, and free amino acids. We found that 383
WT plants grown under high N conditions showed significantly enhanced lateral root 384
growth on the nascent primary root (Fig. 6), as well as elevated chlorophyll levels in the 385
shoots (Fig. 6), compared to low N treatment conditions. By contrast, these responses 386
to high N seen in WT were abrogated in the sdg8-5 deletion mutant (Fig.6). The sdg8-5 387
mutant also showed altered N acquisition and free amino acids compared to WT 388
(Supplemental Fig. S3). The sdg8-5 mutant had lower nitrate acquisition than WT plants 389
under the high N condition (Supplemental Fig. S3). Although the total amount of free 390
amino acids was comparabe between WT and the sdg8-5 deletion mutant 391
(Supplemental Fig. S3), free leucine was significantly higher in the sdg8-5 mutant 392
compared to WT under the low N condition (Supplemental Fig. S3), and gamma-393
aminobutyric acid (GABA), a non-proteinogenic amino acid, was significantly induced by 394
N supply in the sdg8-5 mutant but not in WT (Supplemental Fig. S3). In summary, our 395
data support the idea that deletion of the specific histone methyltransferase encoded by 396
SDG8 affects plant physiological responses to nitrate at many different levels. 397
398
DISCUSSION 399
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14
Chromatin modifications including histone modifications have been reported as an 400
additional regulatory layer of plant responses to environmental fluctuations (Charron et 401
al., 2009; Kim et al., 2010). However, the mechanisms through which a specific histone 402
modifying protein affects gene transcription and/or RNA processing in response to 403
environmental change is largely unknown. In this study, we addressed the role of 404
epigenetics in the regulation of RNA isoforms in response to the nutrient environment by 405
demonstrating that the sdg8-5 mutant, with a deletion of a specific histone 406
methyltransferase, displays alterations in the genome-wide H3K36me3 landscape, 407
transcriptome, and RNA isoform expression in response to nitrate treatments, as well as 408
impaired physiological and metabolic responses to N nutrient treatments. Overall, our 409
study links a specific chromatin modification and histone methyltransferase with RNA 410
processing and nitrate responses in plants. 411
412
Our previous studies showed that loss of H3K36 methylation in the gene body of 413
SDG8 targets in the sdg8-5 mutant is associated with down-regulation of gene 414
expression (Li et al., 2015). Our current study has now tested the hypothesis that the 415
histone methylation marks made by SDG8 in the gene body are also able to impact 416
qualitative transcriptomic changes via affecting RNA isoform switches. This hypothesis 417
was inspired by our previous observation that SDG8 is associated with H3K36me3 in 418
the gene body (Li et al., 2015), indicating a potential role for SDG8 in transcriptional 419
elongation or co-transcriptional splicing. This notion was also supported by a recent 420
study showing that H3K36me3 alters alternative RNA splicing of genes in the 421
temperature response (Pajoro et al., 2017), and by findings that the yeast SET2 protein, 422
a homolog of Arabidopsis SDG8, prevents spurious transcripts from starting in the 423
middle of a gene (Carrozza et al., 2005). Moreover, SETD2, the mammalian homolog of 424
Arabidopsis SDG8, is associated with proper RNA processing including the choice of 425
transcription start site and exon-intron splicing (Simon et al., 2014). We thus tested the 426
hypothesis that SDG8 plays a role in RNA processing in nutrient signaling in 427
Arabidopsis. Our results indeed suggest that the Arabidopsis SDG8 protein affects RNA 428
processing in transcriptional responses to N treatments. 429
430
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15
How do SDG8 and H3K36me3 affect the production of RNA isoforms? In yeast, 431
the H3K36 methylation deposited in the gene body by SET2 (homolog of SDG8) 432
suppresses histone acetylation in the middle of a gene, thus preventing transcription 433
from alternative start sites (Carrozza et al., 2005). It is possible that Arabidopsis SDG8 434
functions similarly to its yeast homolog, and this hypothesis could be tested by 435
performing ChIP-Seq for histone acetylation in the sdg8-5 mutant. Alternatively, 436
H3K36me3 might differentially “mark” histones associated with exons and introns, thus 437
guiding spliceosome machinery for proper splicing. In line with this hypothesis, we found 438
that there is a minor, but significant, enrichment of H3K36me3 deposited by SDG8 in 439
the exons compared to the introns (p-value <0.005), based on our epigenome data (Li 440
et al., 2015). In addition, we speculate that the H3K36me3 marks mediated by SDG8 441
could change the rate of Pol-II mediated transcription, which was shown to affect the 442
fidelity of splicing in yeast (Aslanzadeh et al., 2018). 443
444
To further investigate how SDG8-mediated RNA isoform preference can affect 445
protein function, we focused our functional validation on one putative regulatory protein, 446
CCT101. We found that SDG8 affects the level of H3K36me3 associated with the 447
CCT101 locus. Specifically, a reduced H3K36me3 mark at the CCT101 locus favors the 448
transcription of the short RNA isoform (CCT101.2), while high levels of H3K36me3 are 449
associated with the transcription of the long RNA isoform (CCT101.1). The missing N-450
terminal polypeptide in the short RNA isoform CCT101.2 - relative to the longer isoform 451
CCT101.1 - is not predicted as a signal peptide by SignalP (Nielsen, 2017). Therefore, 452
the biochemical function of the missing N-terminal polypeptide requires further 453
investigation. To this end, we tested whether the protein encoded by the two CCT101 454
RNA isoforms have different regulated genome-wide targets using a transient 455
overexpression system in plant cells called TARGET (Bargmann et al., 2013). Our 456
TARGET studies show that both CCT101 isoforms can regulate hundreds of genes. 457
Specifically, the proteins encoded by the CCT101.1 and CCT101.2 RNA isoforms 458
regulate overlapping, as well as distinct downstream genes. Therefore, the RNA isoform 459
switch between CCT101.1 and CCT101.2 that is affected by SDG8 results in two 460
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16
distinct functional CCT regulatory proteins that appear to regulate different sets of 461
downstream genes. 462
463
How do the genome-wide changes of histone marks that we attribute to SDG8 464
affect the nitrate response in plants at a physiological level? Previous studies showed 465
that levels of SDG8 mRNA are specifically induced in the root pericycle cells by a 466
supply of nitrate (Gifford et al., 2008). Since lateral root initiation occurs in the pericycle, 467
we tested if SDG8 is required for lateral root growth in response to a nitrate signal. We 468
observed that WT plants display an enhanced lateral root growth in the newly grown 469
primary root under high N relative to low N, and this morphological response to N 470
treatments is abolished in the sdg8-5 mutant (Fig. 6). Therefore, SDG8 indeed has a 471
role in mediating lateral root growth in response to N level changes. However, since the 472
H3K36me3 and transcriptome data in our current study were collected from the shoots, 473
an examination of H3K36 status of genes in roots could thus help to elucidate the role of 474
SDG8-mediated histone methylation in sensing and foraging for nitrate. 475
476
One of the many possible roles for SDG8 is that it might serve as an integration 477
point across inputs from photosynthesis, carbon metabolism, and nitrogen metabolism 478
(Thum et al., 2008; Li et al., 2015). Indeed, among the mis-regulated N-responsive 479
genes in the sdg8-5 mutant, GO terms related to response to sugar stimulus are 480
significantly enriched (Supplemental Table S7). In agreement with this role in nutrient 481
crosstalk, we observed that the chlorophyll level is increased by N supply in WT, but not 482
in the sdg8-5 mutant (Fig. 6). Therefore, the sdg8-5 mutant might not be able to provide 483
enough carbon skeletons and reducing power for nitrate assimilation when nitrate level 484
increases. This is supported by the observation that the sdg8-5 mutant has lower nitrate 485
acquisition than WT plants under the high nitrate condition (Supplemental Fig. S3). 486
Another piece of evidence comes from the free amino acid measurements comparing 487
the sdg8-5 mutant and WT plants. GABA, a non-proteinogenic amino acid, has a central 488
position in the interface between carbon and nitrogen metabolism, and is reported to 489
regulate genes involved in nitrogen metabolism and carbon metabolism (Barbosa et al., 490
2010). GABA is synthesized from the nitrogen donor glutamate and degraded into 491
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17
succinate to enter the citric acid cycle to generate carbon molecules (Michaeli and 492
Fromm, 2015). In our study, GABA is significantly induced by N supply in the sdg8-5 493
mutant, but not in WT (Supplemental Fig. S3). Collectively, our results support the 494
notion of altered carbon/nitrogen balance in the sdg8-5 mutant deleted in the H3K36 495
methyltransferase, with impacts on gene expression, RNA processing, and 496
physiological responses to N treatment. 497
498
Conclusions 499
500
Our study shows that nitrate treatments induce genome-wide changes in the histone 501
mark H3K36me3 mediated by SDG8, with impacts on gene expression, RNA 502
processing, and physiological responses. This has revealed a previously overlooked 503
layer of chromatin regulation involved in nutrient signaling. To develop a comprehensive 504
understanding of the scope and function of chromatin regulation in nutrient signaling, it 505
is important that future studies expand to include more epigenetic marks, such as other 506
histone modifications (e.g. histone acetylation), chromatin landscape, and DNA 507
methylation. Such knowledge will reveal how chromatin modifiers could be harnessed 508
and engineered to enable a holistic N response in plants to increase nutrient use 509
efficiency. 510
511
MATERIALS AND METHODS 512
Plant growth for ChIP-Seq and RNA-Seq 513
Arabidopsis (Arabidopsis thaliana) sdg8-5 mutant (aka cli186) and corresponding wild 514
type (WT) (Li et al., 2015) seeds were surface sterilized and grown hydroponically in 515
phytatrays. In detail, approximately 200 seeds were sown onto a nylon mesh supported 516
by a plastic platform in a sterile phytatray filled with 200 ml plant growth media (1× basal 517
MS medium (GIBCO Formula# 97-5068EC), 3 mM sucrose, 0.5 mM NH4 succinate, 0.5 518
g/L MES hydrate, pH 5.8). The phytatrays were kept under white light (50µM•s-1•m-2) in 519
long-day cycle (16h Light/8h Dark) at 22ºC for 16 days. Next, plants were starved for 520
nitrogen for 24 hours in media which is identical to the growth media except for that the 521
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18
0.5 mM NH4 succinate was removed. Next, half of the plants were exposed to nitrate-522
supplied condition (5 mM KNO3 added to the starvation solution). As controls, the other 523
half plants were exposed to the starvation medium with 5 mM KCl added. After 5 hours, 524
the plants were harvested for ChIP-Seq or RNA-Seq (Supplemental Fig. S1). 525
526
ChIP-Seq experiment 527
The H3K36me3 ChIP-Seq was performed as previously described (Li et al., 2015). Two 528
independent biological replicates were collected from shoot tissues. The ChIP-Seq 529
libraries for the first biological replicate were prepared as described in (Li et al., 2015) 530
and sequenced on an Illumina GAII platform with paired-end 75bp format at Cold Spring 531
Harbor Lab (Cold Spring Harbor, NY). The ChIP-Seq libraries for the second replicate 532
were barcoded with NEXTflex DNA Barcodes (Bioo Scientific, Austin, TX) and pooled to 533
sequence on an Illumina HiSeq platform with paired-end 100 bp format at Cold Spring 534
Harbor Lab. 535
536
ChIP-Seq analysis 537
For the first replicate of H3K36me3 ChIP-seq, 20-34 million raw reads were generated 538
for each sample. The raw reads were trimmed for adaptors and low quality using in-539
house Python and Perl scripts, and then mapped to the Arabidopsis genome (TAIR10) 540
using Bowtie (Langmead et al., 2009). On average, 89% of raw reads were mapped to 541
genome, while 78% raw reads mapped to chromosomes. The chromosome mapped 542
reads were used to call significant H3K36me3 islands and significantly differentially 543
methylated islands with H3K36me3 using SICER (Zang et al., 2009) as previously 544
described (Li et al., 2015). For the second replicate, 40-47 million raw reads were 545
generated for each library and 65% were mapped to the genome, while 57% were 546
mapped to chromosomes. The data analysis was performed in the same way as a 547
replicate one. The ChIP-Seq signal of the two replicates share a high similarity 548
(correlation coefficient=0.9 across all the identified islands). To combine the results from 549
the two replicates, a nitrate responsive DMG (differentially methylated gene) has to 550
pass the following criteria: i) identified as significantly different (FDR <0.05) for both 551
replicates, and ii) the mean fold change (N-supplied vs. N-starved) of Enrichment 552
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19
(Enrichment=ChIP/Input) of the two replicates has to be greater than 1.2 for the 553
transient N treatment. We used a relatively low cutoff for fold change that is similar to a 554
previous study (Li et al., 2015) to detect H3K36me3 changes caused by N, because: (1) 555
compared to RNA-Seq analysis that samples multiple copies of a transcript in a cell, 556
histone ChIP-Seq samples only one copy of genomic DNA from a cell, therefore a 557
smaller dynamic range is expected; (2) the nitrate treatment is a transient treatment that 558
lasts for only a few hours; and (3) histone methylation compared to histone acetylation 559
is relatively stable during the time frame of our treatment (in hours) (Barth and Imhof, 560
2010). Nonetheless, changing the fold change cutoff from 1.0 to 2 revealed the same 561
trend, which is that WT shows more H3K36me3 changes in response to nitrate 562
treatment compared to the sdg8-5 mutant (Supplemental Fig. S4). 563
To identify hypomethylated genes, WT and sdg8-5 mutants were compared for 564
N-supplied and N-starved conditions separately, and then the overlap (approximately 565
90% overlap) between two conditions was identified. The analysis regarding biological 566
replicates and statistical cutoff was similar to determining the nitrate-responsive DMGs 567
described above, with fold change of enrichment greater than 2 as the cutoff, because 568
the difference between WT and mutant is a long-term effect compared to the transient N 569
treatment. 570
571
Gene overlap analysis 572
The significance level of overlap between two gene lists was calculated by Genesect 573
tool within the VirtualPlant platform (Katari et al., 2010). Genesect is a non-parametric 574
randomization test to determine whether an overlap between two gene lists is 575
significantly higher or lower than by chance. 576
577
RNA-Seq experiments 578
Three biological replicates of plant shoot tissues were harvested for each genotype and 579
treatment, while each biological replicate is a pool of approximately 200 seedlings. Total 580
RNA samples were extracted using RNeasy Plant Mini kit (Qiagen, Hilden, Germany) 581
following the manufacturer's instruction. The quality and quantity of RNA were 582
determined using a Bioanalyzer (Agilent Genomics, Santa Clara, CA). 9 µg total RNA of 583
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20
each sample was used to prepare an RNA-Seq library using a home-made protocol 584
(Wang et al., 2011). The RNA-Seq libraries were barcoded with the NEXTflex DNA 585
Barcodes. 12 libraries were pooled and sequenced in one lane of the Illumina Hi-Seq 586
platform with the paired-end 100bp format at Cold Spring Harbor Lab. 587
588
RNA-Seq analysis 589
15 million to 20 million read pairs were generated for each RNA-Seq library. The reads 590
were trimmed for quality and adaptor using an in-house Python script. The trimmed 591
reads were then mapped to the Arabidopsis TAIR10 genome using the TopHat suite 592
(Trapnell et al., 2012). On average, 81% of the raw read pairs were mapped to the 593
genome concordantly. Next, two analysis pipelines were performed to answer two 594
different questions: i) to identify differentially expressed genes (DEG) using DESeq2 595
(Love et al., 2014); and ii) to identify alternative splicing genes (ASG) using the cuffdiff 596
function in the Cufflinks package (Trapnell et al., 2012). For the first pipeline, the gene 597
counts were generated by HTSeq (Anders et al., 2015). The raw gene counts were then 598
processed by DESeq2 with the design (~Genotype+Treatment+Genotype:Treatment). 599
Note that DESeq2 deals with normalization and low counts filtering internally (Love et 600
al., 2014). To identify genes that are regulated by the interaction of nitrogen and 601
genotype, p-value for NxG interaction was adjusted for multi-testing with false discovery 602
rate (FDR) controlled at 5%. For the second pipeline to detect alternative spliced 603
isoforms, the mapped reads were assembled into transcriptome (to detect new splicing 604
events), and gene counts were generated using Cufflinks (Trapnell et al., 2012). The 605
cuffdiff function in the Cufflinks package was used to call genes with alternative splicing 606
between N-supplied groups and N-starved groups, in three categories (alternative 607
promoter use, alternative splicing from the same primary transcript, and alternative 608
coding sequence (CDS)) with significance cutoff FDR < 0.1 (Trapnell et al., 2012). The 609
conclusion remains the same if a different statistical cutoff such as FDR 5% is used. 610
Note that cuffdiff only considers genes with high read counts for the alternative splicing 611
analysis, therefore, the number of genes considered for the alternative splicing analysis 612
is ~2000 for alternative promoter use, ~5800 for alternative splicing from the same 613
primary transcript, and ~1800 for alternative CDS output. Therefore, the actual number 614
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21
of significant alternative splicing events is likely to be higher. To confirm the alternative 615
splicing analysis by Cufflinks, we also analyzed the same RNA-Seq data with DEXSeq 616
(Anders et al., 2012) for differential exon usage. The conclusion was also supported by 617
DEXSeq analysis. 618
619
Plant growth for N response phenotyping 620
The sdg8-5 mutant and the corresponding wild type (WT) were surface sterilized and 621
planted on growth medium as below (unless indicated otherwise): 1× nitrate-free 622
Murashige and Skoog medium [Caisson MSP10], 3 mM sucrose, 0.5 mM ammonium 623
succinate, 0.5 g/L sodium MES (sodium 2-(N-Morpholino)ethanesulfonic acid), 100 624
mg/L I-inositol, 0.5 mg/L niacin, 0.5 mg/L pyridoxine HCl, 0.1 mg/L thiamine HCl, and 625
1% (w/v) agar, pH 5.8. The seeds were stratified at 4˚C in dark for four days and then 626
moved to a Percival growth chamber at 22˚C, with a light cycle of 16h light / 8h dark 627
with 50 µM•s-1•m-2 light. The plants were grown vertically on plates. After 10 days, the 628
plants were transferred to two groups of vertical plates, with high N and low N 629
treatments, separately. The high N and low N plates are identical to the growth plates, 630
except for that the 0.5 mM ammonium succinate was replaced by either 5 mM (for high 631
N) or 0.5 mM KNO3 (for low N, with an additional 4.5 mM KCl to balance). For the 15N 632
analysis, the KNO3 was spiked with 1% (v/v) K15NO3 to allow analysis of N uptake. The 633
roots were scanned 5 days after the treatment for root architecture analysis, while the 634
chlorophyll measurement (N=6, individual plants), 15N measurement (N=6, individual 635
plants), and amino acid profiling (N= 3-5, pooled 150 seedlings) were performed 6 days 636
after the treatment. For the amino acid profiling experiment, the plants were first grown 637
with the following nutrients: 1× nitrate-free Murashige and Skoog medium [Caisson 638
MSP10], 3 mM sucrose, 0.5 mM ammonium succinate, 0.5 g/L NaMES, and 1% (w/v) 639
agar, with pH 5.8, and then transferred to high N or low N plates where 0.5 mM 640
ammonium succinate was replaced by either 5 mM KNO3 or 0.5 mM KNO3 641
supplemented with 4.5 mM KCl. 642
643
15N analysis 644
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22
After 6 days of N treatments (with 5 or 0.5 mM KNO3 spiked with 1% v/v K15NO3), plants 645
were analyzed for net 15NO3- uptake. Individual seedlings were first rinsed in 0.1 mM 646
CaSO4, and then placed in pre-weighed tin capsules (CE Elantech, NJ), left at 70˚C 647
oven for 48 hrs. Dry weight was determined and the total nitrogen and atom % 15N were 648
analyzed by continuous-flow isotope ratio mass spectrometer, using a Euro-EA Euro 649
Vector elemental analyzer coupled with an IsoPrime mass spectrometer (GV 650
Instruments). 651
652
Chlorophyll analysis 653
Individual seedling was weighed, flash frozen in 1.5 ml tubes and ground to powder 654
using tissuelyzer (Qiagen). Next, 300 µl methanol was added to each sample and tubes 655
were rotated for 10 min at room temperature. The lysate was then centrifuged at 14,000 656
rpm for 5 minutes, and 200 µl supernatant was transferred and measured with an 657
Infinite M200 plate reader for Absorbance at 750nm, 665nm and 652nm, with methanol 658
as the background controls. The chlorophyll content was calculated as previously 659
described (Porra, 2002). For the high N vs low N experiment, six individual seedlings 660
were analyzed for each genotype/condition. 661
662
Root architecture analysis 663
After transferring the seedlings to high N and low N plates, the end of each primary root 664
was marked with a permanent marker on the plate, to allow measurement of newly 665
grown roots. After 5 days in high N vs low N condition, the roots were scanned, and the 666
resulting images were analyzed using ImageJ (imagej.nih.gov/ij/). The root 667
measurements generated by ImageJ were summarized and analyzed using in-house 668
Perl and R scripts. 669
670
Amino acid profiling 671
Approximately 150 seedlings were pooled for each replicate per genotype and per N 672
condition. In total, three to five replicates were analyzed per genotype and per condition. 673
Amino acid profiling was performed as previously described (McCoy et al., 2018). 674
Briefly, shoots were extracted in 10 ml of methanol. The neutral and basic amino acids 675
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23
were separated from acidic amino acids in order to measure glutamate and aspartate 676
separately from glutamine and asparagine. Free amino acids were then derivatized with 677
heptafluorobutyric acid and analyzed via GC-MS. Pools were quantified in reference to 678
external standards using ⍺-amino-butyrate for neutral and basic amino acids and ⍺-679
amino-adipate for glutamate and aspartate. Pyroglutamate was identified and added to 680
the glutamine abundance. B-cyanoalanine was also identified and added to the 681
asparagine abundance. The resulting measurements were normalized to fresh weights. 682
The significance of difference between nitrate treatments and genotypes was 683
determined by t-test as well as two-way ANOVA (genotype * nitrate). Only amino acids 684
that show significant p-value for genotype difference in both t-test and ANOVA are 685
reported (p<0.05). 686
687
Determining the genome-wide targets of isoforms of CCT 101 688
The cell-based TARGET system for transcription factor perturbation (Bargmann et al., 689
2013) was used to identify the genome-wide targets of different isoforms of CCT101. 690
Plants were grown on vertical petri plates containing ½ MS media with 1 mM KNO3 for 691
10 days prior to the experiment. The shoot protoplast preparation, transient 692
transformation, and cell-sorting were performed as described previously (Bargmann et 693
al., 2013). Protoplasts isolated from shoots were treated with 20 mM KNO3 and 20 mM 694
NH4NO3 prior to the nuclear import of transcription factor induced by dexamethasone. 695
Cells over-expressing either CCT101 isoforms or empty vector controls were collected 696
in triplicate in the same batch of experiment and RNA-Seq libraries were prepared from 697
mRNA using the NEBNext® Ultra™ RNA Library Prep Kit for Illumina. All three isoforms 698
are stably expressed at comparable levels based on RNA-Seq readout. The libraries 699
were pooled and sequenced on the Illumina NextSeq 500 platform for 75 cycles. The 700
RNA-Seq reads were aligned to the TAIR10 genome assembly using TopHat and gene 701
expression was estimated using the GenomicFeatures/GenomicAlignments packages 702
(Lawrence et al., 2013). Genes showing differential expression between the samples 703
with an overexpressed isoform and the samples with empty vector were identified using 704
DESeq2 package (Love et al., 2014) at a significance level of FDR<0.05 and fold 705
change >2. 706
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24
707
Accession numbers 708
The ChIP-Seq and RNA-Seq data for this study can be accessed in NCBI SRA 709
(Accession number SRP149810). SDG8 is encoded by AT1G77300. 710
711
Supplemental Data 712
Supplemental Figure S1. Experimental design for the ChIP-Seq and RNA-Seq 713
experiments. 714
Supplemental Figure S2. Overlaps of the two groups of differentially methylated genes 715
with the SDG8 bound genes. 716
Supplemental Figure S3. Nitrogen uptake and assimilation in WT and the sdg8-5 717
mutant. 718
Supplemental Figure S4. The number of differentially methylated genes between N-719
supplied and N-starved conditions identified using different fold change cutoffs. 720
Supplemental Table S1. Genes with increased H3K36me3 level in N-supplied group 721
compared to N-starved group (A), or vice versa (B), in wild-type plants 722
Supplemental Table S2. Significantly enriched GO terms (Biological process) among 723
the 143 genes whose H3K36me3 increases in response to N-starvation in wild-type, 724
determined by Biomaps tool in VirtualPlant platform. 725
Supplemental Table S3. Significantly enriched GO terms (Biological process) among 726
the 53 genes whose H3K36me3 increases in response to N-supply in wild-type, 727
determined by Biomaps tool in VirtualPlant platform. 728
Supplemental Table S4. Genes with increased H3K36me3 level in N-supplied group 729
compared to N-starved group (A), or vice versa (B), in the sdg8-5 mutant. 730
Supplemental Table S5. 4,058 H3K36 hypomethylated genes that show significantly 731
higher H3K36me3 in WT compared to the sdg8-5 mutant in N-supplied and N-732
starved groups. 733
Supplemental Table S6. Genes that are regulated by the interaction of genotype and 734
nitrogen. 735
Supplemental Table S7. Significantly enriched GO terms among the cluster 1 genes 736
regulated by the interaction of genotype and nitrogen, determined by AgriGO. 737
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25
Supplemental Table S8. Significantly enriched GO terms of cluster 2 genes that are 738
regulated by the interaction of genotype and nitrogen, determined by AgriGO. 739
Supplemental Table S9. Genes with isoform switch between N and KCl in WT and 740
sdg8-5, determined by cuffdiff in Cufflinks. 741
Supplemental Table S10. The target genes of CCT101 and their functional 742
enrichments. 743
744
ACKNOWLEDGEMENTS 745
We thank Dr. David Rhodes for insightful discussion about amino acid analysis. We also 746
thank two undergraduates Ellen Cho and Teresa Qi at New York University for their 747
help in root analysis, and Pascal Tillard in INRA (Montpellier, France) for 15N 748
measurements. This work is supported by Department of Energy (DE-FG02-749
92ER20071 to GMC), National Science Foundation (MCB 1412232) to GMC, USDA 750
National Institute of Food and Agriculture Hatch project numbers 1013620 to YL, and 751
177845 to JRW, and start-up funds from Purdue University to YL and JRW. 752
753
FIGURE LEGENDS 754
755
Figure 1. Nitrogen-responsive H3K36 methylation is attenuated in the sdg8-5 756
deletion mutant. (A) The comparison of genome-wide H3K36me3 patterns between N-757
supplied (+N) and N-depleted (-N) WT plants identified 196 genes with differentially 758
methylated H3K36, including 143 genes that have higher H3K36me3 in -N than in +N, 759
and 53 genes that have higher H3K36me3 in +N than in -N. In the sdg8-5 mutant, only 760
three genes were identified to have higher H3K36me3 in -N than in +N, and 23 genes 761
were identified to have higher H3K36me3 in +N than in -N. There is no overlap between 762
the genes identified in WT and that identified in the sdg8-5 mutant. Therefore, we 763
identified 143 genes that gain H3K36me3 in N starvation condition in WT but not in the 764
sdg8-5 mutant, and 53 genes that gain H3K36me3 in N-supplied condition in WT but in 765
the sdg8-5 mutant. (B) Overlaps of the two groups of differentially methylated genes 766
with the H3K36 hypomethylated genes (i.e. genes with less H3K36me3 in the sdg8-5 767
mutant compared to WT) are shown in the Venn diagram. Asterisks indicate a 768
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26
significant overlap: *** p<0.001 determined by Genesect. (C) Exemplary genes show 769
H3K36me3 responses to nitrate level change in WT but not in the sdg8-5 mutant. 770
771
Figure 2. N-responsive reprogramming of gene expression is altered in the sdg8-5 772
deletion mutant. Expression levels of 215 genes significantly regulated by the 773
interaction of nitrate and SDG8 genotype are shown as a heatmap. Two major clusters 774
of the 215 DEGs were identified: (i) Cluster 1 comprises 126 genes that are more highly 775
induced by N starvation in WT, compared to the sdg8-5 mutant. The Cluster 1 genes 776
share a significant overlap with the genes that display higher H3K36me3 under N-777
starved condition than N-supplied condition. (ii) Cluster 2 contains 89 genes induced by 778
N supply in WT, which is abolished or attenuated in the sdg8-5 mutant. The Cluster 2 779
genes share a significant overlap with genes that exhibit higher H3K36me3 under N-780
supplied condition than N-starved condition. 781
782
Figure 3. Deletion of the histone methyltransferase SDG8 increases N-responsive 783
RNA isoform switching. We identified 144 genes that show N-responsive isoform 784
switching in the sdg8-5 mutant, and only 26 genes that show isoform switching in WT. 785
The comparison between the two gene lists identified 137 genes that are dependent on 786
SDG8 for WT-like RNA processing. The functional enrichment within these 137 genes 787
were determined and visualized using GeneCloud. 788
789
Figure 4. A putative transporter displays RNA isoform switching in response to N. 790
(A) Expression profile of AT3G54830 (encoding a putative transmembrane transporter) 791
shows distinct preference for RNA isoforms under N-supplied condition (+N, green, long 792
RNA isoform) compared to N-starved condition (-N, blue, short RNA isoform). 793
Specifically, a gene-centric view of RNA-Seq sequencing depth is shown with three 794
biological replicates for each condition. (B) Predicted structure of proteins encoded by 795
the long or short mRNA isoform suggested that the encoded protein is likely a putative 796
transmembrane transporter, with distinct structures under N-supplied vs. N-starved 797
conditions. Structures were predicted using Phyre2. 798
799
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27
Figure 5. Histone methylation by SDG8 regulates RNA isoform preference of 800
CCT101 (AT5G53420), which impacts transcriptional regulation. (A) H3K36me3 801
levels at the CCT101 locus are affected by the sdg8-5 deletion and nitrate levels. In WT, 802
the H3K36me3 mark at the CCT101 locus is induced by N starvation. By contrast, in the 803
sdg8-5 mutant, the H3K36me3 mark at the CCT101 locus is greatly reduced and not 804
responsive to nitrate level changes. Two biological replicates of histone ChIP-Seq, each 805
sampled a pool of 200 to 400 seedlings, were used to calculate the H3K36me3 levels. 806
The error bars represent standard error of the mean. ****: p<0.001. The p-values were 807
determined using SICER. (B) CCT101 has four isoforms identified by Cufflinks. Three of 808
them are also annotated by TAIR10 (CCT101.1, CCT101.2, and CCT101.3, as 809
displayed in panel A). The proportion of transcripts for each CCT101 mRNA isoform is 810
shown as pie chart for the genotype and condition tested. In the sdg8-5 mutant but not 811
in the WT, the change of nitrate level induces a significant isoform switch. (C) 812
Overlapping yet different genes are regulated by proteins encoded by isoforms 813
CCT101.1 and CCT101.2. Numbers of genes regulated by each isoform are shown 814
within parentheses. Significantly enriched GO terms are displayed for shared targets 815
between the two isoforms, or unique targets of either isoform. 816
817
Figure 6. Physiological N responses are attenuated in the histone 818
methyltransferase deletion mutant sdg8-5. Root architecture, specifically nascent 819
lateral root growth (A) and chlorophyll content (B), is regulated by nitrogen in the WT 820
plants but not in the sdg8-5 mutant. The sdg8-5 mutants and the corresponding WT 821
seedlings were first grown on ammonium succinate for ten days and then transferred to 822
either low N (0.5 mM KNO3) or high N (5 mM KNO3) medium. Chlorophyll content (N=6, 823
individual plants) and root architecture (N=12-18, individual plants) were measured five 824
to six days after the transfer. The p-values were calculated by Student’s t-test, and the 825
error bars represent standard error of the mean. LR: lateral roots. PR: primary roots. 826
827
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Figure 1.
Figure 1. Nitrogen-responsive H3K36 methylation is attenuated in the sdg8-5 deletion mutant. (A) The comparison of genome-wide H3K36me3 patterns between N-supplied (+N) and N-depleted (-N) WT plants identified 196 genes with differentially methylated H3K36, including 143 genes that have higher H3K36me3 in -N than in +N, and 53 genes that have higher H3K36me3 in +N than in -N. In the sdg8-5 mutant, only 3 genes were identified to have higher H3K36me3 in -N than in +N, and 23 genes were identified to have higher H3K36me3 in +N than in -N. There is no overlap between the genes identified in WT and that identified in the sdg8-5 mutant. Therefore, we identified 143 genes that gain H3K36me3 in N starvation condition in WT but not in the sdg8-5 mutant, and 53 genes that gain H3K36me3 in N-supplied condition in WT but in the sdg8-5 mutant. (B) Overlaps of the two groups of differentially methylated genes with the H3K36 hypomethylated genes (i.e. genes with less H3K36me3 in the sdg8-5 mutant compared to WT) are shown in venn diagram. Asterisks indicate a significant overlap: *** p<0.001 determined by Genesect. (C) Exemplary genes show H3K36me3 responses to nitrate level change in WT but not in the sdg8-5 mutant.
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Figure 2
Figure 2. N-responsive reprogramming of gene expression is altered in the sdg8-5 deletion mutant. Expression levels of 215 genes significantly regulated by the interaction of nitrate and SDG8 genotype are shown as a heatmap. Two major clusters of the 215 DEGs were identified: (i) Cluster 1 comprises 126 genes that are more highly induced by N starvation in WT, compared to the sdg8-5 mutant. The cluster 1 genes share a significant overlap with the genes that display higher H3K36me3 under N-starved condition than N-supplied condition. (ii) Cluster 2 contains 89 genes induced by N supply in WT, which is abolished or attenuated in the sdg8-5 mutant. The cluster 2 genes share a significant overlap with genes that exhibit higher H3K36me3 under N-supplied condition than N-starved condition.
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Figure 3
Figure 3. Deletion of SDG8 increases N-responsive RNA isoform switching. We identified 144 genes that show N-responsive isoform switching in the sdg8-5 mutant, and only 26 genes that show isoform switching in WT. The comparison between the two gene lists identified 137 genes that are dependent on SDG8 for WT-like RNA processing. The functional enrichment within these 137 genes were determined and visualized using GeneCloud.
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Figure 4
Figure 4. A putative transporter displays RNA isoform switching in response to N. (A) Expression profile of AT3G54830 (encoding a putative transmembrane transporter) shows distinct preference for RNA isoforms under N-supplied condition (+N, green, long RNA isoform) compared to N-starved condition (-N, blue, short RNA isoform). Specifically, a gene-centric view of RNA-Seq sequencing depth is shown with three biological replicates for each condition. (B) Predicted structure of proteins encoded by the long or short mRNA isoform suggested that the encoded protein is likely a putative transmembrane transporter, with distinct structures under N-supplied vs. N-starved conditions. Structures were predicted using Phyre2.
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Figure 5.
Figure 5. Histone methylation by SDG8 regulates RNA isoform preference of CCT101 (AT5G53420), which impacts transcriptional regulation. (A) H3K36me3
levels at the CCT101 locus are affected by the sdg8-5 deletion and nitrate levels. In WT,
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the H3K36me3 mark at the CCT101 locus is induced by N starvation. By contrast, in the
sdg8-5 mutant, the H3K36me3 mark at the CCT101 locus is greatly reduced and not
responsive to nitrate level changes. Two biological replicates of histone ChIP-Seq, each
sampled a pool of 200 to 400 seedlings, were used to calculate the H3K36me3 levels.
The error bars represent standard error of the mean. ****: p<0.0001. The p-values were
determined using SICER. (B) CCT101 has four isoforms identified by Cufflinks. Three of
them are also annotated by TAIR10 (CCT101.1, CCT101.2, and CCT101.3, as displayed
in panel A). The proportion of transcripts for each CCT101 mRNA isoform is shown as
pie chart for the genotype and condition tested. In the sdg8-5 mutant but not in the WT,
the change of nitrate level induces a significant isoform switch. (C) Overlapping yet
different genes are regulated by proteins encoded by isoforms CCT101.1 and CCT101.2.
Numbers of genes regulated by each isoform are shown within parentheses. Significantly
enriched GO terms are displayed for shared targets between the two isoforms, or unique
targets of either isoform.
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Figure 6
Figure 6. Physiological N responses are attenuated in the histone methyltransferase deletion mutant sdg8-5. Root architecture, specifically nascent
lateral root growth (A) and chlorophyll content (B), is regulated by nitrogen in the WT
plants but not in the sdg8-5 mutant. The sdg8-5 mutants and the corresponding WT
seedlings were first grown on ammonium succinate for ten days and then transferred to
either low N (0.5 mM KNO3) or high N (5 mM KNO3) medium. Chlorophyll content (N=6,
individual plants) and root architecture (N=12-18, individual plants) were measured five
to six days after the transfer. The p-values were calculated by Student’s t-test, and the
error bars represent standard error of the mean. LR: lateral roots. PR: primary roots.
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