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1 Running Title: RhHB6-RhPR10.1 module regulates flower senescence 1 An ethylene-induced regulatory module delays flower senescence by regulating 2 cytokinin content 3 Lin Wu 1,4 , Nan Ma 1,4 , Yangchao Jia 1 , Yi Zhang 1 , Ming Feng 1 , Cai-Zhong Jiang 2,3 , 4 Chao Ma 1* , Junping Gao 1* 5 6 1 Beijing Key Laboratory of Development and Quality Control of Ornamental Crops, Department 7 of Ornamental Horticulture, China Agricultural University, Beijing 100193, PR China 8 2 Crops Pathology and Genetic Research Unit, United States Department of Agriculture, 9 Agricultural Research Service, Davis, CA, 95616, USA 10 3 Department of Plant Sciences, University of California at Davis, Davis, CA, 95616, USA 11 12 4 These authors contributed equally to this work. 13 * Address correspondence to [email protected] and [email protected]. 14 15 One-sentence summary: A regulatory module by which ethylene and cytokinin interact affects 16 rose flower senescence 17 18 List of author contributions: L.W., N.M., C.M., and J.G. conceived and designed the 19 experiments; L.W., Y.J., Y.Z., and M.F. performed the experiments; N.M., C.J., and C.M. 20 provided technical support, conceptual advice, and analyzed the data; C.M., L.W., N.M., and 21 J. G. wrote the article. 22 23 Funding: This work was supported by the National Natural Science Foundation of 24 China (Grants numbers 31520103913, 31471903, and 31130048). 25 26 The author responsible for distribution of materials integral to the findings presented in this 27 article in accordance with the policy described in the Instructions for Authors 28 (http://www.plantphysiol.org/) is: Junping Gao ([email protected]) & Chao Ma 29 ([email protected]). 30 31 Plant Physiology Preview. Published on November 22, 2016, as DOI:10.1104/pp.16.01064 Copyright 2016 by the American Society of Plant Biologists https://plantphysiol.org Downloaded on December 1, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

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Running Title: RhHB6-RhPR10.1 module regulates flower senescence 1

An ethylene-induced regulatory module delays flower senescence by regulating 2 cytokinin content 3 Lin Wu1,4, Nan Ma1,4, Yangchao Jia1, Yi Zhang1, Ming Feng1, Cai-Zhong Jiang2,3, 4

Chao Ma1*, Junping Gao1* 5 6 1 Beijing Key Laboratory of Development and Quality Control of Ornamental Crops, Department 7 of Ornamental Horticulture, China Agricultural University, Beijing 100193, PR China 8 2Crops Pathology and Genetic Research Unit, United States Department of Agriculture, 9 Agricultural Research Service, Davis, CA, 95616, USA 10 3Department of Plant Sciences, University of California at Davis, Davis, CA, 95616, USA 11 12 4 These authors contributed equally to this work. 13 * Address correspondence to [email protected] and [email protected]. 14 15

One-sentence summary: A regulatory module by which ethylene and cytokinin interact affects 16 rose flower senescence 17 18 List of author contributions: L.W., N.M., C.M., and J.G. conceived and designed the 19 experiments; L.W., Y.J., Y.Z., and M.F. performed the experiments; N.M., C.J., and C.M. 20 provided technical support, conceptual advice, and analyzed the data; C.M., L.W., N.M., and 21 J. G. wrote the article. 22 23 Funding: This work was supported by the National Natural Science Foundation of 24 China (Grants numbers 31520103913, 31471903, and 31130048). 25 26 The author responsible for distribution of materials integral to the findings presented in this 27 article in accordance with the policy described in the Instructions for Authors 28 (http://www.plantphysiol.org/) is: Junping Gao ([email protected]) & Chao Ma 29 ([email protected]). 30

31

Plant Physiology Preview. Published on November 22, 2016, as DOI:10.1104/pp.16.01064

Copyright 2016 by the American Society of Plant Biologists

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ABSTRACT 32

In many plant species, including rose (Rosa hybrida), flower senescence is 33 promoted by the gaseous hormone, ethylene, and inhibited by cytokinin (CTK) class 34 of hormones. However, the molecular mechanisms underlying these antagonistic 35 effects are not well understood. In this current study, we characterized the association 36 between a pathogenesis related PR-10 family gene from rose (RhPR10.1) and the 37 hormonal regulation of flower senescence. Quantitative RT-PCR analysis showed that 38 the RhPR10.1 was expressed at high levels during senescence in different floral 39 organs, including petal, sepal, receptacle, stamen, and pistil, and that expression was 40 induced by ethylene treatment. Silencing of RhPR10.1 expression in rose plants by 41 virus induced gene silencing accelerated flower senescence, which was accompanied 42 by a higher ion leakage rate in the petals, as well as increased expression of the 43 senescence marker gene, RhSAG12. CTK content and the expression of three CTK 44 signaling pathway genes were reduced in RhPR10.1-silenced plants, and the 45 accelerated rate of petal senescence that was apparent in the RhPR10.1-silenced plants 46 was restored to normal levels by CTK treatment. Finally, RhHB6, a 47 homeodomain-leucine zipper I transcription factor, was observed to bind to the 48 RhPR10.1 promoter, and silencing of its expression also promoted flower senescence. 49 Our results reveal an ethylene-induced RhHB6-RhPR10.1 regulatory module that 50 functions as a brake of ethylene-promoted senescence through increasing the CTK 51 content. 52 53 Key words: Rosa hybrida, flower senescence, ethylene, cytokinin, RhPR10.1 54

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INTRODUCTION 55

The terminal phase of flower development is senescence, which is generally 56 characterized the time to petal wilting, or withering, or by the time to turgid petal 57 abscission (van Doorn, 2001). Flower senescence is a highly regulated process that 58 exhibits many of the structural, biochemical, and molecular changes that are 59 hallmarks of programmed cell death. These include a loss of membrane permeability, 60 increase in reactive oxygen species, and decreased levels of protective enzymes, 61 followed by protein degradation, fatty acid breakdown, and degradation of nucleic 62 acids (Wagstaff et al., 2002; Jones et al., 2005; Xu et al., 2006; Tripathi and Tuteja, 63 2007; Yamada et al., 2009; Shahri and Tahir, 2011; Rogers, 2013). 64

The initiation of flower senescence can be triggered by external or internal cues 65 (Gan and Amasino, 1997; Rogers, 2013; Zhang and Zhou, 2013) and phytohormones 66 have been shown to play a pivotal role in triggering and modulating the progression of 67 senescence, often through combinatorial interactions (Beaudoin et al., 2000; van 68 Doorn and Woltering, 2008; Zhang and Zhou, 2013). Among these hormones, 69 ethylene is considered a major regulator of flower senescence (van Doorn, 2001), and 70 in ethylene-sensitive species, flower senescence is associated with a burst of ethylene 71 production, and the coordinated expression of ethylene-responsive genes, including 72 those with regulatory functions (Ichimura et al., 2009; Lerslerwong et al., 2009). For 73 example, in Arabidopsis thaliana, the expression of microRNA164 was shown to be 74 repressed by ethylene, thereby promoting senescence (Li et al., 2013), while in rose 75 (Rosa hybrida), a reduction in transcript abundance of a homeodomain-leucine zipper 76 (HD-Zip) I transcription factor gene, RhHB1, was also observed to accelerate 77 senescence (Lü et al., 2014). In contrast, the expression of another member of the 78 HD-Zip I transcription factor family, HaHB-4, was reported to be up-regulated in 79 response to ethylene in sunflower (Helianthus annuus), consequently delaying the 80 senescence process (Manavella et al., 2006), suggesting that antagonistic mechanisms 81 influence ethylene-induced senescence. 82

Members of another class of hormones, cytokinins (CTKs), have been proposed 83 to act as anti-senescence factors in flowers (Mayak and Halevy, 1970; Eisinger, 1977). 84 Accordingly, it has been reported that rose varieties with long flower longevity 85 contain higher levels of CTKs than those with shorter lived flowers (Mayak and 86 Halevy, 1970), and CTK application has been shown to delay flower senescence in 87 several plant species including carnation (Dianthus caryophyllus), petunia (Petunia 88 hybrid), and rose (Mayak and Halevy, 1970; Mor et al., 1983; Taverner et al., 1999). 89 Conversely, an increase in transcript abundance of two genes encoding CTK 90

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oxidase/dehydrogenase during carnation petal senescence, accelerated CTK 91 degradation and promoted corolla senescence (Hoeberichts et al., 2007). 92

In addition to the individual effects of ethylene and CTK on flower senescence, 93 these two hormones have been reported to interact in senescence. For example, in 94 carnation, declining CTK levels act as a trigger for ethylene production (Eisinger, 95 1977), while treatment of carnation petals with CTK blocked the conversion of 96 exogenously supplied 1-aminocyclopropane-1-carboxylic acid to ethylene (Mor et al., 97 1983; Taverner et al., 1999). In petunia, driving the overexpression of the CTK 98 biosynthesis gene, IPT, with the Senescence-associated gene 12 (SAG12) promoter, 99 resulted in a significant delay in flower senescence, accompanied by reduced ethylene 100 sensitivity (Chang et al., 2003). In addition, it has been reported that the A. thaliana 101 KNOTTED-like homeodomain protein, KNAT2, acts synergistically with CTKs, and 102 antagonistically with ethylene to delay senescence. (Hamant et al., 2002). However, 103 despite such observations exemplifying the antagonistic effects of ethylene and CTKs 104 on flower senescence, the underlying molecular mechanisms and the many details of 105 the genes in the associated regulatory networks are still largely unknown. 106

In this current study of rose flower senescence, we identified a member of 107 pathogenesis-related (PR) PR-10 family as being involved in the crosstalk between 108 ethylene and CTK signaling. PR proteins, which function in a wide range of processes 109 related to signal transduction and antimicrobial activity (Zubini et al., 2009), have 110 been classified into 17 families based on structural features, of these, the PR-10 111 protein family is a large group containing more than 100 members (Somssich et al., 112 1988; Biesiadka et al., 2002; Hashimoto et al., 2004; Xu et al., 2014), which are 113 typically small and localized in the cytosol (Fernandes et al., 2013). PR-10 proteins 114 play multifunctional roles in defense mechanisms against abiotic and biotic stresses, 115 and in developmental regulation via their RNase activity and/or interaction with 116 ligands (Fernandes et al., 2013; Agarwal and Agarwal, 2014). In addition, CTKs can 117 play a role in biological processes that are mediated by PR-10 proteins (Pasternak et 118 al., 2006; Srivastava et al., 2006, 2007; Fernandes et al., 2008; Zubini et al., 2009). It 119 has been proposed that when PR-10 proteins act as RNases, they may degrade certain 120 types of tRNAs containing a CTK moiety, thereby contributing to the regulation of 121 endogenous CTK content (Zubini et al., 2009). In support of this idea, 122 over-expression of the pea (Pisum sativum) PR-10 gene, ABR17, in Brassica napus 123 and A. thaliana enhanced abiotic stress tolerance through degradation of RNA to 124 increase CTK levels (Srivastava et al., 2006, 2007). PR-10 proteins are also capable 125 of binding CTKs; LIPR-10.2 from yellow lupine (Lupinus arboreus), has been shown 126 to bind trans-zeatin, thereby acting as a reservoir of CTK molecules (Fernandes et al., 127

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2008). However, the role of PR-10 proteins in modulating CTK content during flower 128 senescence has not been characterized. 129

Roses are one of the most important ornamental crops worldwide, and its 130 commodity value largely depends on a long vase life of the flower. It is well-known 131 that ethylene promotes rose flower senescence (Ma et al., 2005), while CTKs delay 132 the process (Mayak and Halevy, 1970; Lukaszewska et al., 1994). Here, we report that 133 an ethylene-induced rose PR-10 family gene, RhPR10.1 inhibits ethylene-induced 134 flower senescence via increasing the CTK content. 135 136

RESULTS 137

The Expression of RhPR10.1 is Induced Following Senescence and Exogenous 138 Ethylene Treatment 139

To investigate the function of the PR-10 family genes during flower senescence, 140 seven transcripts encoding putative PR-10 family proteins were identified in an 141 in-house ethylene-treated rose (Rosa hybrida cv. Samantha) petal transcriptome 142 database (http://bioinfo.bti.cornell.edu/cgi-bin/rose_454/index.cgi.): RU05569, RU05592, 143 RU06312, RU26673, RU20207, RU22712, and RU02982 (Fig. S1). Expression 144 analysis by quantitative RT-PCR (qRT-PCR) showed that, of these genes, RU06312 145 exhibited a trend of increasing expression and the highest accumulation of transcripts 146 during petal opening (Fig. S2), and so we targeted this gene for further analysis. 147

We isolated a 766bp RU06312 cDNA sequence with a 477bp predicted open 148 reading frame by rapid-amplification of cDNA ends (RACE), encoding a deduced 149 protein of 159 amino acids with a conserved P-loop motif and Bet v 1 motif (Lebel et 150 al., 2010) (Fig. S3). Sequence alignment showed that RU06312 had a high degree of 151 sequence homology to FaPR10-4 from Fragaria × ananassa (Fig. S3), and the gene 152 corresponding to RU06312 was named RhPR10.1. Phylogenetic analysis showed that 153 RhPR10.1 belongs to major pollen and food allergens subclass of PR10 protein family 154 (Fernandes et al., 2013) (Fig. S3). 155

The expression of RhPR10.1 was evaluated in different organs during flower 156 opening using qRT-PCR. The opening process of cut rose flowers can be divided into 157 six stages (Ma et al., 2005). The expression patterns of RhPR10.1 in all the floral 158 organs, including sepal, stamen, pistil, petal, and receptacle, showed similar trends, 159 with transcript levels increasing progressively from the partially opened flower bud 160 (stage 1) to the onset of petal wilting (stage 6) (Fig. 1A). The expression was higher in 161 sepals in all the flower opening stages (stage 1 to stage 6) than in the other floral 162

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organs (Fig. 1A). When the petals treated with hormones were tested, we observed 163 that exogenous ethylene and CTK significantly induced RhPR10.1 expression, while 164 an abscisic acid (ABA) treatment resulted in a decrease in expression. Gibberellic 165 acid (GA) and brassinosteroid treatments did not alter RhPR10.1 transcript levels (Fig. 166 1B). 167 168

Silencing of RhPR10.1 Accelerates Rose Flower Senescence 169 To investigate the potential role of RhPR10.1 in flower senescence, expression of 170

the gene was silenced using virus-induced gene silencing (VIGS). We constructed a 171 tobacco rattle virus vector (TRV-RhPR10.1) from the 3’ end region of RhPR10.1 (Fig. 172 S1) to specifically silence RhPR10.1 expression in rose plantlets and petal discs. As 173 shown in Fig. 2A, with the exception of RhPR10.1, none of the RhPR10 family genes 174 exhibited reduced expression levels in RhPR10.1-silenced petals compared to the 175 TRV control, thereby confirming the specificity of the gene silencing. 176

The flower lifespan of rose plantlets can be divided into 6 stages: Stage 1 to 4 are 177 defined as the flower opening phase, where the flowers have not fully opened, while 178 stages 5 and 6 are defined as phases with ornamental value, starting from when the 179 flower fully opens until color fading has occurs (Fig. 2C). The duration of the flower 180 opening phase in the RhPR10.1-silenced plant was indistinguishable from that of the 181 TRV control, with stage 1 to stage 4 lasting 6.8±0.6 and 6.9±0.8 days, respectively 182 (Fig. 2B and C). The flower sizes were also not statistically significant difference 183 between TRV control and RhPR10.1-silenced plantlets. However, in the ornamental 184 value phases, RhPR10.1-silencing caused an accelerated senescence phenotype, with 185 stage 5 to 6 lasting only 3.7 ± 0.4 days, compared to 5.1 ± 0.5 days in the TRV-control 186 (Fig. 2B and C). In addition, we observed that the flower lifespans of TRV-control 187 plantlets were indistinguishable from that of wild-type (WT) (Fig. S4), indicating 188 TRV-control did not affect the flower opening and senescence. 189

We further analyzed the effect of RhPR10.1-silencing on senescence of petal 190 discs. In the TRV control, slight color fading occurred at 10 days (Fig. 3A). In 191 contrast, RhPR10.1-silenced discs showed color fading at day 5, and almost all the 192 petal discs were discolored at day 10 (Fig. 3A). In addition, the ion leakage rate and 193 expression of the senescence marker gene, RhSAG12 (Lü et al., 2014) were 194 significantly higher in RhPR10.1-silenced petal discs than in the TRV control (Fig. 3B 195 and C). 196 197

RhPR10.1-Silencing Induced Senescence is Associated with the CTK Pathway 198 To investigate whether RhPR10.1 influences the CTK pathway during senescence, 199

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we measured the expression of three CTK signaling pathway genes, RhRR3, RhRR8, 200 and RhRR9, in RhPR10.1-silenced rose petals. qRT-PCR analysis showed that the 201 transcript abundance of all three genes, along with RhPR10.1, was reduced in petals 202 of RhPR10.1-silenced flowers compared with TRV controls (Fig. 4A). In addition, the 203 expression of the three genes increased in response to exogenous CTK, confirming 204 their involvement in the CTK response pathway (Fig. 4B). We next examined the 205 contents of the biologically active CTKs, trans-zeatin, N6-(∆2-isopentenyl)-adenine 206 (iP), and isopentenyl adenosine (iPA), in the petals of the RhPR10.1-silenced flowers. 207 iPA levels were found to be significantly reduced in the silenced flowers, while 208 trans-zeatin levels were undetectable in both the TRV control and the 209 RhPR10.1-silenced petals (Fig. 4C), and no significant difference was observed in iP 210 contents (Fig. 4C). 211

Since CTK response and content were altered in RhPR10.1-silenced flowers, we 212 speculated that RhPR10.1-mediated flower senescence may function by regulating 213 CTK pathway. We therefore examined the effect of treating RhPR10.1-silenced petals 214 with 6-BA (a synthetic CTK), and found that its application resulted in a slight color 215 fading after 10 days in both the TRV control and RhPR10.1-silenced petal discs (Fig. 216 5A). In addition, similar expression levels of RhSAG12 and ion leakage rates were 217 detected in petal discs from the TRV control and the RhPR10.1-silenced plants (Fig. 218 5B and C). 219 220

RhPR10.1 is Directly Regulated by Transcription Factor RhHB6 221 To better understand the regulatory mechanism of RhPR10.1, a 1,378 bp 222

promoter region upstream of the RhPR10.1 coding sequence was identified. Sequence 223 analysis using the PlantCARE database 224 (http://bioinformatics.psb.ugent.be/webtools/plantcare/html/) suggested that the 225 cis-elements in the promoter of RhPR10.1 included 13 homologous sequences of 226 ARR1AT for CTK response regulator, 5 binding sites for MYB transcription factor, 4 227 binding sites for MYC, and 3 binding site for WRKY (Fig. S5). In addition, a known 228 binding site (CAATTATTA) for the homeodomain leucine zipper (HD-Zip) subfamily 229 I member, ATHB6 (Himmelbach et al., 2002), was located between positions -706 to 230 -715 (Fig. S5). A search of an in-house generated ethylene treated rose petal 231 transcriptome database (http://bioinfo.bti.cornell.edu/cgi-bin/rose_454/index.cgi) revealed 232 three members of the HD-Zip I subfamily: RhHB1 (RU00522) (Lü et al., 2014), 233 RhHB6 (RU12591), and RhHB7 (RU00731), as well as a member of the HD-Zip II 234 subfamily, RhHB2 (RU01217). 235

We conducted a yeast one-hybrid assay, using the cis-element in the RhPR10.1 236

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promoter, CAATTATTA as bait, to assess the interaction of the HD-Zip family 237 members with the RhPR10.1 promoter (Fig. S6). RhHB1, RhHB7, and RhHB2 did 238 not bind to the RhPR10.1 promoter (Fig. S6); however, RhHB6 bound and triggered 239 expression of the LacZ reporter gene (Fig. 6A), but did not bind a mutated RhPR10.1 240 fragment (Fig. 6A). An electrophoretic mobility shift assay (EMSA) was conducted to 241 confirm the interaction of RhHB6 and the RhPR10.1 promoter and RhHB6 was found 242 to bind to the biotin-labeled proRhPR10.1 probe, moreover binding was gradually 243 attenuated by increasing concentrations of unlabeled probe (Fig. 6B). 244 245

Silencing of RhHB6 Accelerates Rose Flower Senescence 246 To investigate the potential function of RhHB6 in rose petal senescence, we 247

evaluated its expression in different flowering stages and following various hormone 248 treatments. As shown in Fig. 7, the expression of RhHB6 increased during flower 249 opening (Fig. 7A), and was induced by ethylene but inhibited by an ABA treatment 250 (Fig. 7B). When RhHB6 was silenced in rose plantlets by VIGS, the expression levels 251 of RhHB6 and RhPR10.1 were lower in petals of the silenced flowers than in those of 252 the TRV control (Fig. 8A). The duration of the flower opening phase of the 253 RhHB6-silenced flowers was similar to that of the TRV control (Fig. 8B and C); 254 however, RhHB6-silencing caused an accelerated senescence phenotype during the 255 ornamental value phases, with stage 5 to 6 only lasting 3.6 ± 0.3 days, compared to 256 5.7 ± 0.6 days for the TRV-control (Fig. 8B and C). 257

258

DISCUSSION 259

RhPR10.1 is Involved in Ethylene-Induced Rose Petal Senescence 260 Flower senescence is accompanied by changes in the levels of endogenous 261

hormones, which regulate networks of signaling events that control the senescence 262 program (van Doorn and Woltering, 2008; Zhang and Zhou, 2013). Ethylene is known 263 to be a crucial accelerant of flower senescence. Based on the results generated in this 264 study, we propose that RhPR10.1 is a member of the network of genes involved in 265 ethylene-induced senescence process in rose flowers. This hypothesis is supported by 266 the expression pattern of RhPR10.1, which was found to be significantly induced by 267 ethylene treatments (Fig. 1B), and by the accelerated flower senescence phenotype of 268 the RhPR10.1 VIGS plants (Fig. 2 and 3). 269

The expression of PR-10 family members is known to increase during 270 senescence, although their function in senescence is not clear. In bean (Phaseolus 271

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vulgaris L.), the transcript levels of the PR-10 protein, Ypr10, were reported to 272 increase at the onset of leaf senescence (Wallter et al., 1996), and in yellow lupine, the 273 expression of the PR-10 gene, LlPR10.1A was induced in senescent leaves (Sikorski 274 et al., 1999). Here, we showed that in rose, the transcript levels of several PR10 275 members also accumulated during flower senescence (Fig. 1A). Although we targeted 276 RhPR10.1 with the highest accumulation among RhPR-10 members for analyzing the 277 function in senescence, it is feasible that other RhPR-10 genes may also play roles in 278 petal senescence, especially those showed trends of increasing expression during 279 flower senescence. Therefore, additional studies are required to determine whether 280 other RhPR-10 members have functions in flower senescence. 281

Intriguingly, functional analysis of the RhPR10.1 gene indicated that the effect 282 of RhPR10.1 on senescence is antagonistic, with reduced RhPR10.1 expression 283 promoting flower senescence (Fig. 2 and 3). In A. thaliana, during the last step of 284 petal senescence, abscission, the expression of BREVIPEDICELLUS (BP)/KNAT1 is 285 induced, and the accelerated petal abscission phenotype of its knockout mutant, bp, 286 indicates that BP/KNAT1 plays an antagonistic role in the petal abscission process 287 (Shi et al., 2011). It has also been shown that in sunflower, increased expression of 288 HaHB-4 suppresses flower senescence (Manavella et al., 2006). These represent 289 antagonistic mechanisms that may be important in preventing premature organ 290 senescence. 291 292

RhPR10.1 Inhibits Ethylene-Induced Rose Flower Senescence through 293 Regulation of CTK Signaling and Content 294

Altered levels of CTKs and the expression of three CTK signaling pathway genes 295 were observed in rose petals with reduced RhPR10.1 expression (Fig. 4), suggesting 296 that RhPR10.1 may play a role in modulating CTK levels. Sequence alignment 297 showed that RhPR10.1 has a conserved P-loop domain, which functions as a 298 nucleotide binding site correlated to RNase activity (Fig. S3A) (Liu and 299 Ekramoddoullah, 2006). Three conserved residues related to ribonucleolytic activity 300 of PR-10 proteins also existed in RhPR10.1 protein, including glutamic acid (E) at 96, 301 148 and tyrosine (Y) or Histidine (H) at 150 amino acid position (Fig. S3A) (Liu et al., 302 2006; Lebel et al., 2010). The two conserved features above suggest that RhPR10.1 303 may have RNase activity. In addition, RhPR10.1 belongs to major pollen and food 304 allergens subclass of PR10 protein family (Fig. S3B). It is known that PR-10 proteins 305 from major pollen and food allergens subclass acting as RNases can degradate certain 306 types of tRNAs (Fernandes et al., 2013), which have been suggested to be a major 307 source of CTK, since isoprenoid CTKs have been identified in tRNA hydrolysates 308

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(Sakakibara, 2006). Moreover, although PR-10 protein has been reported to have 309 capable of binding to trans-zeatin (Fernandes et al., 2008, 2009), here, we found that 310 the levels of trans-zeatin were undetectable in both TRV and RhPR10.1-silenced 311 petals (Fig. 4C), suggesting that RhPR10.1might not interact with trans-zeatin during 312 petal senescence. 313

It has been suggested that PR-10-mediated tRNA degradation may result in the 314 accumulation of different types of CTKs, depending on plant species and 315 environmental conditions (Srivastava et al., 2006, 2007). Ectopic expression of the 316 pea PR-10 family gene, ABR17, in both B. napus and A. thaliana enhanced 317 endogenous CTK levels (Srivastava et al., 2006, 2007), and in ABR17-overexpressing 318 B. napus seedlings, a significant increase in trans-zeatin riboside, but not iP or 319 cis-zeatin, was observed (Srivastava et al., 2006). However, in ABR17-overexpressed 320 A. thaliana seedlings, increases in the concentrations of cis-zeatin and iP were 321 observed in both Murashige and Skoog (MS) medium- and soil-grown plants, while 322 trans-zeatin content was only increased in MS-grown plants (Srivastava et al., 2007). 323 In this current study, we detected a significant reduction in the levels of iPA, but not 324 trans-zeatin or iP, in RhPR10.1-silenced flowers (Fig. 4C), suggesting that different 325 pathways may function in CTK accumulation in rose petals as a consequence of tRNA 326 degradation. In carnation flower, among iso-pentenyladenine derivatives, iPA was 327 most abundant during flower senescence, and application of iPA delayed the onset of 328 flower senescence (van Staden et al., 1990). It suggests that iPA may play an 329 important role in flower senescence. 330 331

RhHB6 Acts Upstream of RhPR10.1 to Regulate Flower Senescence 332 HD-Zip transcription factors have been grouped into four different classes 333

(HD-Zip I to IV) based on sequence similarity criteria, and this grouped is further 334 supported by their intron/exon patterns (Henriksson et al., 2005). Previous studies 335 have reported that HD-Zip I transcription factors are involved in abiotic stress 336 responses (Ariel et al., 2010), fruit ripening (Lin et al., 2008), and leaf and flower 337 senescence (Manavella et al., 2006; Lü et al., 2014). Here we showed that RhHB6 acts 338 as a regulator of RhPR10.1, and that its expression was also up-regulated in senescent 339 rose petals, and involved in ethylene-induced flower senescence (Fig. 7 and 8). A 340 regulatory action of RhHB6/RhPR10.1 is suggested based on the observation that 341 RhHB6 binds to a 9 bp CAATTATTA region of the RhPR10.1 promoter, as revealed 342 by EMSA and yeast one-hybrid analysis (Fig. 6A and B). In A. thaliana, ATHB6 343 recognized the same 9 bp cis-element present in the promoters of its target genes 344 (Himmelbach et al., 2002). In addition, ATHB6 specific targeting of this cis-element 345

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was revealed by the elimination of the interaction when a single nucleotide was 346 mutated in this cis-element (Himmelbach et al., 2002). The effects of 347 RhHB6-silencing in rose (Fig. 8) are consistent with those of RhPR10.1-silencing, in 348 that both accelerated flower senescence (Fig. 2). We conclude that the 349 ethylene-induced RhHB6-RhPR10.1 regulatory module inhibits ethylene-induced 350 flower senescence through increasing the levels of CTKs (Fig. S7). 351 352

MATERIALS AND METHODS 353

Plant Materials and Growth Conditions 354 For expression pattern analyses of RhPR10.1 and RhHB6, cut Rose (Rosa hybrida 355

cv. Samantha) flowers were harvested from a local commercial greenhouse, and 356 transported to the laboratory within 1h. Stems were then re-cut under water to 25cm 357 length and placed in vases with distilled water. The cut flower opening stages were 358 defined as previously described (Ma et al., 2005). Floral organ samples were collected 359 at different opening stages, and petal samples were collected from the same middle 360 whorl of the flowers. 361

For hormone treatments, cut rose flowers at opening stage 2 were placed in vases 362 containing 100 µM 6-benzyl aminopurine (6-BA), 100 µM ABA, 80 µM GA3, or 5 363 µM brassinosteroid for 24 h. Mock samples were placed in 0.1% dimethylsulfoxide. 364 For ethylene treatment, the cut rose flowers at opening stage 2 were exposed to 10 µl 365 L-1 ethylene in an airtight chamber for 24 h. NaOH solution (1M) was also applied in 366 the chamber to prevent the CO2 accumulation (Lü et al., 2014). 367

For the VIGS assay, rose (Rosa hybrida cv. Samantha) plantlets were propagated 368 by in vitro culturing. Rose shoots of at least 2 cm, and including 1 node, were cultured 369 on propagation medium comprising Murashige and Skoog (MS) medium 370 supplemented with 1.0 mg L-1 6-BA, 0.05 mg L-1 α-naphthalene acetic acid (NAA), 371 and 3 mg L-1 GA3 for 30 days, before being transferred to rooting medium, 372 comprising 1/2-strength MS supplemented with 0.1 mg L-1 NAA, for 30 days. Flower 373 opening stages were defined as follows: stage 1, bud with completely opened sepal; 374 stage 2, bud with outmost petal layer loosened; stage 3, flower with outmost petal 375 layer opened; stage 4, flower with inner petal layer loosened; stage 5, fully opened 376 flower; stage 6, flower with faded color, and consequently reduced ornamental value. 377

378 Cloning, Plasmid Construction and Plant Transformation 379

The full length and promoter sequences of RhHB6 and RhPR10.1 were amplified 380

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using RACE (Clontech) and thermal asymmetric interlaced (TAIL)-PCR (Takara) 381 according to the manufacturer’s instructions, respectively. All PCR products were 382 sub-cloned into the pGEM-T easy vector (Promega), and then transformed into 383 Escherichia coli DH5α cells prior to sequencing. 384 The RhHB6 VIGS construct was created by inserting a 365 bp fragment of the 385 RhHB6 3’-untranslated region into the pTRV2 vector, and the RhPR10.1 VIGS 386 construct by inserting a 210 bp fragment (Fig. S1) from the 3’-untranslated region into 387 the pTRV2 vector. 388

In order to express the RhHB6 recombinant protein in E. coli for the EMSA (see 389 below for details), the open reading frame (ORF) of RhHB6 was inserted into the 390 pGEX-4T-2 vector to produce the GST-RhHB6 fusion protein. 391 For the yeast one-hybrid assay (see below), the ORF of RhHB6 was cloned into the 392 EcoR I and Xho I sites of the pJG4-5 vector (Clontech), resulting in the GAD-RhHB6 393 construct. To generate a construct expressing the LacZ reporter gene driven by the 394 wild-type or mutant motif of the RhPR10.1 promoter, a 35 bp wild-type or mutant 395 oligonucleotide was synthesized and ligated into the pLacZi2μ vector (Lin et al., 396 2007). All primers used above are listed in Table S1. 397 398 RNA Extraction and qRT-PCR 399

Total RNA was extracted from rose floral organs as previously described (Xue et 400 al., 2008). 1μg of DNase-treated RNA was used to synthesize cDNA in a 25 μl 401 reaction volume by Superscript Reverse Transcriptase (Invitrogen). qRT-PCR 402 reactions were performed using 1μl cDNA as a template and the Step One PlusTM 403 real-time PCR system (Applied Biosystems) with the KAPATM SYBR® FAST 404 quantitative PCR kit (Kapa Biosystems). RhACT5 was used as an internal control (Pei 405 et al., 2013). 406 407 Sequence Analysis 408

Deduced amino acid sequences were aligned using ClutalX and DNAMAN, and 409 phylogenetic analysis was performed using MEGA. The phylogenetic tree was 410 computed using the Neighbor-Joining algorithm with 1,000 bootstrap replicates. 411 412 VIGS 413

Silencing of RhPR10.1 and RhHB6 in petal discs and plantlets by VIGS was 414 conducted as previously described (Ma et al., 2008; Dai et al., 2012; Tian et al., 2014), 415 but with some modifications. The pTRV1, pTRV2, pTRV-RhPR10.1 and 416 pTRV-RhHB6 vectors were transformed into Agrobacterium tumefaciens strain 417

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GV3101. The transformed A. tumefaciens lines were cultured in Luria-Bertani 418 medium supplemented with 10 mM MES, 20 mM acetosyringone, 50 μg ml-1 419 kanamycin, and 50 μg ml-1 gentamycin sulfate. The cultures were harvested by 420 centrifugation at 4,000 rpm for 10 min, and re-suspended in infiltration buffer (10 421 mM MgCl2, 200 mM acetosyringine, 10 mM MES, pH 5.6) to a final OD600 of 422 approximately 1.5. Mixtures of cultures containing an equal ratio (v/v) of pTRV1 and 423 pTRV2, or pTRV1 and pTRV-RhPR10.1, or pTRV1 and pTRV-RhHB6, were used as 424 TRV control, TRV-RhHB6, and TRV-RhPR10.1 experiments, respectively. The culture 425 mixtures were placed at room temperature in the dark for 3-6 h before vacuum 426 infiltration into the rose petal discs or plantlets. 427

For VIGS in rose petal discs, the petals were collected from the same middle 428 whorl at stage 2 of flower opening, and 1 cm diameter discs were excised from the 429 center of the petals using a hole punch. For VIGS in rose plantlets, plantlets were 430 propagated by in vitro culturing as described above. The plantlets were washed in 431 deionized water, and the roots were placed in water for 2 days to equilibrate. Vacuum 432 infiltration was performed by immersing the discs or plantlets in the bacterial 433 suspension and infiltrating under a vacuum at 0.7 MPa. After release of the vacuum, 434 the discs or plantlets were washed in deionized water. Discs were placed in petri 435 dishes, and plantlets were transplanted into a mixture of vermiculite and nutritive soil 436 (1:1). The discs and plantlets were then placed in the dark at 8°C for 3 days. For RNA 437 extraction, discs were kept in deionized water at 23°C until sampling. The phenotypes 438 of the discs were observed daily until the onset of necrosis. Three independent 439 experiments were performed with 40 petal discs in each experiment. The flowers of 440 the rose plantlets were observed from stage 1 to stage 6. Three independent 441 experiments were performed with 30 plantlets in each experiment. 442

443 Extraction and Quantification of Endogenous Cytokinins 444

The extraction and quantification of endogenous CTKs was performed as 445 previously described (Pan et al., 2010). Rose petals (100 mg) were frozen in liquid 446 nitrogen, ground to a powder in a mortar, and transferred into 2 ml tubes. Extraction 447 solvent (2-propanol/H2O/concentrated HCl (2:1:0.002, vol/vol/vol)) was added to 448 each tube, keeping the ratio of sample:solvent at 1:10 (mg µl-1). Samples were shaken 449 at 100 rpm for 30 min, at 4°C. One ml dichloromethane was added to each sample, 450 and then samples were shaken at 100 rpm for 30 min, at 4°C. After centrifugation at 451 13,000 g for 5 min (4°C), the solvent (approximately 1.5 ml) from the lower phase 452 was transferred into a new 2 ml tube and concentrated (not completely to dryness) 453 using a concentrator (Eppendorf), and re-dissolved in 0.1 ml methanol. Quantitative 454

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analysis of endogenous CTKs in crude rose flower extracts was performed by HPLC 455 electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). 456

457 Measurement of Electrolyte Leakage Rates 458

Electrolyte leakage rates were measured as previously described (Lee et al., 2015), 459 with minor modifications. Briefly, membrane leakage was determined by 460 measurement of electrolytes leaking from rose petal discs. Sixteen rose petal discs 461 from each treatment were immersed in 15 mL of 0.4 M mannitol at room temperature 462 with gentle shaking for 3h, and initial conductivity of the solution was measured with 463 a conductivity meter (DDBJ-350, LeiCi). Total conductivity was determined after 464 sample incubation at 85°C for 20 min. The electrolyte leakage rates were calculated as 465 the percentage of initial conductivity divided by total conductivity. 466

467 Yeast One-Hybrid Assay 468

The yeast one-hybrid assay was performed by transferring LacZ reporter gene 469 constructs into yeast strain EGY48, as described in the Yeast Protocols Handbook 470 (Clontech). Transformants were grown on synthetic dextrose plates lacking uracil and 471 tryptophan, but containing X-gal (5-bromo-4-chloro-3-indilyl-β-D-galactopyranside) 472 to observe the color development of yeast colonies. 473

474 Purification of GST-RhHB6 Recombinant Protein and EMSA 475

The EMSA was performed as previously described (Dai et al., 2012). Expression 476 of the GST-RhHB6 fusion protein was induced in 100 ml cultures of transformed E. 477 coli BL21 cells (see above) by addition of IPTG to a final concentration of 0.2 mM, 478 before the cultures were incubated at 20°C for 6h. The recombinant proteins were 479 extracted from the cells, and were purified using glutathione Sepharose 4B beads 480 according to the manufacturer’s instructions (GE Healthcare). EMSA was performed 481 using a Light Shift chemiluminescent EMSA kit (Pierce) according to the 482 manufacturer’s instructions. Biotin-labeled DNA fragments (wild type fragment in Fig. 483 6A) were synthesized and used as probes, while unlabeled DNA of the same sequence 484 was used as a competitor. 485

486 Accession numbers 487

The GenBank (http://www.ncbi.nlm.nih.gov) accession numbers for rose genes 488 used in this study are as follows: RhPR10.1 (KX462848) and RhHB6 (KX462849). 489

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ACKNOWLEDGEMENTS 490

We thank PlantScribe (www.plantscribe.com) for careful editing of this article. 491

FIGURE LEGENDS 492

Figure 1. Expression patterns of RhPR10.1 in different rose flower organs during 493 various opening stages (A), and in rose petals in response to exogenous hormones 494 (B). (A) Se, sepal; St, stamen; Pi, pistil; Re, receptacle; Pe, petal. (B) Eth, 10 µl L-1 495 ethylene; CTK, 100 µM 6-BA; ABA, 100 µM ABA; GA, 80 µM GA3; BR, 5 µM 496 brassinosteroid. The petals at opening stage 2 were used as material. The results are 497 the means of three biological replicates with standard deviations. Letters indicate 498 significant differences according to Duncan’s multiple range test (P<0.05), and 499 asterisks indicate significant differences according to Student’s t-test (*P<0.05, 500 **P<0.01). 501 Figure 2. Silencing of RhPR10.1 promotes flower senescence. (A) Expression of 502 the RhPR10 genes in RhPR10.1-silenced and TRV control flowers was analyzed by 503 qRT-PCR. (B) The time of flower opening (stage 1-4) and retention of ornamental 504 value (stage 5 and 6) were recorded. (C) Flower phenotypes were recorded and 505 photographed every day. Stages of rose flowering were defined as: stage 1, bud with 506 completely opened sepal; stage 2, bud with the outmost petal layer loosened; stage 3, 507 flower with the outmost petal layer opened; stage 4, flower with the inner petal layer 508 loosened; stage 5, fully opened flower; stage 6, flower with fading color and loss of 509 ornamental value. The results are the means of three biological replicates with 510 standard deviations. Asterisks indicate statistically significant differences (Student’s 511 t-test, *P<0.05, **P<0.01). 512 Figure 3. Silencing of RhPR10.1 promotes senescence in rose petal discs. (A) The 513 color fading phenotypes of petal discs were recorded. (B) Ion leakage rate and (C) 514 expression of RhSAG12 were analyzed in RhPR10.1-silenced and TRV control petals. 515 The results are the means of three biological replicates with standard deviations. 516 Asterisks indicate significant differences (Student’s t-test, *P<0.05, **P<0.01). 517 Figure 4. RhPRh10.1-silencing reduces cytokinin (CTK) signaling and levels in 518 rose petals. (A) Expression of genes related to the CTK signaling pathway in petals 519 of RhPR10.1-silenced and TRV control flowers, and (B) in rose WT petals treated 520 with CTK; (C) CTK contents in petals of RhPR10.1-silenced and TRV control flowers. 521 Values are means (pg g-1 fresh weight) ±SD. The results are the means of three 522 biological replicates with standard deviations. Asterisks indicate statistically 523 significant differences (Student’s t-test, *P<0.05, **P<0.01). 524 Figure 5. The effect of exogenous cytokinin (CTK) treatment on senescence of 525 RhPR10.1-silenced rose petal discs. The phenotypes of petal discs treated with 100 526 µM 6-BA were recorded (A). The ion leakage rates (B) and transcript levels of 527 RhSAG12 (C) in RhPR10.1-silenced and TRV control petal discs treated by 6-BA. 528

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Statistical differences were analyzed by Student’s t-test (P<0.05). 529 Figure 6. RhHB6 binds to a cis-element in the promoter of RhPR10.1. (A) 530 Transactivation activity of RhHB6 to the RhPR10.1 promoter in yeast. Wild type and 531 mutant probes were derived from the RhPR10.1 promoter. The wild-type cis-element 532 and its nucleotide substitutions in the mutant versions are underlined. (B) 533 Electrophoretic mobility shift assay (EMSA) was used to analyze the interaction of 534 GST-RhHB6 and a biotin-labeled probe. Purified protein (3 µg) samples were 535 incubated with 25 pM of the biotin-labeled wild-type probe. Non-labeled probes at 536 10- and 100-fold concentrations were added for the competition test. Images are 537 representative of three independent experiments. 538 Figure 7. Expression of RhHB6 in rose petals at different flower opening stages 539 (A), and in response to exogenous hormones (B). (A) Se, sepal; St, stamen; Pi, pistil; 540 Re, receptacle; Pe, petal. (B) Eth, 10 µl L-1 ethylene; CTK, 100 µM 6-BA; ABA, 100 541 µM ABA; GA, 80 µM GA3; BR, 5 µM brassinosteroid. The petals at stage 2 were 542 used as material. The results are the means of three biological replicates with standard 543 deviations. Letters indicate significant differences according to Duncan’s multiple 544 range test (P<0.05), and asterisks indicate significant differences according to 545 Student’s t-test (*P<0.05, **P<0.01). 546 Figure 8. Silencing of RhHB6 promotes flower senescence. (A) Expression of the 547 RhHB6 and RhPR10.1 genes in RhHB6-silenced and TRV control petals was analyzed 548 by qRT-PCR. (B) The time of flower opening and senescence phases was recorded. (C) 549 The phenotypes of flowers were recorded and photographed every day. The results are 550 the means of three biological replicates with standard deviations. Asterisks indicate 551 statistically significant differences (Student’s t-test, *P<0.05, **P<0.01). 552 553 554

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SUPPLEMENTARY DATA 555

Supplemental Table 556 Table S1. Primer list. 557 558 Supplemental Figures 559 Supplemental Figure S1. Alignment of seven rose PR-10 family cDNA sequences. 560 Supplemental Figure S2. qRT-PCR analysis of the expression of seven PR-10 561 family genes in rose petals at different flower opening stages. 562 Supplemental Figure S3. Alignment of deduced amino acid sequences (A) and a 563 phylogenetic tree (B) of the RhPR10.1 protein and representative PR-10 564 members from other species. 565 Supplemental Figure S4. Flower phenotypes of WT and TRV-control plantlets. 566 Supplemental Figure S5. A pictorial representation of the promoter region of 567 RhPR10.1 with potential cis-element binding sites. 568 Supplemental Figure S6. Transactivation activity of four HD-Zip transcription 569 factors to the RhPR10.1 promoter in yeast. 570 Supplemental Figure S7. Proposed model for how the RhHB6-RhPR10.1 module 571 regulates ethylene-induced flower senescence. 572 573 574 Supplemental Figure S1. Alignment of seven rose PR-10 family cDNA sequences. 575 The fragment underlined in red indicates the 3’ specific fragment of the RU06312 576 gene used for VIGS. 577 Supplemental Figure S2. qRT-PCR analysis of the expression of seven PR-10 578 family genes in rose petals at different flower opening stages. RhACT5 was used as 579 an internal control. The results are the means of three biological replicates with 580 standard deviations. Letters indicate significant differences according to Duncan’s 581 multiple range test (P<0.05). 582 Supplemental Figure S3. Alignment of deduced amino acid sequences (A) and a 583 phylogenetic tree (B) of the RhPR10.1 protein and representative PR-10 584 members from other species. (A) Conserved P-loop motif (GxGGxGxxK, amino 585 acid 47-55) was marked by red square. Bet v 1 signature motif was marked by blue 586 square. Amino acids implied in possible ribonucleolytic activity were marked by 587 asterisk (Liu et al., 2006; Lebel et al., 2010). (B) Phylogenetic tree was generated by 588 the MEGA2 software. Bootstrap values indicate the divergence of each branch. PR-10 589 proteins were classified into two subclasses: major pollen and food allergens and 590

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classic PR-10 (Fernandes et al., 2013). GenBank accession numbers are as follows: 591 Fragaria × ananassa FaPR10-4 (AET44159); Fragaria vesca subsp. vesca FvPru av 592 1-like (XP_004296886), FvPru ar 1-like (XP_004296888); Prunus mume PmPru av 593 1-like (XP_008223280); Malus domestica Mald1.01 (AAX18288), Mald1.02 594 (AAX18291), Mald1.03A (AAX18313), Mald1.03F (AAX18323), Mald1.04 595 (AAX18294), Mald1.05 (AAX18296), Mald1.06A (AAX18299), Mald1.07 596 (AAX18307), Mald1.08 (AAX18310); Betula pendula Betv1.0401 (CAA54482), 597 Betv1.0601 (CAA54484), Betv1.1101 (CAA54694), Betv1.1301 (CAA54696), 598 Betv1.1801 (CAA96540); Lupinus luteus LlPR10.1A (CAA56298), LlPR10.1B 599 (CAA56299), LlPR10.2A (AAF77633), LlPR10.2B (AAF77634), LlPR10.2E 600 (AAP37978); Prunus persica Prup1.01 (ACE80940), Prup1.02 (ACE80942), 601 Prup1.03 (ACE80944), Prup1.04 (ACE80946), Prup1.05 (ACE80948), Prup1.06A 602 (ACE80952). 603 Supplemental Figure S4. Flower phenotypes of WT and TRV-control plantlets. 604 (A) Flower phenotypes were recorded and photographed every day. (B) The 605 expression of TRV coat protein (CP) gene (Tian et al., 2014) was analyzed to confirm 606 virus infiltration in TRV-control plantlets. Stages of rose flowering were defined as: 607 stage 1, bud with completely opened sepal; stage 2, bud with the outmost petal layer 608 loosened; stage 3, flower with the outmost petal layer opened; stage 4, flower with the 609 inner petal layer loosened; stage 5, fully opened flower; stage 6, flower with fading 610 color and loss of ornamental value. 611 Supplemental Figure S5. A pictorial representation of the promoter region of 612 RhPR10.1 with potential cis-element binding sites. 613 Supplemental Figure S6. Transactivation activity of four HD-Zip transcription 614 factors to the RhPR10.1 promoter in yeast. Images are representative of three 615 independent experiments. 616 Supplemental Figure S7. Proposed model for how the RhHB6-RhPR10.1 module 617 regulates ethylene-induced flower senescence. 618 619

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Pasternak, O., Bujacz, G.D., Fujimoto, Y., Hashimoto, Y., Jelen, F., Otlewski, J., Sikorski, M.M., 713 Jaskolski, M. (2006). Crystal structure of Vigna radiata cytokinin-specific binding protein in 714 complex with zeatin. The Plant Cell 18, 2622-2634. 715

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Figure 1. Expression patterns of RhPR10.1 in different rose flower organs during various opening stages (A), and in rose petals in response to exogenous hormones (B). (A) Se, sepal; St, stamen; Pi, pistil; Re, receptacle; Pe, petal. (B) Eth, 10 µl L-1

ethylene; CTK, 100 µM 6-BA; ABA, 100 µM ABA; GA, 80 µM GA3; BR, 5 µM brassinosteroid. The petals at opening stage 2 were used as material. The results are the means of three biological replicates with standard deviations. Letters indicate significant differences according to Duncan’s multiple range test (P<0.05), and asterisks indicate significant differences according to Student’s t-test (*P<0.05, **P<0.01).

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Figure 2. Silencing of RhPR10.1 promotes flower senescence. (A) Expression of the RhPR10 genes in RhPR10.1-silenced and TRV control flowers was analyzed by qRT-PCR. (B) The time of flower opening (stage 1-4) and retention of ornamental value (stage 5 and 6) were recorded. (C) Flower phenotypes were recorded and photographed every day. Stages of rose flowering were defined as: stage 1, bud with completely opened sepal; stage 2, bud with the outmost petal layer loosened; stage 3, flower with the outmost petal layer opened; stage 4, flower with the inner petal layer loosened; stage 5, fully opened flower; stage 6, flower with fading color and loss of ornamental value. The results are the means of three biological replicates with standard deviations. Asterisks indicate statistically significant differences (Student’s t-test, *P<0.05, **P<0.01).

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Figure 3. Silencing of RhPR10.1 promotes senescence in rose petal discs. (A) The color fading phenotypes of petal discs were recorded. (B) Ion leakage rate and (C) expression of RhSAG12 were analyzed in RhPR10.1-silenced and TRV control petals. The results are the means of three biological replicates with standard deviations. Asterisks indicate significant differences (Student’s t-test, *P<0.05, **P<0.01).

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Figure 4. RhPRh10.1-silencing reduces cytokinin (CTK) signaling and levels in rose petals. (A) Expression of genes related to the CTK signaling pathway in petals of RhPR10.1-silenced and TRV control flowers, and (B) in rose WT petals treated with CTK; (C) CTK contents in petals of RhPR10.1-silenced and TRV control flowers. Values are means (pg g-1 fresh weight) ±SD. The results are the means of three biological replicates with standard deviations. Asterisks indicate statistically significant differences (Student’s t-test, *P<0.05, **P<0.01).

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Figure 5. The effect of exogenous cytokinin (CTK) treatment on senescence of RhPR10.1-silenced rose petal discs. The phenotypes of petal discs treated with 100 µM 6-BA were recorded (A). The ion leakage rates (B) and transcript levels of RhSAG12 (C) in RhPR10.1-silenced and TRV control petal discs treated by 6-BA. Statistical differences were analyzed by Student’s t-test (P<0.05).

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Figure 6. RhHB6 binds to a cis-element in the promoter of RhPR10.1. (A) Transactivation activity of RhHB6 to the RhPR10.1 promoter in yeast. Wild type and mutant probes were derived from the RhPR10.1 promoter. The wild-type cis-element and its nucleotide substitutions in the mutant versions are underlined. (B) Electrophoretic mobility shift assay (EMSA) was used to analyze the interaction of GST-RhHB6 and a biotin-labeled probe. Purified protein (3 µg) samples were incubated with 25 pM of the biotin-labeled wild-type probe. Non-labeled probes at 10- and 100-fold concentrations were added for the competition test. Images are representative of three independent experiments.

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Figure 7. Expression of RhHB6 in rose petals at different flower opening stages (A), and in response to exogenous hormones (B). (A) Se, sepal; St, stamen; Pi, pistil; Re, receptacle; Pe, petal. (B) Eth, 10 µl L-1 ethylene; CTK, 100 µM 6-BA; ABA, 100 µM ABA; GA, 80 µM GA3; BR, 5 µM brassinosteroid. The petals at stage 2 were used as material. The results are the means of three biological replicates with standard deviations. Letters indicate significant differences according to Duncan’s multiple range test (P<0.05), and asterisks indicate significant differences according to Student’s t-test (*P<0.05, **P<0.01).

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Figure 8. Silencing of RhHB6 promotes flower senescence. (A) Expression of the RhHB6 and RhPR10.1 genes in RhHB6-silenced and TRV control petals was analyzed by qRT-PCR. (B) The time of flower opening and senescence phases was recorded. (C) The phenotypes of flowers were recorded and photographed every day. The results are the means of three biological replicates with standard deviations. Asterisks indicate statistically significant differences (Student’s t-test, *P<0.05, **P<0.01).

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