Improving Crop Resistance to Abiotic Stress (TUTEJA:PLANT STRESS OMICS O-BK) || Make Your Best - MYB Transcription Factors for Improving Abiotic Stress Tolerance in Crops

21Make Your Best – MYB Transcription Factors for ImprovingAbiotic Stress Tolerance in CropsAndrea Pitzschke

Toohot, too dry, too cold, and too bright. Field crops grow and reproduce in a dynamicand unpredictably changing environment. Adverse climate conditions severelyimpair plant growth and development. In the face of the growing world populationand the climate change, humanity cannot take food production for granted. There isan increasing demand for crops with improved stress tolerance and yield. Under-standing themolecularmechanisms thatmediate stress adaptation and applying thisknowledge to engineering stress-resistant crops is therefore a key to ensure worldfood security.

MYBdomain-containing proteins forma family of transcription factors involved ina diversity of stress-related responses in plants. Interfering with the activity ofindividual family members often correlates with altered stress tolerance. MYBtranscription factors have been thoroughly studied and functionally characterizednot only in the model plant Arabidopsis thaliana but also in other plant species.Heterologous expression approaches and phylogenetic analyses indicate a gooddegree of functional conservation.

This conservation will facilitate knowledge transfer, and thus tomorrow�s farmersmay benefit from today�s discoveries in Arabidopsis. This chapter reviews the role ofMYB transcription factors in abiotic stress signaling. While selected examples fromArabidopsis are described, emphasis is given to MYB proteins in field crops. Thepotentials and limitations of using heterologous expression approaches for geneticengineering of crops with improved stress tolerance are discussed. One part isdevoted to mechanisms that regulate MYB protein abundance and activity. A list ofvaluable data resources (transcription factor databases, transcriptome studies) shallprovide fast access to detailed information and bioinformatics tools for researchersinterested in a particular plant species.

Improving Crop Resistance to Abiotic Stress, First Edition.Edited by Narendra Tuteja, Sarvajeet Singh Gill, Antonio F. Tiburcio, and Renu Tuteja� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

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21.1Introduction

21.1.1Abiotic Stress

As sessile organisms, plants have to endure any condition posed by their environ-ment. Environmental stress is estimated to account for 60–70% of crop yield loss [1].Owing to global warming, abiotic stresses are gaining even more importance asmajor constraints on worldwide crop production. The most widespread types ofabiotic challenges occurring in the field are drought, osmotic, cold, and heat stresses.In addition, other stresses such as those triggered by ozone and ultraviolet (UV) lightare a growing problem. Over the past decades, intensive research has been dedicatedto disentangle the molecular mechanisms and identify key regulators of stressadaptation.

Plant stress responses have been most intensively studied in the model plantArabidopsis thaliana and to various extents also in other species. Principal stressadaptation strategies such as osmolyte accumulation under high-salt conditions,production of anthocyanins asUVprotectants, and the underlying signalingmechan-isms, appear to be largely conserved between species.

21.1.2Abscisic Acid – A Stress Signaling Hormone

Many stress-related signaling pathways are involve in the production of and regu-lation by abscisic acid (ABA) [2, 3]. The importance of this plant hormone asmediatorof a variety of stress adaptation mechanisms is mirrored by the fact that manymutants displaying an altered stress tolerance also show abnormalities in ABAsensitivity or biosynthesis. As regulator of stomatal aperture, ABA directly controlsthe plant�s water balance.

21.1.3(Dis)Similarities of Stress Responses

Although any kind of environmental challenge requires a specific adaptationresponse, the molecular events triggered by diverse abiotic stresses significantlyoverlap. This is, for example, evidenced in studies comparing transcriptome changesupon cold, salt, or heat treatment in Arabidopsis [4], as well as upon cold, drought, orsalt treatment in chickpea [5]. Accordingly, a number of mutants and transgenicplants exhibit altered tolerance to, for example, salt, cold, and drought [4, 6].

Often, enhanced stress tolerance is accompanied by a poor growth or seed yield.This may be explained by the relatively high energy cost required for maintaining apermanent �stress awareness.� For instance, the Arabidopsis eskimomutant not onlytolerates drought, cold, and osmotic stress but also produces fewer seeds [6].Likewise, enhanced stress tolerance conferred by overexpression of a number of

482j 21 Make Your Best – MYB Transcription Factors for Improving Abiotic Stress Tolerance in Crops

MYB genes in various plant species is reflected by severe dwarfism [7–10]. Anincreasing understanding of stress-related signaling mechanisms shall pave the wayfor improving stress tolerance in cropswhilemaintaining normal plant growthundernonstress conditions.

21.2Signal Transduction and Amplification

21.2.1Principle of Signaling Pathways

Plant growth under adverse conditions is a challenging task that requires theestablishment of effective stress signaling pathways and networks. Stress adaptationfollows a general scheme: (1) stress perception; (2) transduction of the �stress signal�to cytoplasmic components, for example, through modification of receptor bindingintracellular proteins; (3) transduction of information into the nucleus; and (4)transcriptional reprogramming, followed by synthesis of stress proteins (e.g., HSPsin heat stress), altered enzyme activities, and metabolic changes (e.g., osmoprotec-tants upon salt stress, anthocyanins as UV protectants, etc.). Up to now, mostsignaling pathways are far from being known in their entirety.

21.2.2Protein Signaling Cascades

Cellular signaling is not only a one-to-one transduction from protein to protein butalso involves concomitant amplification of the stress information. Such amplificationcan be achieved throughmodification of (a homo- or heterogeneous pool of)multipletarget proteins by a singlemolecule of regulatory enzyme. Prominent players in suchcatalysis-based signal amplification in the early stress response are protein kinases,such as those of the SERK [11], CDPKs [12], and MAPK family [13–15].

21.2.2.1 MAPK CascadesAhighly efficient and specific funneling and amplification of perceived stress signalsis achieved if regulatory enzymes are linked in a cascade. Here, mitogen-activatedprotein kinase (MAPK) cascades are prominent examples. MAPK cascades aresignaling modules conserved in eukaryotic species. These cascades minimallyconsist of three types of kinases, encoded by the gene families of MAPK kinasekinases, MAPK kinases, and MAPKs. MAPK modules serve as both signal trans-duction and signal amplification: an MAPK kinase kinase phosphorylates multiplemolecules of its target MAPK kinase, which in turn phosphorylates and therebyactivates a pool of MAPK molecules. The active MAPK then passes on the signalthrough phosphorylation of downstream targets, including transcription factors.Thus, by altering the properties/activities of target transcription factors, MAPKcascades translate signal perception into altered gene expression. Genetic and

21.2 Signal Transduction and Amplification j483

biochemical studies implicate MAPK signaling modules both in biotic and abioticstress responses and in stomata development [13, 15, 16].

21.2.3Transcription Factors

Transcription factors that accumulate or are stimulated by an upstream kinase orother regulatory protein subsequently further amplify the perceived stress signal.Few molecules of active transcription factor can drive massive accumulation oftarget gene transcripts whose translation can yield thousands of thousands ofmolecules of the respective protein. Thus, transcription factors may be consideredas �master switches.� Often, their activity is tightly controlled, so as to preventexpression of target genes when there is no demand. Many transcription factorsaccumulate or are activated at a particular development stage or in a stress-dependent manner. Undoubtedly, transcription factors are very attractive proteinsfor genetic manipulation to generate stress-resistant crops. Plant transcriptionfactors, according to the type of DNA binding domain, are classified into severalfamilies. Many families have been shown to be involved in stress responses.Examples of stress-related transcription factors can be found in all main transcrip-tion factor families, including bZIP proteins, WRKY, AP2, NAC, C2H2 zinc finger,and MYB proteins [17].

While members of the WRKY family are prominent regulators of biotic stressresponses, those of the MYB family primarily act in the signaling of abiotic stresses.In the subsequent chapters, focus will be onMYB transcription factors and their roleas regulators of the abiotic stress response and on their potential for engineeringcrops with improved stress tolerance. Table 21.1 provides an overview of all MYBgenes discussed in the chapter, including information on expression and stress-related phenotypes of mutants and overexpressing plants.

21.2.3.1 MYB Transcription Factors – Structure and PhylogenyMYB proteins form one of the largest families of transcription factors in plants [18],characterized by the conservedDNAbinding domain, theMYBdomain. The name isderived from the first MYB gene identified, the oncogene v-MYB from the avianmyeloblastosis virus. MYB proteins are found in animals, fungi, and plants. Inanimals, they control the development of various types of cancer [19]. TheMYBDNAbinding domain generally consists of up to three imperfect repeats, R1, R2, and R3,each consisting of 51–53 amino acids.

MYB genes in plants form three major subfamilies, depending on the number ofrepeats: according to a nomenclature suggested by Stracke et al. [20], 1-, 2-, and 3Rrepeat-containing plant MYB proteins are referred to as MYB1R (Myb-related),R2R3, and MYB3R factors. The largest group, R2R3-type MYB proteins, is exclu-sively found in plants. Members of this group mainly regulate plant-specificprocesses [20].

The richest pool of plant MYB research data has been generated inA. thaliana andhas been reviewed previously [21]. TheArabidopsis genome encodes for 126MYB and


Table21.1

Overviewof

plan

tMYB

tran

scriptionfactorsdiscussedin

thechap

ter.

Species

Genena

me/ID

Closest

homolog

ue(A.tha

liana

,rice)

a)Geneexpression

(þ,ind

uced;�

,repressed)

Phenotype(þ

,enh

anced;

�,redu

cedtolerance;

o/e,

overex-

pression

;k.o.,knockout)

Com

ment

References

Arabidopsis

AtM

YB2

At2g4

7950

Droug

htþ,

ABA

þo/eABAsensitivity

þ,salt

tolerance

þMicroarrayavailable.

MYC2/MYB2coordi-

natelyactivate

Rd2

2.MYB2o/e

aredw

arfed

[10]

Arabidopsis

AtM

YB4

At4g3

8620

UV�

k.o.

accumulate

sinapate

esters,U

Vresistan

ceþ

o/e

UVresistan

ce�

Tran

scriptional

repressor

ofrate-limitingstep

insinapateestersynthesis

containsrepressormotif

[56]

Arabidopsis

AtM

YB15

AT3G

2325

0ABA,drough

t,salt

þo/edrou

ght,saltþ

ABAsen-

sitivity

þ,ABAsynthesis

genes

þ

[27]

Arabidopsis

AtM

YB44

At5g6

7300

Variousabiotic

stresses

þflagel-

linþ

k.o.

drou

ght�,

salt�,

impaired

stom

atal

closure

o/e

ABAhypersensitive,d

rough

tþ,saltþ,enhan

cedstom

atal

closure

Directly

regu

latedby

MAPK-activated

bZIP

proteinVIP1

[22,

29,6

6]

Arabidopsis

AtM

YB60

AT1G

0881

0Droug

ht�

k.o.

drou

ghtþ

o/ein

lettuce

repressesan

thocyanin

synthesis

[26,

59]

Arabidopsis

AtM

YB96

AT5G

6247

0ABA,d

rough

tþ,

expressedin

guardcells

o/ean

dactivation

-tagg

edmutantABAsensitivity

þ,

drou

ghtþ;pathogen

tolerance

þ;d

warfs

undernormal

con-

dition

sk.o.

Rd22expression

�normal

phen

otype

ABA/auxin/SAcrosstalk

[9,2

8]

(Continu

ed)


Table21.1

(Contin

ued)

Species

Genena

me/ID

Closest

homolog

ue(A.tha

liana

,rice)

a)Geneexpression

(þ,ind

uced;�

,repressed)

Phenotype(þ

,enh

anced;

�,redu

cedtolerance;

o/e,

overex-

pression

;k.o.,knockout)

Com

ment

References

Arabidopsis

AtM

YB10

8(AtBOS1

)AT3G

0649

0

B.cinerea

(necro-

trop

hpathogen

)þ

k.o.

necrotrop

hpathogen

sþ

biotroph

pathogen

s�drou

ght,

salt,

oxidativestress

�

[82]

Resurrection

plan

tCrateros-

tigm

aplan

tagineum

CpM

YB10

AAM43

912

AtM

YB78

AT5G

4962

0Os03g

0315

400

ABA,d

rough

tþ

o/ein

A.tha

liana

salt

þ,

drou

ghtþ,A

BAsensitivity

þ,normal

phen

otypeundernor-

mal

condition

s

Mightregu

late

itsow

nprom

oter

[83]

Rice

OsM

YBS3

AAN6315

4MYB-related

AT5G

4739

0Coldþ

k.o.

cold�o/ecold

þ;o

ther-

wisenormal

phen

otype

Microarrayexists;rolein

long-term

cold

acclim

ation

[37]

Rice

OsM

YB4

BAA23

340

AtM

YB5AT3G

1354

0Coldþ

o/ein

A.tha

liana

andapple

drou

ghtþ,coldþ

dwarfed

apple:

mod

ified

metabolite

accumulation

Nodata

onk.o.

oro/ein

rice

[7,8

]

Rice

OsM

YBR3-2

BAD8176

5MYB3R

-5protein

AT5G

0232

0Coldþ

o/ecold

þpralinelevels

þhigher

G2/M-specificgene

expression

Binds

mitosis-specificcis-

elem

ent

[38]

Wheat

TAMYB1

ABC86

569

AtM

YB44

At5g6

7300

Os02g

0187

70Droug

ht�

ND

[31]

Wheat

TAMYB2

AAT3716

8AtM

YB15

At3g2

3250

Os04g

0517

100

Droug

htþ

ND

[31]

Wheat

TaMYB4TaMYB5

Droug

htþ

ND

[31]

Wheat

TaMYB32

Salt

þND

[32]


Wheat

TaMYBsdu1

AtM

YB20

AT1G

6623

0OS0

6g0258

000

Droug

htþ,salt

þstrongersalt-

indu

cedexpres-

sion

insalt-toler-

antgenotype

[33]

Soybean

GmMYB76

ABH0283

6AtM

YB2At2g4

7190

Os03g

0315

400

Salt

þo/esalt

þ,freezingþ

Hom

odim

er,h

eterod

i-mer

withGmMYB17

7[40]

Soybean

GmMYB92

ABH0284

4AtM

YB111

AT5G

4933

0Os05g

0429

900

Salt

þ,coldþ

o/enormalsalt,

freezing

tolerance

Hom

odim

er[40]

Soybean

GmMYB177

ABH0286

6MYB-like

A.tha

liana

proteinAT5G

1730

0Os06g

0728

700

Salt

þ,

drou

ghtþ

o/esalt

þ,freezingþ

Heterod

imer

with

GmMYB76

[40]

Grapevine

VvM

YBF1

ACV8169

7AtM

YB12

At2g4

7460

Hypothrice

MYB

GI:12

554353

9

Expressed

during

ripening;

corre-

lateswithflavon

olaccumulation

o/ein

A.tha

liana

complem

ents

flavon

ol-defi

cien

tphen

otypeof

Atm

yb12

Specificactivatorof

fla-

vonol

synthase

[48]

Grapevine

VvM

YBPA1

AB2423

02VvM

YBPA2

DQ88

6419

AtM

YB3At1g2

2640

AtM

YB113

AT1G

6637

0OsM

YB3

Os03g

0410

000

Expressed

inyoungberries

o/e(transien

t)proanthocyani-

din

þTran

scriptom

estudy

ofo/eavailable

[53,

54]

Sweetpo

tato

IbMYB1

AB2589

85AtM

YB113

AT1G

6637

0OsM

YB3

Os03g

0410

000

Highlyexpressed

inpu

rple-fleshed

sweetpo

tato

o/ein

A.tha

liana

orpo

tato

anthocyanin

levels

þ(through

indu

ctionof

allstructural

anthocyanin

genes

[55]

(Continu

ed)


Table21.1

(Contin

ued)

Species

Genena

me/ID

Closest

homolog

ue(A.tha

liana

,rice)

a)Geneexpression

(þ,ind

uced;�

,repressed)

Phenotype(þ

,enh

anced;

�,redu

cedtolerance;

o/e,

overex-

pression

;k.o.,knockout)

Com

ment

References

Strawberry

FaMYB1

AF4012

20AtM

YB6AT4G

0946

0Hypothetical

OsI_3

5350

Expressed

inrip-

eningfruits

o/ein

tobacco:

flavon

olan

dan

thocyanin

synthesis-

Con

tainsrepressormotif

[47]

App

leMdM

YB10

ACQ4520

1AtM

YB113

AT1G

6637

0BAD0402

5

Highlyexpressed

inred-fleshed

apple.

Expression

correlates

with

color

developm

ent

o/ein

A.tha

liana

high-sorbitol

tolerance

þ,anthocyaninsþ,

osmop

rotectan

tsþ

[45,

57]

a)Closest

hom

ologueusingNCBIBlastP.


64MYB-related proteins [22]. Until now, to approximately half of allArabidopsisMYBproteins, a functional role could be ascribed [21]. ArabidopsisMYB factors regulate adiversity of processes, including primary and secondary metabolism, cell fate andidentity, and development and stress responses. The identification and functionalcharacterization of MYB proteins in crops is on the rise, and a picture of MYBtranscription factors as key players in plant stress management is emerging.

21.3MYB Proteins in the Model – Abiotic Stress Signaling in Arabidopsis

21.3.1AtMYB2: the Pioneer and Its Partner

The involvement of MYB proteins in the plant response to abiotic stress was firstreported in 1997 [23]. In Arabidopsis, AtMYB2, a MYB-related protein, andAtMYC2, a bHLH transcription factor, function as transcriptional activators inABA-inducible gene expression under drought stress. Coordinately, they activatethe expression of the drought stress marker gene AtRd22 through direct bindingto AtRd22 promoter elements [10, 23]; AtMYB2 and AtMYC2 are synthesized afterthe accumulation of endogenous ABA, pointing to a role at a late stage in the stressresponse. Overexpression of these transcription factors in Arabidopsis resulted inABA hypersensitivity and an enhanced tolerance to osmotic stress [10]. Accord-ingly, several ABA-inducible genes are among the candidate target genes iden-tified by microarray analysis of AtMYC2/AtMYB2 overexpressing plants [10].Although AtMYC2 overexpressing plants under normal conditions are indistin-guishable from wild type, AtMYB2 and AtMYC2/AtMYB2 overexpression resultsin growth retardation.

Meanwhile, Arabidopsis drought stress-related MYB proteins have been charac-terized further. Apparently, they are involved in several aspects of the desiccationresponse, both as tolerance enhancing and as tolerance repressing factors.

21.3.2Arabidopsis MYB Proteins as Stomatal Regulators

Stomata are the major sites of gas exchange. Adequate stomatal movement is pivotalto allowCO2uptake for photosynthesis on the onehand and to restrict excessivewaterloss on the other hand. Stomatal pore aperture is mediated by turgor-driven volumeexchanges of two surrounding guard cells [24]. Both opening and closing of stomatais regulated by the stress hormone ABA and has been reviewed previously [25].Briefly, ABA-stimulated accumulation of reactive oxygen species induces stomatalclosure via activation of plasma membrane calcium channels. Complementarily,ABA inhibits stomatal opening through downregulation of Kþ

in channels andHþ -ATPases.

21.3 MYB Proteins in the Model – Abiotic Stress Signaling in Arabidopsis j489

The first plant transcription factor reported to regulate stomatal movement was aMYB protein Arabidopsis AtMYB60 [26]. AtMYB60 is specifically expressed in guardcells, and its expression decreases during drought. Loss-of-function mutants showconstitutive reduction of stomatal opening and an enhanced drought tolerance.Microarray analysis showed only few genes to be differentially expressed in thesemutants, and many are associated with stress responses. The stomata-specific geneexpression and the fact that lack of functional AtMYB60, at least in the homologoussystem (Arabidopsis) is affecting only stress-associated pathways and not interferingwith growth and development point to a highly specific role. Identifying potentialAtMYB60 orthologues in other plant species and repressing their expression wouldbe a targeted and promising strategy to engineer stomatal activity to help cropssurvive desiccation.

At least three more genes, AtMYB15 [27], AtMYB44, and AtMYB96 [28], areimplicated in drought stress response through stomatal regulation in Arabidopsis.AtMYB15 is expressed in stomatal guard cells. Its expression is induced by ABA,drought, or salt treatments. AtMYB15 overexpressing lines are hypersensitivity toexogenous ABA and display an improved tolerance to drought and salt stresses. ABA-induced induction ofABAbiosynthesis, ABA signaling, andABA-responsive genes isenhanced in these lines.

Overexpression of AtMYB44 enhances ABA sensitivity in stomatal closure [29].AtMYB44 is an immediate stress-responsive genewhose expression is regulated via aMAPK pathway (see Section 21.6.3.2).

AtMYB96 is primarily expressed in the root. An activation-tagged myb96 over-expressing mutant (MYB96ox) exhibited an enhanced drought resistance, accom-panied by a reduction in stomatal opening upon ABA or drought exposure. Theopposite was observed in amyb96 T-DNA insertional knockoutmutant [28]. Stomataldensities were not affected in either of these mutant lines, showing that AtMYB96regulates specifically stomatal opening.

In addition, AtMYB96ox mutants exhibit dwarfed growth with altered leaf shapeand reduced lateral roots. Thus, in contrast toAtmyb60 knockout mutants, enhanceddrought resistance in AtMYB96ox mutants is gained at the expense of reducedgrowth. This growth impairment might in addition be explained by the energy costpaid to an alerted state toward biotic stress:AtMYB96ox plants have elevated levels ofbiotic stress genes and are more resistant to infection with the biotrophic pathogenPseudomonas syringae [9].

21.4Desiccation, Cold, and Osmotic Stress in Crops

21.4.1Wheat

Also in other plant species, abiotic stress responses are controlled throughmultiple MYB proteins. In wheat, 203 MYB and 116 MYB-related genes have been


identified [30], and experimental evidence implicates several family members instress signaling. Aiming at improving wheat harvest yield, the challenge is now topick the most relevant candidates (see also Section 21.7).

Upon wheat desiccation, TaMYB2,MYB4, andMYB5 are induced (treatment with30% PEG), while another gene, TaMYB1, is repressed [31]. Whether the oppositeresponsiveness of TaMYB1 versus TaMYB2, TaMYB4, TaMYB5 is due to a crosstalkbetween the four genes is an interesting question. Another salt stress-induced MYBgene in wheat, TaMYB32, was isolated and mapped [32]. All five genes mentionedabove still await functional characterization. TaMYB2 might be particularly inter-esting: its closest homologue in Arabidopsis is AtMYB15, a drought-inducible genethat is expressed in stomatal guard cells [27] (see above). It will be interesting to seewhether TaMYB2 also displays guard cell-specific expression and, more important,whether similar to AtMYB15 [28], TaMYB2 overexpression also confers salt anddrought stress tolerance.

Astudyexaminingtheexpressionlevelsof10MYBgenesintworecombinant inbredwheat lines that have contrasting salt and drought tolerance revealed four genesresponding to short-term salt treatment [33]. One of these genes, TaMYBsdu1, isupregulated in leaves and roots under long-term drought stress. Moreover,TaMYBsdu1 expression is more strongly induced upon salt treatment in the tolerantthaninthesusceptiblegenotype.TaMYBsdu1couldbeanimportantregulator inwheatadaptation to both salt and drought stresses [33]. Its closest homologues in rice andArabidopsis are a MYB protein of unknown function and AtMYB20/AtMYB85,respectively. Interestingly,AtMYB85 expression is upregulated andAtMYB20 expres-sion downregulated by salt and drought stresses [34, 35]. Neither AtMYB20 norAtMYB85 loss- or gain-of-functionmutants/transgenic plantshavebeen analyzedyet.

21.4.2Rice

Plants growing in temperate zones have developed a strategy, termed cold acclima-tion, to withstand subfreezing conditions. Cold acclimation involves the orchestratedaction of numerous genes whose expression is controlled by a small number oftranscription factors directly responding to the temperature stimulus. Substantialresearch data exist for severalMYBgenes to be involved in low-temperature signalinginArabidopsis [21]. Also, in crops evidence for a role of MYB genes in cold adaptationis growing.

Cold is one if not the limiting factor in rice production. Particularly at the seedlingstage, rice is very sensitive to chilling. Once again, MYB proteins are playing at thefront in stress management.

OsMYBS3 is a single MYB domain-containing protein that had been previouslyimplicated in sugar signaling in rice [36]. Sound evidence exists for an additional roleof this protein asmediator of cold adaptation [37]:OsMYBS3 expression is induced bycold. Loss-of-function results in a cold-hypersensitive phenotype, while transgenicrice constitutively overexpressing OsMYBS3 is more cold tolerant (4 �C for at least1 week). Thus, OsMYBS3 appears to be sufficient and necessary for cold tolerance

21.4 Desiccation, Cold, and Osmotic Stress in Crops j491

in rice. Transcriptome profiling of loss- and gain-of-function plants revealed putativeOsMYBS3 target genes, including various stress-responsive genes. Interestingly,OsMYBS3 was found to repress the well-known DREB1/CBF-dependent coldsignaling pathway. The different kinetics of cold-induced DREB1 (rapid) andOsMYBS3 (slow) expression suggests that distinct pathways act sequentially andcomplementarily for adapting short- and long-term cold stress in rice [37]. The closestOsMYBS3 homologue inArabidopsis, At5g47390, is also induced by salt, as well as byABA and CdCl2 [22], indicating a functional similarity. More important, theenhanced cold tolerance of OsMYBS3 overexpressing plants is not accompanied byyield penalty in normal field conditions [37],makingOsMYBS3 particularly attractivefor improving cold tolerance in other crops (Table 21.3).

Equally promising is another cold-inducible MYB gene from rice, OsMYB4.Constitutive overexpression ofOsMYB4 inArabidopsis conferred enhanced toleranceto several abiotic stresses [8], suggesting that the function is evolutionarily conserved.In fact, it was found later that ectopic expression of OsMYB4 also improves droughtand cold resistance in apple [7]. Despite its promising potential for genetic engi-neering of other stress-resistant crops, OsMYB4 will have to be expressed in a morecontrollable manner (see Section 21.8): both Arabidopsis and apple plants constitu-tively expressingOsMYB4 showed reduced growth under normal conditions [7, 8]. Sofar, no studies on OsMYB4 overexpressing or mutant plants in the homologoussystem, that is, rice, have been reported.

ThericeMYBproteinOsMYB3R-2hasalsobeen implicated in thecold response [38].OsMYB3R-2 specifically binds a mitosis-specific activator cis-element, found in thepromoters of cyclin-genes. Upon cold treatment, transgenic rice plants overexpressingOsMYB3R-2 have higher transcript levels of several G2/M phase-specific genes.Moreover, these plants display an enhanced cold tolerance, accompanied with elevatedlevelsof theosmoprotectantproline.Therefore,OsMYB3R-2wasproposed tomediateacold resistance mechanism through regulation of the cell cycle [38].

21.4.2.1 Thirty-Seven Priority Candidates for Cold Signaling in RiceThe above examples might only be the tip of the iceberg in MYB-regulated ricechilling response. A genome-wide, physiological and whole-plant-level analysisreported by Yun et al. [39] provides amore holistic view of the chilling stress responsemechanism in rice.

Within a 96-h duration at which rice seedlings were exposed to 10 �C, 8668 genesshowed altered expression. The majority of chilling-induced transcription factorswere activated during the initial 24 h. Among these potential main determinants inorchestrating the rice cold response are 37 members of the MYB gene family.

21.4.3Soybean

In soybean, there are at least 156 MYB genes [40]. For 43 of these, altered expressionin response to treatment withABA, salt, drought, and cold stresswas observed. Threecandidates, GmMYB76, GmMYB92, and GmMYB177, were studied in more detail.


Table21.2

(a)Tran

scriptiona

lprofilingstud

ieson

abiotic

stress

respon

sesin

crop

san

d(b)useful

bioinformatictoolsan

ddatabaseson

tran

scriptionfactorsin

Arabidopsisan

dfield

crop

s.

Species

Descriptio

nof

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ments

Reference

(a)

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ana

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ucleotide

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sehttp://w

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tent/supp

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64-10-43

6-s1.zip

[75]

Chickp

eaMicroarrayof

osmotic,cold,

drou

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se[5]

M.truncatula

Tran

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http://bioinform

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tED/.

[70]

Rice

Microarrayof

chillingrespon

seCon

tains37

differen

tiallyexpressedMYBgenes

[39]

(Continu

ed)

21.4 Desiccation, Cold, and Osmotic Stress in Crops j493

Table21.2

(Contin

ued)

Species

Descriptio

nof

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Reference

Soybean

Salt,

cold,d

rough

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MYBgenes

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[40]

Sunflow

erSalt,

cold

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sehttp://w

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[76]

(b)

Arabidopsis

Databaseon

stress-in

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scriptionfactors

http://caps.ncbs.res.in/stifdb

[84]

Rice

Gen

eon

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ku.edu

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[85]

Wheat

Databaseincluding203MYBproteinsaffymetrixID

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http://w

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antes.inra.fr:8180/w

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[30]

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They exhibit different dimerization characteristics and DNA binding preferences.Overexpression of GmMYB76 or GmMYB177, but not GmMYB92, in Arabidopsisresulted in an improved salt and freezing tolerance [40]. Together, this study suggestsa nonredundant function for these threeMYB genes in the plant stress response. Theremaining 40 stress-responsiveMYB genes are further attractive candidates for stresstolerance improvement through genetic engineering.

21.5Colorful MYB Proteins and Their Merits

ExcessiveUVradiation triggers the production of reactive oxygen species, followed bycell death. Many plant species produce flavonols and anthocyanins as effective UVprotectants. Pigmented anthocyanin compounds also play an important reproductiverole as attractants for pollinating insects. On the other hand, accumulation ofanthocyanins can prevent plants from excessive herbivory insect attack [41].

For the human diet, the health-promoting effect of anthocyanins is of greatinterest. Flavonols/anthocyanins offer protection against certain cancers, cardiovas-cular disease, and age-related degenerative diseases; they have anti-inflammatoryactivity, promote visual acuity, and hinder obesity and diabetes [42].

The identification and characterization of key regulators in flavonol production incrops may therefore speed up the development of plants with enhanced UVtolerance, improved pollination, and elevated nutritional/health value.

Anthocyanin biosynthesis is controlled by a distinct clade of R2R3 MYB tran-scription factors [43]. In Arabidopsis, a number of MYB factors are involved in theregulation of the phenylpropanoid pathway, mediating anthocyanin and flavonolbiosynthesis [21]. This pathway appears to be also regulated bymultiple MYB factorsin other plant species, including, for instance, apple [44–46], strawberry [47],grapevine [48], persimmon fruit [49], tomato [50], and sweet potato [51].

Table 21.3 Highly attractiveMYB genes for genetic engineering, also in heterologous plant species.Research data so far indicate that tolerance is not at the expense of growth, development, or seedyield.

Strategy Reported effect Studied so far in species

OsMYB3 overexpression Cold tolerance RiceSnapdragon MYB/MYCoverexpression

Anticancer effect due to antho-cyanin accumulation in fruit

Tomato

IbMYB1 overexpression Potential health-promotingeffect due to anthocyaninaccumulation

Sweet potato

AtMYB60 inhibition Drought tolerance, anthocyaninsynthesis

A. thaliana, lettuce

GmMYB76 or GmMYB177overexpression

Salt and freezing tolerance A. thaliana

21.5 Colorful MYB Proteins and Their Merits j495

21.5.1MYB Proteins for the Human Health

In an impressive scientific report [42], the huge potential of MYB proteins for theengineering of crops with health-promoting activity was demonstrated: ectopicexpression of a bHLH and a MYB transcription factor, which had earlier beenfound to interact to induce anthocyanin biosynthesis in snapdragon [52], in tomatogave rise to fruits with substantially elevated anthocyanin levels. The anthocyaninproduction in these dark-colored transgenic tomato fruits correlated with a massiveincrease in antioxidant capacity. Strikingly, cancer-susceptible mice, which were fedwith these fruits, attained a longer life span [42]. MYB proteins may therefore opennovel approaches for human medicine. Particularly in countries with limited accessto a balanced and healthy diet, anthocyanin-accumulating crops may provide asolution.

21.5.2Anthocyanin Production in Grapevine

For winemakers, sun can be both a blessing and a curse. Quality and quantity ofsun is a major determinant in flavor development in ripening wine berries.However, high light and the associated UV radiation are critical challenges forwine plants. A number of wine MYB transcription factors have been identified andcharacterized to various extents: VvMYBF1 controls flavonol synthesis throughactivation of the flavonol synthase gene VvFLS. Overexpression of VvMYBF1 inArabidopsis complements the flavonol-deficient phenotype of Atmyb12. Thisfinding highlights the strong conservation and interspecies compatibility of plantMYB proteins [48].

VvMYBPA1 was the first transcription factor found to be involved in theregulation of protocyanin production during seed development in grapevine [53].Later on, a second gene with high sequence similarity, VvMYBPA2, was identified.VvMYBPA2 is expressed in berries and leaves. The proanthocyanidin profiles ingrapevine hairy roots ectopically expressing VvMYBPA1 or VvMYBPA2 werealtered. Transcriptomic studies of transformed grapevine organs ectopically expres-sing VvMYBPA1 or VvMYBPA2 revealed putative target genes, including severalenzymes of the flavonoid pathway [54]. Considering the UV-protective role ofanthocyanins and the high light conditions often encountered in vineyards, it willbe a worthy experiment to assess the UV tolerance in VvMYBPA1/2 overexpressingorgans or plants.

21.5.3A Red and Rich Sweet Potato

Purple-fleshed sweet potatoes owe their color – at least partially – to the activity of aMYB transcription factor, IbMYB1 [51]. Transcript levels of this gene are particularlyhigh in the roots of the purple-fleshed variety. IbMYB overexpression leads to a drastic


increase in anthocyanin levels through induction of all structural anthocyanin genes,both in sweet potato and in heterologous plant species (Arabidopsis). IbMYB1 mighttherefore be applicable to the production of anthocyanins as nutritive value [55], but itmight also serve as a UV protectant in other plant species.

21.5.4Negative Control of Anthocyanin Production

The above-described proteins, along with a number of other MYB factors fromdiverse species [43], are all positive regulators of the phenylpropanoid pathway. Incontrast,Arabidopsis AtMYB4 acts as a repressor by inhibiting the expressionof a rate-limiting enzyme in this pathway. Accordingly, Atmyb4mutants accumulate sinapateesters (major soluble phenylpropanoid metabolites) and exhibit an enhanced UVtolerance, while overexpression – in Arabidopsis and tobacco – diminishes sinapateester levels and results inUVhypersensitivity. The transcriptional repressing effect ofAtMYB4 is conferred by a repressor domain located in the C-terminus of theprotein [56].

Similar motifs are found in and apparently confer a repressing effect on a smallnumber of other MYB proteins. FaMYB1, which had been isolated from ripeningstrawberry, represses transcription of anthocyanin-related genes [47]. Heterologousoverexpression of FaMYB1 in tobacco represses flavonol and anthocyanin synthe-sis [47]. Thus, the putative repressor motif contained in FaMYB1 is apparently alsofunctional in the (distant) heterologous species.

21.5.5Anthocyanins, UV Protection, and the Crosstalk with Other Stressors

Apart from their role as major contributors to UV protection and as attractants forpollinating insects, phenylpropanoid-regulating MYB factors might have a huge yetmainly undiscovered potential; a recent study on the involvement of MdMYB10, aregulator of anthocyanin production in apple fruit [45], in osmotic stress protection ispromising: overexpression ofMdMYB10 inArabidopsis resulted in improved growthunder high-sorbitol conditions. Accordingly, levels of anthocyanins and osmopro-tectants were elevated in these plants [57]. A possible two-sided regulation of bothphenylpropanoid pathway and seemingly rather distant stress responses by a singleMYB factor is indicated by Arabidopsis AtMYB60. The gene is repressed upon UVradiation (www.genevestigator.ethz.ch [58]). myb60 mutants are more drought tol-erant, which is due to a constitutive reduction in stomatal opening (see abovesection) [26]. Interestingly, when overexpressed in lettuce, AtMYB60 acts as tran-scriptional repressor of anthocyanin biosynthesis [59]. Recently, also enhancedanthocyanin levels have been reported for AtMYB96 activation-tagged Arabidopsismutants, which are more tolerant to drought [9] and pathogen infection [28]. So far,the salt/drought/temperature tolerance of mutants and transgenic plants withaltered content or activity of MYB transcription factors involved in phenylpropanoidbiosynthesis has hardly been tested. This is worthy of future study.

21.5 Colorful MYB Proteins and Their Merits j497

21.6Regulating the Regulators

Members of theMYB factor family not only take an active part in gene regulation, butmany are also subject to transcriptional regulation themselves (summarized inTable 21.1). In addition,MYB transcription factors are controlled at the posttranscrip-tional and posttranslational levels. This is indicative of a sophisticated network fine-tuningMYB-mediated cellular effects.Under acute conditions, plantsmust be able toquickly prioritize stress responses at the expense of growth. Posttranslational regu-lation ofMYBproteinsmay serve the immediate response, while their stress-inducedtranscriptional accumulation is likely to mediate long-term stress adaptation. Themajor mechanisms controlling MYB expression and activity are described next.

21.6.1Transcriptional Regulation of MYB Genes

ManyMYB factor-encoding genes are expressed in a stimulus-dependentmanner. Asexemplified in the previous sections, transcript accumulation of individual MYBgenesunder certain stress conditions has often (successfully) been taken as indicativeof the process controlled by the respective proteins. Some MYB transcript levels arealtered in response to a diversity of small selection of stimuli or to one type of stressexclusively (www.genevestigator.ethz.ch). A comprehensive survey of transcriptabundance of Arabidopsis MYB genes in response to various hormone and abioticstress treatments, as well as a phylogenetic comparisonwith rice, has highlighted thecomplex regulation of this gene family in both species [22]. An overview of MYBgenes and their response to abiotic stresses canbe found inTable 21.1. Transcriptomeprofiling of plant stress responses and their applicability for MYB research aredescribed in detail in Section 21.7.1.

21.6.2miRNAs

InArabidopsis, a number ofMYB genes are targeted bymicroRNAs.miR159 appears tobeakeyregulator,controllingatleastfourAtMYBproteinsthatareinvolvedinantherandpollen development [21]. Similar posttranscriptional regulatory mechanisms of MYBproteins are likely to exist inotherplant species.The identificationofmiRNAs targetingmultiple MYB proteins in crops is a challenging but promising task. Manipulation ofmiRNAs is a useful tool to repress related, that is, potentially functionally redundantMYB proteins that negatively control stress tolerance. Interesting candidates in thisrespect are AtMYB60 and its putative orthologues (see Section 21.3.2).

21.6.3Posttranslational Regulation of MYB Proteins

Under acute stress conditions, plants need to respond rapidly. Cell damage has to beimmediately prevented, even before de novo synthesis of signaling components is


accomplished. This can be achieved through posttranslational modification ofinactive transcription factors already residing in the cell. Moreover, cells also employthis strategy to quickly inactivate signaling components. Examples of posttransla-tionally modified proteins can be found in all plant transcription factor families.

Particularly under the aspect of employing transcription factors for geneticengineering of plants, possible posttranslational modifications and their effects onprotein activity should be considered. Once again, knowledge aboutmodifications ofMYBproteins in (the better-studied)Arabidopsismay in some cases be applied to theirclosest homologues in other species. Major types of posttranslational modificationare outlined in the following sections.

21.6.3.1 SUMOylationSUMOylation is a type of posttranslationial modification in which the small ubiqui-tin-related modifier (SUMO) is reversibly and covalently conjugated to a lysineresidue in a substrate protein. This process involves three biochemical steps, SUMOE1 activation, E2 conjugation, and E3 ligation, and is reversed through the action ofubiquitin-like SUMO-specific proteases. SUMOylation can lead to changes inprotein–protein interactions, resulting in an altered activity, stability, or localizationof substrate proteins [60–62]. Bioinformatic, genetic, and biochemical analyses ofArabidopsis, rice, tomato, and Medicago indicate that components of the SUMOconjugation and deconjugation systems are conserved in plants [63].

At present, knowledge on SUMOylated proteins in plants is very limited. PutativeSUMOylation targetmotifs (characterized by theminimal consensusmotifYKXE/D, where Y is a large hydrophobic residue; X, any amino acid; E/D, glutamate oraspartate) are found in several plant proteins, including a number of MYBtranscription factors. Modification through SUMOylation has been reported forthe MYB protein AtLAF1. Disruption of the AtLAF1 SUMO motif resulted in analtered nuclear distribution [64]. However, the functional relevance is still unclear.

A sophisticated mutual regulation between MYB factors and SUMOylation wasreported by Miura et al. [63]: the Arabidopsis MYB-like protein AtPHR1 is aSUMOylation target of the SUMO E3 ligase AtSIZ1, involved in the phosphatestarvation response. In turn, SUMOylated AtPHR1 functions as the transcriptionalactivator of AtSIZ1.

Another plant SUMO targetMYBprotein, AtMYB30, has been studied thoroughlyin vitro, and three SUMOylated lysine residues, located at the C-terminus, have beenidentified [65]. The experimental system reported by the authors can be applied to testcandidate proteins, from any species of interest, for SUMOylation. In this context, itis interesting to note that a few MYB proteins from other plant species displayhomology to the SUMOylation-site-spanning region of AtMYB30, and at least someof the SUMOylation motifs are conserved (Pitzschke, unpublished).

21.6.3.2 PhosphorylationPhosphorylation is ameans of reversiblemodification that can change the propertiesof a protein. Protein kinases attach phosphate groups to serine, thereonin, or tyrosineresidues of target substrates, while protein phosphatases reverse this process. Afascinating stress-related transcription factor involved in stress and MYB regulation

21.6 Regulating the Regulators j499

is the bZIP protein AtVIP1. AtVIP1 has been shown to be a target of the stress-activated kinase AtMPK3 [66]. Upon phosphorylation, AtVIP1 migrates into thenucleus where it activates the expression of stress genes, including AtMYB44. Thus,AtVIP1 directly linksMAPK activation to transcriptional reprogramming.Moreover,it further amplifies the stress signal perceived from the MAPK cascade by drivingexpression of further transcription factors, which in turn modify expression ofsubsequent target genes [67]. Interestingly, both Atmpk3 [68] and Atmyb44mutants [29] are impaired in ABA-dependent stomatal closure. Whether this defectin Atmpk3 is primarily due to a nonfunctional VIP1–MYB44 pathway is yet to bedetermined.

Rather than being transcriptionally regulated through MAPKs, some MYB pro-teins are likely to be directly modified by MAPKs: several MYB proteins are amongthe candidates identified through a proteomic approach using high-density proteinmicroarrays to determine phosphorylation targets of Arabidopsis MAPKs [69]. AMAPK phosphorylation-controlled MYB factor has been reported from pine.PtMYB4 has a potential role in xylem differentiation. Phosphorylation had no effecton (in vitro) DNA binding activity of PtMYB4, but it altered its capacity to promotetranscription [70].

21.7Databases and Transcriptome Studies – Resources for MYB Research

Omics-based approaches are promising tools for identifying candidate key regulatorsin the stress response. A number of links to transcriptome profiles and databasesrelevant for abiotic stress/MYB research in crops are listed in Table 21.2.

21.7.1Transcriptomic Profiling of Abiotic Stress Responses in Crops

The characterization of many stress-related plant MYB genes has been preceded byisolation of candidate genes from transcriptome studies. From the vastly increasinginformation extracted from transcriptomic approaches, combined with large-scaleEST sequencing in crops and phylogenetic studies, a global picture is emerging.These rich sources of data, handled in a smart manner, can elucidate stress-specific/cross-species similarities and thus aid the development of more directed approachestoward generating multiple stress-resistant crops. A targeted search for stresssignaling components can, for example, be performed by screening publishedmicroarrays for genes differentially expressed upon a certain stimulus. This willreveal not onlyMYB genes as putative regulators, but also their possible downstreamtargets (i.e., those whose promoters contain MYB binding sites).

A promising and targeted approach to identify MYB factors (and other keycomponents) potentially involved in abiotic stress resistance is the comparison oftranscription profiles between non- and stress-exposed plants or between stress-susceptible and -tolerant cultivars.


The latter has, for example, been applied in tomato, where aMYB gene is amongthe transcription factors that show altered expression specifically in drought-tolerantgenotypes [71]. Likewise, in wheat TaMYBsdu1was found through comparison ofexpression profiles in wheat cultivars exhibiting contrasting salt and droughttolerance (see above) [33]. Similarly, IbMYB was identified as a gene particularlyhighly expressed in purple-fleshed sweet potato (see above) [55].

Changes in expression profiles in response to stresses have been studied innumerous plant species. Not surprisingly, MYB transcription factors are on the�candidate lists� that arose from such screens. A selection of transcriptome studies isgiven below, stressing the versatility of such approaches.

By means of microarray analysis of cold-treated rice plants, a number ofcandidates, including MYB-encoding genes, involved in the chilling response, wereidentified [39]. Transcript and metabolic profiling data are now also available forcommon bean, an important legume for human consumption. Phosphorus defi-ciency is a major limiting factor for nitrogen fixation and is widespread in areaswhere common bean is grown. From the comparison of P-deficient versus controlnodules, a number of candidate genes, including 37 transcription factors, 3 ofwhich are annotated as MYB genes, were identified [72]. In Arabidopsis, at least twoMYB proteins are involved in the phosphate starvation response (AtPHR1 andAtMYB62) [73, 74], once again stressing functional conservation. Given theirwidespread cultivation, excellent characteristics for the human diet (protein rich!),and the suitability for enriching nitrogen content in soil, leguminous plants are veryattractive for research aimed at improving yield/stress tolerance. It is reasonable toassume that MYB proteins act in abiotic stress signaling in probably all legumespecies. In this context, a database of gene expression profiles of salt-treated roots inMedicago truncatula (http://bioinformatics.cau.edu.cn/MtED/) is worth mention-ing. This database provides a number of useful bioinformatics tools and is intendedto help in selecting gene markers to improve abiotic stress resistance inlegumes [70].

A heterologous oligonucleotide microarray approach leads to the identification ofdrought-induced genes in banana. [75]. Many of these, including MYB genes,displayed homology to genes underlying QTL for drought and cold tolerance inrice. Similar experimental approaches may speed up the discovery of key regulatorsin abiotic stress signaling in other crop species. First attempts toward microarraystudies for the identification of abiotic stress-responsive genes in sunflower havebeen reported [76].

Despite the merits of microarray studies, one has to be cautious with datainterpretation. On the one hand, transcript abundance does not always correlatewith protein abundance or activity. On the other hand, most expression studies coveronly one or few time points; therefore, genes with transient transcript accumulationmight bemissed. Even if a stress persists, regulatory proteins (such asMYBs)maynothave to be synthesized continuously. Rather, once they have initiated downstreamsignaling through induction of their target genes, subsequent signaling components�take over.� Transient accumulation of MYB transcription factors is also a means ofpreventing �spillover.�

21.7 Databases and Transcriptome Studies – Resources for MYB Research j501

The fine-tuning of MYB-mediated signaling is exemplified by AtMYB4. AtMYB4negatively regulates the expression of a rate-limiting enzyme in the phenylpropanoidbiosynthesis (see above). The repressing effect exerted by AtMYB4 occurs in a dose-dependent manner. Moreover, upon UV exposure, AtMYB4 expression is down-regulated, indicating that derepression is an important step in acclimation toUV [56].

21.7.2Transcription Factor Databases

Another milestone in plant stress research was the development of transcriptionfactor databases (TFDB) (see Table 21.2). These have become available forA. thaliana,polar rice, Chlamydomonas reinhardtii, and Ostreococcus tauri [77]. A similar fruitfuldatabase and excellent tool providing amore substantial idea on the complexity of theMYB (and other transcription factors) family also exists for wheat [30]. This toolintegrates information on 203 MYB and 116 MYB-related genes.

Another example is the database LegumeTFDB, integrating a diversity of infor-mation for transcription factors ofGlycinemax, Leonurus japonicus, andM. truncatula.Hyperlinks to available expression profiles as well as information on cis-elements intranscription factor promoters will further aid the identification of stress-relatedMYB proteins. [78]. For instance, the database can be searched for all TF genes, in agiven species, in a given TF family, whose promoters carry a stress-associated cis-element(s) of choice. Questioning this database for members of the G. max MYBgene family (333 members) that contain MYBR cis-element(s) returns 87 hits. Incontrast, the larger family of AP2_EREBP transcription factors (405 members)contains proportionally fewer MYBR elements (87/405) (Pitzschke, own observa-tion). The overrepresentation of MYBR elements in promoters ofMYB genes mightbe indicative of a sophisticated crosstalk between severalMYBmembers. In addition,autoregulation (a MYB protein regulating its own expression) is also a likelyexplanation.

21.8Genetic Engineering, Limitations, Optimizations, Practical Considerations

For the genetic engineering of stress-resistant plants, �simply� overexpressing atranscription factor of interest might often not lead to the anticipated result. Tworeasons of failure and means to circumvent these problems are outlined as follows:

1) Overabundance of the transcription factor can have adverse side effects.2) Posttranslational modification is required for full activity.

1) Most transcription factors carry a nuclear localization signal. Both bioinfor-matic and in vivo studies document that the majority indeed localize to thenucleus, that is, the subcellular site of action. Overexpression of a transcriptionfactor of interest that constitutively localizes to the nucleus and that does not


require posttranslational modification or additional (limiting) transcriptionfactors for reaching full activity might result in hyperactivation of stress genesto an undesirable extent, particularly if growth retardation or developmentaldefects are the price of enhanced stress resistance conferred by constitutiveexpression of stress genes. This is exemplified by the dwarfed phenotype ofArabidopsis AtMYB96 overexpressing plants [28]. Moreover, in several casesattempts to generate transgenic plants constitutively overexpressing MYBgenes were unsuccessful, possibly due to the toxicity of excessive amounts ofthe protein (Pitzschke, unpublished) [65].Here, amore controlled approachmight be themethod of choice. Tominimize

negative effects on plant growth due to the use of a constitutive promoter, the useof stress-inducible promoters (rd29A, COR15A) to drive ectopic expression oftranscription factors was previously reported [79, 80]. Alternatively, a chemical-inducible, ecdysone receptor-based system suitable for field applications [81]may be used.

2) In some cases, transcription factors need to be posttranslationally modified toachieve full activity. Such modification can alter the stability, localization,dimerization properties, DNA binding activity, or cis-motif preference (seeSection 21.6.3). If the site and type of modification is known, protein variantscarrying single amino acid exchanges (e.g., mimicking a constitutively phos-phorylated state of a transcription factor) should be considered. This strategy hasbeen successfully pursued for PtMYB4 frompine [70] and theMYB44-regulatingbZIP protein VIP1 [66]).

21.9Outlook

The high functional conservation of MYB transcription factors across species willaccelerate the process of engineering crops with improved stress tolerance. As manystudies have shown, overexpression in heterologous plant species can indeedproduce the anticipated stress phenotype.

However, interfering withMYB transcription factor activity has its limitations thatone should be aware of: enhanced tolerance to one type of stress might be associatedwith a reduced tolerance to other stress types. For instance, Arabidopsis mutantslacking a functional MYB gene, AtBOS1 (AtMYB108), are more tolerant to infectionby a nectrotrophic pathogen (Botryotinia cinerea), but have a reduced resistance todrought, salt, and oxidative stress. [82]. Therefore, in-depth studies of a range of stressresponses will have to be carried out before engineered plants are ready for the field.

Acknowledgments

I thank Georg Seifert for critical comments on the chapter. I gratefully acknowledgefunding by the Austrian Science Foundation FWF (P21851-B09).

21.9 Outlook j503

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