Role of DREBs in regulation of abiotic stress responses in plants

18 Pages·2011·0.42 MB·English

JournalofExperimentalBotany,Vol.62,No.14,pp.4731–4748,2011
doi:10.1093/jxb/err210 AdvanceAccesspublication6July,2011
REVIEWPAPER
Role of DREBs in regulation of abiotic stress responses in
plants
Charu Lata and Manoj Prasad*
NationalInstituteofPlantGenomeResearch,ArunaAsafAliMarg,NewDelhi-110067,India
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*Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected] o
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Received17March2011;Revised4June2011;Accepted7June2011 d
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Abstract http
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Abiotic stresses such as drought, high salinity, and cold are common adverse environmental conditions that ://a
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significantly influence plant growth and productivity worldwide. The phytohormone abscisic acid (ABA) plays an ad
e
important role in physiological and developmental responses as well as in co-ordinating various stress signal m
ic
transduction pathways in plants. DREBs (dehydration responsive element binding) are important plant transcription .o
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factors(TFs)thatregulatetheexpressionofmanystress-induciblegenesmostlyinanABA-independentmannerand p
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play a critical role in improving the abiotic stress tolerance of plants by interacting with a DRE/CRT cis-element om
present in the promoter region of various abiotic stress-responsive genes. This review summarizes recent studies /jx
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highlightingtheroleoftheDRE-bindingfamilyofTFsintheadaptiveresponsestodifferentabioticstressesandtheir /a
structural and functional characters with emphasis on the expression and regulation of DREBs. The practical and rtic
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application value of DREBs in crop improvement, such as stress tolerance engineering as well as marker-assisted -a
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selection(MAS),hasalsobeendiscussed. tra
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Keywords: Abscisicacid,dehydration-responsiveelement-binding,DRE/CRT,marker-assistedselection,over-expression, 2
/1
transcriptionfactors. 4
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Introduction 13
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As sessile organisms, plants are constantly challenged by biochemical modifications in plants including leaf wilting, g
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a wide range of environmental stresses such as drought, reduction in leaf area, leaf abscission, stimulation of root s
high salt, and temperature change. Growth constraints and growth,changesinrelativewatercontent(RWC),electrolytic t on
1
stress due to these environmental changes result in reduced leakage (EL), generation of reactive oxygen species (ROS), 2
A
productivity and significant crop losses. Drought and and accumulation of free radicals which disrupt cellular p
salinity together affect more than 10% of arable land, homeostasis by reacting with lipids, proteins, pigments, and ril 2
0
resulting in a more than 50% decline in the average yields nucleic acids resulting in lipid peroxidation (LP), membrane 1
9
of major crops worldwide (Bray et al., 2000). Response to damage, and the inactivation of enzymes, thus affecting cell
abiotic stresses is a very complex phenomenon as various viability (Bartels and Sunkar, 2005). Besides this, abscisic
stages of plant development can be affected by a particular acid (ABA), a plant growth regulator and stress hormone,
stress and often several stresses simultaneously affects the induces leaf stomata closure to reduce water loss through
plant (Chinnusamy et al., 2004). Therefore, the mechanisms transpiration and decreases the photosynthetic rate in order
underlying stress tolerance and adaptation have long been to improve the water-use efficiency (WUE) of plants.
the focus of intensiveresearch. Molecular responses to abiotic stresses, on the other hand,
Plants usually respond to their changing environment in include stress perception, signal transduction to cellular
a complex, integrated way allowing them to respond and components,geneexpression,and,finally,metabolicchanges
adapttothespecificsetofconditionsandconstraintspresent imparting stress tolerance (Agarwal et al., 2006). The genes
ataparticulartime.Itinvolvesanarrayofphysiologicaland thus induced by stress not only function in protecting cells
ªTheAuthor[2011].PublishedbyOxfordUniversityPress[onbehalfoftheSocietyforExperimentalBiology].Allrightsreserved.
ForPermissions,pleasee-mail:[email protected]
4732 | Lata and Prasad
from stress by the production of important metabolic pro- embryo maturation, seed development, seed and bud
teins but also in regulating the downstream genes for signal dormancy, seed germination, root growth, fruit ripening,
transduction. regulationofstomatalaperture,andtheactivationofstress-
Large-scaletranscriptome analysishasrevealedthatthese responsive genes (Agarwal and Jha, 2010). Increased levels
gene products can broadly be classified into two groups of ABA are triggered by a variety of environmental stresses
(Bohnertetal.,2001;Sekietal.,2002;FowlerandThomashow, such as drought, water stress, salinity, cold, desiccation,
2002). One group constitutes genes that encode proteins to heat stress, and wounding. Further, it is also proved that
protect the cells from the effects of water stress. These genes ABA is a major physiological signal that induces drought
include those that govern the accumulation of compatible and high salinity responses (Gomez et al., 1988; Verslues
solutes (key enzymes for osmolyte biosynthesis like proline, and Bray, 2006; Farooq et al., 2009; Cutler et al., 2010;
betaine, sugars, etc.); passive transport through membranes Hubbard et al., 2010). The action of ABA, therefore, not
and energy-requiring water transport systems (water channel only involves the regulation of developmental pathways but
proteins and membranetransporters); andthe protection and also controls many stress-adaptation responses such as the
stabilization of cell structures from desiccation and damage activation of genes responsible for osmotic adjustment, ion
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byreactiveoxygenspecies(thedetoxificationenzymessuchas compartmentalization, root hydraulic conductivity, the o
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glutathione S-transferase, catalase, superoxide dismutase, regulation of shoot and root growth, limiting transpiration lo
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ascorbate peroxidase, etc.); enzymes for fatty acid metabo- rate and wilting, thus reducing water loss in the plants d
e
lism,proteinaseinhibitors,ferritin,andlipid-transferproteins; (Verslues and Zhu, 2005; Posp´ısˇilova´ et al., 2009). It is also d fro
and other proteins for the protection of macromolecules involved in the modification of gene expression, and m
[LEA (late embryogenesis abundant) protein, osmotin, anti- a number of stress-responsive genes are up-regulated by http
freeze proteins, chaperons, etc.]. It has been suggested that ABA during osmoticimbalance (Ingram and Bartels,1996). s
introduction or over-expression of genes encoding LEA pro- Although several genes are induced in response to ://ac
a
teins, proline synthetase or betaine synthetase, etc. can pro- dehydration and cold stress on exogenous ABA treatment d
e
m
vide toleranceto drought or high salinity in transgenic plants (Zhu, 2002; Shinozaki et al., 2003), there are also many ic
(CushmanandBohnert,2000). genes that do not respond to such treatments (Zhu, 2002; .o
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A second group of genes activated by abiotic stresses Yamaguchi-ShinozakiandShinozaki,2005)suggestingthe .c
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comprises regulatory proteins that further regulate stress existence of both ABA-dependent and -independent signal m
signaltransductionandmodulategeneexpressionand,hence, transduction cascades. DRE/CRT is one of the major /jxb
/a
probablyfunctioninthestressresponse.Theyincludevarious cis-acting elements which function in ABA-responsive or rtic
transcription factors (TFs) such as myelocytomatosis onco- non-responsive gene expression during abiotic stresses le
-a
gene (MYC), myeloblastosis oncogene (MYB), basic leucine (Nakashima and Yamaguchi-Shinozaki, 2010). b
s
zipper (bZIP), NAM, ATAF, and CUC (NAC), dehydration ABA-dependent signalling systems have been described tra
c
responsive element binding (DREB), etc. suggesting the role as pathways that mediate adaptation to stress by the t/6
2
ofvarious transcriptional regulatorymechanismsin the stress activation of at least two different regulons (a cluster of /1
4
signal transduction pathways; protein kinases [mitogen acti- genes controlled by a certain type of TF can be identified: /4
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vated protein (MAP) kinase, calcium-dependent protein (i) the AREB/ABF (ABA-responsive element-binding pro- 31
(CDP)kinase,receptorproteinkinase,etc.];proteinphospha- tein/ABA-binding factor) regulon; and (ii) the MYC/MYB /48
2
tasesandproteinases(phosphoesterasesandphospholipaseC, regulon (Abe et al., 1997; Busk and Page´s, 1998). On the 7
1
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etc.) implicated in the regulation of signal transduction and other hand, ABA-independent regulons are: (i) the CBF/ b
y
gene expression (Agarwal et al., 2006; Shinozaki and DREB (cold-binding factor/dehydration responsive element g
u
Yamaguchi-Shinozaki,2007). binding) regulon; and (ii) the NAC and ZF-HD (zinc-finger es
TheTFsinteractwithcis-elementsinthepromoterregions homeodomain) regulon (Saibo et al., 2009). In addition, t o
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of various stress-related genes to up-regulate the expression previous studies have identified the existence of both ABA- 12
of many downstream genes, thus imparting stress tolerance dependent and -independent pathways of stress response Ap
(AgarwalandJha,2010).IntheArabidopsisthalianagenome and function through members of the AP2/EREBP (ERF) ril 2
0
only, nearly 1500 TFs are reported which are thought to be family of TFs (Yamaguchi-Shinozaki and Shinozaki, 1994; 1
9
involved in stress-responsive gene expression (Riechmann Kizis and Page´s, 2002). Although these different stress
et al., 2000). Microarray analysis data in Arabidopsis and in response pathways usually function independently, it is
several other plants reveal that there are several pathways possible that some level of cross-talk certainly exists
thatindependentlyrespondtoabioticstress(inbothanABA between them (Fig. 1).
dependent and an ABA-independent manner), thus forming Other than the above-mentioned regulons, some other
a highly complex gene network (Fowler and Thomashow, TFs such as WRKY, HARDY, Zinc fingers etc. are also
2002;Umezawaetal.,2006). involved in abiotic stress tolerance responses and key
regulatory networks in plants (for details see Lata et al.,
2011b). However, until now, the best studied group of TFs
Role of ABA in stress-responsive gene expression
in abiotic stresses is the DREB genes since it activates the
ABAisanimportantplanthormone thatplaysaregulatory expression of many target genes that are responsible for
role in many physiological processes in plants, such as controlling correlated characters such as osmoprotection
DREBs in regulation of abiotic stress responses in plants | 4733
but not ABA (Yamaguchi-Shinozaki and Shinozaki, 1994
Drought/Salinity Cold Saleh etal., 2005).
Subsequently, the C-repeat responsive element (CRT)
with the core sequence 5#-CCGAC-3# was identified in the
Signal Perception promoter of cold-inducible genes from Arabidopsis (Saleh
etal.,2005).ThissequenceisrelatedtotheDREmotifandis
Pathways
essential for the induction of several genes by low tempera-
ABA-dependent ABA-independent ture. The CRT motif and the low temperature-responsive
element (LTRE) were identified in the promoters of cold-
regulated genes from Arabidopsis such as kin1, kin2, and
rab18, and were also responsible for the regulation of BN115
MYC/MYB bZIP DREB1/DREB2 NAC
and WCS120 from Brassica napus and wheat, respectively
Transcription factors (Kurkela and Borg-Franck, 1992; Lang and Palva, 1992;
Baker et al., 1994; Jiang et al., 1996; Ouellet et al., 1998).
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These are activated in an ABA-independent manner during o
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MYCRS/MYBRS ABRE DRE/CRT NACRS droughtandcoldstressinABA-deficientandABA-insensitive nlo
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cis-elements of target genes mutantsofArabidopsis.However,ithasalsobeensuggested de
d
that some DRE/CRT motifs can respond to an ABA- fro
dependent pathway (Haake et al., 2002). The DRE2 of the m
maize rab17 promoter, for example, is involved in ABA- http
Target gene dependent responses to osmotic stress with a typical core s
expression motif of 5#-ACCGAC-3# and was identified in the embryos ://ac
a
and leaves (Busk et al., 1997). Additionally, the DRE1 cis- d
e
element (5#-ACCGAG-3#) has also been identified in the mic
Tolerance rab17 promoter mediating an ABA-dependent regulation in .ou
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the embryo, but not in vegetative tissues in response to .c
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Fig.1. Aschematicrepresentationofstresssignalperceptionand drought stress (Busk et al., 1997; Busk and Page`s, 1998 m
geneexpressionviaABA-dependentandindependentpathways Saleh etal., 2005). /jxb
/a
atcellularlevelinplants(basedonwell-knownconcepts). rtic
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Identification, expression, and structural analysis of the -a
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and metabolism (Hussain et al., 2011). Thus, this review DREB transcription factors s
tra
specifically focuses on the DREB proteins and their role in c
regulating the abiotic stress responses in plants with an The DREB proteins namely, DREB1 and DREB2, involved t/62
emphasis on Arabidopsis, grasses, and other crop plants as in two separate signal transduction pathways under low /14
temperature and dehydration, respectively, are important /4
wellastheirutilityincropimprovementprogrammesthrough 7
geneticengineeringandmarker-assistedselection(MAS). ATFPsETthAaLtAin2du(AceP2a)/estehtyolefneabrieostpiconsstirveess-fraecltaotred(EgRenFe)s.pTlahnet 31/48
2
large AP2/ERF family of TFs in Arabidopsis was character- 7
The dehydration responsive elements 13
ized on the basis of the number of repetitions and the b
y
Extensive molecular analyses have revealed the presence of sequence of the AP2 domain (Sakuma et al., 2002). The 145 g
u
specific cis-elements that mediate the activation of various membersofthisfamilyinArabidopsiswereclassifiedintofive es
genes under different environmental stresses. The dehydra- subfamilies namely, DREB, ERF, AP2, RAV, and others t o
n
tion responsive element (DRE) with a 9 bp conserved core (Table1).TheAP2/ERFsuperfamilyofrice,however,canbe 12
sequence (5’-TACCGACAT-3’) was first identified in the divided into three families based on sequence similarity and Ap
promoterofthedrought-responsivegenerd29A(Yamaguchi- numbersofdomains:AP2,ERF(theERFsubfamilyandthe ril 2
0
ShinozakiandShinozaki,1993).Proteinfactorsfromnuclear CBF/DREBsubfamily),andRAV(Nakanoetal.,2006). 1
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extracts of salt-stressed and unstressed Arabidopsis plants DREBgenesplayanimportantroleintheABA-independent
weredesignatedasDREbindingfactor-1orDRBF-1asthey stress-tolerance pathways that induce the expression of various
specifically bind to DRE sequence in gel-shift assays thus, stress-responsive genes in plants. The first isolated cDNAs
resulting in the discovery of the DREB transcription factors encodingDREbindingproteins,CBF1(CRTbindingfactor1),
(Yamaguchi-Shinozaki and Shinozaki, 1994). Since then DREB1AandDREB2Awerefirstisolatedbyusingyeastone-
DREs have been reported to be involved in various abiotic hybrid screening (Stockinger etal.,1997; Liu et al.,1998)from
stress responses through both ABA-dependent and ABA- Arabidopsis. Since then, numerous DREB genes have been
independentpathways(Yamaguchi-ShinozakiandShinozaki, isolated from a number of plants (Table 2). These proteins
1994; Busk et al., 1997; Liu et al., 1998; Kizis and Page`s, specifically bind to the DRE sequence and activate the
2002Dubouzetetal.,2003).Thiselementisessentialforthe expressionofgenesdrivenbyit.
inductionofrd29Ageneexpressionbyosmoticstressessuch DREB1B/CBF1,DREB1A/CBF3,andDREB1C/CBF2genes
as drought and high salinity as well as by low temperature, lieintandemonchromosome4ofArabidopsis(Gilmouretal.,
4734 | Lata and Prasad
Table1. ClassificationofAP2/ERFfamilyoftranscriptionfactors via Arg and Trp residues located in the b-sheet (Magnani
(basedonSakumaetal.,2002). et al., 2004). The structure and characteristics largely hold
trueforAP2domain-containingproteinsfromothercropsas
well. A recent study showed AP2-DNA binding domain of
AP2/ERF No.of Subgroup Additional No.of
subfamilies AP2/ domain/motif genes SiDREB2 from foxtail millet sharing the same conserved
ERF features(Lataetal.,2011a).Twoconservedfunctionalamino
domain acids (valine and glutamic acid) at the 14th and 19th
AP2 2 - 14 residues, respectively, are present in the DNA binding
1 3 domain and are thought tobe crucialsites forthe binding of
DREB 1 A-1toA-6 ConservedWLG 56 DREBs to the DRE core sequences (Liu et al., 1998). An
motif alkaline N-terminal amino acid region, which acts as
ERF 1 B-1toB-6 ConservedWLG 65 a nuclear localization signal (NLS), and a conserved Ser/
motif
Thr-rich region adjacent to the AP2/ERF DNA binding
RAV 1 B3domain 6
domain are also mostly present. Ser/Thr-rich region is
Others 1 LackWLGmotif 1 D
considered to be responsible for phosphorylation of DREB o
(AL079349) wn
Total 145 proteins (Liu et al., 1998; Agarwal et al., 2006). The proteins loa
alsocontainanacidicC-terminalregionwhichispredictedto d
e
be functional in trans-activation activity (Stockinger et al., d fro
1998; Liu et al., 1998). Arabidopsis also contains two DREB2 1997). Most of the positively charged residues are conserved m
proteins namely, DREB2A and DREB2B (Liu et al., 1998). at the N-terminal domain. The NLS ‘PKRPAGRTK- http
DREB1/DREB2-homologous genes have also been isolated FRETRHP’, a DSAW motif immediately flanking the ERF/ s
from several grasses such as rice, wheat, barley, maize, AP2 domain and a conserved LWSY motif at the end of the ://ac
a
sorghum, rye, oat, and perennial ryegrass (Nakashima et al., C-terminal are present in most of the DREB1-type proteins d
e
m
2009). In important cereal crops like wheat and barley, (Congetal.,2008).Similarly,A-2andA-3subgroupproteins ic
a number of CBF homologues have been mapped to the Fr-2 possess a PKK-like NLS sequence RKXPAKGSKKGCM- .o
u
p
chromosomal region (Skinner et al., 2005; Miller et al., 2006). XGKGGPENXXA and RKXXXXKGGPXNXKF almost .c
o
AfunctionalFr-A1alleleplaysanimportantroleinregulating up to the AP2/ERF domain, however, unlike A-1 subgroup m
the CBF-mediated Cor/Lea gene expression in wheat members they do not have a conserved motif close to the /jxb
/a
(Kobayashi et al., 2005). Shukla et al. (2006) reported the C-terminal of the AP2-domain (Zhou et al., 2010). rtic
isolationandcharacterizationofagene(CAP2)fromchickpea A systematic phylogenetic analysis has been carried out, based le
-a
(Cicer arietinum) encoding a novel AP2-family TF which is on the similarities of AP2 domains in the DREB subfamily b
s
relatively small in comparison to most of the well-studied proteins isolated from various plant species, using MEGA 4.0 tra
c
DREB family members. Detailed functional study on small by the Neighbor–Joining method (Fig. 3). The phylogenetic t/6
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AP2 proteins such as CAP2 and soybean GmDREB may analysis showed that all the six groups within the DREB /1
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open up new areas in the plant developmental process. subfamily can easily be classified on the basis ofAP2 domains /4
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Recently, a novel DREB2-like gene, SiDREB2, associated assuggestedbySakumaetal.(2002).Itwasalsoobservedthat 31
with dehydration stress tolerance, has been isolated from AP2 domains can easily dichotomize monocots from dicots. /48
2
foxtailmillet(Setariaitalica)(Lataetal.,2011a). Thisfunctionalconservationmakesthemimportanttargetsfor 7
1
3
Expression of the Arabidopsis DREB1/CBF genes is in- crop improvement for abiotic stress tolerance through genetic b
y
duced by cold, while the DREB2 genes are induced by engineeringandplantbreeding. g
u
dehydration, high-salinity, and heatstressesgenerally(Fig. 2) es
(Liu et al., 1998; Shinwari et al., 1998; Nakashima et al., t o
Roleof DREB1/CBFsin cold-responsivegeneexpression n
2000). However, CBF4/DREB1D, DREB1E/ DDF2, and 12
DREB1F/DDF1 are induced by osmotic stress, suggesting the The Arabidopsis DREB1 subgroup consists of six genes Ap
existence of cross-talk between the DREB1 and the DREB2 (Sakuma et al., 2002). DREB1A/CBF3, DREB1B/CBF1, ril 2
0
pathways(Haakeetal.,2002;Nakashimaetal.,2009). andDREB1C/CBF2arestronglyandtransientlyinducedby 1
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AccordingtothestructurecharacteristicofDREBTFs,the low temperature stresses (Gilmour et al., 1998; Fowler and
subfamily of DREB TFs can be further divided into six Thomashow, 2002). Interestingly, it was observed that the
subgroupsfromA-1toA-6(Sakumaetal.,2002).TheDREB activation of CBF1–CBF3 genes in response to low tempera-
TFs contain a highly conserved AP2/ERF DNA-binding tureisgatedbythecircadianclock,suggestingtheirregulation
domain across the plant kingdom including Arabidopsis, has aspects in common with the regulation of Arabidopsis
rice,soybean,chickpea,tomato,tobacco,andmillets(Lata chlorophyll a/b-binding (CAB) genes (Fowler et al., 2005).
et al., 2011a). The three-dimensional structure of the OsDREB1A, OsDREB1B, OsDREB1C, and OsDREB1D, re-
Arabidopsis AtERF1 AP2 domain (PDB ID: 1GCC) was spectively,havebeenisolatedfromrice(Dubouzetetal.,2003).
solved by a heteronuclear multi-dimensional nuclear mag- A DREB1/CBF-type TF, ZmDREB1A was also identified in
netic resonance (NMR) technique (Allen et al., 1998). The maize(Qinetal.,2004).
domain consists of a three-stranded b-sheet and one a-helix CompetitiveDNAbindingassaysrevealedthatAtDREB1A
running almost parallel to the b-sheet, and it contacts DNA proteincouldbindtobothACCGACandGCCGACwiththe
DREBs in regulation of abiotic stress responses in plants | 4735
Table2. ResponseofDREBgenestovariousstresses.
DREBTFs Species Accessionno. Stressresponse References
DREB1A Arabidopsisthaliana AB007787 Cold Liuetal.,1998
DREB2A Arabidopsisthaliana AB007790 Drought,Salt,ABA Liuetal.,1998
DREB2C Arabidopsisthaliana At2g40340 Salt,Mannitol,Cold Leeetal.,2010
CBF1 Arabidopsisthaliana U77378 Cold Gilmouretal.,1998
CBF2 Arabidopsisthaliana AF074601 Cold Gilmouretal.,1998
CBF3 Arabidopsisthaliana AF074602 Cold Gilmouretal.,1998
CBF4 Arabidopsisthaliana AB015478 Drought,ABA Haakeetal.,2002
OsDREB1A Oryzasativa AF300970 Cold,Salt,Wounding Dubouzetetal.,2003
OsDREB1B Oryzasativa AF300972 Cold Dubouzetetal.,2003
OsDREB1C Oryzasativa AP001168 Drought,Salt,Cold,ABA,Wound Dubouzetetal.,2003
OsDREB1D Oryzasativa AB023482 None Dubouzetetal.,2003
D
OsDREB2A Oryzasativa AF300971 Drought,Salt,faintlytoCold,ABA Dubouzetetal.,2003 o
w
OsDREB1F Oryzasativa Drought,Salt,Cold,ABA Wangetal.,2008 n
lo
OsDREB2B Oryzasativa Heat,Cold Matsukuraetal.,2010 a
d
OsDREB2C Oryzasativa AK108143 None Matsukuraetal.,2010 ed
OsDREB2E Oryzasativa None Matsukuraetal.,2010 fro
m
OsDREBL Oryzasativa AF494422 Cold Chenetal.,2003
h
TaDREB1 Triticumaestivum AAL01124 Cold,Dehydration,ABA Shenetal.,2003 ttp
WCBF2 Triticumaestivum Cold,Drought Kumeetal.,2005 s://a
WDREB2 Triticumaestivum BAD97369 Drought,Salt,Cold,ABA Egawaetal.,2006 c
a
HvDRF1 Hordeumvulgare AY223807 Drought,Salt,ABA XueandLoveridge,2004 d
e
m
HvDREB1 Hordeumvulgare DQ012941 Drought,Salt,Cold Xuetal.,2009
ic
ZmDREB2A Zeamays AB218832 Drought,Salt,Cold,Heat Qinetal.,2007 .o
u
PgDREB2A Pennisetumglaucum AAV90624 Drought,Salt,Cold Agarwaletal.,l.,2007 p.c
SbDREB2 Sorghumbicolor ACA79910 Drought Bihanietal.,2011 om
SiDREB2 Setariaitalica HQ132744 Drought,Salt Lataetal.,2011a /jx
b
CaDREB-LP1 Capsicumannum AY496155 Drought,Salt,Wounding HongandKim,2005 /a
AhDREB1 Artiplexhortensis Salt Shenetal.,2003b rtic
GmDREBa Glycinemax AY542886 Cold,Drought,Salt Lietal.,2005 le-a
GmDREBb Glycinemax AY296651 Cold,Drought,Salt Lietal.,2005 bs
GmDREBc Glycinemax AY244760 Drought,Salt,ABA Lietal.,2005 tra
c
GmDREB Glycinemax AF514908 Drought,Salt Shiqingetal.,2005 t/6
2
GmDREB2 Glycinemax ABB36645 Drought,Salt Chenetal.,2007 /1
4
PpDBF1 Physcomitrellapatens ABA43697 Drought,Salt,Cold,ABA Liuetal.,2007 /4
7
PNDREB1 Arachishypogea FM955398 Drought,Cold Meietal.,2009 3
1
CAP2 Cicerarietinum DQ321719 Drought,Salt,ABA,Auxin Shuklaetal.,2006 /4
8
DvDREB2A Dendrathema EF633987 Drought,Heat,ABA,Cold Liuetal.,2008 27
1
DmDREBa Dendronthema3moriforlium EF490996 Cold,ABA Yangetal.,2009 3
b
DmDREBb Dendronthema3moriforlium EF487535 Cold,ABA Yangetal.,2009 y
g
PeDREB2 Populuseuphratica EF137176 Drought,Salt,Cold Chenetal.,2009 u
e
SbDREB2A Salicorniabrachiata GU592205 Drought,Salt,Heat Guptaetal.,2010 st o
n
1
2
same efficiency; however, OsDREB1A protein showed prefer- exposure to 4 (cid:1)C with maximum expression at 1 h (Medina Ap
ential binding to GCCGAC compared with ACCGAC et al., 1999). CBF1/DREB1B and CBF3/DREB1A tran- ril 2
0
(Sakuma et al., 2002; Dubouzet et al., 2003). However, al- scripts accumulated after 15 min of cold treatment while 1
9
though the Aloe DREB1 can bind to the DRE cis element it CBF2/DREB1C transcripts accumulated at a slower rate
may also bind to other cis-elements effectively and, hence, can with maximum expression after 2.5 h of cold exposure and
function in a new cold-induced signal transduction pathway then gradually declined (Novillo et al., 2004). However, the
(Wang and He, 2007). A similar result was also observed for CBF4 TF in Arabidopsis was rapidly induced during
OsDREBL which did not bind effectively to the CRT/DRE drought and ABA treatment but not by cold stress (Haake
motif(Chenetal.,2003). et al., 2002). OsDREB1A and OsDREB1B were induced
TemporalgeneexpressionstudiesofDREB/CBFgenesin soonaftercoldexposure(within40min)butdonotrespond
various crops have revealed that these are induced by to ABA treatment. OsDREB1A was induced within 5 h of
abiotic stresses particularly low temperature, however, at salt stress whereas OsDREB1C was constitutively expressed
different time periods (Table 2). For example, AtDREB1 during stress. OsDREB1D expression was not detected with
expressed within 10 min at 4 (cid:1)C (Liu et al., 1998). The orwithout any stress (Dubouzetetal.,2003;Agarwalet al.,
transcripts of CBF genes were detectable after 30 min of 2006). OsDREBL also accumulated quickly within 30 min
4736 | Lata and Prasad
Role of DREB2 proteins in drought, salinity, and heat-
responsive gene expression
Cold Dehydration/Salinity/
Heat
Among the DRE-binding proteins, the DREB2 subfamily is
induced by drought and high-salinity stress indicating their
important roleinstress-responsivegene expression(Table2).
The DREB2A and DREB2B were first isolated as cDNAs
ICE1 HOS1 encoding DRE/CRT-binding protein from Arabidopsis (Liu
et al., 1998). However, among the eight DREB2-type
SIZ1 proteins, DREB2A and DREB2B were thought to be major
MYB15 ICE1 transcription factors that function under osmotic stresses
(Nakashima et al., 2000; Sakuma et al., 2002). Though
DREB1 genes are widely investigated in many crops in
response to different abiotic stresses, the studies did not
DREB1/CBF DREB2 proceed as rapidly for DREB2 expression. DREB2 homol- Do
w
ogous genes have been isolated from economically impor- n
lo
tant cereal crops also such as rice, wheat, barley, maize, a
d
e
pearl millet, and foxtail millet (Dubouzet et al., 2003; Xue d
Stress-responsive genes and Loveridge, 2004; Egawa et al., 2006; Agarwal et al., fro
m
CRT/DRE 2sc0r0ip7;tsQwinereetfoauln.,d2t0o07b;eLreagtualaettedalb.,y2a0l1te1ran)a.tiDveREspBli2cintrganin- https
barley, wheat, maize, and rice with most of them showing ://a
c
transactivation abilities in yeast or plant cells (Dubouzet ad
Abioticstress tolerance e
et al., 2003; Egawa et al., 2006; Agarwal et al., 2007; Qin m
ic
et al., 2007). PgDREB2 from pearl millet was shown to be .o
Fig.2. Atypicalrepresentationoftheinductionofabiotic-stress- u
induciblegeneswiththeCRT/DREcis-elementintheirpromoters phosphorylated by total cell extracts and could not bind to p.c
o
DRE/CRT sequence (Agarwal et al.,2007). m
(basedonwell-knownconceptsfromArabidopsis).Transcription
The expression of Arabidopsis DREB2A and its homo- /jx
factormodifyingenzymesarerepresentedassmalltriangles.The b
logue DREB2B were induced by dehydration and high salt /a
starcorrespondstopost-translationalmodificationofDREB2. stress, but not by cold stress and exogenous ABA (Liu rticle
et al., 1998; Nakashima et al., 2000). ABA, mannitol, and -a
b
s
in responsetolowtemperature,butnotinresponsetoABA, cold treatments had little effect on DREB2C expression but tra
c
NaCl,anddehydration(Chenetal.,2003).Theexpressionof an elevated level of DREB2C mRNA was detected after t/6
the WCBF2 gene from wheat was induced rapidly by low 250 mM salt treatment (Lee et al., 2010). OsDREB2A 2/1
4
temperature and drought but not by ABA (Kume et al., transcript was induced within 24 h of dehydration and 250 /4
7
2005). In Arachis hypogaea, PNDREB1 was strongly up- mM salt stress but weakly responded to ABA and cold 31
regulated by treatments with low temperature, and also stress (Dubouzet et al., 2003). A comprehensive analysis of /48
2
responded to dehydration (Mei et al., 2009). However, all five OsDREB2s from rice revealed that OsDREB2A 7
1
Ca-DREBLP1 from hot pepper was rapidly induced by accumulated to the highest levels under the non-stress 3 b
y
dehydration and high salinity but was not at all affected by condition, and its expression was increased slightly by high g
u
coldstress(HongandKim,2005).TheexpressionofPpDBF1 temperature, drought, and high salinity treatments, but not es
was also induced by NaCl, cold, drought, and ABA treat- by low temperature (Matsukura et al., 2010). The t o
n
mentsinPhyscomitrellapatens(Liuetal.,2007). OsDREB2B transcript level was markedly increased espe- 12
The induction of DREB/CBF transcripts is organ-specific cially after 20 min of high temperature and 24 h of low Ap
and proportional to the length of the stress treatment. temperature stress, respectively. The transcript levels of ril 2
0
AhDREB1 was highly expressed in roots but less in stems OsDREB2C and OsDREB2E were low under control 1
9
and leaves in response to salt stress (Shen et al., 2003b). conditions and were not strongly induced by the abiotic
OsDREB1F was constitutively expressed in almost all the stresses (Matsukura et al., 2010). Wheat TaDREB1 and
tissues and organs, including young leaves, young roots, WDREB2, maize ZmDREB2A, and pearl millet PgDREB2
matureleaves,matureroots,youngpanicles,andcalluswith are responsive to cold stress as well, whereas foxtail millet
higher expression in panicles and callus than in the other SiDREB2 was not (Dubouzet et al., 2003; Shen et al.,
tissues (Wang et al., 2008). Expression of the HvDREB1 2003a; Egawa et al., 2006; Agarwal et al., 2007; Qin et al.,
gene in barley leaves was significantly induced by salt, 2007; Lata et al., 2011a). ZmDREB2A also responded to
drought, and low-temperature (Xu et al., 2009). GmDREBa high temperature (Qin et al., 2007). Sorghum SbDREB2
and GmDREBb were also induced by cold, drought, and showed induction at 1 h exposure to drought, after which
salt in leaves of soybean seedlings while expression of expression gradually dropped to basal levels by 24 h in
GmDREBc was high in roots following drought, salt, and transgenic rice (Bihani et al., 2011). The transcript level of
ABA treatments(Li etal.,2005). chickpea CAP2 increased by dehydration, NaCl, ABA, and
DREBs in regulation of abiotic stress responses in plants | 4737
D
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Fig.3. PhylogeneticanalysisofAP2/ERFdomainsofpublishedDREBproteinsinNCBIdatabase.Thephylogenetictreewasgenerated
byMEGA4.0software.Branchlengthsindicatedistance.A-1toA-6indicatethegroupsproposedbySakumaetal.(2002).The
appendedproteinsareasfollows:ArabidopsisthalianaAtDREB1(BAA33434),AtDREB1B(BAA33435),AtDREB1C(BAA33436),RAP2.1
4738 | Lata and Prasad
auxin treatments but not by treatments with low tempera- both AtDREB1A and OsDREB1A in Arabidopsis caused
ture, salicylic acid, and jasmonic acid (Shukla et al., 2006). severe growthretardation underoptimalgrowthconditions.
Chrysanthemum DvDREB2A was significantly affected by The level of stress tolerance and growth retardation in the
heat, low temperature, drought, ABA, and high salt treat- transgenic Arabidopsis over-expressing OsDREB1A was
ments (Liu et al., 2008). Expression of Populus euphratica relatively lower compared with AtDREB1A which might be
PeDREB2 was induced by cold, drought, and high salinity, due to difference in the number of target stress genes
but not by ABA (Chen et al., 2009). The transcript induced (Agarwal et al., 2006).
expression of SbDREB2A from Salicornia brachiata was Over-expression of the Arabidopsis DREB1/CBF genes in
induced by NaCl, drought, and heat stress (Gupta et al., transgenic Brassica napus or tobacco plants induced expres-
2010). sion of downstream genes and increased the freezing
AtDREB2A accumulated in roots, stems, and leaves tolerance of transgenic plants (Jaglo-Ottosen et al., 2001;
under normal growth conditions (Liu et al., 1998). Kasuga etal.,2004).LeCBF1–3(DREB1/CBFhomologues)
DREB2C expression was observed in mature embryo and werereportedtobepresentinatandemarrayinthetomato
the cotyledons of germinating seedlings (Lee et al., 2010). genome (Zhang et al., 2004). Out of these three genes only
D
Almoguera et al. (2009) reported that sunflower HaDREB2 the LeCBF1 gene was found to be cold-inducible and its o
w
n
accumulates in all vegetative tissues. Chrysanthemum constitutiveover-expressionintransgenicArabidopsisplants lo
a
DvDREB2A was expressed in all organs under natural induced expression of DREB1/CBF-targeted genes and d
e
d
conditions with the highest transcript accumulation in flowers increased freezing tolerance. On the other hand, over- fro
while less accumulation was detected in roots, stems, and expression of CBF1/DREB1B in tomato has been shown to m
youngleaves(Liuetal.,2008).SiDREB2,anA-2typeDREB increase the chilling and drought tolerance of transgenic http
family gene was expressed in leaves, roots, and young and tomato plants (Hsieh et al., 2002a, b). A constitutive over- s
mature spikelets of foxtail millet suggesting its role in expression of either LeCBF1 or DREB1A in transgenic ://ac
a
developmentalpathwaysalso(Lataetal.,2011a). tomato plants, however, did not result in increased freezing d
e
m
tolerance (Zhang et al., 2004). ic
The over-expression of DREB1A/CBF3 driven by the .o
Stress tolerance through over-expressing DREBs up
stress-inducible rd29A promoter in transgenic wheat im- .c
o
Functional analyses in vivo are important to understand the proved drought stress tolerance (Pellegrineschi et al., 2004). m
molecular mechanisms of stress tolerance in plants and also Similarly, the constitutive over-expression of CBF3/ /jxb
/a
to provide tools that might improve crop productivity. One DREB1A using the 35S promoter in transgenic rice resulted rtic
important way of achieving tolerance to multiple stress in increased stress tolerance to drought and high salinity le
-a
conditions is to over-express TFs that control multiple without any growth inhibition or phenotypic aberrations b
s
genes from various pathways. In fact, over-expression of (Oh et al., 2005). The constitutive over-expression of CBF4 tra
c
several DREB TFs in transgenic plants using various with resultant induction of cor genes in Arabidopsis t/6
2
promoters has resulted in plants more tolerant to drought, displayed higher tolerance for freezing and drought stress /1
4
salt, heat, and freezingstresses (Table 3). (Haake et al., 2002). The 35S:TaDREB1 rice transgenic /4
7
Transgenic Arabidopsis plants expressing DREB1B/CBF1 plants showed a dwarf phenotype, although the same was 31
or DREB1A/CBF3 under the control of the cauliflower not observed in the corresponding Arabidopsis transgenic /48
2
mosaic virus (CaMV) 35S promoter showed strong toler- plants. Thus, it may be possible that a monocot gene 71
3
ance to freezing, drought, and high salinity stresses suggest- transferred to dicots may not function as effectively as in b
y
ing that DREBs/CBFs target multiple genes (Jaglo-Ottosen themonocots (Shen et al., 2003a). g
u
etal.,1998;Liu etal.,1998;Kasuga etal., 1999). DREB1A/ Transgenic tobacco plants over-expressing PpDBF1 es
CBF3 over-expressing lines accumulated osmoprotectants, showed higher tolerance to salt, drought, and cold stresses t on
such as proline and various sugars, under non-stress (Liuetal.,2007).Theover-expressionoftheAhDREB1gene 12
conditions (Gilmour et al., 2000). Transgenic Arabidopsis led to the accumulation of its putative downstream target Ap
andriceplantsover-expressingOsDREB1Aweretolerantto genes and also conferred a better survival rate to transgenic ril 2
0
low temperatures, high salinity, and drought (Dubouzet tobacco plants under salt stress compared with the wild-type 1
9
et al., 2003; Ito et al., 2006). However, over-expression of plants (Shen et al., 2003b). These results are also consistent
(NP564496),RAP2.10(NP195408),RAP2.4(NP177931),AtCBF1(NP567721),AtDREB2A(AAU93685),AtDREB2B(BAA36706),
AtDREB2C(Q8LFR2),ABI4(AF085279),TINY(AAC29139);OryzasativaOsDREB1A(AAN02486),OsDREB1B(AAX28958),OsDREB2
(AAN02487),OsDREB2B(Q5W6R4),OsDREB2C(Q84ZA1),OsDBF1(AAP56252);ZeamaysZmDREB2A(ACG47772),ZmDBF1
(AAM80486),ZmDBF3(NP001105651);TriticumaestivumTaDBF(AAL37944);SorghumbicolorSbDREB2(ACA79910);Setariaitalica
SiDREB2(HQ132744);HordeumvulgareHvDBF2(AAM13419),HvCBF1(AAX23686),BCBF1(AAK01088),BCBF3(AAK01089),
HvDRF1.1(AAO38209),HvDRF1.3(AAO38211);FestucaarundinaceaFeDREB1(CAG30550);PhyllostachysedulisPeDREB2
(ABY19376);LycopersicumesculentumLeCBF1(AAS77820);NicotianatabacumNtDREB2(ACE73694);Gossypiumhirsutum
GhDREB1A(AAP83936);GlycinemaxGmTINY(ACP40513);CatharanthusroseusORCA1(CAB93939)BrassicajunceaBjDREB1B
(ABX00639),BrassicanapusBnCBF7(AAM18959);ThellungiellahalophilaThDREB2B(ABV08790).
DREBs in regulation of abiotic stress responses in plants | 4739
Table3. StressresponseofoverexpressingDREBsintransgenicplants.
DREBgene Transgenicplants Stressresponse References
AtDREB1A Arabidopsis Droughttolerance Liuetal.,1998
AtDREB1A Tobacco Freezinganddroughttolerance Kasugaetal.,2004
AtDREB1A Wheat Droughttolerance Pellegrineschietal.,2004
AtDREB1A Rice Droughtandsalinitytolerance Ohetal.,2005
AtDREB2A Arabidopsis None Liuetal.,1998
AtDREB1A Potato Salinitytolerance Behnametal.,2006
AtDREB1A Peanut Droughttolerance Bhatnagar-Mathuretal.,2006
AtDREB2A-CA Arabidopsis Droughttolerance Sakumaetal.,2006a
AtCBF1 Arabidopsis Salinitytolerance Jaglo-Ottosenetal.,1998
AtCBF1 Tomato Droughtandfreezingtolerance Hsiehetal.,2002a
AtCBF1 Brassicanapus Freezingtolerance Jaglo-Ottosenetal.,2001
D
AtCBF1 Canola Freezingtolerance Jagloetal.,2001 o
w
AtCBF3 Arabidopsis Freezingtolerance Gilmouretal.,2000 n
lo
AtCBF4 Arabidopsis Freezingtolerance Haakeetal.,2002 a
d
AtDREB2C Arabidopsis Thermotolerance Limetal.,2007 ed
OsDREB1A Arabidopsis Drought,salinityandfreezingtolerance Dubouzetetal.,2003 fro
m
OsDREB2A Arabidopsis None Dubouzetetal.,2003
h
OsDREB2B Arabidopsis Droughtandthermotolerance Matsukuraetal.,2010 ttp
OsDREB1F Rice,Arabidopsis Drought,salinityandfreezingtolerance Wangetal.,2008 s://a
OsDREB1G Rice Droughttolerance Chenetal.,2008 c
a
ZmDREB2A Arabidopsis Droughtandthermotolerance Qinetal.,2007 d
e
m
PgDREB2A Tobacco Hyperionicandhyperosmoticstresses Agarwaletal.,2010
ic
BNCBF5 Brassicanapus Freezingtolerance Savitchetal.,2005 .o
u
BNCBF17 Brassicanapus Freezingtolerance Savitchetal.,2005 p.c
AhDREB1 Wheat Droughtandsalinitytolerance Shenetal.,2003b om
GmDREB Arabidopsis Freezingtolerance Shiqingetal.,2005 /jx
b
LeCBF1 Tomato None Zhangetal.,2004 /a
LeCBF1 Tobacco Freezingtolerance Zhangetal.,2004 rtic
GhDREB1 Tobacco Drought,salinityandfreezingtolerance Shanetal.,2007 le-a
PpDBF1 Tobacco Droughtandsalinitytolerance Liuetal.,2007 bs
CAP2 Tobacco Salinitytolerance Shuklaetal.,2006 tra
c
PeDREB2 Arabidopsis Salinitytolerance Chenetal.,2009 t/6
2
HvDREB1 Arabidopsis Droughttolerance Xuetal.,2009 /1
4
/4
7
3
1
/4
8
2
with the over-expression of OsDREB1F as tolerance to high but only slight tolerance to freezing (Sakuma et al., 2006a). 7
1
3
salinity, drought, and low-temperature is enhanced both in The DREB2A-CA-GFP fusion protein showed a stable b
y
riceandArabidopsistransgenicplants(Wangetal.,2008). expression in the nucleus, however, the same was not the g
u
The over-expression of DREB2A does not result in any casewithDREB2A-FL-GFPfusionprotein,suggestingthat es
phenotypic changes in Arabidopsis transgenic plants, i.e. thecentralregionofDREB2Aisrequiredfortheregulation t o
n
neither any retardation in growth nor any improved stress ofits stability (Nakashima etal.,2009). 12
tolerance (Liu et al., 1998). This suggested that the Although DREB2A and DREB1A were isolated together Ap
DREB2A protein requires post-translational modification (Liuetal.,1998),itwasfoundthatsomeofthedownstream ril 2
0
such as phosphorylation for its activation (Liu et al., 1998). target genes of DREB2A were different from those of 1
9
Busk and Page`s (1998) also reported that phosphorylation DREB1A (Nakashima et al., 2009). The reason for this
may be necessary for the activation of proteins under may be due to the fact that both DREB2A and DREB1A
drought-stressconditions,thusenhancingtheDNA-binding slightly differ in their DNA-binding specificities. While
activity of several transcription regulators. The central DREB2A preferentially binds ACCGAC motifs, DREB1A
region of DREB2A contains a negative regulatory domain specifically has a high affinity to A/GCCGACNT sequences
as revealed by its domain analysis using Arabidopsis (Sakuma et al., 2006a). Microarray analysis of transgenic
protoplasts and that the internal deletion of amino acids Arabidopsis plants over-expressing DREB2A-CA revealed
136 to 165 makes DREB2A constitutively active. Over- that its over-expression not only induced drought- and salt-
expression of this constitutively active form (DREB2A-CA) responsive genes but also heat-shock (HS)-related genes.
resulted in growth retardation in transgenic Arabidopsis These transgenic plants also showed improved thermotol-
plants, up-regulation of many stress-inducible downstream erance which was significantly decreased in DREB2A
genes, and imparted significant tolerance to drought stress knockout plants (Sakuma et al., 2006b). Moreover, it was
4740 | Lata and Prasad
found that transient induction of the DREB2A occurred not enhance longevity in transgenic tobacco. The constitu-
rapidly by HS stress, and that the sGFP-DREB2A (synthetic tively over-expressing HaDREB2 could not increase thermo-
green fluorescent protein-DREB2A) protein accumulated in tolerancein seedlingsor leadtothe accumulationofHSPsat
nucleiofHS-stressedcells.ItwasalsoobservedthatDREB2A normal growth temperatures. By contrast, when HaDREB2
up-regulated genes were down-regulated in DREB2A knock- and HaHSFA9 (sunflower Heat Shock Factor A9) were
out mutants under stress conditions (Yamaguchi-Shinozaki over-expressed together, positive effects on seed longevity
and Shinozaki, 2009). Over-expression of DREB2C was were observed, apart from those observed with over-
also found to induce the expression of many HS stress- expression of HaHSFA9 alone (Almoguera et al., 2009).
inducible genes, ensuring thermotolerance of transgenic Over-expression of CAP2 in tobacco improved growth and
Arabidopsis (Lim et al., 2007). Recently it has been development, and tolerance to dehydration and salt stress of
shown that HsfA3 (an HS TF) regulates expression of the transgenic plants (Shukla et al., 2006). Surprisingly,
many heat-inducible genes downstream of the DREB2A expression of CAP2 cDNA in yeast (Saccharomyces cerevi-
stress-regulatory system and functions in acquiring sae)alsoenhanced heattolerance, withincreased expression
thermo-tolerance under the control of the DREB2A of gene for heat shock factor 1 (Hsf1) and its target yeast
D
transcriptional cascade (Schramm et al., 2008; Yoshida heat shock protein 104 (Hsp 104) suggesting strong o
w
n
et al., 2008). Recent analyses of genetic responses involved evolutionary conservation of the stress response mecha- lo
a
in plant acclimation to high temperature have also in- nisms (Shukla et al., 2009). In another interesting study it d
e
dicated that AtDREB2B, together with AtHSFA3, are the was reported that the recombinant E. coli cells expressing d fro
only two TFs that were specifically induced in thermo- SbDREB2A exhibited better growth in basal LB medium as m
tolerant lines of Arabidopsis. Therefore, it appears that well as in medium supplemented with NaCl, PEG, and http
AtDREB2A and AtDREB2B could have some functional mannitol. The improved growth in recombinant E. coli cells s
redundancy in thermo-tolerance, as mutants for either did could be due to the regulation of stress-regulated functional ://ac
a
not show a defect in heat acclimation (Schramm et al., genes by this TF and certain interactions with transcrip- d
e
m
2008); however, dreb2a-1 mutant plants, showed reduced tional network in the bacterial cells, thus providing stress ic
basal thermotolerance when directly treated at 46 (cid:1)C for tolerance (Gupta etal., 2010). .ou
p
45 min (Larkindale and Vierling, 2008). Taken together, it These observations suggest that the DREB proteins are .c
o
can be concluded that DREB2 regulons function in both important TFs in regulating abiotic stress-related genes and m
osmotic and heat stress responses. playacrucialroleinimpartingstresstolerancetoplants.The /jxb
/a
Theover-expressionofriceOsDREB2Adoesnotresultin DREB1 and DREB2 regulons can thus be used to improve rtic
any phenotypic changes in transgenicArabidopsis, butover- the tolerance of various kinds of agriculturally important le
-a
expression of wheat WDREB2 and maize ZmDREB2A cropplantstodrought,high-salinity,and freezing stressesby b
s
caused such changes in transgenic Arabidopsis and tobacco genetransfer. tra
c
(Dubouzet et al., 2003; Kobayashi et al., 2007; Qin et al., t/6
2
2007). Transgenic Arabidopsis plants over-expressing maize Mechanisms of DREB gene regulation and stress /14
ZmDREB2A were dwarf, and exhibited improved drought /4
tolerance in plants 7
and heat stress tolerance. Microarray analysis of these 31
plants revealed that 28 of 44 up-regulated genes were Intheearliersections,itwasdiscussedthatDREBgenesare /48
2
common with transgenic Arabidopsis over-expressing involved in abiotic stress responses and impart stress 7
1
3
DREB2A-CA (Qin et al., 2007). Over-expression of endurance to plants, but the physiological and biochemical b
y
OsDREB2B in transgenic Arabidopsis showed enhanced basesofsuchresponsesandtheiractivationmechanismsare g
u
expression of DREB2A target genes and improved drought still a matter of investigation. In Arabidopsis a number of es
and heat-shock tolerance (Matsukura et al., 2010). Trans- regulatorygeneswhichareinvolvedinCBFcold-responsive t o
n
genic tobacco plants over-expressing PgDREB2A exhibited pathway have been isolated and analysed by combining 12
enhanced tolerance to both hyperionic and hyperosmotic genetic and molecular approaches (Fig. 2). The gene Ap
stresses. The transgenics also showed higher expression of products which may function directly in transcription are: ril 2
0
downstream genes NtERD10B, HSP70-3, Hsp18p, PLC3, higher expression of osmotically responsive genes1 (HOS1), 1
9
AP2 domain TF, THT1, LTP1,NtHSF2, and pathogen- a negative regulator of CBFs; FIERY2 (FRY2), a transcrip-
regulated (NtERF5) factors with different stress treatments tional repressor; and low expression of osmotically re-
(Agarwal et al., 2010). sponsive genes2 (LOS2), a positive regulator in the
Recently Bihani et al. (2011) reported that the constitu- pathway (Yang et al., 2005). A putative MYC ICE1
tive expression of sorghum SbDREB2 driven by the CaMV (Inducer of CBF expression 1)-like TF may play an
35S promoter led to pleiotropic effects in rice and these important role in activating CBF1/DREB1B, CBF2/
transgenics did not set seed. However, the rd29A:SbDREB2 DREB1C and CBF3/DREB1A (Chinnusamy et al., 2003).
riceplantssetseedandthetransgenicsshowedasignificantly Its activation requires cold-induced phosphorylation, and
higher number of panicles compared with the wild-type rice may be regulated by HOS1 which targets the ICE1 protein
plants. Phenological and agronomic traits were also not forubiquitinationandsubsequentdegradation(Dongetal.,
affected in rd29A:SbDREB2 transgenic rice (Bihani et al., 2006; Saibo et al., 2009). Further, a SIZ1 (a SUMO E3
2011). Over-expression of sunflower HaDREB2 in seeds did ligase)-dependent sumoylation can block ubiquitination of