SnRK2 Protein Kinases—Key Regulators of Plant Response to Abiotic Stresses

14 Pages·2011·0.58 MB·English

OMICS AJournalofIntegrativeBiology Review Article
Volume15,Number12,2011
ªMaryAnnLiebert,Inc.
DOI:10.1089/omi.2011.0091
SnRK2 Protein Kinases—Key Regulators of Plant
Response to Abiotic Stresses
Anna Kulik,1 Izabela Wawer,1,2 Ewa Krzywin´ska,1 Maria Bucholc,1 and Graz_yna Dobrowolska1
Abstract
The SnRK2 family members are plant-specific serine/threonine kinases involved in plant response to abiotic
stresses and abscisic acid (ABA)-dependent plant development. SnRK2s have been classed into three groups;
group 1 comprises kinases not activated by ABA, group 2 comprises kinases not activated or activated very
weaklybyABA,andgroup3compriseskinasesstronglyactivatedbyABA.Sofar,theABA-dependentkinases
belonging to group 3 have been studied most thoroughly. They are considered major regulators of plant
response to ABA. The regulation of the plant response to ABA via SnRK2s pathways occurs by direct phos-
phorylation of various downstream targets, for example, SLAC1, KAT1, AtRbohF, and transcription factors
required for the expression of numerous stress response genes. Members of group 2 share some cellular func-
tions with group 3 kinases; however, their contribution to ABA-related responses is not clear. There are strong
indicationsthattheyarepositiveregulatorsofplantresponsestowaterdeficit.Mostprobablytheycomplement
the ABA-dependent kinases in plant defense against environmental stress. So far, data concerning the physio-
logicalroleofABA-independentSnRK2sareverylimited;itistobeexpectedtheywillbestudiedextensivelyin
the nearest future.
Introduction activated by 5¢-AMP, able to phosphorylate and inactivate
enzymes of lipid and cholesterol synthesis (Carling et al.,
In nature, plants are frequently exposed to harmful 1987;Hardieetal.,1989).InS.cerevisiae,theSNF1genewas
environmentalconditions:abioticandbioticstresses.Inor- discovered by a screen for mutations that caused failure to
dertosurviveplantsmustbeabletosensethechangesoftheir growonsucroseoronnonfermentablecarbonsourcessuchas
environmentandrespondtothesechangesbyvariousdefense glycerol and ethanol (Celenza and Carlson, 1986). It is well
mechanisms.Stresssignalsarerecognizedandtransmittedto establishednowthatAMPKinmammalsandSNF1inyeast
differentcellularcompartmentsbyspecializedsignalingpath- are cellular energy sensors acting as central regulators of
ways in which protein kinases and phosphatases are key cellularenergyhomeostasis.Theycontrolenergybalanceby
components. Among the protein kinases involved in stress regulating metabolic processes and responses to different
signal transduction in plants are those common to all eu- environmentalandmetabolicstresses.AMPKinhibitsseveral
karyotic organisms, for example, mitogen-activated protein anabolic processes (e.g., synthesis of lipids, glycogen, and
kinases(MAPKs)(reviews:Choetal.,2009;Mishraetal.,2006; proteins) and activates catabolic ones (fatty acid oxidation
Pitzschke et al., 2009; Rodriguez et al., 2010), Glycogen syn- and glycolysis) by direct phosphorylation of enzymes con-
thasekinase3(GSK3)(JonakandHirt,2002;Kohetal.,2007), trollingtheseprocesses(forreview,seeHardie,2007).More-
S6kinase(S6K)(Mahfouzetal.,2006),aswellasplantspecific over,AMPKactivatestranscriptionofseveralgenesinvolved
ones,forexample,calcium-dependentproteinkinases(CDPKs) incellularadaptationtostressbymodulatingtheactivityof
(DasandPandey,2010;Hrabaketal.,2003;Wurzingeretal., transcriptionregulators(forreviews,seeCanto´ andAuwerx,
2011)andmostofSNF1-relatedkinases(SnRKs). 2010;Hardie,2007)andalsothroughdirectassociationwith
Plant SnRKs are classified into the SNF1/AMPK family, chromatin and phosphorylation of histone H2B (Bungard
whichcomprisesalsoSNF1(sucrosenonfermenting-1)kinase et al., 2010) and histone deacetylase (McGee et al., 2008).
from the yeast Saccharomyces cerevisiae and mammalian Disorders in AMPK functioning result in serious health
AMPKs (AMP-activated protein kinases). AMPK was origi- problems(forreviews,seeLuoetal.2010;Poelsetal.,2009;
nally defined as a mammalian protein kinase allosterically RafalskiandBrunet,2011;WangandGuan,2009).
1InstituteofBiochemistryandBiophysics,PolishAcademyofSciences,Warsaw,Poland.
2InstituteofGeneticsandBiotechnology,UniversityofWarsaw,Warsaw,Poland.
1
2 KULIKET AL.
The SNF1 kinase exhibits very similar features to AMPK, anabolism were coordinately repressed by KIN10 (Baena-
including structural similarities (both are heterotrimeric), Gonza´lezetal., 2007). Thisfunctional analysis ofArabidopsis
substrate specificity, mode of regulation (reversible phos- KIN10/11providesevidencethatSnRK1s,likeyeastSNF1and
phorylationofaconservedThrinthekinaseactivationloop), mammalian AMPK, have a central role in energy signaling.
andtheroleinregulationofcellularmetabolismandresponse The results also reveal crucial and previously unrecognized
to environmental stresses. SNF1 activity rises dramatically regulatory roles of the plant SnRK1s in controlling a large
during glucose deficit and also to some extent upon alkali- number of genes encoding transcription factors, chromatin
zationofyeastenvironment,exposuretohighconcentration remodeling proteins, and a plethora of signal transduction
ofsodiumions,andoxidativestress(HongandCarlson,2007; components(Baena-Gonza´lez.,2010).
Wilson et al., 1996). SNF1 is the main regulator of the shift TheSnRK2andSnRK3kinaseshavedivergedfurtherfrom
fromfermentationtorespiratorymetabolism,beingaglobal SNF1/AMPKthanmembersoftheSnRK1subfamily.These
regulatoroftranscriptionofgenesinvolvedincarbohydrate twosubfamiliesarespecifictoplantsandareinvolvedinplant
metabolism and respiration. Several excellent reviews on developmentandenvironmentalstresssignaling(forreviews,
structural and biochemical features of AMPK and SNF1 as seeBoudsocqandLauriere,2005;Coelloetal.,2011;Halford
wellastheirfunctionshavebeenpublishedrecently(Hardie, andHey,2009;Harmon,2003;Hrabaketal.,2003).
2007, 2011; Hedbacker and Carlson 2008; Petranovic et al.,
2010;Scottetal.,2009;Zhangetal.,2010a).
SnRK3 Subfamily
SnRKsareplanthomologsofAMPK/SNF1andsimilarly
to them seem to be crucial in linking stress and metabolic Thefirstidentifiedserine/threonineproteinkinase,which
responses (Coello et al., 2011; Halford and Hey, 2009; Hey accordingtoHalfordandHardie(1998)belongstotheSnRK3
etal.,2010;PolgeandThomas,2006).Theyaredividedinto subfamily, was WPK4 from wheat (Sano and Youssefian,
threesubfamilies:SnRK1,SnRK2,andSnRK3(forreviews,see 1994).WPK4transcriptionwasupregulatedbylight,nutrient
HalfordandHardie,1998;Harmon,2003;Hrabaketal.,2003). deprivation, and cytokinins, and downregulated by sucrose
ThisreviewisdedicatedtotheSnRK2subfamily,whoserole (Ikeda et al., 1999; Sano and Youssefian, 1994). Results
inregulationofplantresponsetoabioticstresswasrecently obtainedbyIkedaetal.(2000)stronglysuggestthatWPK4is
recognized responsible for controlling nitrogen metabolism and carbon
assimilationduringperiodsoflowtemperature,inwhichthe
Brief Overviewof SnRK1 andSnRK3 Subfamilies kinase resembles the SnRK1 kinases. WPK4 orthologs are
known in various plants (Chikano et al., 2001; Ohba et al.,
SnRK1 subfamily
2000); their expression is differentially and distinctly regu-
Enzymes belonging to the SnRK1 subfamily structurally latedbyvariousexternalsignals.
andfunctionallyresembletheyeastandanimalSNF1/AMPK Today,itisconsideredthattheSnRK3subfamilycomprises
kinases. SnRK1s show approximately 62% amino acid se- protein kinases interacting with calcineurin B-like (CBL)
quenceidentitywithSNF1andAMPKinthecatalyticdomain calcium-binding proteins. They were discovered indepen-
and 48% overall (Halford et al., 2004). SnRK1 genes from dently by the research groups of Zhu, Kudla, and Luan.
variousplantshavebeenidentifiedandcharacterized.SnRK1 Therefore,differentnamesaregiventothoseenzymesinthe
kinases are involved in the regulation of global metabolism literature:SOS2-likekinasesorPKSs—proteinkinasesrelated
andenergystatusoftheplant,forexample,inresponsetolow toSOS2(Gongetal.,2004;Guoetal.,2001)andCIPKs—CBL-
glucose/high sucrose level, dark period, hypoxia, salinity, interactingproteinkinases(Shietal.,1999).Asimilarsituation
pathogen or herbivore attack (for reviews, see Halford and occurs in the case of the calcium sensors interacting with
Hey,2009;PolgeandThomas,2006).SnRK1sarepostulated them.InArabidopsis,thecalcineurinB-likeproteinsarealter-
to control signaling pathways leading to modulation of ni- natively designated as SOS3-like, SCaBPs, or CBLs (Gong
trogen, sucrose, and lipid metabolism, organogenesis, and etal.,2004;Halfteretal.,2000;LiuandZhu,1998;Shietal.,
senescence.LiketheAMPK/SNF1kinases,SnRK1sregulate 1999). CBLs and CIPKs have been indentified in all higher
the above processes at the transcriptional and posttransla- plant species whose genomes have been sequenced. The
tional levels (for reviews, see Baena-Gonza´lez and Sheen, Arabidopsis genome encodes 26 putative CIPK and 10 CBL
2008;HalfordandHardie,1998;Halfordetal.,2003)Several proteins. CIPKs/SOS2-like kinases/PKSs directly interact
enzymesinvolvedinbasalmetabolism(e.g.,nitratereductase, with CBL/SCaBPs/SOS3-like proteins. Several studies have
HMG-CoAreductase, sucrosephosphate synthase, trehalose revealedthateachCBLinteractswithasubsetofCIPKsand
phosphatesynthase)areknowntobedirectlyphosphorylated each CIPK interacts withone or moreCBLs (forreview, see
and regulated by SnRK1s. Using various molecular biology Batisti(cid:2)candKudla,2009;Gongetal.,2004;Luan,2009;Weinl
techniques,acombinationofsystematicscreens,aswellasby andKudla,2009).AnincreaseofCa2+concentrationtriggered
analyzing public transcriptome databases the KIN10 and by various environmental or cellular stimuli is detected by
KIN11proteinkinases,membersoftheSnRK1subfamilyfrom CBLs:theirconformationchangesuponCa2+ binding,which
Arabidopsis thaliana, have been shown to control convergent in most cases results in activation of the CIPKs interacting
reprogramming of transcriptional response to darkness, with them. The CIPKs play an important role in the plant
changesofsugarlevel,andstressconditions(Baena-Gonza´lez responsestodrought,cold,ABA,sugar,salt,andpHchanges
et al., 2007). KIN10 promotes catabolism by regulating ex- (for reviews, see Coello et al., 2011; Gong et al., 2004;
pression of a broad array of genes involved in a variety of Hrabak et al., 2003; Luan, 2009). The best known signaling
majorcatabolicpathwaysthatprovidealternativesourcesof pathwayinwhichCIPKsareinvolvedistheSOS(saltoverly
energy and metabolites. Conversely, a large set of genes in- sensitive) pathway regulating sensitivity to salinity in
volved in the energy-consuming ribosome biogenesis and A.thaliana.TheSOS3–SOS2kinasecomplex,whichisrequired
SnRK2—ROLE IN PLANT RESPONSETO ABIOTICSTRESSES 3
forintracellularNa+ andK+ homeostasisandplanttolerance Throughtransientexpressioninprotoplastsitwasshownthat
tohighNa+andlowK+environments,appearstocontrolthe all Arabidopsis thaliana SnRK2s, except SnRK2.9, are rapidly
expressionofSOS1andtheactivityofSOS1protein,aplasma activatedbytreatmentwithdifferentosmolytes,suchassu-
membrane Na+/H+ exchanger (antiporter) (Zhu, 2003). crose,mannitol,sorbitol,orNaCl,andsomeofthemalsoby
Moreover,SOS2positivelyregulatesactivitiesoftwovacuolar abscisic acid (ABA), suggesting that these kinases are in-
membrane transporters, the Na+/H+ antiporter AtNHX1 volvedinageneralresponsetoosmoticstress(Boudsocqetal.,
(Qiuetal.,2004)andtheCa2+/H+ antiporterCAX1(Cheng 2004, 2005). In rice, all 10 members are activated by hyper-
et al., 2004), and negatively regulates the highly selective osmoticstressandthreeofthemalsobyABA(SAPK8,SAPK9,
Na+ transporterAtHKT1(Rusetal.,2001).TheSOSpathway and SAPK10). Rice SnRK2 kinases were designated SAPK1
wasfoundtocontrolexpressionofseveralsaltstress-specific through SAPK10, which stands for osmotic Stress/ABA-
tolerancedeterminantgenes(Gongetal.,2001). activated Protein Kinase (Kobayashi et al., 2004). SAPK3 is
ThedataconcerningtheinvolvementofSnRK1andSnRK3 identical with the previously reported rice Ser/Thr protein
kinasesinstressresponsearecombinedinexcellentreviews kinaseREK(Hottaetal.,1998).SAPK1andSAPK2arehighly
byHrabaketal.(2003),Chinnusamyetal.(2004),Boudsocq homologous to wheat (Triticum aestivum) PKABA1 (protein
and Lauriere (2005), Baena-Gonza´lez and Sheen (2008), kinase ABA1) (Anderberg and Walker-Simmons, 1992; Ho-
HalfordandHey(2009),andCoelloetal.(2011). lappa and Walker-Simmons, 1995), and barley (Hordeum
vulgare)HvPKABA1(Yamaguchietal.,2002).
Based on a phylogenetic analysis, the SnRK2 family
SNRK2 Subfamily
membershavebeendividedintothreegroups(Fig.1).Group
Until very recently not much attention was paid to this 1compriseskinasesnotactivatedbyABA,group2—notac-
subfamily. Before the year 2000, there was basically no in- tivatedoractivatedveryweaklybyABA(dependingonplant
formationconcerningSnRK2function,onlyafewdataindi- species), and group 3—strongly activated by ABA. Evolu-
catingthatABAandabioticstressesinducetheexpressionof tionalstudiesbasedonsequencedatafromalgae(Chlamydo-
someSnRK2geneswerepublished(AnderbergandWalker- monasreinhardtii),mossesPhyscomitrellapatensandSelaginella
Simmons, 1992; Holappa and Walker-Simons, 1995). In the
year 2000, SnRK2s started to be recognized as enzymes in-
volved in abiotic stress signal transduction in plants. That
yeartwoarticlesshowingrapidactivationofSnRK2kinases
in response to abiotic stresses were published; activation of
Nicotiana tabacum osmotic stress-activated protein kinase
(NtOSAK)inresponsetohyperosmoticstressintobaccoBY-2
cells (Miko1ajczyk et al., 2000) and AAPK (ABA-activated
proteinkinase)byABAinresponsetodroughtinguardcells
offavabean(Viciafaba)(Lietal.,2000)werepresented.Lietal.
(2000)showedthatAAPKisinvolvedinregulationofanion
channelsandstomatalclosure.Studiesduringthefollowing
years extended our knowledge on SnRK2s—their structure,
mechanismofactivation,andfunction.Theyear2009wasa
veryspecialyearforunderstandingSnRK2sfunction.In2009,
independently,twolaboratoriesobtainedatriplesnrk2.2/3/6
mutant, which allowed to establish the role of ABA-
dependent SnRK2s in plant response to water deficit, seed
maturation,andgermination(FujiiandZhu,2009;Fujitaetal.,
2009;Nakashimaetal.,2009).Moreover,thesameyearsolu-
bleABAreceptorswerediscovered,andthisdiscoveryledto
understanding early events of ABA signaling pathway, in
which SnRK2 kinases are involved. Now, after 10 years of
studies, the SnRK2 subfamily members are considered as
major players in plant response to osmotic stress and ABA-
dependentplantdevelopment(BoudsocqandLauriere,2005;
Fujiietal.,2007,2011;FujiiandZhu,2009;Fujitaetal.,2009;
Nakashimaetal.,2009).TheSnRK2kinaseswereidentifiedin
every plant studied. Ten members of the SnRK2 family are
encoded by the Arabidopsis as well as rice and sorghum ge-
nomes(Boudsocqetal.,2004;Kobayashietal.,2004;Lietal.,
2010). Eleven SnRK2s have been identified in maize (Huai
etal.,2008),andsomeSnRK2representativesweredescribed
FIG. 1. Phylogenetic tree of selected higher plant SnRK2s.
for tobacco (Kelner et al., 2004), wheat (Anderberg and
Group 1—ABA-independent kinases, group 2—kinases not
Walker-Simmons, 1992; Holappa and Walker-Simons, 1995;
dependent or weakly dependent on ABA, group 3—ABA-
Maoetal.,2010;Zhangetal.,2010b,2011),soybean(Monks dependentkinases.TheunrootedUPGMA(unweightedpair
etal.,2001),favabean(LiandAssmann,1996;Lietal.,2000), group method with arithmetic mean) tree was built using
andalgae(Daviesetal.,1999;Gonzalez-Ballesteretal.,2008). ClustalW2 andpresented using TreeView.
4 KULIKET AL.
moellendorffi, and several angiosperm genomes (Arabidopsis, C-terminaldomainconsistsoftwosubdomains,DomainIand
poplar, rice) showed that all higher plants have all three Domain II(Fig. 2). Domain I (about 30amino acids starting
groups of SnRK2, P. patens has only SnRK2s belonging to fromthekinasedomain)ischaracteristicforallSnRK2family
group3,andS.moellendorffikinasesbelongingtogroups2and members and is needed for activation by osmotic stress, in-
3.ChlamydomonasreinhardtiiSnRK2saredistinctfromthoseof dependently of ABA, whereas Domain II (about 40 amino
landplants(Mizoguguchietal.,2010;Umezawaetal.,2009; acids just after Domain I) is specific to the ABA-dependent
and reviews by Hauser et al., 2011; Umezawa et al., SnRK2sonlyandisneededfortheABAresponse(Belinetal.,
2010).These data indicate that group 3 isan ancient form of 2006; Kobayashi et al., 2004; Yoshida et al., 2006). Yoshida
land plant SnRK2, whereas group 1 is the most recent. An- etal.(2006)proposedamodelinwhichDomainIIofSRK2E/
otherclassificationofSnRK2scanalsobefoundinthelitera- OST1/SnRK2.6kinasewasresponsibleforthekinaseactiva-
ture (Halford and Hardie, 1998), which now has only tionbyABAandinteractionwiththeABI1phosphatase.Re-
historical meaning. In that classification the SnRK2 kinases cently,severalindependentinvestigationshaveconfirmedthe
weredividedintotwoclasses,SnRK2aandSnRK2b,basedon interactionbetweenABA-dependentSnRK2s(fromgroup3)
the amino acid composition of their divergent C-terminal with members of the clade A PP2C phosphatases, among
domainsrichinacidicaminoacids,eitherpoly-Asp(SnRK2a) themABI1(Umezawaetal.,2009;Vladetal.,2009),andhave
or poly-Glu (SnRK2b). Groups 2 and 3 of the phylogenic shownthatthesephosphatasesarenegativeregulatorsofthe
classification correspond to the SnRK2a subfamily, whereas ABA-dependentSnRK2s.
group1toSnRK2b(Fig.2).
Regulation of SnRK2 activity
Structure ofSnRK2
Resultspresentedbyseverallaboratoriesprovideevidence
SnRK2s are monomeric serine/threonine protein kinases thatreversiblephosphorylationinthekinaseactivationloop
with a molecular weight around 40kDa (Hardie, 1999; Mi- is responsible for regulation of SnRK2 activity (Belin et al.,
ko1ajczyketal.,2000).Recently,crystalsofmutatedformsof 2006;Burzaetal.,2006;Boudsocqetal.,2007;Kobayashietal.,
SRK2E/OST1/SnRK2.6 unable to bind ATP were obtained 2004; Miko1ajczyk et al., 2000). In 2006, our group demon-
(Yuntaetal.,2011).OnlypreliminaryX-rayanalysishasbeen strated that in response to osmotic stress the activity of
performedsofarandthestructureshouldbesolvedsoon. NtOSAK (Nicotiana tabacum osmotic stress-activated protein
TheaminoacidsequencesofallSnRK2scanbedividedinto kinase), a tobacco ABA-independent SnRK2 (member of
tworegions,theN-terminalhighlyconservedkinasedomain, group1),isregulatedbyreversiblephosphorylationofSer154
related to those of SNF1 and AMPK, and the regulatory and Ser158 within the kinase activation loop. Site-directed
C-terminaldomaincontainingstretchesofacidicaminoacids, mutagenesisstudiesshowedthatphosphorylationofSer-158
either Glu (group 1) or Asp (groups 2 and 3). Further, the wascrucialforthekinaseactivity,whereasphosphorylation
FIG. 2. Structure of Arabidopsis thaliana SnRK2s. (A) Model of SnRK2 structure. Ser/Thr kinase domain—kinase catalytic
domain,I—domaininvolvedinABA-independentactivationinresponsetoosmoticstress,characteristicforallSnRK2,II—
domain needed for ABA-dependent activation of SnRK2s. (B) Alignment of Arabidopsis thaliana SnRK2s with characteristic
domainsmarked:activationloop(greenbox),domainI(violetbox),anddomainII(bluebox).Gapsintroducedtomaximize
alignment are markedwithdashes.
SnRK2—ROLE IN PLANT RESPONSETO ABIOTICSTRESSES 5
of Ser-154 most probably facilitated the phosphorylation on ayamaandUmezawa,2010;Joshi-Sahaetal.,2011;Maetal.,
Ser-158 (Burza et al., 2006). Several other laboratories also 2009;Melcheretal.,2009;Nishimuraetal.,2009,2010;Park
showedthecrucialroleofphosphorylationonaresiduecor- etal.,2009;Santiagoetal.,2009;Umezawaetal.,2009,2010;
responding to Ser-158 of NtOSAK in activation of different Vlad et al., 2009). It should be stressed that the above-
membersoftheSnRK2subfamily(Belinetal.,2006;Boudsocq mentioned mechanism was described only for the ABA-
etal.,2007;Chaeetal.,2007).UsingPro-QDiamond,aspecific dependentSnRK2sbelongingtogroup3.Kinasesfromgroup
dyeforphosphoproteins,Boudsocqetal.(2007)showedthat 2 (Arabidopsis SnRK2.7 and SnRK2.8) interact with PP2C
all activated SnRK2s studied were phosphorylated in re- phosphatasesaswell(Umezawaetal.,2009),butsofartheir
sponse to osmotic stress but the induced changes of phos- inactivationbyPP2Cinvivohasnotbeenshown.TheABA-
phorylationwereclearlydifferentdependingonthekinase.In independent SnRK2s (group 1) do not interact with PP2C
addition,theincreaseoftheglobalphosphorylationlevelin- phosphatasesintheyeasttwohybridsystem(Umezawaetal.,
ducedbyABAapplicationwaslowerthaninthecaseofos- 2009;Vladetal.,2010,ourunpublishedresults).
motic stress, suggesting that different phosphorylation Recently, another negative regulator of SnRK2, SCS
mechanismsmaybeinvolvedinSnRK2activationbyhyper- (SnRK2-interacting Calcium Sensor), has been identified in
osmolarityandABA(Boudsocqetal.,2007).Thesedifferences our laboratory (Bucholc et al., 2010). SCS is a plant-specific
have been recently confirmed (Vlad et al., 2010). These au- calcium-bindingproteinthatinteractswithallSnRK2sstud-
thors studied changes of phosphorylation of two SnRK2 ki- iedbyus,bothABA-dependentandABA-independent,and
nases,SnRK2.6/OST1(ABA-dependentkinasefromgroup3) inhibits their activity. These results present not only a new
and SnRK2.10 (ABA-independent kinase from group 1), in mechanism of regulation of SnRK2s activity but also a link
response to osmotic stress and ABA. They indentified two betweenSnRK2andCa2+ signaling.
importantphosphorylationsiteswithintheactivationloopsof Besides phosphorylation, also other posttranslational
thesekinases,oneofthemcriticalforthecatalyticactivityand modifications can possibly regulate SnRK2 activity in plant
both necessary for the full activation by hyperosmolarity or cells.Ithasbeendemonstratedthatnitricoxide(NO)induces
ABAofbothSnRK2sstudied.ItwasalsoshownthatSnRK2.6 the activity of NtOSAK in tobacco cells and can be a key
was most probably activated through independent phos- component in NtOSAK activation in response to hyper-
phorylation of these two sites, whereas a sequential process osmotic and salinity stresses (Lamotte et al., 2006; Wawer
occurred in SnRK2.10, phosphorylation of one serine being etal.,2010).ThissuggestedthatpresumablyS-nitrosylationof
required for the phosphorylation of the other. The results, NtOSAK could be involved in the regulation of its activity.
indicating that SnRK2.10 is activated by sequential phos- However,S-nitrosylationstudieshaveindicatedthatcysteine
phorylationoftwoserinesinthekinaseactivationloop,arein residuesofNtOSAKdonotundergodirectchemicalmodifi-
agreementwithourpreviousdataconcerningthemechanism cations; it is more likely that some of the S-nitrosylated cel-
ofNtOSAKactivation(Burzaetal.,2006). lular partners of the kinase, for example, GAPDH
So far, an upstream kinase(s) phosphorylating and acti- (glyceraldehyde-3-phosphate dehydrogenase) are responsi-
vatingSnRK2shasnotbeenidentified.Someauthorssuggest ble for the regulation of the activity of NtOSAK by NO
that SnRK2s can be activated by autophosphorylation (Fujii (Waweretal.,2010)byanas-yetunknownmechanism.Sofar,
etal.,2009);however,thisissueremainscontroversial. itisnotknownifNOisinvolvedintheregulationoftheABA-
EventhoughthekinasesupstreamofSnRK2sarestillnot dependentSnRK2s;however,itisprobablebecauseNOisa
known,severalnegativeregulatorsofSnRK2s’activityhave keyelementofABA-dependentstomatalclosure(forreview,
been identified. All SnRK2s are inactivated by dephosphor- seeNeilletal.,2008).
ylation;therefore,obviouslyspecificphosphatasesshouldact ItshouldalsobementionedthatanABA-independentki-
astheirnegativeregulators.Recently,ithasbeenshownthat nase,SnRK2.10hasbeenidentifiedamongproteinsinteract-
membersofcladeAPP2C-typeproteinphosphatasesinteract ingwithphosphatidicacid(Testerinketal.,2004);therefore,it
withABA-dependentSnRK2sandinactivatethemefficiently cannot be ruled out that this phospholipid, whose role in
viadephosphorylationofSerandThrresiduesinthekinase stress signal transduction iswell documented, can be a reg-
activationloop(Umezawaetal.,2009;Vladetal.,2009).This ulatorofatleastsomeoftheSnRK2subfamilymembers.
discoverywasreportedatthesametimeastheidentification Possibleposttranslationalmodificationsinvolved(ormost
of ABA receptors—RCAR/PYR/PYL (RCAR, regulatory probablyinvolved)inregulationofSnRK2kinasesactivityare
componentofABAReceptor/PYR1,pyrabactinresistance1/ presentedinFigure3.
PYL,PYR1-like)(forreviews,seeCutleretal.,2010;Hubbard
etal.,2010;Klingleretal.,2010;Melcheretal.,2010;Umezawa
Regulation ofSnRK2s expression
etal.,2010;Weineretal.,2010).TheidentificationoftheABA
receptorswasaturningpointinstudiesonABAsignalingand SnRK2s are primarily regulated at the posttranslational
ontheregulationofSnRK2sactivity.Thephosphatasesfrom level,byphosphorylation.However,expressionofgenesen-
group A of the PP2C family form a complex with RCAR/ codingthesekinasesisalsoregulatedinresponsetochanges
PYR/PYL proteins, sometimes they are even called ABA oftheplantenvironment.Kabayashietal.(2004)showedthat
coreceptors.IntheabsenceofABA,phosphatasesbelonging expressionofvariousOryzasativaSnRK2sisdifferently reg-
tothecladeAPP2CinteractwithanddephosphorylateABA- ulatedbyABA,mannitol,andNaClindifferentplantorgans
activatedSnRK2kinases(fromgroup3),keepingtheminan (roots, blades, and sheaths); some of them are upregulated,
inactive form. Upon ABA binding the conformation of the whereasothers,onthecontrary,aredownregulated.Inrice,
RCAR/PYR/PYL-PP2Ccomplexchangesresultingininacti- high salt significantly induces SAPK4 expression (Die´dhiou
vation of the phosphatase and its release from the SnRK2 et al., 2008). Triticum aestivum PKABA1 (protein kinase in-
kinase active site. This allows activation of the kinase (Hir- ducedbyabscisicacid1),thefirstclonedSnRK2fromwheat,
6 KULIKET AL.
FIG. 3. Simplified model of SnRK2s pathways involved in plant response to osmotic stress. Model was done based on
publishedandunpublisheddatadescribedinthemaintext.ABA,abscisicacid;NO,nitricoxide;PA,phosphatidicacid;ROS,
reactive oxygen species; unknown SnRK2K(s), an SnRK2 upstream kinase(s)— still unknown; unknown PP, an unknown
phosphoprotein phosphatase dephosphorylating and inactivating ABA-independent SnRK2s; clade A PP2C, clade A PP2C
phosphoprotein phosphatase; SCS, SnRK2-interacting calcium sensor; PYR/PYL/RCAR, soluble ABA receptor; Rboh,
plasma membrane NADPH oxidase; SLAC1, Slow Anion Channel 1; KAT1, inward-rectifying potassium channel; TFs,
transcription factors. Connections represent positive (arrow) and negative (block) regulation. Bold line—direct regulation/
interaction, dashedline—indirect regulation. Question marksindicate currently unknown or not confirmedregulation.
aswellasotherwheatSnRK2genes:TaSnRK2.4,TaSnRK2.7, inducestheexpressionoffoursoybean(Glycinemax)SnRK2
andTaSnRK2.8areupregulatedinresponsetoplanttreatment genes,SPK1,SPK2,SPK3,andSPK4(serine/threonineprotein
withdifferentstressors.Theirexpressionisinducedbypoly- kinase1,2,3,and4)(Monksetal.,2001;Yoonetal.,1997).
ethyleneglycol,NaCl,andcold,andadditionallyexpression Itshouldalsobementionedthatexpressionofatleastsome
ofTaSnRK2.4(fromgroup1),TaSnRK2.8(fromgroup3),and SnRK2scanberegulatedbythelight–darkcycle,forexample,
PKABA1(fromgroup2)isupregulatedbyABA(Anderberg SnRK2.8(Shinetal.,2007),SnRK2.4andSnRK2.1(Parketal.,
and Walker-Simmons, 1992; Holappa and Walker-Simons, 1993).
1995; Mao et al., 2010; Zhang et al., 2010b, 2011). A recent
report by Xuet al. (2009) concerned expression of the W55a
The physiological role ofSnRK2 subfamilymembers;
gene, encoding a protein kinase belonging to the SnRK2
SnRK2 downstream targets
family from wheat, whose amino acid sequence is the most
homologoustoriceSAPK1(group2ofSnRK2s).Expressionof AmpledataindicatethatSnRK2sarepositiveregulatorsof
W55a wasupregulated notonlybydrought,salt, andexog- plant response to drought. AAPK (ABA-activated protein
enouslyappliedABA,butalsobysalicylicacid,ethylene,and kinase)isactivatedbyABAinguardcellsoffavabean(Vicia
methyl jasmonate, whereas it was not responsive to cold faba)inresponsetodroughtandisinvolvedintheregulation
stress.Inmaize,expressionofSnRK2sgeneswasenhancedin of anion channels and stomatal closure (Li and Assmann,
responsetoNaCl(ZmSnRK2.3,ZmSnRK2.6,andZmSnRK2.4), 1996; Li et al., 2000). The closest homolog of AAPK in A.
ABA(ZmSnRK2.2,ZmSnRK2.4,ZmSnRK2.5,ZmSnRK2.7,and thaliana istheSRK2E/OST1/SnRK2.6protein kinase.Sofar,
ZmSnRK2.10), and cold (ZmSnRK2.3, ZmSnRK2.7), whereas this kinase is the most broadly studied kinase in respect to
expression of some of them (ZmSnRK2.5, ZmSnRK2.6 and SnRK2roleinplantresponsetoenvironmentalstresses.The
ZmSnRK2.9)wassuppressedbyheat(Huaietal.,2008;Zou srk2e/ost1/snrk2.6mutantcannotcopewitharapiddecreasein
etal.,2006).Hyperosmolarity(dehydrationandhighsalinity) humidity and has a wilty phenotype (Yoshida et al., 2002).
SnRK2—ROLE IN PLANT RESPONSETO ABIOTICSTRESSES 7
Loss of the kinase function causes an ABA-insensitive phe- on the regulation of gene expression by SnRK2s is signifi-
notype in stomatal closure. It has been shown that OST1 cantlybroader.
mediatestheregulationofstomatalaperturebyABA(Mustilli Some functional redundancy between SnRK2.6, SnRK2.2,
etal.,2002;Yoshidaetal.,2002)andactsupstreamofreactive andSnRK2.3hasbeenpostulated.Allofthemaremembersof
oxygenspecies(ROS)production(Mustillietal.,2002).ROS group3(ABA-dependent SnRK2s)oftheSnRK2familyand
are essential intermediates in ABA signaling in guard cells. arekeyregulatorsofplantresponsetoABA.TheArabidopsis
TheyareneededforregulationofCa2+ influxbyactivationof triplemutantsnrk2.2/snrk2.3/snrk2.6,alsoknownassrk2d/i/e,
Ca2+channels(Kwaketal.,2003;Murataetal.,2001).Itiswell disrupted in three ABA-dependent SnRK2s (SnRK2.2/
establishedthatCa2+ influxisrequiredforstomatalclosure. SRK2D,SnRK2.3/SRK2I,andSnRK2.6/SRK2E),isextremely
Two partially redundant guard cell NADPH oxidases (Atr- insensitivetoABAandexhibitsgreatlyreducedtoleranceto
bohDandAtrbohF)functioninROS-dependentABAsignal- drought, not only by the disruption of stomatal closing but
ing in Arabidopsis (Kwak et al., 2003). Therefore, it is very alsoasaresultofdownregulationofABA-andwaterstress-
likelythatABA-dependentregulationofROSproductionby inducedgenes(FujiiandZhu,2009;Fujitaetal.,2009).Studies
OST1 is achieved through phosphorylation of the plasma involving this mutant indicate that these kinases are major
membraneNADPHoxidases.InthecaseofAtrbohD,itwas regulatorsofABAsignalinginresponsetowaterstress.The
shownthatphosphorylationandCa2+ bindingisneededfor intensityofthetriplemutantphenotypeisfarstrongerthan
itsactivation(Ogasawaraetal.,2008).Sirichandraetal.(2009) thatofanyofthesingleordoublemutants.Previousstudies
showedthattheN-terminalpartoftherecombinantAtrbohF using snrk2.2, snrk2.3, and double snrk2.2/snrk2.3 T-DNA
produced in a bacterial system is phosphorylated by OST1 knockout mutants indicated that SnRK2.2 and SnRK2.3 ki-
in vitro. Moreover, OST1interacts with AtrbohF in bimolec- naseswereinvolvedintheregulationofseeddormancyand
ularfluorescencecomplementation (BIFC)assay,suggesting germination(Fujiietal.,2007).Doublemutantsnrk2.2snrk2.3
that NADPH oxidase could be an SnRK2.6/OST1 substrate was insensitive to ABA in terms of seed dormancy, germi-
in vivo. However, there are no direct data so far presenting nation,andseedlinggrowth,incontrasttothesnrk2.6mutant
phosphorylationandregulationofAtrbohsactivitybySnRK2 (Fujii et al., 2007; Yoshida et al., 2002). Moreover,
in planta. Specific ion channels of the plasma membrane of snrk2.2snrk2.3 was barely affected in leaf water loss, sug-
guardcells,activatedinresponsetoABAareresponsiblefor gestingthat SnRK2.2and SnRK2.3haveonly minor rolesin
the ABA-dependent stomatal closure. Recently, a guard cell thecontrolofstomatalmovementindroughtresponse,which
slow-anion channel 1 (SLAC1), functioning in stomatal re- further distinguished their function from that of SnRK2.6
sponsetohighCO andABA,wasidentifiedandcharacter- (Fujiietal.,2007).However,thephenotypeofthetriplemu-
2
izedatthemolecularlevel(Negietal.,2008;Vahisaluetal., tant clearly showed that all three kinases are partially re-
2008). It appeared that SLAC1 is a critical channel of guard dundant and all of them are important for ABA responses,
cells,responsibleforstomatalmovements.Itwasshownthat both in terms of water stress response, as well as ABA-
the activity of this channel was regulated by reversible dependentdevelopmentalprocesses(seeddevelopmentand
phosphorylation, the phosphorylated form being the active dormancy)(FujiiandZhu,2009;Fujitaetal.,2009;Nakashima
one.ItwasestablishedthatOST1togetherwithCDK21and etal.,2009).
possibly CDPK23 were responsible for the phosphorylation As mentioned above, OST1/SnRK2.6, as well as other
andactivationofSLAC1(Leeetal.,2009;Geigeretal.,2009, ABA-dependent SnRK2s, are involved in the regulation of
2010). Another ion channel in Arabidopsis guard cells medi- expression of stress–response genes. The Arabidopsis triple
ating the aperture of stomata pores is an inward-rectifying mutantsnrk2.2/snrk2.3/snrk2.6showedstrongimpairmentof
potassium channel KAT1 (Pilot et al., 2001). The activity of ABA- and water stress-dependent gene expression under
this channel is also regulated by phosphorylation catalyzed waterdeficitconditions(Fujitaetal.,2009).Ontheotherhand,
bySnRK2.6/OST1andmostprobablybyCDPK(Satoetal., jasmonic acid-dependent genes and those involved in regu-
2009).However,inthecaseofKAT1,thephosphorylationhas lationoffloweringwereupregulated.Thoseauthorsshowed
anegativeeffectonitsactivity.Itshouldbestressed,however, that expression of almost all genes encoding dehydration-
thatincontrasttoSLAC1,downregulationofKAT1activityis responsive late embryogenesis abundant (LEA) proteins as
needed for stomatal closure. The above results indicate that wellasthoseencodinggroupAPP2C(negativeregulatorsof
SnRK2.6/OST1phosphorylatesbothanion(SLAC1)andka- the SnRKs activity) were strongly downregulated in the
tion(KAT1)channelsthatarecrucialforstomatalmovements, snrk2.2/snrk2.3/snrk2.6 mutant under water stress (NaCl or
and these phosphorylations are required for the ABA- ABAtreatment).Similarresultswerealsoobtainedwhenthe
dependentstomatalclosinginresponsetowaterdeficit.Very mutantwasanalyzedin termsofregulation ofgene expres-
recently two other reports concerning OST1, SLAC1 and sionduringseeddevelopmentanddormancy.Stress-related
stomatal closing were published. It was shown that OST1 genes were downregulated in this mutant in comparison to
phosphorylatesandactivatesSLAC1notonlyinresponseto the wild type, whereas those related to photosynthesis and
ABA, but also in response to elevated level of intercellular tetrapyrrole synthesis were upregulated (Nakashima et al.,
CO (Xueetal.2011)orozone(Vahisaluetal.,2010).More- 2009). It was concluded that ABA-dependent SnRK2s
2
over, OST1isresponsiblefor stomataclosingin responseto (SnRK2.2,SnRK2.3,andSnRK2.6)areessentialforthecontrol
pathogens(Melotto,2006).Theregulationofplantresponseto of seed development and dormancy through an extensive
water stress by SnRK2.6/OST1 is not restricted to the regu- controlofgeneexpression.
lationofstomatamovement.Alreadyin2002,Yoshidaetal. ABA-regulated genes have conserved ABA-responsive
presentedresultsindicatingthatthiskinaseisresponsiblefor elements (ABRE) in their promoters, usually in several cop-
thepositiveregulationofexpressionofseveralstressresponse ies. The transcription factors which bind to ABRE and trig-
genes,forexample,RD29BandRD22.Now,ourknowledge ger transcription of the ABA-regulated genes, known as
8 KULIKET AL.
ABRE-binding proteins (AREBs) or ABRE-binding factors as another bZIP protein (ABF1) in vitro (Mizoguchi et al.,
(ABFs), are regulated by phosphorylation. Furihata et al. 2010). These data indicate that EEL and maybe some other
(2006)showedthatArabidopsisABA-dependentSnRK2sfrom bZIPproteinscouldbecellularsubstratesofSnRK2.8.Johnson
group 3 (SnRK2.2, SnRK2.3, and SnRK2.6) phosphorylate et al. (2002) have identified TaABF, a wheat member of the
ABFs in vitro at sites crucial for their activity. That result ABFfamily,thatisspecificallyboundandphosphorylatedby
was confirmed by Fujii et al. (2007). Later on it was shown wheatkinasePKABA1(fromgroup2oftheSnRK2subfamily).
that these kinases play an essential role in phosphorylation ThedatasuggestedthatTaABFmayserveasaphysiological
ofABFs,includingABI5duringseedmaturationandgermi- substrate for PKABA1 in the ABA signal transduction path-
nation (Nakashima et al., 2009), and also in response to way during grain maturation, dormancy, and could lead to
environmental stress. There are several examples of phos- suppression of germination-associated gene expression and
phorylation of ABFs by SnRK2s also in plants other than germinationitself.
Arabidopsis(Johnsonetal.2002;Kobayashietal.,2005).Arice Recentanalyseshaveaddressedthequestionabouttherole
ABF, TRAB1, has been shown to be activated via ABA- ofgroup2and3SnRK2sfromwheat.cDNAsencodingwheat
dependent phosphorylation. SAPK8, SAPK9, and SAPK10 kinases were expressed in Arabidopsis and stress tolerance
(orthologs of SnRK2.2, SnRK2.3, and SnRK2.6) directly analysis of the obtained transgenic plants was performed.
phosphorylateTRAB1inresponsetoABA(Kobayashietal., ExpressionofTaSnRK2.8(fromgroup3)orTaSnRK2.4(from
2005). Not only genes with an ABRE in their promoters are group 2) in Arabidopsis enhanced the plant tolerance to
likely to be regulated by SnRK2s. For example, in both the drought, salinity, and low temperature (Mao et al., 2010;
snrk2.6andobviouslyinthesnrk2.2/snrk2.3/snrk2.6mutantan Zhang et al., 2010b). Expression of TaSnRK2.4 resulted in
impairmentofRD22expressionwasobserved(FujiiandZhu, delayed seedling establishment, longer primary roots, and
2009;Yoshidaetal.,2002).ExpressionofRD22iscontrolledby higher yield in normal growth condition. The authors also
MycandMybtranscriptionfactors;therefore,itcanbespec- suggest that TaSnRK2.4 and TaSnRK2.8 are involved in the
ulated that also the activity of these transcription factors is regulationofcarbonmetabolism,whichiscloselyconnected
modulatedbySnRK2phosphorylation.Moreinformationon withstressresponse.Independently,Xuetal.(2009)showed
the ABA-mediated transcription regulation in response to that transgenic Arabidopsis plants in which W55a (a wheat
osmoticstressinplants,includingregulationbySnRK2,can SnRK2 belonging to group 2) has been expressed exhibited
befoundinaveryrecentreviewbyFujitaetal.(2011). higher tolerance to drought. Even though recent studies in-
There are indications that SnRK2s from group 3 are in- dicatethatsomeSnRK2sbelongingtogroup2,forexample,
volvedintheregulationofplantmetabolism.Recently,Zheng Arabidopsis SnRK2.7 and SnRK2.8, are weakly activated by
etal.(2010)showedthatSnRK2.6participatesintheregula- ABA treatment and upregulate expression of some AREB/
tionofoilsynthesisinArabidopsisseeds,sucrosesynthesisand ABFtargetgenesinresponsetostress(drought,NaCl),they
fattyaciddesaturationinleaves. do not regulate gene expression in response to direct plant
Much less is known on the physiological role of SnRK2s treatmentwithABA;therefore,theirroleinABAsignalingis
fromgroup2(SnRK2.7andSnRK2.8inArabidopsis).Umeza- stillunknown.
waetal.(2004)showedthatSKR2C/SnRK2.8playsasignif- ThereareseveralindicationsthatSnRK2sfromgroup2are
icantroleindroughttolerance.Arabidopsisknockoutmutants involvedintheregulationofplantmetabolism.SnRK2.8isa
srk2c/snrk2.8werehypersensitivetodrought,whereasArabi- positive regulator of plant growth, most probably by phos-
dopsistransgenicplantsoverexpressingSRK2C/SnRK2.8were phorylationandregulationoftheactivityofseveralenzymes
significantlymoretoleranttodroughtthanwild-typeplants. involvedinbasicmetabolism(Shinetal.,2007).Thereisevi-
Thekinasefunctionsmainlyinroottipsandapparentlydoes dencethatexpressionofSnRK2.8isdownregulatedbyN,P,or
not affect stomatal movement, in contrast to SRK2E/ K deprivation, and this downregulation is accompanied by
SnRK2.6/OST1, because the water-loss level of the mutant poorplantgrowth,whereasoverexpressionofcDNAencod-
was similar to that of wild-type plants. On the other hand, ing this kinase results in improved growth of Arabidopsis
root elongation and lateral root number were significantly under normal and nutrient-limited conditions (Shin et al.,
lower in the srk2c/snrk2.8 mutant than in wild-type plants. 2007). Several target proteins have been identified as being
DNA microarray analysis of 35S::SRKC/SnRK2.8-GFP and phosphorylated by SnRK2.8. Certain targets, such as 14-3-3
controlplantsrevealedupregulationofmanystress-responsive proteins,regulateasyetunidentifiedproteins,whereasother
genes,forexample,RD29A,COR15A,DREB1Aintransgenic targets,suchasglyoxalaseIandribose5-phosphateisomer-
plants overexpressing SRKC/SnRK2.8, indicating that, most ase,detoxifybyproductsofglycolysisandcatalyzeoneofthe
probably,theregulationofwaterstresstolerancebySnRK2.8 final steps in carbon fixation, respectively. Also, adenosine
is due to the control of stress-responsive gene expression. kinaseanda60Sribosomalproteinwereconfirmedastargets
Recently, an analysis of the expression profiles of snrk2.7/ ofSnRK2.8.
snrk2.8 double mutant and wild-type seedlings exposed to Sofar,informationontheroleofABA-independentSnRK2
drought stress confirmed previous data indicating that familymembers(group1)isscant.Die´dhiouetal.(2008)have
kinases from group 2 of the SnRK2 subfamily play an im- revealed that overexpression of SAPK4 (a rice SnRK2 from
portantroleindroughtresponse(Mizoguchietal.,2010).The group 1) increases rice tolerance to salinity and oxidative
results indicated that SnRK2.7 and SnRK.8 upregulate the stress. The authors studied seed germination as well as de-
expressionofsomeAREB/ABFtargetgenes,albeittoanex- velopment of rice seedlings and growth of mature plants
tentlowerthan theSnRK2.2/2.3/2.6 kinases.Thoseauthors under salt stress. In plants overexpressing SAPK4 transcript
showedthatSRK2C/SnRK2.8interactswithanABA-related levelsofthevacuolarH+-ATPaseandcatalasewerehigherin
bZIPprotein,EEL.SRK2C/SnRK2.8andEELarecoexpressed comparisontowild-typeplants,whereasthoseencodingthe
inroottipsandSRK2C/SnRK2.8phosphorylatesEELaswell vacular Na+/H+ antiporter OsNHX1 and the Cl- channel
SnRK2—ROLE IN PLANT RESPONSETO ABIOTICSTRESSES 9
OsCLC1werelower.Mostlikelythatwaswhythetransgenic considerationthatinlowerplantsandmostprobablyalsoin
plants exhibited lower oxidative stress and accumulation of higher plants SnRK2s are involved in sulfur metabolism,
Na+ and Cl- ions and overall enhanced salt tolerance which is closely connected with regulation of heavy metal
(Die´dhiou et al., 2008). Zhang et al. (2011) reported that tolerance,onecanexpectthatSnRK2smightbeinvolvednot
TaSnRK2.7 encoding an ABA-independent kinase expressed only in the plant response to water stress and nutrient dep-
inArabidopsisenhancedplantdrought,cold,andsalinitytol- rivationbutalsointheresponsetoheavymetalstress.
erance. It was also shown that two soybean (Glycine max)
ABA-independent SnRK2s, SPK1, and SPK2, were able to
Conclusions
phosphorylate soybean phosphoinositide binding protein
Ssh1p,thatmayfunctionasacomponentofthehyperosmotic AllpresentedfindingsindicateakeyroleofSnRK2kinases
stress response pathway (Kearns et al., 1998; Monks et al., in the regulation of plant response to abiotic stresses and
2001). These examples suggest that ABA-independent nutrient limitation in an ABA-dependent and independent
SnRK2sactaspositiveregulatorsofplantresponsetoabiotic manner. Similarly to SnRK1, they link ABA and stress sig-
stresses.However,theymayhaveothercellularfunctionsas naling with plant metabolism. The ABA-dependent kinases
well.SeveralyearsagoitwasfoundthatMtSK1,ageneen- belongingtogroup3oftheSnRK2subfamilyaremajorreg-
codinganSnRK2kinasefromgroup1inMedicagotruncatula, ulatorsofplantresponsetoABA,bothinenvironmentalstress
was involved in the induction of somatic embryogenesis conditionsaswellasduringplantdevelopment,seedmatu-
(Nolanetal.,2006). ration,andgermination.TheyareacentralhubinABAsig-
Veryrecently,anextremelydifficultlargescaleexperiment naling.RegulationoftheresponsetoABAviaSnRK2soccurs
leadingtoestablishingtheroleofSnRK2s,includingtheABA- bydirectphosphorylationofvariouseffectorproteins,suchas
independentmembers,wasundertakenbyFujiietal.(2011). ionchannels(SLAC1,KAT1),andbyregulationofexpression
TheyconstructedandcharacterizedArabidopsisdecuplemu- ofstressresponsegenes,mostprobablybyphosphorylation
tantscarryingmutationsinall10SnRK2genes,mutantsde- andactivationofABFsandotherspecifictranscriptionfactors.
fective in SnRK2.1/4/5/7/8/9/10 and also quadruple EventhoughABA-dependentkinasesincomparisontoother
snrk2.1/4/5/10andquintuplesnrk2.1/4/5/9/10mutants.Firstof SnRK2s have been studied the most deeply still only a few
all,theresultssuggestthatallSnRK2sareimportantinplant cellularsubstratesofmembersofthisgroupwereidentified.It
response to osmotic stress. The features of those mutants iswellestablishedthatmostSnRK2sarelocalizedbothinthe
(expressionlevelofselectedstress-responsegenes,ABA,and nucleusandcytoplasm,butnothingisknownonthemecha-
proline production in response to osmotic stress and ABA nism of their nuclear–cytoplasmic translocation; their struc-
treatment) have revealed some differences between the mu- turelacksatypicalnuclearlocalizationsignals.
tants studied. The analysis of proline level, for example, in- SnRK2s of group 2 are also positive regulators of plant
dicatedthatABA-independentSnRK2s(SnRK2.1/4/5/9/10) responses to water deficit, most probably by regulation of
couldactasnegativeregulatorsofABA-inducedprolineac- stress response genes. However, this regulation is not as
cumulation. This feature is exactly opposite to that of the strong as in the case of the ABA-dependent SnRK2s from
ABA-dependentkinasesfromgroup3.However,becausethe group3.
data concerning functions of ABA-independent members of The least is known on the physiological role of ABA-
the SnRK2 subfamily is scarce, it is very difficult to make independent SnRK2s. We therefore envision that this group
any general statements in terms of their function; a lot of willbewidelystudiedinthenearestfeature.Sofar,thereare
work remains to be done to establish the role of the ABA- mostlyquestionmarksconcerningtheirrole,mechanism(s)of
independentSnRK2s. regulationoftheiractivityandlocalization(localizationonly
In algae, several SnRK2s involved in response to abiotic ofsomeofthemwasstudied).Veryintriguingisthefactthat
stress have been identified. Two Chlamydomonas reinhardtii group 1 kinases, whose activity is not dependent on ABA,
SnRK2kinases,SNRK2.1andSNRK2.2(previousnameSac3) have an influence on ABA-dependent proline accumulation
are involved in response to sulfur limitation (Davies et al., (Fujiietal.,2011),aswellasotherABA-dependentprocesses
1999; Gonzalez-Ballester et al., 2008). SNRK2.1 positively (ourunpublishedresults).Moreover,phosphoproteomicdata
regulates sulfur-starvation responsive genes, for example, obtainedlastyearbyKlineetal.(2010)showedastrongin-
SAC-like Transporter 1 (SLT1), SAC-like Transporter 2 (SLT2), crease of SnRK2.4 phosphorylation in the kinase activation
Sulfate Transporter 2 (SULTR2) needed for maintaining cell loopinresponsetoABAtreatment.TheincreaseofSnRK2.4
viabilityduringS-deprivation(Gonzalez-Ballesteretal.,2008; phosphorylation was similar to that observed for ABA-
Pootakham et al., 2010). SNRK2.2, on the other hand, is re- dependent kinases—SnRK2.2, SnRK2.3, and SnRK2.6. Why
sponsible for repression of S-inducible genes when cells are thenisthiskinasenotactivatedbyABA?Aretheresomeother
depleted of S (Davies et al., 1999; Gonzalez-Ballester et al., mechanisms, beside phosphorylation, controlling SnRK2s
2008).Itseemslikelythatalsoinhigherplantsatleastsome activity?WhatistheroleofPAandNOinSnRK2sfunction?
SnRK2s are involved in sulfur metabolism as well. Kimura WhichphosphatasesareresponsibleforinactivationofABA-
et al. (2006) showed that mRNA levels of four Arabidopsis independent SnRK2? Is SnRK2 activation dependent on
kinases, both ABA-dependent and ABA-independent, phosphorylationbyansofarunknownupstreamkinase(s)?In
SnRK2.1, SnRK2.3, SnRK2.4, and SnRK2.6, increased in re- which stress signaling pathways, beside osmotic stress, are
sponse to sulfur limitation. Additionally, it was shown that SnRK2sinvolved?Stillmuchworkneedstobedoneinorder
SnRK2.3 controls the production of O-acetyl-L-serine, a pu- to identify the SnRK2s’ cellular substrates and to establish
tativesignalingcompoundofthesulfurstarvationresponse, theconnections,orcrosstalk,ofSnRK2swithothersignaling
andexpressionofthesulfatetransporterSULTR2-2geneun- elementsandvarioushormonalsignalingpathways.Allthese
der limited sulfur supply (Kimura et al., 2006). Taking into issues should be addressed to learn more about the role of
10 KULIKET AL.
SnRK2inplantresponsetoabioticstresses.Droughtandsa- activated kinase is regulated by phosphorylation on Ser-154
linityseverelyaffectplantgrowthandproductivity.Inorder and Ser-158 in the Kinase activation loop. J Biol Chem 281,
to obtain plants with enhanced tolerance to environmental 34299–34311.
stresses the knowledge on plant stress signal transduction Canto´,C.,andAuwerx,J.(2010).AMP-activatedproteinkinase
pathways and plant defense against given stressors is and its downstream transcriptional pathways. Cell Mol Life
required. Therefore, studies on SnRK2s are of high priority, Sci67,3407–3423.
as they seem to be perfect candidates for improvement Carling,D.,Zammit,V.A.,andHardie,D.G.(1987).Acommon
bicycle protein kinase cascade inactivates the regulatory en-
of crop tolerance to environmental stresses and water use
zymes of fatty acid and cholesterol biosynthesis. FEBS Lett
efficiency.
223,217–222.
Celenza, J.L., and Carlson, M. (1986). A yeast gene that is es-
Acknowledgments
sential for release from glucose repression encodes a protein
FinancialsupportfromtheMinistryofScienceandHigher kinase.Science233,1175–1180.
Education (Grant 500/N-COST/2009/0 to GD), EU (COST Chae, M.J., Lee, J.S., Nam, M.H., Cho, K., Hong, J.Y., Yi, S.A.,
FA0605), and FNP (Foundation for Polish Science) (Grant et al. (2007). A rice dehydration-inducible SNF1-related pro-
tein kinase 2 phosphorylates an abscisic acid responsive
TEAM/2008-2/4) fellowship to I.W. is gratefully acknow-
element-binding factor and associates with ABA signaling.
ledged.
PlantMolBiol63,151–169.
Cheng, N.H., Pittman, J.K., Zhu, J.K., and Hirschi, K.D. (2004).
Author DisclosureStatement
The protein kinase SOS2 activates the Arabidopsis H(+)/
Nocompetingfinancialinterestsexist. Ca(2+) antiporter CAX1 to integrate calcium transport and
salttolerance.JBiolChem279,2922–2926.
Chikano,K.,Ogawa,M.,Ikeda,Y.,Koizumi,N.,Kusano,T.,and
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