Small RNAs as big players in plant abiotic stress responses and nutrient

Abiotic stress is one of the primary causes of crop lossesworldwide. Much progress has been made in unravelingthecomplexstressresponsemechanisms,particularlyinthe identification of stress responsive protein-codinggenes. In addition to protein coding genes, recentlydiscovered microRNAs (miRNAs) and endogenous smallinterfering RNAs (siRNAs) have emerged as importantplayersinplantstressresponses.Initialcluessuggestingthat small RNAs are involved in plant stress responsesstem from studies showing stress regulation of miRNAsand endogenous siRNAs, as well as from target predic-tions for some miRNAs. Subsequent studies havedemonstrated an important functional role for thesesmall RNAs in abiotic stress responses. This reviewfocuses on recent advances, with emphasis on integ-ration of small RNAs in stress regulatory networks.Small RNAs are ubiquitous regulators of geneexpressionPlant development, metabolism and stress responses, aswell as a myriad of other functions, depend on the correctregulation of gene expression. This is achieved by multiplemechanisms, with perhaps the most important controlbeing exerted at the level of transcription. However,post-transcriptional events also play a crucial role inregulating gene expression. The stability of mRNAs isregulatedbyavarietyofsignalsactingonspecificsequenceswithin the RNAs. This regulation is often mediated byspecificRNA-bindingproteins(RBPs)thatbindtoelementsintheuntranslatedregions(UTRs)ofmRNAsandregulatethe stability, translation or localization of the mRNA [1–3].TherecentdiscoveryofmicroRNAs(miRNAs,seeGlossary)and small interfering RNAs (siRNAs) revealed anotherubiquitous mode of post-transcriptional regulation. Thesesmall RNAs are known to silence genes post-transcription-ally by guidingtarget mRNAs for degradation or by repres-sing translation [4–8]. The role of miRNAs in controllingdevelopmental processes has been at the forefront of plantmiRNAresearch.Thisisbasedinpartonthefactthatmanyproteins (dcl1, hen1, hyl, se and hst) required for miRNAgeneration and miRNA target genes (phb and ago1)werefirst identified through genetic screens for developmentaldefects[9–15].AcomprehensiveexaminationofmiRNAsinplant development is provided by several excellent recentreviews [5–8,16–18]. In this review, the emerging roles ofmiRNAs and endogenous siRNAs in plant stress responsesare discussed.Overview of the role of miRNAs in plant stressresponsesPlants are sessile organisms that must endure stressfulenvironments. A large proportion of plant genes areregulated by stresses such as drought, soil salinity andextremetemperatures[19–23].Ofthemanygeneregulatorymechanisms such as transcriptional, post-transcriptionaland post-translational regulation, transcriptional regula-tion is the most widely studied mechanism. The actionof specific transcription factors that bind to conservedcis-actingpromoterelementsiswelldocumentedasacauseofchangesingeneexpression,particularlythoseinducedbyabiotic stress [20]. Furthermore, post-transcriptional generegulation under stress conditions has been documentedbefore,althoughtheunderlyingmechanismwasnotknown[24–26]. Considering the important roles of small RNAs inguiding post-transcriptional gene silencing, their involve-ment in stress-regulated gene expression seemed likely[27,28]. The discovery that stress can regulate miRNAlevels, coupled with the identification of stress-associatedgenes as miRNA targets provided clues about the role ofmiRNAs in stress responses. Functional analyses havedemonstrated that several plant miRNAs play vital rolesin plant resistance to abiotic as well as biotic stresses [29–34]. Understanding small RNA-guided stress regulatorynetworks should provide new tools for the genetic improve-mentofplantstresstolerance(Figure1).Indeed,ithasbeenshown recently that manipulation of miRNA-guided generegulation can help to engineer plants that will be morestress-resistant [34].miRNA and oxidative stressUnder normal conditions, plants maintain a delicatebalance between reactive oxygen species (ROS) productionReview