Seventy-fivepreviouslyknownplantmicroRNAs(miRNAs)wereclassifiedinto14familiesaccordingtotheirgenesequenceidentity.Atotalof18,694plantexpressedsequencetags(EST)werefoundintheGenBankESTdatabasesbycomparingallpreviouslyknownArabidopsismiRNAstoGenBank'splantESTdatabaseswithBLASTalgorithms.AfterremovingtheESTsequenceswithhighnumbers(morethan2)ofmismatchednucleotides,atotalof812ESTcontigswereidentified.AfterpredictingandscoringtheRNAsecondarystructureofthe812ESTsequencesusingmFoldsoftware,338newpotentialmiRNAswereidentifiedin60plantspecies,miRNAsarewidespread.SomemicroRNAsmayhighlyconserveintheplantkingdom,andtheymayhavethesameancestorinveryearlyevolution.ThereisnonucleotidesubstitutioninmostmiRNAsamongmanyplantspecies.SomeofthenewidentifiedpotentialmiRNAsmaybeinducedandregulatedbyenvironmentalbioticandabioticstresses.Somemaybepreferentiallyexpressedinspecifictissues,andareregulatedbydevelopmentalswitching.ThesefindingssuggestthatESTanalysisisagoodalternativestrategyforidentifyingnewmiRNAcandidates,theirtargets,andothergenes.AlargenumberofmiRNAsexistindifferentplantspeciesandplayimportantrolesinplantdevelopmentalswitchingandplantresponsestoenvironmentalabioticandbioticstressesaswellassignaltransduction.EnvironmentalstressesanddevelopmentalswitchingmaybethesignalsforsynthesisandregulationofmiRNAsinplants.AmodelformiRNAinductionandexpression,andgeneregulationbymiRNAishypothesized.
