＜正＞ Seedlings of three species of Malus were used to study the expression of mitogen-activatedprotein kinase （MAPK） in response to water stress:Malus hupehensis,a drought-sensitive species;Malussieversii,a drought-tolerant species;and Malus micromalus,a middle type.Results showed that MalusMAPK （MaMAPK,GenBank accession No.AF435805） was expressed in both roots and leaves of seedlingsof the three Malus species treated with 20% polyethylene glycol for different time periods.Expression levelspeaked at 1.5 h after treatment with polyethylene glycol,then decreased to their lowest levels.Liquid kinaseassays indicated that the dynamic changes of MAPK activity were very similar to those of the relativeexpression of MaMAPK mRNA.However,the peak of the former occurred slightly behind the latter.It wasnoticed that,although the kinase activity decreased after the peak,it was still higher than that of the controlduring the whole time period.These results suggested that MaMAPK was regulated not only by water stressat the transcription level,but also by phosphorylation and dephosphorylation at the protein level.In addition,of these three apple species,the highest MAPK activity and MaMAPK expression level was found in M.sieversii,followed by M.micromalus and M.hupehensis,suggesting that MAPK might be correlated withdrought tolerance in these three species.The different expression levels might be one of the molecularmechanisms of the different drought tolerances in Malus.
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