Arabidopsis PKS5 (protein kinase SOS2-like 5),a protein kinase,involves in the pathways responded to the tolerance of salt-alkali stress.To dissect the functions of distinct domains of PKS5 under the stress of external salt and alkali,the characteristics of various PKS5 were determined using the PKS5 point mutants.The results show that:(1)The distinct PKS5 point mutants display the distinguishable phenotypes of growth under the combined salt and alkali stress.In all the PKS5 mutants measured in this study,the pks5-4,pks5-5 and pks5-9 display no differenc of growth of the primary root compared with the wild type (WT) under the salt-alkali stress but the pks5-2,pks5-6,pks5-7 and pks5-8 exhibit the salt-alkali phenotype under the same conditions of salt-alkali stress.The pks5-2 and pks5-8 display the tolerant phenotype of growth of primary root and the pks5-6 and pks5-7 illustrate the sensitive phenotype of growth of primary root under the salt-alkali stress compared with the WT.(2)The RT-PCR analysis of the PKS5 and the point mutated versions demonstrates the point mutations in the PKS5 lead to the alterations of the PKS5 expression.The expressions of the PKS5-2,PKS5-6 and PKS5-7 decrease compared with the WT.(3)The sub-location of PKS5 point mutation versions exhibit no differences compared with the WT.Both the WT and the PKS5 point mutation proteins distribute ubiquitously in the nucleus,cytoplasm and membrane of cells in wild plants.(4)The contents of Na⁺ present the significant differences between the PKS5 point mutants and the WT.The pks5-2 accumulates the less contents of Na⁺ and the pks5-6 and pks5-7 possess the more Na⁺ ions compared with the WT.Taken together,the results in this study show the different point mutations in PKS5 cause the various responses of plants to salt-alkali stress and suggest the diverse domains of PKS5 confer to the distinct functions under the stress of salt and alkali in Arabidopsis.