Effects of Exogenous Proline on Proline Metabolism of Cucumis melo under Salt Stress

The melon(Cucumis melo L.)variety ‘Xuemei'was nutrient solution cultured to investigate the effect of exogenous proline on proline metabolism under salt stress.The content of proline,△1-pyrroline-5-carboxylate synthase(P5CS)enzyme activity,ornithine-oxo-acid transaminase(OAT)enzyme activity,proline dehydrogenase(ProDH)enzyme activity in leaves were measured under salt stress,addition of exogenous proline under salt stress and the controlrespectively,cloning and semi-quantitative expression analysis of OATand ProDH gene were also undertaken.The results showed that compared with the control,the content of proline in the leaves of the seedlings increased significantly under salt stress.The increase of P5 CS enzyme activity was larger than that of OAT enzyme,the amount of OATgene expression did not increase in the majority of time period.ProDH enzyme activity decreased and the amount of ProDH gene expression also decreased in the leaves of melon seedling under salt stress.With the addition of exogenousproline under salt stress.The content of proline further increased,the activities of OAT and ProDH enzyme improved and activity of P5 CS enzyme decreased,the amount of OATexpression increased rapidly,the amount of ProDHexpression increased at first and then fell.This result suggested that proline accumulationin the melon seedlings was mainly through the increase of glutamate pathway and inhibition of proline degradation under salt stress.The adequate exogenous proline under salt stress could enhance the ornithine pathway but had a certain inhibitory effect on the glutamate pathway,and proline was further accumulated to enhance salt tolerance of melon seedling by regulating the synthesis and degradation,two kinds of metabolism ways.