Fuctions Of Conjugated Polyamines In The Embryo Cell Of Developing Wheat Grain Under Drought Stress

Wheat(Triticum aestivum L.) is an important food crop in our state. However, in North China, the wheat output is severely affected due to drought stress(including soil drought and dry hot wind atmospheric drought) during the grain filling stage after pollination.Polyamine, which belongs to short-chain aliphatic amines with low molecular weight, is one of secondary metabolites in organism. It is increasingly proved that polyamines play important roles in many aspects of growth and development of plants under stress environment. Especially in the stage of grain developing, there is active polyamine metabolism in embryos. However, the study of the relationship between polyamines and the tolerance of developing wheat grain to drought stress is still lacking and the functions of conjugated polyamines in the embryo cell of developing wheat grain under drought stress have not yet been documented. Therefore, two wheat cultivars with markedly different drought stress-tolerance(Luomai No.22 cv., drought-tolerant; Yumai No.48 cv.,drought-sensitive) were used as research materials and the developing grain embryos were sampled under rhizosphere drought stress during the grain filling stage after pollination.The changes in three main polyamines in embryo organ were previously detected, and on the basis of it, the functions of covalently and non-covalently conjugated polyamines in plasma membrane, tonoplast and mitochondrial membrane purified from the embryos were studied. Furthermore, the effects of polyamines on the conformations of plasma membrane,vacuole, mitochondrion and cortical microtubules were also preliminarily explored. The results are as follow:In the earlier stage of drought stress, the levels of free polyamines(f PAs), especially free putrescine(f Put), increased markedly in the embryos of developing grains. In the later stage of drought stress, the level of f Put in Luomai No. 22(drought-tolerant) decreased sharply and the levels of free spermidine(f Spd), free spermine(f Spm) and acid insoluble covalently conjugated Put(AISCC-Put) continuously increased markedly, while the levelof f Put in Yumai No.48(drought-sensitive) remained high and the levels of f Spd, f Spm and AISCC-Put increased slightly. These results suggested that tolerance of developing wheat grain to drought stress is attributed not only to the great increase of f Put in embryos in the earlier stage of stress, but also to the easily conversion of f Put to f Spd, f Spm and AISCC-Put. This notion has also been verified by the results of the researches with exogenous polyamines and polyamine biosynthesis inhibitors. Acid soluble covalently conjugated PAs(ASCC-PAs) are little related to the drought resistance of developing grain embryos.In the stage of grain filling and ripening, the tolerance of the developing wheat grain embryos to drought stress is promoted by the high activity of embryo cell plasma membrane(PM) H+-ATPase, which is related to the high levels of non-covalently conjugated(NCC) Spd, NCC Spm, covalently conjugated(CC) Put and CC Spd in embryo PM vesicles. In the stage of grain filling, increased levels of NCC Spd and CC Put in tonoplast vesicles is of positive significance and correlate with maintenance of the H+-ATPase and H+-PPase activities, and enhance drought stress tolerance; the increased levels of NCC Spd and CC Put in mitochondrion inner membrane could improve the drought stress tolerance via maintenance of the hydrolysis activity of H+-ATPase and –SH group content.The conformations of PM, mitochondrion and vacuole of embryo cells transformed abnormally under drought stress, and the abnormal transformation might be prevented by NCC Spd and CC Put in the membranes mentioned above.Cell cortical microtubules of developing wheat grain embryos participated in the response of grains to drought stress, and Spd played an important role in stabilizing conformation of cell cortical microtubules of the embryos under drought stress.In summary, in the earlier stage of drought stress, the level of f Put rapidly increases greatly, and in the later stage, the increased f Put is transformed to f Spd, f Spm and AISCC-Put. Then, the f Spd and f Spm are further converted to NCC Spd and NCC Spm via being non-covalently conjugated to bio-membrane, and the f Put and f Spd are further converted to CC Put and CC Spd via being covalently conjugated to the related proteins.These conjugated PAs play important roles in enhancing / maintaining the activities of the main proteins in bio-membrane and stabilizing conformation of cell, vacuole and mitochondrion. The increased f Spd in the later stage facilitates the conformation stabilization of cell cortical microtubules of the embryos under drought stress.