The molecular mechanism of injury and tolerance related to acid stress in higher plant

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Responses of plant to aluminum(Al) stress in acidic condition were investigated.1) When barley roots were treated for Al stress, H^+-ATPase of plasma membrane decreased. The amount of Ca^<2+> associated with the membrane of the Al-treated sample was approximately half as large as that of the control, while substantial binding of Al to the membrane was observed in the membrane of Al-treated roots.2) The activities of ATP- and PPi-dependent H^+ -pump of tonoplast increased by Al stress. During this process, the increase of ABA and the amount of protein of both pumps were observed.3) With wheat varieties differing in Al tolerance, the efflux of pre-loaded K^+ was investigated. Lowring the pH in duced increasing K^+ efflux in both cultivars, and the rate of K^+ efflux of sensitive cultivar was twice as much as that of tolerant cultivar at pH 4.2. Extracellular low pH may cause an increase in cytoplasmic H^+ concentration followed by the depolarization of the plasma membrane, which may be adjusted by the efflux of K^+ and H^+. It seemed that the activities of H^+ -pump and K^+ channel decreased more in Al sensitive cultivar by Al stress.4) The tobacco cells pre-cultured in a medium deprivated of phosphate exhibited 3 to 7 fold higher growth than the control cells under Al stress. Absorption of Al followed by the decrease of cell growth was only observed in the cells of logarithmic phase.The capability of the cells excluding of absorbed Al increased by the treatment of phosphate starvation.5) During the treatment of cells for phosphate starvation followed by Al treatment, the synthesis of two polypeptides of 45KD and 21KD increased. The gene(s) related to the expression of these peptides were tried to find. cDNA library was constracted and positive clone(s) for the induction of Al tolerance were isolated with differential hybridization technique with proper probes. Several positive clones were isolated and further identification is now under the progress.