Gibberellin-Induced Suppression of Chloroplast Starch Formation from Exogenous Sucrose in Isolated Epidermis of Light-Grown Cucumber Hypocotyls

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The effects of gibberellin A_3(GA_3) on chloroplast starch formation from exogenous sucrose in epidermal strips of light-grown cucumber (Cucumis sativus L. cv. Aonagajibai) hypocotyls were studied. Chloroplast starch formation was measured by counting microscopically the number of I_2-KI stained chloroplasts. In the presence of sucrose (1-50 mM) and in light, chloroplast starch formation occurred rapidly for 5 hr and then the rate declined. GA_3 (100μM) simultaneously supplied with sucrose clearly suppressed both the rate and the degree of starch formation. When the epidermis was preincubated in the dark with sucrose in the presence or absence of GA_3 and then post-incubated in light with buffer only in the presence or absence of GA_3, starch formation was suppressed to the same degree by GA_3 given either in the pre- or post-incubation period. When the epidermis was preincubated in the dark with GA_3 alone and then transferred to light for post-incubation with sucrose alone, GA_3 suppressed the initial rate of starch formation during the post-incubation period. The degradation of chloroplast starch formed in advance from exogenous sucrose in light was not significantly affected by GA_3. These results are discussed in relation to the mechanism of the GA_3 action and also to the GA_3-induced decrease in the osmotic potential of the cell.
日本植物生理学会の論文