The Relationship between Levels of Endogenous Gibberellins and the Response of Vigna Hypocotyls to Exogenous Indole-3-Acetic Acid : GROWTH AND DEVELOPMENT

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Segments excised from the upper and the lower parts of cowpea (Vigna unguiculata L.) hypocotyls were compared in terms of their responses to exogenous indole-3-acetic acid (IAA) in relation to their endogenous levels of gibberellin. Growth of the segments was measured continuously during xylem perfusion with a linear differential transformer. IAA induced a burst of elongation in the upper segments but only slight promotion of growth in the lower segments. Treatment with uniconazole, a potent inhibitor of the biosynthesis of gibberellins, reduced the responsiveness of the upper segments to exogenous IAA to about one half of the control value. Pre-perfusion with GA_3 of such segments for two hours prior to application of IAA, partially restored the responsiveness to IAA. Analysis by GC/MS identified GA_1, GA_4, GA_9, GA_<20> and GA_<51> as native gibberellins in the hypocotyls of cowpea seedlings. Analysis by GC/SIM also showed that the physiologically active gibberellins (GA_1 and GA_4) were located mainly in the upper part of the hypocotyl and the treatment with uniconazole marketly reduced the endogenous level of gibberellins there to less than 11% of the control level. These results suggest that levels of endogenous gibberellins possibly control the response to IAA in these segments.
日本植物生理学会の論文