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Plant and Cell Physiology, 1995, Vol. 36, No. 1 165-171
© 1995

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

Akane Okamoto1, Masayuki Katsumi2, Hideaki Nojiri3, Noboru Murofushi3 and Hisashi Okamoto1

1 Graduate School of Integrated Science, Yokohama City University Yokohama, 236 Japan
2 Department of Biology, International Christian University Tokyo, 118 Japan
3 Department of Applied Biological Chemistry, The University of Tokyo Tokyo 113 Japan

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 GA3 of such segments for two hours prior to application of IAA, partially restored the responsiveness to IAA. Analysis by GC/MS identified GA1, GA4, GA9 GA20 and GA51 as native gibberellins in the hypocotyls of cowpea seedlings. Analysis by GC/SIM also showed that the physiologically active gibberellins (GA1 and GA4) 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.

(Received May 12, 1994; Accepted November 15, 1994)
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