Gibberellin metabolism in excised lettuce hypocotyls: Response to GA9 and the conversion of [3H]GA9

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Abstract
Elongation growth and gibberellin (GA9) metabolism in excised hypocotyls of lettuce (Lactuca sativa L. cv. Arctic) were investigated. Exogenously supplied GA9 stimulates elongation of hypocotyl sections and this response is intermediate between that elicited by GA1 or GA20 and GA4/7 mixture. Although uptake of radioactivity from [3H]GA9 increases with time, this gibberellin does not accumulate in the tissue but is rapidly converted to a compound with HPLC properties resembling those of [3H]GA20. After 2 h incubation in [3H]GA9, the presumptive GA20 represents 90% of the acidic ethyl acetate-soluble radioactivity in the tissue. Radioactivity is also associated with an acidic butanol-soluble fraction containing two components resolvable by HVE. The major component is similar in electrophoretic properties to a GA-glucosyl ether while the other compares to a GA-glucosyl ester. Conversion of [3H]GA9 to its [3H]GA20-like metabolite is reduced by addition of carrier GA9 or GA4/7 at concentrations as low as 1 μM, while GA1, GA3 and L-proline are without effect. Formation of the GA20-like compound can be blocked by the addition of 2,2′-dipyridyl, and this inhibitory effect of dipyridyl can be reversed by addition of Fe2+. At 200 μM dipyridyl, elongation growth as well as [3H]GA9 metabolism are reduced by 80%. The relationship of the metabolism of GA9 to the growth response is discussed.