Growth and Gibberellin A1 Metabolism in Excised Lettuce Hypocotyls 1

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Excised lettuce (Lactuca sativa L. cv. Arctic) hypocotyls retain the ability to elongate in response to exogenously supplied gibberellic acid and gibberellin A1 (GA1). We have studied the relationship between metabolism of GA1 and elongation in this tissue. In 24 hours at 28 C, hypocotyls treated with 3 μm GA1 double in length while controls elongate less than 45%. After an exogenous hormone supply is removed, hypocotyls continue to grow faster than untreated controls, although as the hormone application time is decreased, the GA1 concentration required to effect a given length change increases. [3H]GA1 was used to determine rates of hormone uptake, efflux, and metabolism. In the presence of [3H]GA1, hypocotyls accumulate and metabolize lable for at least 24 hours. When the exogenous label is removed, the amount of acidic GA in the hypocotyl declines rapidly to a constant level while ethyl acetate-insoluble metabolites increase rapidly to a constant level. Lable accumulation and metabolism at any time are proportional to the external GA1 concentration below 50 μm GA1. Chromatographic analysis of radioactive compounds present in tissue extracts suggests that unaltered GA1 is the major component of the acidic ethyl acetate-soluble fraction, and gibberellin A8 is a minor component. The ethyl acetate-insoluble fraction appears to contain an unidentified GA1 metabolite with chromatographic properties similar to those of GA1. The strong retention of accumulated GA1 confirms the possibility of a continuing requirement for GA1 during the sustained response to a GA1 “pulse” but raises the question of accessibility of the stored hormone for growth promotion.

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