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Differential Compartmentation of Gibberellin A₁ and Its Metabolites in Vacuoles of Cowpea and Barley Leaves ¹

John B. Ohlrogge

Lawrence Rappaport

Instituto de Agroquimica y Tecnologia de Alimentos (Consejo Superior de Investigaciones Cientificas), Jaime Roig, 11, Valencia-10, Spain, Northern Regional Research Center, Agricultural Research, Science and Education Administration, United States Department of Agriculture, Peoria, Illinois 61604, Department of Vegetable Crops/Plant Growth Laboratory, University of California, Davis, California 95616

The metabolism and efflux of gibberellin A₁ (GA₁) taken up by leaves of cowpea (Vigna sinensis cv. Blackeye pea No. 5), as well as the distribution of GA₁ metabolites in the protoplasts and vacuoles of cowpea and barley (Hordeum vulgare L. cv. Numar), were studied.

GA₁ is metabolized rapidly in cowpea leaf discs to products tentatively identified as gibberellin A₈ (GA₈) and gibberellin A₈ glucoside (GA₈-glu). After labeling leaf discs with [³H]GA₁ for 1 hour, the release of radioactivity from the leaf was followed. Over a 12-hour period, the level of radioisotope in the tissue declined to about 35% of the original, after which no further release was observed. At this time, almost all of the radioactivity remaining in the leaf was GA₈-glu, while most of the radioactivity which had been released was unmetabolized GA₁.

Mesophyll protoplasts and vacuoles were isolated from cowpea and barley leaves previously fed [³H]GA₁. These protoplasts retain the ability to metabolize GA₁, indicating that neither the leaf structure nor the cell wall is necessary for this metabolism. A higher proportion of GA₈-glu was found in the vacuoles relative to the entire protoplasts. The results obtained suggest that GA₁ metabolites are preferentially compartmentalized in the vacuoles relative to GA₁.

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