Evidence that auxin promotes gibberellin A(1) biosynthesis in pea

In shoots of the garden pea, the bioactive gibberellin (GA(1)) is synthesis ed from GA(20), and the enzyme which catalyses this step (a GA 3-oxidase - PsGA3ox1) is encoded by Mendel's LE gene. It has been reported previously t hat decapitation of the shoot (excision of the apical bud) dramatically red uces the conversion of [H-3]GA(20) to [H-3]GA(1) in stems, and here we show that endogenous GA(1) and PsGA3ox1 transcript levels are similarly reduced . We show also that these effects of decapitation are completely reversed b y application of the auxin indole-3-acetic acid (IAA) to the 'stump' of dec apitated plants. Gibberellin A(20) is also converted to an inactive product , GA(29), and this step is catalysed by a GA 2-oxidase, PsGA2ox1. In contra st to PsGA3ox1, PsGA2ox1 transcript levels were increased by decapitation a nd reduced by IAA application. Decapitation and IAA treatment did not marke dly affect the level of GA(1) precursors. It is suggested that in intact pe a plants, auxin from the apical bud moves into the elongating internodes wh ere it (directly or indirectly) maintains PsGA3ox1 transcript levels and, c onsequently, GA(1) biosynthesis.