Evidence that auxin promotes gibberellin A1 biosynthesis in pea

In shoots of the garden pea, the bioactive gibberellin (GA 1) is synthesised 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 that decapitation of the shoot (excision of the apical bud) dramatically reduces the conversion of [ 3H]GA 20 to [ 3H]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 by application of the auxin indole-3-acetic acid (IAA) to the 'stump' of decapitated 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 contrast to PsGA3ox1, PsGA2ox1 transcript levels were increased by decapitation and reduced by IAA application. Decapitation and IAA treatment did not markedly affect the level of GA 1 precursors. It is suggested that in intact pea plants, auxin from the apical bud moves into the elongating internodes where it (directly or indirectly) maintains PsGA3ox1 transcript levels and, consequently, GA 1 biosynthesis.