Further evidence for feedback regulation of gibberellin biosynthesis in pea

In shoots of the garden pea (Pisum sativum L.), the main bioactive gibberellin (GA) is GA 1, which is synthesised from GA 20 by 3β-hydroxylation. Gibberellin A 20 is produced from GA 19, as part of the process known as GA 20-oxidation. Because these steps are thought to be negatively regulated by GA 1, we compared the metabolism of labelled GA 19 and GA 20 in mutants deficient in GA 1, with that observed in isogenic wild-type (WT) plants. There was a large and specific increase in the 3β-hydroxylation of labelled GA 20 in the GA 1-deficient (dwarf) mutants, compared with the WT. Metabolism experiments did not provide convincing evidence for feedback regulation of 20-oxidation, possibly because GA 19 appears to be metabolised rapidly, even in WT pea shoots. Both 3β-hydroxylase and 20-oxidase transcript levels were markedly higher in the mutants than in isogenic WT lines. The results support previous suggestions that both biosynthetic steps are feedback-regulated by GA 1 in pea.