Further evidence for feedback regulation of gibberellin biosynthesis in pea

In shoots of the garden pea (Pisum sativum L,), the main bioactive gibberel lin (GA) is GA,, which is synthesised from GA(20) by 3 beta-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 muta nts deficient in GA(1) with that observed in isogenic wild-type (WT) plants , There was a large and specific increase in the 3 beta-hydroxylation of la belled GA(20) in the GA(1)-deficient (dwarf) mutants, compared with the WT, Metabolism experiments did not provide convincing evidence for feedback re gulation of 20-oxidation, possibly because GA(19) appears to be metabolised rapidly, even in WT pea shoots. Both 3 beta-hyrdroxylase and 20-oxidase tr anscript levels were markedly higher in the mutants than in isogenic WT lin es. The results support previous suggestions that both biosynthetic steps a re feedback-regulated by GA(1) in pea.