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,, 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 mutants 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 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 3 beta-hyrdroxylase 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.