Feedback regulation of GA5 expression and metabolic engineering of gibberellin levels in arabidopsis

The gibberellin (GA) 20-oxidase encoded by the GA5 gene of Arabidopsis dire cts GA biosynthesis to active GAs, whereas that encoded by the P16 gene of pumpkin endosperm leads to biosynthesis of inactive GAs, Negative feedback regulation of GA5 expression was demonstrated in stems of Arabidopsis by bi oactive GAs but not by inactive GA. In transgenic Arabidopsis plants overex pressing P16, there was a severe reduction in the amounts of C-20-GA interm ediates, accumulation of large amounts of inactive GA(25) and GA(17), a red uction in GA(4) content, and a small increase in GA(1). However, due to fee dback regulation, expression of GA5 and GA4, the gene coding for the subseq uent 3 beta-hydroxylase, was greatly increased to compensate for the effect s of the P16 transgene, Consequently, stem height was only slightly reduced in the transgenic plants.