FEEDBACK-REGULATION OF GIBBERELLIN BIOSYNTHESIS AND GENE-EXPRESSION IN PISUM-SATIVUM L

Treatment of tall and dwarf (3 beta-hydroxylase impaired) genotypes of pea (Pisum sativum L.) with the synthetic, highly active gibberellin (GA), 2,2-dimethyl GA(4), reduced the shoot contents of C-19-GAs, incl uding GA(1), and increased the concentration of the C-20-GA, GA(19). I n shoots of the slender (la cry(s)) mutant, the content of C-19-GAs wa s lower and GA(19) content was higher than in those of the tall line. Metabolism of GA(19) and GA(20) in leaves of a severe (na) GA-deficien t dwarf mutant was reduced by GA treatment. The results suggest feedba ck regulation of the 20-oxidation and 3 beta-hydroxylation reactions. Feed-back regulation of GA 20-oxidation was studied further using a cl oned GA 20-oxidase cDNA from pea. The cDNA, Ps074, was isolated using polymerase chain reaction with degenerate oligonucleotide primers base d on pumpkin and Arabidopsis 20-oxidase sequences. After expression of this cDNA clone in Escherichia coli, the product oxidized GA(12) to G A(15), GA(24) and the C-19-GA, GA(9), which was the major product. The 13-hydroxylated substrate GA(53) was similarly oxidized, but less eff ectively than GA(12), giving mainly GA(44) with low yields of GA(19) a nd GA(20). Ps074 hybridized to polyadenylated RNA from expanding shoot s of pea. Amounts of this transcript were less in the slender genotype than in the tall line and were reduced in GA-deficient genotypes by t reatment with GA(3), suggesting that there is feed-back regulation of GA 20-oxidase gene expression.