Regulation in Lolium temulentum of the metabolism of gibberellin A(20) and gibberellin A(1) by 16,17-dihydro GA(5) and by the growth retardant, LAB 198 999

The ring D-modified gibberellin [GA], 16,17-dihydro GA(5), can retard stem growth in Lolium temulentum L, while promoting flowering (Evans et at, 1994, Planta 193, 107-114). Using [1,2,3-H-3]GA(20) to study the final biosynthetic step to GA(1) (a known effector of shoot elongation in higher plants), it was shown that C-SP-hydroxylation of GA(20) to GA(1) is blocked by 16,17-dihydro GAS but is little affected by GA(5). Another late-stage biosynthetic inhibitor, the acylcyclohexanedione, LAB 198 999, also blocked GA(1) formation. Furthermore, endogenous levels of GA,, built up after application of 16,17-dihydro GAS. Consequently, growth retardation by 16,17-dihydro GA(5) and LAB 198 999 is likely to be the result of their inhibition of GA(20) 3 beta-hydroxylation to GA(1). Another fate for GA(20) in Lolium is its C-2 beta-hydroxylation to growth-inactive GA(29). This conversion was also inhibited by 16,17-dihydro GA(5) but less so by LAB 198 999. The analogous step involving 2 beta-hydroxylation of GA(1) to GA(8) appeared to be insensitive to either growth retardant. When [H-3]GA(20) was injected into the cavity within the young intact sheathing leaves, there was an appreciable metabolism of this GA(20) to GA(1) and thence to GA(8) (ca 10% and 30% respectively within 5 h). For excised shoot tips, however, [3H]GA(20) was converted rapidly and virtually completely to GA, in 3-5 h. Interestingly, with these excised shoot tips, GA(3) and GA(5) as well as 16,17-dihydro GAS when applied via the agar strongly inhibited 2 beta-hydroxylation of GA(20) to GA(29) In contrast, while 16,17-dihydro GAS blocked GA(20) metabolism to GA(29) in intact sheat/stem tissue, this conversion was not inhibited by GAS. These differences in structural specificity for GAs which inhibit 2 beta-hydroxylation as apposed to 3 beta-hydroxylation are in accordance with these two Ring-A hydroxylation steps being catalysed by different enzymes. Finally, the differences in GA(20) metabolism between intact versus excised tissue raise the possibility that tissue wounding with excision enhanced the activity of the GA(20) 2 beta-hydroxylase(s).