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 e t 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 elongatio n 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 le vels of GA,, built up after application of 16,17-dihydro GAS. Conseque ntly, growth retardation by 16,17-dihydro GA(5) and LAB 198 999 is lik ely to be the result of their inhibition of GA(20) 3 beta-hydroxylatio n to GA(1). Another fate for GA(20) in Lolium is its C-2 beta-hydroxyl ation 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 in volving 2 beta-hydroxylation of GA(1) to GA(8) appeared to be insensit ive to either growth retardant. When [H-3]GA(20) was injected into the cavity within the young intact sheathing leaves, there was an appreci able metabolism of this GA(20) to GA(1) and thence to GA(8) (ca 10% an d 30% respectively within 5 h). For excised shoot tips, however, [3H]G A(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 be ta-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, t his conversion was not inhibited by GAS. These differences in structur al specificity for GAs which inhibit 2 beta-hydroxylation as apposed t o 3 beta-hydroxylation are in accordance with these two Ring-A hydroxy lation steps being catalysed by different enzymes. Finally, the differ ences in GA(20) metabolism between intact versus excised tissue raise the possibility that tissue wounding with excision enhanced the activi ty of the GA(20) 2 beta-hydroxylase(s).