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 GA5, can retard stem growth in Lolium temulentum L. while promoting flowering (Evans et al ., 1994, Planta 193, 107–114). Using [1,2,3-3 H]GA20 to study the final biosynthetic step to GA1 (a known effector of shoot elongation in higher plants), it was shown that C-3b-hydroxylation of GA20 to GA1 is blocked by 16,17-dihydro GA5 but is little affected by GA5. Another late-stage biosynthetic inhibitor, the acylcyclohexanedione, LAB 198 999, also blocked GA1 formation. Furthermore, endogenous levels of GA20 built up after application of 16,17-dihydro GA5. Consequently, growth retardation by 16,17-dihydro GA5 and LAB 198 999 is likely to be the result of their inhibition of GA20 3b-hydroxylation to GA1. Another fate for GA20 in Lolium is its C-2b-hydroxylation to growth-inactive GA29. This conversion was also inhibited by 16,17-dihydro GA5 but less so by LAB 198 999. The analogous step involving 2b-hydroxylation of GA1 to GA8 appeared to be insensitive to either growth retardant.
When [3H]GA20 was injected into the cavity within the young intact sheathing leaves, there was an appreciable metabolism of this GA20 to GA1 and thence to GA8 ( ca 10% and 30% respectively within 5 h). For excised shoot tips, however, [3H]GA20 was converted rapidly and virtually completely to GA29 in 3–5 h. Interestingly, with these excised shoot tips, GA3 and GA5 as well as 16,17-dihydro GA5 when applied via the agar strongly inhibited 2b-hydroxylation of GA20 to GA29. In contrast, while 16,17-dihydro GA5 blocked GA20 metabolism to GA29 in intact sheath/stem tissue, this conversion was not inhibited by GA5. These differences in structural specificity for GAs which inhibit 2b-hydroxylation as opposed to 3b-hydroxylation are in accordance with these two Ring-A hydroxylation steps being catalysed by different enzymes. Finally, the differences in GA20 metabolism between intact versus excised tissue raise the possibility that tissue wounding with excision enhanced the activity of the GA20 2b-hydroxylase(s).