Regulation in Lolium temulentum of the Metabolism of Gibberellin A20 and Gibberellin A1 by 16,17-Dihydro GA5 and by the Growth Retardant, LAB 198 999

Abstract

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, Planta193, 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).