Inhibition and promotive effects of desoxygibberellin C and its methyl ester on plant growth

GROWTHRETARDANTS: Effects on Gibberellin Biosynthesis and Other Metabolic Pathways

Plant growth retardants are applied in agronomic and horticultural crops to reduce unwanted longitudinal shoot growth without lowering plant productivity. Most growth retardants act by inhibiting gibberellin (GA) biosynthesis. To date, four different types of such inhibitors are known: (a) Onium compounds, such as chlormequat chloride, mepiquat chloride, chlorphonium, and AMO-1618, which block the cyclases copalyl-diphosphate synthase and ent-kaurene synthase involved in the early steps of GA metabolism. (b) Compounds with an N-containing heterocycle, e.g. ancymidol, flurprimidol, tetcyclacis, paclobutrazol, uniconazole-P, and inabenfide. These retardants block cytochrome P450-dependent monooxygenases, thereby inhibiting oxidation of ent-kaurene into ent-kaurenoic acid. (c) Structural mimics of 2-oxoglutaric acid, which is the co-substrate of dioxygenases that catalyze late steps of GA formation. Acylcyclohexanediones, e.g. prohexadione-Ca and trinexapac-ethyl and daminozide, block particularly 3ss-hydroxylation, thereby inhibiting the formation of highly active GAs from inactive precursors, and (d) 16,17-Dihydro-GA5 and related structures act most likely by mimicking the GA precursor substrate of the same dioxygenases. Enzymes, similar to the ones involved in GA biosynthesis, are also of importance in the formation of abscisic acid, ethylene, sterols, flavonoids, and other plant constituents. Changes in the levels of these compounds found after treatment with growth retardants can mostly be explained by side activities on such enzymes.

Gibberellin Structure-Dependent Interaction between Gibberellins and Deoxygibberellin C in the Growth of Dwarf Maize Seedlings

The effects of 3-deoxygibberellin C (DGC) on the growth-promoting actions of gibberellins A(1), A(2), A(3), A(4), A(5), A(7), A(8), A(9), A(13), A(18), A(19), A(20), and A(23) (GAn) as well as 13-deoxygibberellin A(5) (deoxy-GA(5)) were tested with seedlings of gibberellin-deficient dwarf mutants (d(2) and d(5)) of maize (Zea mays L.). It was found that DGC promoted the actions of gibberellins having both C-1 double bond and C-3 axial hydroxyl group, and it inhibited the action of gibberellins having the saturated ring A and lacking the C-3 axial hydroxyl group, whereas it did not affect that of the ones having the hydroxyl group. The presence of C-2 double bond, as in GA(5) and deoxy-GA(5), diminished the inhibitory action of DGC. The DGC inhibition was alleviated by raising the doses of the relevant GAs, suggesting that it is a competitive inhibition. These results and the finding that the growth of normal maize and rice seedlings are inhibited by DGC indicate that GA(9), GA(19), GA(20) or other gibberellins having ring A of the same structure are involved in the growth of these plants as active form(s) or as intermediate(s) leading to the active form(s).

Metabolism of [(3)H]Gibberellin A 20 by plants of Bryophyllum daigremontianum under long- and short-day conditions

[(3)H]Gibberellin A20 ([(3)H]GA20), a native gibberellin of this plant, was injected into mature leaves of Bryophyllum daigremontianum (Hamet et Perr.) Berg. under long- and short-day conditions. It was converted, in order of decreasing yields, to GA29, 3-epi-GA1 (pseudo GA1), C/D-ring-rearranged GA20, and two minor, unidentified metabolites. Identifications were made by gas-liquid chromatography with radioactive monitoring using three different phases. Metabolism to 3-epi-GA1 was greater under short days, particularly in the treated leaf pair, although the absolute amount of GA29 was greater than that of 3-epi-GA1 under both photoperiods. The levels of radioactive metabolites in the shoots above the treated leaf pair gradually increased over a 51-day period, GA29 reaching 5 times the contten of 3-epi-GA1.