Interconversion of gibberellin A4 to gibberellins A 1 and A 34 by dwarf rice, cultivar Tan-ginbozu

Gibberellins regulate seed germination in tomato by endosperm weakening: a study with gibberellin-deficient mutants

The germination of seeds of tomato [Lycopersicon esculentum (L.) Mill.] cv. Moneymaker has been compared with that of seeds of the gibberellin-deficient dwarf-mutant line ga-1, induced in the same genetic background. Germination of tomato seeds was absolutely dependent on the presence of either endogenous or exogenous gibberellins (GAs). Gibberellin A4+7 was 1000-fold more active than commercial gibberellic acid in inducing germination of the ga-1 seeds. Red light, a preincubation at 2°C, and ethylene did not stimulate germination of ga-1 seeds in the absence of GA4+7; however, fusicoccin did stimulate germination independently. Removal of the endosperm and testa layers opposite the radicle tip caused germination of ga-1 seeds in water. The seedlings and plants that develop from the detipped ga-1 seeds exhibited the extreme dwarfy phenotype that is normal to this genotype. Measurements of the mechanical resistance of the surrounding layers showed that the major action of GAs was directed to the weakening of the endosperm cells around the radicle tip. In wild-type seeds this weakening occurred in water before radicle protrusion. In ga-1 seeds a similar event was dependent on GA4+7, while fusicoccin also had some activity. Simultaneous incubation of de-embryonated endosperms and isolated axes showed that wild-type embryos contain and endosperm-weakening factor that is absent in ga-1 axes and is probably a GA. Thus, an endogenous GA facilitates germination in tomato seeds by weakening the mechanical restraint of the endosperm cells to permit radicle protrusion.

Promotion of flowering in apple trees with gibberellin A4 and C-3 epi-gibberellin A 4

The proportion of spurs flowering on apple trees (Malus domestica Borkh. cv Golden Delicious) displaying a high degree of alternate-year flowering was increased in the "off" year by gibberellin A4 (GA4) and C-3 epi-GA4 applied in the previous year. When applied 4.5 weeks after anthesis amounts of GA4 ranging from 3 to 300 μg per spur and 25 or 50 μg of C-3 epi-GA4 per spur were effective. Treatments with GA4 made seven weeks after anthesis were less effective. A combination of 30 μg GA4 and 30 μg zeatin (6-(4-hydroxy-3-methylbut-trans-2-enylamino)purine) promoted flowering at both treatment times, and tended to be more effective than GA4 alone.

Translocation and metabolism of [(3)H]gibberellins by light-grown Phaseolus coccineus seedlings

Tritiated gibberellins (GAs) A1, A4, A5 and A20 injected into the apical bud of light-grown Phaseolus coccineus L. seedlings were retained in the apical region and underwent little metabolism in 24 h. However, when injected into the hypocotyl, the [(3)H]GAs were extensively metabolised and redistributed. All four GAs were equally effective in promoting subapical elongation in shoot explants irrespective of apical or basal application. The metabolism and export of [(3)H]GA4 by excised organs incubated for 24 h in buffer solution was both quantitatively and qualitatively different to that of the corresponding treated organs of intact plants, suggesting abnormal metabolic conditions in explants. [(3)H]GAs applied to embryonic axes of germinating seeds were translocated into the cotyledons but no translocation in the reverse direction took place.

The effect of light on the gibberellin metabolism and growth of Phaseolus coccineus seedlings

Light inhibits the rate of stem elongation of Phaseolus coccineus L. seedlings. Gibberellin A4 (GA4), an endogenous component of Phaseolus seedlings (Bowen et al., Phytochem. 12, 2935-2941, 1973) promotes stem growth in the light but not in darkness. Dark-grown seedlings contain larger GA pools than light-grown plants. Apically applied [(3)H]GA4 in etiolated bolised more extensively in the light. The slower rate of metabolism of [(3)H]GA4 in etiolated seedlings is not a consequence of isotopic dilution by the endogenous GA4 pool or a lack of penetration of the labelled material. While it can be concluded that the capacity of seedlings to metabolise [(3)H]GA4 is greater in the light than in darkness, it does not necessarily follow that there is a more rapid rate of turnover of endogenous GA4 in light-grown tissues. The results are discussed in relation to the involvement of GAs in the inhibitory effects of ligh on stem elongation.

The conversion of gibberellin A1 to gibberellin A 8 by a cell-free enzyme system

An enzyme system which catalyzes hydroxylation of (3)[H]gibberellin A1 ([(3)H]GA1) to [(3)H]GA8 was obtained from homogenates of germinating bean (Phaseolus vulgaris L.) seed. The system remained in the supernatant after centrifugation at 95000×g for 2 h. Hydroxylation occurred at one of the specifically (3)H-labeled positions (C-2) and, concomitantly, produced [(3)H]water whic was used to measure formation of [(3)H]GA8. NADPH and/or ascorbate are cofactors utilized by this enzyme. EDTA effectively inhibited the system, suggesting a cation requirement.