Enzymes from seeds of Phaseolus vulgaris L.: Hydroxylation of gibberellins A20 and A 1 and 2,3-dehydrogenation of gibberellin A 20.

A time-course study is described relating the enzyme activities for GA20 metabolism with seed development in Phaseolus vulgaris L. Enzyme activity for the 3β-hydroxylation of GA20 to GA1, and for the 2,3-desaturation of GA20 to GA5, was confined to the cotyledons and showed maximal specific activity at 21 d after anthesis. These enzyme activities co-occurred, together with a much lower level of activity for the 2β-hydroxylation of GA20 to GA29. The observed rates of GA1, GA5 and GA29 formation from GA20 were constant under a range of incubation conditions. Enzyme activity for the conversion of GA1 to GA8 was detected only in embryos of seed from 40 d after anthesis. By deuterium-labelling and analysis of the products by gas chromatography-selected ion monitoring it was shown that 2β-hydroxylation of GA1 to GA8 and 3β-hydroxylation of GA20 to GA1 occur with retention of configuration and that the conversion of GA20 to GA5 occurs with loss of the 2β- and 3β-hydrogens. These results establish that GA1 is not formed from GA20 via GA5.