A STEP IN THE BIOSYNTHESIS OF GIBBERELLINS THAT IS CONTROLLED BY THE MUTATION IN THE SEMIDWARF RICE CULTIVAR TAN-GINBOZU

Genetic analysis and a comparison of endogenous levels of gibberellins between the semi-dwarf rice cultivar Tan-ginbozu and the correspondin g normal cultivar Ginbozu have confirmed that Tan-ginbozu is a gibbere llin-deficient mutant and that the semi-dwarfism of Tan-ginbozu is con trolled by a single recessive gene, A step in the biosynthesis of gibb erellins that is blocked by the mutation in Tan-ginbozu had been consi dered to be the synthesis of ent-kaurene or an earlier step. However, the rate of production of ent-kaurene by Tan-ginbozu was almost the sa me as that by Ginbozu. By contrast, accumulation of only a small amoun t of ent-kaurene was detected in Tan-ginbozu, and the amount that accu mulated was similar to that in Ginbozu that had been treated with 6.9 x 10(-8) M uniconazole-P (an effective inhibitor of three oxidative st eps in the pathway from ent-kaurene to ent-kaurenoic acid via ent-kaur enol and ent-kaurenal), The height of the treated Ginbozu plants was r educed to the same as that of Tan-ginbozu plants, Resembling Tan-ginbo zu plants, Ginbozu plants that had been treated with uniconazole-P res ponded well to ent-kaurenoic acid and slightly to ent-kaurene and ent- kaurenol, Since the growth-promoting activity of ent-kaurenal in Tan-g inbozu was similar to that of ent-kaurene, our results suggest that th e mutation in Tan-ginbozu blocks the three oxidative steps whereby ent -kaurene is converted to ent-kaurenoic acid.