GENETIC-REGULATION OF DEVELOPMENT IN SORGHUM-BICOLOR .9. THE MA(3)(R) ALLELE DISRUPTS DIURNAL CONTROL OF GIBBERELLIN BIOSYNTHESIS

The diurnal regulation of gibberellin (GA) concentrations in Sorghum bicolor was studied in a mutant lacking a light-stable 123-kD phytochrome (ma(3)(R)ma(3)(R)), wild-type (ma(3)ma(3),Ma(3)Ma(3)), and heterozygous (ma(3)ma(3)(R)) cultivars. GAs were determined in shoots of 14-d-old plants by gas chromatography-selected ion-monitoring-mass spectrometry. GA(12) levels fluctuated rhythmically in Ma(3)Ma(3), ma(3)ma(3), and ma(3)(R)ma(3)(R); peak levels occurred 3 to 9 h after lights-on. In some experiments, GA(53) levels followed a similar pattern. There was no rhythmicity in levels of GA(19) and GA(8) in any genotype. In ma(3)ma(3) and Ma(3)Ma(3), GA(20) levels increased at lights-on, peaked in the afternoon, and decreased to minimum levels in darkness. In ma(3)(R)ma(3)(R), peak GA(20) levels occurred at lights-on, 9 h earlier than in the wild-type genotypes. The pattern for GA(1) levels closely followed GA(20) levels in all cultivars. One copy of ma(3) restored near wild-type regulation of GA(20) levels. GA rhythms persisted in 25-d-old ma(3)ma(3) plants. Since absence of the 123-kD phytochrome disrupted diurnal regulation of the GA(19) --> GA(20) step, the ma(3)(R)ma(3)(R) genotype may be viewed as being phase shifted in the rhythmic levels of GA(20) and GA(1) rather than as simply overproducing them.