GENETIC-REGULATION OF DEVELOPMENT IN SORGHUM-BICOLOR .7. SHOOT GROWTH, TILLERING, FLOWERING, GIBBERELLIN BIOSYNTHESIS, AND PHYTOCHROME LEVELS ARE DIFFERENTIALLY AFFECTED BY DOSAGE OF THE MA(3)(R) ALLELE

Sorghum [Sorghum bicolor (L.) Moench] homozygous for ma(3)(R) lacks a type II, light-stable phytochrome of 123 kD and has a number of phenotypic characteristics consistent with the absence of functional phytochrome B. We have used plants heterozygous at Ma(3) (Ma(3)/ma(3)(R) and ma(3)/ma(3)(R)) to determine the effect of dosage of ma(3)(R) on plant growth, flowering, gibberellin (GA) levels, and content of the 123-kD phytochrome. Both Ma(3)/ma(3)(R) and ma(3)/ma(3)(R) produced the same number of tillers per plant as their respective homozygous non-ma(3)(R) parents. Height of the heterozygotes was intermediate between the homozygous parents, although it was more similar to the non-ma(3)(R) genotypes. In both field and growth-chamber environments, the timing of floral initiation and anthesis in the heterozygotes also was intermediate, again more similar to non-ma(3)(R) plants. In Ma(3)/ma(3)(R), levels of GA(53), GA(19), CA(20), and GA(1) were almost exactly intermediate between levels detected in Ma(3)/ Ma(3) and ma(3)(R)/ma(3)(R) plants. Immunoblot analysis indicated that there was less of the 123-kD phytochrome in Ma(3)/ma(3)(R) than in homozygous Ma(3), whereas none was detected in ma(3)(R)/ma(3)(R). The degree of dominance of Ma(3) and ma(3) over ma(3)(R) varies with phenotypic trait, indicating that mechanisms of activity of the 123-kD phytochrome vary among the biochemical processes involved in each phenotypic character. Although the heterozygotes were similar to homozygous Ma(3) and ma(3) plants in growth and flowering behavior, Ma(3)/ma(3)(R) contained 50% less of the bioactive GA (GA(1)) than non-ma(3)(R) genotypes. Thus, sensitivity to endogenous GAs also may be regulated by the 123-kD phytochrome. To fully regulate plant growth and development, two copies of Ma(3) or ma(3) are required to produce sufficient quantities of the light-stable, 123-kD phytochrome.