PLANT PHYSIOLOGY , Vol 105, Issue 3 941-948, Copyright © 1994 by American Society of Plant Biologists

DEVELOPMENT AND GROWTH REGULATION

Genetic Regulation of Development in Sorghum bicolor (VIII. Shoot Growth, Tillering, Flowering, Gibberellin Biosynthesis, and Phytochrome Levels Are Differentially Affected by Dosage of the ma3R Allele

K. R. Foster, F. R. Miller, K. L. Childs and P. W. Morgan
Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843-2474

Sorghum [Sorghum bicolor (L.) Moench] homozygous for ma3R 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 Ma3 (Ma3/ma3R and ma3/ma3R) to determine the effect of dosage of ma3R on plant growth, flowering, gibberellin (GA) levels, and content of the 123-kD phytochrome. Both Ma3/ma3R and ma3/ma3R produced the same number of tillers per plant as their respective homozygous non-ma3R parents. Height of the heterozygotes was intermediate between the homozygous parents, although it was more similar to the non-ma3R 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-ma3R plants. In Ma3/ma3R, levels of GA53, GA19, GA20, and GA1 were almost exactly intermediate between levels detected in Ma3/Ma3 and ma3R/ma3R plants. Immunoblot analysis indicated that there was less of the 123-kD phytochrome in Ma3/ma3R than in homozygous Ma3, whereas none was detected in ma3R/ma3R. The degree of dominance of Ma3 and ma3 over ma3R 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 Ma3 and ma3 plants in growth and flowering behavior, Ma3/ma3R contained 50% less of the bioactive GA (GA1) than non-ma3R 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 Ma3 or ma3 are required to produce sufficient quantities of the light-stable, 123-kD phytochrome.

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