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Development and Growth Regulation

Genetic Regulation of Development in Sorghum bicolor¹

V. The ma₃^R Allele Results in Gibberellin Enrichment

Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843, Research School of Chemistry, Australian National University, Canberra ACT 2601, Australia

Sorghum bicolor genotypes, near isogenic with different alleles at the third maturity locus, were compared for development, for responsiveness to GA₃ and a GA synthesis inhibitor, and occurrence and concentrations of endogenous GAs, IAA, and ABA. At 14 days the genotype 58M (ma₃^Rma₃^R) exhibited 2.5-fold greater culm height, 1.75-fold greater total height, and 1.38-fold greater dry weight than 90M (ma₃ma₃) or 100M (Ma₃Ma₃). All three genotypes exhibited similar shoot elongation in response to GA₃, and 58M showed GA₃-mediated hastening of floral initiation when harvested at day 18 or 21. Both 90M and 100M had exhibited hastening of floral initiation by GA₃ previously, at later application dates. Tetcyclacis reduced height, promoted tillering, and delayed flowering of 58M resulting in plants which were near phenocopies of 90M and 100M. Based on bioassay activity, HPLC retention times, cochromatography with ²H₂-labeled standards on capillary column GC and matching mass spectrometer fragmentation patterns (ions [m/z] and relative abundances), GA₁, GA₁₉, GA₂₀, GA₅₃, and GA₃ were identified in extracts of all three genotypes. In addition, based on published Kovats retention index values and correspondence in ion masses and relative abundances, GA₄₄ and GA₁₇ were detected. Quantitation was based on recovery of coinjected, ²H₂-labeled standards. In 14 day-old-plants, total GA-like bioactivity and GA₁ concentrations (nanograms GA/gram dry weight) were two- to six-fold higher in 58M than 90M and 100M in leaf blades, apex samples, and whole plants while concentrations in culms were similar. Similar trends occurred if data were expressed on a per plant basis. GA₁ concentrations for whole plants were about two-fold higher in 58M than 90M and 100M from day 7 to day 14. Concentrations of ABA and IAA did not vary between the genotypes. The results indicate the mutant allele ma₃^R causes a two- to six-fold increase in GA₁ concentrations, does not result in a GA-receptor or transduction mutation and is associated with phenotypic characteristics that can be enhanced by GA₃ and reduced by GA synthesis inhibitor. These observations support the hypothesis that the allele ma₃^R causes an overproduction of GAs which results in altered leaf morphology, reduced tillering, earlier flowering, and other phenotypic differences between 58M and 90M or 100M.

¹ A contribution of the Texas Agricultural Experiment Station (TAES) paper No. 25548 in the Technical Article Series. Supported by U.S. Department of Agriculture Competitive Grant No. 86-CRCR-1-2005 and TAES.

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