Heterosis and the metabolism of gibberellin A20 in sorghum

The correlation between gibberellin (GA) metabolism and growth rate was investigated using two Sorghum bicolor inbred lines, Hegari and AT×623, and their heterotic F₁ hybrid. Previous studies have demonstrated that this hybrid is taller and has substantially greater shoot dry weights and leaf areas than either parental inbred. ³H]GA₂₀ was applied to the leaf whorl of seedlings and after 24 hours, plants were harvested and separated into roots, shoot cylinders containing the apical meristems, and leaf blades. Chromatographic analyses of metabolites indicated the conversions of ³H]GA₂₀ to ³H]GA_{1,8 and 29}. The conversion of ²H]GA₂₀ to ²H]GA₁ was demonstrated by gas chromatography-selected ion monitoring (GC-SIM). Putative glucosyl conjugates of all of the ³H]GAs were also produced and GA₈ was identified by GC-SIM following enzymic cleavage of the putative ³H]GA₈ glucosyl conjugate fraction. Comparing the genotypes, ³H]GA₂₀ metabolism was more rapid in the shoot cylinders of the hybrid than in the shoot cylinders from inbreds. In the hybrid samples, there was a three-fold increase in the putative conjugate(s) of ³H]GA₁ which was the principal metabolite, and increased production of ³H]GA₈ and the putative conjugates of ³H]GA₂₉ and ³H]GA₈. Conversely, levels of the remaining precursor, ³H]GA₂₀, and its putative conjugate(s) were reduced in the hybrid. The rate of GA₂₀ metabolism was thus positively correlated with growth rate across these sorghum genotypes. This correlation supports a promotive role of GA in the regulation of shoot growth and in the expression of heterosis (hybrid vigor) in sorghum.