Effects of Ring D-Modified Gibberellins on Gibberellin Levels and Development in Selected Sorghum bicolor Maturity Genotypes

Abstract

Plants of early flowering mutant and wild type genotypes of Sorghum bicolor were treated with ring D-modified gibberellins (GAs), and the effects on endogenous GA levels were determined. The growth and timing of floral initiation in 58M plants grown under 18-h days (which significantly delays floral initiation in this short day plant) following treatment with these compounds, relative to GA3 and GA5 treatments, were also investigated. Application of the endo-isomer of C16,17-dihydro-GA5 (endo-DiHGA5), the exo-isomer of C16,17-dihydro-GA5 (exo-DiHGA5), and C16α,17-dichloromethanodihydro-GA5 (DMDGA5) altered GA levels in both genotypes. Each ring D-modified GA significantly inhibited shoot growth while significantly decreasing levels of GA1 and increasing levels of its immediate precursor, GA20. Gibberellin A8 levels also decreased. Tillering was not affected by any treatment. For the early flowering genotype 58M, grown under noninductive long days, both dihydro-GA5 isomers promoted floral initiation while shoot growth was strongly inhibited, and floral development was strongly advanced beyond floral stage 4. Gibberellin A3 and GA5, applied under the same conditions, promoted shoot growth slightly and gave ``floral-like'' apical meristems that did not develop past floral stage 1. These results suggest that the reduced shoot growth of sorghum, which follows application of those ring D-modified GAs, is due to their inhibiting the 3β hydroxylation of GA20 to GA1, thereby reducing the GA1 content. That floral initiation was hastened and floral development promoted in genotype 58M by application of both isomers of DiHGA5 are in contrast to the effects of other GA biosynthesis inhibitors, which act earlier in the GA biosynthesis pathway, but are consistent with results seen for long day grasses. This suggests that endo-DiHGA5 and exo-DiHGA5 may be acting directly in promoting floral initiation and subsequent floral apex development of this short day plant under long day conditions.