Role of plant growth substances in MS33-controlled stamen filament growth in Arabidopsis

The rapid growth of stamen filaments just before flower anthesis in Arabidopsis thaliana does not occur in the male sterile33 ( ms33 , formerly known as msZ ) mutant. ms33 filaments were approximately 40% shorter than the wild type (WT), and there was corresponding reduction in the epidermal cell length of filaments. This suggests that MS33 controls the final cell-elongation phase of filament growth. Both low temperatures and gibberellic acid (GA₃) restored filament and cell growth in intact ms33 flowers, but these treatments only had a small promotive effect on WT filaments. Decapitation experiments involving the removal of the anther had the opposite effect on WT and ms33 filaments; growth was inhibited in WT, but was increased in ms33 filaments. In young stamen primordia cultured in vitro, filament growth was less in WT, but more in ms33 , than in respective in vivo produced filaments. Plant growth substances (PGSs), GA₃ and indole-3-acetic acid (IAA) were promotive, zeatin had no effect, and abscisic acid (ABA) and ethrel inhibited filament growth in both intact and decapitated WT and ms33 filaments. Together these observations suggest that MS33 is activated immediately before anthesis and that the MS33 product either regulates temporal biosynthesis of gibberellins (GAs) and/or IAA or makes the filament tissue sensitive to these PGSs, which in turn trigger cell elongation and filament growth. The data also suggest that ms33 mutant anthers contain a relatively high ratio of growth inhibitors to promoters, which inhibits epidermal cell elongation and filament growth.