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

The rapid growth of stamen filaments just before flower anthesis in Arabido psis thaliana does not occur in the male sterile33 (ms33, formerly known as msZ) mutant. ms33 filaments were approximately 40% shorter than the wild t ype (WT), and there was corresponding reduction in the epidermal cell lengt h of filaments, This suggests that MS33 controls the final cell-elongation phase of filament growth. Both low temperatures and gibberellic acid (GA(3) ) restored filament and cell growth in intact ms33 flowers, but these treat ments only had a small promotive effect on WT filaments. Decapitation exper iments involving the removal of the anther had the opposite effect on WT an d ms33 filaments; growth,vas inhibited in WT, but was increased in ms33 fil aments. In young stamen primordia cultured in vitro, filament growth was le ss in WT, but more in ms33, than in respective in vivo produced filaments. Plant growth substances (PGSs), GA(3) and indole-3-acetic acid (IAA) were p romotive, zeatin had no effect, and abscisic acid (ABA) and ethrel inhibite d filament growth in both intact and decapitated WT and ms33 filaments. Tog ether these observations suggest that MS33 is activated immediately before anthesis and that the MS33 product either regulates temporal biosynthesis o f gibberellins (GAs) and/or IAA or makes the filament tissue sensitive to t hese PGSs, which in turn trigger cell elongation and filament growth, The d ata also suggest that ms33 mutant anthers contain a relatively high ratio o f growth inhibitors to promoters, which inhibits epidermal cell elongation and filament growth.