Developmental and physiological characterization of the male sterile33 (ms33) mutant in arabidopsis
The objectives of this study were to characterize the ' male sterile33' ('ms33') mutant in 'Arabidopsis thaliana' at the morphological and developmental levels, and to investigate the possible role of plant hormones in gene-controlled stamen and pollen development. The morphology and growth of 'ms33' and WT plants showed that the 'ms33' mutation not only affects stamen and pollen development, but also has several pleiotropic effects including, aberrant growth of all floral organs, and delays in seed germination, seedling growth and flowering time. Ultrastructural studies showed that in anther of the ' ms33' mutant there was premature degeneration of the tapetum which led to defects in intine maturation, tryphine deposition on the pollen wall, lipid body formation in the pollen, and the production of large, highly vacuolate non-viable pollen. These observations suggested that dehydration of pollen is impaired in the 'ms33' mutant and that it affects pollen viability. In the WT stamen, there was rapid growth of filaments before anthesis which was suppressed in the 'ms33' mutant. This growth was mainly due to cell elongation and was stimulated by GA and IAA. The data suggest that the 'MS33' gene controls the final stage of filament growth by temporally stimulating GA and/or IAA biosynthesis, or their signal transduction pathways. Seed germination was also delayed in the 'ms33' mutant, but this was partially overcome by low temperature and GAs. GA4 was more effective than GA3 in promoting seed germination, as well as seedling and plant growth. The 'ms33' flowers contained relatively low levels of total GAs, in particularly GA4, but a high level of GA3. It is suggested that mutation in 'MS33 ' favors the early 13-hydroxylation pathway of GA biosynthesis resulting in greater accumulation of GA3, instead of the non-13-hydroxylation pathway in WT flowers and high GA4 content. WT flowers also had a higher level of IAA, but lower level of ABA than 'ms33' flowers. Analysis of double mutants of 'ms33' with an ABA-deficient mutant 'aba-1', and a GA-signal transduction mutant ' spy-3', revealed that the inhibition of filament growth, and aberrant pollen development in 'ms33' mutant are not related to high level of ABA, or to a possible blockage in the GA signal transduction pathway.