| Summary: | A critical stage in pollen development is the separation of the microspores following meiosis. In almost all angiosperms the pollen mother cell and resultant microspores are encased in a thick callose wall. At the appropriate developmental stage, the surrounding tapetal cells secrete callase, a complex of beta-1,3-glucanases, which hydrolyse this callose wall. The free microspores then continue their development into mature pollen grains. A Brassica napus cDNA, A6, and two corresponding Arabidopsis thalimin genomic clones, G61 and G62, had been isolated. The A6 cDNA exhibited sequence similarity to beta-1,3-glucanases and was known to be present at the time of microspore release. It was proposed that A6 may represent a component of callase. The scope of this thesis was to characterise the A. thaliana genomic clones with a view to understanding their role in microsporogenesis, and in particular whether they are involved in the crucial process of microspore release. Due to the high degree of sequence similarity between the two genomic clones, work was concentrated on G62. The G62 sequence was compared to beta-1,3-glucanases and found to be similar, but distinct from them. The temporal and spatial expression pattern of G62 was investigated using promoter fusions. It was found to be tapetum-specific and present during the developmental window of callase activity. The enzymatic nature of both A6 and G62 were investigated in an Escherchia coli over-expression system and in planta. No activity was associated with the A6 protein when over-expressed in E. coli, or with the G62 protein when expressed in Nicotiana tabacum. The most likely explanation for these findings was that both A6 and G62 were found to lack an essential catalytic residue.
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