| Proper chromatin condensation and segregation are crucial for maintaining chromosomal integrity, and are mediated by a conserved complex called condensin. The catalytic subunits of the complex are members of the SMC2 (S&barbelow;tructural M&barbelow;aintenance of C&barbelow;hromosomes) and SMC4 gene families.; An Arabidopsis gene, AtCAP-E1, a functional ortholog of the yeast SMC2 gene, and a second highly homologous SMC2 gene, AtCAP-E2, have been characterized. SMC2 expression in Arabidopsis correlated with the mitotic activity of tissues. The two genes are differentially expressed, with AtCAP-E9 accounting for more than 85% of the total SMC2 transcript pool.; The titan3 mutant (Plant J. 29:405--415) is due to a T-DNA insertion into AtCAP-E1 with subtle endosperm defects. A T-DNA insertion mutant of AtCAP-E2 was also identified with no mutant phenotype. A genetic cross between the two SMC2 mutants showed that gametogenesis was compromised in double mutant spores. Embryo lethality was observed for both double homozygous and AtCAP-E1 -/-, AtCAP-E2+/- plants; arrest occurred at the globular stage of embryogenesis with altered planes of cell division in both the suspensor and the embryo. Down regulation of both genes by antisense technology resulted in meristem disorganization and fasciation, and similar phenotypes were observed in AtCAP-E1 +/-, AtCAP-E2-/- plants. These data suggest that threshold levels of SMC2 proteins are required for development and that AtCAP-E2 may have a higher affinity for its target than AtCAP-E1.; The T-DNA insertion mutants of AtCAP-C, the Arabidopsis SMC4 gene, also exhibited embryo lethality and semi-sterility. The arrested embryos showed patterning defects with more than the usual number of cells and altered planes of division in the arrested embryos and the suspensors.; The AtCAP-E1 antibodies cross-reacted with the SMC2 cognates in lily meiocytes and tobacco cells. The lily SMC2 protein was enriched in the nucleolus and the meiotic chromosomes during prophase I. Also, the SMC2 protein levels did not change during the cell cycle in a synchronized tobacco cell culture suggesting that SMC2 proteins might be regulated post-translationally.; This research has demonstrated the importance of SMC2 and SMC4 proteins in plant development. The tools that I have developed should provide the means to further characterize the condensin genes and elucidate their novel functions in plants. |
