Topoisomerase II is Required for Programmed Chromosome Reorganization during Meiosis and Mitosis in Plants

Type II topoisomerases are highly conserved proteins. They resolve catenated and supercoiled structures between two dsDNA molecules by ATP-dependent induction of a transient double-strand break in one of the molecules to enable the other to pass through the broken strand which is subsequently religated. TOPII in Arabidopsis thaliana (At3g23890) corresponds to the alpha version of the homodimeric human protein. In eukaryotes TOPII has a role in many aspects of DNA metabolism. Recently, TOPII was also implicated in mediating crossover interference in budding yeast⁶. We have analysed a hypomorphic topIIa allele of an A. thaliana T-DNA line. These plants exhibit a dwarf phenotype, short roots and reduced fertility. Analysis of meiosis specific RNAi knockdown-line of TOPIIa revealed an almost normal somatic phenotype but 90% reduced fertility. Mitosis and meiosis were analysed cytologically revealing defects in chromosome condensation and segregation together with fragmentation. This included: fragmentation during both meiotic divisions; condensation problems during prophase and metaphase I; a significant increase in the number of chromosomal interlocks. High levels of spontaneous DNA breaks occur during the cell cycle and the plants are hypersensitive to genotoxic agents. Crossover designation and formation was compared to the frequency of chiasma per cell to test for interference alterations. Our data indicate that in common with other organisms AtTOPIIa is involved in a range of cellular processes. Moreover we provide new evidence suggesting a role in interlock resolution during meiosis.