Overexpression of putative topoisomerase 6 genes from rice confers stress tolerance in transgenic Arabidopsis plants.

DNA topoisomerase 6 (TOP6) belongs to a novel family of type II DNA topoisomerases present, other than in archaebacteria, only in plants. Here we report the isolation of full-length cDNAs encoding putative TOP6 subunits A and B from rice (Oryza sativa ssp. indica), preserving all the structural domains conserved among archaebacterial TOP6 homologs and eukaryotic meiotic recombination factor SPO11. OsTOP6A1 was predominantly expressed in prepollinated flowers. The transcript abundance of OsTOP6A2, OsTOP6A3 and OsTOP6B was also higher in prepollinated flowers and callus. The expression of OsTOP6A2, OsTOP6A3 and OsTOP6B was differentially regulated by the plant hormones, auxin, cytokinin, and abscisic acid. Yeast two-hybrid analysis revealed that the full-length OsTOP6B protein interacts with both OsTOP6A2 and OsTOP6A3, but not with OsTOP6A1. The nuclear localization of OsTOP6A3 and OsTOP6B was established by the transient expression of their beta-glucuronidase fusion proteins in onion epidermal cells. Overexpression of OsTOP6A3 and OsTOP6B in transgenic Arabidopsis plants conferred reduced sensitivity to the stress hormone, abscisic acid, and tolerance to high salinity and dehydration. Moreover, the stress tolerance coincided with enhanced induction of many stress-responsive genes in transgenic Arabidopsis plants. In addition, microarray analysis revealed that a large number of genes are expressed differentially in transgenic plants. Taken together, our results demonstrate that TOP6 genes play a crucial role in stress adaptation of plants by altering gene expression.