Enhanced osmotic stress tolerance in Medicago truncatula plants overexpressing the DNA repair gene MtTdp2α (tyrosyl-DNA phosphodiesterase 2)

No information is currently available in plants concerning the tyrosyl-DNA phosphodiesterase 2 (Tdp2) enzyme which in animals is involved in the removal of DNA topoisomerase II-mediated DNA damage and cell proliferation/differentiation signaling. Bioinformatic investigation revealed the occurrence in the plant kingdom of three distinct Tdp2 isoforms, named alpha, beta and gamma. The MtTdp2 alpha gene from Medicago truncatula Gaertn., encoding a protein with putative nuclear localization signal and chloroplast transit peptide, was significantly up-regulated in response to osmotic stress induced by polyethylene glycol. The transgenic M. truncatula lines Tdp2 alpha-13C and Tdp2 alpha-28 overexpressing the MtTdp2 alpha gene were characterised by enhanced tolerance to both osmotic and photo-oxidative stress. According to single cell gel electrophoresis, MtTdp2 alpha gene overexpression prevented accumulation of double strand breaks in absence and presence of osmotic stress. Interestingly, the MtMRE11, MtRAD50 and MtNBS1 genes involved in double strand break sensing/repair were significantly up-regulated in the MtTdp2 alpha-overexpressing plants grown under physiological conditions and no further up-regulation occurred in response the osmotic agent. The Tdp2 alpha-13C and Tdp2 alpha-28 lines also showed significant up-regulation of several genes essential for the control of DNA topology and genome maintenance, such as MtTdp1 alpha, MtTop2 (DNA topoisomerase II) and MtGYR (DNA gyrase). The role of MtTdp2 alpha gene in enhancing the plant response to genotoxic injury under osmotic stress is discussed.