Seed imbibition in Medicago truncatula Gaertn.: Expression profiles of DNA repair genes in relation to PEG-mediated stress.

The expression profiles of genes involved in DNA repair, namely MtTdp1 (tyrosyl-DNA phosphodiesterase), top1 (DNA topoisomerase I), MtTFIIS (transcription elongation factor II-S) and MtTFIIS-like, were evaluated in Medicago truncatula Gaertn. during seed imbibition carried out with the osmotic agent polyethylene glycol (PEG6000, 100g/L). The use of PEG6000 resulted in delayed water up-take by seeds, and reduced levels of oxidative DNA damage, measured in terms of 7,8-dihydro-8-oxoguanine (8-oxo-dG) were observed compared to seeds imbibed with water. The prolonged exposure to PEG6000 caused an increase in DNA oxidative damage; after 24h of treatment with the osmotic agent, the estimated amount of 8-oxo-dG was 1.25-fold higher compared to the value detected in seeds imbibed with water. Three days after imbibition, consistent cell damage and reactive oxygen species (ROS) production were also detected in radicles emerging from the PEG-treated seeds. All of the tested genes were known to be up-regulated during seed imbibition, with the highest transcript levels accumulating at approximately 8-12h of rehydration. Exposure to PEG6000 caused a delayed up-regulation of MtTdp1α and MtTdp1β genes, with transcript peaks occurring at 12-24h, when the highest levels of DNA damage were also recorded. For the top1, MtTFIIS and MtTFIIS-like genes, different expression profiles were observed in response to PEG6000. The possible roles of these genes in the repair response activated during seed imbibition are discussed.