Exogenous proline activated an integrated response of NER and HR pathways to reduce DNA damage in rice seedlings under chromium stress.

The DNA damage induced by hexavalent chromium [Cr(VI)] pollutant causes a genotoxic effect on rice seedlings. Hereby, we examined the effects of exogenous proline (Pro) on the alleviation of DNA damage in rice seedlings under different effective concentrations of Cr(VI). Our results revealed that Cr(VI) stress induced reactive oxygen species (ROS), i.e., HO and O accumulation in rice seedlings, repressed genes expression activated in the homologous recombination (HR) and nucleotide excision repair (NER) pathways, and caused DNA damage. Exogenous application of Pro increased Cr accumulation in rice roots, but decreased Cr accumulation in rice shoots, wherein Pro application decreased ROS accumulation in both tissues of rice seedlings. The comet assays suggested that exogenous application of Pro significantly alleviated the DNA damage in rice seedlings during Cr(VI) treatments, judged by the Olive tail moment and tail DNA. Transcriptional assays revealed that exogenous Pro upregulated the expression level of genes associated with the HR and NER pathways and triggered coordinated actions of both repairing pathways to modulate DNA lesion in rice plants during exposure to Cr(VI). Calculations from gene expression variation factors showed that regulative effect of exogenous application of Pro on DNA repair pathways was highly activated at 2.0 mg Cr/L. The current study revealed that Cr(VI) affect rice plants and exogenous Pro rescue these effects by the activation of HR and NER pathways.