Nitric oxide alleviates the effects of copper-induced DNA methylation, genomic instability, LTR retrotransposon polymorphism and enzyme activity in lettuce

Copper is an essential element for plant growth, but higher concentration can damage proteins, DNA and lipids. Nitric oxide (NO) is a highly reactive and lipophilic molecule and is functional during plant defense responses. Hence, this study aims to evaluate the role of the exogenous NO treatment (0 µM, 200 µM and 300 µM of sodium nitroprusside) on the DNA damage levels, DNA methylation, retrotransposon polymorphism and enzyme activity in lettuce seedlings grown under the non-stress and copper stressed conditions (200 and 400 µM of CuSO 4 ·7H 2 O). The inter-retrotransposon amplified polymorphism (IRAP) and couple restriction enzyme digestion-random amplification (CRED-RA) were applied to define the genomic template stability (GTS) levels, DNA methylation, and retrotransposon polymorphism. Copper stress (400 µM) decreased the POD activity as compared to control, whereas combined application of NO (300 µM) and copper increased it as compared to the plants treated with 400 µM copper and without NO. The results of this study highlighted that copper stress increased genomic template instability, DNA methylation and long terminal repeat retrotransposon polymorphism. However, simultaneous treatment of NO and copper caused a decrease in retrotransposon polymorphism and DNA methylation, and an increase in GTS and enzyme activity accompanied it. The results implied that in the presence of excess copper the exogenous NO treatment mitigate the adverse effects of copper stress on the GTS, DNA methylation and retrotransposon polymorphism in lettuce seedlings by increasing antioxidant enzyme activity.