DNA Repair and Acclimatisation to Damage under Ionizing Radiations

Abstract

Being sessile, plants are continuously exposed to DNA damaging agents present in the environment such as ultraviolet (UV) and ionising radiations (IR). Sunlight acts as an energy source for photosynthetic plants; hence, avoidance of UV radiations (UV-A, UV-B and UV-C) is unpreventable. DNA in particular strongly absorbs UV-B; therefore, it is the most important target for UV-B induced damage. UV-B induces three major classes of DNA lesions such as cyclobutane pyrimidine dimmers (CPDs), pyramiding (6-4) pyrimidone dimers and their Dewar isomers. UV-B induced modifications of DNA purine bases have also been recognized. These comprise the photoproducts that involve, at least, one adenine residue that undergoes photo-cycloaddition reactions with contiguous adenine or thymine upon exposure to UV-B. On the other hand, IR causes water radiolysis, which generates highly reactive hydroxyl radicals (OH.) and causes radiogenic damage to important cellular components. The accumulation of such damages and unrecognised and unrepaired DNA damage may cause fatal mutations which in turn can reduce plant genome stability, growth, and productivity and also threaten the organism’s immediate survival. To maintain genomic integrity under UV/IR exposure, plants however make use of several DNA repair mechanisms. These repair mechanisms can be enzymatic or nonenzymatic. Enzymatic processes include light dependent repair (photoreactivation/photorepair), light independent processes (nucleotide excision repair, base excision repair and mismatch repair), homologous recombination, non-homologous recombination and elimination of active radicals. Active radicals and ROS accumulation are counteracted by enzymatic antioxidants that include a variety of scavangers, such as, superoxide dismutase, ascorbate peroxidase and catalase. Non-enzymatic processes include non-enzymatic antioxidants like ascorbic acid (vitamin C), glutathione (GSH), αtocopherols (vitamin E), carotenoids and flavonoids. Identification and characterization of UV/IR-sensitive mutants in different plant species are expected to achieve more insights into genetic recombination/manipulation in plants.