Molecular mechanisms of DNA damage and repair: progress in plants

Despite stable genomes of all living organisms, they are subject to damage by chemical and physical agents in the environment (e.g., UV and ionizing. radiations, chemical mutagens, fungal and bacterial toxins, etc.) and by free radicals or alkylating agents endogenously generated in metabolism. DNA is also damaged because of errors during its replication. The DNA lesions produced by these damaging agents could be altered base, missing base, mismatch base, deletion or insertion, linked pyrimidines, strand breaks, intra- and inter-strand cross-links. These DNA lesions could be genotoxic or cytotoxic to the cell. Plants are most affected by the UV-B radiation of sunlight, which penetrates and damages their genome by inducing oxidative damage (pyrimidine hydrates) and cross-links (both DNA protein and DNA-DNA) that are responsible for retarding the growth and development. The DNA lesions can be removed by repair, replaced by recombination, or retained, leading to genome instability or mutations or carcinogenesis or cell death. Mostly organisms respond to genome damage by activating a DNA damage response pathway that regulates cell-cycle arrest, apoptosis, and DNA repair pathways. To prevent the harmful effect of DNA damage and maintain the genome integrity, all organisms have developed various strategies to either reverse, excise, or tolerate the persistence of DNA damage products by generating a network of DNA repair mechanisms. A variety of different DNA repair pathways have been reported that include direct reversal, base excision repair, nucleotide excision repair, photoreactivation, bypass, double-strand break repair pathway, and mismatch repair pathway. The direct reversal and photoreactivation require single protein, all the rest of the repair mechanisms utilize multiple proteins to remove or repair the lesions. The base excision repair pathway eliminates single damaged base, while nucleotide excision repair excises a patch of 25- to 32-nucleotide-long oligomer, including the damage. The double-strand break repair utilizes either homologous recombination or nonhomologous endjoining. In plant the latter pathway is more error prone than in other eukaryotes, which could be an important driving force in plant genome evolution. The Arabidopsis genome data indicated that the DNA repair is highly conserved between plants and mammals than within the animal kingdom, perhaps reflecting common factors such as DNA methylation. This review describes all the possible mechanisms of DNA damage and repair in general and an up to date progress in plants. In addition, various types of DNA damage products, free radical production, lipid peroxidation, role of ozone, dessication damage of plant seed, DNA integrity in pollen, and the role of DNA helicases in damage and repair and the repair genes in Arabidopsis genome are also covered in this review.