UV Damaged DNA Repair & Tolerance in Plants

IntroductionIn both natural and agricultural conditions, plants are frequently subjected to unfavourable environments, resulting in varying degrees of stress.UV irradiation, drought, heat shock, chilling/freezing, salinity and oxygen deficiency are a few of the major abiotic factors restricting plant growth and development.An important consequence of stratospheric ozone depletion is increased transmission of solar Ultra Violet (UV) radiation through the earth's atmosphere.This increased incidence of UV irradiation causes detrimental effects to all life forms on earth. UV irradiationThe spectrum of solar electromagnetic radiation striking the earth's atmosphere ranges from 100 nm to 1 mm.This includes the UV spectrum (100-400 nm), visible spectrum (380-780 nm) and infrared spectrum (700 nm-1 mm).The UV spectrum is further subdivided into three catogories: UV-C (100-280 nm), UV-B (280-315 nm), and UV-A (315-400 nm) (Ballaré, 2003).The shortest of these wavelengths, UV-C, is blocked completely by the ozone layer and atmospheric oxygen.In contrast, UV-A is weakly absorbed and directly transmitted to the earth's surface.Wavelengths in the UV-B range are absorbed efficiently though not completely by ozone, as a very small percentage may pass through holes in the ozone.UV-C is extremely harmful, followed by UV-B, while UV-A has milder effects (Batschauer et al., 1999). Plants and UV radiationPlants, due to their non-motile nature, generally have a higher rate of UV tolerance than animals.Plant secondary metabolites aid in defence against both abiotic and biotic stress factors.Plants are capable of reflecting or absorbing harmful UV rays via thick layers of waxy cutin or submerin on the cell walls and intracellular accumulation of chemical substances such as flavanols or phenolics.The biological effects of UV radiation on plants include altered growth responses, reproductive deformities, epigenetic variations, plant susceptibly to biotic factors, premature senescence, damage to the photosynthetic apparatus, and altered conformation of membrane structures.A wide array of genes were found to be induced upon prolonged exposure to low doses of UV-B in the model plant Arabidopsis thaliana (Frohnmeyer & Staiger, 2003;Mackerness, 2000;Ries et al., 2000).Upregulated transcripts include: antioxidant/free radical scavenging enzymes, proteins involved in: DNA repair, translation, E3 ligase system, cell cycle, signal transduction, and secondary www.intechopen.comSelected Topics in DNA Repair 74 metabolites, as well as several other genes with unknown function (Brosché et al., 2002; Jansen et al., 2008).UV-B also results in numerous changes in plant morphology.This signalling cascade is well reviewed elsewhere (Jenkins, 2009).Here we focus on plant responses to UV-induced DNA damage. UV induced DNA damageUV-C/B radiation is directly absorbed by DNA, inducing lesions which inhibit vital cellular functions such as transcription and DNA replication.UV-A is comparatively less efficient in lesion induction but triggers the production of reactive oxygen species (ROS) (Kunz et al., 2006).The primary UV induced DNA lesions include cyclobutane pyrimidine dimers (adjacent pyrimidines covalently linked between C-5 and C-6 carbon atoms) and secondary lesions 6-4 pyrimidine-pyrimidone photoproducts (6-4 PP) (covalent linkage between the C-4 position of a pyrimidine to the C-6 position of an adjacent pyrimidine) (Fig. 1).In order to respond to this damage, plants employ specific mechanisms (Britt, 1999).In light conditions, photoreactivation catalyses dimer monomerizations while during dark conditions, Nucleotide Excision Repair (NER) excises these helix-distorting lesions.Finally, residual lesions are bypassed via translesion synthesis (TLS).

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