**2. DNA damage caused by exposure to sunlight and abiotic stress**

Ironically, although sunlight is obligatory for photosynthesis and survival of plants, it also represents one of the major threats to their genomic integrity. This can be ascribed to at least three reasons:

First, sunlight contains energy rich UV-C (280 to 100 nm), UV-B (290 to 320 nm) and UV-A (320–400 nm) light. Whereas UV-C is filtered out in the atmosphere, UV-B and UV-A can reach earth's surface. Although the amount strongly depends on the latitude and elevation, as well as cloud cover and canopy density, due to their sessile nature plants are exposed throughout the day to this genotoxic stress. UV-light is a strong mutagen that is absorbed by the DNA and may lead to the generation of cyclobutane pyrimidine dimers (CPD) and to a lesser extent pyrimidine (6,4) pyrimidone dimers (Friedberg EC et al., 2006). Both photoproducts are DNA lesions that affect transcriptional processes and result in error-prone replication (Fig. 1) (Friedberg EC et al., 2006). Solar UV light can also indirectly cause DNA damage by ROS production in the nucleus (Iovine et al., 2009). ROS induce a broad range of DNA damage, which includes base and nucleotide modifications, especially in sequences with a high guanosine content, and may even cause strand breaks (Wiseman and Halliwell, 1996; Tuteja et al., 2001; Tuteja and Tuteja, 2001). Although the precise nature of ROS generated by UV-light is not fully resolved, it is well established that oxygenated nucleotides like 8-oxo-guanine that can be caused by the accumulation of hydroxyl radicals (•OH) after prolonged UV exposure in the cell (Yamamoto et al., 1992; Hattori et al., 1996).

Fig. 1. UV light and ROS as genotoxic stress factors.

the cell (Yamamoto et al., 1992; Hattori et al., 1996).

three reasons:

**2. DNA damage caused by exposure to sunlight and abiotic stress**

Ironically, although sunlight is obligatory for photosynthesis and survival of plants, it also represents one of the major threats to their genomic integrity. This can be ascribed to at least

First, sunlight contains energy rich UV-C (280 to 100 nm), UV-B (290 to 320 nm) and UV-A (320–400 nm) light. Whereas UV-C is filtered out in the atmosphere, UV-B and UV-A can reach earth's surface. Although the amount strongly depends on the latitude and elevation, as well as cloud cover and canopy density, due to their sessile nature plants are exposed throughout the day to this genotoxic stress. UV-light is a strong mutagen that is absorbed by the DNA and may lead to the generation of cyclobutane pyrimidine dimers (CPD) and to a lesser extent pyrimidine (6,4) pyrimidone dimers (Friedberg EC et al., 2006). Both photoproducts are DNA lesions that affect transcriptional processes and result in error-prone replication (Fig. 1) (Friedberg EC et al., 2006). Solar UV light can also indirectly cause DNA damage by ROS production in the nucleus (Iovine et al., 2009). ROS induce a broad range of DNA damage, which includes base and nucleotide modifications, especially in sequences with a high guanosine content, and may even cause strand breaks (Wiseman and Halliwell, 1996; Tuteja et al., 2001; Tuteja and Tuteja, 2001). Although the precise nature of ROS generated by UV-light is not fully resolved, it is well established that oxygenated nucleotides like 8-oxo-guanine that can be caused by the accumulation of hydroxyl radicals (•OH) after prolonged UV exposure in Second, ROS are commonly produced as metabolic byproducts in the chloroplasts, peroxisomes, and mitochondria (Foyer and Noctor, 2003). In fact it is estimated for mammals that per day ~180 guanines are oxidized to 8-hydroxyguanine in a single cell (Lindahl, 1993); and it is likely that this rate is even higher in photosynthetically active plants where chloroplasts continuously produce ROS. Furthermore, excessive light exposure as it may occur in mid-day under non-shaded conditions can overexcite the photosynthetic machinery. As a consequence, singlet oxygen (1O2) can be produced from triplet-state chlorophyll in the light- harvesting complex of photosystem II (PSII). In addition, byproducts of photosynthetic activities are superoxide (O2 -) and hydrogen peroxide (H202) that can derive from water-splitting activities of the oxygen-evolving complex of PSII, and superoxide can be generated on the reducing side of PSI by the Mehler reaction (Noctor et al., 2002) (Fig. 1).

Third, heat from the sunlight can lead to failure of the structural composition and enzymatic machinery within the cell. To prevent cellular collapse, plants have developed a variety of protective mechanisms, the most important being the cooling effect of water transpiration through stomata. However, this dependency on water availability, together with their immobility, make plants highly susceptible to water stress conditions that derive from drought, salinity, or cold. Abiotic stress unbalances metabolic processes including photosynthesis, which ultimately causes a general increase in ROS concentration in the cell (Vinocur and Altman, 2005; Jaspers and Kangasjarvi, 2010). Although ROS detoxifying defense mechanisms are in place in the organelles and the cytosol, under the stress conditions described above, these mechanisms may not provide sufficient protection. To avoid excessive mutations over prolonged exposure to abiotic stress, plant cells depend on efficient repair pathways.
