**1. Introduction**

200 Selected Topics in DNA Repair

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Living organisms are continuously exposed to factors that threaten the integrity of their cells. This includes structural and enzymatic components like lipids or proteins, but also their genomes. Damage to genetic material can be critical as unrecognized and unrepaired DNA damage may cause fatal mutations not only threatening the organism's immediate survival but also that of its descendants. These genotoxic factors can derive from their surrounding environment and may include chemicals or ionizing radiation; but DNA damage can also be caused by reactive oxygen species (ROS) that are byproducts of daily metabolism or result from insufficient protection against abiotic stress conditions.

UV light can cause direct DNA damage by generating 6-4 and CPD photoproducts (example given in Fig. 1 is a thymine dimer). UV like most abiotic stress conditions can also generate ROS production in the cell. ROS have a high potential to damage single bases by oxidation (example give is 8-oxoG (Fig. 1)), but are also capable of introducing single or double strand breaks.

In contrast to most animals, plants are sessile organisms that cannot change their location when exposed to unfavorable conditions such as drought or salinity. Plants also face the difficult situation that they depend on sunlight for photosynthesis, a process that on its own constitutively generates ROS (Asada, 1999; Krieger-Liszkay, 2005; Triantaphylides and Havaux, 2009). Sunlight also contains significant amounts of UV-B light, which can contribute to both ROS production in the nucleus as well as directly affecting the DNA structure. Sunlight and high production rates of ROS are two of the main factors that lead to many mutations in plants. Consequently, the current review will focus on mechanisms that plants have in place to recognize and repair damaged DNA caused by either of these factors. We will provide a brief overview on the different classifications of DNA damage that can be expected, how these damages are repaired, and what is known about regulatory and physiological mechanisms that are in place in plants to recognize and respond to DNA damage. Because plants have taken a different evolutionary path than animals and possess some unique features not found in animals, we will compare selected repair and regulatory pathways in animals and plants. Despite their differences, plants and animals share many aspects in damaged DNA recognition and repair, and for this reason we will conclude this chapter by elaborating on some opinions for using plants as powerful and valuable model organisms for animals to understand the underlying processes of DNA repair.

Recognition and Repair Pathways of Damaged DNA in Higher Plants 203

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

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

In plants the main repair pathway for direct DNA damage caused by UV-light that leads to the generation of CPDs and (6–4) photoproducts is based on the activity of photolyases (Jiang et al., 1997). Two types of photolyases have evolved that specifically recognize and repair either type of photodamage. Based on sequence homology, CPD photolyases are grouped into two different classes: while class I CPD photolyases are present in microorganisms, class II enzymes can be found in archaea, eubacteria, some animals (excluding placental mammals), and plants (Kanai et al., 1997). In comparison, (6-4) photolyases have been found in metazoans and plants, and they share sequence similarities

The structure and reaction mechanisms of photolyases have been intensively studied in the last decade, providing us with plentiful data on their function. Photolyases have two types of chromophoric co-factors that are used for photoreactivation (Huang et al., 2006; Ozturk et al., 2008; Hitomi et al., 2009). One chromophore is FADH-, the two electron reduced form of FAD, while the second one can be either methenyltetrahydrofolate (MTHF) or 7,8 didemethyl-8-hydroxy-5-deazariboflavin (8-HDF). MTHF or 8-HDF function as the light harvesting chromophores that absorb blue light (300-600 nm), and transfer the energy to

 (Moldt et al., 2009; Li et al., 2010; Okafuji et al., 2010). Photolyases bind directly to CPD and (6-4) photoproducts, where an electron is transferred from the excited FADH- to

al., 2002) (Fig. 1).

efficient repair pathways.

FADH-

**3. Major repair mechanisms in plants** 

with class I CPD photolyases (Kanai et al., 1997).

**3.1 Photoreactivation by photolyases** 

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