Nitric oxide, nitrate reductase and UV-B tolerance.

Land plants rely on light for photosynthesis and have evolved leaves as light-capturing organs. As an undesired corollary, there is also a considerable absorption of damaging light, in particular ultraviolet B (UV-B) radiation (280–320 nm). In modern times, exposure to UV-B radiation is increasing progressively, due to thinning of the protective ozone layer by human activities, for example the production of halones such as chlorofluorocarbon compounds (CFCs). In the stratosphere, UV-mediated activation of CFCs generates halogen radicals, which initiate a chain reaction of ozone depletion (Andrady et al. 2010). UV-B radiation is dangerous because it impairs genome stability by damaging nucleic acids (Ries et al. 2000), such that UV-B-resistant Arabidopsis mutant uvi1 (UV-B insensitive 1) displays increased genome stability (Tanaka et al. 2002). The impact of UV light on genes and genomes is one of the driving forces of evolution (Tuteja et al. 2001), and plants have evolved various strategies to moderate this stress. Since UV damage often involves the production of free radicals such as reactive oxygen species (ROS) and nitric oxide (NO) (Tossi et al. 2009, Zhang et al. 2011), it is not surprising that the counteracting plant defence systems include various antioxidants such as ascorbate and glutathione (Jansen et al. 1998). While thickening of the leaf surface has been shown to be important at the morphological level (Jenkins 2009, Hayashi et al. 2011), most studies have focused on the production of UV-B reflecting and absorbing ‘sun-block’ chemicals. Flavonoids, as a prominent example, are phenylpropanoid compounds that can efficiently absorb light in the wavelength range between 240 and 545 nm. The role of flavonoids in preventing cellular damage through light irradiation is well established and they accumulate in the vacuoles of epidermal cells in order to minimize the absorption of UV within the mesophyll (Agati et al. 2009). Recently, various studies have pointed to the involvement of NO in response to UV radiation (An et al. 2005, Qu et al. 2006), which appears to act through abscisic acid-mediated steps (Tossi et al. 2009). Mechanisms of NO action in UV protection are now emerging, with a very recent study showing an effect by inducing chalcone synthase (CHS) and chalcone isomerase (CHI) genes linked to polyphenylpropanoid production (Tossi et al. 2011). An elegant study by Zhang et al. (2011) shows a link between nitrate reductase (NR)-mediated NO production and UV-B-induced flavonoid production (Figure 1). During the last decade enormous progress has been made in research on NO in plants, which has been shown to influence various physiological and developmental processes (reviewed in Gupta et al. 2011a, Mur et al. 2006). One of the hottest, and most controversial, topics in NO research is NO synthesis. Plants can produce NO through either oxidative or reductive pathways. The most well established is the NAD(P)H-linked reduction of nitrite by cytosolic NR under normoxic conditions. For efficient NO production, NR requires low concentrations of nitrate and high concentrations of nitrite because NR has a two times higher affinity for nitrite in comparison to nitrate (Rockel et al. 2002). NR-dependent NO increases during hypoxia due to a decrease in pH (Rockel et al. 2002) and is also affected by post-translational modification (Lillo et al. 2004). The existence of a plasma membrane-located nitrite NO reductase has also been demonstrated. Under low oxygen conditions, plant mitochondria can produce NO from nitrite at the site of complex III, IV and AOX (alternative oxidase) (Gupta 2011, Gupta and Igamberdiev 2011). As an alternative, oxidative pathways are based on a nitric oxide synthase (NOS)-like enzyme and hydroxylamine-mediated NO and polyamine-mediated NO are oxidative pathways (Gupta et al. 2011a). In all oxidative NO production pathways, l-arginine acts as a substrate or intermediate. However, the molecular bases of these biochemically defined pathways remain to be elucidated. Among the oxidative pathways, the NOS-like enzyme has been linked Commentary