Plants' responses to Ultra Violet-B radiation
Author: Dhammaprakash P. Wankhede
ICAR-National Bureau of Plant Genetic Resources, Pusa campus, New Delhi

Ultra-violet B (UV-B, 280–320 nm) radiation is a small component of minor component of the solar spectrum and is strongly absorbed by ozone in stratosphere. However, depletion of the ozone layer has led to an increasing amount of UV-B radiation at the earth’s surface. Higher level of UV-B has several harmful consequences for all living beings. Increased UV-B level have several physiological as well as morphological changes such as, reduced plant growth, shortened internodes, and thicken the wax of leaves and cuticular wax, thus, significantly impact agricultural ecosystems (Teramura and Sullivan, 1994; Jansen et al., 1998; Paul and Gwynn-Jones, 2003). UV-B has been shown to induce changes in secondary metabolites, which in turn affects numerous physiological processes. UV-B also induces accumulation of phenolic compounds (UV absorbing compounds) such as flavonoids, and lignin, sinapate esters etc. which as free radical scavengers reduces deleterious effects of irradiation (Rozema et al., 1997). These compounds have direct relation to resistance against higher levels of UV. It has been observed that mutants which fail to accumulate these compounds show hypersensitivity to UV-B radiation, and mutants with higher flavonoid and sinapate concentrations show increased resistance to UV radiation (Li et al., 1993; Jin et al., 2000; Bieza and Lois, 2001).

Nature of the response to UV-B is also dependent on the fluence rate, duration, and wavelength of the UV-B treatment (Brosche and Strid, 2003; Frohnmeyer and Staiger, 2003; Ulm and Nagy, 2005). In general, exposure to high fluence rates and short wavelengths of UV-B is likely to cause stress responses and possibly necrosis. Among the known damages are damage to DNA, proteins, membrane lipids and adverse effect on protein synthesis and photosynthetic reactions. A low fluence rate of UV-B, however, induces regulatory responses in plants. For example, lower levels of UV-B inhibit stem extension, stimulate cotyledon opening, promote the accumulation of flavonoids and regulate the expression of a range of genes (reviewed by Jenkins, 2009). These low fluence UV-B induced regulatory responses may not be stress responses, and are likely to be photomorphogenic in nature, comparable to responses mediated by phytochromes, cryptochromes and phototropins.

Differential gene expression in response to UV-B

A common feature of UV-B induced responses is transcriptional activation or repression of genes upon perception of an external stimulus (Yang et al., 1997). Several genes are known to have altered expression in response to UV-B (Brown et al., 2005; Xu et al., 2006; Brown and Jenkins, 2008). Different types of UV-B exposure regulate different sets of genes (Brosche et al., 2002, Brown and Jenkins, 2008). High fluence rates and short wavelengths of UV-B induce many genes normally expressed in defence, wound or general stress responses (A-H-Mackerness, 2000; Ulm and Nagy, 2005) whereas, low fluence rates shorter exposures and longer wavelengths of UV-B leads to increased expression of several genes involved in UV protection (Frohnmeyer et al., 1999; Jenkins et al., 2001). Transcriptomic studies in maize (Casati et al., 2006) and Arabidopsis (Ulm and Nagy, 2005) show that UV-B regulates a large number of genes concerned with a wide range of cellular processes. In C. roseus, UV induced transcription of genes encoding tryptophan decarboxylase (Tdc), strictosidine synthase (Str) and Mitogen activated protein kinase (CrMPK3) have been observed (Raina et al., 2011, Ramani et al., 2007).

In rice, suppression subtractive hybridization between two rice genotypes, UV-B-resistant-Lemont and the UV-B sensitive-Dular upon UV-B irradiation revealed differential expression of several genes related to defence responses. The genes encoding HECT domain-containing protein, ascorbate peroxidase, receptor-like protein kinase, ubiquitin carrier protein (Rad6), phenylalanine ammonialyase, 4-coumarate, CoA ligase and chalcone synthase showed enhanced expression to different degrees in two rice cultivars (Fang et al., 2009). Additionally, UV induced expression of different phytoalexin biosynthetic genes like OsKS4 (Kaurene senthase 4) is known (Shimura et al., 2007). UV induced expression of Mitogen Activated Protein Kinase cascade genes have also been shown in rice (Wankhede et al. 2013, 2016).

UV-B responsive expression of several genes triggered the initiation of studies for identification of promoter elements and transcription factors involved in these responses. Further, attempts have been made to understand how transcriptional regulation is coupled to UV-B signalling pathways. Detailed studies of DNA sequence elements that regulate transcription of the gene encoding the key flavonoid biosynthesis enzyme chalcone synthase (CHS) did not identify any UV-B-specific element (Kaiser et al., 1995; Hartmann et al., 1998). However, a UV-B-specific element was identified recently in the Arabidopsis ANAC13 gene, which encodes a putative NAC-domain transcription factor (Safrany et al., 2008). This element is necessary for induction by shorter wavelength, higher-fluence rate UV-B.

A majority of genes induced by UV-B encode transcription factors (Kilian et al., 2007) play key roles in UV-B responses. A few transcription factors which show UV induced expression are also involved in regulation of biosynthesis of UV protective phenolic compounds (Cominelli et al., 2008). The Arabidopsis basic leucine-zipper transcription factor ELONGATED HYPOCOTYL 5 (HY5) is required for the UV-B induction of a substantial number of genes including those with vital roles in UV protection (Brown et al., 2005). A few members of rice WRKY gene family also show UV-B inducible expression viz OsWRKY89 (named as WRKY101 by Zhang and Wang, 2005) in addition to MeJA, and infection by blast fungus M. grisea (Wang et al., 2007).

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About Author / Additional Info:
Scientist, Division of Genomic Resources, ICAR-National Bureau of Plant Genetics Resources, New Delhi , India