雾都孤儿℃Ultraviolet-B
Elevated UV-B radiation (UV-B) has pleiotropic effects on plant development, morphology, and physiology. Low influence UV-B stimulates distinct responses, such as the accumulation of UV-absorbing pigments. Low influence of
UV-B was also found to stimulate the transcript levels of a robust set of genes involved in stress responses (Casati and Walbot, 2003; Ulm et al., 2004). UV-B also stimulates production of ROS (Casati and Walbot, 2003) and antioxidant defenses (Rozema et al., 1997; Jansen et al., 1998). It has been proposed that ROS not only function as destructive radicals, but also as signaling molecules during UV-B responses (Casati anf Walbot, 2003). High influence of UV-B photons cause cellular damage by generating photoproducts in DNA (Sinha and H?der, 2002) and direct damage to proteins (Gerhardt et al., 1999). These responses are considered to play a protective role against potential damage by UV irradiation (Solovchenko and Schmitz-Eiberger, 2003). The most effective protection mechanism stimulated under such a light regime is the biosynthesis of flavonoids and other UV-B-absorbing phenolic components (Li et al., 1993; Landry et al., 1995). These flavonoids generally absorb in the 280-315 nm region and thus are capable of acting as UV filters, thereby protecting the underlying photosyn-thetic tissues from damage. Plants subjected artificially to UV-B radiation respond by changes in the pathway of flavonoid synthesis. Changes have been observed not only in the levels of flavonoids in epidermal cells of the adaxial leaf surface, but also in flavonoids in the leaf wax and in leaf hairs (Hirner et al., 2001).
Anthocyanins served as an important class of flavonoids; its accumulation stimulated by low fluence UV-B radiation was observed in maize (Sharma et al., 1999), rice (Reddy et al., 1994), apple fruits (Arakawa et al., 1985), kangaroo paw (Ben-Tal and King, 1997), apple flowers (Dong et al., 1998) Arabidopsis (Christie and Jenkins, 1996), and roses (Maekawa et al., 1980). UV-B increased the accumulation of anthocyanins via stimulating the expression of genes encoding enzymes in the anthocyanin biosynthetic pathway (Fuglevand et al., 1996). CHS, DFR, and F3H showed a positive correlation with anthocyanin accumulation in UV-B-irradiated lettuce leaves (Park et al., 2007). UV-B increased expression of stress response and ribosomal protein genes, whereas photosynthesis-associated genes were down-regulated, which is also a kind of protection mechanism of plants initiated by UV-irradiation. Genes involved in stress responses increasing upon UV-B exposure were observed in maize (Casati and Walbot, 2003) and Arabidopsis (Ulm et al., 2004). CHS and PAL transcripts were shown to accumulate in epidermal cells following UV irradiation using in situ hybridization and immunolo-calization techniques (Schmelzer et al., 1988). Until now, many UV-activated signaling components have been identified (Stratmann et al., 2000); however, the actual signal transduction pathways activated by UV-B radiation are not yet well defined.
Ultraviolet-A
Since UV-A is hardly absorbed by ozone, UV-A radiation
Guo et al. 4969
reaching the earth’s surface is much stronger than other wavelength of UV rays. Sometimes UV-A caused similar responses to low fluence UV-B such as necrosis observed in UV-sensitive Arabidopsis mutants defective in succinic-semialdehyde dehydrogenase (Bouché et al., 2003). Besides, UV-A induction of anthocyanin accumulation was also observed in grape (Kataoka et al., 2003), Arabidopsis (Christie and Jenkins, 1996), eggplant (Toguri et al., 1993), carrot cells (Hirner and Satz, 2000), and kalancho? (Hoffmann, 1999). In addition, it is known that many horticultural crops, including fruits of eggplants (Matsumaru et al., 1971) and petals of Primula malacoides (Kashiwagi et al., 1977), show poor pigmen-tation in a greenhouse covered with UV-A-absorbing films, while the pigmentation was recovered when the UV-A cut film was replaced with UV-B cut films, which are commonly used for the production of these crops.
The accumulation of anthocyanin under UV-A was realized via induction the expression of anthocyanin biosynthesis genes. cDNA microarray made by unique gene fragments of subtraction library showed 81 genes were regulated including cytochrome P450, PAL, F3H, ANS, CHS, DFR and GST gene fragments related to anthocyanidin biosynthesis (Xu and Li, 2006). Phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), and anthocyanidin synthase (ANS) genes increased during a 24 h exposure to UV-A in turnip (Zhou et al., 2007) and in cell culture of carrot (Hirner et al., 2001). The production of anthocyanins by UV-A exposure may have contributed to the protection of the plant tissue from the potential damage by UV absorption, which may be responsible for the observation that UV-A did not have a strong impact on gene expression profiles as observed for low fluence UV-B response.
Photoreceptors
Photoreceptor evolved gradually when the plants evolved to adaptation light. Until now, three types of photoreceptors have been characterized: (1) A red, far-red-reversible chromoprotein, phytochrome for the absorption of red and far-red light (600-750 nm). (2) Cryptochromes that mediate several responses of blue light and UV-A (320-500 nm). More recently, another blue-light-absorbing chromoprotein, phototropin, has been identified as a photoreceptor mediating photo-tropism. (3) Photoreceptors for UV-B (282-320 nm). However, it has not been isolated until now (Giliberto et al., 2005; Briggs and Olney, 2001). All three types of these photoreceptors could potentially sense the UV-A signal (Mancinelli, 1986). UV-A responses in Arabidopsis could be replaced by blue light (Feinbaum et al., 1991; Fuglevand et al., 1996), indicating the involvement of UV-A/blue photoreceptors. However, some anthocyanin biosynthesis genes, i.e. CHS and F3H, are induced only upon UV-A exposure (Zhou et al., 2007). Both UV-B and
4970 Afr. J. Biotechnol.
UV-A/blue pathways involve reversible protein phospho-rylation and require protein synthesis. The UV-B and UV-A/blue light signaling are therefore different from phytochrome signal transduction pathway regulating CHS expression in other species (Christie and Jenkins, 1996).
OTHER ENVIRONMENTAL FACTORS
Biosynthesis of the flavonoid compounds were differentially modulated by environmental (exogenous) and developmental (endogenous) factors. Besides light, the factors involved include various stresses (wound, infection, low temperature), certain growth regulator (ethylene, auxin), and nutritional factors. Many flavonoid compounds are induced in response to wounding or to feeding by herbivores. These flavonoid compounds may act directly at defense compounds or may serve as precursors for the synthesis of lignin, suberin, and other wound-induced polyphenolic barriers (Hahlbrock and Scheel, 1989). Plants also produce phenolic compounds under fungal or bacterial attack or infection. They may also serve to prevent microbial infection in an otherwise nutrient-rich environment (Fukasawa-Akada et al., 1996). Levels of anthocyanins increase following cold stress (Christie et al., 1994) and nutritional stress (notably phosphate limitation). Water deficient increased antho-cyanin accumulation in grape berries, which resulted from earlier and greater expression of the genes controlling flux through the anthocyanin biosynthetic pathway, including F3H, DFR, UFGT and GST(Castellarin et al., 2007). Low nitrogen induces flavonoid and isoflavonoid nod gene inducers and chemoattractants for nitrogen-fixing symbionts, whereas low iron levels can cause increased release of phenolic acids, presumably to help solubilize metals and thereby facilitate their uptake (Graham, 1991).
Phenylalanine ammonia-lyase (PAL) is a key enzyme in phenylpropanoid metabolism and catalyzes the conversion of phenylalanine to trans-cinnamic acid, the first step in the biosynthesis of phenylpropanoid. Changes in PAL activity can occur during growth or may follow traumatic or pathological events or the action of light (Br denfeldt and Mohr, 1988). Red light, acting via phytochrome, stimulates PAL activity in the cotyledons and hypocotyls of tomato seedlings, and exposure to UV-B has been shown to stimulate PAL activity in rice, maize, and turnip (Reddy et al., 1994; Sharma et al., 1999). PAL activity can be induced by various stresses such as chilling, wounding, ozone, pathogen invasion (Lafuente et al., 2003), the plant hormone ethylene, and plant signal molecules, including jasmonic acid, salicylic acid, and MeJA (Campos-Vargas and Saltveit, 2002; Kim et al., 2007). Anthocyanin and PAL activity in strawberry plants is increased when treated with abscisic acid (ABA) (Jiang and Joyce, 2003). It was reported PAL is signifi- cantly induced by abiotic stresses, such as salt, cold and drought in tomato (Guo and Wang, 2008). Chalcone syn- thase (CHS) is the first flavonoid biosynthetic gene isolated from parsley (Kreuzaler et al., 1983). Its synthe-sis can be induced either by long-wavelength light, UV-light or certain chemical and biological compounds, which are called elicitors (Lawton et al., 1983). However, PAL and CHS showed no accumulation after cold treatment in the dark, suggesting that the cold-induced accumulation of PAL and CHS mRNA is light dependent (Leyva et al., 1995). These observations suggest light plays a regula-tory role in chilling and antioxidant signaling pathway. Some environmental stresses induce the accumulation of flavonoid compounds in the plant by activating their biosynthesis as well as inhibiting their oxidation (Rivero et al., 2001). It was reported that the mechanism signal transduction of PAL depends on the adhesion of plasma membrane-cell wall (Wang et al., 2006). Some pathways regulated by defense, salt and oxidative stresses are shared with pathways regulated by UV radiation. How-ever, UV-B radiation can activate additional pathways not shared with other stresses (Casati and Walbot, 2003).
