Grape, one of the most common fruits in the world, is exposed to a variety of bacterial, fungal and viral pathogens (Pearson and Goheen, 1988) leading to a significant decrease in yield (Montesinos, 2007). More than 10,000 species of oomycetes and fungi can cause diseases in plants, leading to a significant decrease in the quantity and quality of plant products (Agrios, 2005). Worldwide, the vast majority of the vine fields are planted with varieties of Vitis vinifera which is sensitive to P. viticola and requires regular fungicidal applications. The causal agent of grapevine an heterothallic oomycete known as Plasmopara viticola, is one of the most destructive and main world pathogens of the vineyards.
Saunders (Gregory, 1862 cit) and Lippincott (Lippincott, 1866) reported that the presence of wounded grapevine leaves provided suitable atmosphere for downy mildew infection. This disease was not clear until 1876 when Farlow (Farlow, 1876) correctly identified this disease and linked it to Peronospora viticola. It was identified that it developed in the absence of ozone, so the primary factor was not P. viticola, but injuries. It was first identified in Europe in 1878. It was introduced to Europe by American grape cuts. After a year, It was observed in france, nothern Italy in 1879. It was spreaded to Eastern Europa, Turkey and Greece, Spain and Portugal in 1881. Since the early 20th century, the disease has been a serious problem for viticultural areas (Gäumann, 1927). Due to favorable weather conditions and adequate control measures were not put into practice, vast of majority of grapevine fields were damaged in Germany, France and Switzerland. Since then, Downy mildew has been the most destructive fungal disease in Central Europe (Müller and Sleumer 1934; Mohr 2005; Agrios, 2005). Reproductive programs have been promoted in many countries to produce grapevines resistant to downy mildew for over 20 years (Doazan, 1980). Most of the grapevine varieties (Vitis vinifera L.) are susceptible to P. viticola, while some other Vitis strains show resistance to pathogens.
Depending on the severity of the disease and the climatic conditions, protection against Plasmopara viticola contains several applications of fungicides between budding / sprouting and fruit ripening (Schnee et al., 2013). Use of numerous fungicide is aimed to increase the production of high-quality yields.However, these fungicides have harmful effects for environment and human health. Even a natural micronutrient used long term in organic farming against downy mildew may cause harmful results due to accumulation in the soil (La Torre et al., 2011). In addition, pesticides increase production costs and cause resistant strains of P. viticola (Aziz et al., 2013; Bavaresco et al., 1997). In fact, secondary metabolits, which is synthesized by plants and exhibited antimicrobial properties, provide to protect themselves against pathogens.
Phytoalexins are antimicrobial and secondary metabolites (Kuc, 1995). They have been reported to have biological activity against most of pathogens and may be supposed as markers of plant disease resistance. From Vitaceae, phytoalexins have been the issue of a large number of studies because of their phytopathological effects (Jeandet et al., 2002). For this purpose, an alternative way is to detect elicitor compounds for induction of plant defense, but only several commercial products with weak effect have been admitted for disease control (Aziz et al., 2003, Saigne-Soulard et al., 2015).Therefore , it has become an urgent need that the development of several strategies such as the application of natural products less toxic and capable of being decomposed by bacteria or other living organisms. Unfortunately, synthetic fungicides have supported the development of resistant isolates of P. viticola (Matasci et al., 2008). Grapevines may produce various mechanisms in response to P. viticola infection, including callose synthase (Gindro et al., 2003), induced peroxidases (Kortekamp and Zyprian, 2003) lignification processes. However, grape-derived phenolic compounds such as stilbenes, found naturally in Vitaceae, have been known to play an important role in plant diseases(Chung et al., 2003). To date, more than 60 stilbenes have been identified in Vitis vinifera and exhibit a broad structural variation from monomers to hexamers (Pawlus et al., 2012; Rivière et al., 2012). Some of the stilbene phytoalexins, such as cis- and transresveratrol (Langcake and Pryce 1977), trans- and cispiceid (Waterhouse and Lamuela-Raventos, 1994), pterostilbene (Pezet and Pont, 1988), and piceatannol (Guerrero et al., 2010; Piotrowska et al., 2012) have been identified in grapes. They are also found in small quantities in lignified organs such as roots and canes, in roots and in seeds (Bavaresco and Fregoni, 2001). The role of the downy mildew infection on stilbene synthesis has been extensively researched (Bavaresco and Fregoni, 2001). It was reported that resveratrol dehydrodimers, the δ-viniferin and ε-viniferin, are the main dimers to be synthesized due to stress factor related to P. viticola (Pezet et al., 2003; Gindro et al., 2006). Pezet et al. (2004b), indicated that resistance was linked to the conversion of resveratrol to viniferin, while susceptibility was linked to piceid formation of resveratrol. It means that resveratrol metabolism may play an significant role in relation to resistance or susceptibility of grapevines. Thus for, the resistance mechanisms in Vitis are complex and include constituent substances that function as fungicide-like molecules (Pezet and Pont, 1992).
Synthesis of stilbenes in grape berries and leaves is stimulated in response to biotic and abiotic stresses factors. It has been reported that at least 20 different stilbene synthase (STS) genes are expressed in grapes following infection with Plasmopara viticola (Richter et al., 2005). The expression of STS genes is often stimulated in response to various biotic (virus, bacteria, fungal infections) and abiotic agents (low and high temperatures, UV-B light, wound) (Sgarbi et al., 2003, Solecka and Kacperska, 2003). In the past decade, it was reported that the activity of BABA (DL-3-Aminobutyric Acid) as a SAR (systemic acquired resistance) activator against P. viticola (Slaughter et al., 2008), and chitosan was reported to promote levels of resveratrol, viniferins and piceids (Aziz et al., 2006). It has been shown that treatment of BABA strongly reduced Plasmopara viticola sporulation and the accumulation of phytoalexins of the stilbene family increased over time after infection. Elevated levels of stilbene accumulation in grapevine leaves is attributed to resistance (Schnee et al., 2008). It has been suggested that the capacity and intensity of grapevines as indicators to generate stilbenes for plant resistance against fungal infection (Pezet et al., 1991). Induction of trans-piceide, transresveratrol, and more importantly, trans- ε-viniferin, trans- δ-viniferin and trans-pterostilbene was reported for a BABA-primed susceptible grapevine genotype (Slaughter et al., 2008). Also, well-known plant activators such as Benzothiadiazole (BTH) led to upregulation of stilbene synthase (Busam et al., 1997a). BTH is a constructional analog of salicylic acid (SA), which is known to play a role in the elucidation of systemic acquired resistance in a wide variety of plant species. Caffeoyl-CoA-O-methyltransferase as an enzyme that functions in lignin biosynthesis Martz et al., 1998), has been reported to be up-regulated by SAR activator BTH in grape and considered to play a role for disease resistance response (Busam et al., 1997a; Pasquer et al., 2005).
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