Plant cell wall acts as a physical barrier to pathogen and is also a matrix where many protein involved in pathogen recognition is delivered (Cantu et al., 2008). Phytoalexins are low molecular weight lipophilic, non-selective (in relation to pathogen toxicity), antimicrobial compounds, which are synthesised and accumulated in plant cells in response to pathogen attack (Smith 1996). Most phytoalexins being lipophilic in nature are able to cross the plasma membrane. Phytoalexin accumulation as defense mechanism involves the same component such as a signal, a receptor and a responsive metabolic system (Smith 1996).                           

Stilbenes: Stilbenes are plant secondary metabolite. All flavonoid and stilbene phytoalexins and derivatives (dihydrophenanthrenes) are formed from the universal phenylpropanoic-polymalonic pathway (Jeandet et al., 2014).  Stilbenes biosynthesis require stilbene synthase. The most widely reported plant stilbene is resveratrol (3,5,4’-trihydroxy-trans-stilbene) (Kozlowska and Czekata 2017). Resistance of plant to pathogen infection is the result of multiple defense responses including constitutive and inducible barriers. Stilbenes participate in both constitutive and inducible defense mechanisms in plants (Chong et al., 2009). Stilbenes with fungicidal potential are formed in plant such as peanut (Arachis hypogea), grapevine (Vitis vinifera) and pine (Pinus sylvestris). Hain et al. (1993) report that regenerated tobacco plants containing stilbene synthase genes are resistant to infection by Botrytis cinerea.  Stilbenes phytoalexins are key defense molecules implicated in the resistance of grapevine cultivars to three major fungal pathogens, Botrytis cinerea (grey mold), Plasmopara viticola (downy mildew) and Erysiphae necator (powdery mildew). Resistant varieties respond rapidly to the infection by producing high concentration of toxic stilbenes, d-viniferin and pterostilbene at the site of infection (Viret et al., 2018). Downy mildew (Plasmopara viticola) is a destructive disease of Vitis vinifera. The leaf disc of resistant variety Bianca of Vitis spp., was inoculated with a suspension of P. viticola.  Chitarrini et al. (2017) observed that after 96 hours of post-inoculation, phenylpropanoids, flavonols, stilbenes and stilbenoids increased. Stilbenes phytoalexin showed antifungal activity against Plasmopara viticola (Dercks and Creasy 1989). The grapevine, Vitis vinifera produces metabolites resveratrol, ε-viniferin, α-viniferin and pterostilbene in response to infection. The latter three compounds are fungitoxic (Langcake 1981). Resveratrol a phytoalexin is produced by plant and provides disease resistance. Many roles have been ascribed to resveratrol and its derivative such as antimicrobial, deterrent or repellent compounds acting as allelochemicals (Jeandet et al., 2012).  Cellular response to resveratrol included rapid alkalinisation, accumulation of pathogenesis related protein 5 (PR5) transcripts, oxidative burst, actin bundling and cell death. Resveratrol in addition to its role as antimicrobial phytoalexin, regulates initiation of HR-related cell death (Chang et al., 2011). Stilbene phytoalexins from Vitaceae has antifungal activity (Jeandet et al., 2002). Pterostilbene is a phytoalexin produced by Vitaceae.  Pterostilbene can act on membrane proteins and interfere in mitochondrial respiration process (Pezet and Pont 1990).   

Lignin:  Lignin is a ubiquitous polymer present in cell wall of all vascular plant, it strengthens the cell wall structure through covalent linkages to cell wall polysaccharides. Lignin       synthesis is often induced at the site of pathogen attack (Sattler and Funnell-Harris 2013). Lignin content and composition have 10 enzymatic step of monolignol pathway, in which lignin monomers are synthesized from amino acid phenylalanine, then oxidatively polymerized into hydroxyphenol- (H-), guiacyl- (G-) or sinapyl- (S-) lignin (Sattler and Funnell-Harris 2013). Lignin deposition is a defense mechanism against pathogens (Nicholson and Hammerschmidt 1992). The accumulation of low molecular- weight phenols leading to the formation of biopolymers restricts the spread of the pathogens (example lignin and callose) (Obermeier et al., 2013). Saprophytic and pathogenic fungi induce lignification but yeast and bacteria are poor elicitors of lignification (Vance et al., 1980). The lignin deposited structure, functions as a physical barrier similar to casparian strip, trapping pathogen and thereby restricting their growth (Lee et al., 2019). Menden et al. (2007) have studied that syringyl-lignin accumulate in wheat cell during hypersensitive response upon fungal penetration. Lignification of walls may restrict diffusion of enzymes and toxins from fungus to the host and water and nutrient from host to fungus (Vance et al., 1980). Induction of lignin synthesis is a part of resistance response. An elicitor induced lignin released as lignin-extensin complex was found to be similar to early developmental lignins (Lange et al., 1995).  Plant defense against the pathogen infection is provided by synthesis of phenolics and their polymerization into cell wall (Matern and Kneusel 1988). Lignin and salicylic acid concentration increased during resistant interaction but abscisic acid reduced accumulation. Abscisic acid suppresses expression of many defense-related genes, including those important for phenylpropanoid biosynthesis and those encoding resistance-related proteins (Mohr and Cahill 2006). Funnell-Harris et al. (2010) suggest that impairing lignin biosynthesis does not reduce resistance to some pathogens.

                                                                                     See Part IV

                                                                                     Continue ……..


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