Plant resistance to pathogen relies on constitutive and pathogen inducible secondary metabolite. Secondary metabolites function as defense and signaling compound (Wink 2003). Plants contain diverse secondary metabolites such as sulfur compounds, saponins, cyanogenic glycosides and glucosinolates (Osbourn 1996), it may also include alkaloids, flavonoids and phenolics (Zaynab et al., 2018). Classes of secondary metabolites found in Arabidopsis thaliana are indole and indole sulfur compounds, glucosinolates, phenyl propanoids, flavonoid, terpenes and fatty acid derivatives and they have an essential role in defense against pathogens (D’Auria and Gershenzon 2005).
Brassica plant synthesizes wide range of sulfur-containing secondary metabolites, including glucosinolates and indole type phytoalexins (Bednarek 2012). Phytoalexins are low molecular weight lipophilic antimicrobial compounds that accumulates around infection site (Hammond-Kosack and Jones 1996). Defensive metabolites produced and stored constitutively in plant tissue are termed phytoanticipins, whereas, those synthesized de novo in response to infection are termed phytoalexins (van Etten et al., 1994). The distinction between a phytoalexin and phytoanticipin is not based on its chemical structure but rather on how it is produced. Phytoanticipins originates from existing precursor which are produced by a healthy plant right from the beginning of its growth and provides protection against the pest. Glucosinolate is an example of such precursors (Zukalova and Vasak 2002). Cyanogenic glycoside and glucosinolate occur as inactive precursors and are activated in response to tissue damage or pathogen attack (Osbourn 1996). Phytoalexins are substances not detectable before infection. The molecule that signals plant to start the process of synthesis of phytoalexins are called elicitors. Phytoalexin camalexin accumulate in response to pathogen invasion and limits its growth. Camalexin disrupts bacterial membranes (Roger et al., 1996).
Glucosinolates are sulfur containing secondary metabolites and are present in many species belonging to Brassicaceae family (Fahey et al., 2001). Glucoside known as glucosinolate is detected in Arabidopsis thaliana (Hogge et al., 1988). Glucosinolates (β-thioglucoside-N-hydroxysulfates) is the precursors of isothiocyanates. At least 120 different glucosinolates have been identified in plants. Glucosinolates and their breakdown products may possess fungicidal, bacteriocidal, nematocidal and allelopathic properties (Fahey et al., 2001). The genes encoding enzymes involved in tryptophan, camalexin and indole glucosinolate biosynthesis are induced in response to Phytophthora brassicae. However, Schlaeppi et al. (2010) observed that disease resistance of Arabidopsis to P. brassicae is established by sequential activity of the phytoanticipin indole glucosinolate and phytoalexin camalexin. Once glucosinolates are released into the soil from root exudate or the decay of plant organs, it may have effect on the rhizosphere community (Halkier and Gershenzon 2006). The growth of ectomycorrhizal species is stimulated by the hydrolysis products of glucosinolates (Zeng et al., 2003).
Phenylpropanoids are a large class of plant secondary metabolites derived from aromatic amino acids phenylalanine in most plants. It includes flavonoid, monolignols, phenolic acids, stilbenes and coumarins (Deng and Lu 2017). Plant root secrete coumarins which can mobilize iron (Tsai and Schmidt 2017). Coumarins (scopolin, scopoletin and ayapin) as probable phytoalexin is induced in response to pathogen attack. Scopolin is known to be less phytotoxic than ayapin and scopoletin. Accumulation of scolopin can be the basis for resistance to Sclerotinia sclerotiorum (Prats et al., 2006). Stilbenes may protect plant tissue against pathogen attack (Lattanzio 2013). Phenylpropanoid serve as low-molecular weight flower pigment, antibiotic (phytoalexin), UV protectant, insect repellent and signal molecule in plant-microbe interaction and also function as polymeric constituents of surface support structures, such as suberin, lignin and other cell component (Hahlbrock and Scheel 1989). One of the end-products of the phenylpropanoid pathway is lignin. Cell wall –bound phenolics appears to be involved in early defense against the fungus whereas, de novo formation of lignin and lignin-like polymer may be important at later stages of infection (Eynck et al., 2009). The plant cell wall strengthening play a role in providing resistance to Camelina sativa against necrotroph Sclerotinia sclerotiorum (Eynck et al., 2012). Phenolic compounds are antifungal, antimicrobial in action and protect plant from disease causing agents (Friend 1979). Another group of flavonoids present in A. thaliana are the anthocyanins, the red, purple and blue pigments of plants. The seed of A. thaliana contain proanthocyanidins which provides protection from pathogens. Flavonoid compound particularly isoflavonoids are main components in defense responses of legumes to pathogens in which they likely have roles in restricting microbial growth (Samac and Graham 2007). Flavonoids are suggested to be auxin transport inhibitors (Brown et al., 2001).
Plant secondary metabolite involved in plant defense affect the susceptibility or resistance trait of the affected plant.
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To be continued………….