Induced resistance of plants to pathogenic microorganisms is characterized by hypersensitive response and accumulation of antimicrobial compound phytoalexins. The phytoalexins with the highest antifungal activity is the isoflavonoid based group of pterocarpans (Jimenez-Gonzalez et al., 2008). Naturally occurring pterocarpans showed antifungal activity towards Monilinia fructicola (Perrin and Cruickshank 1969).
Pterocarpans such as medicarpin from alfalfa (Medicago sativa), pisatin from pea (Pisum sativum) and maackiain from chickpea (Cicer arietinum) are major class of antimicrobial phytoalexins in legumes (Naoumkina et al., 2010). (-)-Pterocarpans are biosynthesized from isoflavones. Soybean produces (-) type pterocarpan phytoalexins that are called glyceollins (Uchida et al., 2017). Phaseolus species which are closely related to Glycine (belonging to Phaseoleae) produce pterocarpan phytoalexin (-)-phaseollin. (-)-Maackian is produced by legumesuch as Maackia, Trifolium and Cicer, whereas, (-)-medicarpin is found in legumes including Glycyrrhiza and Medicago. The isoflavan (-)-vesitol which is biosynthesized by reduction of (-)-medicarpins the major phytoalexin in Lotus (Uchida et al., 2017). Example of (+)-type pterocarpan phytoalexin include (+)-pisatin from pea, (+)-medicarpin from peanut (Arachis hypogea) and (+)-maackiain from Sophora japonica (Van Etten et al., 1983; Finefield et al., 2012; Celoy and Van Etten 2014). Increased medicarpin production leads to effective resistance of alfalfa to the leaf spot pathogen Phoma medicaginis indicating involvement of isoflavonoid in disease resistance (He and Dixon 2000).
The pterocarpan phytoalexin, medicarpin has antifungal activity (Jeandet et al., 2013) towards Fusarium (Kakuda et al., 2016). Medicarpin and maackiain showed antifungal activity towards Fusarium oxysporum f. sp. ciceri the pathogen causing wilt disease in chickpea (Cicer arietinum L.).The pterocarpans were significantly greater in wilt resistant cultivar indicating an association between phytoalexin and resistance (Stevenson et al., 1997). Medicarpin and maackiain phytoalexin inhibit fungal spore germination and germ tube growth (Duczek and Higgins 1976). The germ tube growth of Fusarium roseum was inhibited by maackiain (McMurchy and Higgins 1984). Sophora japonica leaves produced pterocarpanoid phytoalexinsmedicarpin and maackiainin response to inoculated fungus Helminthosporium carbonum (Van Etten et al., 1983). The antimicrobial activity of medicarpin, vestitone, 2’-hydroxyformononetin (2’-OHF), formononetin and diadzein was tested against eight phytopathogenic fungi. Phytophthora megasperma f. sp. medicaginis was strongly inhibited by medicarpin, vestitone and 2’-OHF; whereas, Phoma medicaginis was strongly inhibited by medicarpin and vestitone but 2’-OHF was not a significant inhibitor and the three strains of Nectria haematococca were inhibited to varying extent by medicarpin and its precursors (Blount et al., 1992).
Glyceollin accumulation during infection by Phytophthora sojae and Macrophomina phaseolina contributes to soybean defense response (Lygin et al., 2013). Phytoalexins glyceollins was induced in different varieties of Korean soybean upon fungal infection (Lee et al., 2010). Phaseollin accumulated in bean leaves (Phaseolus vulgaris) inoculated with Pseudomonas mors-prunorum and P. phaseolicola, the isoflavonoid coumestrol also accumulated in infected leaves and inhibited growth of P. mors-prunorum and P. phaseolicola (Lyon and Wood 1975). Rhizoctonia solani hyphae when exposed to phaseollin isoflavonoids resulted in cessation of protoplasmic streaming and shrinkage of hyphal tip protoplasts (Van Etten and Bateman 1971). Therefore it is hypothesised that phaseollin acts on the plasma membrane or affects some process needed for membrane function. The relative role of glyceollin, lignin and the hypersensitive response in controlling the spread of pathogen was studied in soybean cultivars inoculated with Phytophthora sojae (Mohr and Cahill 2001). It was observed that glyceollin restricted the spread of the pathogen.
Naturally occurring coumestans are wedelolactone, coumestrol, aureol, medicagol and flemichapparin C exhibiting wide spectrum of biological activities (Song et al., 2019). Many of their biological effects can be attributed to their action as phytoestrogens and polyphenols (Nehybova et al., 2014). Coumestrol has a strong antioxidant activity and confers partial resistance in soybean plants against Cercospora leaf blight (Silva et al., 2018). Coumestrol accumulated in alfalfa in response to infection by pathogenic fungi (Sherwood et al., 1969). The phytoalexins coumestrol and glyceollins are increased in soybean plants after exposure to Aspergillus species (Boue et al., 2000). Coumestrol shows bactericidal activity towards Bacillus subtilis, B. licheniformis, Staphylococcus aureus, Streptococcus thermophillus and Xanthomonas campestris pv. glycines (Fett and Osman 1982).
Accumulation of isoflavonoid phytoalexin resulted in faster defense response. The bioactive properties of these natural compound may be used as compound inducing plant resistance.
See Part V (Ei) for further information ………
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