Isoflavonoid are a group of secondary metabolites common to the leguminous plants that play a role in plant defense responses and nodulation. Isoflavones have a role in plant disease responses (Dixon 2001) of soybean to pathogen attack (Wegulo et al., 2005). They can be divided into aglycones and glycones. Aglycones isoflavones are without sugar moiety for example genistein, daidzein, glycitein, formononetin and biochanin A (Dixon 2004). Glycosidic isoflavone such as daidzin, genistin, glycitin, ononin etc. are often present in high levels in healthy plants (Murakami et al., 2014; Durango et al., 2018). Absidia coerulea and Absidia glauca transform genistein and biochanin A. As a result of glycosylation of genistein and biochanin A, genistin and sissotrin is obtained (Sordon et al., 2017). Biochanin A and genistein have fungicidal property (Rivera-Vargas et al., 1993). Lotus leaves produce a number of isoflavans a specific class of isoflavonoid as a defense response against pathogens (Lanot and Morris 2005). High concentration of isoflavonoid phytoalexins was detected when cultivars were infected with in Cercospora arachidicola than when they were infected with Puccinia arachidis (Edwards et al., 1995). Weidenborner and Jha (1994) studied the structural-activity of isoflavonoid in relation to antifungal activities and observed that the unreduced structure of isoflavones has less inhibitory effect on the growth of fungi whereas, reduced isoflavones i.e. isoflavans showed very weak activity.
The effect of soybean isoflavones on pathogenic fungi was studied to corelate the fungistatic activity to their composition and structure (Naim et al., 1974). Guo et al. (2011) concluded soybean isoflavones daidzein and genistein that are released in rhizosphere may act as allelochemicals when interaction takes place between root and soil microbial community depending on the duration of monocropping. Continuous monocropping of soybean led to change in the soil microbial community structure which resulted in change of rhizosphere isoflavones. Both the isoflavones can be easily degraded in soil. During the process of biodegradation, diadzein and genistein exerted an effect on soil microbes and subsequently changed the microbial community structure (Guo et al., 2011). The zoospores of Phytophthora sojae arechemotactically attracted to isoflavones genistein and daidzein that are released by soybean roots (Tyler et al., 1996). Isoflavone genistein, daidzein and glicitein glycosides in soybean inhibited growth of Trichoderma lignorum, Rhizoctonia solani, Fusarium oxysporum, Pythium spp., Rhizopus spp. and Sclerotium rolfsii (Naim et al., 1974). The fungicidal activity of isoflavones from soybean and chickpea has been studied and it was observed that isoflavones and isoflavanones are variable in their activity whereas, isoflavans are moderately active inhibitors of fungal growth (Kramer et al., 1984). Armillaria mellea, the fungal pathogen causing root rot is able to convert the antifungal compound genistein from leguminous plant into an intermediate metabolites (Curir et al., 2006). Isoflavonoids of the biochanin A series showed high antifungal activity, while genistein isoflavan and other isoflavan with two hydroxyl and one methoxy group were fungitoxic (Weidenborner et al., 1990). Elicitation of plants with fungal phytotoxin prior to infection resulted in higher levels of prenylated iso-flavones, especially phytoalexins luteone and wighteone and their different glycoconjugates in comparison to those observed in plants infected only with fungal spores (Wojakowska et al., 2015).
Genistein is a free aglycone and inhibits fungal infection and spread of plant disease (Formela et al., 2014). As signal molecules, sugars stimulated accumulation of isoflavones including genistein. Infection with hemibiotrophic fungus Fusarium oxysporum enhanced the synthesis of genistein, wighteone and luteone in cells of embryo axes of yellow lupin with high endogenous level of sucrose, glucose and fructose (Formela et al., 2014).
Soybean leaves on attack by bacterial pathogen accumulate isoflavone aglucones, isoflavone glucosides and glyceollin. Isoflavone aglycones and isoflavone conjugates are induced in soybean leaves not only by pathogens but also by foliar insect herbivory (Murakami et al., 2014). Conjugation of isoflavone in the plant cell is related to storage in the vacuole (Yu et al., 2000). Glyceollin inhibits the growth of soil-borne fungal pathogen Fusarium solani f. sp. glycines (Lozovaya et al., 2004). Infection with zoospores of the phytopathogenic oomycetes Phytophthora megasperma f .sp. glycinea race 1 resulted in accumulation of glyceollin l in soybean (Schmidt et al., 1992).
Isoflavonone production pave way for rice plants to enter into symbiotic relationship with rhizobia. The membrane component play a role in the interaction of symbiotic or pathogenic bacteria with their host organisms, the function of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compound and at the same time to allow the influx of nutrient molecules (Nikaido 2003).
Isoflavone production affects rhizosphere structure of microbial community thereby affecting growth and health of plants.
See Part V (F) for further information ………
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