Isoflavonoid play a role in plant defense responses such as acting as precursors for phytoalexins biosynthesis (Ahmad et al., 2017). Phytoalexins and phytoanticipins, of which isoflavonoids form a major part, are the compounds used in defence. Aspergillus niger and Streptomyces sp. are the most adaptable fungi and actinomycetes respectively and are used as biotransformation strains because of their ability to use various isoflavones and flavonoids as substrate for biosynthesis and biotransformation (Wang et al., 2020). The symbiotic Rhizobium and Bradyrhizobium cells when are outside the plant are exposed to isoflavonoid phytoalexins that are normally associated with pathogenic infection (Phillips and Kapulnik 1995). As a part of defense response against pathogens, Lotus leaves have been reported to produce a number of isoflavans, a specific class of isoflavonoid (Lanot and Morris 2005).
Isoflavonoids are subdivided into following groups: Isoflavan, isoflavone, isoflavanone, isoflav-3-ene, isoflavanol, rotenoid, coumestane, 3-arylcoumarin, coumaronochromene, coumaronochromone and pterocarpan (Marais et al., 2006). Isoflavan inhibit mycelial growth (Colpas et al., 2003).
Isoflavonoid is derived from flavonone intermediate naringenin that is universally present in plants and play a role in plant defense responses (Sreevidya et al., 2006). Isoflavonoid inhibit pathogens and also serve as chemoattractants, promoters of microbial growth and inducers of nodulation genes in Rhizobium and Bradyrhizobium (Dakora and Phillips 1996). Isoflavonoid and 5-deoxy(iso)flavonoids produced by legumes act as plant defensive compound against pathogenic microorganisms and as chemical signal against symbiotic nitrogen fixation (Aoki et al., 2000). Glyceollin was inhibitory to the soil borne fungal pathogen Fusarium solani f. sp. glycines causing disease in soybean (Lozovaya et al., 2004).
The prenylation of flavonoids enhances their antibacterial, antifungal and other biological activities by increasing their lipophilicity and membrane permeability (Dixon and Pasinetti 2010). The prenylated flavonoid mobilizes from plastid to apoplast by transporting across both plastidial and plasma membranes (Sasaki et al., 2011). Prenylated isoflavones are distributed in legume plants. They possess antibacterial, antifungal and antioxidant activities (Sasaki et al., 2011). Ethylene mediated accumulation of isoflavonoids against the root pathogens in legumes was observed by Liu et al.(2017). Fungus inoculated leaflets of 55 Trifolium species were examined for the presence of isoflavonoid and non-flavonoid phytoalexins and the isoflavonoid derivatives belonging to the pterocarpen (Medicarpin, maackiain and 4-methoxymaackiain) and isoflavan (vesititol, isovestitol, sativan, isosativan and arvensan) classes were isolated from 50 species (Ingham 1978).
Isoflavonoids in soybean are grouped in five classes (Durango et al., 2018):
- Malonyl-glycosides (malonyl-genistin, malonyl-daidzin)
- Aglycones (daidzein, genistein and formononetin)
- Glycosides (daidzin, genistin)
Coumestrol, genistein show antifungal property (Rivera-Vargas et al., 1993). Genistein is a bacteriostatic agent rather than a bactericidal compound. Action of genistein indicated changed cell morphology (formation of filamentous cells) of Vibrio harveyi, inhibition in DNA, RNA and protein synthesis (Ulanowska et al., 2006). Glyceollin and coumesterol inhibited soybean bacterial leaf pathogen Xanthomonas campestris pv. glycines and secondary bacterial invaders (Fett and Osman 1982). Kim et al.(2010) demonstrated glyceollins derived from soybean seeds elicited with Aspergillus sojae possess antifungal property and can become an alternative to synthetic fungicide for controlling certain fungal diseases. Glyceollins antifungal effect was observed against Fusarium oxysporum, Phytophthora capsici, Sclerotinia sclerotiorum and Botrytis cinerea. Application of salicylic acid and isonicotinic acids resulted in higher concentration of isoflavonoid in soybean seedlings. Malonyl-daidzin, coumestrol and glyceollins were present in extract from soybean induced by these elicitors (Durango et al., 2018). Zacharius and Kalan (1990) studied isoflavonoid in soybean cell suspension when challenged with fungal and bacterial elicitors. They observed culture that darkened showed gradual loss of viability and accumulated the phytoalexin glyceollin. Such culture had higher level of isoflavonoid than those that did not darken or produced glyceollin under biotic stress. Isoflavones and isoflavanones accumulation in Phaseolus, Vigna and Lablab seedings were found on application of Rhizopus oryzae (Aisyah et al., 2016).
The hypersensitive response or wounding in soybean require activation of immediate surrounding cells to deploy defense responses to the cell wall glucan elicitor from the pathogen Phytophthora sojae. These proximal defense responses include accumulation of phenolic polymer and glyceollin phytoalexins (Abbasi et al., 2001). Soybean phenylpropanoid defense response to the wall glucan elicitor from Phytophthora sojae include accumulation of phenolic polymer and glyceollin in cells adjacent to the point of treatment and accumulation of conjugates of the isoflavones, diadzein and genistein in distal cell. Daidzein (is a glyceollin precursor) and genistein is both toxic to Phytophthora sojae are implicated in local potentiation for the glyceollin response suggesting induced distal defense potentiation against P. sojae (Park et al., 2002). Several pathogenic microorganisms are sensitive to isoflavonoid.
See Part V (E) for further information ………
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