Disease resistance in plants depends on the activation of more than one biochemical pathway. The antimicrobial phytoalexins produced by Phaseolus vulgaris are the prenylated isoflavonoids kievitone and phaseollidin (Turbek et al., 1992).
Phaseollin and phaseollidin accumulated as a defense response in bean against the infection caused by Colletotrichum lindemuthianum. However, phaseollidin accumulated earlier than phaseollin and at greater concentration (Soriano-Richards et al., 1998). Phaseollin caused loss of electrolytes and betacyanin from beet root storage tissue and killed beet leaf protoplasts. The death of cell by phaseollin may result in loss of integrity of the tonoplast which allows the release of harmful plant metabolites or hydrolytic enzymes into cytoplasm (Hargreaves 1980). Bean plant response to nine different races of Pseudomonas syringae pv phaseolicola causing bacterial halo blight. The accumulation of isoflavonoid, phaseollin in bean restricted the bacterial growth (Bozkurt and Soylu 2011). Hypersensitive response in Phaseolus vulgaris was observed when leaf tissue was inoculated with pseudomonas phaseolicola which showed high concentration of isoflavonoid phytoalexins phaseollin (pterocarpan), phaseollinisoflavan (isoflavan) and kievitone (isoflavanone) (Gnanamanickam and Patil 1977).
Kievitone is an isoflavanone and occurs in hypocotyls of Phaseolus vulgaris L. infected with Rhizoctonia solani (Smith et al., 1973). It may contribute to disease resistance by restricting the spread of fungus Rhizoctonia solani in invaded tissue (Smith et al., 1975). High molecular weight cell wall component from Colletotrichum lindemuthianum elicited browning and phytoalexin accumulation. Kievitone was the main phytoalexin produced by the cotyledons (Theodorou and Smith 1979). The mycelial growth of Aphanomyces euteiches, Rhizoctonia solani and Fusarium solani f. sp. phaseoli was inhibited by kievitone (Smith 1976). Phaseollinisoflavan and kievitone are a strong bactericidal compound (Wyman and Van Etten 1978).
Neoflavonoid is a group of C-15 naturally occurring compounds which are related structurally and biogenetically to the flavonoids and to the isoflavonoids. Neoflavonoid is found in Guttiferae, the Papilionoideae (subfamily of Leguminosae) and is also present in Rubiaceae, Passifloraceae and Polypodiaceae (Donnelly and Sheridan 1988). They are based on 4-phenylcoumarin skeleton. Neoflavonoid include 4-arylcoumarins (neoflavones), 4-arylchromanes, dalbergiones and dalbergiquinols (Garazd et al., 2003). Depending on the position of phenyl ring three major classes of flavonoids can be identified: flavonoids (phenyl ring at the C-2 position), isoflavonoids (phenyl ring at the C-3 position) and neoflavonoids (phenyl ring at the C-4 position) (Sarbu et al., 2019). Beside these three major classes a fourth category is identified as minor flavonoid which consists of chalcones and aurones that do not have benzopyran backbone (Sarbu et al., 2019). Derivatives of neoflavonoid also named as 4-arylcoumarins possess biological activity like antimicrobial, antioxidant, cytotoxic etc. (Wang et al., 2017).
Neoflavones display antibacterial activity. Structural activity exhibited function related to antimicrobial activity. Lactone opened ring in neoflavonoids showed effective inhibition (Son et al., 2018). Garazd et al.(2003) present physico-chemical data of neoflavones, mesuagin and mesuarin neoflavones exhibit antibacterial activity. Neoflavonoid S(+)-3’-hydroxy-4’,2,4,5-tetramethoxydalbergiquinol and a benzofuran together with two known neoflavonoids from Dalbergia melanoxylon were tested for their inhibitory activity against microorganisms. All compounds showed weak activity or were inactive (Lin et al., 2020). The open-chain neoflavonoids can be subdivided according to their oxidation level giving dalbergiquinol, dalbergione and benzophenone groups (Donnelly and Sheridan 1988). It is interesting to compare the compounds of different oxidation levels isolated from one Dalbergia species.
According to the molecular structure the neoflavonoids can be divided into dalbergiphenols, dalbergiones, dalbergins, benzophenones and other types (Liu et al., 2017). The three neoflavonoids as listed:
are evaluated for inhibitory activity against three fungal and seven bacterial strains. However none of the three neoflavonoids showed potential antimicrobial activities in vitro (Wang et al., 2020). Neoflavonoid and various other isoflavonoids prevent colonization of plant by plant pathogens. They act as natural defense molecules for plant.
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