Symbiotic nitrogen fixation is a key to the nitrogen nutrition to the legumes. The most important agents for the symbiotic nitrogen fixation are the bacteria of the genus Rhizobium, which invade the root hairs of leguminous plant and develop nodules on the roots, where nitrogen fixation occurs.Rhizobium promotes growth of plants by fixing nitrogen from the atmosphere and is also a biocontrol agent which inhibits growth of pathogens. The biocontrol effect is due to the secretion of secondary metabolites.
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The present study describes the physiological, biochemical characterization and antagonistic activity of Rhizobium species isolated from root nodules of leguminous plant. The Rhizhobium sp. were rod shaped, gram negative and mucous producing. Rhizobium sp. was unable to hydrolysis the gelatin and showed sensitivity against antifungal Flucanozole.Antifungal activity of ten different Rhizobium isolates was tested against three pathogenic fungi. Some of these fungi are potential phytopathogens on legumes. Inhibition zones were observed, hence Rhizobium can be used as biocontrol agent .Keywords: Biocontrol effect, antifungal activity, antagonistic activity, inhibition zone Rhizobium sp.
Chemicals control plant diseases but contaminate the soil environment , degrade its fertility and also defile underground water, causing health risk. Thus, biocontrol agents emerge as an alternate to those chemicals which are inexpensive , ecofreindly and have no harmful effects on human , animals and plants (Deshwal et al. (2003). Rhizobia promote the growth of plants either directly through N2 fixation, supply of nutrients, synthesis of phytohormones and solubilization of minerals or indirectly as a biocontrol agent by inhibiting the growth of pathogens.
Due to the secretion of secondary metabolites such as antibiotics and HCN by rhizobia, they have the capacity to restrict the growth of fungal pathogen. In iron stress conditions in rhizobia, siderophore production provides an added advantage, resulting in the exclusion of pathogen due to iron starvation(Deshwal et al. (2003).Chakraborty and Purkayastha, (1984) reported that the charcoal rot fungus caused by the infection of Macrophomina phaseolina in soybean was inhibited by some rhizobitoxine – producing strains of Bradyrhizobium japonicum.
Phytophthora megasperma causes root-rot in soybeans, whose infectious activity was reduced by Rhizobium (Tu J.C, 1978). Rhizobial mechanism such as improvement in intake of plant nutrients by altering root morphology, production of siderophore (Antoun et al, (1998); Arora et al., 2001; Chabot et al., 1996) to meet the iron requirement of the plant under iron stressed conditions and lowering of ethylene through ACC deaminase enzyme are some of the examples with direct positive effects on non leguminous plant growth. Seven pathogenic microorganisms of soybean shows Inhibition of growth of by twenty B. japonicum strains studied by Balasundaram and Sarbhoy, 1988.
The fast growing rhizobial strains were found to completely inhibit the growth of white sclerotia of S. rolfsii.Several species of Rhizobium and Bradyrhizobium have shown antagonistic activity against M. phaseolina by inhibiting their growth (Deshwal VK et al., 2003; Arora NK et al.,1998). The rhizobia having biocontrol potential showed more competency in root hair infection in host plants as compared to non- biocontrol rhizobia. Rhizobia also appear to influence the plant defense mechanism by stimulating the production of phytoalexins by plants. Antibiotics produced by rhizobia have been found to play an important role in disease control. HCN, a secondary metabolite produced by several microorganisms, has deleterious effect on the growth of some microbes (Knowles CJ,1996).Studies conducted on numerous plant microbe interaction have shown that such antagonistic rhizobacteria could function by competition and antibiosis i.e. by producing antimicrobial compounds like bacteriocin (Rodelas et al., 1998; Joseph et al., 1983) but also indirectly induces systemic resistance against plant diseases. Rhizobia infect the roots of legumes and induce the formation of nodules, where nitrogen fixation takes place.
The enzyme system of bacteria supplies constant source of reduced nitrogen to the host plant and the plant in turn provides nutrients and energy for the activities of the bacteria (Singh et al., 2008). It has also been evaluated that Rhizobium increases plant growth by various ways such as production of plant growth hormones, vitamins, siderophores, by solubilisation of insoluble phosphates, induction of systemic disease resistance and enhancement in stress resistance (Hussain et al.,2009).
Some Rhizobium spp. have shown antimicrobial activities towards Pseudomonas sp., (Kacem et al., 2009) Aspergillus niger (Yuttavanichakul et al., 2012) Huang and Erickson, 2007) Phytophthora cinnamomi (Malajczuk et al., 1984) Fusarium solani f. sp. Phaseoli (Buonassisi et al., 1986) Fusarium oxysporum f. sp. Lentis (Essalmani and Lahlou, 2003) with varying degree of growth inhibition. In the present study Rhizobium was isolated from the root nodules of soybean and its antagonistic activity was studied against pathogenic fungi such as Aspergillus niger and Fusarium oxysporum.
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