Antagonism in Rhizobacteria: Application for Biocontrol of Soil-borne Plant Pathogens
Rhizobacteria play an important role in plant defense and could be promising sources of biocontrol agents. Antagonism between soil microorganisms is a common phenomenon. A number of investigators’ have documented antagonism of fungi, actinomycetes and true bacteria to root rot pathogens (Ortiz-Castro et al., 2009). Soil bacteria, particularly rhizospheric ones with antagonistic properties, reveal biological control effectiveness to some plant diseases, and are the most likely for development of biological control agents (BCAs) (Essghaier et al., 2009).
The rhizosphere of plant is a zone of intense microbial activity, and some bacteria from this zone, termed rhizobacteria, exhibit active root colonization in the presence of the existing native microfolora. Various species of rhizospheric bacteria like Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Arthrobacter, Bacillus and Serratia and endophytic bacteria like Pseudomonas, Bacillus, Xanthomonas and Erwinia have been reported to be associated with soil crops. Bacteria are estimated to occupy between 7% and 15% of the total root surface area (Gray and Smith, 2005). Rhizo-bacteria that exert beneficial effects on plant development are referred to as plant growth-promoting rhizo-bacteria (PGPR) (Kloepper and Schroth, 1978) because their application is often associated with stimulation of plant growth. It was found that bacteria that produced more compounds associated to phytopathogen biocontrol and/or plant growth had a higher efficacy for biocontrol (Mota et al., 2017).
Biological control, a bioeffector method with other living organisms to control pests (insects, mites, weeds, and plant diseases) (Flint et al., 1998), has been considered as effective approaches. The use of biological control measure is one of the best strategies available to combat the pests and diseases in an ecofriendly manner and much experimental work is being carried out all over world to assess its commercial acceptability and applicability.
Interest in biological control of plant pathogens has increased considerably over the past years, partly as a response to public concern about the use of hazardous chemical pesticides (Raaijmahers et al., 2002). Most approaches for biocontrol of plant diseases have used single biocontrol agents as antagonists to a single pathogen (Siddiqui et al., 2000). It is likely that most cases of naturally occurring biological control result from mixtures of antagonists for example, mixture of antagonists are considered to account for protection in disease- suppressive soils (Lemanceau and Alabouvette, 1991).
Biological control offers an important alternative to synthetic chemicals. The use of bacteria like Pseudomonas sp., Bacillus sp., and various other bacteria has been investigated because of their properties to produce antifungal metabolites and protect plants from fungal infection (Gupta and Kaushal, 2017). Bacillus sp. is considered safe biological agent and their potential as biocontrol agent is considered to be high (Kim et al., 2004; Etesami and Alikhani, 2017). Bacteria are also used for biocontrol of other bacterial or fungal pathogen, viruses and nematodes (Shalaby and Sedik, 2008; Alsohiby et al., 2016; Murphy ET AL., 2000; Saman, 2009). The mechanisms of biocontrol adopted by rhizobacteria include the production of antibiotics, bacteriocins, siderophores, hydrolytic enzymes and other metabolites, phytoalexins production, interference in quorum sensing, reduction in ethylene production, and induction of systemic resistance.
A comparative study was conducted and Bacillus thuringiensis isolates were identified from different localities of city Gujranwala which could be used as biocontrol agents (Yunus et al., 2016a). Similarly from other investigations B. subtilis isolates were found to produce protease enzyme (Yunus et al., 2017a) and Esterase enzyme (Yunus et al., 2017b). A total of 56 isolates, belonging to four different strains of antibiotic producing bacteria; B. subtilis (30), B. licheniformis (9), Streptomyces (12) and Actinomycetes (5) were found from soil samples. These results suggest that soil isolates, having antibiotic producing capability can be used commercially after proper standardization (Yunus et al., 2016b).
To improve efficacy of biological control, however, understanding of the mechanisms of action, nutrition, and ecology of biocontrol agents is needed. Such knowledge will lead to substantial progress in selection of superior strains, mass production, and appropriate formation of biocontrol organisms (Fravel, 1988). In vivo biocontrol agent selection is not a simple task owing to the diversity of agents and interactions with the host plant. Thus, it is required to develop efficient selection approaches to diminish costs and increase the possibility of choosing organisms that can be produced in a large scale at low cost and that retain their viability and efficiency for long periods.