**3. Pathogen control mechanisms**

Phytopathogens are those organisms which have the potential to adversely affect growth, development as well as the physiological activities of the crop. Any deviation in the environment which favors the proliferation of these phytopathogens result in a rapid outbreak of the diseases, leading to the crop destruction. Thus, reducing the yield and causing considerable loss of productivity. To prevent the development of disease it is necessary to control the pathogen mostly when their level is low. The organisms involved in biocontrol process are called as biocontrol agents and most of the biocontrol agents such as bacteria, fungi, algae, and nematodes which are found in root zone i.e., rhizosphere could influence various properties of soil and plants and thus act as defense mechanism against attack by pathogens [21]. It has been reported that there are some beneficial bacteria which can bring some changes in the rhizosphere as well as in the plants, leading to the enhancement in the plant growth, development and productivity and as such protect the plant from outbreak of various diseases [22]. Rhizobium being one of the categories of microorganisms which comprises of bacteria which can develop the symbiotic relationship with leguminous plants. Thus, are regarded as important nitrogen fixing organisms which play

*DOI: http://dx.doi.org/10.5772/intechopen.102657 Potential Applications of Rhizobacteria as Eco-Friendly Biological Control, Plant Growth…*

significant role in the maintenance of soil fertility [23]. However, many species of rhizobia are also reported to reduce the development of various disease-causing fungi, thereby increasing the yield of legume crops [24]. Several rhizobial strains such as *Rhizobium leguminosarum*, *Sinorhizobiummeliloti* and *Bradyrhizobium japonicum* have the ability to suppress soil-borne pathogens such as *Rhizoctonia solani, Pythium spp*., *Fusarium spp.,* and *Macrophominaphaseolina* in both legumes and non-legumes thus can be used as biocontrol against various soil-borne diseases [25]. Godebo et al. [26] suggested that *rhizobium* species can be used as biocontrol agents, since it inhibited the growth of aphanomyces in vitro in pea. *Rhizobia in combination with Tricoderma spp. can act as potential biocontrol agent* [27]*.* Colonization behavior of *Sinorhizobium meliloti* in the alfalfa rhizosphere reported to be useful for biocontrol. The application of *Pseudomonas maltophilia* in combination with *Mesorhizobium* and PSB was reported to be more beneficial as it showed the reduction in root rot incidence [28]. The study above shows that significant reports have been presented which favors the use of rhizobia as biocontrol agent against soil-borne pathogens, apart from being responsible for biological nitrogen fixation, thus acting as a befitting alternate measure over chemical treatments to control the spread of various plant diseases. Rhizobium is an effective biocontrol agent which helps in bringing down the growth of phytopathogens by implementing various mechanisms which include phytohormone production, siderophore production, production of antibiotics, HCN production, production of lytic enzymes, metabolite production and phytoalexin production and induction of systemic resistance [29].

### **3.1 Lytic enzyme production**

Rhizobia produces several lytic enzymes which are responsible for degrading the cell wall of pathogens and as such are considered as an efficient source for biocontrol. Lytic enzymes produced by the rhizobia for biocontrol involves chitinases, cellulases, β-1,3-glucanase β-1,4-glucanase, β-1,6-glucanase, proteases, pectinase and amylases [30]. These enzymes are known to cause lysis of the fungal and bacterial cell walls and thus helps in controlling the population of plant pathogens [31]. Chitinase is a lytic enzyme which causes the lysis of pathogenic fungal cell wall through the disintegration of chitin in the cell wall of fungi and bacteria. This process involves the breakdown of glycosidic bond in chitin thus, reducing the chitin polymer into monomer. Endochitinase cleaves chitin randomly at internal points within the polymer of chitin and releases low molecular weight multimers and dimers. Exochitinase causes hydrolysis of chitin and releases di-acetylchitobiose with no monosaccharide or oligosaccharides formed. Protease is another lytic enzyme which prevents the protein of pathogen to effect plant cells as protease have the capacity to cause the breakdown of proteins of phytopathogens into smaller polypeptides or single amino acids. Some of the protease also involved in inactivation of extracellular enzymes of phytopathogenic fungi. Cellulases is another enzyme which causes the decomposition of cellulose. This reaction involves the hydrolysis of the 1, 4-β-D-glucosidic linkages in cellulose. The degradation of cellulose involves conversion of the cellulose into β-glucose which occurs by the combined action of important cellulolytic enzymes like cellulose / endoglucanases, exo-cellobiohydrolase/exo-glucanases and β-glucosidases. Cellulose is thereby converted into β-glucose by the synergetic act of all these cellulolytic enzymes. Glucanase are enzymes which causes hydrolysis of polysaccharide made of glucose subunits. This process involves two possible mechanisms viz., cleaving the glucose residues from the non-reducing end in sequence and breaking

the linkages along the polysaccharide chain at random points and smaller oligosaccharides are released [32]. Among all these enzymes, chitinase are considered to be the most important ones as it acts as prime constituent of biocontrol and protect the plant against phytopathogens. It has been reported that rhizobial isolates producing chitinase results in inhibition of pathogenic microbes [24]. Damping-off of fava bean (*Vicia faba*) was reduced when rhizobium spp. capable of producing chitinase was applied as seed treatment either separately or along with mycorrhizal fungi [24]. Rhizobium strains isolated from *Sesbania sesban* has been reported to be produce chitinase. Rhizobium sp. Strain RS12, which have the ability to produce chitinase controlled the diseases of chickpea caused by *F*. *oxysporum*, *S*. *sclerotiorum* and *M. phaseolina* by reducing the growth and development of mycelia [33]. Plant diseases caused by several phytopathogens like *A. niger, F. solani, F. oxysporium, B. cinerea*and *R. solani* were reported to be controlled by chitinase from rhizobia, thus the latter was regarded as efficient biocontrol agent (34). Ability of rhizobia to produce lytic enzymes such as chitinase, β-1, 3 glucanase, protease, and lipase which bring about the lysis of pathogenic fungal and bacterial cell walls was also reported in various plants [3]. In fava (*V. faba*) bean infection caused by fungal mycelia of *F. solani* was reduced significantly by chitinase, protease and lipase [34].
