**4. Mechanism in plant growth promotion**

Modern agriculture is experiencing a number of challenges *viz*., poor soil fertility, serious pathogen and pest attacks, climate changes. Agricultural production must be sustainable and at the same time eco-friendly. This could be achieved by using environmentally sound approaches such as use of bio-fertilizers, bio-pesticides and by returning the crop residues to the soil thereby increasing the organic matter content of the soil. Application of crop residues to the soil resulted in increased yields compared to control [76]. Microbial inoculants which have been used for centuries, is a safer and relatively cheaper tool for promoting plant growth and improving soil health properties by different mechanisms [22]. Nitrogen fixing rhizobium bacteria live in association with legumes, infect them and form nodules in its roots. In case of non-legume crops they interact asymbiotically [77]. They are found in the rhizosphere to make use of the nutrients as the latter has plentiful nutrients oozed from roots of plants. They either have a direct or indirect control over plant growth,

by synthesizing phytohormones, control pathogen infestation by influencing the production of several enzymes like cellulase, protease, lipase and other such productions thereby inducing whole plant resistance against pests or by soil nutrient enrichment through their nitrogen fixation and phosphate solubilizing ability. Microbial inoculants have multiple beneficial effects, particularly as plant growth promoters (PGP). Not only this but PGPR also help in combating a variety of abiotic stresses like temperature stress, salinity as well as drought stress, heavy metal toxicity and other types of abiotic stresses [3]. According to their closeness and interaction with the plant roots Rhizospheric bacteria have been classified as: (1) rhizosphere occupying bacteria (2) bacteria's forming colonies at the surface of roots (3) bacteria's living inside the roots (endophytes); and (4) bacteria's residing in the cells of root nodules. Bacteria's that belong to these groups are known as plant growth promoting rhizobacteria (PGPR) [78]. The bacteria belonging to 1 to 3 categories as extracellular PGPR (ePGPR) while the 4th category was named as intracellular PGPR (iPGPR). The ePGR includes following genera: *Bacillus*, *Pseudomonas*, *Erwinia*, *Caulobacter*, *Serratia*, *Arthrobacter*, *Micrococcus*, *Flavobacterium*, *Chromobacterium*, *Agrobacterium*, and *Hyphomicrobium* whereas *Rhizobium*, *Bradyrhizobium*, *Sinorhizobium*, *Azorhizobium*, *Mesorhizobium* and *Allorhizobium* belong to iPGR category. To strengthen the use of soil rhizobia for the attainment of sustainable and eco-friendly production methods a basic understanding of their functioning and means by which they facilitate plant growth is needed.
