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15Strains of Pseudomonas, Bacillus and Azospirillum with plant growth promoting ability were 16 checked for their zinc solubilizing ability. Efficient zinc solubilizers were checked for their ability 17 to produce nano-scale zinc particles. The nanoparticles from the cell-free culture filtrates obtained 18 from these strains were characterized for particle size, Zeta potential and functional groups. 19Presence of Zn nanoparticles in the bacterial culture filtrate was confirmed by particle distribution 20 and Scanning electron microscope (SEM) analysis. Most properties of nanoparticles are size 21 dependent. Zinc nanoparticles were observed to be spherical in shape and size ranged from 52.0 22 to 106.0 nm. Zeta potential of the Zn nanoparticles was estimated to understand the stability of 23 the particles. The measured zeta potentials varied from -14.5mV to +179.10 mV indicating high 24 stability and dispersion of the zinc nanoparticles. FTIR peaks at different wave numbers depicted 25 the role of functional groups of proteins in the biosynthesis of Zn nanoparticles. This finding 26 opens a new area of research focusing on microbe-microbe interactions in rhizosphere and plant-27 microbe interactions at rhizosphere apart from biosynthesis of nanoparticles, which has major 28 2 applications. To our knowledge, this is the first report of production of nanoparticles as part of 29 nutrient mobilization by plant growth promoting rhizobacteria.30

With a substantial decline in the use of synthetic chemicals, the growing demand for agricultural production is a critical concern in today's world. The use of plant growth-promoting rhizobacteria (PGPR) has been found to be an environmentally sound way of increasing agricultural productivity by promoting plant growth either through a direct or indirect mechanism. PGPRs are commonly occurring soil microbes that colonize the root system, which is an ideal location for interactions with plant microbes. PGPRs can provide an enticing way of reducing the use of toxic chemicals and can affect plant growth and development, either through releasing plant growth regulators or other bioactive stimulants and by taking up nutrients through fixation and mobilization, minimizing adverse effects of microbial pathogens on crops by using numerous mechanisms. In addition, they also play a significant role in soil fertility. This chapter aims to explore the diversified plant growth mechanisms that promote rhizobacteria in fostering crop yields and promoting sustainable agriculture.

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