At a molecular level, plants are rather leaky. They are perpetually sloughing off cells, leaking moisture, nutrients, carbohydrates and other compounds. Because the plant roots provide food, shelter and energy, areas of very high biological diversity are found directly on roots and the areas next to them. This area is collectively called the rhizosphere. The role of rhizobia, mycorrhizae, biological control organisms, and the whole of the soil food web in maintaining soil quality for crop production have been reported by various researchers. However, rhizobia and mycorrhizae are just a small portion of the total biodiversity in the soil. Although our knowledge has increased, we remain woefully ignorant about soil biology. There is a group of soil bacteria known as plant growth promoting rhizobacteria (PGPR). Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Inoculations of crop plants with certain strains of PGPR at an early stage of development improve biomass production through direct effects on root and shoot growth. Inoculation of agricultural crops with PGPR may result in multiple effects on early-season plant growth, as seen in the enhancement of seedling germination, plant vigor, plant height, shoot weight, and nutrient content of shoot tissues. PGPR are reported to influence the growth, yield and nutrient uptake by an array of mechanisms. There has been much research interest in PGPR and there is now an increasing number of PGPR being commercialized for various crops. Several reviews have discussed specific aspects of growth promotion by PGPR. In this review, we have discussed various bacteria which act as PGPR, mechanisms and the desirable properties exhibited by them.
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