Abstract

Plant growth in agricultural soils is influenced by many abiotic and biotic factors. There is a thin layer of soil immediately surrounding plant roots that is an extremely important and active area for root activity and metabolism which is known as rhizosphere. The rhizosphere concept was first introduced by Hiltner to describe the narrow zone of soil surrounding the roots where microbe populations are stimulated by root activities. A large number of microorganisms such as bacteria, fungi, protozoa and algae coexist in the rhizosphere. Bacteria are the most abundant among them. Plants select those bacteria contributing most to their fitness by releasing organic compounds through exudates creating a very selective environment where diversity is low. Since, bacteria are the most abundant microorganisms in the rhizosphere, it is highly probable that they influence the plants physiology to a greater extent, especially considering their competitiveness in root colonization. The present review deals with the following topics: Plant growth promoting rhizobacteria (PGPR), occurrence of PGPR, nitrogen fixation by PGPR, Bacillus species, Pseudomonas species, Production of plant growth promoting substances by PGPR isolates, PGPR as biocontrol agent and antagonistic activity of PGPR isolates against phytopathogens.

Key words: Biocontrol, plant growth promoting rhizobacteria (PGPR), Pseudomonas fluorescens, Bacillus subtilis.