PGPR and its Use

Some bacteria are associated with roots of crop plants and exert beneficial effects on their hosts and are denoted to as plant growth-promoting rhizobacteria (PGPR) (Kloepper et al., 1980). PGPR may promote plant growth directly through improving uptake of minerals and water or the production of growth-stimulating compounds, but in many cases improved growth can be attributed to the suppression of deleterious microorganisms that are harmful to the plant (Schippers et al., 1987; Glick et al., 1999). PGPR can, thus, promote plant growth by suppressing diseases caused by various soil-borne pathogens (Van Loon and Glick, 2004).

Genera of PGPR include Azotobacter, Azospirillum, Pseudomonas, Acetobacter, Burkholderia, Bacillus, Paenibacillus, and some are members of the Enterobacteriaceae. Direct use of microorganisms to promote plant growth and to control plant pests continues to be an area of rapidly expanding research. Rhizosphere colonization is one of the first steps in the pathogenesis of soil borne microorganisms. It is also crucial for the microbial inoculants used as biofertilizers, biocontrol agents, phytostimulators, and bioremediators. Pseudomonas spp. are often used as model root-colonizing bacteria (Lugtenberg et al., 2001). The beneficial effects of these rhizobacteria have been variously attributed to their ability to produce various compounds including phytohormones, organic acids, siderophores, fixation of atmospheric nitrogen, phosphate solubilization, antibiotics and some other unidentified mechanisms (Glick, 1995). Motile rhizobacteria may colonize the rhizosphere more profusely than the non-motile organisms resulting in better rhizosphere activity and nutrient transformation. They also eliminate deleterious rhizobacteria from the rhizosphere by niche exclusion thereby better plant growth (Weller, 1988). Induced systemic resistance has been reported to be one of the mechanisms by which PGPR control plant diseases through the manipulation of the host plant’s physical and biochemical properties.