An Overview of Plant Growth Promoting Rhizobacteria (PGPR) for Sustainable Agriculture

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Soil bacteria beneficial to plant growth usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. The mechanisms of PGPR-mediated enhancement of crop growth includes (i) a symbiotic and associative nitrogen fixation; (ii) solubilization and mineralization of other nutrients; (iii) production of hormones e.g. auxin i.e. indole acetic acid (IAA), abscisic acid (ABA), gibberellic acid and cytokinins; (iv) production of ACC-deaminase to reduce the level of ethylene in crop roots thus enhancing root length and density; (v) ability to produce antagonistic siderophores, ß-1-3-glucanase, chitinases, antibiotics, fluorescent pigment and cyanide against pathogens and (vi) enhanced resistance to drought and oxidative stresses by producing water soluble vitamins niacin, thiamine, riboflavin, biotin and pantothenic acid. Increased crop production through biocontrol is an indirect mechanism of PGPR that results in suppression of soil born deleterious microorganisms. Biocontrol mechanisms involved in pathogen suppression by PGPR include substrate competition, antibiotic production, and induced systemic resistance in the host. PGPR can play an essential role in helping plants to establish and grow in nutrient deficient conditions. Their use in agriculture can favour a reduction in agro-chemical use and support ecofriendly crop production. Trials with rhizosphere-associated plant growth-promoting P-solubilizing and N2-fixing microorganisms indicated yield increase in rice, wheat, sugar cane, maize, sugar beet, legumes, canola, vegetables and conifer species. A range of beneficial bacteria including strains of Herbaspirillum, Azospirillum and Burkholderia are closely associated with rhizosphere of rice crops. Common bacteria found in the maize rhizosphere are Azospirillum sp., Klebsiella sp., Enterobacter sp., Rahnella aquatilis, Herbaspirillum seropedicae, Paenibacillus azotofixans, and Bacillus circulans. Similarly, strains of Azotobacter, Azorhizobium, Azospirillum, Herbaspirillum, Bacillus and Klebsiella can supplement the use of urea-N in wheat production either by BNF or growth promotion. The commonly present PGPR in sugarcane plants are Azospirillum brasilense, Azospirillum lipoferum, Azospirillum amazonense, Acetobacter diazotrophicus, Bacillus tropicalis, Bacillus borstelensis, Herbaspirillum rubrisubalbicans and Herbaspirillum seropedicae. Symbiotic N2-fixing bacteria collectively known as Rhizobia are currently classified into six genera; Rhizobium, Allorhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium and Sinorhizobium and 91 species. Their inoculation may increase nodulation and N2-fixation in legumes. All these Rhizobiumn spp. can minimize chemical N fertilizers by BNF, but only if conditions for expression of N2-fixing activity and subsequent transfer of N to plants are favourable. In this Chapter, PGPR role has been discussed in the process of crop growth promotion, their mechanisms of action and their importance in crop production on sustainable basis.