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Chapter presents a discussion on the term PGPR which underlines the need to have a uniform definition to be used by all authors. The actual biodiversity of PGPR will be illustrated by examples of genera and species chosen from the literature and their mechanisms of action for the following different groups: diazotrophs, bacilli, pseudomonads, and rhizobia. As PGPR are introduced in an ecosystem where intense interactions are taking place, we describe how plants, mycorrhiza, and soil fauna can influence the microbial diversity in the rhizosphere. Finally, the beneficial interactions between PGPR and symbiotic microorganisms in the -legume symbiosis, and in mycorrhizal plants are discussed. Interactions of PGPR with protozoa and nematodes are also examined.

Plant growth-promoting rhizobacteria can suppress diseases through antagonism between the bacteria and soil-borne pathogens, as well as by inducing a systemic resistance in the plant against both root and foliar pathogens. The generally non-specific character of induced resistance constitutes an increase in the level of basal resistance to several pathogens simultaneously, which is of benefit under natural conditions where multiple pathogens may be present. Specific strains induce systemic resistance in e.g. carnation, cucumber, radish, tobacco and , as evidenced by an enhanced defensive capacity upon challenge inoculation. Although some bacterial strains are equally effective in inducing resistance in different plant species, others show specificity, indicating specific recognition between bacteria and plants at the root surface. In carnation, radish and , the O-antigenic side chain of the bacterial outer membrane lipopolysaccharide acts as an inducing determinant, but other bacterial traits are also involved. Pseudobactin siderophores have been implicated in the induction of resistance in tobacco and , and another siderophore, pseudomonine, may explain induction of resistance associated with salicylic acid (SA) in radish. Although SA induces phenotypically similar systemic acquired resistance (SAR), it is not necessary for the systemic resistance induced by most rhizobacterial strains. Instead, rhizobacteria-mediated induced systemic resistance (ISR) is dependent on jasmonic acid (JA) and ethylene signaling in the plant. Upon challenge inoculation of induced plants with a pathogen, leaves expressing SAR exhibit a primed expression of SA-, but not JA/ethylene-responsive defense-related genes, whereas leaves expressing ISR are primed to express JA/ethylene-, but not SA-responsive genes. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways, as well as extend protection to a broader spectrum of pathogens than ISR or SAR alone.

Plant growth promoting rhizobacteria (PGPR) play a vital role in crop protection, growth promotion and in the improvement of soil health. Some well known PGPR strains are , and species. The primary mechanism of biocontrol by PGPR involves the production of antibiotics such as phenazine-1-carboxyclic acid, 2,4-diacetyl phloroglucinol, oomycin, pyoluteorin, pyrrolnitrin, kanosamine, zwittermycin-A, and pantocin. A cascade of endogenous signals such as sensor kinases, N-acyl homoserine lactones and sigma factors regulates the synthesis of antibiotics. The genes responsible for the synthesis of antibiotics are highly conserved. The antibiotics pertain to polyketides, heterocyclic nitrogenous compounds and lipopeptides have broad-spectrum action against several plant pathogens, affecting crop plants. In addition to direct antipathogenic action, they also serve as determinants in triggering induced systemic resistance (ISR) in the plant system. Though antibiotics play a vital role in disease management, their role in biocontrol is questioned due to constraints of antibiotic production under natural environmental conditions. Environmental and other factors that suppress the antimicrobial action of antibiotics have to be studied to exploit the potential of antibiotics of PGPR in crop protection.

Plant growth promoting rhizobacteria (PGPR) are indigenous to soil and the plant rhizosphere and play a major role in the biocontrol of plant pathogens. PGPR can profoundly improve seed germination, root development and water utilization by plants. These rhizobacteria can stimulate plant growth directly by producing growth hormones and improving nutrient uptake or indirectly by changing microbial balance in the rhizosphere in favour of beneficial microorganisms. They can suppress a broad spectrum of bacterial, fungal and nematode diseases. PGPR can also provide protection against viral diseases. The use of PGPR has become a common practice in many regions of the world. Although significant control of plant pathogens has been demonstrated by PGPR in laboratory and greenhouse studies, results in the field have been inconsistent. Recent progress in our understanding of their diversity, colonizing ability, mechanisms of action, formulation and application should facilitate their development as reliable biocontrol agents against plant pathogens. Some of these rhizobacteria may also be used in integrated pest management programmes. Greater application of PGPR is possible in agriculture for biocontrol of plant pathogens and biofertilization.

Many bacteria and fungi can enhance plant growth. The present review is limited to plant growth promoting rhizobacteria (PGPR). However, it includes endophytic bacteria that show plant growth enhancing activity as well. Also the best studied bacterial mechanisms of plant growth promotion are discussed, with a special emphasis on biological nitrogen fixation and synthesis of phytohormones, including less understood mechanisms like inhibition of plant ethylene synthesis, degradation of organic-P compounds, phenazine-related mineral solubilization, and synthesis of lumichrome. In addition, examples of PGPR genes that show activation in the interaction with plants, and beneficial events resulting from plant-bacterial interactions like stress relief and enhancement of other ecological associations are presented. Plant growth promoting activity and more precisely, crop yield enhancement are the final effects of the different mechanisms that PGPR possess and are the applicative goal of the agricultural microbiology research. Despite the undoubted economic and ecological benefits of utilizing some PGPR species as biofertilizers, the application of such a species must be very carefully assessed because of their importance as opportunistic pathogens in nosocomial infections and in patients with other diseases. On this basis, PGPR species must be selected for producing safe biofertilizers. Strain selection, as also the number of the bacterial cells, and characteristics of the bacterial cultures used in the production of biofertilizers, as well as, results of inoculation of different crops and cultivars with under field conditions are also included in the discussion.

Plant growth regulators (PGRs) are organic substances that influence the physiology and development of plants at very low concentrations. Cytokinins are one of the five major groups of PGRs or phytohormones and regulate cytokinesis in plant cells. Soil microorganisms are capable of synthesizing PGRs such as cytokinins in pure culture, soil and in association with plant tissues. This chapter reviews the structure and function of cytokinins in plant tissues and their production by plant growth promoting rhizobacteria (PGPR). A role for microbially-produced cytokinins in plant growth and development is proposed. Cytokinin production by PGPR is an innovative alternative to enhance plant growth and may be a sustainable approach to improve the yield and quality of agricultural crops. However further research is necessary to understand the principles underlying cytokinin production by rhizobacteria and to develop cytokinin-producing inoculants for practical application by growers.

Use of plant growth promoting rhizobacteria (PGPR) for the benefits of agriculture is gaining worldwide importance and acceptance and appears to be the trend for the future. PGPR are bioresources which may be viewed as a novel and potential tool for providing substantial benefits to the agriculture. These beneficial, free-living bacteria enhance emergence, colonize roots, stimulate growth and enhance yield. PGPR are known to induce resistance against various plant pathogens in different crops ranging from cereals, pulses, ornamentals, vegetables, plantation crops, spices and some trees. Most studies have emphasized exploration and potential benefits of PGPR in agriculture, horticulture and forestry. The plausible mechanisms adopted by these rhizobacteria in growth promotion and resistance, though abundantly documented but still remains to be fully explored. Integrated use of PGPR allows the combination of various mechanisms thereby enhancing their beneficial abilities. However, their use has not been to the full potential due to inconsistency in their performance and their commercialization limited to few developed countries. Use of PGPR as bioinoculants, biofertilizers and biocontrol agents, advantages and disadvantages, practical potential in improved agriculture and future prospects are also discussed.

The fungus f. sp. () causes foot and root rot of tomato, which can be controlled by various microbes including and non-pathogenic Microbes labeled with autofluorescent protein (AFP) markers can be visualized in live samples using confocal laser scanning microscopy (CLSM). This enables the simultaneous determination of both pathogen and biocontrol agent in the tomato rhizosphere and provides a better understanding of the biocontrol processes. Results of CLSM suggest that mechanisms of biocontrol of plant pathogens include inhibition of spore germination, competition for niches and nutrients, antibiosis, predation, parasitism, and induction of host resistance.

Plants are invaded by a large number of pathogens and they resist pathogen attacks with preformed defenses and by inducing defense responses. Nature is bestowed with many biocontrol agents including plant growth promoting rhizobacteria (PGPR) and species. PGPR colonise the rhizosphere and regulate plant growth by inducing defense responses in plants via an induced systemic resistance (ISR) and/or a systemic acquired resistance (SAR), increase the availability of nutrients to plants, produce growth hormones, suppress phytopathogens, release volatile compounds, secrete antimicrobial metabolites and decrease phytotoxic microbial communities in the rhizosphere. controls phytopathogenic fungi by secreting cell wall-degrading enzymes, antibiosis and stimulating plants to produce their own anti-microbial compounds. Though genome sequencing has already been done for some symbiotic and phytopathogenic bacteria, genome sequencing of five PGPR has been only established recently. K84 and four strains of , Pf0-1, Pf-5, Q8r1 and SBW-25, are being sequenced. The utilization of proteomics to explore biocontrol agents and their mechanisms in plant disease management is in the stage of infancy. It has the potential to revolutionize the way research is conducted on the biocontrol agents and plant defense mechanisms. The interaction between a biocontrol agent, a phytopathogen and a plant brings significant changes to the plant proteome and metabolism. Recently, globular and organellar proteomics approaches have been employed to study the changes in plant proteome after treating with biocontrol agent. In addition to biocontrol agents, proteomics studies on plant defense mechanisms against fungal, bacterial and viral pathogens are also discussed.

The export oriented agricultural and horticultural crops depends on the export of residue free produce and has created a great potential and demand for the incorporation of biopesticides in crop protection. To ensure the sustained availability of biocontrol agent’s mass production technique and formulation development protocols has to be standardized to increase the shelf life of the formulation. It facilitates the industries to involve in commercial production of plant growth promoting rhizobacteria (PGPR). PGPR with wide scope for commercialization includes , and other spp. The potential PGPR isolates are formulated using different organic and inorganic carriers either through solid or liquid fermentation technologies. They are delivered either through seed treatment, bio-priming, seedling dip, soil application, foliar spray, fruit spray, hive insert, sucker treatment and sett treatment. Application of PGPR formulations with strain mixtures perform better than individual strains for the management of pest and diseases of crop plants, in addition to plant growth promotion. Supplementation of chitin in the formulation increases the efficacy of antagonists. More than 33 products of PGPR have been registered for commercial use in greenhouse and field in North America. Though PGPR has a potential scope in commercialization, the threat of certain PGPR ( and ) to infect human beings as opportunistic pathogens has to be clarified before large scale acceptance, registration and adoption of PGPR for pest and disease management.