The PGPR and plant interactions benefit plants in different ways such as: improvement in seed germination rate, shoot and root weights, root development, leaf area, yield, protein content, hydraulic activity, chlorophyll content and nutrient uptake including phosphorus, potassium and nitrogen (Adesemoye and Kloepper, 2009). The mechanisms of PGPR in enhancing plant growth are not completely understood. But some mechanisms have been studied on how PGPR can promote and enhance plant growth including: increasing the supply or availability of primary nutrients to the host plant and also solubilisation of mineral phosphates and other nutrients. PGPR have ability to solubilizing potassium and phosphorus minerals and avoid environmental pollution hazards (Parmar and Sindhu, 2013). Nitrogen (N), potassium (K) and Phosphorus (P) are vital nutrient required for the development and growth of plant as well as microorganisms but they are not available in the form which plant and other organisms can utilize. Some PGPR have the ability to fix nitrogen, convert insoluble potassium (K) and phosphorus (P) to an accessible form, like orthophosphateand this increases plant yields they also aid in stimulation of nutrient uptake (Ranjan et al., 2013).
PGPR promotes the growth of plant directly by producing phytohormone (Egamberdiyeva, 2007), Examples of plant hormones are gibberellins, cytokinins, abscisic acid, and auxins. At very low concentrations, plant hormones influence plant physiological processes such as, host’s root respiration rate, root abundance and metabolism. Traits of phytohormone production are spread among some different groups of bacteria such as Azospirillum spp., Azotbacter paspal, Azotobacter vinelandii, , Azotobacter chroococcum, Bacillus megaterium, Acetobacter, Bacillus megaterium, Herbaspirillum Bacillus subtilis, and Rhizobium spp., produce indoleacetic acid, cytokinins and gibberellins in growth media (Ahemad and Kibret, 2014).
Phytohormone syntheses by PGPR influence plant physiological processes. Gibberellins are known to influence seed germination, flowering, stem elongation and development and fruit setting of plants (Saharan and Nehra, 2011). Auxins particulally indole acetic acid (IAA) and indole acetamide (IAM) also influence tissue differentiation, root development and responses to light and gravity. Another mechanism for growth promotion by PGPR (Glick et al., 2007) is lowering of ethylene levels in plants and this is done by synthesising enzyme called 1-amino-cyclopropane-1-carboxylate (ACC) deaminase that hydrolyzes the ethylene precursor ACC (Glick et al., 2007).
PGPR indirectly promotes growth through the elimination of pathogens by the production of antibiotics, e.g., phenazine antibiotics, siderophores (Suresh et al., 2010), hydrogen cyanide (HCN), protease, 2, 4- diacetylphloroglucinol (DAPG); phosphate and solubilizing enzymes. Siderophores also influence sequestering of iron for plants, biological control (Ahemad and Kibret, 2014), and help in delayed senescence. They can also enhance plants’ tolerance to adverse environmental stresses, such as drought and salt stress, nutrient deficiency, heavy metal contaminations and weed infestation (Sivasakthi et al., 2013).
Nitrogen is vital nutrient for plant growth and development. It is abundantly present as atmospheric nitrogen. Plants utilize nitrogen in the form of ammonia and nitrate and biological nitrogen fixation naturally provides nitrogen for plants and other organisms. Biological nitrogen fixation (BNF) is a process that involves the changes of the atmospheric nitrogen (N2) into a form called ammonia that can be used by plant (Hayat et al, 2010).
Several bacterial species associate with the plant rhizosphere and they belong to genera Azospirillum, Acinetobacter, Alcaligenes, Arthrobacter, Burkholderia , Bacillus, Erwinia, Serratia, Enterobacter, Pseudomonas, Flavobacterium, and Rhizobium. (Sharma et al., 2011). Biological nitrogen fixation contributes 180 X 106 metric tons per year globally and it has been studied that symbiotic associations’ syntheses 80% and the rest by free-living or associative systems.
Bacteria and Archaea have the ability to reduce and derive appreciable and viable amounts of nitrogen from the atmospheric reservoir to enrich and enhance the soil. Some organisms include Rhizobium, a symbiotic nitrogen fixer and also obligate symbiotic in leguminous plants (Hayat et al, 2010). Frankia associates in non-leguminous trees, and other non-symbiotic (free-living, associative or endophytic) N2-fixers are Azospirillum, Acetobacter, cyanobacteria, Azoarcus, Azotobacter and diazotrophicus. Several rhizobacteria aid to derive full benefit from root exudates by their ability to attach to the root surfaces (rhizoplane). Associative interactions between plants and microorganisms must have resulted through co evolution, the use of rhizobacteria as bio inoculants must be pre-adapted, so that it is sustainable in agricultural system for a very long-term (Hayat et al, 2010; Sharma et al., 2011).
Some microorganisms are Non-symbiotic nitrogen fixers, they live on rhizospherial environment and fix atmospheric nitrogen for plant in the soil. Some examples of non-symbiotic nitrogen-fixing bacteria include , Azospirillum, Achromobacter, Azoarcus sp., Azotobacter sp. Azomonas, Acetobacter, Arthrobacter, (Santi et al., 2013), Acetobacter, AlcaligenesBacillus, Beijerinckia, Herbaspirillium sp., Gluconacetobacter diazotrophicus, Corynebacterium, Enterobacter, Clostridium, Rhodospirillum, Rhodopseudomonas , Klebsiella, Pseudomonas, Derxia, RhodoPseudomonas and Xanthobacter (Santi et al., 2013).