A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha(-1), 90 kg N + 60 kg P2O5 ha(-1), 120 kg N ha(-1) and 120 kg N + 60 kg P2O5 ha(-1). Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12-14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.

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Biofertilizers contribute in N(2) fixation, P solubilization, phytohormone production and thus enhance plant growth. Beneficial plant-microbe interactions and the stability and effectiveness of biofertilizer depend upon the establishment of bacterial strains in the rhizosphere of the plant. This interaction depends upon many factors, one of them being plant exudates. Root exudates are composed of small organic molecules like carbonic acids, amino acids or sugars etc., which are released into the soil and bacteria can be attracted towards these exudates due to chemotaxis. The chemotactic behaviour of Azotobacter strains was studied using cotton (Desi HD 123 and American H 1098) and wheat (WH 711) seedlings and the root exudates of these two plants were chemically characterized. Analysis of the root exudates revealed the presence of sugars and simple polysaccharides (glucose), amino acids (glutamate, lysine) and organic acids (citric acid, succinic acid, maleic acid, malonic acid). Differences between cotton cultivars in root exudates were observed which influenced chemotactic response in Azotobacter. These results indicate colonization with rhizobacteria which implies that optimal symbionts, on the sides of both plant cultivar and bioinoculant bacteria can lead to better plant growth under cultivation conditions.

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Soil bacteria belonging to the genus Azotobacter, Pantoea and some unidentified soil isolates were tested in vitro for phytohormone production under laboratory and soil conditions. The German wheat variety Munk was inoculated by several soil bacteria with exogenously applied hormones (IAA, 2,4-D) and a flavonoid (naringenin) with a half of the amount of recommended doses of fertilizers under greenhouse conditions. Most of the soil bacteria tested were able to produce indole acetic acid (IAA), and stimulated a lateral root development and colonization by the addition of 2,4-D and IAA. A formation of paranodules on roots as a result of crack entry invasion was observed with 2,4-D as well as with IAA. We were able to reisolate the organism from the paranodules and could establish the same results. Analyses for root exudates and in vitro phytohormone production by various bacterial isolates were also carried out, revealing that 2,4-D can be replaced either by high IAA producing bacteria or by exogenous application of IAA. Bacterial survival in the rhizosphere as well as the root and shoot weight of wheat plants were positively affected also by the addition of IAA, 2,4-D and naringenin. A. brasilense (Sp7) partly due to the development of a protected niche. These paranodule structures derived from the induction of the initials of the lateral roots are quite dissimilar to root nodules particularly when colonized by non-symbiotic bacteria. Protoplasts of the bacteria (L-forms of Azotobacter, Pseudomonas syringae and Bacillus polymyxa) were suggested to have the ability to penetrate the cell wall and membrane structures of living plant cells and to colonize plant tissues (Cocking et al. 1990). Also some microorganisms colonize in the intercellular spaces as seen in apoplast of stem of sugarcane (Cocking 2003). Despite encouraging results with Azospirillum, Azotobacter etc., some controversy still exists about the mechanism of bacterial root interactions. Reports suggest that the colonization of these bacteria is caused by factors like N 2 fixation, siderophores, ammonia excretion, phytohormones (Lakshminarayana 1993) and antifungal properties etc. (Verma et al. 2001), collectively enhancing the root proliferation, increase in the lateral roots and root hair formation. Dobbelaere et al. (1999) suggested that plant growth substances are one of the key factors observed in plant growth promotion. Tien et al. (1979) reported that the morphological changes due to bacterial inoculations can be mimicked by applying a combination of plant growth substances.Keeping all these factors in mind, we made an attempt to screen different soil bacteria for the production of various phytohormones, their colonization on German winter wheat (var. Munk) with or without exogenously applied natural and synthetic plant hormones and the concomitant changes in root morphology or formation of paranodules under controlled greenhouse conditions in addition to plant growth parameters. Natural hormone like IAA, synthetic-2,4-D (2,4-dichlorophen...