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Rhizosphere engineering with beneficial plant growth promoting bacteria offers great promise for sustainable crop yield. Potato is an important food commodity that needs large inputs of nitrogen and phosphorus fertilizers. To overcome high fertilizer demand (especially nitrogen), five bacteria, i.e., Azospirillum sp. TN10, Agrobacterium sp. TN14, Pseudomonas sp. TN36, Enterobacter sp. TN38 and Rhizobium sp. TN42 were isolated from the potato rhizosphere on nitrogen-free malate medium and identified based on their 16S rRNA gene sequences. Three strains, i.e., TN10, TN38, and TN42 showed nitrogen fixation (92.67-134.54 nmol h(-1)mg(-1) protein), while all showed the production of indole-3-acetic acid (IAA), which was significantly increased by the addition of L-tryptophan. Azospirillum sp. TN10 produced the highest amount of IAA, as measured by spectrophotometry (312.14 mu g mL(-1)) and HPLC (18.3 mu g mL(-1)). Inoculation with these bacteria under axenic conditions resulted in differential growth responses of potato. Azospirillum sp. TN10 incited the highest increase in potato fresh and dry weight over control plants, along with increased N contents of shoot and roots. All strains were able to colonize and maintain their population densities in the potato rhizosphere for up to 60 days, with Azospirillum sp. and Rhizobium sp. showing the highest survival. Plant root colonization potential was analyzed by transmission electron microscopy of root sections inoculated with Azospirillum sp. TN10. Of the five test strains, Azospirillum sp. TN10 has the greatest potential to increase the growth and nitrogen uptake of potato. Hence, it is suggested as a good candidate for the production of potato biofertilizer for integrated nutrient management.

International audience; This study focuses on the chemotaxis, colonization and rice growth promoting ability of indole acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing rhizobacteria Rhizobium leguminosarum bv. trifolii SN10, previously isolated from root nodules of Trifolium alexandrium L. We show here that R. leguminosarum bv. trifolii SN10 promote the growth of four different rice varieties grown in India in terms of biomass, root branching and N content. In addition, using scanning electron microscopy and viable cell counts, we provide evidence that the bacteria successfully colonize the root surface of the rice variety which showed maximum growth promotion upon inoculation. Not only this, R. leguminosarum bv. trifolii SN10 exhibit a strong chemotaxis response towards the rice seed and root exudates despite the presence of a bacteriostatic phenolic compound, 7-hydroxycoumarin (umbelliferone). Further, R. leguminosarum bv. trifolii SN10 secretion of phytohormones such as IAA and ACC deaminase suggest the potential of the plant growth promoting rhizobacteria to be used as biofertilizer to enhance rice crop production in the subcontinent.

Pseudomonas putida strain angstrom 313, a deleterious rhizosphere bacterium, reduced pea nitrogen content when inoculated alone or in combination with Rhizobium leguminosarum bv. viceae on plants in the presence of soil under greenhouse conditions. When plants were grown gnotobiotically in liquid media, mixed inocula of angstrom 313 and rhizobia gave a higher proportion of small evenly distributed nodules when compared with a single rhizobial inoculation. In addition, the rhizobial root establishment was reduced by angstrom 313 irrespective of inoculum, density, indicating that angstrom 313 has the capacity to interact with the early rhizobial infection process. When pea seedlings were simultaneously inoculated with angstrom 313 and rhizobia, angstrom 313 colonised the root hairs to the same extent as the rhizobia, according to analysis by immunofluorescence microscopy. This suggests that the root hair colonisation trait of P. putida interferes with the onset of the symbiotic process. (c) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.