Stimulation of nitrogen fixation and induction of flavonoid-like compounds by rhizobacteria

Abstract

Bacteria belonging to fluorescent Pseudomonas and to the spore-forming Bacillus groups, predominantly found in the rhizosphere and rhizoplane of healthy chickpea plants, were studied in order to determine their effect on effective strains of Rhizobium sp. 'Cicer' (Ca181 and Ca313) and their plant growth-promoting ability under aseptic and soil conditions. Co-inoculation of the rhizobacteria with effective Rhizobium strains of chickpea resulted in a significant increase in nodule weight, root and shoot biomass and total plant nitrogen when grown either in sterilized chillum jars or under pot culture conditions. The Rhizobium stimulatory Pseudomonas sp. 'CRP55b' showed maximum increase in all the symbiotic parameters. On co-inoculation with 'Ca181' and 'Ca313', Pseudomonas sp. 'CRP55b' and 'CRS68' resulted in significant increases in nodule weight, root and shoot biomass and total plant nitrogen. The nodule stimulating rhizobacteria enhanced levels of flavonoid-like compounds in roots on seed bacterization. Also, ethyl acetate extracts of culture supernatant fluids when applied to seeds resulted in enhancement of flavonoids in roots, suggesting that the rhizobacteria have a direct influence on root flavonoids which might be an additional factor in nodule promotion by these bacteria. These rhizobacteria also produced fluorescent compounds with absorption maxima at 252 nm, similar to those of plant flavonoids but having a different Rf value. Whether such fluorescent compounds act as signal molecules for induction of plant flavonoids in plant roots requires further study.