Role of heavy metal resistant Ochrobactrum sp. and Bacillus spp. strains in bioremediation of a rice cultivar and their PGPR like activities.

The present study demonstrates the metal toxicity ameliorating and growth promoting abilities of three different bacterial isolates when applied to rice as host plant. The three bacterial strains included a cadmium resistant Ochrobactrum sp., a lead resistant Bacillus sp. and an arsenic resistant Bacillus sp. designated as CdSP9, PbSP6, and AsSP9, respectively. When these isolates were used as inocula applied to metal-treated rice plants of variety Satabdi, the germination percentage, relative root elongation (RRE), amylase and protease activities were increased. The toxic effect of metal was reduced in presence of these bacteria. The overall biomass and root/shoot ratio were also enhanced by bacterial inoculation. Hydroponic studies showed that the superoxide dismutase (SOD) activity and malondialdehyde (MDA) level, which had been increased in the presence of metal stress in rice roots, were lowered by the bacterial inoculation. In addition, all three strains were 1-aminocyclopropane-1-carboxylate (ACC) deaminase and catalase positive, whereas siderophore producing ability was lacking in PbSP6. However, both PbSP6 and AsSP9 were protease positive and could hydrolyse starch. The data indicate that these bacteria have promise for bioremediation as well as for plant growth promotion.

Synergism of VAM and Rhizobium on production and metabolism of IAA in roots and root nodules of Vigna mungo.

Mature and healthy root nodules of Vigna mungo appeared to contain higher amount of indole-acetic acid (IAA) than non-nodulated roots. Dual effect of VAM fungus, Glomus fasciculatum and the nitrogen-fixing bacteria, Rhizobium sp. on the nodulation of roots of V. mungo was studied. It was recorded that the roots which were inoculated simultaneously with both the symbionts i.e., G. fasciculatum and Rhizobium exhibited greater amount of IAA production than the non-inoculated roots. A tryptophan pool present in the mature nodules and young leaves might serve as a precursor for IAA production in the roots and in the nodules. Activity of IAA-metabolizing enzymes, such as IAA oxidase, peroxidase, and polyphenol oxidase was investigated which indicates the active metabolism of IAA in roots and nodules. The Rhizobium symbiont isolated from fresh nodules of V. mungo produced significant amount of IAA under in vitro condition when tryptophan was added to the medium as precursor. Present study represents some beneficial effects of Rhizobium and G. fasciculatum on the production and metabolism of IAA in roots and nodules of V. mungo. The important physiological implication of the study on IAA production and its metabolism in Rhizobium-Legume-VAM tripartite symbiosis is certainly representing a new approach to satisfy the hormonal balance in the host plant.

Production and metabolism of indole acetic acid in roots and root nodules of Phaseolus mungo.

The mature root nodules of Phaseolus mungo (L.), a leguminous pulse, contain higher amount of indole acetic acid (IAA) than non-nodulated roots. The tryptophan pool present in the mature nodule and young roots might serve as a precursor for the IAA production. Presence of IAA metabolising enzymes - IAA oxidase and peroxidase - indicate the metabolism of IAA in the nodules and roots. In culture, the symbiont, isolated from the nodules, produced a high amount of IAA, when tryptophan was supplied in the medium as a precursor. The symbiont preferred l-isomer over the dl- or d-isomer of tryptophan for IAA production. The important physiological implication of the IAA production in the legume-Rhizobium symbiosis is discussed.