Beneficial effects of indigenous Cd-tolerant and Cd-sensitive Glomus mosseae associated with a Cd-adapted strain of Brevibacillus sp in improving plant tolerance to Cd contamination

In this study, we investigated the basic interactions between plant, rhizosphere microorganisms and Cd pollution. The effectiveness of the inoculation of an indigenous Cd-adapted arbuscular mycorrhizal (AM) strain of Glomus mosseae and/or a Cd-adapted bacterium (Brevibacillus sp.), both isolated from Cd-contaminated soil, was assayed in Trifolium plants growing in Cd-polluted soil. The behavior of the Cd-adapted G. mosseae strain was compared with that of a collection G. mosseae isolate (BEG 119). Results showed the functional compatibility of autochthonous microorganisms, which resulted in a biomass increase of 545% (shoot) and 456% (roots) compared to non-mycorrhizal plants. Single AM fungi (autochthonous or reference strains) and bacterial inoculation also proved highly effective in increasing plant biomass and nutrition. G. mosseae (BEG 119, reference strain) was less efficient than Brevibacillus sp. in increasing shoot growth. Co-inoculation with both microorganisms increased to the highest extent root biomass and symbiotic structures (nodules and AM colonization), and this may be responsible for the beneficial effect found. While plant acquisition of N and P were consistently enhanced by the application of the Cd-adapted autochthonous Brevibacillus sp. plus AM fungus, the Cd uptake by Trifolium plants decreased in dual AM fungus-bacterium treatments. Thus, the combined microbial inoculation conferred tolerance to Cd by increasing nutrient status and rooting development, and by decreasing Cd availability and uptake by the plant. The indigenous bacterial isolate was able to growth at increasing Cd concentrations (from 25 to 100 mug g(-1) Cd) while the growth of a reference, non-Cd-adapted, bacterial strain fell to zero in medium having 25 mug g(-1) Cd, indicating the tolerance to Cd of the indigenous bacterium. The beneficial effect of inoculated microorganisms on symbiotic and nutritional plant values is relevant for the increased growth and nutrition of plants growing in Cd-contaminated soils. The inoculation of suitable symbiotic and saprophytic rhizosphere microorganisms isolated from Cd-polluted soils plays an important role in the development and metal tolerance by plants and in soil bioremediation. (C) 2003 Elsevier B.V. All rights reserved.