Involvement of the bacterium Azospirillum brasilense in wheat tolerance to cadmium

Cadmium is known to cause severe inhibition of plant growth. Some plant growth-promoting rhizobacteria were reported to be capable of decreasing partially the toxicity of Cd for barley. We studied the ability of the rhizobacteria Azospirillum brasilense, strains Sp7 and Sp245, to reduce Cd toxicity for wheat in hydroponic culture. Since one of the wheat lectins (wheat germ agglutinin, WGA) is involved in plant response to some kinds of environmental stress and is also essential for plant-bacterial interactions, the second aim of this work was to elucidate whether Cd stress could induce synthesis of WGA in wheat plants. The capacity of A. brasilense to decrease Cd toxicity for wheat was found to depend on the Cd concentration and on the wheat variety. Thus, when the CdCl2 concentration in the plant growth medium was 2 mg/l, both strains diminished Cd toxicity for the wheat cultivar Saratovskaya 52; however, they gave no effect with the other three varieties used. At 8 mg/l CdCl2, the inhibition of plant growth was significant (by 82-85%), and inoculation with bacteria did not decrease the toxicity of Cd. It was found that the presence of Cd in the plant growth medium caused an increase in the concentration of WGA in wheat roots. The enhanced level of WGA was retained during the whole period of plant cultivation with cadmium. The results obtained suggest that A. brasilense is capable of decreasing partially the toxicity of Cd for wheat, and WGA is probably involved in plant response to cadmium stress.