Isolation and identification of cadmium-resistant growth-promoting strains and their growth-promoting effects on soybean

Abstract:

[Background] South China suffers from serious cadmium (Cd) pollution. Symbiosis with beneficial microorganisms can relieve Cd toxicity and improve crop resistance to Cd through direct or indirect mechanisms, and thereby promote crop growth. Cd-resistant growth-promoting bacterial inoculants have broad application prospects. [Objective] Growth-promoting bacteria that can resist Cd and promote soybean growth are screened from the roots or rhizosphere of Cd-contaminated plants in South China, which is expected to enrich beneficial microbial resources in soybean production. [Methods] Strains were isolated from the roots or rhizosphere of the plants with the streak plate method, and preliminarily identified through the analysis of physiological and biochemical characteristics and 16S rRNA gene sequence. Meanwhile, pot experiment was used to explore the effects of these strains on soybean growth under Cd stress. Malondialdehyde (MDA) content and total antioxidant capacity (T-AOC) were determined to explore Cd-resistant mechanism of these strains. [Results] Four strains D1, D2, D3, and D4 were isolated, which all can solubilize phosphate and produce indole-3-acetic acid (IAA) and siderophore. They belong to Acinetobacter, Microbacterium, Paenibacillus, and Providencia, respectively. The pot experiment on soybean Baxi 10 showed that these four strains had Cd-resistant capacity and could promote soybean growth. Shoot dry weight, root dry weight, and plant height of soybean inoculated with D4 were increased by 28%, 35% and 31%, respectively, in the absence of Cd. In the presence of 20 mg/kg-CdCl₂·5/2H₂O, shoot dry weight of soybean plants inoculated with D1, D2, D3, and D4 was raised by 35%, 55%, 53%, and 43%, respectively. The MDA content in the shoots of soybean plants inoculated with D2 and D4 was decreased by 23% and 29%, respectively. The T-AOC in the shoots of soybean plants inoculated with D1 and D4 was elevated by 11% and 13%, respectively. [Conclusion] The screened strains are expected to be developed into microbial fertilizers and applied into Cd-contaminated farmland for improvement of crop growth and yield. Meanwhile, it lays a theoretical basis for the study of Cd-resistant mechanisms of plant growth-promoting bacteria.

Key words:plant growth-promoting bacteria;cadmium toxicity;soybean;isolation and identification;growth-promoting mechanism