Effects of Pseudomonas TCd-1 inoculation on Cd uptake, rhizosphere soils enzyme activities and Cd bioavailability in rice (Oryza sativa) varieties with different Cd tolerance

Heavy metal contamination in rice (Oryza sativa) is a serious problem. Microbial remediation is a promising technique to reduce Cd accumulation in rice. To explore the rhizosphere-associated ecological mechanism of Pseudomonas TCd-1-induced reduction of Cd uptake in rice, two rice varieties, high Cd-tolerant variety ‘Teyou 671’ and low Cd-tolerant variety ‘Baixiang 139’, were used. A set of soil culture pots treated with 10 mg∙kg⁻¹ Cd were employed to evaluate the effects of Pseudomonas TCd-1 inoculation on rice Cd uptake and enzymes activities in rhizosphere soils. The results showed that the Cd content in different parts of both the high and low Cd-tolerant rice varieties significantly decreased (P<0.05) after inoculation of Pseudomonas TCd-1, and the bioconcentration factor (BCF) of Cd decreased by 35.14% and 47.79% (P<0.05), respectively. However, no significant changes were found in the translocation factor (TF). Meanwhile, in rhizosphere soils of the high and low Cd-tolerant rice varieties, the content of exchangeable Cd decreased by 15.89% and 23.81% (P<0.05), Fe-Mn oxide bound Cd increased by 39.58% and 28.81% (P<0.05), and organic matter Cd increased by 36.11% and 25.00% (P<0.05), respectively. In addition, the activities of acid phosphatase, urease, saccharase, cellulase, and catalase significantly increased by 26.74%, 12.07%, 62.50%, 81.17%, and 5.13%, respectively; while the polyphenol oxidase activity decreased by 12.40% in the rhizosphere soils of low Cd-tolerant rice variety. In rhizosphere soils of high Cd-tolerant rice variety, the activities of acid phosphatase, urease, sucrase, cellulase, and polyphenol oxidase decreased by 7.19%, 9.39%, 25.53%, 16.20%, and 11.44%, respectively; while catalase activity increased by 5.13%. There were significant differences in enrichment characteristics, rhizosphere soil enzymes activities, and the proportions of different chemical forms of Cd in rhizosphere soils of different Cd-tolerant rice varieties after inoculation with the TCd-1 strain. The results indicated that inoculation with the strain may partly remediate the changes in soil enzymes activities caused by Cd pollution. In conclusion, Pseudomonas TCd-1 can improve Cd tolerance and inhibit Cd uptake and accumulation in rice, mainly by reducing the bioavailability of soil Cd and restoring the changes in soil enzymes activities caused by Cd pollution.