Cadmium-tolerant facultative endophytic Rhizobium larrymoorei S28 reduces cadmium availability and accumulation in rice in cadmium-polluted soil

Abstract

In this study, a metal-tolerant facultative endophytic strain S28 isolated from rice seeds was evaluated for the impacts on Cd concentration in medium, rice biomass, and Cd concentration and accumulation in rice in Cd-polluted soil. Inoculation with strain S28 decreased the Cd concentration and increased the pH and putrescine concentrations in solution. Strain S28 increased rice dry weight by 18%–49% at different growth stages of rice compared to the controls. Strain S28 reduced the MgCl 2 -extractable (available) Cd concentration by 11%–25% and increased the carbonate- and organic matter-bound (unavailable) Cd contents by 21%–133% at rice maturation stage in the Cd-polluted soil, respectively, compared to the controls. At the rice maturity stage, strain S28 also increased the relative abundance of Geobacter species related to Cd availability by 28% compared with the control. Inoculation with strain S28 reduced the Cd contents in roots, above-ground parts, and grains of rice plants by 18%–23%, 32%–37%, and 54%, respectively, compared to the controls. Furthermore, the cell counts of strain S28 in the rhizosphere soil and root tissue were 2.5 × 10 5 and 3.1 × 10 4 CFU g −1 , respectively. These findings indicated that the facultative endophytic S28 strain colonized the rice rhizosphere and interior, decreased Cd availability, and increased the Geobacter species abundance, resulting in decreased Cd uptake in rice different tissues. Our results may supply a new understanding on the mechanisms involved in facultative endophytic bacteria-mediated bioremediation and decreased rice Cd accumulation in Cd-contaminated soils. • Arginine decarboxylase-producing endophytic strain S28 was obtained from rice seeds. • S28 immobilized Cd and increased putrescine production in Cd-added solution. • S28 increased the biomass and reduced rice tissue Cd uptake in Cd-polluted soil. • S28 reduced Cd availability by increasing carbonate- and organic matter-bound Cd. • S28 reduced Cd availability by increasing the Geobacter species relative abundance.