Effects of Pseudomonas aeruginosa on the growth and cadmium accumulation in rice (Oryza sativa L.) seedling under Cd stress

Heavy metal contamination in rice is a serious problem focused by people all over the world. Microbial passivation is considered as one of effective measures to inhibit heavy metal from environmental into the biological cycle. To investigate the mitigative effects of Pseudomonas aeruginosa on rice seedling exposed to cadmium stress conditions, a set of hydroponic experiments by adding 20 μmol·L^-1 cadmium solution were conducted, and the treatment without added cadmium was used as control (CK). The effects of different treatments on plant growth, cadmium content and cadmium accumulation of rice were determined. The treatments included adding with different volume of bacteria suspension, bacterial carrier A (diatomite, size 1-3 mm), carrier B (diatomite, size 3-6 mm), carrier C (activated carbon, specific surface area 1 000 m²·g^-1) and microbial inoculum A, B and C, which prepared by carrier A, B and C saturated with bacteria suspension, respectively. The results showed that the growth of root length, plant height and biomass of rice were significantly inhibited by treating with 20 μmol·L^-1 cadmium. But, the growth of rice was promoted by adding with P. aeruginosa suspension and microbial inoculum A, B and C. In contrast to the treatment only with cadmium (0.523 g·plant^-1), total biomass of rice increased by 38.5% to 67.3%, and the highest value was tested in the treatment with inoculum B. Cadmium contents in root, stem-sheath, leaf, as well as in the above parts of rice reduced in all above treatments. Under the treatments of microbial inoculum A, B, C and activated carbon, cadmium contents in above parts of rice significantly declined by 45.9%, 47.9%, 59.9% and 59.9%, the transportation factors decreased by 16.7%, 25.0%, 33.3% and 33.3%, as well the bio-concentration factors reduced by 48.1%, 48.8%, 58.8% and 60.9%, respectively. In addition, the cadmium accumulated in rice seedling reduced by 19.2%, 9.5% and 24.3% under the treatments with activated carbon, microbial inoculum A and inoculum C, respectively. While, cadmium accumulation increased by 15.0%, 30.4%, 14.8% and 16.8% under the treatments with microbial inoculum B, bacteria suspension A, B and C, which contained 56.4 mL, 45.3 mL and 28.4 mL bacteria, respectively. It meant that the bacteria promote cadmium accumulation in rice by enlarging biomass of rice. In conclusion, P. aeruginosa exhibited the capacities of promoting rice growth, reducing cadmium content, inhibiting cadmium transportation, as well as cadmium availability of rice under cadmium stress conditions. Higher cadmium passivation abilities of microbial inoculum A, B and C in this study offer us a new promising microbial strain for bio-remediation of environmental cadmium pollution.