[Immobilization of Heavy Metals by Phosphorus-solubilizing Bacteria and Inhibition of Cd and Pb Uptake by Wheat].

Phosphorus-solubilizing microorganisms convert insoluble phosphorus in the soil into phosphorus that can be absorbed by plants. Soluble phosphate combines with heavy metals to form precipitation, reducing the content of available heavy metals, thereby reducing the absorption of heavy metals by crops, which plays an important role in the remediation of heavy metal-contaminated soil. The effects of the immobilization of Cd and Pb and the release of PO43- by the phosphorus-solubilizing bacterium Klebsiella sp. M2 were studied through solution culture experiments. In addition, the effects of strain M2 on wheat uptake of Cd and Pb and its microbiological mechanism were also explored through pot experiments. The results showed that strain M2 reduced the concentrations of Cd and Pb and increased the concentration of PO43- in the solution through cell wall adsorption and induced phosphate precipitation. Pot experiments showed that compared to those in the CK group and inactivated strain M2 group, inoculation with live strain M2 significantly increased （123%-293%） the contents of Ca2-P and Ca8-P in rhizosphere soil, decreased the content of DTPA-Cd （34.48%） and DTPA-Pb （36.72%） in wheat rhizosphere soil, and thus hindered the accumulation of Cd and Pb in wheat grains. Moreover, high-throughput sequencing results showed that strain M2 significantly increased the diversity of wheat rhizosphere bacterial communities； increased the relative abundance of Proteobacteria, Gemmatimonadetes, and Bacteroidota in wheat rhizosphere soil； and increased the proportion of heavy metal-immobilizing and phosphorus-promoting bacteria in wheat rhizosphere soil （mainly Sphingomonas, Nocardioides, Bacillus, Gemmatimonas, and Enterobacter）. These bacterial genera played an important role in immobilizing heavy metals and preventing wheat from absorbing heavy metals. These results provide bacterial resources and theoretical basis for the bioremediation of heavy metal-contaminated farmland.

Polyamine-producing bacteria inhibit the absorption of Cd by spinach and alter the bacterial community composition of rhizosphere soil.

Polyamines (PAs) are small aliphatic nitrogenous bases with strong biological activity that participate in plant stress response signaling and the alleviation of damage from stress. Herein, the effects of the PA-producing bacterium Bacillus megaterium N3 and PAs on the immobilization of Cd and inhibition of Cd absorption by spinach and the underlying mechanisms were studied. A solution test showed that strain N3 secreted spermine and spermidine in the presence of Cd. Both strain N3 and the PAs (spermine+spermidine) immobilized Cd and increased the pH of the solution. Untargeted metabolomics results showed that strain N3 secreted PAs, N1-acetylspermidine, 3-indolepropionic acid, indole-3-acetaldehyde, cysteinyl-gamma-glutamate, and choline, which correlated with plant growth promotion and Cd immobilization. A pot experiment showed that rhizosphere soil inoculation with strain N3 and PAs improved spinach dry weight and reduced spinach Cd absorption compared with the control. These positive effects were likely due to the increase in rhizosphere soil pH and NH4+-N and PA contents, which can be attributed primarily to Cd immobilization. Moreover, inoculation with strain N3 more effectively inhibited the absorption of Cd by spinach than spraying PAs, mainly because strain N3 enabled a better relative abundance of bacteria (Microvirga, Pedobacter, Bacillus, Brevundimonas, Pseudomonas, Serratia, Devosid, and Aminobacter), that have been reported to have the ability to resist heavy metals and produce PAs. Strain N3 regulated the structure of rhizosphere functional bacterial communities and inhibited Cd uptake by spinach. These results provide a theoretical basis for the prevention of heavy metal absorption by vegetables using PA-producing bacteria.