Individual and combined inoculation of plant growth-promoting endophytic bacteria enhance Ricinus communis L. to remediate Cu/Cd polluted soil.

Castor (Ricinus communis L.) has potential for phytoremediation of heavy metal-polluted soil due to its high accumulating capacity. In this study, four endophytic bacteria strains (TR16, TR18, TR21 and YS3), were used to investigate the effect of their individual and combined inoculations on castor's Cu/Cd uptake and the biochemical characteristics in the rhizosphere for two soils. Results showed that the highest Cu/Cd uptake by castor was obtained in treatments H1 and H2. Inoculation treatments significantly increased the activity of sucrase, acid and alkaline phosphatase in rhizosphere soil. Tartaric acid and dissolved organic carbon (DOC) levels significantly increased, enhancing soil Cu/Cd bioavailability. Strains TR18, TR21 and YS3 could be successfully colonized in rhizosphere soil. Endophyte inoculation increased the relative abundance of bacteria involved in soil nutrient cycling with varying effects. Root transcriptome analysis indicated that phenylpropane biosynthesis, glutathione metabolism, cysteine and methionine metabolism, and antioxidant enzyme activity play an important role in enhancing castor tolerance to heavy metal. TR21 inoculation could improve plant resistance to Cu/Cd in the root by regulating the synthesis of several secondary metabolites, including lignin, suberin, flavonol and coumarin, and that was beneficial to the sequestration of Cu/Cd in the castor root.