Increased Cd And Cu Accumulation And Heavy Metal-tolerance Of Plants By ACC-deaminase Producing Bacteria And The Mechanisms

Phytoremediation is paid ever-increasing attention due to its green environment, less cost and environmental protection for remediation of heavy metal-contaminated soils. A large number of microorganisms which could colonize plant tissue interiors or rhizosphere soils, can affect growth and heavy metal accumulation of plants and metal forms in heavy metal-polluted environments by many approaches, and then improve phytoremediation efficiency. Screening PGPB (plant growth-promoting bacteria) and studying the effects of them on improving heavy metal accumulation and the mechanisms were very meaningful in clarifying mechanisms of plant heavy metal accumulation further and developing plant-microbe remediation.16 bacterial strains were isolated from rhizosphere soils and the roots of heavy metal-tolerant Commelina communis growing on copper mine and were resistant to both Cd and Cu (20 mg·L-1), and another 16 bacterial strains kept in our laboratory. In this paper,20 Cd and Cu (20 mg·L-1)-resistant and ACC deaminase-producing bacterial strains were screened out, of which,16S rDNA sequence analysis of 16 the isolates showed that they belonged to three major groups:α-,β-,γ-Proteobacteria, Actinobacteria and Firmicutes, including 10 genera. Cupriavidus was the firstly reported ACCD-producing bacteria. Three highly efficient strains (Enterobacter agglomerans Z1, Bacillus megaterium JL35 and 2Csa-24) were selected based on the promotion of plant growth and heavy metal accumulation. Biological characteristics of the strains Z1, JL35 and 2Csa-24 were studied. The results showed that the three strains could produce ACC deaminase, IAA and siderophore, strains Zl and JL35 also produce arginine decarboxylase; the strains could be resistant to a variety of heavy metals (Pb and Zn:100-500 mg·L-1; Ni:25-50 mg·L-1) and antibiotics (Km:25-100 mg·L-1; Amp, Spe and Cm:50-200 mg·L-1; Str:20-100 mg·L-1; Tc: 10 mg·L-1; Rif:25-200 mg·L-1); strain 2CSa-24 was identified as Cupriavidus nector 2CSa-24 based on the physiological and biochemical characteristics and 16S rDNA sequence analysis and was firstly reported as the ACCD-production bacteria.The effects of the strains Z1, JL35 and 2Csa-24 on growth of maize and rape and accumulation of Cd and Cu were investigated under aqueous culture with Cd and Cu stresses. The results showed that the three strains could enhance accumulation of Cd or Cu by maize and rape mainly through increasing plant biomass and uptake of Cd and Cu. The mechanisms of the strains Z1, JL35 and 2CSa-24 in promoting plant growth and accumulation and tolerance of Cd and Cu were explored by studying the effects of the three strains on activities of SOD and POD and contents of soluble protein in roots of maize and rape and contents of AsA, GSH, TBARS and free Pro in plant leaves. The results showed that the bacterial inoculation reduced plant cell damage and decreased TBARS contents, especially at high concentration of Cd (maize:Cd 20 mg·L-1; rape:Cd 10 mg·L-1). An increase in SOD activity and soluble protein in roots of maize and rape and improvement of AsA contents in leaves of rape were seen by the inoculation of the strains. The bacterial inoculation could improve or significantly improve GSH level in plants. The inoculation treatments also had certain effects on POD activity and Pro contents of the tested plants.Pot experiment showed that inoculation of the tested strains Z1, JL35 and 2CSa-24 promoted growth of maize and rape and increased NH4O Ac-extracted copper in rhizosphere soil of maize and water soluble copper in rhizosphere soil of rape. Strains Z1, JL35 and 2CSa-24 could colonize rhizosphere and root interiors of the rape and maize well and the colonization numbers in rhizosphere and roots were 103-104 cfu.g-1 soil and 102-103 cfu·g-1 FW, respectively. Effects of inoculation of strains Z1, JL35 and 2CSa-24 on micro-ecological environment in rhizosphere and the mechanisms of the strains in improving accumulation and tolerance of cadmium and copper were researched under pot condition. The results showed that the bacterial inoculation increased the soluble sugar contents and the invertase activity, decreased soil urease activity commonly and had little effect on pH of soil. The inoculation with the strains could increase accumulation of heavy metals mainly through enhancing antioxidant enzyme and non-enzyme systems. Different strains had different impacts on plants, and then their effects on physiological characteristics of plants were different. Inoculation with the same strain may have different effects on different parts of the same plant. The results above indicated that ways and mechanisms were different in alleviating heavy metals damage by different strains.