Abstract:Two ACC deaminase-producing endophytic bacteria (K1-6 and K3-9) were obtained from the roots of Cu-tolerant Sorghum sudanens grown in Cu mine wasteland. The biological characteristics of the two endophytic bacterial strains were characterized. Strains K1-6 and K3-9 showed resistance to high levels of Cu and antibiotics, solubilized inorganic phosphate, and produced indole acetic acid (IAA). Strain K3-9 could also produce siderophores and arginine decarboxylase. Strains K1-6 and K3-9 had also the characteristics of acid or alkali and salt tolerance and temperature resistance. Strains K1-6 and K3-9 were identified as Rhizobium sp. K1-6 and Enterobacter aerogenes K3-9 based on the analyses of their 16S rDNA gene sequences, respectively. Plate culture experiment was used to investigate the effects of the two endophytic bacterial strains on the growth and Cu accumulation of Sorghum sudanens grown in 0 and 4 mg/L of Cu added as CuSO4. The results showed that significant (P<0.05) increase was observed in the root (ranging from 10.6% to 45.5%) and above-ground tissue (ranging from 13% to 40%) dry weight of the bacteria-inoculated Sorghum sudanens compared with the control. Inoculation with strain K1-6 was found to significantly increase Cu contents of the roots and above-ground tissues of Sorghum sudanens compared with the control, however, no significant difference was observed in Cu contents of the roots and above-ground tissues between the strain K3-9-inoculated and control plants. Furthermore, total Cu uptakes of the roots and above-ground tissues of the plants inoculated with strain K1-6 were increased by 88% and 114% (P<0.05) respectively compared with the control, while total Cu uptakes of the roots was increased by 44% in the presence of strain K3-9 compared with the control. Notably, Cu content of the roots was 16-23-fold higher than that of the above-ground tissues of the plants inoculated with the two strains. The above results showed that the endophytic bacterial strains K1-6 and K3-9 from the roots of Cu-tolerantSorghum sudanens have the innate multiple plant growth-promoting traits, can increase the growth and Cu tolerance of Sorghum sudanens, and enhance Cu accumulation in the roots. In addition, the increases in the growth, Cu accumulation and tolerance of Sorghum sudanens are dependent on the strains.