Phytoremediation Of Chromium Contaminated Soil With Amaranthus Retroflexus L.Promoted By Its Endophytic Bacteria

Chromium is used by many different industries in a large scale, including metallurgical process, electroplating, pigments production, tanning, etc. The tanning industries are the main contribution of anthropogenic Cr pollution sources to environment above all of them. Although an addition of small amounts of Cr on plants has positive effects of stimulating growth and yield, higher accumulation in plants leads to toxicity on various activities of plants, inducing structural alterations and may even lead to their complete damage. There has been a considerable concern in recent years about using phytoremediation to treat heavy metal-contaminated soils. Phytoremediation is an environmentally friendly and economically feasible remediation technology for mitigating soil contamination in agricultural lands. In recent years, more and more researchers have endeavored to utilize plant growth-promoting bacteria to facilitate the removal of heavy metal-contaminated soils. Plant growth-promoting bacteria could synthesize siderophores which can dissolve and sequester iron from the soil and provide it to plant, synthesizing several different phytohormones such as auxins which can promote various stages of plant growth, having ability to the solubilization of minerals such as phosphorus which would become more available for plants growth. In this paper, chromium-resistant plant growth-promoting endophytic bacteria was screened out of Amaranthus retroflexus L. which has grown in tannery sludge. The primary objective of this work that analysising the effects of phytoremediation for Cr-contaminated soils by Amaranthus retroflexus L. combining with endophytic bacteria and preliminary exploring the mechanism of interaction between endophytes and phytoremediation. Major conclusions were summarized as follow:1.15 chromium-resistant strains which can produce indole-3-acetic acid (IAA), siderophores were isolated from the root of Amaranthus retroflexus L. Through quantitative analysis of plant growth-promoting characteristic that included production of IAA, siderophores and solubilization of phosphorus, it found that strain G8 has the strongest ability to produce IAA reaching 17.80 mg·L-1. Those strains which have the strongest ability to produce siderophores were strain G1, G2, G3 and G9,5 strains which have the ability to dissolve phosphorus, namely G4, G7, G11, G14 and G15.2. Evaluating 15 chromium-resistant strains of their plant growth-promoting characteristic,5 representative strains were selected as the tested strains including strain G1, G3, G8, G14 and G15. The rice seedlings were inoculated with the tested strains by hydroponic experiments to verified its ability of plant growth-promoting. The results showed that strain G1, G3, G8can promote the growth of rice. Rice total root length, total root surface area, number of tips increase significantly compared with non-inoculated control. The effect of plant growth-promoting is remarkable.3. The result of tested bacterium suspension were inoculated in sterilized contaminated soil showed that different strains exert different effects on soil pH value, strain G3, G8 can realized Cr mobilization in soil. The concentration of soil available Cr were significantly increased by 18.7% and 14.5%, respectively.4. The endophytic bacterial strain G3 and G8 were preliminary identified as Pseudomonas sp. CZGSD7 (99% similarity) and Pseudomonas extremorientalis (100% similarity) based on the 16S rDNA gene sequence analysis, respectively.5. Pot experimental results demonstrated that Amaranthus retroflexus L. have a great accumulation capacity for Cr. The Cr concentration in aerial parts and root were 1144.1 mg·kg-1 and 2284.2 mg·kg-1, respectively. Furthermore, the available Cr concentration of rhizosphere soil was significantly increased by 55.7% compared with original soil. Compared with the uninoculated control, inoculation with strain G8 was found to significantly (p< 0.05) increase the aerial parts (up to 54.1%) and root (up to 71.3%) biomass of A.retroflexus L. respectively, and strain G8 significantly increase the Cr concentration by 9.2% in the root of A. retroflexus L, the total Cr uptake in plant was increased by 69.5%. Experimental results demonstrated that endophytic bacteria G8 could be isolated from root of A. retroflexus L and chosen as the bioinoculant for effective phytoremediation of Cr-contaminated soil.