Characterization of endophytic Rahnella sp. JN6 from Polygonum pubescens and its potential in promoting growth and Cd, Pb, Zn uptake by Brassica napus
Highlights
► Bacterium JN6 was isolated from root of Polygonum pubescens. ► Strain JN6 showed very high Cd, Pb, Zn tolerance and mobilization. ► Strain JN6 produced IAA, siderophore, ACC deaminase, and also solubilized P. ► Strain JN6 promoted growth and Cd, Pb, Zn uptake by rape. ► Strain JN6 showed high level of colonization in tissue interior of rapes.
Introduction
Industrial operations such as mining, smelting and combustion of fossil fuel, cause accumulation of metals in different natural resources such as soil, water and air. Excessive heavy metals in the contaminated soils cannot be degraded to harmless products and hence persist in the environment indefinitely, which has been considered as one of the most severe environmental problems (Deng et al., 2010a, Rajkumar et al., 2009). Consequently, the development of a remediation strategy for metal-polluted soils is urgent for environmental restoration and human health. Phytoremediation, using plants to remove pollutants from the environment or to render them harmless, is considered as an esthetic, cost-effective and eco-friendly technology (Shin et al., 2012). Nevertheless, several factors restrict the application of phytoremediation including (a) low growth rate and small biomass of the plants, (b) phytotoxicity of heavy metals, and (c) limited metal uptake (Li et al., 2007). This has prompted us to explore an alternative strategy to improve the efficiency of phytoremediation of heavy metal-contaminated soils.
Recent attention has been paid to the possibilities of enhancing the biomass and metal uptake by metal-accumulating plants inoculated with metal-tolerant and plant growth-promoting endophytic bacteria (PGPE) (Ma et al., 2011a, Luo et al., 2012). The PGPE can assist their host plants in improving plant growth through various mechanisms including the production of plant growth promoting (PGP) substances such as indole-3-acetic acid (IAA), siderophores, 1-aminocyclopropane-1-carboxylic (ACC) deaminase, or phosphate solubilization (Ryan et al., 2008). Moreover, certain PGPE have also been shown to alter heavy metal availability to the plant by producing siderophores and organic acids (Saravanan et al., 2007, Long et al., 2011). Polygonum pubescens is a newly discovered Mn-hyperaccumulator, which is widely distributed in the mining wastelands with high concentrations of Mn, Cd, Pb, Zn and Cr in China (Deng et al., 2010a). P. pubescens has the ability to tolerate and accumulate high concentrations of Cd, and may have some potential for the phytoremediation of Cd-contaminated soils (Deng et al., 2010b). Although heavy metal-tolerance PGPE have been reported to occur widely in tissue interiors of various metal hyperaccumulator or non-hyperaccumulator plants (Sheng et al., 2008, Ma et al., 2011b), little is known about the metal tolerant characteristics of endophytic bacteria from P. pubescens, which may be crucial for establishing close interactions with the plant species and for enhancing the efficiency of phytoremediation of metal-contaminated soils.
The objectives of this study were to isolate and characterize metal-tolerant PGPE from P. pubescens grown in a multimetal-polluted soil, and test the potential of PGPE to promote plant growth and accumulation of Cd, Pb and Zn in rapes for improving the efficiency of phytoremediation in Cd/Pb/Zn-polluted soils.
Section snippets
Isolation and selection of heavy metal-resistant endophytic bacteria
The plants Polygonum pubescens were collected from a heavy metal-contaminated site near an electroplating factory located in Guangzhou City, China, in July 2010. The properties and heavy metal contents of the soil were: pH 6.31 (1:1 w/v water), organic matter 1.85%, lead 354 mg kg−1, cadmium 48 mg kg−1, zinc 376 mg kg−1, copper 56 mg kg−1, nickel 37 mg kg−1, manganese 937 mg kg−1. Plant samples were washed thoroughly with tap water, and then separated into roots, stems and leaves. Plant samples were
Isolation, selection and identification of endophytic bacteria
Seven bacterial strains were isolated from root and stem tissues of P. pubescens based on their different morphologies on SLP agar medium added with 50 mg L−1 of Cd. In order to obtain PGPE, all isolates were qualitatively tested for a number of important PGP activities. Here, the isolate, namely JN6 from the root of P. pubescens, possessed multiple PGP properties and was selected for subsequent studies. Analysis of 1400 bp 16S rDNA sequence of strain JN6 using the BLASTn program at NCBI showed
Discussion
Metal-tolerant bacteria could survive in the tissue interiors of plants grown on the metal-polluted soils and could be isolated and selected for their potential application in the microbe-assisted phytoremediation (Rajkumar et al., 2009). The surface sterilization protocol for isolating endophytic bacteria is effective in removing epiphytic microorganisms, and the bacterial isolates can be considered as true endophytic bacteria. In this study, a metal-tolerant endophytic bacterium Rahnella sp.
Conclusions
Effective phytoremediation could be accomplished by bacteria having the potential of solubilizing heavy metals and promoting plant growth in contaminated soils. The present study indicates that the Cd, Pb and Zn-tolerant and -solubilizing PGPE could be isolated from P. pubescens grown in a heavy metal-contaminated site and chosen as a bioinoculant for the effective phytoremediation in the soils contaminated by heavy metals (Cd, Pb, Zn). Application of PGPE Rahnella sp. JN6 could significantly
Acknowledgments
The project was supported by the NSFC (National Natural Science Foundation of China) – Guangdong United Foundation (U0833004) and the Ministry of Environmental Protection of China (No. 201109020).
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