Impacts of cadmium on the diversity of endophytic bacteria associated with hyperaccumulating and non-hyperaccumulating ecotypes of Sedum alfredii

Abstract:

The effects of heavy metal stress on the community structure of endophytic bacteria in plants are not well understood. To date, few studies have investigated the response of community structure and diversity of endophytic bacteria in hyperaccumulating plants to heavy metals present in soil rhizosphere. [Objective] The objectives of this study were to explore the changes and differences in community structure and diversity of endophytic bacteria in roots, stems, and leaves of hyperaccumulating (HE) and non-hyperaccumulating (NHE) ecotypes of Sedum alfredii under different levels of Cd²⁺ contamination; as well as to explain their differences in ability to tolerate and accumulate the available cadmium by the two ecotypes of Sedum alfredii, based on the mutual relationship between plant and endophyte. [Methods] The community structures of endophytic bacteria in the roots, stems, and leaves of the two ecotypes of Sedum alfredii grown on soils with different Cd²⁺ concentrations were analyzed by Illumina high-throughput sequencing technique. [Results] High concentrations of Cd²⁺ inhibited the growth of NHE Sedum alfredii and decreased the richness and diversity of endophytic bacteria, whereas it promoted the growth of HE Sedum alfredii and increased the richness of endophytic bacteria in stems and roots. Among the three tested soils, the endophytic bacteria in leaves, stems, and roots of two ecotypes of Sedum alfredii were all dominated by Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria. For HE Sedum alfredii, the relative abundances of Gammaproteobacteria, Negativicutes, and Clostridia in leaves as well as Alphaproteobacteria in stems increased significantly with the soil Cd²⁺ concentration, whereas the relative abundances of Clostridia decreased significantly. For NHE Sedum alfredii, with the increase in soil Cd²⁺ concentration, the relative abundances for Alphaproteobacteria, Gammaproteobacteria, and Clostridia in leaves did not show significant change, while those for Negativicutes in stems as well as Betaproteobacteria and Clostridia in roots decreased significantly, and those for Negativicutes in roots increased significantly. In highly Cd²⁺-contaminated soils (50 mg/kg), the relative abundances of Sphingomonas in leaves and Veillonella in stems of HE Sedum alfredii were higher than those for NHE Sedum alfredii. Meanwhile, the first, second, and third predominant endophytic bacteria (i.e., Veillonella, Sphingomonas, and Prevotella) that were present in the roots of HE Sedum alfredii did not appear in roots of NHE Sedum alfredii.[Conclusion] The Cd²⁺-contamination level in soils had significant impacts on the community structure of endophytic bacteria in leaves, stems, and roots of the two ecotypes of Sedum alfredii.