Effects of cadmium accumulation on the structure and co-occurrence network of endophytic bacterial community in Blumea balsamifera

To probe into the impacts of organ cadmium accumulation on endophytic bacteria in Blumea balsamifera, the methods of high-throughput sequencing of 16S rRNA gene V3-V4 region and molecular ecological network analysis were employed to study the effects of Cd accumulation in root, stem and leaf of B. balsamifera on the community characteristics of endophytic bacteria under different exogenous cadmium treatments(0 and 2.0 mg·kg-1). The results were as follows:(1)In comparison with the control group without exogenous cadmium addition(0 mg·kg-1, Cd0), the treatment group with cadmium spiking(2.0 mg·kg-1, Cd2)in soils promoted plant growth and the cumulative Cd contents in root, stem and leaf, with the order of leaf(16.75 mg·kg-1)> stem(11.99 mg·kg-1)> root(3.96 mg·kg-1).(2)α diversity analysis showed that the endophytic bacteria richness(Sobs, Ace and Chao1 indices)and diversity(Shannon and Simpson indices)for organs were the highest in root, followed by stem and leaf under Cd0 and Cd2 treatments. Additionally, the indices of richness and diversity of endophytic bacteria for each organ under Cd2 treatment were superior to those under Cd0.(3)At the phylum level, Proteobacteria, Actinobacteriota and Firmicutes were the dominant bacterial phyla in all the organs for both treatments; at the genus level, Delftia was the main bacterial genus with the relative abundance ranged from 53.0% to 92.7% and 57.1% to 89.2% in the plant organs of Cd0 and Cd2, respectively; certain similarities existed among the endophytic bacterial community structures of root, stem and leaf of B. balsamifera, and Cd2 increased the proportion of mutual endophytic bacterial genera in root, stem and leaf and that of unique endophytic ones in each organ(except for the root).(4)Linear discriminant analysis Effect Size(LEfSe)analysis showed that there existed the differences on endophytic bacterial genera residing in different organs within the same treatment and also the same organ between treatments.(5)Redundancy analysis(RDA)showed that the contents of rhizosphere soil Cd and plant organ Cd were significantly correlated with the composition of endophytic bacterial community.(6)Co-occurrence network analysis clarified that cadmium accumulation in B. balsamifera complicated the interaction network of endophytic bacteria occurring in root and leaf, and enhanced the competition among endophytic bacterial species in root and stem, and the symbiosis in leaf. In summary, exogenous Cd treatment affected the community structure and interaction mode of endophytic bacteria in B. balsamifera organs.