Endophytic bacterial community of Stellera chamaejasme L. and its role in improving host plants' competitiveness in grasslands.

Stellera chamaejasme has become a problematic weed in northern and south-western grasslands of China. To evaluate a possible role of endophytes in its strong competitive capacity, the endophytic bacterial community of S. chamaejasme was investigated by culture-dependent and independent methods, and the growth-promoting traits of some culturable isolates as well as the benefit of endophyte ST3CS3 (Brevundimonas sp.) on host plants growth were studied. The results showed that 823 OTUs were generated with a 97% similarity level in the culture-independent study. They were classified into 29 phyla, 61 classes, 147 orders, 237 families and 440 genera. Among them, Pseudomonas and Ralstonia were the most dominant genera in belowground parts (G) (64.25%) and aboveground parts (S) (26.54%) respectively. The diversity and species richness of endophytes in S were significantly higher than that of G (P < 0.001, t-test). Contrary to this, the number of culturable bacteria in S was a little lower than that of G (P > 0.05, t-test). Totally, 176 isolates belonging to 30 morphotypes were obtained in the culture-dependent study. Among them, Acinetobacter was the most dominant genus in G (51.30%), then followed by Pseudomonas (6.09%) and Brevundimonas (6.09%), while Lysinibacillus (21.31%) was the most dominant genus in S, followed by Pseudomonas (11.48%). Growth-promoting trait tests indicated that 93.65% of the tested isolates (63) exhibited nitrogen-fixing, IAA-synthesizing, phosphorus or potassium solubilizing capacity, in which 77.97% belonged to Proteobacteria, a phylum found to contain more active isolates. Pot experiments demonstrated that endophyte ST3CS3 can significantly improve host plants growth and increase its nitrogen and chlorophyll content (P < 0.01, t-test). Therefore, we suggest that strong competitiveness of S. chamaejasme may in part be due to possession of high ratios of plant growth-promoting proteobacterial endophytes such as Pseudomonas, Acinetobacter and Brevundimonas.