Root exudate sesquiterpenoids from the invasive weed Ambrosia trifida regulate rhizospheric Proteobacteria

The adaption of Ambrosia trifida to the environment to which it has been introduced is crucial to its successful invasion. Microbial diversity analyses suggested that the abundance of Proteobacteria was relatively high in rhizospheric soil surrounding A. trifida roots. Three of these bacterial taxa were isolated and identified as Acinetobacter sp. LHD-1, Pseudomonas sp. LHD-12, and Enterobacter sp. LHD-19. Furthermore, three sesquiterpenoids were authenticated as the main metabolites in the root exudates of A. trifida, and include one new germacrane sesquiterpenoid (1E,4E)-germacrdiene-6β,15-diol (2) and two known sesquiterpenoids, (E)-4β,5α-epoxy-7αH-germacr-1(10)-ene-2β,6β-diol (1) and (2R)-δ-cadin-4-ene-2,10-diol (3). Their chemical structures were elucidated using NMR spectroscopy and single crystal X-ray diffraction analyses. In UPLC-MS/MS analyses, compounds 1-3 showed values of 10.29 ± 2.21, 0.02 ± 0.01, and 0.78 ± 0.52 μg/g FW, respectively, in A. trifida rhizospheric soil. Interestingly, those compounds were able to inhibit the growth of Acinetobacter sp. LHD-1 and promote the growth of Enterobacter sp. LHD-19 where concentrations were close to those secreted into rhizospheric soil. Furthermore, the rhizospheric bacteria Acinetobacter sp. LHD-1 and Enterobacter sp. LHD-19 were able to regulate the growth of A. trifida seedlings in potted planting verification experiments. Interestingly, root exudate sesquiterpenoids could also improve the concentration of IAA in Enterobacter sp. LHD-19, indicating that this bacterium may promote plant growth through regulating the IAA pathway. These results provided new evidence for the rapid adaptation of plants to new environments, allowing their invasive behavior.