Bacterial assembly as part of immune system in Paulownia responses to witches’ broom caused by phytoplasmas

Purpose Paulownia witches’ broom (WB) usually affects part of the infected tree, rarely spreading throughout the tree. WB was infected by phytoplasmas, but the associated microbiome assembly remains unknown. This study aims to clarify the possible mediating mechanism of WB outbreaks in Paulownia. Methods We collected leaves, branches, roots and rhizosphere soil of Paulownia trees with or without WB, and analyzed the bacterial community on the basis of 16S rRNA high-throughput sequencing. Results All leaf and branch samples contained phytoplasmas, but high phytoplasma levels were required for WB. Low bacterial community diversity and richness, a lack of beneficial bacteria, and an unstable bacterial network might be conducive to a WB outbreak. Phytoplasmas were more abundant in the roots of diseased trees than in the roots of healthy trees. Moreover, phytoplasma infections altered the bacterial community composition, but did not induce morphological changes. Phytoplasma did not present in the rhizosphere soil of diseased trees, but their bacterial community composition changed comparing with the corresponding samples of healthy trees. Asymptomatic leaves and branches of diseased trees recruited beneficial bacteria and increased the bacterial network stability, which might contribute to the restriction of phytoplasma infections and WB development further. The abundance of several beneficial bacteria in the endosphere was significantly negative correlation with that of phytoplasmas, implying their inhibitory effects on phytoplasmas. Conclusion Our findings have clarified the possible mechanism mediating WB outbreaks in Paulownia trees. Furthermore, we demonstrated that Paulownia trees inhibit phytoplasma infections by modulating the endosphere and rhizosphere bacterial communities.