Elevated ozone concentration and nitrogen addition increase poplar rust severity by shifting the phyllosphere microbial community

The tropospheric ozone and nitrogen deposition are two major environmental pollutants. Numerous studies have focused on the negative impacts of elevated O 3 and the complementary effect of soil N addition to tree physiological characteristics. However, it was notoriously ignored of how elevated O 3 with N addition affect tree immunity in face of pathogen infection, as well as of the important roles of phyllosphere microbiome community in host-pathogen-environment interplay. Here, we examined the effects of elevated O 3 and soil N addition on poplar leaf rust ( Melampsora larici-populina ) severity of two susceptible hybrid poplars (clone ‘107’: Populus euramericana cv. ‘74/76’; clone ‘546’: P. deltoides ⊆ P. cathayana ) in Free-Air-Controlled-Environment plots, besides, the link between Mlp -susceptibility and changes in microbial community was determined using Miseq amplicon sequencing. Rust severity of clone ‘107’ significantly increased under elevated O 3 or N addition only, however, the negative impact of elevated O 3 could be significantly alleviated when simultaneously conducting N addition, likewise, this trade-off was also found in its phyllosphere microbial α -diversity responding to elevated O 3 and N addition. However, the rust severity of clone ‘546’ did not significantly differ in the cases of elevated O 3 and N addition. Mlp infection altered microbial community composition and increased its sensitivity to elevated O 3 assessed by significantly different abundance of taxa. Elevated O 3 and N addition reduced the complexity of microbial community, which may explain the increased severity of poplar rust. These findings demonstrated that poplars need shifting phyllosphere microbial associations to optimize plant immunity in response to environmental changes. Importance Exploitation of the interaction mechanisms between host plants and pathogens is the essential basis in disease control. However, while much was known about the molecular determinants in pathogenesis process in the past decades, less is known about the role of nonpathogenic microbial community in plant-pathogen interaction, especially when some host plants are currently encountering severe environmental stresses, such as elevated ozone concentration and superfluous nitrogen addition. Thus, we targeted at the widespread and detriment rust disease (poplar-poplar rust) to dissect the influences of elevated ozone and nitrogen addition on rust disease severity and how phyllosphere microbial groups interacting with both poplars and rust pathogen under these biotic stresses. Our findings could be used to prescribe ecological information about poplar plantation in areas with high levels of ozone pollution and better understand the important roles of microbiome in plant heath.