Increased nitrogen deposition may promote plant invasion through interfering plant-pathogen interactions

Intensified anthropogenic activities have led to a documented increase in atmospheric nitrogen deposition, which has been shown to impact the growth and herbivore resistance of invasive plant species, thereby contributing to their invasion success. However, the extent to which elevated nitrogen deposition influences plant-pathogen interactions within the context of biological invasions remains unexplored. This study initially investigated the differences in leaf lesion areas between the invasive weed Alternanthera philoxeroides and its local congener, A. sessilis, in different terrestrial ecosystems that experienced various levels of atmospheric nitrogen deposition. Additionally, a laboratory experiment was conducted to assess the changes in pathogen resistance of these two species in response to simulated nitrogen deposition, following artificial infection with a leaf pathogen. The study also investigated alterations in leaf defensive chemicals and phyllosphere microbial communities between the two species to elucidate the underlying mechanisms. The findings revealed that A. philoxeroides demonstrated superior pathogen resistance compared to A. sessilis under both field and laboratory conditions. Leaf lesion areas of both species exhibited inverse correlations with atmospheric nitrogen deposition levels in the field, as well as with leaf nitrogen content in the laboratory. Further correlation analyses and in vitro toxicity assessments indicated that the observed differences in pathogen resistance between the two species could be attributed to opposing changes in leaf total flavonoid concentrations and shifts in the composition of the endophytic phyllosphere microbiome in response to increased nitrogen deposition. Subsequent microbiome analyses have revealed that the phyllosphere of A. philoxeroides accumulated a greater abundance of ecologically beneficial microbes within its endophytes, which function as biological protectants for the plant. These findings imply that elevated atmospheric nitrogen deposition may enhance the invasiveness of A. philoxeroides, mediated through interactions between the plant and its associated microbes.