Water Stress and Disruption of Mycorrhizae Induce Parallel Shifts in Phyllosphere Microbiome Composition

Water and nutrient limitation are key stressors that affect plant health and ecosystem function. These environmental factors impact both soil- and root-associated microbial communities, and systemically alter plant physiology, but it is less clear how they affect aboveground plant-microbe interactions. Through experimental manipulations in the field and growth chamber, we examined the interacting effects of irrigation, soil fertility, and root mycorrhizal associations on bacterial and fungal communities of the tomato phyllosphere (Solanum lycopersicum). Both water stress and mycorrhizal disruption reduced bacterial richness within plants, homogenized bacterial community diversity among plants, and reduced the relative abundance of dominant fungal taxa. We observed striking parallelism in the individual microbial taxa affected by irrigation and mycorrhizal associations. Given the increasingly understood role of the phyllosphere in shaping plant health and pathogen susceptibility, these results offer an additional mechanism by which belowground conditions shape plant fitness.