Foliar fungal endophyte communities remain unaltered under urban air-pollution but differentially express stress-related genes

Air pollution caused by tropospheric ozone contributes to the decline of forest ecosystems; for instance, coniferous forests dominated by sacred fir, Abies religiosa (Kunth) Schltdl. & Cham., in the peri-urban forests of Mexico City. Individual trees within these forests exhibit variation in their response to ozone exposure, such as the presence or absence of visible symptoms in needles. Using RNA-Seq metatranscriptomic data and ITS2 metabarcoding, we investigated whether such symptom variation correlates with the taxonomic and functional composition of fungal endophytes from needles collected in a highly polluted area in the surroundings of Mexico City. Our findings indicate that ozone-related symptoms do not significantly correlate with changes in the taxonomic composition of fungal endophytes. However, 21 putative proteins were differentially expressed in fungal endophytes from asymptomatic needles, including eight genes previously associated with resistance to oxidative stress. These results suggest that fungal endophyte communities likely play a role in mitigating the oxidative burst caused by tropospheric ozone to sacred fir. Our study illustrates the feasibility of using RNA-Seq data, accessible from global sequence repositories of plants, for the characterization of fungal endophyte communities and gene expression.