Potential effects of micro- and nanoplastics on phyllosphere microorganisms and their evolutionary and ecological responses.

Plastic pollution is among the most urgent environmental and social challenges of the 21st century, and their influxes in the environment have altered critical growth drivers in all biomes, attracting global concerns. In particular, the consequences of microplastics on plants and their associated soil microorganisms have gained a large audience. On the contrary, how microplastics and nanoplastics (M/NPs) may influence the plant-associated microorganisms in the phyllosphere (i.e., the aboveground portion of plants) is nearly unknown. We, therefore, summarize evidence that may potentially connect M/NPs, plants, and phyllosphere microorganisms based on studies on other analogous contaminants such as heavy metals, pesticides, and nanoparticles. We show seven pathways that may link M/NPs into the phyllosphere environment, and provide a conceptual framework explaining the direct and indirect (soil legacy) effects of M/NPs on phyllosphere microbial communities. We also discuss the adaptive evolutionary and ecological responses, such as acquiring novel resistance genes via horizontal gene transfer and microbial degradation of plastics of the phyllosphere microbial communities, to M/NPs-induced threats. Finally, we highlight the global consequences (e.g., disruption of ecosystem biogeochemical cycling and impaired host-pathogen defense chemistry that can lead to reduced agricultural productivity) of altered plant-microbiome interactions in the phyllosphere in the context of a predicted surge of plastic production and conclude with pending questions for future research priorities. In conclusion, M/NPs are very likely to produce significant effects on phyllosphere microorganisms and mediate their evolutionary and ecological responses.