Beyond defense: microbial modifications of plant specialized metabolites alter and expand their ecological functions

Plant specialized metabolites (PSMs) are compounds that are not involved in primary metabolism but instead confer other roles for the plant host, often related to ecological interactions. In the field of plant-microbe interactions, many PSMs have traditionally been considered for their roles in shaping interactions with pathogens. However, it is increasingly clear that 'defensive' PSMs have broader functions in regulating assembly and functions of plant-associated microbes, a phenomenon that is best studied in the rhizosphere. PSMs, however, are secreted throughout plants to mediate interactions with the environment. In this Tansley insight, we argue that these molecules also play outsize roles in shaping microbial community assembly and functions in the phyllosphere. Additionally, we argue that it is important to consider how microbial activity transforms PSMs, because this may shape how plants interact with the environment. Increased attention to these effects and improved strategies to understand them across scales will lead to insights into how microbial responses to PSMs shape broader plant interactions in the environment.