Root-exuded coumarin shapes the root microbiome

Roots of land plants grow through soil, one of the most microbially rich and diverse environments on earth (1). A long history of work has revealed that plant roots interact with these microbes and help establish microbial communities distinct from the surrounding soil (2). This is partly because roots provide a food source of exuded sugars, metabolites, and dead plant material, which some microbes are better suited to exploit than others (3). It is also partly because the root immune response and secreted antimicrobials provide obstacles to colonization, to which only some microbes are tolerant (4). Thus, roots control microbes not only with “carrots” but also with “sticks.” Because plant genetic variation affects root structure, composition, and exudates, many studies have tried to address the extent of genetic control plants have over their associated root microbiomes (5). These studies have generally shown that plant genotype has an effect on the root microbiome but that this is a small influence relative to environmental effects such as soil composition (4). Much of the work ends here, because the next steps are even more difficult. They involve identifying specific genes whose products interact with root-associated microbes, demonstrating conclusively the relevance of those interactions, and explaining how the genes defined function. The paper in PNAS by Stringlis et al. (6) represents a significant step because it does all these things. The authors use a variety of techniques, from quantifying root exudates to metagenome sequencing in natural soil, to show that an exuded coumarin involved in the plant’s iron starvation response reshapes the microbiome and may also protect plants from pathogenic fungi. Furthermore, this effect is triggered by beneficial bacteria. … [↵][1]1To whom correspondence should be addressed. Email: paulojt{at}email.unc.edu. [1]: #xref-corresp-1-1