Do stomatal traits modulate leaf microbiome assembly?

Elucidating the factors controlling plant microbiome assembly is a major research goal in plant biology given a growing awareness of microbial community contributions to host plant health and fitness. While stomata have long been recognized to influence pathogen colonization, less is known about whether or how stomatal traits regulate diverse communities of nonpathogenic microbes that make up the majority of the leaf microbiome. In this Viewpoint, we propose that stomata are a primary filter by which plants influence the assembly of leaf-associated microbial communities. We discuss three nonmutually exclusive hypotheses for how stomatal traits influence leaf microbes, including preliminary support for each based on published studies of foliar fungi and bacteria. The stomatal density hypothesis argues that a greater density of pores increases the rate of microbial entry into the leaf, while the stomatal function hypothesis posits that the duration and speed of stomatal opening and closing regulate microbial access into the leaf. The stomatal covariation hypothesis recognizes that many other leaf traits covary with stomatal traits and thus could contribute to observed relationships between stomatal traits and leaf microbiome structure. Finally, we propose research priorities to improve our understanding of stomatal control over leaf microbiome assembly.