Stomatal Defense a Decade Later

Abstract

Plants and microbes have long coevolved in a constant battle to overcome the mechanisms of defense and attack from both sides. Plants have developed means to prevent pathogen attack by hampering invasion of plant tissues and actively warding off pathogen colonization. On the other hand, pathogens have evolved strategies to mask their presence and/or evade host defenses. Plant-microbe interaction starts with molecular recognition of each other, leading to a cascade of signaling events with the final output of plant resistance or susceptibility to the pathogen. In this molecular war, epidermis of plants is the first barrier that pathogens need to overtake. Natural openings on the leaf surface, such as stomata, provide an entry site to pathogens. Plants have evolved a mechanism to close stomata upon sensing microbe-associated molecular patterns (MAMPs). This mechanism is known as stomatal defense. A decade has passed since the discovery of stomatal defense, and the field has expanded considerably with significant understanding of the basic mechanisms underlying the process. Here, we give a perspective of these findings and their implications in the understanding of plant-microbe interactions. It has been long recognized that infection of plants by foliar pathogens involves pathogen penetration into inner tissues, a niche conducive for living, where they obtain water and nutrients from internal cells. Routes for pathogen penetration into the leaves include stomatal pores, hydathodes, and wounds (either accidental or direct breaching of the cuticle by the pathogen or its vector). The vast majority of contemporary experiments designed to understand mechanisms of pathogenesis in plants has relied on inoculation by artificial wounding or direct infiltration of inocula into the apoplast. Although these are valuable approaches to dissect plant diseases, they preclude thorough understanding of a key step for the establishment of disease: pathogen internalization into the host plant. In the last decade, we came to realize how dynamic and complex this entry process is. It requires active, inducible responses on both the host and the pathogen, as well as specific environmental conditions at the time of penetration. Perhaps these strict requirements contribute to the fact that widespread diseases are rare events in nature. A large number of pathogens use the stomatal pore as a site for penetration into inner leaf tissues. In fact, some pathogens, such as the bacterium Xanthomonas campestris pv armoraciae (Hugouvieux et al., 1998), the oomycete Plasmopara viticola (Allègre et al., 2007), and species of the fungus Puccinia (Shafiei et al., 2007; Grimmer et al., 2012) are specialized to internalize into leaves only through stomata. Earlier observations provided some clues that stomatal closure might diminish bacterial disease severity in a biologically relevant context. For instance, reduced number of lesions developed on dark- or abscisic acid (ABA)-treated tomato (Solanum lycopersicum) plants after inoculation with X. campestris pv vesicatoria (Ramos and Volin, 1987). Furthermore, direct comparison of disease severity after surface-inoculation and apoplastic-infiltration in this pathosystem revealed that bacterium penetration through stomata may be a control point for disease progression (Ramos and Volin, 1987). Characterization of either bacterial or plant mutants also provided indications for possible stomatal control of bacterial infection. The coronatine (COR)-deficient mutant of Pseudomonas syringae pv tomato (Pst) causes less disease when inoculated on the leaf surface than when inoculated directly into the apoplast of Arabidopsis (Arabidopsis thaliana) or tomato (Mittal and Davis, 1995). Similarly, the flagellin-sensitive2 (fls2) mutant of Arabidopsis, which lacks the receptor for bacterial flagellin, is more susceptible than the wild-type plant only when surface-inoculated with Pst DC3000 (Zipfel et al., 2004). These previous studies set the foundation for a direct demonstration that guard cells surrounding the stomatal pore can sense microbes and close the pore, a process that is now known as stomatal defense or stomatal immunity (Melotto et al., 2006; Sawinski et al., 2013). A number of recent reviews have discussed topics ranging from signaling networks that regulates stomatal defense to mechanisms of endogenous and exogenous signal integration in guard cells (Arnaud and Hwang, 2015; Murata et al., 2015; Cotelle and Leonhardt, 2016; Lee et al., 2016). In addition, the use of thermoimaging technology in crop breeding programs and precision agriculture to assess pathogen-induced stomatal movement has been highlighted (Ishimwe et al., 2014; Singh et al., 2016). Furthermore, stomatal closure in response to fungus- and plant-derived elicitors (e.g. chitin, chitosan, and oligogalacturonic acid) and stomatal opening in response to fungal metabolites (e.g. fusicoccin) has been extensively reviewed (Grimmer et al., 2012;