A novel regulator of stomatal immunity in tomato

The ability of a pathogen to successfully colonize a plant is determined in large part by its capacity to rapidly enter the host tissue. While most fungal pathogens secrete cuticle- and cell wall-degrading enzymes or use mechanical force to breach the epidermis (Mendgen et al., 1996), phytopathogenic bacteria are devoid of mechanisms that allow them to penetrate plant tissues.

For this reason, bacterial aggressors rely on lenticels, root cracks, wounds and other natural surface openings to gain access to their hosts. Stomata also provide a natural entry point for bacteria. Leaf tissues can become vulnerable to bacterial invasion during periods of intense photosynthetic activity and transpiration, as these physiological processes depend on widely open stomata.

To hinder bacterial invasion through these pores, plants have evolved sophisticated mechanisms that perceive bacterial attack and close their stomata, a defense strategy usually referred to as stomatal immunity (Sawiski et al., 2013). Little is known about the molecular elements involved in stomatal immunity in economically relevant plant-pathogen systems.

In an issue of Plant Physiology, Guzman et al. (2020) shed light on this topic by uncovering the role of a novel regulator of stomatal immunity, the Tomato Atypical Receptor Kinase1 (TARK1). TARK1 is a leucine-rich repeat receptor-like kinase (LRR-RLK) that was previously identified by the same research group as a target for virulence factors produced by Xanthomonas euvesicatoria (Xe), the causal agent of bacterial spot disease in economically important plants such as tomato and peppers (Kim et al., 2009).

Humans have been dealing with phytopathogenic bacteria since we began domesticating plants and these microbial aggressors remain a major obstacle to reliable and secure food production (Savary et al., 2019). Evidence suggests that relentless selection for important agronomical traits was accompanied by a massive reduction in the capacity of domesticated plants to defend themselves against pests and pathogens (Chen et al., 2015; Moreira et al., 2018). Thus, it is imperative that we decipher the molecular basis of mechanisms in economically important plants (such as tomato) to fend off bacterial attack. Work by Guzman et al. (2020) has uncovered a novel regulator of stomatal immunity in tomato, TARK1. Although future work is needed to determine the exact mechanism of TARK1 function and how it participates in stomatal defense, this study provides important future avenues for the development of pathogen-resistant crops.