A sophisticated network of signaling pathways regulates stomatal defenses to bacterial pathogens

Guard cells are specialized cells forming stomatal pores at the leaf surface for gas exchanges between the plant and the atmosphere. A decade ago, stomata have been shown to play an important role in plant defense as a part of the innate immune response. Indeed, plants actively close their stomata upon contact with microbes thereby preventing pathogen entry into the leaves and the subsequent colonization of host tissues. In this review, we will present current knowledges of molecular mechanisms and signaling pathways implicated in stomatal defenses with a particular attention on plant-bacteria interactions. Stomatal defense responses begin from the perception of pathogen-associated molecular patterns (PAMPs) and activate a signaling cascade involving the production of secondary messengers such as reactive oxygen species (ROS), nitric oxide (NO) and calcium for the regulation of plasma membrane ion channels. The analyses on downstream molecular mechanisms implicated in PAMP-triggered stomatal closure have revealed extensive interplays with components regulating hormonal signaling pathways. We will also discuss on strategies deployed by pathogenic bacteria to counteract stomatal immunity through the example of the phytotoxin coronatine.