Salty or sweet? Guard cell signaling and osmotic control under saline conditions

Sodium ions (Na⁺) negatively affect most land plant species as a consequence of both general osmotic effects and specific ion toxicity. Na⁺ travels through the plant body via the transpiration stream, and this results in direct exposure of guard cells to apoplastic Na⁺. This article summarizes extant literature on short- and long-term stomatal conductance responses to salinity, stomatal movements of isolated guard cells in response to NaCl, and guard cell ion content following NaCl exposure. We discuss the potential role of Na⁺ as an osmoticum that itself could drive guard cell swelling and stomatal opening, possible contributions of carbohydrates in guard cell osmoregulation following NaCl exposure, and the role of ABA in stomatal responses to salinity. We highlight responses of glycophytes vs halophytes when comparative information is available. Based on our integration of published information, we propose that salt sensitivity arises in part by glycophytic guard cell uptake of Na⁺ and resultant short-term stomatal opening that promotes plant desiccation and toxic accumulation of Na⁺ in the leaf. However, many aspects of guard cell Na⁺ sensing, transport, and response remain unknown, and new studies are required both to test this hypothesis and to address numerous knowledge gaps.