Apoplastic metabolomics reveals sugars as mesophyll messengers regulating guard cell ion transport under red light

Here we sought to identify mesophyll messenger(s) that promote stomatal opening under red light. We first established a method to isolate leaf apoplastic fluids from Arabidopsis thaliana and Vicia faba and ascertained that the apoplastic fluid enhances red light-induced stomatal opening. We then applied multiple targeted and non-targeted metabolomics approaches, detecting 464 metabolites as components of the plant apoplastic metabolome – a rich resource for future studies. We identified several small and organic molecules as red light-stimulated apoplastic metabolites in both species and tested the impacts of these candidate mesophyll messengers on stomatal responses. We found that low concentrations of sugars, including sucrose, strongly enhanced stomatal opening under red light. The sucrose enhancement effect was diminished in the H+ ATPase null mutant, aha1-6, and phosphopeptide immunohistochemical assays demonstrated sugar upregulation of H+ ATPase phosphorylation in wild-type, which is correlated with increased ATPase activity. Our whole cell patch clamp assays revealed inhibition of anion efflux through slow anion channels by low (1-3 mM) sucrose concentrations. These regulatory impacts on ion transporters are explanatory of the observed enhancement of stomatal opening. This comprehensive study thus answers the decades-long question concerning the presence, identity, and mechanistic impact of the mesophyll messenger.