Genetic screen to saturate guard cell signaling network reveals a role of GDP L fucose metabolism in stomatal closure

Guard cells regulate plant gas exchange by controlling the aperture of stomatal pores. Stomatal closure involves a multi-input signaling network that governs the activity of ion channels, which in turn regulate guard cell turgor pressure and volume. We describe a forward genetic screen to identify novel components involved in stomatal movements. Through an ozone-sensitivity approach combined with gas exchange analysis, 130 mutants of established stomatal regulators and 76 novel mutants impaired in stomatal closure were identified. One of the novel mutants was mapped to MURUS1 (MUR1), the first enzyme in de novo GDP-L-fucose biosynthesis. Defects in synthesis or import of GDP L Fuc into the Golgi apparatus resulted in impaired stomatal closure to multiple stimuli. Stomatal phenotypes observed in mur1 were independent from the canonical guard cell signaling and instead could be related to altered mechanical properties of guard cell walls. Impaired fucosylation of xyloglucan, N linked glycans and arabinogalactan proteins did not explain the aberrant function of mur1 stomata, however the stomatal phenotypes in mur1 can partially be attributed to defective dimerization of rhamnogalactouronan-II. In addition to providing the genetic framework for future studies on guard cell signaling, our work emphasizes the impact of fucose metabolism on stomatal movement.