The formin protein RMD regulates rice stomatal opening by modulating tonoplast-endoplasmic reticulum associations

The stoma is key to plant growth and development, with highly controlled functions. However, the regulatory mechanisms of stomata remain largely unknown in monocot crop species. In this study, we reveal that guard cell physiology is regulated by actin cytoskeleton-mediated changes to tonoplast-endoplasmic reticulum (ER) contact sites in rice. This mechanism is mediated via the formin protein Rice Morphology Determinant and the vesicle-associated membrane protein-associated protein VAP1 at intersections of the ER and actin filaments. Notably, monocot (rice) guard cells contain two distinct types of vacuoles, where the central vacuole size and stomatal aperture are regulated by the actin cytoskeleton. Furthermore, VAP1 localized at both the ER and central vacuole, which closely aligned with the ER and actin cytoskeleton. Genetic interactions of vap1 and rmd-1 led to defects in rice stomatal aperture, which impacted light-induced stomatal opening. Our results significantly extend our knowledge of crop stomatal regulation.