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Abstract Guard cell movements depend, in part, on the remodelling of vacuoles from a highly fragmented state to a fused morphology during stomata opening. Indeed, full opening of plant stomata requires vacuole fusion to occur. Fusion of vacuole membranes is a highly conserved process in eukaryotes, with key roles played by two multi-subunit complexes: HOPS (homotypic fusion and vacuolar protein sorting) and SNARE (soluble NSF attachment protein receptor). HOPS is a vacuole tethering factor that is thought to chaperone SNAREs from apposing vacuole membranes into a fusion-competent complex capable of rearranging membranes. In plants, recruitment of HOPS subunits to the tonoplast has been shown to require the presence of the phosphoinositide phosphatidylinositol 3-phosphate. However, chemically depleting this lipid induces vacuole fusion. To resolve this counter-intuitive observation regarding the role of HOPS in regulating plant vacuole morphology, we defined a quantitative model of vacuole fusion dynamics and used it to generate testable predictions about HOPS-SNARE interactions. We derived our model by using simulation-based inference to integrate prior knowledge about molecular interactions with limited, qualitative observations of emergent vacuole phenotypes. By constraining the model parameters to yield the emergent outcomes observed for stoma opening—as induced by two distinct chemical treatments—we predicted a dual role for HOPS and identified a stalled form of the SNARE complex that differs from phenomena reported in yeast. We predict that HOPS has contradictory actions at different points in the fusion signalling pathway, promoting the formation of SNARE complexes, but limiting their activity. 

Abstract Stomata are dynamic pores on plant surfaces that regulate photosynthesis and are thus of critical importance for understanding and leveraging the carbon-capturing and food-producing capabilities of plants. However, our understanding of the molecular underpinnings of stomatal kinetics and the biomechanical properties of the cell walls of stomatal guard cells that enable their dynamic responses to environmental and intrinsic stimuli is limited. Here, we built multiscale models that simulate regions of the guard cell wall, representing cellulose fibrils and matrix polysaccharides as discrete, interacting units, and used these models to help explain how molecular changes in wall composition and underlying architecture alter guard wall biomechanics that gives rise to stomatal responses in mutants with altered wall synthesis and modification. These results point to strategies for engineering guard cell walls to enhance stomatal response times and efficiency.