(285d) Quantitative, Multi-Scale Modeling Using Qualitative Data Identifies a Plant-Specific Dual Role for Hops in Regulated Vacuole Fusion

Stomata are the pores on a leaf surface that regulate gas exchange. Each stoma consists of 2 guard cells whose movements regulate pore opening and thereby control CO₂ fixation and water loss. Guard cell movements depend critically on the remodeling of cell vacuoles, which have been observed to change morphology from a highly fragmented state to a fused state during stomata opening. The evolution of vacuole morphology requires a membrane fusion mechanism that responds rapidly to environmental signals, allowing plants to respond to diurnal cues or environmental stresses such as drought. With guard cells being both large and responsive to external signals, stomata represent a unique system in which to delineate mechanisms of membrane fusion and fission.

To resolve a counter-intuitive observation regarding the role of HOPS in regulating vacuole morphology, we derived a quantitative model of vacuole fusion dynamics and used it to generate testable predictions about HOPS-SNARE interactions. We derived our model from limited - and, initially, qualitative - data by integrating statistical inference with quantitative fluorescence imaging and mechanistic modeling. The dynamic model predicted the evolution of vacuole morphology as it arises from intra-cellular signaling events that include: cytosol-to-membrane recruitment, chaperoned protein complexation, and complex disassembly.

By constraining the model parameters to yield the emergent outcomes observed for stoma opening (as induced by two distinct signals), 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 apparently contradictory actions at different points in the fusion signaling pathway, promoting the formation of SNARE complexes, but limiting their activity.