Temperature signals drive grass secondary cell wall thickening.

Secondary cell wall thickening is essential for plant structural development, providing the mechanical strength and rigidity required for upright growth. However, direct observation of this process in its endogenous developmental context within living plants has remained limited. Cellulose, the predominant component of secondary walls and the most abundant biopolymer on Earth, is synthesized at the plasma membrane by complexes containing CELLULOSE SYNTHASE A (CESA) proteins. Despite its central role, the precise timing and regulation of cellulose deposition during plant development remain unclear. To address this gap, we developed a real-time bioluminescence imaging system in the model grass Brachypodium distachyon using a luciferase transcriptional reporter driven by the CESA8 cis-regulatory region. Bioluminescence imaging revealed a consistent spatial pattern of CESA8 expression within elongating internodes, coinciding with regions undergoing secondary wall deposition and progressive increases in cellulose crystallinity. Time-lapse imaging showed that expression follows a robust daily rhythm driven by temperature cycles, independent of light or endogenous circadian signals. Temperature-pulse experiments uncovered rapid, transient inverse responses that were accurately predicted by a mathematical model based on an incoherent feedforward loop. CESA8 expression correlated strongly with stem elongation, linking structural reinforcement with temperature-driven shoot growth in grasses.