Leaf starch metabolism sets the phase of stomatal rhythm

In leaves, stomata open during the day to favour CO2 entry for photosynthesis, and close at night to prevent inefficient transpiration of water vapour. Plants’ days and nights are paced by light availability and anticipated by the circadian clock, but how rhythmic stomatal movements are interlocked with the environment and the physiology of the plant remains elusive. Leaf transitory starch appears as a plausible integrator therein. Here, we developed PhenoLeaks, a pipeline to analyse the diel (24-h) dynamics of transpiration, and used it to screen a series of Arabidopsis mutants impaired in starch metabolism. We show that the diel dynamics of transpiration are driven by a sinusoidal, endogenous rhythm that overarches days and nights. We uncover that a number of severe mutations in starch metabolism affect the endogenous rhythm through a phase shift, resulting in delayed stomatal movements throughout the daytime and reduced stomatal preopening during the night. Nevertheless, analysis of tissue-specific mutations revealed that neither guard-cell nor mesophyll-cell starch metabolism are strictly required for normal diel patterns of transpiration. We propose that leaf starch metabolism affects the endogenous stomatal rhythm by modulating cross-tissue sugar homeostasis, which interacts with the circadian clock that in turn affects guard-cell ion transport. One-sentence summary The PhenoLeaks pipeline for monitoring diel transpiration dynamics reveals that leaf starch metabolism sets the timing of the endogenous stomatal rhythm.