Guard cell photorespiration controls stomata behavior and development.

Photorespiration is traditionally viewed as a limitation to photosynthetic efficiency. However, it is mandatory for safeguarding the Calvin-Benson-Bassham cycle from inhibitory byproducts through Rubisco-mediated oxidative misfire and is tightly integrated with primary metabolism. Whether photorespiratory metabolism directly regulates guard cell function and stomatal behavior remains a matter of intense debate. We manipulated the photorespiration-restricted pathway entry enzyme 2-phosphoglycolate phosphatase (PGLP1) specifically in Arabidopsis thaliana (Arabidopsis) guard cells and assessed effects on growth, photosynthesis, carbohydrate allocation, cell-specific H2O2 accumulation, and stomatal traits under photorespiratory conditions, including exogenous 2-phosphoglycolate (2-PG) feeding. Altered guard cell PGLP1 protein expression consistently affected plant growth, photosynthetic performance, and stomatal movement. PGLP1 perturbation induced guard cell-specific starch and H2O2 accumulation patterns, both of which are central components involved in driving optimal stomatal behavior. Further, altered stomatal size was observed, a phenotype that was recapitulated by external 2-PG application to wild-type plants. Efficient photorespiratory metabolism is essential for proper guard cell function and acclimation to changing CO2 : O2 ratios. Our findings uncover a direct metabolic link between photorespiration and stomatal behavior, revealing an unexpected role for this ancient pathway in controlling gas exchange, photosynthesis, and potentially plant productivity and resilience.