Guard cell starch and malate metabolism facilitate stomatal opening in response to low <scp> CO ₂ </scp>

Guard cells adjust turgor to regulate stomatal aperture, integrating ion transport with metabolic adjustments. The effect of reduced CO₂ on stomatal movement, expected under enhanced photosynthesis, remains largely unexplored. We exposed Arabidopsis thaliana plants to low CO₂ before light onset and analyzed guard cell starch turnover, malate (Mal) accumulation, and stomatal conductance in wild-type and mutant lines. Low CO₂ triggered rapid starch degradation and increased Mal levels in guard cells, accelerating stomatal opening. Mutants defective in starch degradation or hexose uptake showed impaired responses. These findings demonstrate that low CO₂ rewires guard cell metabolism through both internal and external carbon sources, enhancing the responsiveness to light and offering new avenues for stomatal bioengineering.