Abstract
Stomatal opening and closing are driven by ion fluxes that cause changes in guard cell turgor and volume. This process is, in turn, regulated by environmental and hormonal signals, including light and the phytohormone abscisic acid (ABA). Here, we present genetic evidence that expression of $PHO1$ in guard cells of Arabidopsis thaliana is required for full stomatal responses to ABA. $PHO1$ is involved in the export of phosphate into the root xylem vessels and, as a result, the $pho1$ mutant is characterized by low shoot phosphate levels. In leaves, $PHO1$ was found expressed in guard cells and up‐regulated following treatment with ABA. The $pho1$ mutant was unaffected in production of reactive oxygen species following ABA treatment, and in stomatal movements in response to light cues, high extracellular calcium, auxin, and fusicoccin. However, stomatal movements in response to ABA treatment were severely impaired, both in terms of induction of closure and inhibition of opening. Micro‐grafting a $pho1$ shoot scion onto wild‐type rootstock resulted in plants with normal shoot growth and phosphate content, but failed to restore normal stomatal response to ABA treatment. $PHO1$ knockdown using RNA interference specifically in guard cells of wild‐type plants caused a reduced stomatal response to ABA. In agreement, specific expression of $PHO1$ in guard cells of $pho1$ plants complemented the mutant guard cell phenotype and re‐established ABA sensitivity, although full functional complementation was dependent on shoot phosphate sufficiency. Together, these data reveal an important role for phosphate and the action of $PHO1$ in the stomatal response to ABA.