Blue light (BL) receptor phototropins activate the plasma membrane H(+)-ATPase in guard cells through phosphorylation of a penultimate threonine and subsequent binding of the 14-3-3 protein to the phosphorylated C-terminus of H⁺-ATPase, mediating stomatal opening. To date, detection of the phosphorylation level of the guard cell H⁺-ATPase has been performed biochemically using guard cell protoplasts (GCPs). However, preparation of GCPs from Arabidopsis for this purpose requires >5,000 rosette leaves and takes >8 h. Here, we show that BL-induced phosphorylation of guard cell H⁺-ATPase is detected in the epidermis from a single Arabidopsis rosette leaf via an immunohistochemical method using a specific antibody against the phosphorylated penultimate threonine of H⁺-ATPase. BL-induced phosphorylation of the H⁺-ATPase was detected immunohistochemically in the wild type, but not in a phot1-5 phot2-1 double mutant. Moreover, we found that physiological concentrations of the phytohormone ABA completely inhibited BL-induced phosphorylation of guard cell H⁺-ATPase in the epidermis, and that inhibition by ABA in the epidermis is more sensitive than in GCPs. These results indicate that this immunohistochemical method is very useful for detecting the phosphorylation status of guard cell H⁺-ATPase. Thus, we applied this technique to ABA-insensitive mutants (abi1-1, abi2-1 and ost1-2) and found that ABA had no effect on BL-induced phosphorylation in these mutants. These results indicate that inhibition of BL-induced phosphorylation of guard cell H⁺-ATPase by ABA is regulated by ABI1, ABI2 and OST1, which are known to be early ABA signaling components for a wide range of ABA responses in plants.