Stomatal pores surrounded by a pair of guard cells in the plant epidermis control gas exchange for photosynthesis in response to light, CO2, and phytohormone abscisic acid [1, 2]. Phototropins (phot1 and phot2) are plant blue-light receptor kinases and mediate stomatal opening via activation of the plasma membrane H+-ATPase [3, 4]. However, the signaling mechanism from phototropins to the H+-ATPase has yet to be determined. Here, we show that FLOWERING LOCUS T(FT) is expressed in guard cells and regulates stomata! opening. We isolated an scs (suppressor of closed-stomata phenotype in phot1 phot2) 1-1 mutant of Arabidopsis thaliana and showed that scs1-1 carries a novel null early flowering 3 (elf3) allele in a phot1 phot2 background. scs1-1 (elf3 phot1 phot2 triple mutant) had an open-stomata phenotype with high H+-ATPase activity and showed increased levels of FT mRNA in guard cells. Transgenic plants overexpressing FT in guard cells showed open stomata, whereas a loss-of-function FT allele, ft-1, exhibited closed stomata and failed to activate the H+-ATPase in response to blue light. Our results define a new cell-autonomous role for FT and demonstrate that the flowering time genes ELF3 and FT are involved in the regulation of H+-ATPase by blue light in guard cells.