New fava bean guard cell signaling mutant impaired in ABA-induced stomatal closure

We isolated a mutant from Vicia faba L. cv. House Ryousai. It wilts easily under strong light and high temperature conditions, suggesting that its stomatal movement may be disturbed. We determined responses of mutant guard cells to some environmental stimuli. Mutant guard cells demonstrated an impaired ability to respond to ABA in 0.1 mM CaCl2 and stomata did not close in the presence of up to 1 mM ABA, whereas wild-type stomata closed when exposed to 10 muM ABA. Elevating external Ca2+ caused a similar degree of stomatal closure in the wild type and the mutant. A high concentration of CO2 (700 mul liter(-1)) induced stomatal closure in the wild type, but not in the mutant. On the basis of these results, we propose the working hypothesis that the mutation occurs in the region downstream of CO2 and ABA sensing and in the region upstream of Ca2+ elevation. The mutant is named fia (fava bean impaired in ABA-induced stomatal closure).