The Compartmentation of Abscisic Acid and beta-d-Glucopyranosyl Abscisate in Mesophyll Cells.

beta-d-Glucopyranosyl abscisate (ABA-GE) is synthesized in Xanthium strumarium L. leaves during water stress. Following recovery from stress, the amount of ABA-GE does not decline. These observations led to the hypothesis that ABA-GE is sequestered in the vacuole where it is metabolically inert. The localization of abscisic acid (ABA) and ABA-GE was investigated by a dimethyl sulfoxide (DMSO) compartmentation method and by direct isolation of vacuoles.With the DMSO compartmentation method it was shown that in Xanthium mesophyll cells ABA was in a compartment not accessible to DMSO, presumably the chloroplast, whereas ABA-GE was in a compartment accessible to DMSO, presumably the vacuole. Neutral red, which accumulates in the vacuoles, showed a similar DMSO concentration dependence for its release from the cells as ABA-GE.Vacuoles isolated from Vicia faba L. leaf protoplasts contained 22% of the total ABA and 91% of the ABA-GE. Some of the ABA in the vacuole preparations was probably due to cytoplasmic contamination. These findings indicate that ABA-GE is sequestered in the vacuoles of mesophyll cells where the conjugated form of ABA is removed from the active ABA pool.