Transport of ABA from the site of biosynthesis to the site of action.

There is substantial evidence that abscisic acid (ABA) moves within plants. ABA has been considered as a root-derived signaling molecule that induces stomatal closure in response to dry soil conditions. It has been also reported that ABA synthesized in vegetative tissues is translocated to the seeds. The transport of ABA is an important factor in determining the endogenous concentrations of the hormone at the site of action, and hence, it is an important process in physiological responses. However, the molecular mechanisms that regulate ABA transport are not fully understood. Recent studies using Arabidopsis indicate that ABA is actively synthesized in leaf vascular tissues in response to drought, and that ABA is subsequently transported to the guard cells to close stomata. Identification of the transporters that mediate ABA export from the inside to the outside of the cells at the site of ABA biosynthesis (vascular tissues) and ABA uptake into the cells at the site of action (guard cells), respectively, in this species indicates an active mechanism to regulate ABA transport. Although Arabidopsis represents only one model plant, these findings are useful to discuss common or different regulatory mechanisms among different species and to improve our total understanding of the regulation of ABA transport.