ABA Transport and Distribution in Relation to Its Function in Plants

Abstract

Abscisic acid (ABA) is a phytohormone with key roles in various physiological processes, including germination, stomatal movement, and biotic and abiotic stress responses. Given that the sites of ABA biosynthesis and action are separated in plant cells, ABA transport is an important step in ABA signaling. ABA exhibits distinct patterns of compartmentalization, and its level in a specific compartment is not only determined by its rate of biosynthesis and catabolism, but also by the rates of ABA export and import. Compartmentalization ensures that the physiology of the plant is not affected by ABA under normal conditions and that redistribution of ABA in response to stress stimuli triggers ABA stress signaling. ABA is transported between cells or tissues and also systemically in whole plants. The expression of genes encoding key enzymes in the ABA biosynthesis pathway in vascular tissues and the overlap in the expression patterns of genes encoding enzymes involved in ABA biosynthesis and ABA transport implies the importance of ABA transport, particularly ABA systemic transport, in ABA functioning. The root-to-shoot transport of ABA further shows that ABA transport has critical roles in plant systemic signaling, and the molecular identification and functional characterization of ABA transporters provided direct evidence that ABA transport is required for ABA signaling. The ion-trap mechanism by pH and ABA transporters may operate synergistically, but in different manners, to control ABA transport and distribution. Further investigations on ABA transport and localization will contribute to our understanding of mechanisms that modulate ABA signaling in plants.