ABA Transport and Plant Water Stress Responses

Plants close stomata to limit water loss on water deficit. It is evident that ABA is required in this process based on the phenotypes of mutants defective in ABA biosynthesis or signal transduction. ABA may be translocated from the sites of biosynthesis, such as roots and leaf vascular tissues, to the guard cells. Recent identification of multiple transmembrane ABA transporters indicates that the movement of this hormone within a plant is actively regulated in an intercellular network. ABA regulates various molecular events in different organs and tissues as well as guard cells to tolerate water stress dependent on environmental conditions. However, the dynamics of ABA transport within a plant must be analyzed and discussed in a more spatiotemporally physiological context. To understand the integrative networks of signaling molecules, the sites of their biosynthesis and action must be clarified, particularly for phytohormones such as abscisic acid (ABA). The relationship between the sites of ABA biosynthesis and transport has been discussed extensively in the context of guard cells and stomatal regulation. However, guard cells are not the only site of ABA action. Recent studies have reported multiple sites of ABA biosynthesis and multiple ABA transporters, indicating that ABA transport regulation is not unidirectional but rather forms complex networks. Therefore, it is important to determine how multiple ABA sources coordinately contribute to individual biological processes under various physiological conditions. To understand the integrative networks of signaling molecules, the sites of their biosynthesis and action must be clarified, particularly for phytohormones such as abscisic acid (ABA). The relationship between the sites of ABA biosynthesis and transport has been discussed extensively in the context of guard cells and stomatal regulation. However, guard cells are not the only site of ABA action. Recent studies have reported multiple sites of ABA biosynthesis and multiple ABA transporters, indicating that ABA transport regulation is not unidirectional but rather forms complex networks. Therefore, it is important to determine how multiple ABA sources coordinately contribute to individual biological processes under various physiological conditions.