Long-Distance Signals Produced by Water-Stressed Roots

Abstract

Roots can sense small changes in soil water status and rapidly communicate this over long distances throughout the plant. This sets in motion numerous response mechanisms for water conservation and drought tolerance, largely facilitated by the hormone ABA. Despite impressive advances in the molecular mechanisms by which ABA mediates such plant responses, long-distance signaling of soil water status remains relatively poorly understood. Recent results refute the long-held hypothesis of ABA biosynthesis in roots as the primary signal, at least in the initial stage of water stress communication. This chapter examines the involvement of leaf ABA biosynthesis, pH-mediated ABA redistribution, and ABA conjugate catabolism in communicating soil water status. In addition, the chapter presents current knowledge on other xylem-borne signaling compounds such as cytokinins, 1-aminocyclopropane-1-carboxylic acid, inorganic ions, and organic acids and their possible interactions with ABA in long-distance signaling of water stress.