A possible stress physiological role of abscisic acid conjugates in root‐to‐shoot signalling

Abstract

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.

Plant Water Relations: Absorption, Transport and Control Mechanisms

“…After the synthesis of ABA in the roots, it is transported through the xylem to the leaves alone or conjugated with glucose (the latter being more appropriate for transport over long distances). The proportion of each form of transport (alone or conjugated) is variable between species (Sauter et al, 2002). Once in the leaf, conjugated ABA is hydrolysed into its free form by the apoplastic enzyme β-D-glucosidase, inducing stomatal closure through a signalling system in the guard cells of chloroplasts (Yokota et al, 2006).…”

Effects of Soil Temperature on Growth and Root Function in Rice

“…5). The role and physiological signifi cance of hormones in the xylem exudate as stress signals are widely recognized (Bano et al, 1993;Davies and Zhang, 1991;Sauter and Hartung, 2002). The ABA level has been shown to be increased by water stress, and inhibit transpiration through stomatal closure (Davies and Zhang, 1991).…”

Vacuolar Transport of Abscisic Acid Glucosyl Ester Is Mediated by ATP-Binding Cassette and Proton-Antiport Mechanisms in Arabidopsis

“…ABA-GE levels were shown to substantially increase during dehydration and specific seed developmental and germination stages (Boyer and Zeevaart, 1982;Hocher et al, 1991;Chiwocha et al, 2003). Furthermore, ABA-GE is present in the xylem sap, where it was shown to increase under drought, salt, and osmotic stress (Sauter et al, 2002). Apoplastic ABA b-glucosidases in leaves have been suggested to mediate the release of free ABA from xylem-borne ABA-GE (Dietz et al, 2000).…”