ABA Conjugates and Their Physiological Roles in Plant Cells - Abscisic Acid: Metabolism, Transport and Signaling - page 74

Chapter 5
ABA Conjugates and Their Physiological
Roles in Plant Cells

Zheng-Yi Xu, Yun-Joo Yoo and Inhwan Hwang

Abstract The phytohormone abscisic acid (ABA) is involved in many important
physiological processes such as stomatal closure, seed dormancy, plant growth and
development, and responses to various environmental stresses. Along with ABA,
various conjugated forms of ABA, which are inactive in ABA-related physiology,
have been identified in plant cells. One of the most abundant forms of ABA con-
jugates is an ABA-glucosyl ester named as ABA-GE. The conjugate forms have
long been thought to be by-products that are generated by one of the catabolic
processes involved in lowering cellular ABA levels. However, recent studies pro-
vide evidence for the role of ABA-GE as a reservoir for the rapid production of
active ABA in a compartmentalized fashion. ABA-GE can be converted to active
ABA through a one-step hydrolysis by two ʲ -glucosidases: AtBG1 and AtBG2,
that localize to the ER and vacuole, respectively. Moreover, ABA produced from
ABA-GE by these enzymes is crucial for proper adaptation to abiotic stresses.
Thus, ABA-GE plays a crucial role in ABA-related plant physiology, which is
exerted through the hydrolysis by ʲ -glucosidases. In addition, the physiologi-
cal role of ABA-GE is critically dependent on additional activities including de
novo biosynthesis of ABA, conjugation of ABA with glucose by ABA UDP-
glucosyltransferases, compartmentalization into subcellular organelles and long-
distance transportation between tissues.

Keywords ABA-glucosyl ester · Physiological functions · Metabolism · Transport