ABA Conjugates and Their Physiological Roles in Plant Cells - Abscisic Acid: Metabolism, Transport and Signaling - page 75

5.1 Introduction
The phytohormone ABA is a 15-carbon sesquiterpenoid which functions in series
of physiological processes including seed dormancy, stomatal conductance, and
early embryo morphogenesis as well as in the synthesis of seed storage proteins.
In addition, ABA also is involved in various abiotic and biotic stress responses.
Under different environmental conditions, ABA modulates expression of a large
number of genes so as to optimize plant growth and developmental strategies to
these conditions (Thomashow 1999 ; Shinozaki and Yamaguchi-Shinozaki 2000 ;
Zhu 2001 ; Bray 2002 ).
For these physiological responses, alteration of cellular ABA levels is essential
and this is achieved through two opposing processes: biosynthesis and catabolism
of ABA. De novo ABA biosynthesis starts from carotenoids and includes many
intermediates (Nambara and Marion-Poll 2005 ). In the lengthy biosynthetic pro-
cess, the intermediate steps leading to xanthoxin occur in chloroplasts, whereas
the final two steps for conversion of xanthoxin to ABA occur in the cytoplasm
(Cheng et al. 2002 ; Gonzalez-Guzman et al. 2002 ; Seo et al. 2004 ). On the other
hand, the catabolic processes of ABA consist of two different pathways: hydrox-
ylation and conjugation. In Arabidopsis, enzymes encoded by the AtCYP707A
gene family ( CYP707A1, 2, 3 and 4 ) mediate hydroxylation of ABA to produce
hydroxylated ABA which is first metabolized to PA (Saito et al. 2004 ), then fur-
ther converted to DPA by isomerization. In the conjugation pathway, glucose is
added to ABA to produce ABA-glucosyl ester which exhibits little or no biological
activity and is stored in the vacuole and ER (Vilaró et al. 2006 ). However, recent
studies showed that inactive ABA-GE can be converted to active ABA by a one-
step hydrolysis carried out by certain ʲ -glucosidase (Dietz et al. 2000 ; Sauter et al.
2002 ; Lee et al. 2006 ; Xu et al. 2012 , 2013 ). This indicates that ABA-GE is not a
simple by-product produced in the inactivation process of ABA but rather plays a
crucial role as the stored form of ABA.
5.2 Generation of ABA-GE
Until now, the most well-known conjugated form of ABA is ABA-GE which
exhibits little or no biological activity (Vilaró et al. 2006 ). ABA-GE is generated
from ABA in the course of the inactivation. Consistent with this notion, the con-
centration of ABA-GE increases substantially under stress conditions (Sauter et al.
2002 ). Moreover, repeated water stress treatment caused increased ABA-GE levels
in Xanthium strumarium leaf (Zeevaart 1983 ).
In the cell, ABA-GE is produced by ABA UDP-glucosyltransferase (UGT)
through esterification of ABA with glucose (Zeevaart and Creelman 1998 ; Lim
et al. 2005 ; Priest et al. 2006 ; Piotrowska and Bajguz 2011 ). In Arabidopsis ,
the UGT family consists of more than 100 homologous proteins which can
be divided into 12 subfamilies (Lorenc-Kukula et al. 2004 ; Ross et al. 2004 ;