Agriculture Reference
In-Depth Information
AtBG1 and AtBG2 sharing a high-sequence similarity show both overlapping and
also specific expression patterns. The AtBG1 expression is specifically restricted
to seed cells, hydathodes, and vascular tissues in leaf tissues (Lee et al. 2006 ). In
contrast, AtBG2 showed a more broad-expression pattern, such as veins of coty-
ledons and leaf tissues, the vascular bundles of the hypocotyl, and primary and
secondary root tissues (Xu et al. 2012 ). Moreover, both of them are induced under
osmotic stress conditions.
5.4 Compartmentalization of ABA-GE
Location of AtBG1 and AtBG2 in the ER and vacuole, respectively, implies that
ABA-GE is stored in the ER and vacuole. Previous studies have shown that nearly
20 % of total ABA is stored in the vacuole as glucosyl ester forms (Harris and
Dugger 1986 ; Xiong and Zhu 2003 ; Verslues and Zhu 2007 ; Piotrowska and
Bajguz 2011 ). In the cytosol, high levels of ABA are lowered through a process of
glucose conjugation to ABA-GE by the cytosolic ABA-specific UGTs, and subse-
quently, ABA-GE is stored in the vacuole (Fig. 5.2 ). Vacuolar storage of ABA-GE
requires transportation of ABA-GE from the cytosol to the vacuole across the
tonoplast, which most likely occurs via energy-consuming transporters because
ABA-GE is highly impermeable to biomembranes. Recently, it has been shown
that vacuolar ABCC-type ABC transporters, AtABCC1 and AtABCC2, have the
ABA-GE transport activity when expressed in yeast although in vivo evidence is
still lacking in plants (Fig. 5.2 ) (Burla et al. 2013 ).
In contrast to vacuolar ABA-GE, there is little information on the storage of
ABA-GE in the ER. However, the fact that AtBG1 localized to the ER hydrolyzes
ABA-GE to ABA, which is critical for plant development and stress responses,
strongly suggests that ABA-GE is also stored in the ER. In addition, this finding
implies the existence of an ABA-GE transporter at the ER membrane. Unfortunately,
most of the aspects regarding ABA-GE in the ER remain elusive. For example, how
much of ABA-GE is stored in the ER and which type of transporters is involved in
the ABA-GE transportation across the ER membrane remains elusive (Fig. 5.2 ).
Despite the lack of information on these aspects, it is clear that ABA-GE stored in
the ER is crucial for cellular homeostasis of ABA not only for abiotic stress condi-
tions but also under normal physiological conditions as evidenced by the fact that
AtBG1 plays a crucial role in various aspects of ABA-related plant physiology.
5.5 Long-distance Transport of ABA-GE
In ABA-related plant physiology, an important unanswered question is whether
ABA is transported from one tissue to another via long-distance transportation
and whether such transport is necessary in plants. The fact that genes involved
Search WWH ::




Custom Search