ABA Conjugates and Their Physiological Roles in Plant Cells - Abscisic Acid: Metabolism, Transport and Signaling - page 81

in ABA biosynthesis are expressed in most vegetative tissues including leaves
and roots argue against a physiological role of the long-distance transportation
of ABA. However, various biosynthetic genes such as NCED3 and ABA2 are
highly expressed in vascular tissues (Endo et al. 2008 ). Moreover, AtBG1 and
AtBG2 which are involved in the hydrolysis of ABA-GE are also expressed in
the vascular tissues. Thus, this type of the gene expression pattern is consistent
with the notion that ABA is transported from one tissue to other via the xylem.
Another important question regarding long-distance transportation, if indeed
this occurs in plants, is the question as to which form of ABA, active ABA, or
inactive ABA-GE, is involved in the long-distance transport. In the case of
active ABA, long-distance transport through the xylem in stems may not be
easy because ABA in the xylem sap can diffuse to the surrounding parenchyma
cells, especially under acidification of xylem saps, which leads to significant
loss of ABA during transport through the xylem. In contrast, the biochemical
character of ABA-GE is more fitted to long-distance transportation because of
its extremely low biomembrane permeability. Consistent with this notion, the
concentration of ABA conjugates increases in barely xylem saps under salinity
stress conditions (Dietz et al. 2000 ). ABA-GE in the xylem could have originated
from external and internal sources (Sauter et al. 2002 ). In addition, it has also
been shown that ABA-GE was detected in the soil solution at higher concentra-
tions than ABA (Sauter and Hartung 2002 ; Sauter et al. 2002 ; Jiang and Hartung
2008 ). However, only when the exodermis is absent are the Casparian bands
of the exodermis perfect barriers for ABA-GE, the external ABA-GE could be
transported into the apoplast of the root cortex (Hartung et al. 2002 ; Jiang and
Hartung 2008 ). Internally, the release of ABA-GE synthesized in the cytoplasm
of cortex and parenchyma cells into the xylem turns out to be the rate-limiting
process because of the extremely low permeability of plant plasma membranes
(Baier et al. 1990 ; Jiang and Hartung 2008 ). In this sense, a transporter for
ABA-GE is necessary and should play a crucial role in loading ABA-GE to the
xylem under stress conditions. Indeed, Sidler et al. ( 1998 ) provided evidence that
a AtPGP1 transporter located in the plasmalemma in both roots and shoots of
Arabidopsis seedlings is involved in hormonally regulated developmental pro-
cesses. However, the mechanism of ABA-GE transport across the plasma mem-
brane remains unclear.
It is generally believed that ABA-GE is transported from the root tissues
to leaf tissues. At the leaf tissues, ABA-GE is subject to two different path-
ways. One of them involves apoplastic glucosidases that could directly hydro-
lyze ABA-GE to release active ABA, which is then involved in regulating
stomatal opening and closing, or plant development. In another pathway, puta-
tive ABA-GE transporters might conduct the active transport of ABA-GE into
the cytosol of plant cells and then subsequently from the cytosol to subcellu-
lar organelles such as the ER and vacuole where ABA-GE is stored, or hydro-
lyzed to active ABA by ABA-GE-specific ʲ -glucosidases depending on cellular
conditions.