ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 61

and catabolism. It has been suggested that the level of xanthophyll precursors is
several hundred times higher than the ABA level under non-stressed conditions
(Zeevaart and Creelman 1988). Under stressed conditions, the accelerated rate of
ABA biosynthesis would likely lead to the depletion of ABA precursors, such that
content of ABA precursors may become a limiting factor in ABA accumulation.
In support of this notion, a study by Ren et al. ( 2007 ) demonstrated that the water
stress-induced ABA in maize roots depends on the delivery of ABA precursors
from the leaves to the roots (Ren et al. 2007 ).
From a molecular perspective, the changes in ABA content at specific sites are
generally thought to be determined by key enzymes in the ABA biosynthesis and
catabolism pathway. ABA arises from the xanthophyll cleavage product. The first
step of ABA biosynthesis, which is catalyzed by zeaxanthin epoxidase (ZEP), is
the conversion of zeaxanthin to all-trans-violaxanthin, and the second step, which
is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED), is the production of
xanthoxin through the oxidative cleavage of 9-cis-violaxanthin and/or 9′-cisneox-
anthin. The later events of ABA biosynthesis are dominated by a major pathway
that includes the conversion of xanthoxin to abscisic aldehyde, in a reaction that
is catalyzed by a short-chain dehydrogenase/reductase (ABA2), and the conver-
sion of abscisic aldehyde to ABA, which is catalyzed by ABA aldehyde oxidase
(AAO3). ABA catabolism includes two major pathways, i.e., the catabolic path-
way of ABA hydroxylation and the conjugation of ABA (Nambara and Marion-
Poll 2005 ). In the ABA catabolic pathway, 8′-hydroxylation is believed to be the
key regulator of ABA levels. ABA 8′-hydroxylation is catalyzed by a cytochrome
P450 monooxygenase (P450). In Arabidopsis, P450 CYP707A genes that encode
ABA 8′-hydroxylases have been identified (Kushiro et al. 2004 ; Saito et al. 2004 ).
ABA conjugation is largely through the conjugation of the carboxyl and hydroxyl
groups of ABA with glucose to produce ABA glucosyl ester (ABA-GE). The
release of ABA from ABA-GE, which is catalyzed by beta-glucosidase, has been
reported to play an important role in the regulation of ABA accumulation.
The first two steps of the ABA biosynthesis pathway are believed to occur in
plastids and the later steps occur in the cytosol (Seo and Koshiba 2002 ). However,
evidence suggests that ABA is largely distributed in chloroplasts rather than in
the cytosol. This indicates that the sites of ABA biosynthesis may differ from
those of the cellular distribution of ABA. Tan et al. ( 2003 ) have identified nine
NCED genes in the complete genome sequence. Spatial expression analysis using
promoter::GUS fusions in transgenic Arabidopsis plants revealed that different
AtNCED proteins have different expression patterns, but that localization to chlo-
roplasts is a distinguishing characteristic of all AtNCEDs. Furthermore, several
AtNCEDs localized largely to vascular tissues. Using a similar approach, Cheng
et al. ( 2002 ) reported that ABA2 is largely localized to vascular tissues in both
roots and leaves in Arabidopsis seedlings. Similar to the observations for the local-
ization of NCED and ABA2, Koiwai et al. ( 2004 ) reported that AAO3 is abundant
in vascular tissues, especially in phloem companion cells and xylem parenchyma
cells. These studies suggest that chloroplasts in leaves and vascular tissues play
important roles in ABA biosynthesis under unstressed conditions.