Agriculture Reference
In-Depth Information
way begins with isopentyl pyrophosphate (IPP) which is the biological isoprene unit and the
precursor of all terpenoids, as well as many plant hormones. The next step is the epoxida‐
tion of zeaxanthin and antheraxanthin to violaxanthin which is catalyzed by zeaxanthin ep‐
oxidase (ZEP), which was first identified in tobacco [28]. After a series of violaxanthin
modifications which are controlled by the enzyme ABA4, violaxanthin is converted into 9-
cis-epoxycarotenoid [29]. Oxidative cleavage of the major epoxycarotenoid 9-cis-neoxanthin
by the 9-cis-epoxycarotenoid dioxygenase (NCED) yields a C15 intermediate - xanthoxin
[30]. This step is the last one that occurs in the plastid. Xanthoxin is exported to the cyto‐
plasm where two-step reaction via ABA-aldehyde takes place. The first step is catalyzed by
a short-chain alcohol dehydrogenase/reductase (SDR) that is encoded by the
AtABA2
(
ABA
deficient 2
) gene [31-33] and generates ABA aldehyde. Then the ABA aldehyde oxidase
(AAO) with the molybdenum cofactor (MoCo) catalyzes the last step in the biosynthesis
pathway - the conversion of ABA-aldehyde into ABA [34].
Drought stress has been shown to up-regulate
NCED3
expression in Arabidopsis [14], maize
[21], tomato [35], bean [15] and avocado [36]. A significant increase in NCED transcript lev‐
els can be detected within 15 to 30 min after leaf detachment or dehydration treatment [15;
37], indicating activation of NCED genes can be fairly quick. Cheng et al. [32] reported that
the
AtNCED3
gene (and
AtZEP
(
Zeaxanthin Epoxidase
) and
AtAAO3
(
ABA aldehyde oxidase
))
could be induced in the Landsberg erecta background by ABA and studies in rice showed
that
OsNCED3
expression was induced by dehydration [38]. Immunohistochemical analysis,
using antibodies raised against AtNCED3, revealed that the protein is accumulated in the
leaf vascular parenchyma cells in response to drought stress. it was not detected under non-
stressed conditions. These data indicate that the drought induction of ABA biosynthesis oc‐
curs primarily in vascular tissues and that vascular-derived ABA might trigger stomatal
closure via transport to guard cells [39].
AtNCED3
expression is up-regulated by drought
conditions across observed species and decreases after rehydration. At the same time, the
expression level of
AtCYP707A1
,
2
,
3
and
4
(
CYTOCHROME P450, FAMILY 707, SUBFAMI‐
LY A, POLYPEPTIDE 1
,
2
,
3
,
4
) were induced by rehydration [40-41]. These genes, which en‐
code the hydroxylases that are responsible mostly for ABA catabolism, were identified in
Arabidopsis, rice [42], barley [43], wheat [44] and soybean [45].
OsABA8ox1
(
ABA-8-hydroxy‐
lase 1
) expression is induced dramatically by rehydration, which can lead to a decrease in
the ABA content in rice leaves [42].
The balance between active and inactive ABA is very important for plant stress response
and is achieved not only by biosynthesis and catabolism reactions, but also by conjuga‐
tion and deconjugation. ABA can be inactivated at the C-1 hydroxyl group by different
chemical compounds that form various conjugates and accumulate in vacuoles or in the
apoplastic space [46]. The most widespread conjugate is ABA glucosyl ester (ABA-GE)
which is catalyzed by ABA glucosyltransferase [47-48]. Lee et al [49] identified the
AtBG1 (BETA-1,3-GLUCANASE 1) protein which is responsible for the release of ABA
from ABA-GE. Their findings showed that ABA de-conjugation plays a significant role in
providing an ABA pool for plants that allows them to adjust to changing physiological
and environmental conditions.
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