Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 147

10.2 effect of abiotic stress on
aBa biosynthesis, catabolism
and transport

activity (Zhou et al., 2004), but the hydroxylation reac-
tion triggers further steps of inactivation (Zeevaart &
Creelman, 1988; Cutler & Krochko, 1999). ABA conju-
gation and deconjugation are clearly dynamic processes
that play a key role in controlling the amount of biologi-
cally active ABA under both non-stressful and stressful
conditions (Verslues & Zhu 2007). In the conjugation
pathway, ABA is conjugated with glucose by ABA-
glucosyltransferase (ABAGTase) (Figure 10.1). It has
been reported that other conjugates with the hydroxyl
groups of ABA and its hydroxylated catabolites could be
physiologically inactive and accumulate in vacuoles
(Bray & Zeevaart, 1985; Lehmann & Glund, 1986).
However, recently a role for the ABA conjugate, abscisic
acid glucose ester (ABA-GE), was proposed: it could be
involved in long-distance ABA transport (Hartung et al.,
2002; Wilkinson & Davies, 2002). ABA-GE had previ-
ously been thought to be a physiologically inactive
by-product stored in the vacuoles (Zeevaart & Creelman,
1983). ABA-GE cannot migrate passively through
plasma membranes, and the mechanisms underlying the
transport of conjugates of ABA remain incompletely
understood, as do the functions of conjugated forms of
ABA (Schachtman & Goodger, 2008). However, ABA-GE
was reported as an allelopathic substance of Citrus junos
(Kato et al., 2003), and soil in agricultural fields contains
higher ABA-GE content (up to 30 nM) than ABA. It has
been hypothesized that ABA-GE is taken up by the root
(Saito et al., 2004). The ABA-GE glucosidase activity is
increased by salt stress and is inhibited competitively by
ABA-GE or zeatin riboside. Nevertheless, recently Llanes
et al. (2014) proposed that ABA and ABA-GE from roots
and leaves work together to create and intensify a specific
stress signal. This mechanism had not been previously
reported for legumes. ABA-GE was the major metabolite
found in the roots of the legume P. strombulifera under
salt stress. Both the roots and leaves of Na 2 SO 4 -treated
plants at −1.9 and −2.6 MPa showed high amounts of
ABA-GE and free ABA, suggesting the presence of a
system for transport of ABA-GE from roots to leaves.
These plants showed a high glucosidase activity in the
roots and mainly in the leaves, suggesting that ABA-GE
could be a reservoir of free ABA in roots as well as an
ABA-transport form from roots to leaves in this species.
In NaCl-treated P. strombulifera ABA-GE peaked in roots
at 12 and 24 h, whereas in leaves there were no differ-
ences from the controls. Thus, roots could act as a
reservoir of free ABA (Llanes et al., 2014). However, few

In higher plants, the synthesis of ABA follows an
'indirect' pathway via cleavage of a C 40 carotenoid pre-
cursor, followed by a two-step conversion of the
intermediate xanthoxin to ABA via ABA aldehyde. The
epoxidation of zeaxanthin and antheraxanthin to vio-
laxanthin occurs in the plastids. This is the first key step
in ABA biosynthesis pathway and it is catalysed by zea-
xanthin epoxidase (ZEP). Molecular identification of this
enzyme in tobacco was reported by Marin et al. (1996).
The violaxanthin is converted to 9- cis -epoxycarotenoid
through structural modifications. Epoxycarotenoid 9- cis -
neoxanthin undergoes an oxidative cleavage by 9- cis -
epoxycarotenoid dioxygenase (NCED) resulting in a C 15
intermediate called xanthoxin (Schwartz et al., 1997).
This reaction, which is the first relevant step in the ABA
biosynthetic pathway, is followed by the export of xan-
thoxin to the cytosol. Here, xanthoxin is converted to
ABA-aldehyde and finally to free ABA through two
reactions. The AtABA2 gene encodes a short-chain
alcohol dehydrogenase/reductase (SDR) that catalyses
the first reaction step in ABA aldehyde generation. The
enzyme ABA aldehyde oxidase (AAO) catalyses the final
step in ABA biosynthesis (Rook et al., 2001; Gonzalez-
Guzman et al., 2002, Cheng et al., 2002).
The biosynthesis of ABA can be regulated directly or
indirectly by certain environmental signals. However,
the environmental signals that dramatically activate
ABA biosynthesis are drought and salt stress. Under
these abiotic stresses there is an increase in ABA levels
resulting mainly from enhanced de novo biosynthesis.
Both drought and salt stress induce ABA biosynthesis
by controlling the transcriptional regulation of genes
encoding enzymes of the ABA biosynthetic pathway
(Nambara & Marion-Poll, 2005). Therefore, it is
essential to understand how ABA biosynthesis is regu-
lated by transcriptional regulation of ABA biosynthetic
genes and by the specific activities of ABA biosyn-
thesis enzymes.
The catabolism of ABA is divided into two pathways,
hydroxylation and glucose conjugation. There are three
different methyl groups on the carbon skeleton of ABA
(C-7, C-8 and C-9), but the C-8 position is the predomi-
nant position for the hydroxylation reaction. These three
forms of ABA hydroxylate still have substantial biological