Download Abscisic Acid and Antioxidant Defense in Plant Cells

Acta Botanica Sinica
植 物 学 报
2004, 46 (1): 1－9
http://www.chineseplantscience.com
．Review．
Abscisic Acid and Antioxidant Defense in Plant Cells
JIANG Ming-Yi1*, ZHANG Jian-Hua2
(1. College of Life Sciences, Nanjing Agricultural University, Nanjing 210095, China;
2. Department of Biology, Hong Kong Baptist University, Hong Kong, China)
Abstract: Water stress is one of the most important environmental factors that affect plant growth and
development, and limit plant production. Plants can respond and adapt to water stress by perceiving the
stimulus, generating and transmitting the signals, and initiating various defense mechanisms. The plant
hormone abscisic acid (ABA), as a stress signal, plays important roles in the regulation of plant responses
to water stress. ABA not only regulates water balance by inducing stomatal closure, but also enhances
water stress tolerance by inducing the expression of genes that encode dehydration tolerance proteins.
Increasing evidence indicates that ABA-enhanced water stress tolerance is related to the induction of
antioxidant defense systems by ABA. In this review, recent advances on the roles of ABA in the induction
of the generation of reactive oxygen species (ROS), the expression of antioxidant enzyme genes, and the
capacity of antioxidant defense systems are presented. Special attention is given to the cross-talk mechanisms between Ca2+ and ROS that originates from NADPH oxidase in the ABA-induced antioxidant defense
in plants.
Key words: abscisic acid (ABA); antioxidant defense; reactive oxygen species (ROS); signal transduction;
water stress
Plants respond to environmental challenges by altering
their cellular metabolism and invoking various defense
mechanisms. Survival under these stress conditions depends on the plant’s ability to perceive the stimulus, generate and transmit signals, and instigate biochemical
changes that adjust the metabolism accordingly (Rao et
al., 1997; Shinozaki and Yamaguchi-Shinozaki, 1997). One
important regulator of plant responses to abiotic stresses
is the phytohormone abscisic acid (ABA). Under drought,
cold or salt stress condition, plants accumulate increased
amounts of ABA, with drought stress having the most
prominent effect. ABA plays important roles in the induction of plant tolerance to these stress conditions (Shinozaki
and Yamaguchi-Shinozaki, 1997; Finkelstein et al., 2002;
Xiong et al., 2002; Zhu, 2002). Increasing evidence indicates that one mode of ABA action is related to oxidative
stress in plant cells. In this paper, we shall focus on the
roles of ABA in the induction of the generation of reactive
oxygen species (ROS), the expression of antioxidant defense genes, the capacity of antioxidant defense systems,
and the possible mechanisms involved in ABA-induced
antioxidant defense.
1 ABA-Induced ROS Generation
In ABA-induced stomatal closure of guard cells of
Received 26 Feb. 2003 Accepted 30 Jun. 2003
* Author for correspondence. E-mail: <[email protected]>.
Arabidopsis, ABA treatments induced a rapid increase in
the production of H2O2 (Pei et al., 2000). Treatment with 1
µmol/L ABA increased the production of H2O2 by 36.8%,
and treatment with 50 µmol/L ABA increased by 49%. Similar results were also observed in ABA-induced stomatal
closure of guard cells of Arabidopsis (wild type and abi21 mutant; Murata et al., 2001) and Vicia faba (Zhang et al.,
2001). In maize embryo cells, H2O2 levels significantly increased in response to 100 µmol/L ABA at 0.5 up to 3 h, at
4 h after ABA treatment, H2O2 levels returned to the control
level (Guan et al., 2000). ABA-induced increases in H2O2
have also been reported for rice roots (Lin and Kao, 2001).
Our recent studies not only showed that ABA induces increases in the generation of O－2. and H2O2 in maize leaves
(Jiang and Zhang, 2001), but also further indicated that
water stress-induced ROS generation results, at least in
part, from the accumulation of ABA induced by water stress
(Jiang and Zhang, 2002a; 2002b). These results suggest
that ABA-induced ROS production may be of more general
importance for ABA signal transduction in plants.
The source of ROS induced by ABA is gaining
attentions. There are many potential sources of ROS, including intracellular sources such as chloroplasts, mitochondria and peroxisomes, plasma membrane NADPH
oxidase, cell wall peroxidase, and apoplastic oxalate
２
oxidase and amine oxidases in plants (Grant and Loake,
2000; Mittler, 2002; Neill et al., 2002b; Vranováet al., 2002).
It has been shown that one source of ROS generation induced by ABA may be the plasma membrane NADPH
oxidase, which transfers electrons from cytoplasmic NADPH
to O2 to form O－2. , followed by dismutation of O－2. to H2O2. In
guard cells of Arabidopsis, ABA-induced H2O2 production and H2O2-activated Ca2+ channels are important mechanisms for ABA-induced stomatal closing (Pei et al., 2000).
Diphenylene iodonium (DPI), a well-known inhibitor of neutrophil NADPH oxidase, partially inhibits ABA-induced
stomatal closing. In guard cells of V. faba, ABA-induced
H2O2 production is partly inhibited by 10 µmol/L DPI
(Zhang et al., 2001). Cytosolic NADPH is required for ABA
activation of Ca2+ channels in ABA-induced stomatal closure in Arabidopsis (Murata et al., 2001). These results
suggest that NADPH oxidase contributes to early ABA
signaling. However, DPI at the higher concentrations can
also affect other enzymes potentially involved in the generation of ROS, including extracellular peroxidases and nitric oxide synthase (Bolwell et al., 1998; Frahry and Schopfer,
2001; Orozco-Cárdenas et al., 2001). Furthermore, an NAD
(P)H oxidase-peroxidase can also use NAD(P)H as an electron donor to produce ROS (Papadakis and RoubelakisAngelakis, 1999; Frahry and Schopfer, 2001). Our recent
study, using two-phase fractionated plasma membrane extracts and several widely-used neutrophil NADPH oxidase
inhibitors, such as DPI, imidazole and pyridine, has demonstrated that NADPH oxidase is involved in ABA- and water
stress-induced ROS production, and water stress-induced
ABA accumulation plays an important role in the regulation of NADPH oxidase activity in maize leaves (Jiang and
Zhang, 2002c). In addition to NADPH oxidase, cell wall
NADH-peroxidase and diamine oxidase (Lin and Kao, 2001),
and light reaction in chloroplasts (Zhang et al., 2001) may
also contribute to ABA-induced production of ROS.
2 Expression of Genes Encoding Antioxidant
Enzymes Induced by ABA
It has been documented that ABA can induce the expression of antioxidant genes encoding Cu/Zn-superoxide
dismutase (SOD) (Guan and Scandalios, 1998a; Kaminaka
et al., 1999), Mn-SOD (Zhu and Scandalios, 1994; Bueno et
al., 1998; Kaminaka et al., 1999) and Fe-SOD (Kaminaka et
al., 1999), and catalase (CAT) (Anderson et al., 1994; Guan
and Scandalios, 1998b; Guan et al., 2000) in plants. The
expressions of Sod and Cat genes in response to ABA
depend on plant species or varieties. For example, in rice
seedlings, ABA induced an increase at the transcript levels
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of SodA1, which encodes Mn-SOD, SodB, which encodes
Fe-SOD, and SodCc1 and SodCc2, which encodes cytosolic Cu/Zn-SODs, in a dose-dependent manner (Kaminaka et
al., 1999). But in tobacco BY-2 cell suspensions, ABA treatments only increased the accumulation at the transcript
level of Mn-SOD gene, and did not change the transcript
level of Fe-SOD gene and reduced that of cytosolic Cu/ZnSOD gene (Bueno et al., 1998). In an inbred maize line, the
transcript of Cat3 was induced 3.6-fold by ABA in coleoptiles and 3.3-fold in mesocotyls (Anderson et al., 1994). In
another inbred maize line, however, the Cat3 transcript decreased after 2 h of ABA treatment in leaves (Guan and
Scandalios, 1998b). Moreover, the expressions of Sod and
Cat genes are different in response to ABA at different
developmental stages in plants. For example, in maize, the
transcripts of Sod4 and Cat1 accumulated in response to
ABA in developing and germinating embryos, and in young
leaves; the Sod4A transcript showed no increase in response
to ABA in developing and germinating embryos, but increased in young leaves; Cat2 and Cat3 transcripts were
up-regulated only at very high ABA concentrations (10－3
mol/L) during late embryogenesis and in response to various concentrations of ABA in germinating embryos, and
the Cat2 transcript increased in response to ABA and the
Cat3 transcript decreased in young leaves (Guan and
Scandalios, 1998a; 1998b). These results suggest that the
Sod and Cat genes in maize are regulated by ABA in a
multilayered fashion.
Water stress can induce ABA accumulation and the expression of Sod and Cat genes in plants. There should be
a connection between water stress-induced ABA accumulation and the expression of these antioxidant genes in
plants. Utilizing the ABA-deficient maize mutant vp5, the
connection between them has been examined. It has been
shown that the transcripts of Sod3.2, Sod3.3, Sod3.4, Sod4,
and Cat1 are only increased in wild-type embryos in response to osmotic stress; however, the transcripts of these
genes increased to the same levels in the wild-type and vp5
embryos in response to ABA (Zhu and Scandalios, 1994;
Guan and Scandalios, 1998a; 1998b). These results suggest that the increases of these Sod and Cat transcripts in
response to high osmoticum are mediated by an increase at
endogenous ABA levels.
3 Effect of ABA on Enzymatic and Non-enzymatic Antioxidants
ABA not only induces the expression of antioxidant
defense genes, but also enhances the activities of antioxidant enzymes in plants. It has been shown that ABA
JIANG Ming-Yi et al.: Abscisic Acid and Antioxidant Defense in Plant Cells
increases the activities of total SOD, Cu/Zn-SOD, Mn-SOD,
Fe-SOD, CAT, ascorbate peroxidase (APX), and glutathione
reductase (GR) in plants (Anderson et al., 1994; Prasad et
al., 1994a; Bueno et al., 1998; Gong et al., 1998; Guan and
Scandalios, 1998b; Bellaire et al., 2000). In tobacco BY-2
cell cultures, ABA treatment increased the activities of total SOD, Cu/Zn-SOD, Mn-SOD, Fe-SOD, CAT, and APX,
and reduced the activity of GR (Bueno et al., 1998). In maize
coleoptiles (Gong et al., 1998) and cotton callus tissue
(Bellaire et al., 2000), ABA treatment increased the activities of all antioxidant enzymes including SOD, peroxidase,
CAT, APX and GR tested in these studies. Our recent studies show that ABA not only induces the increases in the
activities of these antioxidant enzymes in leaves of maize
seedlings, but also induces the increases in the contents of
non-enzymatic antioxidants such as ascorbate, reduced
glutathione, α-tocopherol and carotenoids (Jiang and
Zhang, 2001; 2002a). These results suggest that ABA can
induce the capacity of whole antioxidant defense systems
including enzymatic and non-enzymatic constituents in
plants.
The relationship between ABA accumulation and antioxidant defense in plants under stress conditions has been
also investigated. Pretreatment with ABA enhances the
capacity of antioxidant defense systems in plants exposed
to environmental stresses such as chilling (Anderson et
al., 1994; Prasad et al., 1994a), high temperature (Gong et
al., 1998), NaCl stress (Bellaire et al., 2000), and water stress
(Jiang and Zhang, 2002a). The increase in antioxidant defense systems is closely related to the stress tolerance. A
recent study has provided genetic evidence for the involvement of ABA in the protection against oxidative damage in
Arabidopsis exposed to heat stress (Larkindale and Knight,
2002). Pretreatment with ABA reduced the level of lipid
peroxidation and enhanced the survival in Arabidopsis exposed to heat stress. Utilizing the ABA-insensitive mutant
abi-1, which carries a mutation in a protein phosphatase
required for sensing ABA, an increased lipid peroxidation
(approximately 2-fold greater) and a reduced survival
(reducing survival to 0, from a 40% control value) were
shown. These results suggest that ABA is truly used by
plants to mediate protection against stress-induced oxidative damage. Furthermore, pretreatments with some ABA
biosynthesis inhibitors such as fluridone, which inhibits
the action of phytoene desaturase, an essential enzyme for
the conversion of phytoene to lycopene in the pathway of
carotenoids biosynthesis, and tungstate, which blocks the
formation of ABA from ABA-aldehyde by impairing ABAaldehyde oxidase, suppressed the stress-induced increases
３
in antioxidant defense systems in plants exposed to NaCl
stress (Bellaire et al., 2000) and water stress (Jiang and
Zhang, 2002a; 2002b; 2002c). These results imply that endogenous ABA is involved in the stress-induced up-regulation of antioxidant defense systems in plants.
4 Signaling Role of ROS in ABA-Induced Antioxidant Defense
Several lines of evidence indicate that ROS is involved
in ABA-induced antioxidant defense in plants. First, time
course of changes in ROS and antioxidant defense induced
by ABA has shown that a significant increase in the generation of ROS precedes that of antioxidant defense systems (Jiang and Zhang, 2001). Second, Cat1 expression is
induced by the presence of H2O2, and the accumulation
pattern for each of the three Cat transcripts in response to
H2O2 is similar to that of the ABA response (Guan et al.,
2000). Third, a block in the increase in the generation of
ROS induced by ABA also prevents the enhancement in
antioxidant defense systems, when the plants were pretreated with the NADPH oxidase inhibitors DPI, imidazole
and pyridine, and the ROS scavengers Tiron and
dimethylthiourea (DMTU) (Jiang and Zhang, 2002b; 2002c).
These results clearly suggest that H2O2 plays an important
intermediary role in the ABA signal transduction pathway
leading to the induction of antioxidant defense systems.
Furthermore, using these ROS manipulators and the ABA
biosynthesis inhibitor tungstate, it has also been shown
that under mild water stress, water stress-induced ABA
accumulation triggers the increased generation of ROS,
which, in turn, leads to the up-regulation of the antioxidant
defense systems in plants (Jiang and Zhang, 2002b; 2002c).
NADPH oxidase is involved in water stress-induced ROS
production and antioxidant defense systems, and the enzyme activity is, at least in part, regulated by ABA induced
by water stress in plants exposed to water stress (Jiang and
Zhang, 2002c).
However, ROS may also act in the upstream of ABA
signaling under water stress. It has been proposed that an
oxidative burst (ROS) might function as one of the triggers
of the water-stress responses and ABA might function in
the downstream of ROS to regulate gene expression as well
as physiological and biochemical responses during water
stress (Shinozaki and Yamaguchi-Shinozaki, 1997). A recent
study showed that in rapid dehydrated root tissues of wheat
seedlings, the induction of ABA by dehydration was
strongly blocked by ROS scavengers such as Tiron or ASC,
and ROS generators diethyldithiocarbamic acid, xanthinexanthine oxidase and triazole also induced ABA
４
accumulation (Zhao et al., 2001). These data suggest that
ROS induced by water stress might serve as signals and is
involved in water stress-induced ABA biosynthesis.
However, in a similar study, it has been shown that water
stress-induced ABA accumulation was neither affected by
ROS scavengers, tested with dimethyl sulfoxide and
melatonin, nor by the direct treatment with O－2. or H2O2 in
excised maize leaf and root tissues (Jia and Zhang, 2000).
These contradictory results may be related to the excised
root or leaf tissues and very rapid, severe dehydration.
Using intact plants excised at the base of the stem, our data
showed that oxidative stress induced by paraquat or H2O2
treatments does not affect ABA content in maize leaves,
regardless of either a mild or a severe oxidative stress (Jiang
and Zhang, 2002b). Furthermore, pretreatment with the ROS
scavengers, Tiron and DMTU, and the NADPH oxidase
inhibitor DPI also did not affect the content of ABA in the
leaves of maize plants exposed to mild water stress. These
data suggest that ROS are not involved in ABA biosynthesis in plants under mild water stress. Nevertheless, a more
strict examination is required under physiological conditions for elucidating whether ROS is involved in the accumulation of ABA induced by water stress.
ROS is inevitably produced in higher plant cells during
normal metabolism. Biotic and abiotic stresses often lead
to an increased generation of ROS. Overproduction of ROS
can damage proteins, DNA and lipids, potentially disrupting cell function and causing mutations. On the other hand,
ROS also plays a positive role in normal plant growth and
development and in a plant’s response to stress (Murphy
and Auh, 1996; Papadakis and Roubelakis-Angelakis, 1999;
Desikan et al., 2001; Frahry and Schopfer, 2001; Schopfer
et al., 2001). Abundant evidence has been shown that ROS,
especially H2O2 and O－2. , are involved in cellular signaling
process as secondary messengers to induce a number genes
and proteins involved in stress defenses, including SOD,
CAT, APX, GR, glutathione peroxidase, guaiacol peroxidase,
glutathione-S-transferase and pathogenesis-related protein
(Levine et al., 1994; Prasad et al., 1994b; Lamb and Dixon,
1997; Karpinski et al., 1999; Morita et al., 1999; Desikan et
al., 2001; Mittler, 2002; Neill et al., 2002b; 2002c; Vranováet
al., 2002).
Plasma membrane NADPH oxidase is thought to use
cytosolic NADPH to reduce O2 at the apoplastic membrane
face (Lamb and Dixon, 1997; Grant and Loake, 2000; Neill et
al., 2002c; Pastori and Foyer, 2002). H2O2 produced by the
dismutation of O－2. may be transported from the apoplast to
the cytosol through water channels (aquaporins; Mittler,
2002; Neill et al., 2002c; Pastori and Foyer, 2002). H2O2
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generated in chloroplasts, mitochondria and peroxisomes
may also move into cytosol (Neill et al., 2002c; Shigeoka et
al., 2002). The cytosolic H2O2 directly triggers local signal
transduction events, and then induces gene expression. In
support of this assumption, stresses that result in the enhanced production of ROS at the chloroplast induce the
expression of cytosolic but not chloroplastic APX genes
(Karpinski et al., 1997; Yoshimura et al., 2000). However,
the mechanisms about how ROS to induce the expression
of antioxidant genes remain to be elucidated. Neither signaling pathway(s) nor transcription factors and promoter
elements specific for the redox regulation have been identified in plants to date (Neill et al., 2002b; Vranováet al.,
2002). It is possible, however, that, in some cases, H2O2 can
interact directly with target proteins; for example, by oxidizing cysteine residues and thereby altering protein
conformation. Alternatively, H2O2 may be detected by a
cellular receptor or sensor such as His kinase. Its detection
results in the activation of a mitogen-activated protein kinase (MAPK) cascade, which then activates transcription
factors. Either way, activated transcription factors would
subsequently interact with cognate H2O2-response elements and modulate the expression of antioxidant defense
genes (Desikan et al., 2001; Mittler, 2002; Neill et al., 2002b;
2002c; Vranováet al., 2002).
5 Cross-Talk Between Calcium (Ca) and ROS
Ca2+ has been shown involving in ABA signal transduction in plant cells. ABA stimulates the increases in cytosolic Ca2+ by inducing both Ca2+ influx from the extracellular space and Ca2+ release from intracellular stores (Allen
et al., 2000; Pei et al., 2000; Murata et al., 2001). In maize
seedlings, ABA-induced increases in the activities of SOD,
CAT, APX and GR can be prevented by the pretreatments
with the Ca2+ chelator ethylene glycol-bis (β-amino ethyl
ether)-N, N, N', N'-tetraacetic acid (EGTA) and the Ca2+ channel blockers La3+ and verapamil, indicating the involvement of Ca2+ in ABA-induced antioxidant defense (Gong et
al., 1998; Jiang and Zhang, 2003). In Arabidopsis, ABAinduced ROS production triggers the influx of Ca2+ and the
increase in cytosolic Ca2+, which induces stomatal closing
(Pei et al., 2000; Murata et al., 2001). However, in some
biotic and abiotic stresses, these stresses trigger a Ca2+
influx, and the increased cytosolic Ca2+ stimulates the production of ROS, which induces the physiological response
(Chen and Li, 2001; Yang and Poovaiah, 2002). Moreover,
Ca2+ has a signal function upstream as well as downstream
of ROS in plant responses to pathogens (Lamb and Dixon,
1997; Bowler and Fluhr, 2000). Our recent study has shown
JIANG Ming-Yi et al.: Abscisic Acid and Antioxidant Defense in Plant Cells
that Ca2+ acts in the upstream as well as downstream of
ROS production in ABA signal transduction pathway leading to the induction of antioxidant defense system, and the
signal interaction between Ca2+ and ROS plays a pivotal
role in ABA-induced antioxidant defense (Fig.1; Jiang and
Zhang, 2003).
A cross talk between Ca2+ and ROS originates, at least
in part, from the communication between Ca2+ and NADPH
oxidase in ABA signaling (Fig.1). A striking feature of the
plant NADPH oxidase homologues is the presence of two
Ca2+-binding EF hand motifs, suggesting that Ca2+ may
play an important role in the regulation of NADPH oxidase
activity (Keller et al., 1998; Grant and Loake, 2000; Sagi and
Fluhr, 2001). Ca2+ may regulate NADPH oxidase activity by
activating the gp91phox subunit of NADPH oxidase directly,
or indirectly via phosphorylation, following the Ca2+-mediated activation of a specific Ca2+-dependent protein kinase
(CDPK), or activating the production of NADPH via NAD
kinase regulated by calmodulin (CaM) (Grant and Loake,
2000; Sagi and Fluhr, 2001; Neill et al., 2002b). Although it
was shown that treatment of tomato cells with race-specific
elicitors induced the translocation of three cytosolic regulatory proteins, p67phox, p47phox and rac2, from the cytosol
to the plasma membrane (Xing et al., 1997), which is thought
to be a key point of NADPH oxidase regulation in neutrophils (Henderson and Chappell, 1996), several recent studies have questioned the conclusion. Using novel in-gel
NADPH oxidase activity assay (Sagi and Fluhr, 2001), and
knockout mutants (Torres et al., 2002) and antisense constructs (Simon-Plas et al., 2002) of NADPH oxidase genes,
it has been demonstrated that plant NADPH oxidase, unlike the mammalian version, can produce O－2. in the absence
of additional cytosolic components. In vitro experiments
showed that the plant plasma membrane NADPH oxidase
is regulated directly by Ca2+ (Sagi and Fluhr, 2001; Jiang
and Zhang, 2003). ABA does not directly regulate the activity of NADPH oxidase, but requires additional cytosolic
components (Jiang and Zhang, 2002c). The pretreatments
with the Ca2+ chelator EGTA and the Ca2+ channel blockers
La3+ and verapamil almost completely suppressed the ABAinduced increases in the activity of NADPH oxidase, the
production of ROS, and the activities of antioxidant enzymes in maize plants (Jiang and Zhang, 2003), suggesting
that Ca2+ plays a pivotal role in the regulation of NADPH
oxidase activity and antioxidant enzyme activity in the ABA
signal transduction.
The mechanism that Ca2+ regulates antioxidant defense
is still open. It has been shown that Ca2+ binds to CaM, a
ubiquitous calcium-binding protein, and the Ca2+/CaM com-
５
plex stimulate the activities of antioxidant enzymes such as
CAT (Yang and Poovaiah, 2002) and SOD (Gong and Li,
1995). However, an increase in cytosolic Ca2+ mediated by
H2O2 also brings about a reduction in the activity of SOD
in tobacco (Price et al., 1994). Our data showed that Ca2+induced increases in the activity of NADPH oxidase, the
production of ROS, and the activities of antioxidant enzymes were almost fully blocked by the pretreatments with
the NADPH oxidase inhibitors DPI, imidazole and pyridine
(Jiang and Zhang, 2003), suggesting that Ca2+-stimulated
ROS production, which originates mainly from NADPH
oxidase, contributes to the induction of antioxidant enzyme
activity in plant cells (Fig.1). On the other hand, Ca2+
overload, which causes toxic levels of ROS production and
results in cellular oxidative damage (Chen and Li, 2001),
may be a causative factor of the reduction in the activity of
antioxidant enzyme activity. However, the increases in the
activities of antioxidant enzymes induced by oxidative stress
by paraquat, which binds to the thylakoid membrane of the
chloroplasts and transfers the electrons to O2 in a chain
reaction that causes continuous formation of O－2. , can be
fully blocked by the pretreatments with the Ca2+ chelator
and the Ca2+ channel blockers (Jiang and Zhang, 2003),
indicating that Ca2+ is a stringent requirement for the ROSinduced antioxidant enzyme activity (Fig.1). Although it
Fig.1. Schematic representation of a testable model for the
interaction of water stress, abscisc acid (ABA), reactive oxygen
species (ROS), Ca2+, and antioxidant defense systems in plants.
６
has been suggested that H2O2 sensing may be linked to
changes at the levels of Ca2+ and CaM, and to the activation or induction of a Ca2+-CaM kinase that can activate
the activity of transcription factors, which results in the
induction of antioxidant defense (Mittler, 2002), no calciumdependent protein kinases have been shown to be regulated by ROS (Neill et al., 2002b). Further investigations are
needed on this matter.
6 Conclusions and Future Developments
It has been well documented that ABA can result in the
increased generation of ROS, induce the expression of antioxidant genes, and enhance the capacity of antioxidant
defense systems in plants. ABA-dependent signal transduction pathway is an important one in the induction of
antioxidant defense under water stress. Water stress-induced ABA accumulation triggers the increased generation of ROS, which originates, at least in part, from the
NADPH oxidase pathway, resulting in the induction of antioxidant defense systems against oxidative damage in
plants. A cross talk between Ca2+ and ROS plays a pivotal
role in the ABA-induced antioxidant defense. Available information suggests the existence of intracellular networks
rather than linear pathways in ABA signal transduction
leading to the induction of antioxidant defense in plants
(Fig.1). However, many questions remain to be answered.
How does ABA induce the generation of ROS? What contributions to the cellular H2O2 pool are made by the various
sources of ROS induced by ABA? What concentrations of
ROS are toxic to plant cells? Is the diffusion of H2O 2
through water channels regulated? What are the sensors
of ABA and ROS in plants? How does ROS induce the
expression of antioxidant genes? How is ABA-induced
antioxidant defense regulated? Recently, several studies
have demonstrated that NO is an essential signaling intermediate in ABA-induced stomatal closure (Desikan et al.,
2002; Garcí
a-Mata and Lamattina, 2002; Neill et al., 2002a).
However, it is unclear whether NO is involved in ABAinduced antioxidant defense. The use of various mutants
including ABA-insensitive and ABA-deficient mutants,
mutants with altered ability to produce or scavenger ROS
and ROS-signaling mutants, and post-genomic developments in transcriptomics and proteomics, together with the
development of cellular markers that enable the non-destructive quantification of different ROS in the different
cellular components, will no doubt advance considerably
our understanding of mechanisms about how ABA induces
antioxidant defense in plant cells.
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