全 文 ：Received 19 May 2003 Accepted 12 Aug. 2003
Supported by the Sino-UK Science and Technology Collaboration Fund and National Natural Science Foundation of China (370100100).
* Author for correspondence. Tel: +86 (0)21 62932002; Fax: +86 (0)21 62824073; Email: or com>.
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.Review.
Acta Botanica Sinica
植 物 学 报 2004, 46 (2): 127-136
MAP Kinase Cascades Responding to Environmental Stress in Plants
YU Shun-Wu1, TANG Ke-Xuan1, 2*
(1. Fudan-SJTU-Nottingham Plant Biotechnology Research and Development Center, Plant Biotechnology Research Center,
School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200030, China;
2. Fudan-SJTU-Nottingham Plant Biotechnology Research and Development Center, State Key Laboratory of Genetic Engineering,
School of Life Sciences, Morgan-Tan International Center for Life Sciences, Fudan University, Shanghai 200433, China)
Abstract: Plant mitogen-activated protein kinases (MAPKs) are involved in growth, development and
responses to endogenous and environmental cues, which link stimuli that are activated by external
sensors to cellular responses. In Arabidopsis, as a model, all of MAP kinase genes have been listed and
classified. Based on the Arabidopsis MAPK families, a number of MAP kinase genes in other plant species
have been recently isolated and classified. Most of the cloned MAP kinase genes can be activated by a
variety of stress stimuli including pathogen infection, wounding, temperature, drought, salinity, osmolarity,
UV irradiation, ozone and reactive oxygen species. Some tools and strategies are used to investigate their
functions and signal pathways under different environmental stresses, indicating complexity and crosstalk
of plant MAP kinase signaling pathways. It is still necessary to explore more novel tools and strategies to
clarify MAPK signaling pathways, and how to apply the MAPK cascade to improve the resistance of crop to
abiotic and biotic stress.
Key words: environmental stress; hormone; mitogen-activated protein kinase (MAPK); signal trans-
duction
Under environmental stress, plant has developed com-
plex signaling networks to sense environmental signals and
adapt to unfavorable conditions. Mitogen-activated pro-
tein kinase (MAPK) signaling pathways are one of most
important and conserved approaches, which are ubiqui-
tous modules of signal transduction in human, yeast and
plant. MAPKs are known to regulate cell growth and death,
differentiation, the cell cycle and environmental stress
response. MAPK cascades commonly include three pro-
tein kinase MAPK kinase kinase (MAPKKK), MAPK ki-
nase (MAPKK) and MAPK, which covalently attach phos-
phate to the side chain of serine, threonine or tyrosine of
specific proteins inside cells. Through phosphorylation,
MAPKs regulate cellular activities ranging from gene
expression, mitosis, movement, metabolism and programmed
death (Johnson and Lapadat, 2002). MAPK-catalyzed phos-
phorylation of substrate proteins functions as a switch to
turn on or off various ways to adapt to various responses.
These substrates include transcription factors, protein ki-
nases and cytoskeletal proteins. Under various stimuli,
MAPK cascades specifically and sequentially activate ki-
nases or serve as substrates to respond to changes in the
environment.
The completion and analysis of Arabidopsis genome
sequencing have revealed components of Arabidopsis
MAPK cascades (Ichimura et al., 2002). MAPKs serve as
phosphorylation substrates for MAPKK which phospho-
rylate MAPKs on threonine and tyrosine residues in T(E/
D)Y motif. MAPKKKs are the third component of the
phosphorelay system, which phosphorylate two serine/
threonine residues in the conserved S/T-X5-S/T motif and
activate specific MAPKKs. MAPKKKs have distinct mo-
tifs in their sequences that selectively confer their activa-
tion through physical interaction and/or phosphorylation
by the receptor itself in response to different stimuli. In the
past six years, a large number of genes of MAPK cascades
from various species have been isolated and their func-
tions have been described. In this update, MAP kinase
signalling cascades in Arabidopsis conferring resistance
to both bacterial and fungal pathogens have been defined
in detail (Asai et al., 2002). Liu et al. (2000) have described
MAPKs and their signal transductions in higher plants. In
this review, we focus on describing the current view of the
complexity of plant MAPK cascades in environmental stress
and recent advances in this field.
1 MAP Kinase Family Members
Thanks to the completion of the Arabidopsis whole
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004128
genome sequencing, MAPK group from nine countries has
defined the complement of MAPK family members in
Arabidopsis (Ichimura et al., 2002). They displayed and
aligned the whole family of MAPK cascades from
Arabidopsis and other species published before May 2002.
In Arabidopsis genome, 20 MAPKs, 10 MAPKKs and 80
MAPKKKs (Table 1) were identified and a unified nomen-
clature for Arabidopsis MAPKs and MAPKKs has been
proposed (Ichimura et al., 2002) .
1.1 MAPKs
Based on alignment, plant MAPKs are divided into at
least four subfamilies (A-D) that are all involved in envi-
ronmental responses. Recently molecular identification of
most known genes focuses on the A and B subfamilies that
have a TEY phosphorylation motif in their active sites. Rep-
resentative genes of subfamily A have been identified
as AtMAPK3 (Mizoguchi et al., 1996; Kovtun et al., 2000),
AtMAPK6 (Kovtun et al., 2000; Desikan et al., 2001),
NtSIMK (Munnik et al., 1999), MsMMK4 (Jonak et al.,
1996), NtSIPK (Salicylic acid-induced) and NtWIPK (wound-
induced) (Seo et al., 1995). Subfamily B has been found to
be involved in both disease resistance (AtMAPK4 (Kovtun
et al., 2000; Petersen et al., 2000), MsMMK2, MsMMK3
(Cardinale et al., 2002)), and abiotic stress (AtMAPK4
(Ichimura et al., 2000)). Information on the subfamily C
MAPKs was less known than A and B except for OsMAPK4
(response to environmental stresses) (Miko et al., 2000)
and AtMAPK7 (circadian-rhythm-regulated expression)
(Schaffer et al., 2001). Subfamily D MAPKs have the TDY
motif, but lack CD domain that functions as a docking site
for MAPKKs. The subfamily functions were only described
in a few number examples. For examples, OsBWMK1 is
induced by pathogen stimuli (Schoenbeck et al., 1999), and
OsWJUMK1 is expressed in encountering diverse envi-
ronmental stress and developmental regulation (Agrawal
et al., 2003a). Recently, we have successfully isolated two
MAPK genes BnMPK3 and BnMPK9 from Brassica napus.
BnMPK3 and BnMPK9 belonged to A and D subfamilies
respectively and were confirmed to respond to environ-
mental stresses (data not shown).
1.2 MAPKKs
Although MAPKKs that have been molecularly identi-
fied are less than 30, they can be divided into four different
groups: A, B, C and D. Plant MKKs have the consensus
sequence S/T-X5-S/T for the phosphorylation site and a
putative MAPK docking site K/R-K/R-K/R-X1-6-L/I-X-L/
V/I at N-terminal. Currently only A and C type MKKs have
been demonstrated that they can be activated by multiple
abiotic stresses (AtMKK1 (Matsuoka et al., 2002),
MsSIMKK (Kiegerl et al., 2000; Cardinale et al., 2002)) and
pathogen stimuli as well as pathogen-like chemical
(MsPRKK (Cardinale et al., 2002), NtMEK2 (Asai et al.,
2002), AtMKK4 and AtMKK5 (Ren et al., 2002)). NtMEK1
activates the cell cycle-regulated p43Ntf6 MAPK (Calderini
et al., 2001). There is no functional evidence about B and D
type MKKs although the B-type MAPKKs have nuclear
transport 2 (NTF2) domains (Ichimura et al., 2002).
1.3 MAPKKKs
Compared with MAPKs and MKKs, the MAPKKK fam-
ily forms the largest and most complex group of MAPK
pathway components. In Arabidopsis, there are 80 puta-
tive MAPKKKs that can be divided into three subfamilies.
Among 21 members of putative MEKK-like genes,
AtMEKK1 and AtMEKK4-like genes are involved in
osmotic, touch and disease-response (Mizoguchi et al.,
1996; Meyers et al., 1999). NPK1-related and AtMAP3Ke1-
related protein kinase may play a role in cell division
(Nishihama et al., 2001; 2002; Jouannic et al., 2001). ZIK-
like protein kinases have 11 members. Largest subfamily of
Raf-like protein kinase includes AtCTR1 (Kieber et al., 1993),
AtEDR1 (Frye et al., 2001) and ATN1-like protein (http://
www.arabidopsis.org/info/genefamily/ MAPKKK.html).
Table 1 Arabidopsis mitogen-activated protein kinase (MAPK) signaling components
Number Kinase class Number Member Signature motif
MAPK 20 A 3 MAPK3, 6, 10 MTEYVVTRWYRAPELLL
B 5 MAPK4, 5, 11, 12, 13
C 4 MAPK1, 2, 7, 14
D 8 MAPK8, 9, 15, 16, 17, 18, 19, 20 MTDYVATRWYRAPELCG
MAPKK 10 A 3 MKK1, 2, 6 VGTYXYMSPER
B 1 MKK3 VGTVXYMSPER
C 2 MKK4, 5 VGTIXYMSPER
D 4 MKK7, 8, 9, 10 VGT(C/F)XYMSPER
MAPKKK 80 MEKK 21 MAPKKK1-21 G(T/S)PX(W/Y/F)MAPEV
ZIK 11 ZIK1-11 GTPEFMAPE(L/V)Y
Raf 48 Raf1-48 GTXX(W/Y)MAPE
YU Shun-Wu et al.: MAP Kinase Cascades Responding to Environmental Stress in Plants 129
2 Plant MAPK Pathways
Plants respond to biotic and abiotic stress by undergo-
ing a variety of cellular changes including oxygen bursts,
ion fluxes, production of salicylic acid, abscisic acid and
phytoalexin, and expression of defense-related genes.
Through several stimuli including cold, drought, wound-
ing and various peptide elicitors, as well as other kinase
inhibitor and enzyme, the plant MAPK cascade descrip-
tion suggests that signaling events converge into a con-
served MAPK cascade (Asai et al., 2002; Johnson and
Lapadat, 2002) (Fig.1).
Currently dominating methods for specific pathway
studies are yeast two-hybrid, in vitro interaction and tran-
sient expression in protoplasts. Compared to human MAPK
cascades (Johnson and Lapadat, 2002), the plant MAPK
cascades are more complex. Based on yeast two-hybrid
results, interaction between MEK1 and AtMEKK1/2 was
detected (Ichimura et al., 1998; Mizoguchia et al., 1998).
However, transient expression assays showed that
AtMEKK1 could activate AtMEK4/5 (Asai et al., 2002).
One AtMEKK1 could activate the expression of four MEKs.
One MAPKK (MsSIMKK and MsPRKK) could activate
up to three different types of MAPKs under different envi-
ronmental stresses in alfalfa (Cardinale et al., 2002). There
are similar reports in tobacco and Arabidopsis that NtMEK2
could activate both SIPK and WIPK and AtMKK4/5 could
activate MPK3/6 (Asai et al., 2002). From previous reports,
AtMEKK1 could activate A and C type MEK (Zhang and
Klessig, 2001; Asai et al., 2002; Ichimura et al., 2002; Jonak
et al., 2002). It was shown from most known cascades from
diverse species that C type MEK could interact with A type
MAPK (Jonak et al., 2002 ). But other interactions between
MEK and MAPK were less known except that A type MEK
(AtMEK1/2) could activate AtMAPK4 (B type MAPK) (Fig.
2).
MAP kinase can phosphorylate myelin basic protein
(MBP) and be recognized by anti-mammalian/plant MAPK
Fig.1. Conserved MAPK cascades in human and plant. Detailed elucidations are shown in a review (Johnson and Lapadat, 2002) and
an article (Asai et al., 2002).
Fig.2. Interactions between different MAPK cascade families in
environmental stress. Broken lines indicate unknown pathways
and arrows denote identified pathways.
antibodies. Therefore identification of MAP kinase is gen-
erally performed by two-dimensional MAP kinase in-gel
kinase assay and specific antibody Northern blot analysis.
Otherwise functional complementation of yeast relative
mutants is often used. During functional research of MAPK
cascades, MAPKs are found to have phosphorylational
abilities that are regulated by other factors although they
are activated in response to stimuli (Fig.3). Protein phos-
phatases can reciprocally and rapidly alter the behavior of
cells together with MAPKs when cells respond to various
environmental changes. Protein phosphatases remove the
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004130
phosphates that were transferred to the protein substrate
by the MAPK and inhibit the action of MAPKs (Sheng,
1998; Ulm et al., 2001; Meskiene et al., 2003). Acting on the
MEKK, ubiquitylation can inhibit the ability of the protein
kinase to phosphorylate substrates and regulate down-
stream signaling pathways (Witowsky et al., 2003). How-
ever there is a positive regulation of the MAPKs that NDPK2
interacts with MAPKs and enhances the MBP phosphory-
lation activity (Moon et al., 2003). It is not known if there
are other uncharacterized regulators to affect the function
of MAPK cascades.
Diverse stress signaling and mitotic regulatory signal-
ing converge at MAPK cascades. Furthermore, the exist-
ence of a lot of putative MAPKKKs and MAPKs and a
small quantity of MEKs suggests that MAPKKs function
as convergence points. U0126 and PD098059 are organic
compounds that have been identified as inhibitor of MEKs
in libraries (http://www.promega.com/pnotes/69/7542_06/
7542_06.html). In our laboratory, we also found the two
organic compounds as inhibitors of MAPKs are specific
(data not published). They provide an effective means to
investigate the pathway of the MAPK cascade.
3 MAPK Cascades and the Pathogen Stimuli
Suffered from pathogen attack, plant produces defense
responses to activate diverse signal pathway to regulate
gene expression and cellular processes. Several MAPK
cascades are identified to be associated with the induction
of pathogen attack. To avoid the possibility that pathogen
or natural elicitors cause multiple elicitors to activate mul-
tiple parallel MAPK cascades, fungal elicitors (chitosan,
Avr and CWD), harpin and Flg 22 of bacteria, and helicases
of virus are commonly used to activate the function of di-
verse MAPK signal pathways in various species (Zhang
and Klessig, 2001; Link et al., 2002). Excluding the expres-
sion of these simple genes under pathogen stimuli, chains
composed of two or more MAP kinase genes have been
elucidated in Arabidopsis, alfalfa and tobacco that are all
established conveniently by the system of protoplast tran-
sient expression.
In alfalfa, fungal elicitors can induce SIMKK and PRKK
(Cardinale et al., 2002). However SIMKK mediates patho-
gen elicitor signaling and salt stress, and PRKK transmits
only elicitor-induced MAPK (SIMK, MMK3 and SAMK)
(Fig.4). But PRKK is unable to activate any MAPK upon
salt stress. In contrast, SIMKK activates SIMK and MMK3
in response to elicitor, but it activates only SIMK upon salt
stress.
Tobacco MAPKKK, Nicotiana Protein Kinase 1 (NPK1),
can interfere with the function of the disease-resistance
(Jin et al., 2002). NtMEK2, a MAPK, was identified to be
upstream of salicylic acid-induced protein kinase (SIPK)
and wouding-induced protein kinase (WIPK) which con-
trols two defense genes encoding key enzymes in the phy-
toalexin and salicylic acid biosynthesis pathway.
Overexpression of a constitutively active mutant of NtMEK2
induces hypersensitive response (HR)-like cell death (Yang
et al., 2001). The activation of MAPKs is one of the earliest
responses in plants challenged by pathogens or elicitors.
SIPK and WIPK can be induced not only by pathogen and
be expressed to precede HR, but also by osmotic stresses
(Droillard et al., 2000). It is still not known whether other
upstream MEKs can also activate SIPK and WIPK, or
NtMEK2-SIPK/WIPK is the crossroad of several transduc-
tion pathways initiated by elicitor or osmotic stimuli (Fig.
4).
In Arabidopsis, two mutants of MAPK cascades have
been discovered and their functions are indicated to be
involved into disease defense. But interestingly
Arabidopsis mpk4 and edr1 mutants activated upon patho-
gen challenge, and MPK4 and EDR1 were negative regula-
tors of pathogen responses (Zhang et al., 1998; Frye et al.,
2001). mpk4 mutant plants were dwarfed, and exhibited
constitutively elevated levels of SA, constitutive expres-
sion of pathogenesis-related (PR) genes and increased
pathogen resistance (Petersen et al., 2000). Resistance me-
diated by the edr1 mutation was correlated with induction
of several defense responses (Frye et al., 2001).
Overexpression of a kinase-deficient form of the EDR1 gene
Fig.3. MAPK cascade in stress signaling. Both biotic and abi-
otic stimuli activate MAPK pathway. Ubiquitylation, several
serine/threonine protein phosphatases (PPases) (PP1, PP2A/C),
MAP kinase phosphatases (MKPs) and protein tyrosine phos-
phatases (PTPases) may negatively regulate MAPK cascade, and
NDP kinase (NDPK) enhances the role of MAPKs. The arrows
indicate activation. The line indicates enhancement and the bar
denotes inhibition.
YU Shun-Wu et al.: MAP Kinase Cascades Responding to Environmental Stress in Plants 131
enhanced powdery mildew resistance and ethylene-in-
duced senescence (Tang and Innes, 2002). EDR1 is thus a
negative regulator of ethylene responses and negatively
regulates SA-inducible defense responses. Although MPK4
and EDR1 are associated with pathogen attack, it is still
necessary to investigate what roles their negative or posi-
tive regulations play virtually. A similar report, showed sup-
pression of OsMAPK5 (Oryza sativa MAPK5) expression
and its kinase activity resulted in the constitutive expres-
sion of PR genes in the dsRNAi transgenic plants and sig-
nificantly enhanced resistance to pathogens (Xiong and
Yang, 2003). However, in drought, salt and cold tolerance,
the suppression of OsMAPK5 resulted in reduction, but
the overexpression resulted in increase (Xiong and Yang,
2003) . These results strongly suggest that OsMAPK5 can
positively regulate drought, salt and cold tolerance and
negatively modulate PR gene expression. To date, a large
number of genes in MAPK cascade have been isolated and
characterized from dicot and monocot species, but no more
reports support the dualism of other MAPKs.
Currently in Arabidopsis, the flagellin MAPK cascade
has been identified by Arabidopsis mesophyll protoplast
transient expression assays (Asai et al., 2002). Flg22, 22-
amino acid peptide elicitor derived from pathogenic bacte-
rial flagellin conserved domain, binds to FLS2 (LRR recep-
tor kinase) and induces defence responses in leaves (Sheen,
2001). Combined with transient expression assays, FLS2, a
putative sensor of bacterial flagellin, was placed upstream
of MAPK cascades. Simultaneously a complete plant
MAPK cascade (MEKK1-MKK4/5-MPK3/6) functions as
signaling components to activate WRKY 22/29 transcrip-
tion factor of defence response (Fig.4). Further
Agrobacterium-mediated transient transformation study
substantiated that constitutively activated flagellin MAPK
or WRKY29 conferred resistance of Arabidopsis leaves to
infection by the bacterial pathogen Pseudomonas syringae
and the fungal pathogen Botryis cinerea (Asai et al., 2002).
Although the complete MAPK cascade confirmed their
conservation between plant and other eukaryotes, more
details are unknown such as how to interact between FLS2
and AtMEKK1, and between WRKY22/29 and AtMPK3/6.
4 MAPK Cascades and Osmotic Stress
Cell shrinkage from osmotic stress causes mechanical
stimulation to the plant cell. A putative transmembrane hy-
brid-type histidine kinase (AtHK) was identified to func-
tion as an osmosensor and transmit the stress signal to a
downstream MAPK cascade (Urao et al., 1999). However,
it remains to be investigated how AtHK1 interacts with
MAPK cascades and functions in osmosensing in
Arabidopsis. In downstream of AtHK1, only AtMEKK1 was
identified to be induced by touch, cold, and water stress in
Arabidopsis thaliana (Sheen, 2001; Yang et al., 2001). Al-
though AtMEKK1 mediated flagellin signaling by the acti-
vation of AtMKK4/5 and AtMAPK3/6, specific protein-
protein interactions of AtMEKK1-AtMKK1/2-AtMAPK4
Fig.4. MAPK signaling pathways for pathogen response in plants. Details of MAPK pathways in Arabidopsis are shown in the
reports (Kovtun et al., 2000; Asai et al., 2002). Details of alfalfa and tobacco MAPK pathways are shown in the reports (Yang et al.,
2001; Cardinale et al., 2002). Constitutively active mutant of NtMEK2 induces hypersensitive response (HR)-like cell death.
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004132
were detected by using yeast two-hybrid system (Ichimura
et al., 1998; Mizoguchia et al., 1998). AtMEKK1 was also
found to be activated by wounding, cold, drought and high
salt stress and had an elevated protein kinase activity to
phosphorylate AtMAPK4 (Matsuoka et al., 2002). On the
other side, MAPK4 was associated with pathogen stimuli
(Petersen et al., 2000; Desikan et al., 2001). The character-
istic of MAPK4 is similar to OsMAPK5 that negatively
regulates downstream gene expression in biotic stress and
positively regulates gene expression in abiotic stress. How-
ever the detailed function of MAPK4 remains to be
confirmed. From these studies, we can construct one sig-
naling pathway of MAPK cascade of osmotic stress (Fig.
5).
Extreme temperatures, drought and salt stress are con-
sidered to be different forms of osmotic stress (Jonak et al.,
1996). In cold treatment, membrane rigidification can in-
duce SAMK, and membrane fluidization can activate
HAMK (heat-activated MAPK) (Sangwan et al., 2002). It
indicates that both cold and heat are sensed by structural
changes in the plasma membrane that translates the signal
via cytoskeleton, Ca2+ fluxes and calcium dependent pro-
tein kinases (CDPKs) into the activation of distinct MAPK
cascades. A number of MAP kinase genes have been found
to be involved in drought and salt stress (Zhang and
Klessig, 2001; Jonak et al., 2002; Agrawal et al., 2003b) and
their functions are complex that can be activated by differ-
ent environmental stresses. Tobacco MAPKs SIPK and
WIPK are known to be induced by SA and wounding. Both
SIPK and WIPK are involved in the tobacco mosaic virus
infection (Zhang and Klessig, 2001), but they all appear to
be located at the crossroad of complex signaling networks.
The SIPK is also activated by a hyperosmotic stress, indi-
cating that the same kinase plays a role both in hypo- and
hyperosmotic signaling pathway, as well as elicitor signals.
Nevertheless WIPK is induced by hypoosmolarity but not
by hyperosmolarity (Droillard et al., 2000). Although
AtMEKK1 and its downstream AtMPK3/6 were confirmed
to be part of a complete flagellin MAPK cascade (Asai et
al., 2002), AtMEKK1 might be part of the signal transduc-
tion pathway involved in osmotic stress (Covic et al., 1999).
In addition, AtMPK3 and AtMPK6 are all involved with
osmotic stresses using Arabidopsis thaliana cell suspen-
sion assay (Droillard et al., 2002). Therefore plant MAP
kinase signaling is complex and crosstalk.
In alfalfa, assays of specific peptide antibodies revealed
alfalfa p44MMK4 kinase was transiently posttranslationally
activated by drought and cold treatments (Jonak et al.,
1996). However high salt and heat shock did not induce
activation of p44MMK4, indicating the existence of dis-
tinct mechanisms to medicate different stresses in alfalfa
(Jonak et al., 1996). Cardinale et al. (2002) reported that
SIMKK strongly activated SIMK and MMK3 upon elicitor
treatment. Moreover, in salt stress-treated cells SIMKK
enhanced the activation of SIMK but not of MMK3 (Fig.
5). These results indicate that intracellular signal also influ-
ences interaction between protein and the outcome of sig-
naling chains. It was found that the signal selection was
involved with scaffold proteins. The yeast MAPK scaffold
ste5 was tolerant to major stereochemical perturbations;
simple tethering heterologous protein could functionally
replace native kinase recruitment interactions to mediate
alternative signal pathway (Park et al., 2003). Scaffolds such
as ste5 are modular and flexible organizing centers that can
control the flow of information in signaling or transcription.
Then plant MAPK scaffold protein SIMKK in the salt and
elicitor stimuli can mediate different signal transduction
pathway that shares identical components. The elucida-
tion of scaffold mechanism would be helpful to understand
OsMAPK5’s negative and positive roles in salt and patho-
gen stress.
5 MAPK Cascades, Other Stresses, and Phy-
tohormone
Many abiotic environmental stresses such as wounding,
ozone, drought and UV often result in the activation of
adaptive responses that is similar to the result of reactive
oxygen species (ROS) response of plants (Zhang and
Klessig, 2001). The production of ROS is also one of the
earliest responses in plants that are resisting pathogen in-
fection (Zhang and Klessig, 2001). Kovtun et al. (2000) re-
ported that H2O2 could activate AtMAPKKK, ANP1 and
AtMPK3/6 (Fig.4). Besides these abiotic stresses may all
activate MAPK cascades, heavy metal also dramatically
Fig.5. Schematic illustration of MAPK pathways for osmotic
responses in plants. The signaling pathways in Arabidopsis and
alfalfa are summarized on the basis of the reports (Mizoguchia et
al., 1998; Kovtun et al., 2000; Cardinale et al., 2002). The arrows
denote MAPK pathway of osmotic responses. The broken ar-
rows indicate MAPK pathway of pathogen response.
YU Shun-Wu et al.: MAP Kinase Cascades Responding to Environmental Stress in Plants 133
induce expression of OsEDR1 (Kim et al., 2003) and
OsWJUMK1 (Agrawal et al., 2003a), and genotoxic stress
is associated with MAP kinase (Ulm et al., 2002).
When encountering abiotic stresses, plants often pro-
duce secondary defense signals and phytohormone to regu-
late cellular metabolism. Studies on mutant CTR1 (Raf-like
MAPKKK) revealed that CTR1 was a negative regulator of
ethylene signaling that was involved with MAPK cascades
in hormone signaling (Kieber et al., 1993). But ethylene can
also distinctly up-regulate the expression of OsEDR1
(MAPKKK) (Kim et al., 2003) and OsMSRMK3 (Agrawal
et al., 2003a) of Oryza sativa, and the expression of ERK1
(MAPK) of pea (Pisum sativum) (Moshkov et al., 2003).
Abscisic acid (ABA) mediates plant responses to envi-
ronmental stress, particularly to water stress. Besides
jasmonic acid (JA), SA, ethylene and H2O2, ABA can result
in the anabatic expression of OsEDR1(Kim et al., 2003) and
OsMSRMK3 (Agrawal et al., 2003a). In barley aleurone
protoplasts, the tyrosine phosphatase inhibi tor
phenylarsine oxide can completely block ABA-induced
MAP kinase activation and rab16 gene expression
(Knetsch et al., 1996). It was also evident that AtMPK3
was activated by ABA in vivo (Lu et al., 2002) and ABA
signal was transmitted to the transcriptional apparatus
through MAPK signaling. However it is still not known
how important MAPK cascade route is in ABA signal trans-
mission to trigger growth arrest.
Auxin is an essential phytohormone that regulates di-
verse processes such as cell division and expansion,
embryogenesis, meristem formation, root and leaf
patterning, tropism and reproduction. Different MAPKs
have recently been shown to be expressed during plant cell
proliferation and developmental processes (Coronado et
al., 2002). It was shown that auxin treatment of Arabidopsis
roots transiently induced increases in MAPK-like kinase
(Mockaitis et al., 2002). In auxin-resistant 4 (axr4) mutants,
MAPK activation by auxin, but not by salt stress, was sig-
nificantly impaired (Mockaitis et al., 2002). However, cur-
rent evidence showed that H2O2-induced MAPK cascade
induced specific stress-responsive gene expression, but it
blocked the action of auxin (Mizoguchia et al., 1998). The
data indicate a molecular link between oxidative stress and
auxin signal transduction. Further investigation revealed
that the knockout mutation of ANP led to upregulation of
stress responses and no apparent change in auxin-regu-
lated gene expression (Patrick et al., 2002). It was reported
that NPK1 could activate a MAPK cascade that led to the
suppression of early auxin response gene transcription and
overexpression of truncated NPK1 produced seeds defec-
tive in embryo and endosperm development (Kovtun et al.,
1998). Before-mentioned results show that conflicts be-
tween mutants and overexpression as well as between in-
tact plants and individual protoplasts occur. It suggests
that oneself complexity of MAPK cascade and the com-
plexity between MAPK and auxin cause different results in
higher plants. The most important problem is emptiness of
basic transfer chain between MAPK and auxin.
6 Conclusion
Plant MAPK cascades are thought to play a key role in
biotic- and abiotic-stress responses, hormone responses,
cell division and development. A number of relative genes
have been identified and new information continually
emerges to enrich the MAPK cascade. The first MAP ki-
nase signaling module has been identified to redound to
understand the whole MAPK cascade. It is known that
MAPK (OsMAPK5) plays different roles in different envi-
ronmental stresses and a MAPK can be activated by differ-
ent upstream MAPK kinases or activate different down-
stream MAPK. These complexity and crosstalk suggest
that scaffold proteins play important roles to define the
specificity of MAPK signaling pathway, whereas we al-
most know nothing about plant scaffold proteins. Although
yeast two-hybrid system and transient expression of pro-
toplasts have been used to analyze interactions of MAP
kinases, and a combinational approaches of loss-of-func-
tion and gain-of-function have helped us to understand
the complex roles of MAPK in plant defense responses, it
remains to need novel approaches and strategies to define
specific functions and characters of signal transduction of
many plant genes that encode MAPK pathway components.
How to effectively apply the MAPK cascade to improve
the resistance of crop to abiotic and biotic stress is an
important aspect in agriculture, too.
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(Managing editor: HE Ping)