3.1 Historical Background
Sturgill and Ray (1986) for the first time discovered MAPK in animal cells and named it as microtu-bule-associated protein-2 kinase (MAP-2 kinase). It was renamed as mitogen-activated protein kinase (MAP kinase) 1 by Rossomando et al. (1987)
Fig. 14.4 Signaling through CBL/CIPK and protein phosphorylation by Ca2+ signatures
as mitogen was found to activate the group of proteins and this kinase was found to be related to these proteins (reviewed by Sanan-Mishra et al. 2006; Sinha et al. 2011) . In 1990, it was reported as serine/tyrosine kinase that belonged to a multigene family (Gotoh et al. 1990). MAP kinase genes (MsERK1) in plant system was for the first time reported from alfalfa in 1993 (Duerr et al. 1993) and D5 kinase in pea (Stafstrom et al.
1993). After that they were reported from various plants like tobacco, Arabidopsis, etc. (Jonak et al.
1994). Three major groups of MAPKs are found in yeast and animals: (1) extracellular signal-regulated kinases (ERK) (Cobb et al. 1994), (2) c-Jun amino (NH2)-terminal kinases or stress-activated protein kinases (JNS/SAPK) (Davis 1994), and (3) high osmolarity glycerol response or p38 kinases (Hog/p38) (Landry and Huot 1995). MAP kinase genes reported in plants belong to the ERK subfamily (Hirt 2000) and transmit a broader range of stimuli (Ligterink and Hirt 2001). Stafstrom et al. (1993) reported that the first MAP kinase gene isolated from pea has 41% identity with plant cdc2 kinases and other kinases involved in osmosensing. MAP kinases generally function as a cascade in which MAPKKK phosphoralates and activates MAPKK which in turn activates MAPK. All the three kinases are interlinked together and are also called extracellular receptor kinases (Hirt 2000; Sanan-Mishra et al. 2006). Different plant MAPKs recognize different substrates (Jonak et al. 2002) because of the high similarity in catalytic domain and little similarity in N-termini. The activation domain of most of the plant MAPKs contains TEY (Thu-Glu-Tyr) sequence and is similar to ERK/MAP kinases group of mammals and yeast (Hirt 2000) . The activation domain of some MAPKs possesses TDY (Thr-Asn-Tyr) sequence and is closer to the p38/Hog group of mammals and yeast (Tena et al. 2001). No plant MAPK has been found which has TPY (Thr-Pro-Tyr) sequence at its activation domain. There are three functionally linked protein kinase viz.: MAP3K, MAP2K, and MAPKs. 60 MAP3Ks, 10 MAP2Ks, and 20 MAPK have been reported in Arabidopsis thaliana by Group et al. (2002) .
It is also known as MAPKKKs or MEKKs. MAP3Ks constitute a diverse family of kinases and are divided into two subfamilies viz. MEKK1 and RAF-like kinases. The MEKK1-like subfamily members are similar to mammalian MEKK1 and to yeast STE11 and BCK1 and RAF-like kinases are similar to mammalian RAF1 MAPK (Group et al. 2002) . About 8-10 algal MAP3Ks are found in Chlamydomonas and Volvox and about 40-60 in Sorghum and Populous. One of the important things in MEKK-like kinases is having a conserved catalytic domain and Arabidopsis has ten members in this group. Arabidopsis has been reported to have 80 putative MAPKKKs whereas rice has 75 members (Rao et al. 2010). Plant species having homologs of MAPKKKs have been identified, including the MEKK-like protein kinases, oxidative stress-activated MAP triplekinase 1 (OMTK1) from alfalfa (Nakagami et al. 2004), ANP1, ANP2, ANP3 (Kovtun et al. 2000) , YDA (Lukowitz et al. 2004) from Arabidopsis, NPK1 (Nicotiana protein kinase 1) from tobacco (Nishihama et al. 2001), Raf-like protein kinase, EDR1 (enhanced disease resistance 1), and CTR1 (constitutive triple response 1) from Arabidopsis (Frye et al. 2001; Kieber et al. 1993). MEKK-like proteins have been found to participate in canonical MAP kinase cascades that activate downstream MAP2Ks (Rodriguez et al. 2010). The two RAF-like MAP3Ks are CTR1 and EDR1, and these two RAF-like MAP3Ks are found to participate in ethylene-mediated signaling and defense responses (Frye et al. 2001; Huang et al. 2003). It has also been reported that CTR1 and EDR1 do not participate in a canonical MAPK cascade (Rodriguez et al. 2010).
It is also known as MEKs and MKKs and is divided into four groups viz. Groups A, B, C, and D (Hamel et al. 2006). MKK1 and MKK2 belong to Group A and act upstream of the MAPK MAK4 (Ichimura et al. 1998) . There are several reports about the involvement of MKK2 in response to cold and salinity stress and apart from this both MKK1 and MKK2 mediate innate immunity responses (Meszaros et al. 2006; Qiu et al. 2008). Group B includes MKK3. One of the distinguishing features of this group is nuclear transfer factor (NTF) domain (Hamel et al. 2006). Steggerda and Paschal (2002) reported that NTF enhances the nuclear import of cargo proteins, which suggests that plant MAP2Ks with NTF domains are involved in cyto-plasmic nuclear trafficking. MKK3 has been found to participate in cascades that are elicited by pathogens and are dependent on jasmonic acid (JA) signaling (Doczi et al. 2007; Takahashi et al. 2007). Group C has MKK4 and MKK5 and Group D has rest of the kinases from MKKs 7-10. In general, all plant phyla appear to use a more limited number of MKKs compared to other MAPK components. Rodriguez et al. (2010) have reported a single MKK each for the algae Chlamydomonas and VolvoX; This indicates that the same MAP2K may function in several different MAPK modules. Genetic analysis has shown that closely related pairs of plant MAP2Ks have similar functions, e.g., MKK1 and MKK2 are proposed to activate MAPK MPK4 (Qiu et al. 2008), whereas MKK4 and MKK5 act upstream of MPK3 and MPK6, apparently in a redundant manner (Asai et al. 2002). In rice system, MKK genes exhibit differential regulation under different abiotic stresses (Kumar et al. 2008).
These are also known as MPKs and have been divided into four groups (A-D) (Group et al. 2002) . The activation domain of Groups A, B, and C contains TEY (Thu-Glu-Tyr) sequence similar to ERK kinases of animals. The activation domain of Group D contains TDY (Thr-Asn-Tyr) sequence (Rodriguez et al. 2010). MPK3 and MPK6 belong to Group A and have been reported to have a role in developmental processes and also shows responses against biotic and abiotic stresses (Zhang and Klessig 2001; Seo et al. 2007; Sinha et al. 2011). MPK4 belongs to Group B and has a role in pathogen defense and abiotic stress responses (Andreasson et al. 2005; Brodersen et al. 2006; Qiu et al. 2008). The characteristic feature of Group D MAPKs is a C-terminal docking domain that may act as a docking site for MAP2Ks (Yoo et al. 2008).
The mitogen-activated protein kinase (MAPK/ MPKs) cascades transducer environmental cues into intracellular responses. The stimulated plasma membrane receptors activate MAP kinase kinase kinase and then the sequential phosphory-lation ensues as MAP3Ks activate downstream MAP kinase kinase that in turn activates MAPKs (Sinha et al. 2011) (Fig. 14.5). MAPKs then target various effector proteins in the cytoplasm or nucleus, which include other kinases, enzymes, or transcription factors (Khokhlatchev et al. 1998; Sinha et al. 2011; Wurzinger et al. 2011).
Abiotic stress is responsible for the activation of MAPK genes and increased MAPK activity. MEKK1, MKK2, MPK4, and/or MPK6 have been activated during salt, drought, and cold stress (Teige et al. 2004; Sinha et al. 2011). Temperature stress (26-38°C) induces 50 kDa MAP kinase in tomatoes. Induction of MPK3 during cold and salinity in Arabidopsis has also been reported by different workers. Ichimura et al. (2000) reported that MPK4 and MPK6 are activated during low-temperature and osmotic stress. The role of several MAP kinases in response to salinity has also been reported by Sanan-Mishra et al. (2006) . Matsuoka et al. (2002) demonstrated the role of MAPK kinase (MKK1) in abiotic stress signaling. Multiple abiotic stresses such as wounding, cold, drought, and salinity activated MKK1 which activates downstream MPK4 (Matsuoka et al. 2002) . In Arabidopsis a specific MAPKK kinase, ANP1, was activated by H2O2. This ANP1 activates MPK3 and MPK6 and its positive regulator nucleoside diphosphate kinase (NDP) 2 (Moon et al. 2003). Overexpression of ANP1 shows tolerance to heat shock, freezing, and salt stress tin transgenic plants (Kovtun et al. 2000). Plants
expressing AtNDPK2 show reduction in H2O2 accumulation and tolerance to multiple stresses like cold, salt, and oxidative stress. Munnik et al. (1999) demonstrated that a 46 kDa MAP kinase named SIMK activated during moderate hyperosmotic stress in alfalfa. At severe hyperosmotic stress, a smaller kinase gets activated and the activation of SIMK was not observed at severe hyperosmotic stress suggesting that the two kinases function at different stress levels. Mikolajczyk et al. (2000) showed that a salicylic acid-induced protein kinase (SIPK) was activated in tobacco cells under hyperosmotic stress. In tobacco, MAP kinase is activated by multiple stresses like hyperosmotic, hypoosmotic, salicylic acid, and fungal elicitors. A 40 kDa kinase was activated in Arabidopsis by salt stress in a calcium and ABA-independent manner (Hoyos and Zhang 2000). MPK4 acts as a negative regulator in plant defense mechanisms. Tang et al. (2005) demonstrated that Arabidopsis EDR1 acts as negative regulator of disease resistance and drought. The edr1 mutant containing a kinase-deficient form of the EDR1 gene exhibits enhanced resistance to pathogens. The edr1 mutants could also enhance stress responses and spontaneous necrotic lesions under drought conditions (Tang et al. 2005). Accumulation of H2O2, superoxide anions, and hydroxyl radicals during abiotic stress causes oxidative burst in cells (Samuel and Ellis 2000). Plants can withstand this oxidative stress by production of antioxidant enzymes like catalase, which decompose H2O2 in the cells. Xing et al. (2008) demonstrated that AtMKKl mediates ABA-induced CAT1 expression in Arabidopsis thaliana. The Arabidopsis mutants mkk1 and mpk6 were altered in their responses to ABA and desiccation stress and the results showed that MKK1-MPK6 regulate H2O2 metabolism by CAT1 (Xing et al. 2008). Nakagami et al. (2004) reported that MEKK1-MPK4 cascades are an important part of ROS metabolism. During abiotic stress, two signaling events must occur to induce defense responses in plant cells: one is the inhibition of negative regulators such as EDR1 and the other is activation of positive regulators (Tena et al. 2001). LeMPK3 isolated from Lycopersicon peruvianum is homologous to AtMPK3 and is activated by UV-B radiations (Holley et al. 2003). Mayrose et al. (2004) showed that LeMPK3 is involved in mechanical stress and wounding in tomatoes. CbMAPK3 isolated from Chorispora bungeana shows identity of MPK3 and is activated by cold, salt, and ABA. H2O2 activated a novel MAPKKK OMTK1 in alfalfa which activates the MMK3 pathway. In tobacco, the overexpression of NtMEK2 stimulates the gene expression for defense and the generation of reactive oxygen species, which are led by the stimulation of two endogenous MAPKs, SIPK and WIPK (Yang et al. 2001; Ren et al. 2002) ; In rice system, Rao et al. (2011) showed activation of OsMPK3 and OsMPK4 by arsenic stress. While OsMPK3 was activated only in leaves, both OsMPK3 and MPK4 showed activation in roots. Recently, MPK3 from rice and pea were reported to function as effector molecules of the stress-regulated beta subunit of pea heterotri-meric G-proteins (Bhardwaj et al. 2011).
H2O2 induces 2MAPK-like activities in Arabidopsis and are independent of ethylene and jasmonic acid (Grant et al. 2000). It is not clear that these MAPKs belong to AtMPK3 and AtMPK6. Jonak et al. (1999) have reported that AtMPK3 and AtMPK6 have similarity to wound and SA-induced protein kinases (WIPK and SIPK) of tobacco, respectively. It was also found that only SIPK is activated in tobacco by ozone and H2O2.
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