Introduction
A general introduction of the MAPK cascades and plant salt tolerance
Plant survival under adverse environmental conditions generally depends on a combination of stress adaptive metabolic and structure variations into internal growth factors (Golldack, Li, Mohan, & Probst, 2014).
Abiotic environmental aspects, including deficiency of water and salinity are critical plant stressors with severe influence on the plant growth and yield. Consequently, they result into severe losses for farmers (Agarwal, Shukla, Gupta, & Jha, 2013). The control processes of plant drought and salt changes account for the regulation of water fluctuation and cell osmotic changes, which occur through biological processes of osmoprotectants (Agarwal et al., 2013).
These processes are however extremely complex. Salinity initiated difference of cell ion homeostasis is adjusted through ion inflow and out flow leading to vacuolar ion appropriation at the plasma membrane. More important, drought and salinity have furthermore main damaging effects on the cellular energy resource and redox homeostasis, which are controlled through an inclusive re-arrangement of plant major metabolism and changed cellular structures.
Sign transduction paths in plants have been extremely developed. At the same time, these pathways are extremely complicated to show every minor details for analyses. A simple explanation for such intricacy is that plants are generally permanently located and experiences all forms of variations, biotic or abiotic being located at a single place (Golldack et al., 2014). Signaling pathways are prompted in reaction to environmental changes involving different forms of stressors, and the most recent research about molecular and genetic has shown that these pathways involve a wide range of different responses (Agarwal et al., 2013).
It has been previously shown that abiotic stress reaction is an intricate trait controlled by several plant genes. In the last two decades, for example, major biological studies have significantly demonstrated both the expression of single genes or proteins and numerous forms of genes or gene products all at the same time, leading to a demonstration of genome-wide expression approaches for effective comprehension of these intricate traits.
Plant genomes are particularly known for numerous protein kinase signaling elements they have. It is therefore expected that the related signal transduction structures will be extremely dedicated and intricate. This observation is true. For instance, MAPKs/MPKs are greatly conserved protein members of the kinase group. With over 20 members of the family, the relationships are complex. With several sub-families, the dual-distinct MAPK kinases (MKK) act as the main triggers of MPKs via phosphorylation. The Arabidopsis genome converts ten members of the MKK gene family, including the last one, MKK10, which does not have the fully defined -S/T-X3-5-S/T- motif that characterizes eukaryotic MAPK kinases (Lee, Huh, Bhargava, & Ellis, 2008).
Several findings have offered evidence for the participation of plant MPKs in different forms of biotic and abiotic stress reactions, as well as phytohormone signaling and growth modeling. Nonetheless, outlining practical MKK-MPK module relations by linking a specific triggered MPK to a given upstream MKK is still a major obstacle. Since notable instances for activation of several MPKs using a single MKK and evidence of one MKK possessing abilities to trigger a specific MPK, multiple ways through which MKK-MPK signaling components could possibly be constructed have been found.
Phenotype-centered studies in Arabidopsis have offered minimal insight into these associations. For instance, a single MPK (MPK4) has been improved as a loss-of-function mutant (Lee et al., 2008). The inability to recover mutations in other numerous families or families belonging to MKKs in such studies could show that there is substantial practical redundancy within the MAPK signaling structures, that the phenotypic results of loss-of-function changes are understated or restricted, or that the loss-of-function genotypes are not feasible (Lee et al., 2008).
From most signaling pathways engaged in abiotic stress reaction in plants, mitogen activated protein kinase (MAPK) cascade among the most prominent pathways. MAPK module is known to connect external stimuli with multiple cell reactions and is gradually well preserved in the middle of eukaryotic organisms (Sinha, Jaggi, Raghuram, & Tuteja, 2011). MAPK cascades are protected signaling components located in every eukaryotes, which transfer environmental and growth cues into intracellular reactions. A MAPK cascade is marginally made up of at least three elements, including MAP kinase kinase kinases (MAP3Ks/MAPKKKs/MEKKs), MAP kinase kinase (MAP2Ks/MAPKKs/MEKs/MKKs) and MAP kinases (MAPKs/MPKs) (Sinha et al., 2011, p. 196; Conroy et al., 2013, p. 1).
In a reaction to stress, activated plasma membrane triggers MAP3Ks or MAP kinase kinases, kinase kinases (MAP4Ks) (Sinha et al., 2011, p. 196). Consequently, MAP4Ks acquire the role of adaptors joining upstream signaling movement to the major MAPK cascades. MAP3Ks are serine/threonine kinases phosphorylating with dual amino acids in the S/T-X3-5-S/T center of the MAP2K stimulation ring (Sinha et al., 2011).
MAP2Ks phosphorylate MAPKs on threonine and tyrosine deposits are found at the preserved T-X-Y motif. MAPKs have been recognized as serine/threonine kinases with abilities to produce organic phosphate in multiple elements, such as other kinases and/or transcript elements. Support proteins, shared docking domains and adaptor or anchoring proteins can facilitate the development and inclusion of a specific MAPK cascade (Sinha et al., 2011). MKPs, which are MAPK phosphatases, are engaged in the time-contingent regulator in the closure of the path after signaling.
MAPK are recognized for their roles in controlling plant development and growth (Conroy et al., 2013). As noted above, three main enzymes are involved in the formation of MAPK. However, MAPKKs/MKKs depend on a general triggering motif, which is common activation motif, S/TXXXXXS/T and elevated distinction for the downstream MAPKs (Conroy et al., 2013).
Among several species, including Arabidopsis thaliana, it has been demonstrated that MAPK cascades engaged in signaling pathways triggered by abiotic stresses such as cold, salinity, touch, wounding, heat, Ultraviolet rays, osmotic shock, and heavy metals, among others (Sinha et al., 2011, p. 196). In these processes, it has been identified that about “20 MAPKs, 10 MAPKKs and 60 MAPKKKs” (Conroy et al., 2013, p. 1) are observed. AtMEKK1-AtMKK1 and AtMKK2-AtMPK4 have been completely recognize as pathways, and they react to different forms of stresses, such as pathogen invasion, salt, wounds, or low temperatures. In addition, AtMEKK1-AtMKK4/AtMKK5-AtMPK3/AtMP has also been recognized as stress pathway. MAP is known to influence a vital number of kinases have.
However, over 60 MAPK cascade genes with unidentified roles have are also known to exist. This implies that the number of possible engaged MAPKs interacting across all levels of a MAPK cascade is vast. Majorities of MAPKKs are known to trigger extremely few members of the MAPK groups, but there do seems to be exclusions resulting in cases of superfluous activations, as observed in AtMKK1/AtMKK2 triggering AtMPK4, AtMKK4/AtMKK5 triggering AtMPK3/AtMPK6 and AtMKK1/AtMKK2 being able to activate AtMPK6 (Conroy et al., 2013).
A comprehensive pathway for AtMKK1 appears to be available. AtMEKK1 is known to phosphorylate and trigger AtMKK1 in a two-hybrid system of yeast while AtMPK4 is linked to AtMKK1 when a similar yeast two-hybrid experiment is conducted. Based on the first recognition of the AtMEKK1-
AtMKK1-AtMPK4 pathway, many other influencing factors have been noted. AtMKK1 acts a major play in the control of AtMPK4 and AtMPK3, as well as AtMPK6. The most notably noticed pathways where AtMKK1 is prominent include “wounding, bacterial pathogen response, cold, drought, salt stress, oxidative stress, touch, and abscisic acid (ABA)” (Conroy et al., 2013, p. 1). Stresses such as wounding have been acknowledged, although with some conflicting data. Conversely, other stresses, such as salt, have encountered diverse responses. While some studies associate AtMKK1 with the activation of AtMPK4 in salt stress, other researchers have rejected this claim and asserted that AtMKK1 is not engaged in the salt reaction (Conroy et al., 2013).
MAPK Pathway in Arabidopsis
Mitogen-activated protein kinase cascades are gradually protected signaling modules with vital control roles in eukaryotes, including “yeasts, worms, flies, frogs, mammals, and plants” (Sheen, 2010, p. 1). Literature has demonstrated that plant MAPKs are triggered by various factors, including abiotic stresses, pathogens and pathogen-derived elicitors, and plant hormones (Sheen, 2010).
The Arabidopsis genome and EST order studies have shown huge gene families encoding MAPKs and their adjacent upstream regulators, MAPKKs and MAPKKKs (Sheen, 2010). Nevertheless, not much is known about the composition of plant MAPK cascades and the precise functions that specific MAPK cascade genes play in a given plant signal transfer pathways. A thorough model developed from genomic data to create a set of MAPK-, MAPKK-, and MAPKKK-related clones in association with transitory expression analysis to identify the role of all Arabidopsis MAPK cascade genes engaged in vital plant signaling pathways has been adopted.
It appears that the roles of MAPK cascades in plant signal transduction pathways are most likely to be well maintained, as studies based on the Arabidopsis genome elements tend to provide a wide range of implications and applications for different plant species.
Sheen (2010) used recent studies that concentrated on five MAPK cascades. Some of these studies also accounted for “osmotic stress responses, Flg22 peptide-mediated defense responses, the ethylene pathway, and auxin and H2O2 signaling” (Sheen, 2010, p. 1). The whole approach involves the recognition and cloning of the broad set of Arabidopsis genes encoding “MAPKs, MAPKKs, MAPKKKs, and appropriate protein phosphatases” (Sheen, 2010, p. 1).
Transitory manifestation of epitope-associated MAPKs, MAPKKs, and MAPKKKs was used to identify specific MAPK cascades that reacted to a particular abiotic or biotic stress or hormone signals in Arabidopsis and maize protoplasts. Essentially vigorous and leading negative forms of these MAPKKKs and MAPKKs are used to identify specific reporter genes associated with particular signaling pathways. The DNA array technology is adopted to evaluate MAPK-associated gene expression arrangements, and to show novel downstream targets and crosstalk between certain MAPK cascades.
Determination of Arabidopsis knockout mutants consistent with the main MAPK-associated controlling genes is started in search studies. It is expected that transgenic Arabidopsis, maize and soybean plants, which display excessive certain types of genetically modified MAPKKK and MAPKK genes will be developed. In such cases, the resultant plant is usually assessed for the exhibition of agronomically useful traits. Experimental design adopted by Sheen (2010) is founded on Arabidopsis and maize protoplast science. The transitory characteristic of the protoplast systems ensures that direct role analysis of plant genes is achieved at an exceptional rate and at relatively low cost.
This method exploits the Arabidopsis genome structure that is developed as a component of “the NSF Plant Genome Research Program, which includes genome and EST sequencing, DNA microarray gene expression technology, and knockout mutant libraries” (Sheen, 2010, p. 1). Such experimental designs are known to reveal the gene functions that usually difficult to analyze using by outmoded genetic and biochemical methods because of redundancy, destruction, or minimal levels of expression. The exposition and influence of MAPK cascades in plants normally show essentially vital signaling processes and offer novel solutions for crop improvement in stress tolerance, disease resistance, and yield enhancement.
In Arabidopsis, there are 80 MAPKKK, 10 MKK and 20 MPK genes [3, 4]. The ten MKK genes in Arabidopsis may activate different MPKs and hence may integrate cross-talk of different signaling pathways (Liang, et al., 2013).
The Arabidopsis genome consists of 60 MKKKs, 10 MKKs, and 20 MPKs (Xu et al., 2008, p. 26996). However, in this case, the focus is on 10 MKKs. The comparatively scarce MKKs means that different signal transduction pathways congregate at the MKK level in MAPK cascades. The ten MKK genes are grouped into four categories based on their to protein sequence orientations. The ten MKK genes in Arabidopsis are known to trigger various MPKs and, thus, could incorporate cross-talk of various signaling pathways (Liang et al., 2013). In their classification, the ten MKKs are found in four major groups consisting of A, B, C and D, depending on their S/TxxxxxS/T consensus domain and D sites (Liang et al., 2013).
The MKKs found in group A are MKK1, MKK2, and MKK6. MKK1 takes part in protection responses, while MKK2 intercedes the control of cold and salt stress signaling. Further, MKK6 is engaged in the plant cell division. The MKK3 is identified as the sole member of group B. It plays a function in jasmonate-facilitated developmental signaling and in pathogen protection reaction signaling. Members of the group consist of MKK4 and MKK5, which are majorly engaged in the stomatal development pathway and protection response signaling pathway.
In group D, there are four MKKs, but MKK7 acts as a blocker of polar auxin movement. Further, MKK7 is also engaged in the production of the mobile signal of complete acquired resistance. To this end, no transcripts are available for MKK8 and MKK10 (Xu et al., 2008). MKK9 is identified as a negative regulator of seed germination under stress. In addition, recent studies have shown that MKK9 triggers MPK3/MPK6, which subsequently influence phosphorylates and positively controls EIN3 steadiness, resulting into transcription of the initial ERF (ethylene response factor) genes.
Studies have demonstrated that the ten MKK genes have varied salt tolerance rates. For instance, other genes have positive salt stress tolerance, whereas others do not have any relationships with stress salt.
MKK1/2-MPK4/6 cascades are known to take active role roles in the reaction to salt and cold stress, as well as infection of plants (Zhang et al., 2016). Studies in biochemical and genetic analyses demonstrate that MKK2 significantly influence the cold and salt stress response in Arabidopsis. Full genome analysis of plants showing active MKK2 established a vital change in expression of over 150 genes encoding proteins involved in transcriptional regulation, defense, signaling, and metabolism, most of which intersection with several others recently noted marker genes for cold and salt stress reaction in Arabidopsis (Teige et al., 2004, p. 141).
It noted that mkk4 that are not effective in the MPK3/MPK6-upstream control MAPK kinase are less tolerant to osmotic stress, which usually involve salt. On the other hand, MKK4 overexpression that is complemented by MPK3/MPK6 hyperactivation, increases stress tolerance (Pitzschke, Datta, & Persak, 2014). MKK5 also reacted in response to salt stress. It is noted that overexpression of MKK5 in wild-type plants increased their tolerance to salty environments, whereas mkk5 mutant showed increased sensitivity to salt stress during sprouting in media with salt. In addition, MPK6 also worked alongside the MKK5-mediated iron superoxide dismutase (FSD) signaling pathway in salt stress (Xing, Chen, Jia, & Zhang, 2015).
It has also been observed that MKK6 expression level is higher during responses to tissues and the stresses specifically in rice, but not in Arabidopsis (Xing et al., 2015). This shows that MKK6 as a critical role to play in integrating upstream signals for the necessary cellular responses (Xing et al., 2015).
Some studies have shown that MKK7 is a negative regulator of auxin, and over-expression and repression studies have confirmed this function Arabidopsis.
It also observed that the role of MKK8 and the atypical MKK10 are not currently known (Colcombet & Hirt, 2008). It has been demonstrated that a mutation in MKK9 is associated with seedling stress tolerance, depicting that MKK9 may act as a negative regulator of abiotic stress responses (Colcombet & Hirt, 2008).
Analysis of protein-protein interactions between Arabidopsis
Given that the nature of protein kinase activities relies on direct physical association between the enzyme and its bait protein, it is believed that that the ability of specific proteins to associate successfully with each other ultimately influence every level of specific information found within the entire Arabidopsis MKK/MPK structure. To assess this notion, some thorough studies involving yeast two-hybrid screen have been conducted using the ten Arabidopsis MKKs as specific inducement proteins, and other twenty MPKs as target proteins. Hence, the results are critical for explaining protein-protein interactions detected in this Y2H assay.
Arabidopsis.
The many comprehensive MAPK cascade roles in abiotic stresses include of the MAP3K MEKK1 activating MKK2 and MPK4/MPK6. For instance, for the mkk2, cold and salt trigger MPK4, whereas MPK6 is impaired and mkk2 mutant plants are associated with hypersensitivity to cold and salt stresses.
In the ten MKK genes, nine of the ten Arabidopsis MKK proteins were noted to associate with at least a single MPK protein (Lee et al., 2008). However, the exception was MKK8, which was the only non-interacting MKK family member, and it showed structurally intact features related to its kinase domain and specific MAPK kinase sequence motifs. The MKK8 gene is normally expressed at only extremely low levels in most studies (Lee et al., 2008).
The MKK8 gene alleged orthologue in Populus trichocarpa is the same as silent, reflecting a gene that could be losing its organic functionality. Several other MKKs were noted to associate with two or more MPK elements, and in many instances, these findings confirmed earlier results of MKK-MPK interactions. For instance, it was established that MKK1 and MKK2, were two closely linked MAPKKs because these two MKKs interacted with MPK 4 and MPK11, which are considered as two paralogous MPKs. The Interaction between MPK4 and MKK1 (MEK1) had previously been noted in some earlier studies depicting plant MKK-MPK relationships, whereas a recent study also demonstrated that MKK2 was capable of interacting with MPK4 (Lee et al., 2008). However, none of these studies evaluated MPK11.
MKK2 seems to have a broader scope of interactions than its other family members do because apart from MPK4 and MPK11, it can also interact with MPK6, MPK10, and significantly more uncertainly with MPK13.
The interface with MPK6 had been previously noted previously and some recent studies have shown a weak interaction between MKK2 and MPK13 (Lee et al., 2008). Lee et al. (2008) have however noted observed a precise interaction in MKK2-MPK10, which Teige et al. (2004) had failed to test. Teige et al. (2004) focused on MKK2-MPK5 interaction, which other researchers have failed to include in their analyses. A quick lab phosphorylation analyses showed that, in addition to powerfully phosphorylating MPK4 and MPK10, genetic recombination of CAMKK2 showed extremely poor activity alongside MPK11, MPK6, and MPK13.
MKK3 is a distinguishing monophyletic plant MAPKK whose extended C-ending part shows similarity to yeast NTF2 proteins. In the recent past, it was observed that that MKK3 had ability to interact with MPK1, MPK2, MPK7, and MPK14 in some experiments, and Lee et al. (2008) confirmed this observation. In addition, the previous study also showed that MKK3 had ability to use the same four MPKs as substrates.
However, functional association showed in planta appeared to rely on the type of the upstream stimuli being regulated. Remarkably, it was also anticipated that MKK3 could create a functional signaling pathway with a dissimilar MPK like MPK6 within the environment of jasmonic acid signal transfer. While this model seemed to be driven by genetic proof, the functional ability of MKK3 to rely on MPK6 as the primary substrate could not be determined.
As previously observed, MKK4 and MKK5 are members of the Group C MAPKKs that seem to be vital to the ability of plants to react to a wide of environmental stresses, including salt. Evidence obtained from several laboratory studies and actual studies on plants have shown that the downstream targets of MKK4 are thought to be MPK3 and MPK6 (Lee et al., 2008). Further, it is observed that MKK4 was noted to interact only with MPK3 and MPK6 from most the MPKs. MKK5, on the contrary, was noted to interact only with MPK6. Based on this observation, it was claimed that both MKK4 and MKK5 were redundant.
That is, the study findings alluded that while MKK4 and MKK5 do have overlapping roles, they could also play different functions in some other contexts. It is experimented that the overexpression of MKK5 enhances salt tolerance because of MKK5 mediated iron superoxide dismutase (FSD) in salt stress while MEKK1-MKK5/MKK4-MPK6 pathway was believed to relate with salt tolerance in Arabidopsis (Xing et al., 2015). Taken conclusively, these findings showed that Arabidopsis MKK5 might be a vital factor in both abiotic and biotic stress signaling. However, the precise roles and comprehensive mechanisms of MKK5-mediated signaling are poorly understood.
Studies have shown that MKK6 was proposed to rely on MPK13 as its substrate in a signal transduction pathway engaged in the control of cytokinesis (Lee et al., 2008). However, this pathway was studied using Nicotiana thoroughly. It is imperative to observe that most of the supporting evidence for the role of this pathway emanated from genetic studies, but collective unusual expression of Arabidopsis MKK6 in an mpk1 mutant yeast supported the mutant’s signal transduction deficiency, whereas MPK13 activation was noted in the MKK6-expressing yeast.
Remarkably, MKK6 was observed to interact strongly with MPK4. An intermediary level of interaction was also identified between MKK6 and MPK6. However, attempts to show substrate roles through experimentations resulted in an unusual pattern where the autophosphorylation association of MPK4, MPK 6, and other MPKs was sturdily stifled by co-incubation with recombinant CAMKK6. Ultimately, no direct phosphorylation interactions of MPKs could be observed. It was noted that the lack of direct kinase activity against MPK4, MPK6, and other MPKs could presumably not be linked to malfunctioning MKK6 protein.
The genes of MKK7-inhibited and overexpression indicate that signaling through this MKK supports both disease resistance and polar auxin transport (Lee et al., 2008). However, there is no current MPK substrate reported for MKK7. This implies that there is no interaction and salt tolerance in Arabidopsis with MKK7. Lee et al. (2008) found out that MKK7 interacted with MPK2 and MPK15, but when recombinant proteins were introduced, there was no noticeable activity of CAMKK7 against MPK2.
MKK9 was lately reported to act in the control of ethylene signaling (Lee et al., 2008). In these instances, MKK9 serves downstream of the CTR1 MAPKKK, as well as upstream of MPK6. Inquisitively, as opposed to acknowledged MAPK cascades based on successive activation activities, this projected signaling element appears to engage CTR1 inactivity of MKK9 by an unknown processes.
However, it was observed that MKK9 did not interrelate with MPK6 in two-yeast hybrid study. It was observed to interact with MPK10, MPK17, and MPK20, which three MPKs with no known biological roles. In the study, it was noted that recombinant CAMKK9 could also phosphorylate MPK10 and MPK20. In addition, MPK6 could also act as a substrate. However, most important, it was established that MKK9 did not associate with other MPK family members during Y2H system.
Finally, MKK10, which is the last member of the MKK family member, lacks elements of the MKK consensus motif and may not function effectively biological, was noted to interact with MPK17 in the Y2H experiment. To date, there is no known case of salt tolerance in Arabidopsis in MKK10 gene.
Although most Arabidopsis MKKs have been identified to interact with and/or phosphorylate one or more MPKs, it is observed that, for most of the 20 MPKs, including MPK5, MPK8, MPK9, MPK16, MPK18, and MPK19, there were neither MKK interactions nor substrate association noticed (Lee et al., 2008). The yeast two-hybrid method (Y2H) is a prominent method for analyzing protein-protein interactions. It reveals both false positive and false negative results.
Further, various Y2H methodologies can also lead in other new findings. Hence, while not all associations were detected, for instance, a MKK2-MPK5 interaction was not noticed, in this study, further studies may show such outcomes based on assays used in a given study. Largely, however, the findings of this analysis on these interactions are extremely consistent with other findings in different studies. As such, this consistency offers confidence that the interactions noted here are valid for protein-protein interaction of salt tolerance in Arabidopsis.
In Arabidopsis, past research has established that the MEKK1 (a MAPKKK) is amassed in as a reaction to environmental stresses, including high salinity. Yeast two-hybrid analyses showed protein-protein interactions between various MKK member family, including “MEKK1 and MKK2/MEK1 (MAPKKs), between MKK2/MEK1 and MPK4 (a MAPK), and between MPK4 and MEKK1” (Lee et al., 2008, p. 1037). Further studies demonstrated that environmental stress signals are conveyed to not less than two MAPK cascades. One of the cascades is the MPK4 cascade, which accounts for MEKK1-MEK1/MKK2-MPK4) and the other accounts for MPK6 and p44MAPK. In the presence of salt or cold stress, MAPK pathway associates with MEKK1 as an upstream trigger of MKK2 and the downstream MAPKs MPK4 and MPK6 (Lee et al., 2008). MKP1 is associated with a negative role in salt stress signaling via MAPKs (MPK6 and MPK4) (Lee et al., 2008).
It is demonstrated that expression patterns of MKKs under hormone influences or abiotic stresses showed their difference roles in stress responses. That is, every member of the MKKs different groups had different roles to play in relation to stress responses.
Focusing on MKK3 and MKK9
Hypothesis
The MKK3 and MKK9 relate to the salt tolerance
MKK3 is positive regulate for salt tolerance. MKK3 is considered a vital element of plant Reactive oxygen species (ROS) metabolism and is a part of JA and ABA stress signaling. AtMKK3 is linked with AtMPK7 and AtMPK8 since it mediates ROS signaling and regulates AtMPK6 in response to stresses. Further, AtMKK3 has a critical function in ABA stress signaling, which consequently triggers the activity of the group C MAPKs, such as AtMPK1, AtMPK2, AtMPK7 and AtMPK14.
While the Arabidopsis thaliana genome has genes encoding some 20 mitogen-activated protein kinases (MAPKs) and 10 MAPK kinases (MAPKKs), many of these genes are not clearly understood based on their roles. In this study, it is shown that the function of MKK3 and MKK9 also relate to salt tolerance. Transgenic Pro MKK3: GUS lines demonstrated basic expression in vascular tissues that was intensely brought about by infection (Pst DC3000) rather than by abiotic stresses.
The development of virulent Pst DC3000 was escalated in mkk3 knockout plants and declined in MKK3-overexpressing ones. In addition, MKK3 overexpression lines indicated elevated expression of many genes. Based on the yeast two-hybrid analysis, MKK3 was observed as an upstream activator of the group C genes, including MAPKs MPK1, MPK2, MPK7, and MPK14 (Dóczi et al., 2007). While flagellin-based flg22 peptide intensely triggered MPK6, it had low activation effect of MPK7.
Comparatively, H(2)O(2) triggered both MPK6 and MPK7, but only MPK7 activation was increased by MKK3. In support of the idea that MKK3 controls the expression of PR genes, ProPR1: GUS expression was importantly increased by notable expression of MKK3-MPK7. Overall, the study showed that the MKK3 pathway had a significant function in pathogen defense. Further, it revealed the relevance and intricacy of MAPK signaling in plant stress responses (Dóczi et al., 2007).
MKK3 was considered as an upstream controller of MPK7 in the H2O2 signaling pathway that functions individually of the flagellin signaling pathway (Hirt & Pitzschke, 2009). Based on the environmental condition, it seems that MKK3 can focus on either MPK6 or MPK7 (Hirt & Pitzschke, 2009). Steroid-triggered manifestation of essentially active MKK3 or MKK4 in transgenic plants causing the buildup of H2O2 and of the stress-related and senescence-related plant hormone ethylene (Hirt & Pitzschke, 2009).
Conversely, the transcriptome features of these plants do not meaningfully overlap, showing that MKK3 and MKK4 have diverse functions in controlling gene manifestation. According to the finding that MKK3 phosphorylates MPK6 lab studies and that mkk3 and mpk6 knockout mutants are oversensitive to events associated with the wound in plant hormone jasmonic acid, MKK3 has been attributed a function in the jasmonic acid-triggered activation of MPK6 (Hirt & Pitzschke, 2009). The same condition applies in yeast and animals. That is, the privileged assemblage of MKK3 with MPK6 or MPK7 might be aided by specific proteins, which also motivate stimulus-based stimulation.
MKK9 is relatively new and not widely studied, and some findings show that it is negative regulator of abiotic stress responses (Xu et al., 2008). As such, new studies have shown that a mutation in MKK9 leads in increased seedling stress tolerance, depicting that MKK9 may function as a salt tolerance gene. Most recently, it was noted that MKK9-MPK3/MPK6 cascades favored ethylene-insensitive3 (EIN3)-mediated transcription activities. These findings show that MKK9 is a kinase that has some distinct roles and that its ATP-binding location is vital for its kinase roles.
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