392 Plant mitogen-activated protein kinase signaling cascades Guillaume Tena*, Tsuneaki Asai†, Wan-Ling Chiu‡ and Jen Sheen§ Mitogen-activated protein kinase (MAPK) cascades have components that link sensors/receptors to target genes emerged as a universal signal transduction mechanism that and other cellular responses. connects diverse receptors/sensors to cellular and nuclear responses in eukaryotes. Recent studies in plants indicate that In the past few years, it has become apparent that mitogen- MAPK cascades are vital to fundamental physiological functions activated protein kinase (MAPK) cascades play some of the involved in hormonal responses, cell cycle regulation, abiotic most essential roles in plant signal transduction pathways stress signaling, and defense mechanisms. New findings have from cell division to cell death (Figure 1). MAPK cascades revealed the complexity and redundancy of the signaling are evolutionarily conserved signaling modules with essen- components, the antagonistic nature of distinct pathways, and tial regulatory functions in eukaryotes, including yeasts, the use of both positive and negative regulatory mechanisms. worms, flies, frogs, mammals and plants. The recent enthu- siasm for plant MAPK cascades is backed by numerous Addresses studies showing that plant MAPKs are activated by hor- Department of Molecular Biology, Massachusetts General Hospital, mones, abiotic stresses, pathogens and pathogen-derived Department of Genetics, Harvard Medical School, Wellman 11, elicitors, and are also activated at specific stages during the 50 Blossom Street, Boston, Massachusetts 02114, USA cell cycle [2]. Until recently, studies of MAPK cascades in *e-mail: [email protected] †e-mail: [email protected] plants were focused on cDNA cloning [3,4] and used a ‡e-mail: [email protected] MAPK in-gel assay, MAPK and tyrosine-phosphate anti- §e-mail: [email protected] bodies, and kinase inhibitors to connect signals to MAPKs Current Opinion in Plant Biology 2001, 4:392–400 [2]. The recent completion of the Arabidopsis thaliana (At) genome sequence presents a new opportunity to identify 1369-5266/01/$ — see front matter and isolate the large gene families encoding MAPKs and © 2001 Elsevier Science Ltd. All rights reserved. their immediate upstream regulators, MAPK kinases Abbreviations (MAPKKs) and MAPKK kinases (MAPKKKs) on the basis At Arabidopsis thaliana of sequence conservation (Table 1). Various combinations CTR1 CONSTITUTIVE TRIPLE RESPONSE 1 of this large set of genes provide the diversity and specificity DSP dual specificity MAPK phosphatase EDR1 ENHANCED DISEASE RESISTANCE 1 necessary to transmit a broad spectrum of signals in plants. ERK extracellular signal-regulated kinase The challenge ahead is to develop additional tools to better MAPK (or MPK) mitogen-activated protein kinase define the components of plant MAPK cascades and to MAPKK MAPK kinase determine the specific roles of individual MAPK cascade MAPKKK MAPKK kinase MEK/MKK MAPK kinase genes in particular signal transduction pathways. MEKK MAPKK kinase MKP MAPK phosphatase A general background to MAPK signaling cascades has MMK Medicago MAPK been provided by excellent reviews on organisms from Ms Medicago sativa NPK1 Nicotiana protein kinase yeast, to mammals, to plants [2,5,6]. In this review, we offer Nt Nicotiana tabacum the first glance at the coding capacity for MAPK cascade PTP phosphotyrosine phosphatase genes in the Arabidopsis genome (Table 1), and focus on SA salicylic acid the most recent progress in the identification of plant SAMK stress-activated MAPK SAR systemic acquired resistance MAPK, MAPKK, and MAPKKK genes in specific signal SIMK salt-induced MAPK transduction pathways (Figure 1). We also discuss the SIMKK SIMK kinase emerging feature of antagonistic interactions between SIPK salicylic-acid-inducible protein kinase MAPK signaling cascades (Figure 2). WIPK wound-inducible protein kinase Plant MAPK cascade genes Introduction Known eukaryotic MAPKs can be divided into three Plants possess integrated signaling networks that mediate main subfamilies on the basis of their structural charac- the perception of and responses to the hormones, nutri- teristics, which often correlate with their functions in ents, and environmental cues and stresses that govern distinct signal transduction pathways [7]. However, all plant growth and development. Our current knowledge plant MAPK genes described so far belong to a single of plant signal transduction pathways has come from the group, the so-called extracellular signal-regulated kinase identification of the sensors and receptors that perceive (ERK) subfamily. In mammals, members of this the signal, and of the transcription factors and target subfamily are mainly responsible for the transduction of genes that coordinate the response [1]. What is missing mitogenic signals but, in plants, ERKs seem to have from our understanding of most plant signal transduction evolved in such a way as to be able to transmit a broader pathways, however, is the identity of the regulatory range of stimuli [8]. Plant MAP kinase signaling cascades Tena et al. 393 Figure 1 Plant Arabidopsis Tobacco Alfalfa Arabidopsis Tobacco Signal Bacterial Oxidative Fungal Salt Abiotic Mitosis elicitor stress elicitor stress stresses cytokinesis Sensor FLS2? AtHK1? MAPKKK AtMEKK1 AtANP1 AtMEKK1 NtNPK1 (NtNPK1) (AtANP1) AtMKK4/5 NtMEK2 MsSIMKK AtMKK1/2 NtMEK1 ∗∗ MAPKK (NtMEK2) (AtMKK4/5) (AtMKK4/5) (NtSIPKK) (MsSIMKK) (MsSIMKK) (NtMEK2) AtMPK3/6 AtMPK3/6 NtSIPK/WIPK MsSIMK AtMPK4 NtNtf6 ∗∗ MAPK (NtWIPK/SIPK) (AtMPK3/6) (AtMPK6) (MsMMK2) (MsMMK3) (MsSAMK/SIMK ∗) (MsSAMK/SIMK ∗) (NtSIPK) Target GST6, HSP18.2 (+) HMGR(+) PAL(+) ??? genes GH3, ER7 (–) PAL(+) Negative AtMKP1? AtMKP1? ? ? MsMP2C? ? regulator AtPTP1? AtPTP1? Current Opinion in Plant Biology MAPK cascades in diverse plant signal transduction pathways. NtNQK1, and tobacco NtNTF6 is NtNRK1. The negative regulators A general schematic presentation of signal transduction pathways is shown here are limited to known MAPK-specific phosphatases: dual- shown on the left. FLS2 is the putative receptor for the flagellin peptide specificity MAPK phosphatase (AtMKP1), phosphotyrosine elicitor flg22. AtHK1 is the putative histidine kinase osmosensor. The phosphatase (AtPTP1), and protein phosphatase 2C (MsMP2C). ER7, functionally defined MAPK-cascade components are shown in bold. auxin-inducible enhancer; GH3 auxin-inducible promoter; GST, MAPK, MAPKK and MAPKKK homologs in three plant species, tobacco glutathione-S-transferase; HMGR, 3-hydroxy-3-methylglutaryl CoA (Nt), alfalfa (Ms) and Arabidopsis (At) are shown. *Alfalfa MsSAMK is reductase; HSP, heat shock protein; PAL, phenylalanine ammonia-lyase; MsMMK4, MsSIMK is MsMMK1. **Tobacco NtMEK1 is the same as (+) positive regulation; (–) negative regulation. In terms of physiological roles, the best-characterized plant a combination of physiological, genetic and/or biochemical MAPK proteins are from Medicago sativa (alfalfa), Nicotiana data. A global survey of the MAPK cascade gene expression tabacum (tobacco) and Arabidopsis. A whole-genome survey patterns using customized microarrays in Arabidopsis could of the MAPK genes and the genes encoding their upstream help to establish their functionality. A more systematic regulators (MAPKKs and MAPKKKs) is, however, currently nomenclature of MAPK cascade genes would also facilitate possible only in Arabidopsis (Table 1). According to our information exchange among research groups working on sequence search criteria with highly conserved signature different plant species (see Figure 1 and Table 1). motifs, there are 23 MAPK and 9 MAPKK genes in the Arabidopsis genome (Table 1). The sequences of putative MAPK cascades in hormonal responses MAPKKK genes [4] are more divergent than those of other Extensive studies have linked MAPKs with mitogenic members of the MAPK cascades. In the Arabidopsis stimuli in mammalian cells, thus providing the generic genome, a comprehensive database search has identified name for this subclass of protein kinases. In plants, auxin more than 25 MAPKKK gene candidates on the basis of (as a mitogen) has previously been linked to MAPK acti- sequence conservation in the kinase domain (Table 1). The vation in tobacco BY-2 cells [14]; however, at that time, the MAPKKK gene candidates can be divided into at least two extreme sensitivity of plant MAPKs to mechanical stress main subfamilies, typified by their mammalian homologs: was not recognized. Other researchers were unable to Raf-like (e.g. AtCTR1 and AtEDR1) and MEKK-like show auxin activation of MAPK using the same system, (e.g. AtANP1 and AtMEKK1) [9,10•,11••–13••]. The true but did stimulate a strong MAPK-like activation by simply functional proof of any MAPK cascade component requires re-suspending BY-2 cells in fresh medium or treating the 394 Cell signalling and gene regulation Table 1 MAPK cascades components in the Arabidopsis genome. Total Minimal signature cDNA Function described Subfamilies number motif described Members Act. Loop MAPK 23 TxYVxxRWYRAPE 9 3: Group 1: AtMPK3, AtMPK6 3 TEY AtMPK3, 4, 6 Group 2: AtMPK4, AtMPK5 5 TEY [10•,11••,43••] Group 3: AtMPK1, AtMPK2, AtMPK7 4 TEY Group 4: AtMPK8, AtMPK9 8 TDY Group 5: AtMHK 3 TEY Members MAPKK 9 [ST]xxGTxxYMxPER 5 4: Group 1: AtMEK1, AtMKK2 3 AtMEK1, AtMKK2 Group 2: AtMKK4, AtMKK5 2 AtMKK4, 5 [25] Group 3: AtMKK3 1 (T Asai, G Tena, Group 4: No described cDNA 3 unpublished data) Putative members MAPKKK