The alfalfa (Medicago sativa) TDY1 gene encodes a mitogen-activated protein kinase homolog

Genome-Wide Characterization of the Mitogen-Activated Protein Kinase Gene Family and Their Expression Patterns in Response to Drought and Colletotrichum Gloeosporioides in Walnut (Juglans regia)

Mitogen-activated protein kinases (MAPKs) are a family of Ser/Thr (serine/threonine) protein kinases that play very important roles in plant responses to biotic and abiotic stressors. However, the MAPK gene family in the important crop walnut (Juglans regia L.) has been less well studied compared with other species. We discovered 25 JrMAPK members in the Juglans genome in this study. The JrMAPK gene family was separated into four subfamilies based on phylogenetic analysis, and members of the same subgroup had similar motifs and exons/introns. A variety of cis-acting elements, mainly related to the light response, growth and development, stress response, and hormone responses, were detected in the JrMAPK gene promoters. Collinearity analysis showed that purification selection was the main driving force in JrMAPK gene evolution, and segmental and tandem duplications played key roles in the expansion of the JrMAPK gene family. The RNA-Seq (RNA Sequencing) results indicated that many of the JrMAPK genes were expressed in response to different levels of Colletotrichum gloeosporioides infection. JrMAPK1, JrMAPK3, JrMAPK4, JrMAPK5, JrMAPK6, JrMAPK7, JrMAPK9, JrMAPK11, JrMAPK12, JrMAPK13, JrMAPK17, JrMAPK19, JrMAPK20, and JrMAPK21 were upregulated at the transcriptional level in response to the drought stress treatment. The results of this study will help in further investigations of the evolutionary history and biological functions of the MAPK gene family in walnut.

QTL Mapping and Data Mining to Identify Genes Associated With the Sinorhizobium fredii HH103 T3SS Effector NopD in Soybean

In some legume-rhizobium symbioses, host specificity is influenced by rhizobial type III effectors-nodulation outer proteins (Nops). However, the genes encoding host proteins that interact with Nops remain unknown. In this study, we aimed to identify candidate soybean genes associated with NopD, one of the type III effectors of Sinorhizobium fredii HH103. The results showed that the expression pattern of NopD was analyzed in rhizobia induced by genistein. We also found NopD can be induced by TtsI, and NopD as a toxic effector can induce tobacco leaf death. In 10 soybean germplasms, NopD played a positively effect on nodule number (NN) and nodule dry weight (NDW) in nine germplasms, but not in Kenjian28. Significant phenotype of NN and NDW were identified between Dongnong594 and Charleston, Suinong14 and ZYD00006, respectively. To map the quantitative trait locus (QTL) associated with NopD, a recombinant inbred line (RIL) population derived from the cross between Dongnong594 and Charleston, and chromosome segment substitution lines (CSSLs) derived from Suinong14 and ZYD00006 were used. Two overlapping conditional QTL associated with NopD on chromosome 19 were identified. Two candidate genes were identified in the confident region of QTL, we found that NopD could influence the expression of Glyma.19g068600 (FBD/LRR) and expression of Glyma.19g069200 (PP2C) after HH103 infection. Haplotype analysis showed that different types of Glyma.19g069200 haplotypes could cause significant nodule phenotypic differences, but Glyma.19g068600 (FBD/LRR) was not. These results suggest that NopD promotes S. fredii HH103 infection via directly or indirectly regulating Glyma.19g068600 and Glyma.19g069200 expression during the establishment of symbiosis between rhizobia and soybean plants.

Connection the Rhizomicrobiome and Plant MAPK Gene Expression Response to Pathogenic Fusarium oxysporum in Wild and Cultivated Soybean

Little known the connections between soybeans mitogen-activated protein kinase (MAPK) gene expression and the rhizomicrobiome upon invasion of the root pathogen Fusarium oxysporum. To address this lack of knowledge, we assessed the rhizomicrobiome and root transcriptome sequencing of wild and cultivated soybean during the invasion of F. oxysporum. Results indicated F. oxysporum infection enriched Bradyrhizobium spp. and Glomus spp. and induced the expression of more MAPKs in the wild soybean than cultivated soybean. MAPK gene expression was positively correlated with Pseudomonadaceae but negatively correlated with Sphingomonadaceae and Glomeraceae in both cultivated and wild soybean. Specifically, correlation profiles revealed that Pseudomonadaceae was especially correlated with the induced expression of GmMAKKK13-2 (Glyma.14G195300) and GmMAPK3-2 (Glyma.12G073000) in wild and cultivated soybean during F. oxysporum invasion. Main fungal group Glomeraceae was positively correlated with GmMAPKKK14-1 (Glyma.18G060900) and negatively correlated with GmRaf6-4 (Glyma.02G215300) in the wild soybean response to pathogen infection; while there were positive correlations between Hypocreaceae and GmMAPK3-2 (Glyma.12G073000) and between Glomeraceae and GmRaf49-3 (Glyma.06G055300) in the wild soybean response, these correlations were strongly negative in the response of cultivated soybean to F. oxysporum. Taken together, MAPKs correlated with different rhizomicrobiomes indicating the host plant modulated by the host self-immune systems in response to F. oxysporum.

Identification and Characterization of Mitogen-Activated Protein Kinase (MAPK) Genes in Sunflower (Helianthus annuus L.)

Mitogen-Activated Protein Kinase (MAPK) genes encode proteins that regulate biotic and abiotic stresses in plants through signaling cascades comprised of three major subfamilies: MAP Kinase (MPK), MAPK Kinase (MKK), and MAPKK Kinase (MKKK). The main objectives of this research were to conduct genome-wide identification of MAPK genes in Helianthus annuus and examine functional divergence of these genes in relation to those in nine other plant species (Amborella trichopoda, Aquilegia coerulea, Arabidopsis thaliana, Daucus carota, Glycine max, Oryza sativa, Solanum lycopersicum, Sphagnum fallax, and Vitis vinifera), representing diverse taxonomic groups of the Plant Kingdom. A Hidden Markov Model (HMM) profile of the MAPK genes utilized reference sequences from A. thaliana and G. max, yielding a total of 96 MPKs and 37 MKKs in the genomes of A. trichopoda, A. coerulea, C. reinhardtii, D. carota, H. annuus, S. lycopersicum, and S. fallax. Among them, 28 MPKs and eight MKKs were confirmed in H. annuus. Phylogenetic analyses revealed four clades within each subfamily. Transcriptomic analyses showed that at least 19 HaMPK and seven HaMKK genes were induced in response to salicylic acid (SA), sodium chloride (NaCl), and polyethylene glycol (Peg) in leaves and roots. Of the seven published sunflower microRNAs, five microRNA families are involved in targeting eight MPKs. Additionally, we discussed the need for using MAP Kinase nomenclature guidelines across plant species. Our identification and characterization of MAP Kinase genes would have implications in sunflower crop improvement, and in advancing our knowledge of the diversity and evolution of MAPK genes in the Plant Kingdom.

Identification of Soybean Genes Whose Expression is Affected by the Ensifer fredii HH103 Effector Protein NopP

In some legume⁻rhizobium symbioses, host specificity is influenced by rhizobial nodulation outer proteins (Nops). However, the genes encoding host proteins that interact with Nops remain unknown. We generated an Ensifer fredii HH103 NopP mutant (HH103ΩNopP), and analyzed the nodule number (NN) and nodule dry weight (NDW) of 10 soybean germplasms inoculated with the wild-type E. fredii HH103 or the mutant strain. An analysis of recombinant inbred lines (RILs) revealed the quantitative trait loci (QTLs) associated with NopP interactions. A soybean genomic region containing two overlapping QTLs was analyzed in greater detail. A transcriptome analysis and qRT-PCR assay were used to identify candidate genes encoding proteins that interact with NopP. In some germplasms, NopP positively and negatively affected the NN and NDW, while NopP had different effects on NN and NDW in other germplasms. The QTL region in chromosome 12 was further analyzed. The expression patterns of candidate genes Glyma.12g031200 and Glyma.12g073000 were determined by qRT-PCR, and were confirmed to be influenced by NopP.

Genome-wide Identification of Jatropha curcas MAPK, MAPKK, and MAPKKK Gene Families and Their Expression Profile Under Cold Stress

Mitogen-activated protein kinase (MAPK) cascades are fundamental signal transduction modules in all eukaryotic organisms, controlling cell division, growth, development, and hormone signaling. Additionally, they can be activated in response to a variety of biotic and abiotic stressors. Although the evolution and expression patterns of MAPK cascade families have been systematically investigated in several model plants (e.g., Arabidopsis, rice, and poplar), we still know very little about MAPK, MAPKK, and MAPKKK families in Jatropha curcas, an economically important species. Therefore, this study performed genome-wide identification and transcriptional expression analysis of these three families in J. curcas. We identified 12 J. curcas MAPK (JcMAPKs), 5 JcMAPKKs, and 65 JcMAPKKKs. Phylogenetic analysis classified all JcMAPKs and JcMAPKKs into four subgroups, whereas JcMAPKKKs were grouped into three subfamilies (MEKK, RAF, and ZIK). Similarities in exon/intron structures supported the evolutionary relationships within subgroups and subfamilies. Conserved motif analysis indicated that all J. curcas MAPK cascades possessed typical, 200–300 amino-acid protein kinase domains. MAPK cascade genes were presented throughout all 11 chromosomes. Gene duplication analysis suggested that after JcMAPK and JcMAPKKK diverged, 3 and 19 tandem duplicates occurred under strong purifying selection. Furthermore, RNA-seq and qRT-PCR analyses revealed that some MAPK cascade genes are predominantly expressed in specific tissues. Moreover, their expression levels significantly increased under cold treatment. Our results should provide insight into the roles of MAPK cascade genes in regulating J. curcas stress responses and in hormonal signal transduction. Furthermore, these data have important applications in the genetic improvement of J. curcas.

Evolutionary history of mitogen-activated protein kinase (MAPK) genes in Lotus, Medicago, and Phaseolus

Mitogen-Activated Protein Kinase (MAPK) genes encode proteins that mediate various signaling pathways associated with biotic and abiotic stress responses in eukaryotes. The MAPK genes form a 3-tier signal transduction cascade between cellular stimuli and physiological responses. Recent identification of soybean MAPKs and availability of genome sequences from other legume species allowed us to identify their MAPK genes. The main objectives of this study were to identify MAPKs in 3 legume species, Lotus japonicus, Medicago truncatula, and Phaseolus vulgaris, and to assess their phylogenetic relationships. We used approaches in comparative genomics for MAPK gene identification and named the newly identified genes following Arabidopsis MAPK nomenclature model. We identified 19, 18, and 15 MAPKs and 7, 4, and 9 MAPKKs in the genome of Lotus japonicus, Medicago truncatula, and Phaseolus vulgaris, respectively. Within clade placement of MAPKs and MAPKKs in the 3 legume species were consistent with those in soybean and Arabidopsis. Among 5 clades of MAPKs, 4 founder clades were consistent to MAPKs of other plant species and orthologs of MAPK genes in the fifth clade-"Clade E" were consistent with those in soybean. Our results also indicated that some gene duplication events might have occurred prior to eudicot-monocot divergence. Highly diversified MAPKs in soybean relative to those in 3 other legume species are attributable to the polyploidization events in soybean. The identification of the MAPK genes in the legume species is important for the legume crop improvement; and evolutionary relationships and functional divergence of these gene members provide insights into plant genome evolution.

Modifying crops to increase cell wall digestibility

Improving digestibility of roughage cell walls will improve ruminant animal performance and reduce loss of nutrients to the environment. The main digestibility impediment for dicotyledonous plants is highly lignified secondary cell walls, notably in stem secondary xylem, which become almost non-digestible. Digestibility of grasses is slowed severely by lignification of most tissues, but these cell walls remain largely digestible. Cell wall lignification creates an access barrier to potentially digestible wall material by rumen bacteria if cells have not been physically ruptured. Traditional breeding has focused on increasing total dry matter digestibility rather than cell wall digestibility, which has resulted in minimal reductions in cell wall lignification. Brown midrib mutants in some annual grasses exhibit small reductions in lignin concentration and improved cell wall digestibility. Similarly, transgenic approaches down-regulating genes in monolignol synthesis have produced plants with reduced lignin content and improved cell wall digestibility. While major reductions in lignin concentration have been associated with poor plant fitness, smaller reductions in lignin provided measurable improvements in digestibility without significantly impacting agronomic fitness. Additional targets for genetic modification to enhance digestibility and improve roughages for use as biofuel feedstocks are discussed; including manipulating cell wall polysaccharide composition, novel lignin structures, reduced lignin/polysaccharide cross-linking, smaller lignin polymers, enhanced development of non-lignified tissues, and targeting specific cell types. Greater tissue specificity of transgene expression will be needed to maximize benefits while avoiding negative impacts on plant fitness.cauliflower mosiac virus (CaMV) 35S promoter.

Isolation and molecular characterization of the Triticum aestivum L. ethylene-responsive factor 1 (TaERF1) that increases multiple stress tolerance

ERF transcription factors play important roles in regulating gene expression under abiotic and biotic stresses. The first member of the ERF gene family in wheat (Triticum aestivum L.) was isolated by screening a drought-induced cDNA library and designated as T. aestivum ethylene-responsive factor 1 (TaERF1), which encoded a putative protein of 355 amino acids with a conserved DNA-binding domain and a conserved N-terminal motif (MCGGAIL). The TaERF1 gene was located on chromosome 7A. Protein interaction assays indicated that TaERF1, with a putative phosphorylation site (TPDITS) in the C-terminal region, was a potential phosphorylation substrate for TaMAPK1 protein kinase. Deletion of the N-terminal motif enhanced the interaction of TaERF1 with TaMAPK1. The predicted TaERF1 protein contained three putative nuclear localization signals (NLSs), and three NLSs modulated synergistically the activity of subcellular localization. As a trans-acting factor, TaERF1 was capable of binding to the GCC-box and CRT/DRE elements in vitro, and of trans-activating reporter gene expression in tobacco (Nicotiana tabacum L.) leaves. Transcription of the TaERF1 gene was induced not only by drought, salinity and low-temperature stresses and exogenous ABA, ethylene and salicylic acid, but also by infection with Blumeria graminis f. sp. tritici. Furthermore, overexpression of TaERF1 activated stress-related genes, including PR and COR/RD genes, under normal growth conditions, and improved pathogen and abiotic stress tolerance in transgenic plants. These results suggested that the TaERF1 gene encodes a GCC-box and CRT/DRE element binding factor that might be involved in multiple stress signal transduction pathways.

A novel rice MAPK gene, OsBIMK2, is involved in disease-resistance responses

The mitogen-activated protein kinase (MAPK) cascades play important roles in transmission of extracellular signals to the downstream effector proteins through a mechanism of protein phosphorylation. In this study, we isolated and identified a novel rice MAPK gene, OSBIMK2 ( ORYZAE SATIVA L. BTH-Induced MAP Kinase 2). The OSBIMK2 encodes a 506 amino acid protein with molecular weight of 63 kD. The recombinant OSBIMK2 expressed in ESCHERICHIA COLI showed an autophosphorylation activity IN VITRO. OSBIMK2 is a single-copy gene in the rice genome. Expression of OSBIMK2 was activated upon treatment with benzothiadiazole (BTH), which is capable of inducing disease resistance in rice. Expression of OsBIMK2 was also up-regulated during early stage after inoculation with MAGNAPORTHE GRISEA in BTH-treated rice seedlings and during an incompatible interaction between M. GRISEA and a blast-resistant rice genotype. Over-expression of the rice OSBIMK2 gene in transgenic tobacco resulted in an enhanced disease resistance against tomato mosaic virus and a fungal pathogen, ALTERNARIA ALTERNATA. These results suggest that OSBIMK2 plays a role in disease resistance responses.

MAP-ping genomic organization and organ-specific expression profiles of poplar MAP kinases and MAP kinase kinases

Background

As in other eukaryotes, plant mitogen-activated protein kinase (MAPK) cascades are composed of three classes of hierarchically organized protein kinases, namely MAPKKKs, MAPKKs, and MAPKs. These modules rapidly amplify and transduce extracellular signals into various appropriate intracellular responses. While extensive work has been conducted on the post-translational regulation of specific MAPKKs and MAPKs in various plant species, there has been no systematic investigation of the genomic organization and transcriptional regulation of these genes.

Results

Ten putative poplar MAPKK genes (PtMKKs) and 21 putative poplar MAPK genes (PtMPKs) have been identified and located within the poplar (Populus trichocarpa) genome. Analysis of exon-intron junctions and of intron phase inside the predicted coding region of each candidate gene has revealed high levels of conservation within and between phylogenetic groups. Expression profiles of all members of these two gene families were also analyzed in 17 different poplar organs, using gene-specific primers directed at the 3'-untranslated region of each candidate gene and real-time quantitative PCR. Most PtMKKs and PtMPKs were differentially expressed across this developmental series.

Conclusion

This analysis provides a complete survey of MAPKK and MAPK gene expression profiles in poplar, a large woody perennial plant, and thus complements the extensive expression profiling data available for the herbaceous annual Arabidopsis thaliana. The poplar genome is marked by extensive segmental and chromosomal duplications, and within both kinase families, some recently duplicated paralogous gene pairs often display markedly different patterns of expression, consistent with the rapid evolution of specialized protein functions in this highly adaptive species.

Signaling through MAP kinase networks in plants

Protein phosphorylation is the most important mechanism for controlling many fundamental cellular processes in all living organisms including plants. A specific class of serine/threonine protein kinases, the mitogen-activated protein kinases (MAP kinases) play a central role in the transduction of various extra- and intracellular signals and are conserved throughout eukaryotes. These generally function via a cascade of networks, where MAP kinase (MAPK) is phosphorylated and activated by MAPK kinase (MAPKK), which itself is activated by MAPKK kinase (MAPKKK). Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as response to various stresses. In plants, MAP kinases are represented by multigene families and are organized into a complex network for efficient transmission of specific stimuli. Putative plant MAP kinase cascades have been postulated based on experimental analysis of in vitro interactions between specific MAP kinase components. These cascades have been tested in planta following expression of epitope-tagged kinases in protoplasts. It is known that signaling for cell division and stress responses in plants are mediated through MAP kinases and even auxin, ABA and possibly ethylene and cytokinin also utilize a MAP kinase pathway. Most of the biotic (pathogens and pathogen-derived elicitors) including wounding and abiotic stresses (salinity, cold, drought, and oxidative) can induce defense responses in plants through MAP kinase pathways. In this article we have covered the historical background, biochemical assay, activation/inactivation, and targets of MAP kinases with emphasis on plant MAP kinases and the responses regulated by them. The cross-talk between plant MAP kinases is also discussed to bring out the complexity within this three-component module.

Identification of mycorrhiza-regulated genes with arbuscule development-related expression profile

Suppressive subtractive hybridisation was applied to the analysis of late stage arbuscular mycorrhizal development in pea. 96 cDNA clones were amplified and 81, which carried fragments more than 200 nt in size, were sequence analysed. Among 67 unique fragments, 10 showed no homology and 10 were similar to sequences with unknown function. RNA accumulation of the corresponding 67 genes was analysed by hybridisation of macro-arrays. The cDNAs used as probes were derived from roots of wild type and late mutant pea genotypes, inoculated or not with the AM fungus Glomus mosseae. After calibration, a more than 2.5-fold mycorrhiza-induced RNA accumulation was detected in two independent experiments in the wild type for 25 genes, 22 of which seemed to be induced specifically during late stage AM development. Differential expression for 7 genes was confirmed by RT-PCR using RNA from mycorrhiza and from controls of a different pea cultivar. In order to confirm arbuscule-related expression, the Medicago truncatula EST data base was screened for homologous sequences with putative mycorrhiza-related expression and among a number of sequences with significant similarities, a family of trypsin inhibitor genes could be identified. Mycorrhiza-induced RNA accumulation was verified for five members by real-time PCR and arbuscule-related activation of the promoter could be shown in transgenic roots for one of the genes, MtTi 1.

Differential display analysis of gene expression in the cytoplasm of giant cells induced in tomato roots by Meloidogyne javanica

SUMMARY Giant cells induced by root-knot nematodes are highly specialized cells which function as transfer cells and provide nutrients to support the growth and reproduction of the nematode. Changes in the overall pattern of gene expression in giant cells occur during the formation and maintenance of the nematode feeding cells. Differential display analysis has been carried out to detect changes in gene expression in giant cells induced in tomato roots by Meloidogyne javanica, using mRNA isolated directly from mature giant cell cytoplasm, compared to non-infected root tissue. Eighty-one differential displayed bands were generated, and of these, 73 were up-regulated and 8 were down-regulated. Twenty-seven sequences were obtained by direct sequencing of the bands, and 16 fragments were further analysed by real-time quantitative RT-PCR. The most highly up-regulated transcript increased 56-fold in giant cells, and the greatest down-regulation was 11-fold. A time course of expression of the highest and lowest expressed transcripts was also undertaken by quantitative RT-PCR using giant cell enriched tissue. These showed similar changes in expression, but values were dramatically reduced. This result shows the importance of analysing giant cell cytoplasm directly, rather than starting with giant cell enriched tissue, to obtain accurate information on changes in gene expression in nematode feeding cells. Sequenced transcripts showed significant homology to mitogen-activated protein kinase, S-adenosylmethionine decarboxylase, cysteine synthase, cytochrome c reductase subunit, and ribosomal proteins. The expression analysed reflects the high metabolic rate in mature giant cells rather than processes of giant cell induction.

BWMK1, a rice mitogen-activated protein kinase, locates in the nucleus and mediates pathogenesis-related gene expression by activation of a transcription factor

Mitogen-activated protein kinase (MAPK) cascades are known to transduce plant defense signals, but the downstream components of the MAPK have as yet not been elucidated. Here, we report an MAPK from rice (Oryza sativa), BWMK1, and a transcription factor, OsEREBP1, phosphorylated by the kinase. The MAPK carries a TDY phosphorylation motif instead of the more common TEY motif in its kinase domain and has an unusually extended C-terminal domain that is essential to its kinase activity and translocation to the nucleus. The MAPK phosphorylates OsEREBP1 that binds to the GCC box element (AGCCGCC) of the several basic pathogenesis-related gene promoters, which in turn enhances DNA-binding activity of the factor to the cis element in vitro. Transient co-expression of the BWMK1 and OsEREBP1 in Arabidopsis protoplasts elevates the expression of the beta-glucuronidase reporter gene driven by the GCC box element. Furthermore, transgenic tobacco (Nicotiana tabacum) plants overexpressing BWMK1 expressed many pathogenesis-related genes at higher levels than wild-type plants with an enhanced resistance to pathogens. These findings suggest that MAPKs contribute to plant defense signal transduction by phosphorylating one or more transcription factors.

Mitogen-activated protein kinase cascades in plants: a new nomenclature

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes, including yeasts, animals and plants. These protein phosphorylation cascades link extracellular stimuli to a wide range of cellular responses. In plants, MAPK cascades are involved in responses to various biotic and abiotic stresses, hormones, cell division and developmental processes. Completion of the Arabidopsis genome-sequencing project has revealed the existence of 20 MAPKs, 10 MAPK kinases and 60 MAPK kinase kinases. Here, we propose a simplified nomenclature for Arabidopsis MAPKs and MAPK kinases that might also serve as a basis for standard annotation of these gene families in all plants.

Expression of Oryza sativa MAP kinase gene is developmentally regulated and stress-responsive

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. In plants, there is evidence for MAPKs playing a role in the signalling of abiotic stresses, pathogens, plant hormones, and cell cycle cues. The large number and divergence of plant MAPKs indicates that this ancient mechanism of signal transduction is extensively used in plants. However, there have been no reports of classical MAPK module in rice. In this report, we have isolated a MAPK from rice (Oryza sativa) termed OsMAPK2. The cloned cDNA is 1457 nucleotides long and the deduced amino acid sequence comprised 369 amino acid residues. Sequence analysis revealed that the predicted amino acid sequence is 72% identical to tobacco wound-induced protein kinase (WIPK). Southern analysis suggested a single OsMAPK2 gene in rice. Analysis at the mRNA level has shown that OsMAPK2 is expressed in all plant organs and high relative amounts of OsMAPK2 were detected in the mature panicles in comparison with in the immature panicles. In suspension-cultured cells, the OsMAPK2 mRNA transcript increased markedly upon temperature downshift from 26 degrees C to 4 degrees C and sucrose starvation. In contrast, the OsMAPK2 mRNA level rapidly declined in rice cell challenged by high temperature. A similarly rapid response of OsMAPK2 was observed in stress-treated seedlings, demonstrating that response of the MAPK pathway occurs also in intact plants. These results suggest that this OsMAPK2 may function in the stress-signalling pathway as well as panicle development in rice.

MAPK cascades in plant defense signaling

The Arabidopsis genome encodes approximately 20 different mitogen-activated protein kinases (MAPKs) that are likely to be involved in growth, development and responses to endogenous and environmental cues. Several plant MAPKs are activated by a variety of stress stimuli, including pathogen infection, wounding, temperature, drought, salinity, osmolarity, UV irradiation, ozone and reactive oxygen species. Recent gain-of-function studies show that two tobacco MAPKs induce the expression of defense genes and cause cell death. By contrast, loss-of-function studies of other MAPK pathways revealed negative regulation of disease resistance. This 'push-and-pull' regulation by different MAPK pathways might provide a more precise control of plant defense responses.

Recent advances in plant MAP kinase signalling

Mitogen activated protein kinases (MAPK) are important mediators in signal transmission, connecting the perception of external stimuli to cellular responses. MAPK cascades are involved in signalling various biotic and abiotic stresses, like wounding and pathogen infection, temperature stress or drought, but are also involved in mediating the action of some plant hormones, such as ethylene and auxin. Moreover, MAPKs have been implicated in cell cycle and developmental processes. In Arabidopsis mutant screens and in vivo assays several components of plant MAPK cascades have been identified. This review gives an update of recent advances in plant MAPK signalling and discusses the emerging mechanisms of some selected MAPK pathways.

MAP kinase signal transduction pathways in plants

The mitogen-activated protein kinase (MAP kinase) signal transduction cascades are routes through which eukaryotic cells deliver extracellular messages to the cytosol and nucleus. These signalling pathways direct cell division, cellular differentiation, metabolism, and both biotic and abiotic stress responses. In plants, MAP kinases and the upstream components of the cascades are represented by multigene families, organized into different pathways which are stimulated and interact in complex ways. Experimental strategies for the analysis of MAP kinase cascades include the yeast two-hybrid system; using this approach in vitro interactions between specific MAP kinase cascade components have been analysed and putative plant cascades postulated. Transient transformation of protoplasts with epitope-tagged kinases has allowed cascades to be tested in planta. There is clear evidence for the involvement of MAP kinases in plant cell division and in the regulation of auxin signalling. Biotic (pathogens and pathogen-derived elicitors from fungi, bacteria and viruses) and abiotic stresses including wounding, mechanical stimulation, cold, drought and ozone can elicit defence responses in plants through MAP kinase pathways. There are data suggesting that ABA signalling utilizes a MAP kinase pathway, and probably ethylene and perhaps cytokinins do so also. The objective of this paper is to review this rapidly advancing field. Contents Summary 67 I. Introduction 68 II. Background 68 III. MAP kinase targets and targeting specificity 69 IV. Assays and inhibitors 70 V. Two well characterized MAP kinase pathways, Hog1 and Sevenless 71 VI. MAP kinases in plants 73 VII. MAP kinases and cell division 76 VIII. MAP kinases and plant hormones 76 IX. MAP kinase and abiotic stress 78 X. MAP kinase and biotic stress 80 XI. Future perspectives for MAP kinase research in plants 83 Acknowledgements 84 References 84.

Negative regulation of defense responses in plants by a conserved MAPKK kinase

The enhanced disease resistance 1 (edr1) mutation of Arabidopsis confers resistance to powdery mildew disease caused by the fungus Erysiphe cichoracearum. Resistance mediated by the edr1 mutation is correlated with induction of several defense responses, including host cell death. Double mutant analysis revealed that all edr1-associated phenotypes are suppressed by mutations that block salicylic acid (SA) perception (nim1) or reduce SA production (pad4 and eds1). The NahG transgene, which lowers endogenous SA levels, also suppressed edr1. In contrast, the ein2 mutation did not suppress edr1-mediated resistance and associated phenotypes, indicating that ethylene and jasmonic acid-induced responses are not required for edr1 resistance. The EDR1 gene was isolated by positional cloning and was found to encode a putative MAP kinase kinase kinase similar to CTR1, a negative regulator of ethylene responses in Arabidopsis. Taken together, these data suggest that EDR1 functions at the top of a MAP kinase cascade that negatively regulates SA-inducible defense responses. Putative orthologs of EDR1 are present in monocots such as rice and barley, indicating that EDR1 may regulate defense responses in a wide range of crop species.

Negative regulation of defense responses in plants by a conserved MAPKK kinase

The enhanced disease resistance 1 (edr1) mutation of Arabidopsis confers resistance to powdery mildew disease caused by the fungus Erysiphe cichoracearum. Resistance mediated by the edr1 mutation is correlated with induction of several defense responses, including host cell death. Double mutant analysis revealed that all edr1-associated phenotypes are suppressed by mutations that block salicylic acid (SA) perception (nim1) or reduce SA production (pad4 and eds1). The NahG transgene, which lowers endogenous SA levels, also suppressed edr1. In contrast, the ein2 mutation did not suppress edr1-mediated resistance and associated phenotypes, indicating that ethylene and jasmonic acid-induced responses are not required for edr1 resistance. The EDR1 gene was isolated by positional cloning and was found to encode a putative MAP kinase kinase kinase similar to CTR1, a negative regulator of ethylene responses in Arabidopsis. Taken together, these data suggest that EDR1 functions at the top of a MAP kinase cascade that negatively regulates SA-inducible defense responses. Putative orthologs of EDR1 are present in monocots such as rice and barley, indicating that EDR1 may regulate defense responses in a wide range of crop species.