Chitin is a major component of fungal cell walls and serves as a microbe-associated molecular pattern (MAMP) for the detection of various potential pathogens in innate immune systems of both plants and animals. We recently showed that chitin elicitor-binding protein (CEBiP), plasma membrane glycoprotein with LysM motifs, functions as a cell surface receptor for chitin elicitor in rice. The predicted structure of CEBiP does not contain any intracellular domains, suggesting that an additional component(s) is required for signaling through the plasma membrane into the cytoplasm. Here, we identified a receptor-like kinase, designated CERK1, which is essential for chitin elicitor signaling in Arabidopsis. The KO mutants for CERK1 completely lost the ability to respond to the chitin elicitor, including MAPK activation, reactive oxygen species generation, and gene expression. Disease resistance of the KO mutant against an incompatible fungus, Alternaria brassicicola, was partly impaired. Complementation with the WT CERK1 gene showed cerk1 mutations were responsible for the mutant phenotypes. CERK1 is a plasma membrane protein containing three LysM motifs in the extracellular domain and an intracellular Ser/Thr kinase domain with autophosphorylation/myelin basic protein kinase activity, suggesting that CERK1 plays a critical role in fungal MAMP perception in plants.plant immunity ͉ host-pathogen interaction ͉ N-acetylchitooligosaccharide P lants trigger various defense reactions against invading pathogens upon perception of so-called microbe-associated molecular patterns [MAMPs; also known as pathogenassociated molecular patterns (PAMPs)]. MAMP recognition has been implicated to play a major role in the nonhost or basal resistance that makes most plants immune to most potential pathogens (1, 2). In the course of coevolution between hosts and parasites, it seems that pathogens developed various virulence factors to overcome MAMP-mediated defense responses, whereas plants evolved resistance-gene-mediated defense system to detect such factors (3-5). Recent studies also revealed the presence of a striking similarity in the MAMP-mediated immunity between plants and animals (2, 6).Despite the importance of MAMP-mediated immunity in plants, molecular machineries involved in the perception of MAMPs and their signal transduction have been poorly understood. So far, only the receptor molecules for two bacterial MAMPs, flg22 and EF-Tu, have been identified (7,8). Both of these receptors, FLS2 and EFR, are receptor kinases with a leucine-rich repeat in the extracellular domain. However, much less information is available for the perception of fungal MAMPs. Chitin and its fragments, chitin oligosaccharides or N-acetylchitooligosaccharides, are typical fungal MAMPs that trigger various defense responses in both monocots and dicots, indicating the presence of a conserved machinery to perceive these oligosaccharides in a wide range of plant species (9, 10). A recent finding that chitin also induces immune responses in mice (11) suggest...

The Arabidopsis mitogen-activated protein kinase (MAPK) kinase 2 (MKK2) and the downstream MAPKs MPK4 and MPK6 were isolated by functional complementation of osmosensitive yeast mutants. In Arabidopsis protoplasts, MKK2 was specifically activated by cold and salt stress and by the stress-induced MAPK kinase kinase MEKK1. Yeast two-hybrid, in vitro, and in vivo protein kinase assays revealed that MKK2 directly targets MPK4 and MPK6. Accordingly, plants overexpressing MKK2 exhibited constitutive MPK4 and MPK6 activity, constitutively upregulated expression of stress-induced marker genes, and increased freezing and salt tolerance. In contrast, mkk2 null plants were impaired in MPK4 and MPK6 activation and were hypersensitive to salt and cold stress. Full genome transcriptome analysis of MKK2-overexpressing plants demonstrated altered expression of 152 genes involved in transcriptional regulation, signal transduction, cellular defense, and stress metabolism. These data identify a MAP kinase signaling cascade mediating cold and salt stress tolerance in plants.

;Protein phosphorylation has pivotal roles in ABA and osmotic stress signaling in higher plants. Two protein phosphatase genes, ABI1 and ABI2, are known to regulate these signaling pathways in Arabidopsis. The identity of ABAactivated protein kinases required for the ABA signaling, however, remains to be elucidated. Here we demonstrate that two protein kinases, p44 and p42, were activated by ABA in Arabidopsis T87 cultured cells, and at least one protein kinase, p44, was activated not only by ABA but also by low humidity in Arabidopsis plants. Analysis of T-DNA knockout mutants and biochemical analysis using a specific antibody revealed that the p44 is encoded by a SnRK2-type protein kinase gene, SRK2E. The srk2e mutation resulted in a wilty phenotype mainly due to loss of stomatal closure in response to a rapid humidity decrease. ABAinducible gene expression of rd22 and rd29B was suppressed in srk2e. These results show that SRK2E plays an important role in ABA signaling in response to water stress.

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RAR1 and its interacting partner SGT1 play a central role in plant disease resistance triggered by a number of resistance (R) proteins. We identified cytosolic heat shock protein 90 (HSP90), a molecular chaperone, as another RAR1 interacting protein by yeast twohybrid screening. RAR1 interacts with the N-terminal half of HSP90 that contains the ATPase domain. HSP90 also specifically interacts with SGT1 that contains a tetratricopeptide repeat motif and a domain with similarity to the cochaperone p23. In Arabidopsis, the HSP90 inhibitor geldanamycin reduces the hypersensitive response and abolishes resistance triggered by the R protein RPS2 against Pseudomonas syringae pv. tomato DC3000 (avrRpt2). One of four Arabidopsis cytosolic HSP90 isoforms, AtHSP90.1 is required for full RPS2 resistance and is rapidly induced upon pathogen challenge. We propose that RAR1 and SGT1 function closely with HSP90 in chaperoning roles that are essential for disease resistance.

SummaryMitogen-activated protein kinase (MAP kinase, MAPK) cascades play pivotal roles in signal transduction of extracellular stimuli, such as environmental stresses and growth regulators, in various organisms. Arabidopsis thaliana MAP kinases constitute a gene family, but stimulatory signals for each MAP kinase have not been elucidated. Here we show that environmental stresses such as low temperature, low humidity, hyper-osmolarity, touch and wounding induce rapid and transient activation of the Arabidopsis MAP kinases ATMPK4 and ATMPK6. Activation of ATMPK4 and ATMPK6 was associated with tyrosine phosphorylation but not with the amounts of mRNA or protein. Kinetics during activation differ between these two MAP kinases. These results suggest that ATMPK4 and ATMPK6 are involved in distinct signal transduction pathways responding to these environmental stresses.

The plant hormone jasmonic acid (JA) plays a key role in the environmental stress responses and developmental processes of plants. Although ATMYC2/JASMONATE-INSENSITIVE1 (JIN1) is a major positive regulator of JA-inducible gene expression and essential for JA-dependent developmental processes in Arabidopsis thaliana, molecular mechanisms underlying the control of ATMYC2/JIN1 expression remain largely unknown. Here, we identify a mitogen-activated protein kinase (MAPK) cascade, MAPK KINASE 3 (MKK3)-MAPK 6 (MPK6), which is activated by JA in Arabidopsis. We also show that JA negatively controls ATMYC2/JIN1 expression, based on quantitative RT-PCR and genetic analyses using gain-of-function and loss-offunction mutants of the MKK3-MPK6 cascade. These results indicate that this kinase unit plays a key role in JA-dependent negative regulation of ATMYC2/JIN1 expression. Both positive and negative regulation by JA may be used to fine-tune ATMYC2/JIN1 expression to control JA signaling. Moreover, JA-regulated root growth inhibition is affected by mutations in the MKK3-MPK6 cascade, which indicates important roles in JA signaling. We provide a model explaining how MPK6 can convert three distinct signals-JA, pathogen, and cold/salt stress-into three different sets of responses in Arabidopsis.

Innate immunity signaling pathways in both animals and plants are regulated by mitogen-activated protein kinase (MAPK) cascades. An Arabidopsis MAPK cascade (MEKK1, MKK4/MKK5, and MPK3/MPK6) has been proposed to function downstream of the flagellin receptor FLS2 based on biochemical assays using transient overexpression of candidate components. To genetically test this model, we characterized two mekk1 mutants. We show here that MEKK1 is not required for flagellin-triggered activation of MPK3 and MPK6. Instead, MEKK1 is essential for activation of MPK4, a MAPK that negatively regulates systemic acquired resistance. We also showed that MEKK1 negatively regulates temperature-sensitive and tissue-specific cell death and H 2 O 2 accumulation that are partly dependent on both RAR1, a key component in resistance protein function, and SID2, an isochorismate synthase required for salicylic acid production upon pathogen infection.