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Plants utilize tightly regulated mechanisms to defend themselves against pathogens. Initial recognition results in activation of specific Resistance (R) proteins that trigger downstream immune responses, in which the signaling networks remain largely unknown. A point mutation in SUPPRESSOR OF NPRTI CONSTITUTIVEI (SNC1), a RESISTANCE TO PERONOSPORA PARASITICA4 R gene homolog, renders plants constitutively resistant to virulent pathogens. Genetic suppressors of sncl may carry mutations in genes encoding novel signaling components downstream of activated R proteins. One such suppressor was identified as a novel loss-of-function allele of ENHANCED RESPONSE TO ABSCISIC ACID1 (ERAI), which encodes the β-subunit of protein farnesyltransferase. Protein farnesylation involves attachment of C15-prenyl residues to the carboxyl termini of specific target proteins. Mutant eral plants display enhanced susceptibility to virulent bacterial and oomycete pathogens, implying a role for farnesylation in basal defense. In addition to its role in sncl-mediated resistance, cml affects several other R-protein-mediated resistance responses against bacteria and oomycetes. ERA1 acts partly independent of abscisic acid and additively with the resistance regulator NON-EXPRESSOR OF PR GENES1 in the signaling network. Defects in geranylgeranyl transferase I, a protein modification similar to farnesylation, do not affect resistance responses, indicating that farnesylation is most likely specifically required in plant defense signaling. Taken together, we present a novel role for farnesyltransferase in plant-pathogen interactions, suggesting the importance of protein farnesylation, which contributes to the specificity and efficacy of signal transduction events. [ABSTRACT FROM AUTHOR]

The plant defense hormone salicylic acid (SA) is perceived by two classes of receptors, NPR1 and NPR3/NPR4. They function in two parallel pathways to regulate SA-induced defense gene expression. To better understand the roles of the SA receptors in plant defense, we systematically analyzed their contributions to different aspects of Arabidopsis (Arabidopsis thaliana) plant immunity using the SA-insensitive npr1-1 npr4-4D double mutant. We found that perception of SA by NPR1 and NPR4 is required for activation of N-hydroxypipecolic acid biosynthesis, which is essential for inducing systemic acquired resistance. In addition, both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) are severely compromised in the npr1-1 npr4-4D double mutant. Interestingly, the PTI and ETI attenuation in npr1-1 npr4-4D is more dramatic compared with the SA-induction deficient2-1 (sid2-1) mutant, suggesting that the perception of residual levels of SA in sid2-1 also contributes to immunity. Furthermore, NPR1 and NPR4 are involved in positive feedback amplification of SA biosynthesis and regulation of SA homeostasis through modifications including 5-hydroxylation and glycosylation. Thus, the SA receptors NPR1 and NPR4 play broad roles in plant immunity. [ABSTRACT FROM AUTHOR]

Arabidopsis (Arabidopsis thaliana) MAP KINASE (MPK) proteins can function in multiple MAP kinase cascades and physiological processes. For instance, MPK4 functions in regulating development as well as in plant defense by participating in two independent MAP kinase cascades: the MEKK1-MKK1/MKK2-MPK4 cascade promotes basal resistance against pathogens and is guarded by the NB-LRR protein SUMM2, whereas the ANPs-MKK6-MPK4 cascade plays an essential role in cytokinesis. Here, we report a novel role for MKK6 in regulating plant immune responses. We found that MKK6 functions similarly to MKK1/MKK2 and works together with MEKK1 and MPK4 to prevent autoactivation of SUMM2-mediated defense responses. Interestingly, loss of MKK6 or ANP2/ANP3 results in constitutive activation of plant defense responses. The autoimmune phenotypes of mkk6 and anp2 anp3 mutant plants can be largely suppressed by a constitutively active mpk4 mutant. Further analysis showed that the constitutive defense response in anp2 anp3 is dependent on the defense regulators PAD4 and EDS1, but not on SUMM2, suggesting that the ANP2/ANP3-MKK6-MPK4 cascade may be guarded by a TIR-NB-LRR protein. Our study shows that MKK6 has multiple functions in plant defense responses in addition to cytokinesis. [ABSTRACT FROM AUTHOR]

Salicylic acid (SA) serves as a critical signaling molecule in plant defense. Two transcription factors, SARD1 and CBP60g, control SA biosynthesis through regulating pathogen-induced expression of Isochorismate Synthase1, which encodes a key enzyme for SA biosynthesis. Here, we report that Pattern-Triggered Immunity Compromised Receptor-like Cytoplasmic Kinase1 (PCRK1) and PCRK2 function as key regulators of SA biosynthesis. In the pcrk1 pcrk2 double mutant, pathogen-induced expression of SARD1, CBP60g, and ICS1 is greatly reduced. The pcrk1 pcrk2 double mutant, but neither of the single mutants, exhibits reduced accumulation of SA and enhanced disease susceptibility to bacterial pathogens. Both PCRK1 and PCRK2 interact with the pattern recognition receptor FLS2, and treatment with pathogen-associated molecular patterns leads to rapid phosphorylation of PCRK2. Our data suggest that PCRK1 and PCRK2 function downstream of pattern recognition receptor in a signal relay leading to the activation of SA biosynthesis. [ABSTRACT FROM AUTHOR]

Plant nucleotide-binding leucine-rich repeat (NB-LRR) proteins serve as intracellular sensors to detect pathogen effectors and trigger immune responses. Transcription of the NB-LRR-encoding Resistance (R) genes needs to be tightly controlled to avoid inappropriate defense activation. How the expression of the NB-LRR R genes is regulated is poorly understood. The Arabidopsis (Arabidopsis thaliana) suppressor of npr1-1, constitutive 1 (snc1) mutant carries a gain-of-function mutation in a Toll/Interleukin1 receptor-like (TIR)-NB-LRR-encoding gene, resulting in the constitutive activation of plant defense responses. A snc1 suppress screen identified modifier of snc1,9 (mos9), which partially suppresses the autoimmune phenotypes of snc1. Positional cloning revealed that MOS9 encodes a plant-specific protein of unknown function. Expression analysis showed that MOS9 is required for the full expression of TIR-NB-LRR protein-encoding RECOGNITION OF PERONOSPORA PARASITICA 4 (RPP4) and SNC both of which reside in the RPP4 cluster. Coimmunoprecipitation and mass spectrometry analyses revealed that MOS9 associate with the Set1 class lysine 4 of histone 3 (H3K4) methyltransferase Arabidopsis Trithorax-Related7 (ATXR7). Like MOS9, ATXR7 is also required for the full expression of SNC1 and the autoimmune phenotypes in the snc1 mutant. In atxr7 mutant plants expression of RPP4 is similarly reduced, and resistance against Hyaloperonospora arabidopsidis Emwal is compromised. Consistent with the attenuated expression of SNC1 and RPP4, trimethylated H3K4 marks are reduced around the promoters of SNC1 and RPP4 in mos9 plants. Our data suggest that MOS9 functions together with ATXR7 to regulate the expression of SNC1 and RPP4 through H3K4 methylation, which plays an important role in fine-tuning their transcription levels and functions in plant defense. [ABSTRACT FROM AUTHOR]

In fungi and metazoans, extracellular signals are often perceived by G-protein-coupled receptors (GPCRs) and transduced through heterotrimeric G-protein complexes to downstream targets. Plant heterotrimeric G proteins are also involved in diverse biological processes, but little is known about their upstream receptors. Moreover, the presence of bona fide GPCRs in plants is yet to be established. In Arabidopsis (Arabidopsis thaliana), heterotrimeric G protein consists of one Gα subunit (G PROTEIN α-SUBUNIT1), one Gβ subunit (ARABIDOPSIS G PROTEIN β-SUBUNIT1 [AGB1]), and three Gγs subunits (ARABIDOPSIS G PROTEIN γ-SUBUNIT1 [AGG1], AGG2, and AGG3). We identified AGB1 from a suppressor screen of BAK1-interacting receptor-like kinasel-1 (bit1-1), a mutant that activates cell death and defense responses mediated by the receptor-like kinase (RLK) SUPPRESSOR OF BIRI-1. Mutations in AGB1 suppress the cell death and defense responses in bir1-1 and transgenic plants overexpressing SUPPRESSOR OF BIRI-1. In addition, agb1 mutant plants were severely compromised in immunity mediated by three other RLKs, FLAGELLIN-SENSITIVE2 (FLS2), Elongation Factor-TU RECEPTOR (EFR), and CHITIN ELICITOR RECEPTOR KINASE1 (CERK1), respectively. By contrast, G PROTEIN α-SUBUNIT1 is not required for either cell death in bir1-1 or pathogen-associated molecular pattern-triggered immunity mediated by FLS2, EFR, and CERK1. Further analysis of agg1 and agg2 mutant plants indicates that AGG1 and AGG2 are also required for pathogen-associated molecular pattern-triggered immune responses mediated by FLS2, EFR, and CERK1, as well as cell death and defense responses in birl-1. We hypothesize that the Arabidopsis heterotrimeric G proteins function as a converging point of plant defense signaling by mediating responses initiated by multiple RLKs, which may fulfill equivalent roles to GPCRs in fungi and animals. [ABSTRACT FROM AUTHOR]

Plants utilize a large number of immune receptors to recognize pathogens and activate defense responses. A small number of these receptors belong to the receptor-like protein family. Previously, we showed that a gain-of-function mutation in the receptor-like protein SNC2 (for Suppressor of NPR1, Constitutive2) leads to constitutive activation of defense responses in snc2- 1D mutant plants. To identify defense signaling components downstream of SNC2, we carried out a suppressor screen in the snc2-1D mutant background of Arabidopsis (Arabidopsis thaliana). Map-based cloning of one of the suppressor genes, BDA1 (for bian da; "becoming big" in Chinese), showed that it encodes a protein with amino-terminal ankyrin repeats and carboxylterminal transmembrane domains. Loss-of-function mutations in BDA1 suppress the dwarf morphology and constitutive defense responses in snc2-1D nprl-1 (for nonexpressor of pathogenesis-related genes1,1) and also result in enhanced susceptibility to bacterial pathogens. In contrast, a gain-of-function allele of bdal isolated from a separate genetic screen to search for mutants with enhanced pathogen resistance was found to constitutively activate cell death and defense responses. These data suggest that BDA1 is a critical signaling component that functions downstream of SNC2 to regulate plant immunity. [ABSTRACT FROM AUTHOR]

Systemic acquired resistance (SAR) is a defense mechanism induced in the distal parts of plants after primary infection. It confers long-lasting protection against a broad spectrum of microbial pathogens. Lack of high-throughput assays has hampered the forward genetic analysis of SAR. Here, we report the development of an easy and efficient assay for SAR and its application in a forward genetic screen for SAR-deficient mutants in Arabidopsis (Arabidopsis thaliana). Using the new assay for SAR, we identified six flavin-dependent monooxygenasel, four AGD2-Iike defense response protein1, three salicylic acid induction-deficient2, one phytoalexin deficient4, and one avrPphB-susceptible3 alleles as well as a gain-of-function mutant of CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR3 designated camta3-3D. Like transgenic plants overexpressing CAMTA3, camta3-3D mutant plants exhibit compromised SAR and enhanced susceptibility to virulent pathogens, suggesting that CAMTA3 is a critical regulator of both basal resistance and SAR. [ABSTRACT FROM AUTHOR]

The MOS4-associated complex (MAC) is a highly conserved nuclear protein complex associated with the spliceosome. We recently purified the MAC from Arabidopsis (Arabidopsis thaliana) nuclei, identified its potential components by mass spectrometry, and showed that at least five core proteins in the MAC are required for defense responses in plants. Here, we report the characterization of a putative RNA-binding protein identified in the MAC named MAC5A and its close homolog MAC5B. We confirmed that MAC5A is a component of the MAC through coimmunoprecipitation with the previously described MAC protein CELL DIVISION CYCLE5 from Arabidopsis. In addition, like all other characterized MAC proteins, MAC5A fused to the Green Fluorescent Protein localizes to the nucleus. Double mutant analysis revealed that MAC5A and MAC5B are unequally redundant and that a double mac5a mac5b mutant results in lethality. Probably due to this partial redundancy, mac5a and mac5b single mutants do not exhibit enhanced susceptibility to virulent or avirulent pathogen infection. However, like other MAC mutations, mac5a-1 partially suppresses the autoimmune phenotypes of suppressor of npr1-1, constitutive1 (snc1), a gain-of-function mutant that expresses a deregulated Resistance protein. Our results suggest that MAC5A is a component of the MAC that contributes to snc1- mediated autoimmunity. [ABSTRACT FROM AUTHOR]

Arabidopsis (Arabidopsis thaliana) suppressor of npr1-1, constitutive1 (snc1) contains a gain-of-function mutation in a Toll/interleukin receptor-nucleotide binding site-leucine-rich repeat Resistance (R) protein and it has been a useful tool for dissecting R-protein-mediated immunity. Here we report the identification and characterization of snc4-1D, a semidominant mutant with snc1-like phenotypes, snc4-1D constitutively expresses defense marker genes PR1, PR2, and PDF1.2, and displays enhanced pathogen resistance. Map-based cloning of SNC4 revealed that it encodes an atypical receptor-like kinase with two predicted extracellular glycerophosphoryl diester phosphodiesterase domains. The snc4-1D mutation changes an alanine to threonine in the predicted cytoplasmic kinase domain. Wild-type plants transformed with the mutant snc4-1D gene displayed similar phenotypes as snc4-1D, suggesting that the mutation is a gain-of-function mutation. Epistasis analysis showed that NON-RACE-SPECIFIC DISEASE RESISTANCE1 is required for the snc4-1D mutant phenotypes. In addition, the snc4-1D mutant phenotypes are partially suppressed by knocking out MAP KINASE SUBSTRATE1, a positive defense regulator associated with MAP KINASE4. Furthermore, both the morphology and constitutive pathogen resistance of snc4-1D are partially suppressed by blocking jasmonic acid synthesis, suggesting that jasmonic acid plays an important role in snc4-1D-mediated resistance. Identification of snc4-1D provides us a unique genetic system for analyzing the signal transduction pathways downstream of receptor-like kinases. [ABSTRACT FROM AUTHOR]

Plant Resistance (R) genes encode immune receptors that recognize pathogens and activate defense responses. Because of fitness costs associated with maintaining R protein-mediated resistance, expression levels of R genes have to be tightly regulated. However, mechanisms on how R-gene expression is regulated are poorly understood. Here we show that MODIFIER OF snc1, 1 (MOS1) regulates the expression of SUPPRESSOR OF npr1-1, CONSTITUTIVE1 (SNC1), which encodes a Toll/interleukin receptor-nucleotide binding site-leucine-rich repeat type of R protein in Arabidopsis (Arabidopsis thaliana). In the mos1 loss-of-function mutant plants, snc1 expression is repressed and constitutive resistance responses mediated by snc1 are lost. The repression of snc1 expression in mos1 is released by knocking out DECREASE IN DNA METHYLATION1. In mos1 mutants, DNA methylation in a region upstream of SNCI is altered. Furthermore, expression of snc1 transgenes using the native promoter does not require MOS1, indicating that regulation of SNC1 expression by MOS1 is at the chromatin level. Map-based cloning of MOS1 revealed that it encodes a novel protein with a HLA-B ASSOCIATED TRANSCRIPT2 (BAT2) domain that is conserved in plants and animals. Our study on MOS1 suggests that BAT2 domain-containing proteins may function in regulation of gene expression at chromatin level. [ABSTRACT FROM AUTHOR]

Systemic acquired resistance (SAR) is a plant immune response induced by local necrotizing pathogen infections. Expression of SAR in Arabidopsis (Arabidopsis thaliana) plants correlates with accumulation of salicylic acid (SA) and up-regulation of Pathogenesis-Related (PR) genes. SA is an essential and sufficient signal for SAR. In a genetic screen to search for negative regulators of PR gene expression and SAR, we found a new mutant that is hypersensitive to SA and exhibits enhanced induction of PR genes and resistance against the virulent oomycete Hyaloperonospora arabidopsidis Noco2. The enhanced pathogen resistance in the mutant is Nonexpressor of PR genesi independent. The mutant gene was identified by map-based cloning, and it encodes a protein with high homology to Replication Factor C Subunit3 (RFC3) of yeast and other eukaryotes; thus, the mutant was named rfc3-1. rfc3-1. mutant plants are smaller than wild-type plants and have narrower leaves and petals. On the epidermis of true leaves, there are fewer cells in rfc3-TI compared with the wild type. Cell production rate is reduced in rfc3-TI mutant roots, indicating that the mutated RFC3 slows down cell proliferation. As Replication Factor C is involved in replication-coupled chromatin assembly, our data suggest that chromatin assembly and remodeling may play important roles in the negative control of PR gene expression and SAR. [ABSTRACT FROM AUTHOR]