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Summary: Arabidopsis Col‐0 RPP2A and RPP2B confer recognition of Arabidopsis downy mildew (Hyaloperonospora arabidopsidis [Hpa]) isolate Cala2, but the identity of the recognized ATR2Cala2 effector was unknown.To reveal ATR2Cala2, an F2 population was generated from a cross between Hpa‐Cala2 and Hpa‐Noks1. We identified ATR2Cala2 as a non‐canonical RxLR‐type effector that carries a signal peptide, a dEER motif, and WY domains but no RxLR motif. Recognition of ATR2Cala2 and its effector function were verified by biolistic bombardment, ectopic expression and Hpa infection.ATR2Cala2 is recognized in accession Col‐0 but not in Ler‐0 in which RPP2A and RPP2B are absent. In ATR2Emoy2 and ATR2Noks1 alleles, a frameshift results in an early stop codon. RPP2A and RPP2B are essential for the recognition of ATR2Cala2. Stable and transient expression of ATR2Cala2 under 35S promoter in Arabidopsis and Nicotiana benthamiana enhances disease susceptibility.Two additional Col‐0 TIR‐NLR (TNL) genes (RPP2C and RPP2D) adjacent to RPP2A and RPP2B are quantitatively required for full resistance to Hpa‐Cala2. We compared RPP2 haplotypes in multiple Arabidopsis accessions and showed that all four genes are present in all ATR2Cala2‐recognizing accessions. [ABSTRACT FROM AUTHOR]

Summary: The oomycete Albugo candida causes white blister rust, an important disease of Brassica crops. Distinct races of A. candida are defined by their capacity to infect different host plant species. Each A. candida race encodes secreted proteins with a CX2CX5G ('CCG') motif that are polymorphic and show presence/absence variation, and are therefore candidate effectors.The White Rust Resistance 4 (WRR4) locus in Arabidopsis thaliana accession Col‐0 contains three genes that encode intracellular nucleotide‐binding domain leucine‐rich repeat immune receptors. The Col‐0 alleles of WRR4A and WRR4B confer resistance to multiple A. candida races, although both WRR4A and WRR4B can be overcome by the Col‐0‐virulent race 4 isolate AcEx1.Comparison of CCG candidate effectors in avirulent and virulent races, and transient co‐expression of CCG effectors from four A. candida races in Nicotiana sp. or A. thaliana, revealed CCG effectors that trigger WRR4A‐ or WRR4B‐dependent hypersensitive responses.We found eight WRR4A‐recognised CCGs and four WRR4B‐recognised CCGs, the first recognised proteins from A. candida for which the cognate immune receptors in A. thaliana are known. This multiple recognition capacity potentially explains the broad‐spectrum resistance to several A. candida races conferred by WRR4 paralogues. We further show that of five tested CCGs, three confer enhanced disease susceptibility when expressed in planta, consistent with A. candida CCG proteins being effectors. [ABSTRACT FROM AUTHOR]

Summary: Most land plant genomes carry genes that encode RPW8‐NLR Resistance (R) proteins. Angiosperms carry two RPW8‐NLR subclasses: ADR1 and NRG1. ADR1s act as 'helper' NLRs for multiple TIR‐ and CC‐NLR R proteins in Arabidopsis. In angiosperm families, NRG1 co‐occurs with TIR‐NLR Resistance (R) genes. We tested whether NRG1 is required for signalling of multiple TIR‐NLRs.Using CRISPR mutagenesis, we obtained an nrg1a‐nrg1b double mutant in two Arabidopsis accessions, and an nrg1 mutant in Nicotiana benthamiana.These mutants are compromised in signalling of all TIR‐NLRs tested, including WRR4A, WRR4B, RPP1, RPP2, RPP4 and the pairs RRS1/RPS4, RRS1B/RPS4B, CHS1/SOC3 and CHS3/CSA1. In Arabidopsis, NRG1 is required for the hypersensitive cell death response (HR) and full oomycete resistance, but not for salicylic acid induction or bacterial resistance. By contrast, nrg1 loss of function does not compromise the CC‐NLR R proteins RPS5 and MLA. RPM1 and RPS2 (CC‐NLRs) function is slightly compromised in an nrg1 mutant. Thus, NRG1 is required for full TIR‐NLR function and contributes to the signalling of some CC‐NLRs.Some NRG1‐dependent R proteins also signal partially via the NRG1 sister clade, ADR1. We propose that some NLRs signal via NRG1 only, some via ADR1 only and some via both or neither. [ABSTRACT FROM AUTHOR]

Summary: The oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) causes downy mildew disease on Arabidopsis. To colonize its host, Hpa translocates effector proteins that suppress plant immunity into infected host cells. Here, we investigate the relevance of the interaction between one of these effectors, HaRxL106, and Arabidopsis RADICAL‐INDUCED CELL DEATH1 (RCD1). We use pathogen infection assays as well as molecular and biochemical analyses to test the hypothesis that HaRxL106 manipulates RCD1 to attenuate transcriptional activation of defense genes. We report that HaRxL106 suppresses transcriptional activation of salicylic acid (SA)‐induced defense genes and alters plant growth responses to light. HaRxL106‐mediated suppression of immunity is abolished in RCD1 loss‐of‐function mutants. We report that RCD1‐type proteins are phosphorylated, and we identified Mut9‐like kinases (MLKs), which function as phosphoregulatory nodes at the level of photoreceptors, as RCD1‐interacting proteins. An mlk1,3,4 triple mutant exhibits stronger SA‐induced defense marker gene expression compared with wild‐type plants, suggesting that MLKs also affect transcriptional regulation of SA signaling. Based on the combined evidence, we hypothesize that nuclear RCD1/MLK complexes act as signaling nodes that integrate information from environmental cues and pathogen sensors, and that the Arabidopsis downy mildew pathogen targets RCD1 to prevent activation of plant immunity. [ABSTRACT FROM AUTHOR]

Plants use a variety of innate immune regulators to trigger cell death and defense responses against pathogen attack., We identified pepper ( Capsicum annuum) GLYCINE-RICH RNA-BINDING PROTEIN1 (CaGRP1) as a RECEPTOR-LIKE CYTOPLASMIC PROTEIN KINASE1 (CaPIK1)-interacting partner, based on bimolecular fluorescence complementation and coimmunoprecipitation analyses as well as gene silencing and transient expression analysis., CaGRP1 contains an N-terminal RNA recognition motif and a glycine-rich region at the C-terminus. The CaGRP1 protein had DNA- and RNA-binding activity in vitro. CaGRP1 interacted with CaPIK1 in planta. CaGRP1 and CaGRP1-CaPIK1 complexes were localized to the nucleus in plant cells. CaPIK1 phosphorylated CaGRP1 in vitro and in planta. Transient coexpression of CaGRP1 with CaPIK1 suppressed the CaPIK1-triggered cell death response, accompanied by a reduced CaPIK1-triggered reactive oxygen species (ROS) burst. The RNA recognition motif region of CaGRP1 was responsible for the nuclear localization of CaGRP1 as well as the suppression of the CaPIK1-triggered cell death response. CaGRP1 silencing in pepper conferred enhanced resistance to Xanthomonas campestris pv vesicatoria ( Xcv) infection; however, CaPIK1-silenced plants were more susceptible to Xcv., CaGRP1 interacts with CaPIK1 and negatively regulates CaPIK1-triggered cell death and defense responses by suppressing ROS accumulation. [ABSTRACT FROM AUTHOR]

Plant defense against microbial pathogens is coordinated by a complex regulatory network. Cysteine/histidine-rich DC1 domain proteins mediate a variety of cellular processes involved in plant growth, development and stress responses., We identified a pepper ( Capsicum annuum) cysteine/histidine-rich DC1 domain protein gene, Ca DC1, which positively regulates plant defense during microbial infection, based on gene silencing and transient expression in pepper, as well as ectopic expression in Arabidopsis., Induction of Ca DC1 by avirulent Xanthomonas campestris pv vesicatoria ( Xcv) infection was pronounced at both transcriptional and translational levels in pepper leaves. Purified Ca DC1 protein bound to both DNA and RNA in vitro, especially in the presence of Zn2+. Ca DC1 was localized to both the nucleus and the cytoplasm, which was required for plant cell death signaling. The nuclear localization of Ca DC1 was dependent on the divergent C1 ( DC1) domain. Ca DC1 silencing in pepper conferred increased susceptibility to Xcv infection, which was accompanied by reduced salicylic acid accumulation and defense-related gene expression. Ectopic expression of Ca DC1 in Arabidopsis enhanced resistance to Hyaloperonospora arabidopsidis., Ca DC1 binds both RNA and DNA and functions as a positive regulator of plant cell death and SA-dependent defense responses. [ABSTRACT FROM AUTHOR]

Arabidopsis Col-0 RPP2A and RPP2B confer recognition of Arabidopsis downy mildew (Hyaloperonospora arabidopsidis [Hpa]) isolate Cala2, but the identity of the recognized ATR2 Cala2 effector was unknown. To reveal ATR2 Cala2 , an F 2 population was generated from a cross between Hpa-Cala2 and Hpa-Noks1. We identified ATR2 Cala2 as a non-canonical RxLR-type effector that carries a signal peptide, a dEER motif, and WY domains but no RxLR motif. Recognition of ATR2 Cala2 and its effector function were verified by biolistic bombardment, ectopic expression and Hpa infection. ATR2 Cala2 is recognized in accession Col-0 but not in Ler-0 in which RPP2A and RPP2B are absent. In ATR2 Emoy2 and ATR2 Noks1 alleles, a frameshift results in an early stop codon. RPP2A and RPP2B are essential for the recognition of ATR2 Cala2 . Stable and transient expression of ATR2 Cala2 under 35S promoter in Arabidopsis and Nicotiana benthamiana enhances disease susceptibility. Two additional Col-0 TIR-NLR (TNL) genes (RPP2C and RPP2D) adjacent to RPP2A and RPP2B are quantitatively required for full resistance to Hpa-Cala2. We compared RPP2 haplotypes in multiple Arabidopsis accessions and showed that all four genes are present in all ATR2 Cala2 -recognizing accessions., (© 2024 The Authors. New Phytologist © 2024 New Phytologist Foundation.)

The oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) causes downy mildew disease on Arabidopsis. To colonize its host, Hpa translocates effector proteins that suppress plant immunity into infected host cells. Here, we investigate the relevance of the interaction between one of these effectors, HaRxL106, and Arabidopsis RADICAL-INDUCED CELL DEATH1 (RCD1). We use pathogen infection assays as well as molecular and biochemical analyses to test the hypothesis that HaRxL106 manipulates RCD1 to attenuate transcriptional activation of defense genes. We report that HaRxL106 suppresses transcriptional activation of salicylic acid (SA)-induced defense genes and alters plant growth responses to light. HaRxL106-mediated suppression of immunity is abolished in RCD1 loss-of-function mutants. We report that RCD1-type proteins are phosphorylated, and we identified Mut9-like kinases (MLKs), which function as phosphoregulatory nodes at the level of photoreceptors, as RCD1-interacting proteins. An mlk1,3,4 triple mutant exhibits stronger SA-induced defense marker gene expression compared with wild-type plants, suggesting that MLKs also affect transcriptional regulation of SA signaling. Based on the combined evidence, we hypothesize that nuclear RCD1/MLK complexes act as signaling nodes that integrate information from environmental cues and pathogen sensors, and that the Arabidopsis downy mildew pathogen targets RCD1 to prevent activation of plant immunity., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Plant defense against microbial pathogens is coordinated by a complex regulatory network. Cysteine/histidine-rich DC1 domain proteins mediate a variety of cellular processes involved in plant growth, development and stress responses. We identified a pepper (Capsicum annuum) cysteine/histidine-rich DC1 domain protein gene, CaDC1, which positively regulates plant defense during microbial infection, based on gene silencing and transient expression in pepper, as well as ectopic expression in Arabidopsis. Induction of CaDC1 by avirulent Xanthomonas campestris pv vesicatoria (Xcv) infection was pronounced at both transcriptional and translational levels in pepper leaves. Purified CaDC1 protein bound to both DNA and RNA in vitro, especially in the presence of Zn(2+). CaDC1 was localized to both the nucleus and the cytoplasm, which was required for plant cell death signaling. The nuclear localization of CaDC1 was dependent on the divergent C1 (DC1) domain. CaDC1 silencing in pepper conferred increased susceptibility to Xcv infection, which was accompanied by reduced salicylic acid accumulation and defense-related gene expression. Ectopic expression of CaDC1 in Arabidopsis enhanced resistance to Hyaloperonospora arabidopsidis. CaDC1 binds both RNA and DNA and functions as a positive regulator of plant cell death and SA-dependent defense responses., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)