Multiple R-like genes are negatively regulated by BON1 and BON3 in arabidopsis

Copine proteins are required for brassinosteroid signaling in maize and Arabidopsis

Copine proteins are highly conserved and ubiquitously found in eukaryotes, and their indispensable roles in different species were proposed. However, their exact function remains unclear. The phytohormone brassinosteroids (BRs) play vital roles in plant growth, development and environmental responses. A key event in effective BR signaling is the formation of functional BRI1-SERK receptor complex and subsequent transphosphorylation upon ligand binding. Here, we demonstrate that BONZAI (BON) proteins, which are plasma membrane-associated copine proteins, are critical components of BR signaling in both the monocot maize and the dicot Arabidopsis. Biochemical and molecular analyses reveal that BON proteins directly interact with SERK kinases, thereby ensuring effective BRI1-SERK interaction and transphosphorylation. This study advances the knowledge on BR signaling and provides an important target for optimizing valuable agronomic traits, it also opens a way to study steroid hormone signaling and copine proteins of eukaryotes in a broader perspective.

How plants sense and respond to osmotic stress

Drought is one of the most serious abiotic stresses to land plants. Plants sense and respond to drought stress to survive under water deficiency. Scientists have studied how plants sense drought stress, or osmotic stress caused by drought, ever since Charles Darwin, and gradually obtained clues about osmotic stress sensing and signaling in plants. Osmotic stress is a physical stimulus that triggers many physiological changes at the cellular level, including changes in turgor, cell wall stiffness and integrity, membrane tension, and cell fluid volume, and plants may sense some of these stimuli and trigger downstream responses. In this review, we emphasized water potential and movements in organisms, compared putative signal inputs in cell wall-containing and cell wall-free organisms, prospected how plants sense changes in turgor, membrane tension, and cell fluid volume under osmotic stress according to advances in plants, animals, yeasts, and bacteria, summarized multilevel biochemical and physiological signal outputs, such as plasma membrane nanodomain formation, membrane water permeability, root hydrotropism, root halotropism, Casparian strip and suberin lamellae, and finally proposed a hypothesis that osmotic stress responses are likely to be a cocktail of signaling mediated by multiple osmosensors. We also discussed the core scientific questions, provided perspective about the future directions in this field, and highlighted the importance of robust and smart root systems and efficient source-sink allocations for generating future high-yield stress-resistant crops and plants.

In situ deletions reveal regulatory components for expression of an intracellular immune receptor gene and its co-expressed genes in Arabidopsis

Intracellular immune receptor nucleotide-binding leucine-rich repeats (NLRs) are highly regulated transcriptionally and post-transcriptionally for balanced plant defence and growth. NLR genes often exist in gene clusters and are usually co-expressed under various conditions. Despite of intensive studies of regulation of NLR proteins, cis-acting elements for NLR gene induction, repression or co-expression are largely unknown due to a larger than usual cis-region for their expression regulation. Here we used the CRISPR/Cas9 genome editing technology to generate a series of in situ deletions at the endogenous location of a NLR gene SNC1 residing in the RPP5 gene cluster. These deletions that made in the wild type and the SNC1 constitutive expressing autoimmune mutant bon1 revealed both positive and negative cis-acting elements for SNC1 expression. Two transcription factors that could bind to these elements were found to have an impact on the expression of SNC1. In addition, co-expression of two genes with SNC1 in the same cluster is found to be mostly dependent on the SNC1 function. Therefore, SNC1 expression is under complex local regulation involving multiple cis elements and SNC1 itself is a critical regulator of gene expression of other NLR genes in the same gene cluster.

Plant latent defense response to microbial non-pathogenic factors antagonizes compatibility

Unlike microbe-associated molecular patterns (MAMPs) that are readily targeted by host immunity, microbial non-pathogenic factors (NPFs) appear negligible as they do not elicit defense. Little is known about whether and how NPFs may be monitored by hosts to control compatibility. Herein, a forward genetic screening isolated an Arabidopsis mutant with a loss of plant-rhizobacteria mutualism, leading to the disclosure of a plant latent defense response (LDR) to NPFs. The activation of LDR in the mutant, named rol1 for regulator of LDR 1, is triggered by several non-pathogenic volatile organic compounds and antagonizes plant compatibility with the beneficial bacterium Bacillus amyloliquefaciens GB03. The activation of LDR in rol1 is mediated through the prokaryotic pathway of chloroplastic lipid biosynthesis. The rol1 root microbiome showed a reduced proportion of the Bacillaceae family. We propose that, parallel to the forefront immunity to MAMPs, LDR to certain NPFs provides a hidden layer of defense for controlling compatibility with commensal or beneficial microbes.

In situ deletions reveal regulatory components for expression of an intracellular immune receptor gene and its co-expressed genes in Arabidopsis

Intracellular immune receptor nucleotide-binding leucine-rich repeats (NLRs) are highly regulated transcriptionally and post-transcriptionally for balanced plant defense and growth. NLR genes often exist in gene clusters and are usually co-expressed under various conditions. Despite intensive studies of the regulation of NLR proteins, cis-acting elements for NLR gene induction, repression or co-expression are largely unknown due to a larger than usual cis-region for their expression regulation. Here we used the CRISPR/Cas9 genome editing technology to generate a series of in situ deletions at the endogenous location of an NLR gene SNC1 residing in the RPP5 gene cluster. These deletions that made in the wild type and the SNC1 constitutive expressing autoimmune mutant bon1 revealed both positive and negative cis-acting elements for SNC1 expression. Two transcription factors that could bind to these elements were found to have an impact on the expression of SNC1. In addition, co-expression of two genes with SNC1 in the same cluster is found to be mostly dependent on the SNC1 function. Therefore, SNC1 expression is under complex local regulation involving multiple cis-elements and SNC1 itself is a critical regulator of gene expression of other NLR genes in the same gene cluster.

Genetic architecture of the metabolic pathway of salicylic acid biosynthesis in Populus

Salicylic acid (SA) is a vital hormone for adaptive responses to biotic and abiotic stresses, which facilitates growth-immunity trade-offs in plants. However, the genetic regulatory networks underlying the metabolic pathway of SA biosynthesis in perennial species remain unclear. Here, we integrated genome-wide association study (GWAS) with metabolite and expression profiling methodologies to dissect the genetic architecture of SA biosynthesis in Populus. First, we quantified nine intermediate metabolites of SA biosynthesis in 300 unrelated Populus tomentosa Carr. individuals. Then, we used a systematic genetic strategy to identify candidate genes for constructing the genetic regulatory network of SA biosynthesis. We focused on WRKY70, an efficient transcription factor, as the key causal gene in the regulatory network, and combined the novel genes coordinating the accumulation of SA. Finally, we identified eight GWAS signals and eight expression quantitative trait loci situated in a selective sweep, and showed the presence of large allele frequency differences among the three geographic populations, revealing that candidate genes subject to selection were involved in SA biosynthesis. This study provides an integrated strategy for dissecting the genetic architecture of the metabolic pathway of SA biosynthesis in Populus, thereby enhancing our understanding of genetic regulation of SA biosynthesis in trees, and accelerating marker-assisted breeding efforts toward high-resistance elite varieties of Populus.

The truncated TNL receptor TN2-mediated immune responses require ADR1 function

The loss of function of exocyst subunit EXO70B1 leads to autoimmunity, which is dependent on TIR-NBS2 (TN2), a truncated intracellular nucleotide-binding and leucine-rich repeat receptor (NLR). However, how TN2 triggers plant immunity and whether typical NLRs are required in TN2-activated resistance remain unclear. Through the CRISPR/Cas9 gene editing system and knockout analysis, we found that the spontaneous cell death and enhanced resistance in exo70B1-3 were independent of the full-length NLR SOC3 and its closest homolog SOC3-LIKE 1 (SOC3-L1). Additionally, knocking out SOC3-L1 or TN2 did not suppress the chilling sensitivity conferred by chilling sensitive 1-2 (chs1-2). The ACTIVATED DISEASE RESISTANCE 1 (ADR1) family and the N REQUIREMENT GENE 1 (NRG1) family have evolved as helper NLRs for many typical NLRs. Through CRISPR/Cas9 gene editing methods, we discovered that the autoimmunity of exo70B1-3 fully relied on ADR1s, but not NRG1s, and ADR1s contributed to the upregulation of TN2 transcript levels in exo70B1-3. Furthermore, overexpression of TN2 also led to ADR1-dependent autoimmune responses. Taken together, our genetic analysis highlights that the truncated TNL protein TN2-triggered immune responses require ADR1s as helper NLRs to activate downstream signaling, revealing the importance and complexity of ADR1s in plant immunity regulation.

The Effect of Temperature on the Hypersensitive Response (HR) in the Brassica napus-Leptosphaeria maculans Pathosystem

Temperature is considered one of the crucial environmental elements in plant pathological interactions, and previous studies have indicated that there is a relationship between temperature change and host-pathogen interactions. The objective of this research is to investigate the link between temperature and the incompatible interactions of the host and pathogen. In this study, two Leptosphaeria maculans isolates (HCRT75 8-1 and HCRT77 7-2) and two Brassica napus genotypes (Surpass400 and 01-23-2-1) were selected. The selected B. napus genotypes displayed intermediate and resistant phenotypes. The inoculated seedlings were tested under three temperature conditions: 16 °C/10 °C, 22 °C/16 °C and 28 °C/22 °C (day/night: 16 h/8 h). Lesion measurements demonstrated that the necrotic lesions from the 28 °C/22 °C treatment were enlarged compared with the other two temperature treatments (i.e., 16 °C/10 °C and 22 °C/16 °C). The results of expression analysis indicated that the three temperature treatments displayed distinct differences in two marker genes (PATHOGENESIS-RELATED (PR) 1 and 2) for plant defense and one temperature-sensitive gene BONZAI 1 (BON1). Additionally, seven dpi at 22 °C/16 °C appeared to be the optimal pre-condition for the induction of PR1 and 2. These findings suggest that B. napus responds to temperature changes when infected with L. maculans.

Hybrid Incompatibility of the Plant Immune System: An Opposite Force to Heterosis Equilibrating Hybrid Performances

Hybridization is a core element in modern rice breeding as beneficial combinations of two parental genomes often result in the expression of heterosis. On the contrary, genetic incompatibility between parents can manifest as hybrid necrosis, which leads to tissue necrosis accompanied by compromised growth and/or reduced reproductive success. Genetic and molecular studies of hybrid necrosis in numerous plant species revealed that such self-destructing symptoms in most cases are attributed to autoimmunity: plant immune responses are inadvertently activated in the absence of pathogenic invasion. Autoimmunity in hybrids predominantly occurs due to a conflict involving a member of the major plant immune receptor family, the nucleotide-binding domain and leucine-rich repeat containing protein (NLR; formerly known as NBS-LRR). NLR genes are associated with disease resistance traits, and recent population datasets reveal tremendous diversity in this class of immune receptors. Cases of hybrid necrosis involving highly polymorphic NLRs as major causes suggest that diversified R gene repertoires found in different lineages would require a compatible immune match for hybridization, which is a prerequisite to ensure increased fitness in the resulting hybrids. In this review, we overview recent genetic and molecular findings on hybrid necrosis in multiple plant species to provide an insight on how the trade-off between growth and immunity is equilibrated to affect hybrid performances. We also revisit the cases of hybrid weakness in which immune system components are found or implicated to play a causative role. Based on our understanding on the trade-off, we propose that the immune system incompatibility in plants might play an opposite force to restrict the expression of heterosis in hybrids. The antagonism is illustrated under the plant fitness equilibrium, in which the two extremes lead to either hybrid necrosis or heterosis. Practical proposition from the equilibrium model is that breeding efforts for combining enhanced disease resistance and high yield shall be achieved by balancing the two forces. Reverse breeding toward utilizing genomic data centered on immune components is proposed as a strategy to generate elite hybrids with balanced immunity and growth.

A Truncated Singleton NLR Causes Hybrid Necrosis in Arabidopsis thaliana

Hybrid necrosis in plants arises from conflict between divergent alleles of immunity genes contributed by different parents, resulting in autoimmunity. We investigate a severe hybrid necrosis case in Arabidopsis thaliana, where the hybrid does not develop past the cotyledon stage and dies 3 weeks after sowing. Massive transcriptional changes take place in the hybrid, including the upregulation of most NLR (nucleotide-binding site leucine-rich repeat) disease-resistance genes. This is due to an incompatible interaction between the singleton TIR-NLR gene DANGEROUS MIX 10 (DM10), which was recently relocated from a larger NLR cluster, and an unlinked locus, DANGEROUS MIX 11 (DM11). There are multiple DM10 allelic variants in the global A. thaliana population, several of which have premature stop codons. One of these, which has a truncated LRR-PL (leucine-rich repeat [LRR]-post-LRR) region, corresponds to the DM10 risk allele. The DM10 locus and the adjacent genomic region in the risk allele carriers are highly differentiated from those in the nonrisk carriers in the global A. thaliana population, suggesting that this allele became geographically widespread only relatively recently. The DM11 risk allele is much rarer and found only in two accessions from southwestern Spain-a region from which the DM10 risk haplotype is absent-indicating that the ranges of DM10 and DM11 risk alleles may be nonoverlapping.

Multiple insertions of COIN, a novel maize Foldback transposable element, in the Conring gene cause a spontaneous progressive cell death phenotype

Similar progressive leaf lesion phenotypes, named conring for "concentric ring," were identified in 10 independently derived maize lines. Complementation and mapping experiments indicated that the phenotype had the same genetic basis in each line - a single recessive gene located in a 1.1-Mb region on chromosome 2. Among the 15 predicted genes in this interval, Zm00001d003866 (subsequently renamed Conring or Cnr) had insertions of four related 138 bp transposable element (TE) sequences at precisely the same site in exon 4 in nine of the 10 cnr alleles. The 10th cnr allele had a distinct insertion of 226 bp of in exon 3. Genetic evidence suggested that the 10 cnr alleles were independently derived, and arose during the derivation of each line. The four TEs, named COINa (for COnring INsertion) through COINd, have not been previously characterized and consist entirely of imperfect 69-bp terminal inverted repeats characteristic of the Foldback class of TEs. They belong to three clades of a family of maize TEs comprising hundreds of sequences in the genome of the B73 maize line. COIN elements preferentially insert at TNA sequences with a preference for C and G nucleotides in the immediately flanking 5' and 3' regions, respectively. They produce a three-base target site duplication and do not have homology to other characterized TEs. We propose that Cnr is an unstable gene that is mutated insertionally at high frequency, most commonly due to COIN element insertions at a specific site in the gene.

BONZAI Proteins Control Global Osmotic Stress Responses in Plants

Hyperosmotic stress caused by drought and salinity is a significant environmental threat that limits plant growth and agricultural productivity. Osmotic stress induces diverse responses in plants including Ca²⁺ signaling, accumulation of the stress hormone abscisic acid (ABA), reprogramming of gene expression, and altering of growth. Despite intensive investigation, no global regulators of all of these responses have been identified. Here, we show that the Ca²⁺-responsive phospholipid-binding BONZAI (BON) proteins are critical for all of these osmotic stress responses. A Ca²⁺-imaging-based forward genetic screen identified a loss-of-function bon1 mutant with a reduced cytosolic Ca²⁺ signal in response to hyperosmotic stress. The loss-of-function mutants of the BON1 gene family, bon1bon2bon3, are impaired in the induction of gene expression and ABA accumulation in response to osmotic stress. In addition, the bon mutants are hypersensitive to osmotic stress in growth inhibition. BON genes have been shown to negatively regulate plant immune responses mediated by intracellular immune receptor NLR genes including SNC1. We found that the defects of the bon mutants in osmotic stress responses were suppressed by mutations in the NLR gene SNC1 or the immunity regulator PAD4. Our findings indicate that NLR signaling represses osmotic stress responses and that BON proteins suppress NLR signaling to enable global osmotic stress responses in plants.

Developmentally regulated activation of defense allows for rapid inhibition of infection in age-related resistance to Phytophthora capsici in cucumber fruit

Background

Age-related resistance (ARR) is a developmentally regulated phenomenon conferring resistance to pathogens or pests. Although ARR has been observed in several host-pathogen systems, the underlying mechanisms are largely uncharacterized. In cucumber, rapidly growing fruit are highly susceptible to Phytophthora capsici but become resistant as they complete exponential growth. We previously demonstrated that ARR is associated with the fruit peel and identified gene expression and metabolomic changes potentially functioning as preformed defenses.

Results

Here, we compare the response to infection in fruit at resistant and susceptible ages using microscopy, quantitative bioassays, and weighted gene co-expression analyses. We observed strong transcriptional changes unique to resistant aged fruit 2–4 h post inoculation (hpi). Microscopy and bioassays confirmed this early response, with evidence of pathogen death and infection failure as early as 4 hpi and cessation of pathogen growth by 8–10 hpi. Expression analyses identified candidate genes involved in conferring the rapid response including those encoding transcription factors, hormone signaling pathways, and defenses such as reactive oxygen species metabolism and phenylpropanoid biosynthesis.

Conclusion

The early pathogen death and rapid defense response in resistant-aged fruit provide insight into potential mechanisms for ARR, implicating both pre-formed biochemical defenses and developmentally regulated capacity for pathogen recognition as key factors shaping age-related resistance.

Initial responses of the trap-crop, Solanum sisymbriifolium, to Globodera pallida invasions

Many researchers today are looking for mechanisms underlying plant defenses against nematodes by identifying differentially expressed genes in domesticated hosts. In order to provide a different perspective, we analyzed the root transcriptome of an undomesticated non-host species, Solanum sisymbriifolium Lamark (SSI) before and after Globodera pallida infection. Utilizing RNAseq analyses, we identified changes in the expression of 277 transcripts. Many of these genes were not annotated; however, the annotated set included peroxidases, reactive oxygen species-producing proteins, and regulators of cell death. Importantly, 60% of the nematode-responsive genes did not respond to physical damage to root tissues, or to exogenous treatments with either salicylic acid or methyl jasmonate. Based on this, we speculate that the majority of changes in SSI gene expression were promoted by either nematode effectors, pathogen-associated molecular patterns (PAMPs), or by exposure to untested endogenous signaling molecules such as ethylene, or by exposure to multiple stimuli. This study incorporates our findings into a model that accounts for part of this plant's unusual resistance to nematodes.

Nuclear pore complex components have temperature-influenced roles in plant growth and immunity

Nuclear pore complexes (NPCs) are main channels controlling nucleocytoplasmic transport and are composed of approximately 30 nucleoporins (NUPs). Emerging evidence suggests that some NUP genes have specialized functions that challenge the traditional view of NPCs as structures of uniform composition. Here, we analysed the role of six outer-ring components of NPC at normal and warm growth temperatures by examining their loss-of-function mutants in Arabidopsis thaliana. All six NUP subunits, NUP85, NUP96, NUP 133, NUP 160, SEH1 and HOS1, have a non-redundant temperature-influenced function in one or more of the processes, including rosette growth, leaf architecture and intracellular immune receptor-mediated disease resistance. At the molecular level, NUP85 and NUP133 are required for mRNA export only at warm temperature and play a larger role in the localization of transcription factor at warm temperature. In addition, NUP96 and HOS1 are essential for the expression of high temperature-responsive genes, which is correlated with their larger activity in facilitating nuclear accumulation of the transcription factor PIF4 at warm temperature. Our results show that subunits of NPC have differential roles at different temperatures, suggesting the existence of temperature-influenced NPC complexes and activities.

Current views on temperature-modulated R gene-mediated plant defense responses and tradeoffs between plant growth and immunity

Elevated ambient temperatures will likely be a key consequence of climate change over the next few decades. Adverse climatic changes could make crop plants more vulnerable to a number of biotic and abiotic stresses, which would have a major impact on worldwide food production in the future. Recent studies have indicated that elevated temperatures directly and/or indirectly affect plant-pathogen interactions. Elevated temperatures alter multiple signal transduction pathways related to stress responses in the host plant. High temperatures can also influence plant pathogenesis, but little is known about the molecular mechanisms associated with such effects. An improved understanding of the molecular genetic mechanisms involved in plant immune responses under elevated temperatures will be essential to mitigate the adverse effects of climate change to ensure future food security. In this review, we discuss recent advances in our understanding of the effects of temperature on resistance (R) gene and/or regulators of R genes in plant defense responses and summarize current evidence for tradeoffs between plant growth and immunity.

The plant hypersensitive response: concepts, control and consequences

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.

Protein kinase-mediated signalling in priming: Immune signal initiation, propagation, and establishment of long-term pathogen resistance in plants

"Priming" in plant phytopathology describes a phenomenon where the "experience" of primary infection by microbial pathogens leads to enhanced and beneficial protection of the plant against secondary infection. The plant is able to establish an immune memory, a state of systemic acquired resistance (SAR), in which the information of "having been attacked" is integrated with the action of "being prepared to defend when it happens again." Accordingly, primed plants are often characterized by faster and stronger activation of immune reactions that ultimately result in a reduction of pathogen spread and growth. Prerequisites for SAR are (a) the initiation of immune signalling subsequent to pathogen recognition, (b) a rapid defence signal propagation from a primary infected local site to uninfected distal parts of the plant, and (c) a switch into an immune signal-dependent establishment and subsequent long-lasting maintenance of phytohormone salicylic acid-based systemic immunity. Here, we provide a summary on protein kinases that contribute to these three conceptual aspects of "priming" in plant phytopathology, complemented by data addressing the role of protein kinases crucial for immune signal initiation also for signal propagation and SAR.

The Pseudomonas Syringae Effector AvrPtoB Associates With and Ubiquitinates Arabidopsis Exocyst Subunit EXO70B1

Many bacterial pathogens secret effectors into host cells to disable host defenses and thus promote infection. The exocyst complex functions in the transport and secretion of defense molecules, and loss of function of the EXO70B1 subunit leads to autoimmunity by activation of a truncated Toll/interleukin-1 receptor-nucleotide-binding sequence protein (TIR-NBS2; herein referred to as TN2). Here, we show that EXO70B1 is required for pathogen-associated molecular pattern-triggered immune responses in Arabidopsis thaliana. The effector AvrPtoB, an E3 ligase from Pseudomonas syringae pv. tomato (Pto) strain DC3000, associates with EXO70B1. AvrPtoB ubiquitinates EXO70B1 and mediates EXO70B1 degradation via the host's 26S proteasome in a manner requiring E3 ligase activity. AvrPtoB enhances Pto DC3000 virulence by overcoming EXO70B1-mediated resistance. Moreover, overexpression of AvrPtoB in Arabidopsis leads to autoimmunity, which is partially dependent on TN2. Expression of TN2 in tobacco (Nicotiana tabacum and Nicotiana benthamiana) triggers strong and rapid cell death, which is suppressed by co-expression with EXO70B1 but reoccurs when co-expressed with AvrPtoB. Taken together, our data highlight that AvrPtoB targets the Arabidopsis thaliana EXO70 protein family member EXO70B1 to manipulate the defense molecule secretion machinery or immunity.

Modulation of ACD6 dependent hyperimmunity by natural alleles of an Arabidopsis thaliana NLR resistance gene

Plants defend themselves against pathogens by activating an array of immune responses. Unfortunately, immunity programs may also cause unintended collateral damage to the plant itself. The quantitative disease resistance gene ACCELERATED CELL DEATH 6 (ACD6) serves to balance growth and pathogen resistance in natural populations of Arabidopsis thaliana. An autoimmune allele, ACD6-Est, which strongly reduces growth under specific laboratory conditions, is found in over 10% of wild strains. There is, however, extensive variation in the strength of the autoimmune phenotype expressed by strains with an ACD6-Est allele, indicative of genetic modifiers. Quantitative genetic analysis suggests that ACD6 activity can be modulated in diverse ways, with different strains often carrying different large-effect modifiers. One modifier is SUPPRESSOR OF NPR1-1, CONSTITUTIVE 1 (SNC1), located in a highly polymorphic cluster of nucleotide-binding domain and leucine-rich repeat (NLR) immune receptor genes, which are prototypes for qualitative disease resistance genes. Allelic variation at SNC1 correlates with ACD6-Est activity in multiple accessions, and a common structural variant affecting the NL linker sequence can explain differences in SNC1 activity. Taken together, we find that an NLR gene can mask the activity of an ACD6 autoimmune allele in natural A. thaliana populations, thereby linking different arms of the plant immune system.

Plants defend themselves against pathogens by activating immune responses. Unfortunately, these can cause unintended collateral damage to the plant itself. Nevertheless, some wild plants have genetic variants that confer a low threshold for the activation of immunity. While these enable a plant to respond particularly quickly to pathogen attack, such variants might be potentially dangerous. We are investigating one such variant of the immune gene ACCELERATED CELL DEATH 6 (ACD6) in the plant Arabidopsis thaliana. We discovered that there are variants at other genetic loci that can mask the effects of an overly active ACD6 gene. One of these genes, SUPPRESSOR OF NPR1-1, CONSTITUTIVE 1 (SNC1), codes for a known immune receptor. The SNC1 variant that attenuates ACD6 activity is rather common in A. thaliana populations, suggesting that new combinations of the hyperactive ACD6 variant and this antagonistic SNC1 variant will often arise by natural crosses. Similarly, because the two genes are unlinked, outcrossing will often lead to the hyperactive ACD6 variants being unmasked again. We propose that allelic diversity at SNC1 contributes to the maintenance of the hyperactive ACD6 variant in natural A. thaliana populations.

Silencing of copine genes confers common wheat enhanced resistance to powdery mildew

Powdery mildew, caused by the biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt), is a major threat to the production of wheat (Triticum aestivum). It is of great importance to identify new resistance genes for the generation of Bgt-resistant or Bgt-tolerant wheat varieties. Here, we show that the wheat copine genes TaBON1 and TaBON3 negatively regulate wheat disease resistance to Bgt. Two copies of TaBON1 and three copies of TaBON3, located on chromosomes 6AS, 6BL, 1AL, 1BL and 1DL, respectively, were identified from the current common wheat genome sequences. The expression of TaBON1 and TaBON3 is responsive to both pathogen infection and temperature changes. Knocking down of TaBON1 or TaBON3 by virus-induced gene silencing (VIGS) induces the up-regulation of defence responses in wheat. These TaBON1- or TaBON3-silenced plants exhibit enhanced wheat disease resistance to Bgt, accompanied by greater accumulation of hydrogen peroxide and heightened cell death. In addition, high temperature has little effect on the up-regulation of defence response genes conferred by the silencing of TaBON1 or TaBON3. Our study shows a conserved function of plant copine genes in plant immunity and provides new genetic resources for the improvement of resistance to powdery mildew in wheat.

Overlapping and differential roles of plasma membrane calcium ATPases in Arabidopsis growth and environmental responses

Plant cells have multiple plasma membrane (PM)-localized calcium ATPases (ACAs) pumping calcium ions out of the cytosol. Although the involvement of some of these ACAs in plant growth and immunity has been reported, their individual and combined functions have not been fully examined. Here, we analysed the effects of single and combined mutations of four ACA genes, ACA8, ACA10, ACA12, and ACA13, in a number of processes. We found that these four genes had both overlapping and differential involvements in vegetative growth, inflorescence growth, seeds setting, disease resistance and stomatal movement. Disruption of any of these four genes reduces seed setting, indicating their contribution to the overall fitness of the plants. While ACA10 and ACA8 play major roles in vegetative growth and immunity, ACA13 and ACA12 are also involved in these processes especially when the function of ACA10 and/or ACA8 is compromised. The loss of ACA13 and ACA10 function in combination with a reduction in function of ACA8 leads to seedling death at bolting, revealing the essential role of their collective function in plant growth. Taken together, this study indicates a highly tuned calcium system involving these PM-localized calcium pumps in plant growth and environmental responses.

Genome-Wide Transcriptome Analysis Reveals the Comprehensive Response of Two Susceptible Poplar Sections to Marssonina brunnea Infection

Marssonina leaf spot disease of poplar (MLDP), caused by the hemibiotrophic pathogen Marssonina brunnea, frequently results in damage to many poplar species. In nature, two formae speciales of M. brunnea exist that are susceptible to different poplar subgenera. Marssonina brunnea f. sp. monogermtubi infects poplar hosts from Populus sect. Aigeiros (Aig), while M. brunnea f. sp. multigermtubi always infects poplar hosts from Populus sect. Leuce Duby (Leu). Based on the fungal penetration structures, a comprehensive transcriptomic approach was used to investigate the gene expression patterns of these two poplar subgenera at three crucial infection stages. MLDP significantly altered the expression patterns of many genes involved in mitogen activated protein kinase (MAPKs) and calcium signaling, transcription factors, primary and secondary metabolism, and other processes in both poplar subgenera. However, major differences in gene expression were also observed between the two poplar subgenera. Aig was most responsive at the initial infection stage, while Leu largely interacted with M. brunnea at the necrotrophic phase. Furthermore, the differentially expressed genes (DEGs) involved in pathways related to biotic stress also differed substantially between the two poplar subgenera. Further analysis indicated that the genes involved in cell wall metabolism and phenylpropanoid metabolism were differentially expressed in the progression of the disease. By examining the expression patterns of genes related to the defense against disease, we found that several genes annotated with causing hypersensitive cell death were upregulated at the necrotrophic phase of MLDP, inferring that plant immune response potentially happened at this infection stage. The present research elucidated the potential molecular differences between the two susceptible interaction systems in MLDP and provided novel insight into the temporal regulation of genes during the susceptible response. To the best of our knowledge, this study also constitutes the first to reveal the molecular mechanisms of poplar in response to the transition of hemibiotrophic fungal pathogens from the biotrophic phase to the necrotrophic phase.

The Arabidopsis Chromatin-Remodeling Factor CHR5 Regulates Plant Immune Responses and Nucleosome Occupancy

ATP-dependent chromatin-remodeling factors use the energy of ATP hydrolysis to alter the structure of chromatin and are important regulators of eukaryotic gene expression. One such factor encoded by CHR5 (Chromatin-Remodeling Factor 5) in Arabidopsis (Arabidopsis thaliana) was previously found to be involved in regulation of growth and development. Here we show that CHR5 is required for the up-regulation of the intracellular immune receptor gene SNC1 (SUPPRESSOR OF npr1-1, CONSTITUTIVE1) and consequently the autoimmunity induced by SNC1 up-regulation. CHR5 functions antagonistically with another chromatin-remodeling gene DDM1 (DECREASED DNA METHYLATION 1) and independently with a histone mono-ubiquitinase HUB1 (HISTONE MONOUBIQUITINATION 1) in SNC1 regulation. In addition, CHR5 is a positive regulator of SNC1-independent plant immunity against the bacterial pathogen Pseudomonas syringae. Furthermore, the chr5 mutant has increased nucleosome occupancy in the promoter region relative to the gene body region at the whole-genome level, suggesting a global role for CHR5 in remodeling nucleosome occupancy. Our study thus establishes CHR5 as a positive regulator of plant immune responses including the expression of SNC1 and reveals a role for CHR5 in nucleosome occupancy which probably impacts gene expression genome wide.

A look at plant immunity through the window of the multitasking coreceptor BAK1

Recognition of microbe- and danger-associated molecular patterns (MAMPs and DAMPs, respectively) by pattern recognition receptors (PRRs) is central to innate immunity in both plants and animals. The plant PRRs described to date are all cell surface-localized receptors. According to their ligand-binding ectodomains, each PRR engages a specific coreceptor or adaptor kinase in its signaling complexes to regulate defense signaling. With a focus on the coreceptor RLK BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) and related SOMATIC EMBRYOGENESIS RECEPTOR KINASEs (SERKs), here we review the increasing inventory of BAK1 partners and their functions in plant immunity. We also discuss the significance of autoimmunity triggered by BAK1/SERK4 disintegration in shaping the strategies for attenuation of PRR signaling by infectious microbes and host plants.

Temperature-dependent autoimmunity mediated by chs1 requires its neighboring TNL gene SOC3

Toll/interleukin receptor (TIR)-nucleotide binding site (NB)-type (TN) proteins are encoded by a family of 21 genes in the Arabidopsis genome. Previous studies have shown that a mutation in the TN gene CHS1 activates the activation of defense responses at low temperatures. However, the underlying molecular mechanism remains unknown. To genetically dissect chs1-mediated signaling, we isolated genetic suppressors of chs1-2 (soc). Several independent soc mutants carried mutations in the same TIR-NB-leucine-rich repeat (LRR) (TNL)-encoding gene SOC3, which is adjacent to CHS1 on chromosome 1. Expression of SOC3 was upregulated in the chs1-2 mutant. Mutations in six soc3 alleles and downregulation of SOC3 by an artificial microRNA construct fully rescued the chilling sensitivity and defense defects of chs1-2. Biochemical studies showed that CHS1 interacted with the NB and LRR domains of SOC3; however, mutated chs1 interacted with the TIR, NB and LRR domains of SOC3 in vitro and in vivo. This study reveals that the TN protein CHS1 interacts with the TNL protein SOC3 to modulate temperature-dependent autoimmunity.

Identification and analysis of copine/BONZAI proteins among evolutionarily diverse plant species

Copines are evolutionarily conserved calcium-dependent membrane-binding proteins with potentially critical biological functions. In plants, the function of these proteins has not been analyzed except for in Arabidopsis thaliana where they play critical roles in development and disease resistance. To facilitate functional studies of copine proteins in crop plants, genome-wide identification, curation, and phylogeny analysis of copines in 16 selected plant species were conducted. All the identified 32 plant copines have conserved features of the two C2 domains (C2A and C2B) and the von Willebrand factor A (vWA) domain. Different from animal and protozoa copines, plant copines have glycine at the second residue potentially acquiring a unique protein myristoylation modification. Phylogenetic analysis suggests that copine was present as one copy when evolving from green algae to basal flowering plants, and duplicated before the divergence of monocots and dicots. In addition, gene expression and protein localization study of rice copines suggests both conserved and different properties of copines in dicots and monocots. This study will contribute to uncovering the role of copine genes in different plant species.

Constitutive Negative Regulation of R Proteins in Arabidopsis also via Autophagy Related Pathway?

Even though resistance (R) genes are among the most studied components of the plant immunity, there remain still a lot of aspects to be explained about the regulation of their function. Many gain-of-function mutants of R genes and loss-of-function of their regulators often demonstrate up-regulated defense responses in combination with dwarf stature and/or spontaneous leaf lesions formation. For most of these mutants, phenotypes are a consequence of an ectopic activation of R genes. Based on the compilation and comparison of published results in this field, we have concluded that the constitutively activated defense phenotypes recurrently arise by disruption of tight, constitutive and multilevel negative control of some of R proteins that might involve also their targeting to the autophagy pathway. This mode of R protein regulation is supported also by protein-protein interactions listed in available databases, as well as in silico search for autophagy machinery interacting motifs. The suggested model could resolve some explanatory discrepancies found in the studies of the immunity responses of autophagy mutants.

IBR5 Modulates Temperature-Dependent, R Protein CHS3-Mediated Defense Responses in Arabidopsis

Plant responses to low temperature are tightly associated with defense responses. We previously characterized the chilling-sensitive mutant chs3-1 resulting from the activation of the Toll and interleukin 1 receptor-nucleotide binding-leucine-rich repeat (TIR-NB-LRR)-type resistance (R) protein harboring a C-terminal LIM (Lin-11, Isl-1 and Mec-3 domains) domain. Here we report the identification of a suppressor of chs3, ibr5-7 (indole-3-butyric acid response 5), which largely suppresses chilling-activated defense responses. IBR5 encodes a putative dual-specificity protein phosphatase. The accumulation of CHS3 protein at chilling temperatures is inhibited by the IBR5 mutation. Moreover, chs3-conferred defense phenotypes were synergistically suppressed by mutations in HSP90 and IBR5. Further analysis showed that IBR5, with holdase activity, physically associates with CHS3, HSP90 and SGT1b (Suppressor of the G2 allele of skp1) to form a complex that protects CHS3. In addition to the positive role of IBR5 in regulating CHS3, IBR5 is also involved in defense responses mediated by R genes, including SNC1 (Suppressor of npr1-1, Constitutive 1), RPS4 (Resistance to P. syringae 4) and RPM1 (Resistance to Pseudomonas syringae pv. maculicola 1). Thus, the results of the present study reveal a role for IBR5 in the regulation of multiple R protein-mediated defense responses.

Resistance (R) genes play central roles in recognizing pathogens and triggering plant defense responses. CHS3 encodes a TIR-NB-LRR-type R protein harboring a C-terminal LIM domain. A point mutation in CHS3 activates the defense response under chilling stress. Here we identified and characterized ibr5-7, a mutant that suppresses the chilling-induced defense responses of chs3-1. We observed that the enhanced defense responses and cell death in the chs3-1 mutant are synergistically dependent on IBR5 and HSP90. IBR5 physically interacts with CHS3, forming a complex with SGT1b/ HSP90. Moreover, IBR5 is also involved in the R-gene resistance mediated by SNC1, RPS4 and RPM1. Thus, IBR5 plays key roles in regulating defense responses mediated by multiple R proteins.

A truncated NLR protein, TIR-NBS2, is required for activated defense responses in the exo70B1 mutant

During exocytosis, the evolutionarily conserved exocyst complex tethers Golgi-derived vesicles to the target plasma membrane, a critical function for secretory pathways. Here we show that exo70B1 loss-of-function mutants express activated defense responses upon infection and express enhanced resistance to fungal, oomycete and bacterial pathogens. In a screen for mutants that suppress exo70B1 resistance, we identified nine alleles of TIR-NBS2 (TN2), suggesting that loss-of-function of EXO70B1 leads to activation of this nucleotide binding domain and leucine-rich repeat-containing (NLR)-like disease resistance protein. This NLR-like protein is atypical because it lacks the LRR domain common in typical NLR receptors. In addition, we show that TN2 interacts with EXO70B1 in yeast and in planta. Our study thus provides a link between the exocyst complex and the function of a 'TIR-NBS only' immune receptor like protein. Our data are consistent with a speculative model wherein pathogen effectors could evolve to target EXO70B1 to manipulate plant secretion machinery. TN2 could monitor EXO70B1 integrity as part of an immune receptor complex.

To die or not to die? Lessons from lesion mimic mutants

Programmed cell death (PCD) is a ubiquitous genetically regulated process consisting in an activation of finely controlled signaling pathways that lead to cellular suicide. Although some aspects of PCD control appear evolutionary conserved between plants, animals and fungi, the extent of conservation remains controversial. Over the last decades, identification and characterization of several lesion mimic mutants (LMM) has been a powerful tool in the quest to unravel PCD pathways in plants. Thanks to progress in molecular genetics, mutations causing the phenotype of a large number of LMM and their related suppressors were mapped, and the identification of the mutated genes shed light on major pathways in the onset of plant PCD such as (i) the involvements of chloroplasts and light energy, (ii) the roles of sphingolipids and fatty acids, (iii) a signal perception at the plasma membrane that requires efficient membrane trafficking, (iv) secondary messengers such as ion fluxes and ROS and (v) the control of gene expression as the last integrator of the signaling pathways.

Modulation of plant immunity by light, circadian rhythm, and temperature

Plants perceive and integrate intrinsic and extrinsic signals to execute appropriate responses for maximal survival and reproductive success. Plant immune responses are tightly controlled to ensure effective defenses against pathogens while minimizing their adverse effects on plant growth and development. Plant defenses induced in response to pathogen infection are modulated by abiotic signals such as light, circadian rhythm, and temperature. The modulation occurs on specific key components of plant immunity, indicating an intricate integration of biotic and abiotic signals. This review will summarize very recent studies revealing the intersection of plant defenses with light, circadian rhythm and temperature. In addition, it will discuss the adaptive value and evolutionary constraints of abiotic regulation of plant immunity.

Arabidopsis thaliana resistance to fusarium oxysporum 2 implicates tyrosine-sulfated peptide signaling in susceptibility and resistance to root infection

In the plant Arabidopsis thaliana, multiple quantitative trait loci (QTLs), including RFO2, account for the strong resistance of accession Columbia-0 (Col-0) and relative susceptibility of Taynuilt-0 (Ty-0) to the vascular wilt fungus Fusarium oxysporum forma specialis matthioli. We find that RFO2 corresponds to diversity in receptor-like protein (RLP) genes. In Col-0, there is a tandem pair of RLP genes: RFO2/At1g17250 confers resistance while RLP2 does not. In Ty-0, the highly diverged RFO2 locus has one RLP gene conferring weaker resistance. While the endogenous RFO2 makes a modest contribution to resistance, transgenic RFO2 provides strong pathogen-specific resistance. The extracellular leucine-rich repeats (eLRRs) in RFO2 and RLP2 are interchangeable for resistance and remarkably similar to eLRRs in the receptor-like kinase PSY1R, which perceives tyrosine-sulfated peptide PSY1. Reduced infection in psy1r and mutants of related phytosulfokine (PSK) receptor genes PSKR1 and PSKR2 shows that tyrosine-sulfated peptide signaling promotes susceptibility. The related eLRRs in RFO2 and PSY1R are not interchangeable; and expression of the RLP nPcR, in which eLRRs in RFO2 are replaced with eLRRs in PSY1R, results in constitutive resistance. Counterintuitively, PSY1 signaling suppresses nPcR because psy1r nPcR is lethal. The fact that PSK signaling does not similarly affect nPcR argues that PSY1 signaling directly downregulates the expression of nPcR. Our results support a speculative but intriguing model to explain RFO2's role in resistance. We propose that F. oxysporum produces an effector that inhibits the normal negative feedback regulation of PSY1R, which stabilizes PSY1 signaling and induces susceptibility. However, RFO2, acting as a decoy receptor for PSY1R, is also stabilized by the effector and instead induces host immunity. Overall, the quantitative resistance of RFO2 is reminiscent of the better-studied monogenic resistance traits.

The AvrB_AvrC domain of AvrXccC of Xanthomonas campestris pv. campestris is required to elicit plant defense responses and manipulate ABA homeostasis

Plant disease induced by Xanthomonas campestris pv. campestris depends on type III effectors but the molecular basis is poorly understood. Here, AvrXccC8004 was characterized, and it was found that the AvrB_AvrC domain was essential and sufficient to elicit defense responses in an Arabidopsis-resistant ecotype (Col-0). An upregulation of genes in responding to the AvrB_AvrC domain of AvrXccC8004 was shown in a profile of host gene expression. The molecular changes were correlated with morphological changes observed in phenotypic and ultrastructural characterizations. Interestingly, the abscisic acid (ABA)-signaling pathway was also a prominent target for the AvrB_AvrC domain of AvrXccC8004. The highly elicited NCED5, encoding a key enzyme of ABA biosynthesis, was increased in parallel with ABA levels in AvrXccC8004 transgenic plants. Consistently, the X. campestris pv. campestris 8004 ΔavrXccC mutant was severely impaired in the ability to manipulate the accumulation of ABA and induction of ABA-related genes in challenged leaves. Moreover, exogenous application of ABA also enhanced the susceptibility of Arabidopsis to the X. campestris pv. campestris strains. These results indicate that the AvrB_AvrC domain of AvrXccC8004 alone has the activity to manipulate ABA homeostasis, which plays an important role in regulating the interactions of X. campestris pv. campestris and Arabidopsis.

Perturbation of cell cycle regulation triggers plant immune response via activation of disease resistance genes

The Arabidopsis gene OSD1 (Omission of the Second Division) and its homolog UVI4 (UV-B-Insensitive 4) are negative regulators of anaphase-promoting complex/cyclosome (APC/C), a multisubunit ubiquitin E3 ligase that regulates the progression of cell cycles. Here we report the isolation of an activation tagging allele of OSD1 as an enhancer of a mutant of BON1 (BONZAI1), a negative regulator of plant immunity. Overexpression of OSD1 and UVI4 each leads to enhanced immunity to a bacterial pathogen, which is associated with increased expression of disease resistance (R) genes similar to the animal NOD1 receptor-like immune receptor genes. In addition, the reduction of function of one subunit of the APC complex APC10 exhibited a similar phenotype to that of overexpression of OSD1 or UVI4, indicating that altered APC function induces immune responses. Enhanced immune response induced by OSD1 overexpression is dependent on CYCB1;1, which is a degradation target of APC/C. It is also associated with up-regulation of R genes and is dependent on the R gene SNC1 (Suppressor of npr1-1, constitutive 1). Taken together, our findings reveal an unexpected link between cell cycle progression and plant immunity, suggesting that cell cycle misregulation could have an impact on expression of genes, including R genes, in plant immunity.

Assessing the diverse functions of BAK1 and its homologs in arabidopsis, beyond BR signaling and PTI responses

Plants possess a variety of extracellular leucine-rich repeats receptor-like kinases (LRR-RLKs) to coordinate developmental programs with responses to environmental changes. Out of sixteen families of LRR-RLKs in Arabidopsis, the LRR-RLKII family consists of fourteen individual members, including five Arabidopsis thaliana somatic embryogenesis receptor kinases (AtSERKs). BAK1/AtSERK3 was first identified as a dual co-receptor of BRI1 and FLS2, mediating BR signaling and pathogen-associated molecular pattern (PAMP) triggered immunity (PTI), respectively. Since its identification, many researchers have attempted to elucidate the phosphorylation mechanisms between receptor complexes and identify additional components that interact with receptor complexes to transduce the signaling downstream. Relatively detailed early events in complex formation, phosphorylation sites on the BRI1/BAK1 complex and BAK1-interacting proteins, such as BIK1 and PUB13, have been identified. Small receptor complexes consisting of BAK1 and BIR1 or BAK1 and AtSERK4 regulate cell death during steady state conditions. Moreover, the redundant and distinct functions of AtSERK proteins and other members of the LRR-RLKII family have been revealed. This review focuses on the integration of the information from the most recent studies concerning BAK1 and its homologs.

Plant innate immunity: perception of conserved microbial signatures

Plants and animals sense conserved microbial signatures through receptors localized to the plasma membrane and cytoplasm. These receptors typically carry or associate with non-arginine-aspartate (non-RD) kinases that initiate complex signaling networks cumulating in robust defense responses. In plants, coregulatory receptor kinases have been identified that not only are critical for the innate immune response but also serve an essential function in other regulatory signaling pathways.

The impact of temperature on balancing immune responsiveness and growth in Arabidopsis

Plants have evolved polymorphic immune receptors to recognize pathogens causing disease. However, triggering of resistance needs to be tuned to the local environment to maintain a balance between defense and growth. We consider here the impact of temperature as a key environmental factor influencing immune pathway activation in Arabidopsis. Genetic compensatory and molecular buffering mechanisms affecting the diversification, functionality and subcellular dynamics of immune receptors, reveal multiple points at which temperature intersects with host resistance signaling systems, including a role of at least one receptor in sensing temperature change. Analysis of temperature-dependent autoimmunity caused by allelic mismatches in hybrids of evolutionary diverged Arabidopsis accessions is illuminating processes by which plants maintain 'poise' between immune responsiveness and fitness in natural populations.

BON1 interacts with the protein kinases BIR1 and BAK1 in modulation of temperature-dependent plant growth and cell death in Arabidopsis

The Arabidopsis copine gene BON1 encodes a calcium-dependent phospholipid-binding protein involved in plant growth homeostasis and disease resistance. However, the biochemical and molecular mechanisms by which BON1 modulates plant growth and defense responses are not well understood. Here, we show that BON1 interacts physically with the leucine-rich-repeat receptor-like kinases BIR1 (BAK1-interacting receptor-like kinase 1) and pathogen-associated molecular pattern (PAMP) receptor regulator BAK1 in vitro and in vivo. Additionally, bon1 and bir1 mutants exhibit synergistic interaction. While a bir1 null mutant has similar growth and cell-death defects compared with bon1, a bir1 bon1 double mutant displays more severe phenotypes than does the single mutants. The bon1-1 and bir1-1 phenotypes are partially suppressed by overexpression of BIR1 and BON1, respectively. Furthermore, the bir1 phenotype is attenuated by a loss-of-function mutation in the resistance (R) gene SNC1 (Suppressor of npr1-1, constitutive 1), which mediates defense responses in bon1. Intriguingly, BON1 and BIR1 can be phosphorylated by BAK1 in vitro. Our findings suggest that BIR1 functions as a negative regulator of plant resistance and that BON1 and BIR1 might modulate both PAMP- and R protein-triggered immune responses.

The Arabidopsis resistance-like gene SNC1 is activated by mutations in SRFR1 and contributes to resistance to the bacterial effector AvrRps4

The SUPPRESSOR OF rps4-RLD1 (SRFR1) gene was identified based on enhanced AvrRps4-triggered resistance in the naturally susceptible Arabidopsis accession RLD. No other phenotypic effects were recorded, and the extent of SRFR1 involvement in regulating effector-triggered immunity was unknown. Here we show that mutations in SRFR1 in the accession Columbia-0 (Col-0) lead to severe stunting and constitutive expression of the defense gene PR1. These phenotypes were temperature-dependent. A cross between srfr1-1 (RLD background) and srfr1-4 (Col-0) showed that stunting was caused by a recessive locus in Col-0. Mapping and targeted crosses identified the Col-0-specific resistance gene SNC1 as the locus that causes stunting. SRFR1 was proposed to function as a transcriptional repressor, and SNC1 is indeed overexpressed in srfr1-4. Interestingly, co-regulated genes in the SNC1 cluster are also upregulated in the srfr1-4 snc1-11 double mutant, indicating that the overexpression of SNC1 is not a secondary effect of constitutive defense activation. In addition, a Col-0 RPS4 mutant showed full susceptibility to bacteria expressing avrRps4 at 24°C but not at 22°C, while RLD susceptibility was not temperature-dependent. The rps4-2 snc1-11 double mutant showed increased, but not full, susceptibility at 22°C, indicating that additional cross-talk between resistance pathways may exist. Intriguingly, when transiently expressed in Nicotiana benthamiana, SRFR1, RPS4 and SNC1 are in a common protein complex in a cytoplasmic microsomal compartment. Our results highlight SRFR1 as a convergence point in at least a subset of TIR-NBS-LRR protein-mediated immunity in Arabidopsis. Based on the cross-talk evident from our results, they also suggest that reports of constitutive resistance phenotypes in Col-0 need to consider the possible involvement of SNC1.

The Arabidopsis downy mildew resistance gene RPP8 is induced by pathogens and salicylic acid and is regulated by W box cis elements

Plants disease resistance (R) genes encode specialized receptors that are quantitative, rate-limiting defense regulators. R genes must be expressed at optimum levels to function properly. If expression is too low, downstream defense responses are not activated efficiently. Conversely, overexpression of R genes can trigger autoactivation of defenses with deleterious consequences for the plant. Little is known about R gene regulation, particularly under defense-inducing conditions. We examined regulation of the Arabidopsis thaliana gene RPP8 (resistance to Hyaloperonospora arabidopsidis, isolate Emco5). RPP8 was induced in response to challenge with H. arabidopsidis or application of salicylic acid, as shown with RPP8-Luciferase transgenic plants and quantitative reverse-transcription polymerase chain reaction of endogenous alleles. The RPP1 and RPP4 genes were also induced by H. arabidopsidis and salicylic acid, suggesting that some RPP genes are subject to feedback amplification. The RPP8 promoter contains three W box cis elements. Site-directed mutagenesis of all three W boxes greatly diminished RPP8 basal expression, inducibility, and resistance in transgenic plants. Motif searches indicated that the W box is the only known cis element that is statistically overrepresented in Arabidopsis nucleotide-binding leucine-rich repeat promoters. These results indicate that WRKY transcription factors can regulate expression of surveillance genes at the top of the defense-signaling cascade.

Copine A plays a role in the differentiation of stalk cells and the initiation of culmination in Dictyostelium development

Background

Copines are calcium-dependent phospholipid-binding proteins found in diverse eukaryotic organisms. We are studying the function of copines in Dictyostelium discoideum, a single-celled amoeba that undergoes cell differentiation and morphogenesis to form multicellular fruiting bodies when placed in starvation conditions. Previously, we showed that Dictyostelium cells lacking the copine A (cpnA) gene are not able to complete the developmental cycle, arresting at the slug stage. The aim of this study is to further characterize the developmental defect of the cpnA- cells.

Results

Time-lapse imaging revealed that cpnA- cells exhibited delayed aggregation and made large mounds that formed one large slug as compared to the smaller slugs of the wild-type cells. While the prespore cell patterning appeared to be normal within the cpnA- slugs, the prestalk cell patterning was different from wild-type. When cpnA- cells were mixed with a small percentage of wild-type cells, chimeric fruiting bodies with short stalks formed. When a small percentage of cpnA- cells was mixed with wild-type cells, the cpnA- cells labeled with GFP were found located throughout the chimeric slug and in both the stalk and sporehead of the fruiting bodies. However, there appeared to be a small bias towards cpnA- cells becoming spore cells. When cpnA- cells were developed in buffer containing EGTA, they were also able to differentiate into either stalk or spore cells to form fruiting bodies with short stalks.

Conclusions

Our results indicate that CpnA is involved in the regulation of aggregation, slug size, and culmination during Dictyostelium development. More specifically, CpnA appears to be involved in the function and differentiation of prestalk cells and plays a role in a calcium-regulated signaling pathway critical to triggering the initiation of culmination.

Temperature modulates plant defense responses through NB-LRR proteins

An elevated growth temperature often inhibits plant defense responses and renders plants more susceptible to pathogens. However, the molecular mechanisms underlying this modulation are unknown. To genetically dissect this regulation, we isolated mutants that retain disease resistance at a higher growth temperature in Arabidopsis. One such heat-stable mutant results from a point mutation in SNC1, a NB-LRR encoding gene similar to disease resistance (R) genes. Similar mutations introduced into a tobacco R gene, N, confer defense responses at elevated temperature. Thus R genes or R-like genes involved in recognition of pathogen effectors are likely the causal temperature-sensitive component in defense responses. This is further supported by snc1 intragenic suppressors that regained temperature sensitivity in defense responses. In addition, the SNC1 and N proteins had a reduction of nuclear accumulation at elevated temperature, which likely contributes to the inhibition of defense responses. These findings identify a plant temperature sensitive component in disease resistance and provide a potential means to generate plants adapting to a broader temperature range.