SAG101 forms a ternary complex with EDS1 and PAD4 and is required for resistance signaling against turnip crinkle virus

EDS1, PAD4, and SAG101 are common regulators of plant immunity against many pathogens. EDS1 interacts with both PAD4 and SAG101 but direct interaction between PAD4 and SAG101 has not been detected, leading to the suggestion that the EDS1-PAD4 and EDS1-SAG101 complexes are distinct. We show that EDS1, PAD4, and SAG101 are present in a single complex in planta. While this complex is preferentially nuclear localized, it can be redirected to the cytoplasm in the presence of an extranuclear form of EDS1. PAD4 and SAG101 can in turn, regulate the subcellular localization of EDS1. We also show that the Arabidopsis genome encodes two functionally redundant isoforms of EDS1, either of which can form ternary complexes with PAD4 and SAG101. Simultaneous mutations in both EDS1 isoforms are essential to abrogate resistance (R) protein-mediated defense against turnip crinkle virus (TCV) as well as avrRps4 expressing Pseudomonas syringae. Interestingly, unlike its function as a PAD4 substitute in bacterial resistance, SAG101 is required for R-mediated resistance to TCV, thus implicating a role for the ternary complex in this defense response. However, only EDS1 is required for HRT-mediated HR to TCV, while only PAD4 is required for SA-dependent induction of HRT. Together, these results suggest that EDS1, PAD4 and SAG101 also perform independent functions in HRT-mediated resistance.     

Arabidopsis SENESCENCE-ASSOCIATED GENE101 stabilizes and signals within an ENHANCED DISEASE SUSCEPTIBILITY1 complex in plant innate immunity

Plant innate immunity against invasive biotrophic pathogens depends on the intracellular defense regulator ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1). We show here that Arabidopsis thaliana EDS1 interacts in vivo with another protein, SENESCENCE-ASSOCIATED GENE101 (SAG101), discovered through a proteomic approach to identify new EDS1 pathway components. Together with PHYTOALEXIN-DEFICIENT4 (PAD4), a known EDS1 interactor, SAG101 contributes intrinsic and indispensable signaling activity to EDS1-dependent resistance. The combined activities of SAG101 and PAD4 are necessary for programmed cell death triggered by the Toll-Interleukin-1 Receptor type of nucleotide binding/leucine-rich repeat immune receptor in response to avirulent pathogen isolates and in restricting the growth of normally virulent pathogens. We further demonstrate by a combination of cell fractionation, coimmunoprecipitation, and fluorescence resonance energy transfer experiments the existence of an EDS1-SAG101 complex inside the nucleus that is molecularly and spatially distinct from EDS1-PAD4 associations in the nucleus and cytoplasm. By contrast, EDS1 homomeric interactions were detected in the cytoplasm but not inside the nucleus. These data, combined with evidence for coregulation between individual EDS1 complexes, suggest that dynamic interactions of EDS1 and its signaling partners in multiple cell compartments are important for plant defense signal relay.     

Enhanced Disease Susceptibility 1 and Salicylic Acid Act Redundantly to Regulate Resistance Gene-Mediated Signaling

Resistance (R) protein–associated pathways are well known to participate in defense against a variety of microbial pathogens. Salicylic acid (SA) and its associated proteinaceous signaling components, including enhanced disease susceptibility 1 (EDS1), non–race-specific disease resistance 1 (NDR1), phytoalexin deficient 4 (PAD4), senescence associated gene 101 (SAG101), and EDS5, have been identified as components of resistance derived from many R proteins. Here, we show that EDS1 and SA fulfill redundant functions in defense signaling mediated by R proteins, which were thought to function independent of EDS1 and/or SA. Simultaneous mutations in EDS1 and the SA–synthesizing enzyme SID2 compromised hypersensitive response and/or resistance mediated by R proteins that contain coiled coil domains at their N-terminal ends. Furthermore, the expression of R genes and the associated defense signaling induced in response to a reduction in the level of oleic acid were also suppressed by compromising SA biosynthesis in the eds1 mutant background. The functional redundancy with SA was specific to EDS1. Results presented here redefine our understanding of the roles of EDS1 and SA in plant defense.     

New insights into resistance protein-mediated signaling against turnip crinkle virus in Arabidopsis

The Arabidopsis-turnip crinkle virus (TCV) system is one of the few tractable plant-virus systems that allow simultaneous characterization of host components required for basal- and/or resistance (R) protein-mediated defenses. Another unique feature is that hypersensitive response (HR) and resistance can be studied as two distinct phenotypes in this pathosytem. The R protein HRT confers HR to TCV but requires a recessive locus rrt to confer resistance. The pathways leading to HR and resistance are mutually exclusive. HRT interacts with EDS1, which potentiates HR to TCV and is also required for resistance signaling. HRT-mediated signaling is also dependent on the EDS1-interacting proteins PAD4 and SAG101, which form binary and ternary complexes with EDS1. HRT-mediated resistance is also dependent on light and more specifically on the blue-light photoreceptors, cryptochromes (CRY) and phototropins (PHOT). Of these, CRY2 and PHOT2 are required for the stability of HRT. HRT is degraded in a proteasome-dependent manner, which correlates with its interaction with the E3 ubiquitin ligase, COP1. Together, these results suggest that components of light signaling modulate plant defense against TCV by regulating the stability of, and signaling mediated by, the R protein HRT.     

Discrimination of Arabidopsis PAD4 activities in defense against green peach aphid and pathogens

The Arabidopsis (Arabidopsis thaliana) lipase-like protein PHYTOALEXIN DEFICIENT4 (PAD4) is essential for defense against green peach aphid (GPA; Myzus persicae) and the pathogens Pseudomonas syringae and Hyaloperonospora arabidopsidis. In basal resistance to virulent strains of P. syringae and H. arabidopsidis, PAD4 functions together with its interacting partner ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) to promote salicylic acid (SA)-dependent and SA-independent defenses. By contrast, dissociated forms of PAD4 and EDS1 signal effector-triggered immunity to avirulent strains of these pathogens. PAD4-controlled defense against GPA requires neither EDS1 nor SA. Here, we show that resistance to GPA is unaltered in an eds1 salicylic acid induction deficient2 (sid2) double mutant, indicating that redundancy between EDS1 and SID2-dependent SA, previously reported for effector-triggered immunity conditioned by certain nucleotide-binding-leucine-rich repeat receptors, does not explain the dispensability of EDS1 and SID2 in defense against GPA. Mutation of a conserved serine (S118) in the predicted lipase catalytic triad of PAD4 abolished PAD4-conditioned antibiosis and deterrence against GPA feeding, but S118 was dispensable for deterring GPA settling and promoting senescence in GPA-infested plants as well as for pathogen resistance. These results highlight distinct molecular activities of PAD4 determining particular aspects of defense against aphids and pathogens.     

Phloem-based resistance to green peach aphid is controlled by Arabidopsis PHYTOALEXIN DEFICIENT4 without its signaling partner ENHANCED DISEASE SUSCEPTIBILITY1

Green peach aphid (GPA) Myzus persicae (Sülzer) is a phloem-feeding insect with an exceptionally wide host range. Previously, it has been shown that Arabidopsis thaliana PHYTOALEXIN DEFICIENT4 (PAD4), which is expressed at elevated levels in response to GPA infestation, is required for resistance to GPA in the Arabidopsis accession Columbia. We demonstrate here that the role of PAD4 in the response to GPA is conserved in Arabidopsis accessions Wassilewskija and Landsberg erecta. Electrical monitoring of aphid feeding behavior revealed that PAD4 modulates a phloem-based defense mechanism against GPA. GPA spends more time actively feeding from the sieve elements of pad4 mutants than from wild-type plants, and less time feeding on transgenic plants in which PAD4 is ectopically expressed. The activity of PAD4 in limiting phloem sap uptake serves as a deterrent in host-plant choice, and restricts aphid population size. In Arabidopsis defense against pathogens, all known PAD4 functions require its signaling and stabilizing partner EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1). Bioassays with eds1 mutants alone or in combination with pad4 and with plants conditionally expressing PAD4 under the control of a dexamethasone-inducible promoter reveal that PAD4-modulated defense against GPA does not involve EDS1. Thus, a PAD4 mode of action that is uncoupled from EDS1 determines the extent of aphid feeding in the phloem.     

Evidence of salicylic acid pathway with EDS1 and PAD4 proteins by molecular dynamics simulation for grape improvement

Biotic stress is a major cause of heavy loss in grape productivity. In order to develop biotic stress-resistant grape varieties, the key defense genes along with its pathway have to be deciphered. In angiosperm plants, lipase-like protein phytoalexin deficient 4 (PAD4) is well known to be essential for systemic resistance against biotic stress. PAD4 functions together with its interacting partner protein enhanced disease susceptibility 1 (EDS1) to promote salicylic acid (SA)-dependent and SA-independent defense pathway. Existence and structure of key protein of systemic resistance EDS1 and PAD4 are not known in grapes. Before SA pathway studies are taken in grape, molecular evidence of EDS1: PAD4 complex is to be established. To establish this, EDS1 protein sequence was retrieved from NCBI and homologous PAD4 protein was generated using Arabidopsis thaliana as template and conserved domains were confirmed. In this study, computational methods were used to model EDS1 and PAD4 and simulated the interactions of EDS1 and PAD4. Since no structural details of the proteins were available, homology modeling was employed to construct three-dimensional structures. Further, molecular dynamic simulations were performed to study the dynamic behavior of the EDS1 and PAD4. The modeled proteins were validated and subjected to molecular docking analysis. Molecular evidence of stable complex of EDS1:PAD4 in grape supporting SA defense pathway in response to biotic stress is reported in this study. If SA defense pathway genes are explored, then markers of genes involved can play pivotal role in grape variety development especially against biotic stress leading to higher productivity.     

Salicylic acid-independent ENHANCED DISEASE SUSCEPTIBILITY1 signaling in Arabidopsis immunity and cell death is regulated by the monooxygenase FMO1 and the Nudix hydrolase NUDT7

Arabidopsis thaliana ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) controls defense activation and programmed cell death conditioned by intracellular Toll-related immune receptors that recognize specific pathogen effectors. EDS1 is also needed for basal resistance to invasive pathogens by restricting the progression of disease. In both responses, EDS1, assisted by its interacting partner, PHYTOALEXIN-DEFICIENT4 (PAD4), regulates accumulation of the phenolic defense molecule salicylic acid (SA) and other as yet unidentified signal intermediates. An Arabidopsis whole genome microarray experiment was designed to identify genes whose expression depends on EDS1 and PAD4, irrespective of local SA accumulation, and potential candidates of an SA-independent branch of EDS1 defense were found. We define two new immune regulators through analysis of corresponding Arabidopsis loss-of-function insertion mutants. FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) positively regulates the EDS1 pathway, and one member (NUDT7) of a family of cytosolic Nudix hydrolases exerts negative control of EDS1 signaling. Analysis of fmo1 and nudt7 mutants alone or in combination with sid2-1, a mutation that severely depletes pathogen-induced SA production, points to SA-independent functions of FMO1 and NUDT7 in EDS1-conditioned disease resistance and cell death. We find instead that SA antagonizes initiation of cell death and stunting of growth in nudt7 mutants.     

Green peach aphid infestation induces Arabidopsis PHYTOALEXIN-DEFICIENT4 expression at site of insect feeding

The Arabidopsis thaliana PHYTOALEXIN-DEFICIENT4 (PAD4) protein, which has homology to lipases, is required for phloem-based resistance against the green peach aphid (GPA; Myzus persicae Sülzer). PAD4 modulates antibiotic and antixenotic defenses against GPA. PAD4 in conjunction with its interacting partner ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) also functions in basal resistance to bacterial and oomycete pathogens by promoting salicylic acid (SA)-dependent and SA-independent defenses. By contrast, neither EDS1 nor SA is required for PAD4-controlled defense against GPA. Distinct molecular activities of PAD4 are involved in different aspects of Arabidopsis defense against GPA and pathogens. Histochemical analysis of plants containing a PAD4_p:GUS chimera, which expresses the GUS reporter from the PAD4 promoter, indicated strong PAD4 promoter activity at the site of penetration of the vasculature by the insect stylet. GUS activity was also observed in non-vascular tissues of GPA-infested leaves, thus raising the possibility that a combination of distinct PAD4 activities in vascular and non-vascular tissues contribute to Arabidopsis defense against GPA.     

Direct interaction between the Arabidopsis disease resistance signaling proteins, EDS1 and PAD4

The Arabidopsis EDS1 and PAD4 genes encode lipase-like proteins that function in resistance (R) gene-mediated and basal plant disease resistance. Phenotypic analysis of eds1 and pad4 null mutants shows that EDS1 and PAD4 are required for resistance conditioned by the same spectrum of R genes but fulfil distinct roles within the defence pathway. EDS1 is essential for elaboration of the plant hypersensitive response, whereas EDS1 and PAD4 are both required for accumulation of the plant defence-potentiating molecule, salicylic acid. EDS1 is necessary for pathogen-induced PAD4 mRNA accumulation, whereas mutations in PAD4 or depletion of salicylic acid only partially compromise EDS1 expression. Yeast two-hybrid analysis reveals that EDS1 can dimerize and interact with PAD4. However, EDS1 dimerization is mediated by different domains to those involved in EDS1-PAD4 association. Co-immunoprecipitation experiments show that EDS1 and PAD4 proteins interact in healthy and pathogen-challenged plant cells. We propose two functions for EDS1. The first is required early in plant defence, independently of PAD4. The second recruits PAD4 in the amplification of defences, possibly by direct EDS1-PAD4 association.     

The disease resistance signaling components EDS1 and PAD4 are essential regulators of the cell death pathway controlled by LSD1 in Arabidopsis

Specific recognition of pathogens is mediated by plant disease resistance (R) genes and translated into a successful defense response. The extent of associated hypersensitive cell death varies from none to an area encompassing cells surrounding an infection site, depending on the R gene activated. We constructed double mutants in Arabidopsis between positive regulators of R function and a negative regulator of cell death, LSD1, to address whether genes required for normal R function also regulate the runaway cell death observed in lsd1 mutants. We report here that EDS1 and PAD4, two signaling genes that mediate some but not all R responses, also are required for runaway cell death in the lsd1 mutant. Importantly, this novel function of EDS1 and PAD4 is operative when runaway cell death in lsd1 is initiated through an R gene that does not require EDS1 or PAD4 for disease resistance. NDR1, another component of R signaling, also contributes to the control of plant cell death. The roles of EDS1 and PAD4 in regulating lsd1 runaway cell death are related to the interpretation of reactive oxygen intermediate-derived signals at infection sites. We further demonstrate that the fate of superoxide at infection sites is different from that observed at the leading margins of runaway cell death lesions in lsd1 mutants.     

In silico study of interaction between rice proteins enhanced disease susceptibility 1 and phytoalexin deficient 4, the regulators of salicylic acid signalling pathway

Enhanced disease susceptibility 1 (EDS1), a plant-specific protein has homology with the eukaryotic lipase in their N-terminal halves and a unique domain at its C-termini. EDS1 is known to be an important regulator of biotic stress and an essential component of basal immunity. EDS1 interacts with its positive co-regulator phytoalexin deficient 4 (PAD4), resulting in mobilization of the salicylic acid defence pathway. Limited information regarding this interaction in rice is available. To study this interaction, a model of EDS1 and PAD4 proteins from rice was generated and validated with Accelrys DS software version 3.1 using bioinformatics interface. The in silico docking between the two proteins showed a significant protein-protein interaction between rice EDS1 and PAD4, suggesting that they form a dimeric protein complex, which, similar to that in Arabidopsis, is perhaps also important for triggering the salicylic acid signalling pathway in plants.     

The chimeric Arabidopsis CYCLIC NUCLEOTIDE-GATED ION CHANNEL11/12 activates multiple pathogen resistance responses

To investigate the resistance signaling pathways activated by pathogen infection, we previously identified the Arabidopsis thaliana mutant constitutive expresser of PR genes22 (cpr22), which displays constitutive activation of multiple defense responses. Here, we identify the cpr22 mutation as a 3-kb deletion that fuses two cyclic nucleotide-gated ion channel (ATCNGC)-encoding genes, ATCNGC11 and ATCNGC12, to generate a novel chimeric gene, ATCNGC11/12. Genetic, molecular, and complementation analyses suggest that ATCNGC11/12, as well as ATCNGC11 and ATCNGC12, form functional cAMP-activated ATCNGCs and that the phenotype conferred by cpr22 is attributable to the expression of ATCNGC11/12. However, because overexpression of ATCNGC12, but not ATCNGC11, suppressed the phenotype conferred by cpr22, the development of this phenotype appears to be regulated by the ratio between ATCNGC11/12 and ATCNGC12. Analysis of knockout lines revealed that both ATCNGC11 and ATCNGC12 are positive mediators of resistance against an avirulent biotype of Hyaloperonospora parasitica. Through epistatic analyses, cpr22-mediated enhanced resistance to pathogens was found to require NDR1-dependent and EDS1/PAD4-dependent pathways. In striking contrast, none of these pathways was required for cpr22-induced salicylic acid accumulation or PR-1 gene expression. These results demonstrate that NDR1, EDS1, and PAD4 mediate other resistance signaling function(s) in addition to salicylic acid and pathogenesis-related protein accumulation. Moreover, the requirement for both NDR1-dependent and EDS1/PAD4-dependent pathways for cpr22-mediated resistance suggests that these pathways are cross-regulated.     

Genes controlling expression of defense responses in Arabidopsis.

In the past year, two regulatory defense-related genes, EDS1l and COl1, have been cloned. Several other genes with regulatory functions have been identified by mutation, including DND1, PAD4, CPR6, and SSl1. It has become clear that jasmonate signaling plays an important role in defense response signaling, and that the jasmonate and salicylic acid signaling pathways are interconnected.     

LSD1‐, EDS1‐ and PAD4‐dependent conditional correlation among salicylic acid,hydrogen peroxide,water use efficiency and seed yield in Arabidopsis thaliana

In Arabidopsis thaliana, LESION SIMULATING DISEASE 1 (LSD1), ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and PHYTOALEXIN DEFICIENT 4 (PAD4) proteins are regulators of cell death (CD) in response to abiotic and biotic stresses. Hormones, such as salicylic acid (SA), and reactive oxygen species, such as hydrogen peroxide (H₂O₂), are key signaling molecules involved in plant CD. The proposed mathematical models presented in this study suggest that LSD1, EDS1 and PAD4 together with SA and H₂O₂ are involved in the control of plant water use efficiency (WUE), vegetative growth and generative development. The analysis of Arabidopsis wild‐type and single mutants lsd1, eds1, and pad4, as well as double mutants eds1/lsd1 and pad4/lsd1, demonstrated the strong conditional correlation between SA/H₂O₂ and WUE that is dependent on LSD1, EDS1 and PAD4 proteins. Moreover, we found a strong correlation between the SA/H₂O₂ homeostasis of 4‐week‐old Arabidopsis leaves and a total seed yield of 9‐week‐old plants. Altogether, our results prove that SA and H₂O₂ are conditionally regulated by LSD1/EDS/PAD4 to govern WUE, biomass accumulation and seed yield. Conditional correlation and the proposed models presented in this study can be used as the starting points in the creation of a plant breeding algorithm that would allow to estimate the seed yield at the initial stage of plant growth, based on WUE, SA and H₂O₂ content.     

The immune components ENHANCED DISEASE SUSCEPTIBILITY 1 and PHYTOALEXIN DEFICIENT 4 are required for cell death caused by overaccumulation of ceramides in Arabidopsis

Sphingolipids have key functions in plant membrane structure and signaling. Perturbations of plant sphingolipid metabolism often induce cell death and salicylic acid (SA) accumulation; SA accumulation, in turn, promotes sphingolipid metabolism and further cell death. However, the underlying molecular mechanisms remain unclear. Here, we show that the Arabidopsis thaliana lipase-like protein ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and its partner PHYTOALEXIN DEFICIENT 4 (PAD4) participate in sphingolipid metabolism and associated cell death. The accelerated cell death 5 (acd5) mutants accumulate ceramides due to a defect in ceramide kinase and show spontaneous cell death. Loss of function of EDS1, PAD4 or SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2) in the acd5 background suppressed the acd5 cell death phenotype and prevented ceramide accumulation. Treatment with the SA analogue benzothiadiazole partially restored sphingolipid accumulation in the acd5 pad4 and acd5 eds1 double mutants, showing that the inhibitory effect of the pad4-1 and eds1-2 mutations on acd5-conferred sphingolipid accumulation partly depends on SA. Moreover, the pad4-1 and eds1-2 mutations substantially rescued the susceptibility of the acd5 mutant to Botrytis cinerea. Consistent with this, B. cinerea-induced ceramide accumulation requires PAD4 or EDS1. Finally, examination of plants overexpressing the ceramide synthase gene LAG1 HOMOLOGUE2 suggested that EDS1, PAD4 and SA are involved in long-chain ceramide metabolism and ceramide-associated cell death. Collectively, our observations reveal that EDS1 and PAD4 mediate ceramide (especially long-chain ceramide) metabolism and associated cell death, by SA-dependent and SA-independent pathways.     

Constitutive salicylic acid-dependent signaling in cpr1 and cpr6 mutants requires PAD4

Salicylic acid (SA)-dependent signaling controls activation of a set of plant defense mechanisms that are important for resistance to a variety of microbial pathogens. Many Arabidopsis mutants that display altered SA-dependent signaling have been isolated. We used double mutant analysis to determine the relative positions of the pad4, cpr1, cpr5, cpr6, dnd1 and dnd2 mutations in the signal transduction network leading to SA-dependent activation of defense gene expression and disease resistance. The pad4 mutation causes failure of SA accumulation in response to infection by certain pathogens, while the other mutations cause constitutively high levels of SA, defense gene expression and resistance. The cpr1 pad4, cpr5 pad4, cpr6 pad4, dnd1 pad4 and dnd2 pad4 double mutants were constructed and assayed for stature, presence of spontaneous lesions, resistance to Pseudomonas syringae and Peronospora parasitica, SA levels, expression of PAD4, PR-1 and PDF1.2, and accumulation of camalexin. We found that the effects of the cpr1 and cpr6 mutations on SA-dependent gene expression are completely dependent on PAD4 function. In contrast, SA accumulation in the lesion-mimic mutant cpr5 is partially PAD4-independent, while in dnd1 and dnd2 mutants it is completely PAD4-independent. A model describing a possible arrangement of activities in the signal transduction network is presented.