Formation of NPR1 Condensates Promotes Cell Survival during the Plant Immune Response.

In plants, pathogen effector-triggered immunity (ETI) often leads to programmed cell death, which is restricted by NPR1, an activator of systemic acquired resistance. However, the biochemical activities of NPR1 enabling it to promote defense and restrict cell death remain unclear. Here we show that NPR1 promotes cell survival by targeting substrates for ubiquitination and degradation through formation of salicylic acid-induced NPR1 condensates (SINCs). SINCs are enriched with stress response proteins, including nucleotide-binding leucine-rich repeat immune receptors, oxidative and DNA damage response proteins, and protein quality control machineries. Transition of NPR1 into condensates is required for formation of the NPR1-Cullin 3 E3 ligase complex to ubiquitinate SINC-localized substrates, such as EDS1 and specific WRKY transcription factors, and promote cell survival during ETI. Our analysis of SINCs suggests that NPR1 is centrally integrated into the cell death or survival decisions in plant immunity by modulating multiple stress-responsive processes in this quasi-organelle.

NPR1, a key immune regulator for plant survival under biotic and abiotic stresses.

Nonexpressor of pathogenesis-related genes 1 (NPR1) was discovered in Arabidopsis as an activator of salicylic acid (SA)-mediated immune responses nearly 30 years ago. How NPR1 confers resistance against a variety of pathogens and stresses has been extensively studied; however, only in recent years have the underlying molecular mechanisms been uncovered, particularly NPR1's role in SA-mediated transcriptional reprogramming, stress protein homeostasis, and cell survival. Structural analyses ultimately defined NPR1 and its paralogs as SA receptors. The SA-bound NPR1 dimer induces transcription by bridging two TGA transcription factor dimers, forming an enhanceosome. Moreover, NPR1 orchestrates its multiple functions through the formation of distinct nuclear and cytoplasmic biomolecular condensates. Furthermore, NPR1 plays a central role in plant health by regulating the crosstalk between SA and other defense and growth hormones. In this review, we focus on these recent advances and discuss how NPR1 can be utilized to engineer resistance against biotic and abiotic stresses.

High air humidity dampens salicylic acid pathway and NPR1 function to promote plant disease.

The occurrence of plant disease is determined by interactions among host, pathogen, and environment. Air humidity shapes various aspects of plant physiology and high humidity has long been known to promote numerous phyllosphere diseases. However, the molecular basis of how high humidity interferes with plant immunity to favor disease has remained elusive. Here we show that high humidity is associated with an "immuno-compromised" status in Arabidopsis plants. Furthermore, accumulation and signaling of salicylic acid (SA), an important defense hormone, are significantly inhibited under high humidity. NPR1, an SA receptor and central transcriptional co-activator of SA-responsive genes, is less ubiquitinated and displays a lower promoter binding affinity under high humidity. The cellular ubiquitination machinery, particularly the Cullin 3-based E3 ubiquitin ligase mediating NPR1 protein ubiquitination, is downregulated under high humidity. Importantly, under low humidity the Cullin 3a/b mutant plants phenocopy the low SA gene expression and disease susceptibility that is normally observed under high humidity. Our study uncovers a mechanism by which high humidity dampens a major plant defense pathway and provides new insights into the long-observed air humidity influence on diseases.

A vigilant gliding bird protects plants.

The plant hormone salicylic acid (SA) receptor NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1) plays a critical role for plant defense against biotrophic and hemi-biotrophic pathogens. In a milestone paper, Kumar, Zavaliev, Wu et al. unraveled the structural basis for the assembly of an enhanceosome by NPR1 in activating the expression of plant defense genes.

Puncta-localized TRAF domain protein TC1b contributes to the autoimmunity of snc1.

Immune receptors play important roles in the perception of pathogens and initiation of immune responses in both plants and animals. Intracellular nucleotide-binding domain leucine-rich repeat (NLR)-type receptors constitute a major class of receptors in vascular plants. In the Arabidopsis thaliana mutant suppressor of npr1-1, constitutive 1 (snc1), a gain-of-function mutation in the NLR gene SNC1 leads to SNC1 overaccumulation and constitutive activation of defense responses. From a CRISPR/Cas9-based reverse genetics screen in the snc1 autoimmune background, we identified that mutations in TRAF CANDIDATE 1b (TC1b), a gene encoding a protein with four tumor necrosis factor receptor-associated factor (TRAF) domains, can suppress snc1 phenotypes. TC1b does not appear to be a general immune regulator as it is not required for defense mediated by other tested immune receptors. TC1b also does not physically associate with SNC1, affect SNC1 accumulation, or affect signaling of the downstream helper NLRs represented by ACTIVATED DISEASE RESISTANCE PROTEIN 1-L2 (ADR1-L2), suggesting that TC1b impacts snc1 autoimmunity in a unique way. TC1b can form oligomers and localizes to punctate structures of unknown function. The puncta localization of TC1b strictly requires its coiled-coil (CC) domain, whereas the functionality of TC1b requires the four TRAF domains in addition to the CC. Overall, we uncovered the TRAF domain protein TC1b as a novel positive contributor to plant immunity.

A fungal CFEM-containing effector targets NPR1 regulator NIMIN2 to suppress plant immunity.

Colletotrichum fructicola causes a broad range of plant diseases worldwide and secretes many candidate proteinous effectors during infection, but it remains largely unknown regarding their effects in conquering plant immunity. Here, we characterized a novel effector CfEC12 that is required for the virulence of C. fructicola. CfEC12 contains a CFEM domain and is highly expressed during the early stage of host infection. Overexpression of CfEC12 suppressed BAX-triggered cell death, callose deposition and ROS burst in Nicotiana benthamiana. CfEC12 interacted with apple MdNIMIN2, a NIM1-interacting (NIMIN) protein that putatively modulates NPR1 activity in response to SA signal. Transient expression and transgenic analyses showed that MdNIMIN2 was required for apple resistance to C. fructicola infection and rescued the defence reduction in NbNIMIN2-silenced N. benthamiana, supporting a positive role in plant immunity. CfEC12 and MdNPR1 interacted with a common region of MdNIMIN2, indicating that CfEC12 suppresses the interaction between MdNIMIN2 and MdNPR1 by competitive target binding. In sum, we identified a fungal effector that targets the plant salicylic acid defence pathway to promote fungal infection.

SA and NHP glucosyltransferase UGT76B1 affects plant defense in both SID2- and NPR1-dependent and independent manner.

KEY MESSAGE: The small-molecule glucosyltransferase loss-of-function mutant ugt76b1 exhibits both SID2- or NPR1-dependent and independent facets of enhanced plant immunity, whereupon FMO1 is required for the SID2 and NPR1 independence. The small-molecule glucosyltransferase UGT76B1 inactivates salicylic acid (SA), isoleucic acid (ILA), and N-hydroxypipecolic acid (NHP). ugt76b1 loss-of-function plants manifest an enhanced defense status. Thus, we were interested how UGT76B1 genetically integrates in defense pathways and whether all impacts depend on SA and NHP. We study the integration of UGT76B1 by transcriptome analyses of ugt76b1. The comparison of transcripts altered by the loss of UGT76B1 with public transcriptome data reveals both SA-responsive, ISOCHORISMATE SYNTHASE 1/SALICYLIC ACID INDUCTION DEFICIENT 2 (ICS1/SID2)- and NON EXPRESSOR OF PR GENES 1 (NPR1)-dependent, consistent with the role of UGT76B1 in glucosylating SA, and SA-non-responsive, SID2/NPR1-independent genes. We also discovered that UGT76B1 impacts on a group of genes showing non-SA-responsiveness and regulation by infections independent from SID2/NPR1. Enhanced resistance of ugt76b1 against Pseudomonas syringae is partially independent from SID2 and NPR1. In contrast, the ugt76b1-activated resistance is completely dependent on FMO1 encoding the NHP-synthesizing FLAVIN-DEPENDENT MONOOXYGENASE 1). Moreover, FMO1 ranks top among the ugt76b1-induced SID2- and NPR1-independent pathogen responsive genes, suggesting that FMO1 determines the SID2- and NPR1-independent effect of ugt76b1. Furthermore, the genetic study revealed that FMO1, ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), SID2, and NPR1 are required for the SA-JA crosstalk and senescence development of ugt76b1, indicating that EDS1 and FMO1 have a similar effect like stress-induced SA biosynthesis (SID2) or the key SA signaling regulator NPR1. Thus, UGT76B1 influences both SID2/NPR1-dependent and independent plant immunity, and the SID2/NPR1 independence is relying on FMO1 and its product NHP, another substrate of UGT76B1.

Inhibition of NPR1 Leads to Shoot Growth Improvement under Low-Calcium Conditions in Arabidopsis.

Under low-Ca conditions, plants accumulate salicylic acid (SA) and induce SA-responsive genes. However, the relationship between SA and low-Ca tolerance remains unclear. Here, we demonstrated that the inhibition or suppression of nonexpressor of pathogenesis-related 1 (NPR1) activity, a major regulator of the SA signaling pathway in the defense response, improves shoot growth under low-Ca conditions. Furthermore, mutations in phytoalexin-deficient 4 (PAD4) or enhanced disease susceptibility 1 (EDS1), which are upstream regulators of NPR1, improved shoot growth under low-Ca conditions, suggesting that NPR1 suppressed growth under low-Ca conditions. In contrast, growth of SA induction-deficient 2-2 (sid2-2), which is an SA-deficient mutant, was sensitive to low Ca levels, suggesting that SA accumulation by SID2 was not related to growth inhibition under low-Ca conditions. Additionally, npr1-1 showed low-Ca tolerance, and the application of tenoxicam-an inhibitor of the NPR1-mediated activation of gene expression-also improved shoot growth under low Ca conditions. The low-Ca tolerance of double mutants pad4-1, npr1-1 and eds1-22 npr1-1 was similar to that of the single mutants, suggesting that PAD4 and EDS1 are involved in the same genetic pathway in suppressing growth under low-Ca conditions as NPR1. Cell death and low-Ca tolerance did not correlate among the mutants, suggesting that growth improvement in the mutants was not due to cell death inhibition. In conclusion, we revealed that NPR1 suppresses plant growth under low-Ca conditions and that the other SA-related genes influence plant growth and cell death.

The expression of the NPR1-dependent defense response pathway genes in Persea americana (Mill.) following infection with Phytophthora cinnamomi.

A plant's defense against pathogens involves an extensive set of phytohormone regulated defense signaling pathways. The salicylic acid (SA)-signaling pathway is one of the most well-studied in plant defense. The bulk of SA-related defense gene expression and the subsequent establishment of systemic acquired resistance (SAR) is dependent on the nonexpressor of pathogenesis-related genes 1 (NPR1). Therefore, understanding the NPR1 pathway and all its associations has the potential to provide valuable insights into defense against pathogens. The causal agent of Phytophthora root rot (PRR), Phytophthora cinnamomi, is of particular importance to the avocado (Persea americana) industry, which encounters considerable economic losses on account of this pathogen each year. Furthermore, P. cinnamomi is a hemibiotrophic pathogen, suggesting that the SA-signaling pathway plays an essential role in the initial defense response. Therefore, the NPR1 pathway which regulates downstream SA-induced gene expression would be instrumental in defense against P. cinnamomi. Thus, we identified 92 NPR1 pathway-associated orthologs from the P. americana West Indian pure accession genome and interrogated their expression following P. cinnamomi inoculation, using RNA-sequencing data. In total, 64 and 51 NPR1 pathway-associated genes were temporally regulated in the partially resistant (Dusa®) and susceptible (R0.12) P. americana rootstocks, respectively. Furthermore, 42 NPR1 pathway-associated genes were differentially regulated when comparing Dusa® to R0.12. Although this study suggests that SAR was established successfully in both rootstocks, the evidence presented indicated that Dusa® suppressed SA-signaling more effectively following the induction of SAR. Additionally, contrary to Dusa®, data from R0.12 suggested a substantial lack of SA- and NPR1-related defense gene expression during some of the earliest time-points following P. cinnamomi inoculation. This study represents the most comprehensive investigation of the SA-induced, NPR1-dependent pathway in P. americana to date. Lastly, this work provides novel insights into the likely mechanisms governing P. cinnamomi resistance in P. americana.

Salicylic acid regulates phenolic acid biosynthesis via SmNPR1-SmTGA2/SmNPR4 modules in Salvia miltiorrhiza.

Phenolic acids are the main active ingredients in Salvia miltiorrhiza, which can be used for the treatment of many diseases, particularly cardiovascular diseases. It is known that salicylic acid (SA) can enhance phenolic acid content, but the molecular mechanism of its regulation is still unclear. Nonexpresser of PR genes 1 (NPR1) plays a positive role in the SA signaling pathway. In this study, we identified a SmNPR1 gene that responds to SA induction and systematically investigated its function. We found that SmNPR1 positively affected phenolic acid biosynthesis. Then, we identified a novel TGA transcription factor, SmTGA2, which interacts with SmNPR1. SmTGA2 positively regulates phenolic acid biosynthesis by directly up-regulating SmCYP98A14 expression. After double-gene transgenic analysis and other biochemical assays, it was found that SmNPR1 and SmTGA2 work synergistically to regulate phenolic acid biosynthesis. In addition, SmNPR4 forms a heterodimer with SmNPR1 to inhibit the function of SmNPR1, and SA can alleviate this effect. Collectively, these findings elucidate the molecular mechanism underlying the regulation of phenolic acid biosynthesis by SmNPR1-SmTGA2/SmNPR4 modules and provide novel insights into the SA signaling pathway regulating plant secondary metabolism.

HOS15 is a transcriptional corepressor of NPR1-mediated gene activation of plant immunity.

Transcriptional regulation is a complex and pivotal process in living cells. HOS15 is a transcriptional corepressor. Although transcriptional repressors generally have been associated with inactive genes, increasing evidence indicates that, through poorly understood mechanisms, transcriptional corepressors also associate with actively transcribed genes. Here, we show that HOS15 is the substrate receptor for an SCF/CUL1 E3 ubiquitin ligase complex (SCFHOS15) that negatively regulates plant immunity by destabilizing transcriptional activation complexes containing NPR1 and associated transcriptional activators. In unchallenged conditions, HOS15 continuously eliminates NPR1 to prevent inappropriate defense gene expression. Upon defense activation, HOS15 preferentially associates with phosphorylated NPR1 to stimulate rapid degradation of transcriptionally active NPR1 and thus limit the extent of defense gene expression. Our findings indicate that HOS15-mediated ubiquitination and elimination of NPR1 produce effects contrary to those of CUL3-containing ubiquitin ligase that coactivate defense gene expression. Thus, HOS15 plays a key role in the dynamic regulation of pre- and postactivation host defense.

High-Density Chitosan Induces a Biochemical and Molecular Response in Coffea arabica during Infection with Hemileia vastatrix.

The coffee industry faces coffee leaf rust caused by Hemileia vastratix, which is considered the most devastating disease of the crop, as it reduces the photosynthetic rate and limits productivity. The use of plant resistance inducers, such as chitosan, is an alternative for the control of the disease by inducing the synthesis of phytoalexins, as well as the activation of resistance genes. Previously, the effect of chitosan from different sources and physicochemical properties was studied; however, its mechanisms of action have not been fully elucidated. In this work, the ability of food-grade high-density chitosan (0.01% and 0.05%) to control the infection caused by the pathogen was evaluated. Subsequently, the effect of high-density chitosan (0.05%) on the induction of pathogenesis-related gene expression (GLUC, POX, PAL, NPR1, and CAT), the enzymatic activity of pathogenesis-related proteins (GLUC, POX, SOD, PPO, and APX), and phytoalexin production were evaluated. The results showed that 0.05% chitosan increased the activity and gene expression of ß-1,3 glucanases and induced a differentiated response in enzymes related to the antioxidant system of plants. In addition, a correlation was observed between the activities of polyphenol oxidase and the production of phytoalexin, which allowed an effective defense response in coffee plants.

Overexpression of a small GTP-binding protein Ran1 in Arabidopsis leads to promoted elongation growth and enhanced disease resistance against P. syringae DC3000.

Plants resist infection through an innate immune response, which is usually associated with slowing of growth. The molecular mechanisms underlying the trade-off between plant growth and defense remain unclear. The present study reveals that growth/defense trade-offs mediated by gibberellin (GA) and salicylic acid (SA) signaling pathways are uncoupled during constitutive overexpression of transgenic AtRAN1 and AtRAN1Q72L (active, GTP-locked form) Arabidopsis plants. It is well known that the small GTP-binding protein Ran (a Ras-related nuclear protein) functions in the nucleus-cytoplasmic transport of proteins. Although there is considerable evidence indicating that nuclear-cytoplasmic partitioning of specific proteins can participate in hormone signaling, the role of Ran-dependent nuclear transport in hormone signaling is not yet fully understood. In this report, we used a combination of genetic and molecular methods to reveal whether AtRAN1 is involved in both GA and SA signaling pathways. Constitutively overexpressed AtRAN1 promoted both elongation growth and the disease resistance response, whereas overexpression of AtRAN1Q72L in the atran2atran3 double mutant background clearly inhibited elongation growth and the defense response. Furthermore, we found that AtRAN1 coordinated plant growth and defense by promoting the stability of the DELLA protein RGA in the nucleus and by modulating NPR1 nuclear localization. Interestingly, genetically modified rice (Oryza sativa) overexpressing AtRAN1 exhibited increased plant height and yield per plant. Altogether, the ability to achieve growth/defense trade-offs through AtRAN1 overexpression provides an approach to maximizing crop yield to meet rising global food demands.

The novel pathogen-responsive glycosyltransferase UGT73C7 mediates the redirection of phenylpropanoid metabolism and promotes SNC1-dependent Arabidopsis immunity.

Recent studies have shown that global metabolic reprogramming is a common event in plant innate immunity; however, the relevant molecular mechanisms remain largely unknown. Here, we identified a pathogen-induced glycosyltransferase, UGT73C7, that plays a critical role in Arabidopsis disease resistance through mediating redirection of the phenylpropanoid pathway. Loss of UGT73C7 function resulted in significantly decreased resistance to Pseudomonas syringae pv. tomato DC3000, whereas constitutive overexpression of UGT73C7 led to an enhanced defense response. UGT73C7-activated immunity was demonstrated to be dependent on the upregulated expression of SNC1, a Toll/interleukin 1 receptor-type NLR gene. Furthermore, in vitro and in vivo assays indicated that UGT73C7 could glycosylate p-coumaric acid and ferulic acid, the upstream metabolites in the phenylpropanoid pathway. Mutations that lead to the loss of UGT73C7 enzyme activities resulted in the failure to induce SNC1 expression. Moreover, glycosylation activity of UGT73C7 resulted in the redirection of phenylpropanoid metabolic flux to biosynthesis of hydroxycinnamic acids and coumarins. The disruption of the phenylpropanoid pathway suppressed UGT73C7-promoted SNC1 expression and the immune response. This study not only identified UGT73C7 as an important regulator that adjusts phenylpropanoid metabolism upon pathogen challenge, but also provided a link between phenylpropanoid metabolism and an NLR gene.

AUXIN RESPONSE FACTOR 16 (StARF16) regulates defense gene StNPR1 upon infection with necrotrophic pathogen in potato.

KEY MESSAGE: We demonstrate a new regulatory mechanism in the jasmonic acid (JA) and salicylic acid (SA) mediated crosstalk in potato defense response, wherein, miR160 target StARF16 (a gene involved in growth and development) binds to the promoter of StNPR1 (a defense gene) and negatively regulates its expression to suppress the SA pathway. Overall, our study establishes the importance of StARF16 in regulation of StNPR1 during JA mediated defense response upon necrotrophic pathogen interaction. Plants employ antagonistic crosstalk between salicylic acid (SA) and jasmonic acid (JA) to effectively defend them from pathogens. During biotrophic pathogen attack, SA pathway activates and suppresses the JA pathway via NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1). However, upon necrotrophic pathogen attack, how JA-mediated defense response suppresses the SA pathway, is still not well-understood. Recently StARF10 (AUXIN RESPONSE FACTOR), a miR160 target, has been shown to regulate SA and binds to the promoter of StGH3.6 (GRETCHEN HAGEN3), a gene proposed to maintain the balance between the free SA and auxin in plants. In the current study, we investigated the role of StARF16 (a miR160 target) in the regulation of the defense gene StNPR1 in potato upon activation of the JA pathway. We observed that a negative correlation exists between StNPR1 and StARF16 upon infection with the pathogen. The results were further confirmed through the exogenous application of SA and JA. Using yeast one-hybrid assay, we demonstrated that StARF16 binds to the StNPR1 promoter through putative ARF binding sites. Additionally, through protoplast transfection and chromatin immunoprecipitation experiments, we showed that StARF16 could bind to the StNPR1 promoter and regulate its expression. Co-transfection assays using promoter deletion constructs established that ARF binding sites are present in the 2.6 kb sequence upstream to the StNPR1 gene and play a key role in its regulation during infection. In summary, we demonstrate the importance of StARF16 in the regulation of StNPR1, and thus SA pathway, during JA-mediated defense response upon necrotrophic pathogen interaction.

Key residues for maintaining architecture, assembly of plant hormone SA receptor NPR1.

Salicylic acid (SA) is a pivotal hormone required for the development of resistance to many pathogens in plants. As an SA receptor, NPR1(Nonexpressor of Pathogenesis-Related Genes 1) plays a key regulatory role in the plant immune response. The function of NPR1 is dependent on the alteration of its oligomer-to-monomer. Research in recent years has proven that NPRs perceive SA and regulate the expression of downstream defense genes, but the mechanism of NPR1 oligomer-to-monomer conversion remains unclear. In this paper, we mainly studied the oligomerization of NPR1. By mutation experiments on some residues in the BTB domain involved in protein interactions, we found that the residue His80 plays a key role in the oligomerization of NPR1. We also found that NPR1, interacting with zinc ions at a ratio close to 1:1, was independent of the residue His80. These findings may help us to understand the conformational conversion of NPR1.

Ammonium and nitric oxide condition the establishment of Arabidopsis Ler/Kas-2 immune-related hybrid incompatibility.

MAIN CONCLUSION: High ammonium suppresses hybrid incompatibility between Ler and Kas-2 accessions through lowering nitric oxide levels and nitrate reductase activity required for autoimmunity. The immune-related hybrid incompatibility (HI) between Landsberg erecta (Ler) and Kashmir-2 (Kas-2) accessions is due to a deleterious genetic interaction between the RPP1 (RECOGNITION OF PERONOSPORA PARASITICA1)-like Ler locus and Kas-2 alleles of the receptor-like kinase SRF3 (STRUBBELIG RECEPTOR FAMILY 3). The genetic incompatibility is temperature-dependent and leads to constitutive activation of the salicylic acid (SA) pathway, dwarfism and cell death at 14-16 °C. Here we investigated the effect of nutrition on the occurrence of Ler/Kas-2 HI and found that high ammonium suppresses Ler/Kas-2 incompatible phenotypes independently of the ammonium/nitrate ratio. Ammonium feeding leads to compromised disease resistance to Pseudomonas syringae pv. tomato DC3000, lower total SA, nitric oxide and nitrate reductase activity in Ler/Kas-2 incompatible hybrids. In addition, we find that Ler/Kas-2 incompatibility is dependent on NPR1 (NONEXPRESSER OF PR GENES 1) and nitric oxide production. Overall, this work highlights the effect of nutrition on the expression of incompatible phenotypes independently of temperature.

MADS2 regulates priming defence in postharvest peach through combined salicylic acid and abscisic acid signaling.

MADS-box genes play well-documented roles in plant development, but relatively little is known regarding their involvement in defence responses. In this study, pre-treatment of peach (Prunus persica) fruit with ß-aminobutyric acid (BABA) activated resistance against Rhizopus stolonifer, leading to a significant delay in the symptomatic appearance of disease. This was associated with an integrated defence response that included a H2O2 burst, ABA accumulation, and callose deposition. cDNA library screening identified nucleus-localized MADS2 as an interacting partner with NPR1, and this was further confirmed by yeast two-hybrid, luciferase complementation imaging, and co-immunoprecipitation assays. The DNA-binding activity of NPR1 conferred by the NPR1-MADS2 complex was required for the transcription of SA-dependent pathogenesis-related (PR) and ABA-inducible CalS genes in order to gain the BABA-induced resistance, in which MAPK1-induced post-translational modification of MADS2 was also involved. In accordance with this, overexpression of PpMADS2 in Arabidopsis potentiated the transcription of a group of PR genes and conferred fungal resistance in the transgenic plants. Conversely, Arabidopsis mads2-knockout lines showed high sensitivity to the fungal pathogen. Our results indicate that MADS2 positively participates in BABA-elicited defence in peach through a combination of SA-dependent NPR1 activation and ABA signaling-induced callose accumulation, and that this defence is also related to the post-translational modification of MADS2 by MAPK1 for signal amplification.

Lack of NPR1 Increases Vascular Endothelial Adhesion through Induction of Integrin Beta 4.

Natriuretic peptide receptor 1 (NPR1) serves as a modulator of vascular endothelial homeostasis. Interactions between monocytes and endothelial cells may initiate endothelium dysfunction, which is known as an early hallmark of atherosclerosis. In this study, we performed RNA-sequencing analysis for the aorta of Npr1 knockout (Npr1+/-) mice and found that differentially expressed genes were significantly related to cell adhesion. This result was supported by an increased expression of intercellular adhesion molecule 1 (ICAM-1) in the aortic endothelium of Npr1+/- mice. Moreover, we observed that the knockdown of NPR1 increased ICAM-1 expression and promoted THP-1 monocyte adhesion to human umbilical vein endothelial cells (HUVECs). NPR1 overexpression decreased ICAM-1 expression and inhibited the adhesion of monocytes to HUVECs treated by TNF-α (a cell adhesion inducer). Further analysis showed that adhesion-related genes were enriched in the focal adhesion signaling pathway, in which integrin beta 4 (Itgb4) was determined as a key gene. Notably, ITGB4 expression increased in vascular endothelium of Npr1+/- mice and in NPR1-knockdown HUVECs. The deficiency of ITGB4 decreased ICAM-1 expression and attenuated monocyte adhesion to NPR1-knockdown endothelial cells. Additionally, a reduced NPR1 and an increased ITGB4 expression level were found in an atherosclerosis mouse model. In conclusion, our findings demonstrate that NPR1 deficiency increases vascular endothelial cell adhesion by stimulating ITGB4 expression, which may contribute to the development of atherosclerosis.

RING-Type E3 Ubiquitin Ligases AtRDUF1 and AtRDUF2 Positively Regulate the Expression of PR1 Gene and Pattern-Triggered Immunity.

The importance of E3 ubiquitin ligases from different families for plant immune signaling has been confirmed. Plant RING-type E3 ubiquitin ligases are members of the E3 ligase superfamily and have been shown to play positive or negative roles during the regulation of various steps of plant immunity. Here, we present Arabidopsis RING-type E3 ubiquitin ligases AtRDUF1 and AtRDUF2 which act as positive regulators of flg22- and SA-mediated defense signaling. Expression of AtRDUF1 and AtRDUF2 is induced by pathogen-associated molecular patterns (PAMPs) and pathogens. The atrduf1 and atrduf2 mutants displayed weakened responses when triggered by PAMPs. Immune responses, including oxidative burst, mitogen-activated protein kinase (MAPK) activity, and transcriptional activation of marker genes, were attenuated in the atrduf1 and atrduf2 mutants. The suppressed activation of PTI responses also resulted in enhanced susceptibility to bacterial pathogens. Interestingly, atrduf1 and atrduf2 mutants showed defects in SA-mediated or pathogen-mediated PR1 expression; however, avirulent Pseudomonas syringae pv. tomato DC3000-induced cell death was unaffected. Our findings suggest that AtRDUF1 and AtRDUF2 are not just PTI-positive regulators but are also involved in SA-mediated PR1 gene expression, which is important for resistance to P. syringae.