New clues in the nucleus: transcriptional reprogramming in effector-triggered immunity

StoMYB41 positively regulates the Solanum torvum response to Verticillium dahliae in an ABA dependent manner

MYB transcription factor despite their solid involvement in growth are potent regulator of plant stress response. Herein, we identified a MYB gene named as StoMYB41 in a wild eggplant species Solanum torvum. The expression level of StoMYB41 was higher in root than the tissues including stem, leaf, and seed. It induced significantly by Verticillium dahliae inoculation. StoMYB41 was localized in the nucleus and exhibited transcriptional activation activity. Silencing of StoMYB41 enhanced susceptibility of Solanum torvum against Verticillium dahliae, accompanied by higher disease index. The significant down-regulation of resistance marker gene StoABR1 comparing to the control plants was recorded in the silenced plants. Moreover, transient expression of StoMYB41 could trigger intense hypersensitive reaction mimic cell death, darker DAB and trypan blue staining, higher ion leakage, and induced the expression levels of StoABR1 and NbDEF1 in the leaves of Solanum torvum and Nicotiana benthamiana. Taken together, our data indicate that StoMYB41 acts as a positive regulator in Solanum torvum against Verticillium wilt.

SlWRKY16 and SlWRKY31 of tomato, negative regulators of plant defense, involved in susceptibility activation following root-knot nematode Meloidogyne javanica infection

The involvement of WRKY transcription factors in plant-nematode interactions, and in particular, how these WRKYs participate in regulating the complex morphological and physiological changes occurring after nematode infection, are the topic of active research. We characterized the functional role of the unstudied tomato WRKY genes SlWRKY16 and SlWRKY31 in regulating tomato roots' response to infection by the root-knot nematode Meloidogyne javanica. Using promoter-GUS reporter gene fusions and qRT-PCR, we show that both SlWRKYs are predominantly expressed during the first half of the parasitic life stages, when feeding-site induction and construction occur. Expression of SlWRKY16 increased sharply 15 days after inoculation, whereas SlWRKY31 was already induced earlier, but reached its maximum expression at this time. Both genes were downregulated at the mature female stage. To determine biological function, we produced transgenic lines overexpressing SlWRKY16 and SlWRKY31 in tomato hairy roots. Overexpression of both genes resulted in enhanced M. javanica infection, reflected by increased galling occurrence and reproduction. Expression profiling of marker genes responsive to defense-associated phytohormones indicated reductions in salicylic acid defense-related PR-1 and jasmonic acid defense-related PI in inoculated roots overexpressing SlWRK16 and SlWRKY31, respectively. Our results suggest that SlWRKY16 and SlWRKY31 function as negative regulators of plant immunity induced upon nematode infection.

Jasmonate regulates plant resistance to Pectobacterium brasiliense by inducing indole glucosinolate biosynthesis

Pectobacterium brasiliense (P. brasiliense) is a necrotrophic bacterium that causes the soft rot disease in Brassica rapa. However, the mechanisms underlying plant immune responses against necrotrophic bacterial pathogens with a broad host range are still not well understood. Using a flg22-triggered seedling growth inhibition (SGI) assay with 455 Brassica rapa inbred lines, we selected six B. rapa flagellin-insensitive lines (Brfin2-7) and three B. rapa flagellin-sensitive lines (Brfs1-3). Brfin lines showed compromised flg22-induced immune responses (oxidative burst, mitogen-activated protein kinase (MAPK) activation, and seedling growth inhibition) compared to the control line R-o-18; nevertheless, they were resistant to P. brasiliense. To explain this, we analyzed the phytohormone content and found that most Brfin lines had higher P. brasiliense-induced jasmonic acid (JA) than Brfs lines. Moreover, MeJA pretreatment enhanced the resistance of B. rapa to P. brasiliense. To explain the correlation between the resistance of Brfin lines to P. brasiliense and activated JA signaling, we analyzed pathogen-induced glucosinolate (GS) content in B. rapa. Notably, in Brfin7, the neoglucobrassicin (NGBS) content among indole glucosinolates (IGS) was significantly higher than that in Brfs2 following P. brasiliense inoculation, and genes involved in IGSs biosynthesis were also highly expressed. Furthermore, almost all Brfin lines with high JA levels and resistance to P. brasiliense had higher P. brasiliense-induced NGBS levels than Brfs lines. Thus, our results show that activated JA-mediated signaling attenuates flg22-triggered immunity but enhances resistance to P. brasiliense by inducing indole glucosinolate biosynthesis in Brassica rapa. This study provides novel insights into the role of JA-mediated defense against necrotrophic bacterial pathogens within a broad host range.

The Jujube TCP Transcription Factor ZjTCP16 Regulates Plant Growth and Cell Size by Affecting the Expression of Genes Involved in Plant Morphogenesis

Jujube production is threatened by jujube witches’ broom (JWB) disease, which is caused by JWB phytoplasma. The jujube TCP transcription factor (TF) ZjTCP16 may be involved in the interaction of jujube plants with JWB phytoplasma. In this study, qRT-PCR proved that the expression pattern of ZjTCP16 was altered by JWB phytoplasma. The gene functions of ZjTCP16 were analyzed by its overexpression in Arabidopsis and jujube, as well as knock-down in. The overexpression of ZjTCP16 in Arabidopsis and jujube resulted in dwarfism and small leaves, while the zjtcp16 CRISPR mutants were higher than the WT. Microscopic observation of paraffin sections of jujube stems showed that ZjTCP16 affected the size of cells. The interactions of ZjTCP16 with ZjAS2 and ZjLOB in both the cytoplasm and nucleus were demonstrated by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. Yeast one-hybrid (Y1H) assays and qRT-PCR further confirmed that ZjTCP16 affected the expression of genes involved in leaf morphogenesis and cell proliferation (ZjAS1, ZjKNAT1, ZjKNAT2 and ZjKNAT6) at the mRNA level through the ZjAS2 and ZjLOB pathways. In conclusion, ZjTCP16 regulates plant growth and cell size by altering the expression pattern of morphogenesis-related genes in jujube.

Recent Advances in Effector-Triggered Immunity in Plants: New Pieces in the Puzzle Create a Different Paradigm

Plants rely on multiple immune systems to protect themselves from pathogens. When pattern-triggered immunity (PTI)-the first layer of the immune response-is no longer effective as a result of pathogenic effectors, effector-triggered immunity (ETI) often provides resistance. In ETI, host plants directly or indirectly perceive pathogen effectors via resistance proteins and launch a more robust and rapid defense response. Resistance proteins are typically found in the form of nucleotide-binding and leucine-rich-repeat-containing receptors (NLRs). Upon effector recognition, an NLR undergoes structural change and associates with other NLRs. The dimerization or oligomerization of NLRs signals to downstream components, activates "helper" NLRs, and culminates in the ETI response. Originally, PTI was thought to contribute little to ETI. However, most recent studies revealed crosstalk and cooperation between ETI and PTI. Here, we summarize recent advancements in our understanding of the ETI response and its components, as well as how these components cooperate in the innate immune signaling pathways. Based on up-to-date accumulated knowledge, this review provides our current perspective of potential engineering strategies for crop protection.

CaCBL1 Acts as a Positive Regulator in Pepper Response to Ralstonia solanacearum

Bacterial wilt caused by Ralstonia solanacearum is an important disease of pepper (Capsicum annuum), an economically important solanaceous vegetable worldwide, in particular, under high temperature (HT) conditions. However, the molecular mechanism underlying pepper immunity against bacterial wilt remains poorly understood. Herein, CaCBL1, a putative calcineurin B-like protein, was functionally characterized in the pepper response to R. solanacearum inoculation (RSI) under HT (RSI/HT). CaCBL1 was significantly upregulated by RSI at room temperature (RSI/RT), HT, or RSI/HT. CaCBL1-GFP fused protein targeted to whole epidermal cells of Nicotiana benthamiana when transiently overexpressed. CaCBL1 silencing by virus-induced gene silencing significantly enhanced pepper susceptibility to RSI under RT or HT, while its transient overexpression triggered hypersensitive response mimic cell death and upregulation of immunity-associated marker genes, including CabZIP63, CaWRKY40, and CaCDPK15, the positive regulators in the pepper response to RSI or HT found in our previous studies. In addition, by chromatin immunoprecipitation PCR and electrophoretic mobility shift assay, CaCBL1 was found to be directly targeted by CaWRKY40, although not by CaWRKY27 or CaWRKY58, via the W-box-2 within its promoter, and its transcription was found to be downregulated by silencing of CaWRKY40 while it was enhanced by its transient overexpression. These results suggest that CaCBL1 acts as a positive regulator in pepper immunity against R. solanacearum infection, constituting a positive feedback loop with CaWRKY40.

Blast resistance gene Pi54 over-expressed in rice to understand its cellular and sub-cellular localization and response to different pathogens

Rice blast resistance gene, Pi54 provides broad-spectrum resistance against different strains of Magnaporthe oryzae. Understanding the cellular localization of Pi54 protein is an essential step towards deciphering its place of interaction with the cognate Avr-gene. In this study, we investigated the sub-cellular localization of Pi54 with Green Fluorescent Protein (GFP) as a molecular tag through transient and stable expression in onion epidermal cells (Allium cepa) and susceptible japonica cultivar rice Taipei 309 (TP309), respectively. Confocal microscopy based observations of the onion epidermal cells revealed nucleus and cytoplasm specific GFP signals. In the stable transformed rice plants, GFP signal was recorded in the stomata, upper epidermal cells, mesophyll cells, vascular bundle, and walls of bundle sheath and bulliform cells of leaf tissues. These observations were further confirmed by Immunocytochemical studies. Using GFP specific antibodies, it was found that there was sufficient aggregation of GFP::Pi54protein in the cytoplasm of the leaf mesophyll cells and periphery of the epidermal cells. Interestingly, the transgenic lines developed in this study could show a moderate level of resistance to Xanthomonas oryzae and Rhizoctonia solani, the causal agents of the rice bacterial blight and sheath blight diseases, respectively. This study is a first detailed report, which emphasizes the cellular and subcellular distribution of the broad spectrum blast resistance gene Pi54 in rice and the impact of its constitutive expression towards resistance against other fungal and bacterial pathogens of rice.

The Plant Noncanonical Antiviral Resistance Protein JAX1 Inhibits Potexviral Replication by Targeting the Viral RNA-Dependent RNA Polymerase

Understanding the innate immune mechanisms of plants is necessary for the breeding of disease-resistant lines. Previously, we identified the antiviral resistance gene JAX1 from Arabidopsis thaliana, which inhibits infection by potexviruses. JAX1 encodes a unique jacalin-type lectin protein. In this study, we analyzed the molecular mechanisms of JAX1-mediated resistance. JAX1 restricted the multiplication of a potexviral replicon lacking movement-associated proteins, suggesting inhibition of viral replication. Therefore, we developed an in vitro potato virus X (PVX) translation/replication system using vacuole- and nucleus-free lysates from tobacco protoplasts, and we revealed that JAX1 inhibits viral RNA synthesis but not the translation of the viral RNA-dependent RNA polymerase (RdRp). JAX1 did not affect the replication of a resistance-breaking mutant of PVX. Blue native polyacrylamide gel electrophoresis of fractions separated by sucrose gradient sedimentation showed that PVX RdRp constituted the high-molecular-weight complex that seems to be crucial for viral replication. JAX1 was detected in this complex of the wild-type PVX replicon but not in that of the resistance-breaking mutant. In addition, JAX1 interacted with the RdRp of the wild-type virus but not with that of a virus with a point mutation at the resistance-breaking residue. These results suggest that JAX1 targets RdRp to inhibit potexviral replication.IMPORTANCE Resistance genes play a crucial role in plant antiviral innate immunity. The roles of conventional nucleotide-binding leucine-rich repeat (NLR) proteins and the associated defense pathways have long been studied. In contrast, recently discovered resistance genes that do not encode NLR proteins (non-NLR resistance genes) have not been investigated extensively. Here we report that the non-NLR resistance factor JAX1, a unique jacalin-type lectin protein, inhibits de novo potexviral RNA synthesis by targeting the huge complex of viral replicase. This is unlike other known antiviral resistance mechanisms. Molecular elucidation of the target in lectin-type protein-mediated antiviral immunity will enhance our understanding of the non-NLR-mediated plant resistance system.

CaLRR-RLK1, a novel RD receptor-like kinase from Capsicum annuum and transcriptionally activated by CaHDZ27, act as positive regulator in Ralstonia solanacearum resistance

BACKGROUND

Bacterial wilt caused by Ralstonia solanacearum is one of the most important diseases in pepper worldwide, however, the molecular mechanism underlying pepper resistance to bacterial wilt remains poorly understood.

RESULTS

Herein, a novel RD leucine-rich repeat receptor-like kinase, CaLRR-RLK1, was functionally characterized in immunity against R. solanacearum. CaLRR-RLK1 was targeted exclusively to plasma membrane and was up-regulated by R. solanacearum inoculation (RSI) as well as by the exogenous application of salicylic acid (SA), methyl jasmonate (MeJA) or ethephon (ETH). The silencing of CaLRR-RLK1 led to enhanced susceptibility of pepper plants to RSI, accompanied by down-regulation of immunity-related genes including CaACO1, CaHIR1, CaPR4 and CaPO2. In contrast, transient overexpression of CaLRR-RLK1 triggered hypersensitive response (HR)-like cell death and H2O2 accumulation in pepper leaves, manifested by darker trypan blue and DAB staining respectively. In addition, the ectopic overexpression of CaLRR-RLK1 in tobacco plants enhanced resistance R. solanacearum, accompanied with the immunity associated marker genes including NtPR2, NtPR2, NtHSR203 and NtHSR515. Furthermore, it was found that CaHDZ27, a positive regulator in pepper response to RSI in our previous study, transcriptionally activated CaLRR-RLK1 by direct targeting its promoter probably in a CAATTATTG dependent manner.

CONCLUSION

The study revealed that CaLRR-RLK1 confers pepper resistance to R. solanacearum as the direct targeting of CaHDZ27.

Capsicum annuum HsfB2a Positively Regulates the Response to Ralstonia solanacearum Infection or High Temperature and High Humidity Forming Transcriptional Cascade with CaWRKY6 and CaWRKY40

The responses of pepper (Capsicum annuum) plants to inoculation with the pathogenic bacterium Ralstonia solanacearum and to high-temperature-high-humidity (HTHH) conditions were previously found to be coordinated by the transcription factors CaWRKY6 and CaWRKY40; however, the underlying molecular mechanism was unclear. Herein, we identified and functionally characterized CaHsfB2a, a nuclear-localized heat shock factor involved in pepper immunity to R. solanacearum inoculation (RSI) and tolerance to HTHH. CaHsfB2a is transcriptionally induced in pepper plants by RSI or HTHH and by exogenous application of salicylic acid (SA), methyl jasmonate (MeJA), ethylene (ETH), or abscisic acid (ABA). Virus-induced gene silencing (VIGS) of CaHsfB2a significantly impaired pepper immunity to RSI, hampered HTHH tolerance, and curtailed expression of immunity- and thermotolerance-associated marker genes such as CaHIR1, CaNPR1, CaABR1, and CaHSP24. Likewise, transient overexpression of CaHsfB2a in pepper leaves induced hypersensitive response (HR)-like cell death and H2O2 accumulation and upregulated the above-mentioned marker genes as well as CaWRKY6 and CaWRKY40. Chromatin immunoprecipitation (ChIP) and microscale thermophoresis (MST) analysis revealed that CaHsfB2a bound the promoters of both CaWRKY6 and CaWRKY40. In a parallel experiment, we determined by ChIP-PCR and MST that CaHsfB2a was regulated directly by CaWRKY40 but indirectly by CaWRKY6. Cumulatively, our results suggest that CaHsfB2a positively regulates plant immunity against RSI and tolerance to HTHH, via transcriptional cascades and positive feedback loops involving CaWRKY6 and CaWRKY40.

CaWRKY40b in Pepper Acts as a Negative Regulator in Response to Ralstonia solanacearum by Directly Modulating Defense Genes Including CaWRKY40

WRKY transcription factors (TFs) have been implicated in plant growth, development, and in response to environmental cues; however, the function of the majority of pepper WRKY TFs remains unclear. In the present study, we functionally characterized CaWRKY40b, a homolog of AtWRKY40, in pepper immunity. Ralstonia solanacearum inoculation (RSI) in pepper plants resulted in downregulation of CaWRKY40b transcript, and green fluorescent protein (GFP)-tagged CaWRKY40b was localized to the nuclei when transiently overexpressed in the leaves of Nicotiana benthamiana. Virus-induced gene silencing (VIGS) of CaWRKY40b significantly decreased pepper’ susceptibility to RSI. Consistently, the transient over-expression of CaWRKY40b-SRDX (chimeric repressor version of CaWRKY40b) triggered cell death, as indicated by darker trypan blue and DAB staining. CaWRKY40b targets a number of immunity-associated genes, including CaWRKY40 JAR, RLK1, EIN3, FLS2, CNGIC8, CDPK13, and heat shock cognate protein 70 (HSC70), which were identified by ChIP-seq and confirmed using ChIP-real time PCR. Among these target genes, the negative regulator HSC70 was upregulated by transient overexpression of CaWRKY40b and downregulated by silencing of CaWRKY40b, whereas other positive regulators as well as two non-target genes, CaNPR1 and CaDEF1, were downregulated by the transient overexpression of CaWRKY40b and upregulated by CaWRKY40b silencing or transient overexpression of CaWRKY40b-SRDX. In addition, CaWRKY40b exhibited a positive feedback regulation at transcriptional level by directly targeting the promoter of itself. In conclusion, the findings of the present study suggest that CaWRKY40b acts as a negative regulator in pepper immunity against R. solanacearum by transcriptional modulation of a subset of immunity-associated genes; it also represses immunity in the absence of a pathogen, and derepresses immunity upon pathogen challenge.

MOS6 and TN13 in plant immunity

The Arabidopsis nuclear transport receptor IMPORTIN-α3/MOS6 (MODIFIER OF SNC1, 6) is required for constitutive defense responses of the auto-immune mutant snc1 (suppressor of npr1-1, constitutive 1) and contributes to basal disease resistance, suggesting a role in nuclear import of defense-regulatory cargo proteins. We recently showed that MOS6 selectively interacts with TN13, a TIR-NBS protein involved in basal resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 lacking the effectors AvrPto and AvrPtoB. Consistent with a predicted N-terminal transmembrane domain, TN13 localizes to the endoplasmic reticulum (ER) and the nuclear envelope (NE) where it interacts with MOS6 in a transient expression assay. Here, we propose a model that summarizes the subcellular localization, association and function of TN13 and MOS6 in plant defense signaling.

Functional diversification of structurally alike NLR proteins in plants

In due course of evolution many pathogens alter their effector molecules to modulate the host plants' metabolism and immune responses triggered upon proper recognition by the intracellular nucleotide-binding oligomerization domain containing leucine-rich repeat (NLR) proteins. Likewise, host plants have also evolved with diversified NLR proteins as a survival strategy to win the battle against pathogen invasion. NLR protein indeed detects pathogen derived effector proteins leading to the activation of defense responses associated with programmed cell death (PCD). In this interactive process, genome structure and plasticity play pivotal role in the development of innate immunity. Despite being quite conserved with similar biological functions in all eukaryotes, the intracellular NLR immune receptor proteins happen to be structurally distinct. Recent studies have made progress in identifying transcriptional regulatory complexes activated by NLR proteins. In this review, we attempt to decipher the intracellular NLR proteins mediated surveillance across the evolutionarily diverse taxa, highlighting some of the recent updates on NLR protein compartmentalization, molecular interactions before and after activation along with insights into the finer role of these receptor proteins to combat invading pathogens upon their recognition. Latest information on NLR sensors, helpers and NLR proteins with integrated domains in the context of plant pathogen interactions are also discussed.

Functional and Promoter Analysis of ChiIV3, a Chitinase of Pepper Plant, in Response to Phytophthora capsici Infection

Despite the involvement of many members of the chitinase family in plant immunity, the precise functions of the majority of the members remain poorly understood. Herein, the gene ChiIV3 in Capsicum annuum encoding a chitinase protein containing a chitin binding domain and targeting to the plasma membrane was found to be induced by Phytophthora capsici inoculation (PCI) and applied chitin treatment. Besides its direct inhibitory effect on growth of Phytophthora capsici (P. capsici), ChiIV3 was also found by virus-induced gene silencing (VIGS) and transient overexpression (TOE) in pepper plants to act as a positive regulator of plant cell death and in triggering defense signaling and upregulation of PR (pathogenesis related) genes against PCI. A 5' deletion assay revealed that pChiIV3-712 to -459 bp was found to be sufficient for ChiIV3' response to PCI. Furthermore, a mutation assay indicated that W-box-466 to -461 bp in pChiIV3-712 to -459 bp was noted to be the PCI-responsible element. These results collectively suggest that ChiIV3 acts as a likely antifungal protein and as a receptor for unidentified chitin in planta to trigger cell death and defense signaling against PCI.

The Verticillium-specific protein VdSCP7 localizes to the plant nucleus and modulates immunity to fungal infections

Fungal pathogens secrete effector proteins to suppress plant basal defense for successful colonization. Resistant plants, however, can recognize effectors by cognate R proteins to induce effector-triggered immunity (ETI). By analyzing secretomes of the vascular fungal pathogen Verticillium dahliae, we identified a novel secreted protein VdSCP7 that targets the plant nucleus. The green fluorescent protein (GFP)-tagged VdSCP7 gene with either a mutated nuclear localization signal motif or with additional nuclear export signal was transiently expressed in Nicotiana benthamiana, and investigated for induction of plant immunity. The role of VdSCP7 in V. dahliae pathogenicity was characterized by gene knockout and complementation, and GFP labeling. Expression of the VdSCP7 gene in N. benthamiana activated both salicylic acid and jasmonate signaling, and altered the plant's susceptibility to the pathogens Botrytis cinerea and Phytophthora capsici. The immune response activated by VdSCP7 was highly dependent on its initial extracellular secretion and subsequent nuclear localization in plants. Knockout of the VdSCP7 gene significantly enhanced V. dahliae aggressiveness on cotton. GFP-labeled VdSCP7 is secreted by V. dahliae and accumulates in the plant nucleus. We conclude that VdSCP7 is a novel effector protein that targets the host nucleus to modulate plant immunity, and suggest that plants can recognize VdSCP7 to activate ETI during fungal infection.

Nucleoporin NUP88/MOS7 is required for manifestation of phenotypes associated with the Arabidopsis CHITIN ELICITOR RECEPTOR KINASE1 mutant cerk1-4

Arabidopsis nucleoporin MOS7/NUP88 was identified in a forward-genetic screen for components that contribute to auto-immunity of the deregulated Resistance (R) gene mutant snc1, and is required for immunity to biotrophic and hemi-biotrophic pathogens. In a recent study, we showed that MOS7 is also essential to mount a full defense response against the necrotrophic fungal pathogen Botrytis cinerea, suggesting that MOS7 modulates plant defense responses to different types of pathogenic microbes. Here, we extend our analyses of MOS7-dependent plant immune responses and report the genetic requirement of MOS7 for manifestation of phenotypes associated with the CHITIN ELICITOR RECEPTOR KINASE1 (CERK1) mutant cerk1-4.

Structure-informed insights for NLR functioning in plant immunity

To respond to foreign invaders, plants have evolved a cell autonomous multilayered immune system consisting of extra- and intracellular immune receptors. Nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs) mediate recognition of pathogen effectors inside the cell and trigger a host specific defense response, often involving controlled cell death. NLRs consist of a central nucleotide-binding domain, which is flanked by an N-terminal CC or TIR domain and a C-terminal leucine-rich repeat domain (LRR). These multidomain proteins function as a molecular switch and their activity is tightly controlled by intra and inter-molecular interactions. In contrast to metazoan NLRs, the structural basis underlying NLR functioning as a pathogen sensor and activator of immune responses in plants is largely unknown. However, the first crystal structures of a number of plant NLR domains were recently obtained. In addition, biochemical and structure-informed analyses revealed novel insights in the cooperation between NLR domains and the formation of pre- and post activation complexes, including the coordinated activity of NLR pairs as pathogen sensor and executor of immune responses. Moreover, the discovery of novel integrated domains underscores the structural diversity of NLRs and provides alternative models for how these immune receptors function in plants. In this review, we will highlight these recent advances to provide novel insights in the structural, biochemical and molecular aspects involved in plant NLR functioning.

Express yourself: Transcriptional regulation of plant innate immunity

The plant immune system is a complex network of components that function together to sense the presence and activity of potential biotic threats, and integrate these signals into an appropriate output, namely the transcription of genes that activate an immune response that is commensurate with the perceived threat. Given the variety of biotic threats a plant must face the immune response must be plastic, but because an immune response is costly to the plant in terms of energy expenditure and development it must also be under tight control. To meet these needs transcriptional control is exercised at multiple levels. In this article we will review some of the latest developments in understanding how the plant immune response is regulated at the level of transcription. New roles are being discovered for the long-studied WRKY and TGA transcription factor families, while additional critical defense functions are being attributed to TCPs and other transcription factors. Dynamically controlling access to DNA through post-translational modification of histones is emerging as an essential component of priming, maintaining, attenuating, and repressing transcription in response to biotic stress. Unsurprisingly, the plant's transcriptional response is targeted by pathogen effectors, and in turn resistance proteins stand guard over and participate in transcriptional regulation. Together, these multiple layers lead to the observed complexity of the plant transcriptional immune response, with different transcription factors or chromatin components playing a prominent role depending on the plant-pathogen interaction being studied.

The Arabidopsis immune regulator SRFR1 dampens defences against herbivory by Spodoptera exigua and parasitism by Heterodera schachtii

Plants have developed diverse mechanisms to fine tune defence responses to different types of enemy. Cross-regulation between signalling pathways may allow the prioritization of one response over another. Previously, we identified SUPPRESSOR OF rps4-RLD1 (SRFR1) as a negative regulator of ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1)-dependent effector-triggered immunity against the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 expressing avrRps4. The use of multiple stresses is a powerful tool to further define gene function. Here, we examined whether SRFR1 also impacts resistance to a herbivorous insect in leaves and to a cyst nematode in roots. Interestingly, srfr1-1 plants showed increased resistance to herbivory by the beet army worm Spodoptera exigua and to parasitism by the cyst nematode Heterodera schachtii compared with the corresponding wild-type Arabidopsis accession RLD. Using quantitative real-time PCR (qRT-PCR) to measure the transcript levels of salicylic acid (SA) and jasmonate/ethylene (JA/ET) pathway genes, we found that enhanced resistance of srfr1-1 plants to S. exigua correlated with specific upregulation of the MYC2 branch of the JA pathway concurrent with suppression of the SA pathway. In contrast, the greater susceptibility of RLD was accompanied by simultaneously increased transcript levels of SA, JA and JA/ET signalling pathway genes. Surprisingly, mutation of either SRFR1 or EDS1 increased resistance to H. schachtii, indicating that the concurrent presence of both wild-type genes promotes susceptibility. This finding suggests a novel form of resistance in Arabidopsis to the biotrophic pathogen H. schachtii or a root-specific regulation of the SA pathway by EDS1, and places SRFR1 at an intersection between multiple defence pathways.

Go in for the kill: How plants deploy effector-triggered immunity to combat pathogens. [Corrected]

Plant resistance (R) proteins perceive specific pathogen effectors from diverse plant pathogens to initiate defense responses, designated effector-triggered immunity (ETI). Plant R proteins are mostly nucleotide binding-leucine rich repeat (NB-LRR) proteins, which recognize pathogen effectors directly or indirectly through sophisticated mechanisms. Upon activation by effector proteins, R proteins elicit robust defense responses, including a rapid burst of reactive oxygen species (ROS), induced biosynthesis and accumulation of salicylic acid (SA), a rapid programmed cell death (PCD) called hypersensitive response (HR) at the infection sites, and increased expression of pathogenesis-related (PR) genes. Initiation of ETI is correlated with a complex network of defense signaling pathways, resulting in defensive cellular responses and large-scale transcriptional reprogramming events. In this review, we highlight important recent advances on the recognition of effectors, regulation and activation of plant R proteins, dynamic intracellular trafficking of R proteins, induction of cell death, and transcriptional reprogramming associated with ETI. Current knowledge gaps and future research directions are also discussed in this review.

Transcriptional networks in plant immunity

Next to numerous abiotic stresses, plants are constantly exposed to a variety of pathogens within their environment. Thus, their ability to survive and prosper during the course of evolution was strongly dependent on adapting efficient strategies to perceive and to respond to such potential threats. It is therefore not surprising that modern plants have a highly sophisticated immune repertoire consisting of diverse signal perception and intracellular signaling pathways. This signaling network is intricate and deeply interconnected, probably reflecting the diverse lifestyles and infection strategies used by the multitude of invading phytopathogens. Moreover it allows signal communication between developmental and defense programs thereby ensuring that plant growth and fitness are not significantly retarded. How plants integrate and prioritize the incoming signals and how this information is transduced to enable appropriate immune responses is currently a major research area. An important finding has been that pathogen-triggered cellular responses involve massive transcriptional reprogramming within the host. Additional key observations emerging from such studies are that transcription factors (TFs) are often sites of signal convergence and that signal-regulated TFs act in concert with other context-specific TFs and transcriptional co-regulators to establish sensory transcription regulatory networks required for plant immunity.

TCP three-way handshake: linking developmental processes with plant immunity

The TCP gene family encodes plant-specific transcription factors involved in growth and development. Equally important are the interactions between TCP factors and other pathways extending far beyond development, as they have been found to regulate a variety of hormonal pathways and signaling cascades. Recent advances reveal that TCP factors are targets of pathogenic effectors and are likely to play a vital role in plant immunity. Our focus is on reviewing the involvement of TCP in known pathways and shedding light on other linkages in the nexus of plant immunity centered around TCP factors with an emphasis on the convergence of effectors, interconnected hormonal networks, utility of the circadian clock, and the potential mechanisms by which pathogen defense may occur.

Arabidopsis flowering locus D influences systemic-acquired-resistance- induced expression and histone modifications of WRKY genes

A plant that is in part infected by a pathogen is more resistant throughout its whole body to subsequent infections--a phenomenon known as systemic acquired resistance (SAR). Mobile signals are synthesized at the site of infection and distributed throughout the plant through vascular tissues. Mechanism of SAR development subsequent to reaching the mobile signal in the distal tissue is largely unknown. Recently we showed that flowering locus D (FLD) gene of Arabidopsis thaliana is required in the distal tissue to activate SAR. FLD codes for a homologue of human-lysine-specific histone demethylase. Here we show that FLD function is required for priming (SAR induced elevated expression during challenge inoculation) of WRKY29 and WRKY6 genes. FLD also differentially influences basal and SAR-induced expression of WRKY38, WRKY65 and WRKY53 genes. In addition, we also show that FLD partly localizes in nucleus and influences histone modifications at the promoters of WRKY29 and WRKY6 genes. The results altogether indicate to the possibility of FLD's involvement in epigenetic regulation of SAR.

The blast resistance gene Pi54of cloned from Oryza officinalis interacts with Avr-Pi54 through its novel non-LRR domains

The dominant rice blast resistance gene Pi54 cloned by map-based cloning approach from indica rice cultivar Tetep confers broad spectrum resistance to Magnaporthe oryzae. In this investigation, an orthologue of Pi54 designated as Pi54of was cloned from Oryza officinalis conferring high degree of resistance to M. oryzae and is functionally validated. We have also characterized the Pi54of protein and demonstrate its interaction with AVR-Pi54 protein. The Pi54of encoded ∼43 kDa small and unique cytoplasmic LRR family of disease resistance protein having unique Zinc finger domain overlapped with the leucine rich repeat regions. Pi54of showed Magnaporthe-induced expression. The phylogenetic and western blot analysis confirmed orthologous nature of Pi54 and Pi54of genes, whereas the identity of protein was confirmed through MALDI-TOF analysis. The in silico analysis showed that Pi54of is structurally more stable than other cloned Pi54 proteins. The molecular docking revealed that Pi54of protein interacts with AVR-Pi54 through novel non-LRR domains such as STI1 and RhoGEF. The STI1 and GEF domains which interact with AVR-Pi54 are also components of rice defensome complex. The Pi54of protein showed differential domain specificity while interacting with the AVR protein. Functional complementation revealed that Pi54of transferred in two rice lines belonging to indica and japonica background imparts enhanced resistance against three highly virulent strains of M. oryzae. In this study, for the first time, we demonstrated that a rice blast resistance gene Pi54of cloned from wild species of rice provides high degree of resistance to M. oryzae and might display different molecular mechanism involved in AVRPi54-Pi54of interaction.

The conformational and subcellular compartmental dance of plant NLRs during viral recognition and defense signaling

Plant innate immune response against viruses utilizes intracellular Nucleotide Binding domain Leucine Rich Repeat (NLR) class of receptors. NLRs recognize different viral proteins termed elicitors and initiate diverse signaling processes that induce programmed cell death (PCD) in infected cells and restrict virus spread. In this review we describe the recent advances made in the study of plant NLRs that detect viruses. We describe some of the physical and functional interactions these NLRs undertake. We elaborate on the intra-molecular and homotypic association of NLRs that function in self-regulation and activation. Nuclear role for some viral NLRs is discussed as well as the emerging importance of the RNAi pathway in regulating the NLR family.

The Arabidopsis immune adaptor SRFR1 interacts with TCP transcription factors that redundantly contribute to effector-triggered immunity

The plant immune system must be tightly controlled both positively and negatively to maintain normal plant growth and health. We previously identified SUPPRESSOR OF rps4-RLD1 (SRFR1) as a negative regulator specifically of effector-triggered immunity. SRFR1 is localized in both a cytoplasmic microsomal compartment and in the nucleus. Its TPR domain has sequence similarity to TPR domains of transcriptional repressors in other organisms, suggesting that SRFR1 may negatively regulate effector-triggered immunity via transcriptional control. We show here that excluding SRFR1 from the nucleus prevented complementation of the srfr1 phenotype. To identify transcription factors that interact with SRFR1, we screened an Arabidopsis transcription factor prey library by yeast two-hybrid assay and isolated six class I members of the TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factor family. Specific interactions were verified in planta. Although single or double T-DNA mutant tcp8, tcp14 or tcp15 lines were not more susceptible to bacteria expressing AvrRps4, the triple tcp8 tcp14 tcp15 mutant displayed decreased effector-triggered immunity mediated by the resistance genes RPS2, RPS4, RPS6 and RPM1. In addition, expression of PATHOGENESIS-RELATED PROTEIN2 was attenuated in srfr1-4 tcp8-1 tcp14-5 tcp15-3 plants compared to srfr1-4 plants. To date, TCP transcription factors have been implicated mostly in developmental processes. Our data indicate that one function of a subset of TCP proteins is to regulate defense gene expression in antagonism to SRFR1, and suggest a mechanism for an intimate connection between plant development and immunity.

WRKY transcription factors: Jack of many trades in plants

WRKY transcription factors are one of the largest families of transcriptional regulators found exclusively in plants. They have diverse biological functions in plant disease resistance, abiotic stress responses, nutrient deprivation, senescence, seed and trichome development, embryogenesis, as well as additional developmental and hormone-controlled processes. WRKYs can act as transcriptional activators or repressors, in various homo- and heterodimer combinations. Here we review recent progress on the function of WRKY transcription factors in Arabidopsis and other plant species such as rice, potato, and parsley, with a special focus on abiotic, developmental, and hormone-regulated processes.

The blast resistance gene Pi54of cloned from Oryza officinalis interacts with Avr-Pi54 through its novel non-LRR domains

The dominant rice blast resistance gene Pi54 cloned by map-based cloning approach from indica rice cultivar Tetep confers broad spectrum resistance to Magnaporthe oryzae. In this investigation, an orthologue of Pi54 designated as Pi54of was cloned from Oryza officinalis conferring high degree of resistance to M. oryzae and is functionally validated. We have also characterized the Pi54of protein and demonstrate its interaction with AVR-Pi54 protein. The Pi54of encoded ∼43 kDa small and unique cytoplasmic LRR family of disease resistance protein having unique Zinc finger domain overlapped with the leucine rich repeat regions. Pi54of showed Magnaporthe-induced expression. The phylogenetic and western blot analysis confirmed orthologous nature of Pi54 and Pi54of genes, whereas the identity of protein was confirmed through MALDI-TOF analysis. The in silico analysis showed that Pi54of is structurally more stable than other cloned Pi54 proteins. The molecular docking revealed that Pi54of protein interacts with AVR-Pi54 through novel non-LRR domains such as STI1 and RhoGEF. The STI1 and GEF domains which interact with AVR-Pi54 are also components of rice defensome complex. The Pi54of protein showed differential domain specificity while interacting with the AVR protein. Functional complementation revealed that Pi54of transferred in two rice lines belonging to indica and japonica background imparts enhanced resistance against three highly virulent strains of M. oryzae. In this study, for the first time, we demonstrated that a rice blast resistance gene Pi54of cloned from wild species of rice provides high degree of resistance to M. oryzae and might display different molecular mechanism involved in AVRPi54-Pi54of interaction.