Immune receptor complexes at the plant cell surface

Alternaria solani core effector Aex59 is a new member of the Alt a 1 protein family and is recognized as a PAMP

Early blight caused by Alternaria solani is a destructive disease in potato production. Here, through systematically screening of an effector protein pool consisting of 115 small cysteine-containing candidate Aex (Alternariaextracellular proteins) in A. solani, we identified a core effector protein named Aex59, a pathogen-associated molecular pattern (PAMP) molecule. Aex59 is uniquely present in the Ascomycota of fungi and can activate defense responses in multiple plants. Targeted gene disruption showed that Aex59 is a virulence factor and participates in spore development. Perception of Aex59 in Nicotiana benthamiana does not depend on the receptor-like kinases Brassinosteroid-associated kinase1 (BAK1) and Suppressor of BIR1-1 (SOBIR1), which are required for multiple pattern recognition receptors (PRR) pathways. Sequence analysis revealed that Aex59 is a new member of the Alt a 1 protein family and is a potential molecular marker capable of aiding in the classification of the fungi Alternaria spp.

Fine-tuning of the dual-role transcription factor WRKY8 via differential phosphorylation for robust broad-spectrum plant immunity

Plants perceive pathogen-associated molecular patterns (PAMPs) using plasma-membrane-localized pattern recognition receptors (PRRs) to activate broad-spectrum pattern-triggered immunity. However, the regulatory mechanisms that ensure robust broad-spectrum plant immunity remain largely unknown. Here, we reveal that the transcription factor WRKY8 has a dual role in the transcriptional regulation of PRR genes: repressing expression of the nlp20/nlp24 receptor gene RLP23 while promoting that of the chitin receptor gene CERK1. SsNLP1 and SsNLP2, two nlp24-type PAMPs from the destructive fungal pathogen Sclerotinia sclerotiorum, activate two calcium-elicited kinases, CPK4 and CPK11, which phosphorylate WRKY8 and thus release its inhibition on RLP23 to promote accumulation of RLP23 transcripts. Meanwhile, SsNLPs activate the RLCK-type kinase PBL19, which phosphorylates WRKY8 and thus enhances accumulation of CERK1 transcripts. Intriguingly, RLP23 is repressed at later stage by PBL19-mediated phosphorylation of WRKY8, thus avoiding excessive immunity and enabling normal growth. Our findings unveil a plant strategy of "killing two birds with one stone" to elicit robust broad-spectrum immunity. This strategy is based on PAMP-triggered fine-tuning of a dual-role transcription factor to simultaneously amplify two PRRs that recognize PAMPs conserved across a wide range of pathogens. Moreover, our results reveal a novel plant strategy for balancing the trade-off between growth and immunity by fine-tuning the expression of multiple PRR genes.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

A new chromosome-scale genome of wild Brassica oleracea provides insights into the domestication of Brassica crops

The cultivated diploid Brassica oleracea is an important vegetable crop, but the genetic basis of its domestication remains largely unclear in the absence of high-quality reference genomes of wild B. oleracea. Here, we report the first chromosome-level assembly of the wild Brassica oleracea L. W03 genome (total genome size, 630.7 Mb; scaffold N50, 64.6 Mb). Using the newly assembled W03 genome, we constructed a gene-based B. oleracea pangenome and identified 29 744 core genes, 23 306 dispensable genes, and 1896 private genes. We re-sequenced 53 accessions, representing six potential wild B. oleracea progenitor species. The results of the population genomic analysis showed that the wild B. oleracea populations had the highest level of diversity and represents the most closely related population to modern-day horticultural B. oleracea. In addition, the WUSCHEL gene was found to play a decisive role in domestication and to be involved in cauliflower and broccoli curd formation. We also illustrate the loss of disease-resistance genes during selection for domestication. Our results provide new insights into the domestication of B. oleracea and will facilitate the future genetic improvement of Brassica crops.

Release of a ubiquitin brake activates OsCERK1-triggered immunity in rice

Plant pattern-recognition receptors perceive microorganism-associated molecular patterns to activate immune signalling1,2. Activation of the pattern-recognition receptor kinase CERK1 is essential for immunity, but tight inhibition of receptor kinases in the absence of pathogen is crucial to prevent autoimmunity3,4. Here we find that the U-box ubiquitin E3 ligase OsCIE1 acts as a molecular brake to inhibit OsCERK1 in rice. During homeostasis, OsCIE1 ubiquitinates OsCERK1, reducing its kinase activity. In the presence of the microorganism-associated molecular pattern chitin, active OsCERK1 phosphorylates OsCIE1 and blocks its E3 ligase activity, thus releasing the brake and promoting immunity. Phosphorylation of a serine within the U-box of OsCIE1 prevents its interaction with E2 ubiquitin-conjugating enzymes and serves as a phosphorylation switch. This phosphorylation site is conserved in E3 ligases from plants to animals. Our work identifies a ligand-released brake that enables dynamic immune regulation.

Salivary Protein Cyclin-Dependent Kinase-like from Grain Aphid Sitobion avenae Suppresses Wheat Defense Response and Enhances Aphid Adaptation

Aphids are insect pests that suck phloem sap and introduce salivary proteins into plant tissues through saliva secretion. The effector of salivary proteins plays a key role in the modulation of host plant defense responses and enhancing aphid host adaptation. Based on previous transcriptome sequencing results, a candidate effector cyclin-dependent kinase-like (CDK) was identified from the grain aphid Sitobion avenae. In this study, the function of SaCDK in wheat defense response and the adaptation of S. avenae was investigated. Our results showed that the transient overexpression of SaCDK in tobacco Nicotiana benthamiana suppressed cell death triggered by mouse pro-apoptotic protein-BAX or Phytophthora infestans PAMP-INF1. SaCDK, delivered into wheat cells through a Pseudomonas fluorescens-mediated bacterial type III secretion system, suppressed callose deposition in wheat seedlings, and the overexpression of SaCDK in wheat significantly decreased the expression levels of salicylic acid and jasmonic acid signaling pathway-related genes phenylalanine ammonia lyase (PAL), pathogenesis-related 1 protein (PR1), lipoxygenase (LOX) and Ω-3 fatty acid desaturase (FAD). In addition, aphid bioassay results showed that the survival and fecundity of S. avenae were significantly increased while feeding on the wheat plants carrying SaCDK. Taken together, our findings demonstrate that the salivary protein SaCDK is involved in inhibiting host defense response and improving its host adaptation, which lays the foundation to uncover the mechanism of the interaction of cereal aphids and host plants.

Functional screening of the Arabidopsis 2C protein phosphatases family identifies PP2C15 as a negative regulator of plant immunity by targeting BRI1-associated receptor kinase 1

Genetic engineering using negative regulators of plant immunity has the potential to provide a huge impetus in agricultural biotechnology to achieve a higher degree of disease resistance without reducing yield. Type 2C protein phosphatases (PP2Cs) represent the largest group of protein phosphatases in plants, with a high potential for negative regulatory functions by blocking the transmission of defence signals through dephosphorylation. Here, we established a PP2C functional protoplast screen using pFRK1::luciferase as a reporter and found that 14 of 56 PP2Cs significantly inhibited the immune response induced by flg22. To verify the reliability of the system, a previously reported MAPK3/4/6-interacting protein phosphatase, PP2C5, was used; it was confirmed to be a negative regulator of PAMP-triggered immunity (PTI). We further identified PP2C15 as an interacting partner of BRI1-associated receptor kinase 1 (BAK1), which is the most well-known co-receptor of plasma membrane-localized pattern recognition receptors (PRRs), and a central component of PTI. PP2C15 dephosphorylates BAK1 and negatively regulates BAK1-mediated PTI responses such as MAPK3/4/6 activation, defence gene expression, reactive oxygen species bursts, stomatal immunity, callose deposition, and pathogen resistance. Although plant growth and 1000-seed weight of pp2c15 mutants were reduced compared to those of wild-type plants, pp2c5 mutants did not show any adverse effects. Thus, our findings strengthen the understanding of the mechanism by which PP2C family members negatively regulate plant immunity at multiple levels and indicate a possible approach to enhance plant resistance by eliminating specific PP2Cs without affecting plant growth and yield.

Receptor-like kinases OsRLK902-1 and OsRLK902-2 form immune complexes with OsRLCK185 to regulate rice blast resistance

Receptor-like kinases (RLKs) are major regulators of the plant immune response and play important roles in the perception and transmission of immune signals. RECEPTOR LIKE KINASE 902 (RLK902) is at the key node in leucine-rich repeat receptor-like kinase interaction networks and positively regulates resistance to the bacterial pathogen Pseudomonas syringae in Arabidopsis. However, the function of RLK902 in fungal disease resistance remains obscure. In this study, we found that the expression levels of OsRLK902-1 and OsRLK902-2, encoding two orthologues of RLK902 in rice, were induced by Magnaporthe oryzae, chitin, and flg22 treatment. osrlk902-1 and osrlk902-2 knockout mutants displayed enhanced susceptibility to M. oryzae. Interestingly, the osrlk902-1 rlk902-2 double mutant exhibited similar disease susceptibility, hydrogen peroxide production, and callose deposition to the two single mutants. Further investigation showed that OsRLK902-1 interacts with and stabilizes OsRLK902-2. The two OsRLKs form a complex with OsRLCK185, a key regulator in chitin-triggered immunity, and stabilize it. Taken together, our data demonstrate that OsRLK902-1 and OsRLK902-2, as well as OsRLCK185 function together in regulating disease resistance to M. oryzae in rice.

SERKs and NIKs: Coreceptors or signaling hubs in a complex crosstalk between growth and defense?

SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES (SERKs) and NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASES (NIKs) belong to superfamily II of leucine-rich repeat receptor-like kinases, which share cytosolic kinase conservation and a similar ectodomain configuration. SERKs have been extensively demonstrated to function as coreceptors of receptor-like kinases, which sense biotic or developmental signals to initiate specific responses. NIKs, on the other hand, have emerged as downstream components in signaling cascades, not functioning as coreceptors but rather serving as hubs that converge information from both biotic and abiotic signals, resulting in a unified response. Like SERKs, NIKs play a crucial role as information spreaders in plant cells, forming hubs of high centrality. However, unlike SERKs, which function as coreceptors and assemble paired receptor-specific responses, NIKs employ a shared signaling circuit to transduce diverse biotic and abiotic signals into the same physiological response. Therefore, this review highlights the concept of signaling hubs that differ from coreceptors in signaling pathways.

Dual phosphorylation of DGK5-mediated PA burst regulates ROS in plant immunity

Phosphatidic acid (PA) and reactive oxygen species (ROS) are crucial cellular messengers mediating diverse signaling processes in metazoans and plants. How PA homeostasis is tightly regulated and intertwined with ROS signaling upon immune elicitation remains elusive. We report here that Arabidopsis diacylglycerol kinase 5 (DGK5) regulates plant pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). The pattern recognition receptor (PRR)-associated kinase BIK1 phosphorylates DGK5 at Ser-506, leading to a rapid PA burst and activation of plant immunity, whereas PRR-activated intracellular MPK4 phosphorylates DGK5 at Thr-446, which subsequently suppresses DGK5 activity and PA production, resulting in attenuated plant immunity. PA binds and stabilizes the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), regulating ROS production in plant PTI and ETI, and their potentiation. Our data indicate that distinct phosphorylation of DGK5 by PRR-activated BIK1 and MPK4 balances the homeostasis of cellular PA burst that regulates ROS generation in coordinating two branches of plant immunity.

Host-pathogen interaction between pitaya and Neoscytalidium dimidiatum reveals the mechanisms of immune response associated with defense regulators and metabolic pathways

BACKGROUND

Understanding how plants and pathogens regulate each other's gene expression during their interactions is key to revealing the mechanisms of disease resistance and controlling the development of pathogens. Despite extensive studies on the molecular and genetic basis of plant immunity against pathogens, the influence of pitaya immunity on N. dimidiatum metabolism to restrict pathogen growth is poorly understood, and how N. dimidiatum breaks through pitaya defenses. In this study, we used the RNA-seq method to assess the expression profiles of pitaya and N. dimidiatum at 4 time periods after interactions to capture the early effects of N. dimidiatum on pitaya processes.

RESULTS

The study defined the establishment of an effective method for analyzing transcriptome interactions between pitaya and N. dimidiatum and to obtain global expression profiles. We identified gene expression clusters in both the host pitaya and the pathogen N. dimidiatum. The analysis showed that numerous differentially expressed genes (DEGs) involved in the recognition and defense of pitaya against N. dimidiatum, as well as N. dimidiatum's evasion of recognition and inhibition of pitaya. The major functional groups identified by GO and KEGG enrichment were responsible for plant and pathogen recognition, phytohormone signaling (such as salicylic acid, abscisic acid). Furthermore, the gene expression of 13 candidate genes involved in phytopathogen recognition, phytohormone receptors, and the plant resistance gene (PG), as well as 7 effector genes of N. dimidiatum, including glycoside hydrolases, pectinase, and putative genes, were validated by qPCR. By focusing on gene expression changes during interactions between pitaya and N. dimidiatum, we were able to observe the infection of N. dimidiatum and its effects on the expression of various defense components and host immune receptors.

CONCLUSION

Our data show that various regulators of the immune response are modified during interactions between pitaya and N. dimidiatum. Furthermore, the activation and repression of these genes are temporally coordinated. These findings provide a framework for better understanding the pathogenicity of N. dimidiatum and its role as an opportunistic pathogen. This offers the potential for a more effective defense against N. dimidiatum.

The Globodera rostochiensis Gr29D09 Effector with a Role in Defense Suppression Targets the Potato Hexokinase 1 Protein

The potato cyst nematode (Globodera rostochiensis) is an obligate root pathogen of potatoes. G. rostochiensis encodes several highly expanded effector gene families, including the Gr4D06 family; however, little is known about the function of this effector family. We cloned four 29D09 genes from G. rostochiensis (named Gr29D09v1/v2/v3/v4) that share high sequence similarity and are homologous to the Hg29D09 and Hg4D06 effector genes from the soybean cyst nematode (Heterodera glycines). Phylogenetic analysis revealed that Gr29D09 genes belong to a subgroup of the Gr4D06 family. We showed that Gr29D09 genes are expressed exclusively within the nematode's dorsal gland cell and are dramatically upregulated in parasitic stages, indicating involvement of Gr29D09 effectors in nematode parasitism. Transgenic potato lines overexpressing Gr29D09 variants showed increased susceptibility to G. rostochiensis. Transient expression assays in Nicotiana benthamiana demonstrated that Gr29D09v3 could suppress reactive oxygen species (ROS) production and defense gene expression induced by flg22 and cell death mediated by immune receptors. These results suggest a critical role of Gr29D09 effectors in defense suppression. The use of affinity purification coupled with nanoliquid chromatography-tandem mass spectrometry identified potato hexokinase 1 (StHXK1) as a candidate target of Gr29D09. The Gr29D09-StHXK1 interaction was further confirmed using in planta protein-protein interaction assays. Plant HXKs have been implicated in defense regulation against pathogen infection. Interestingly, we found that StHXK1 could enhance flg22-induced ROS production, consistent with a positive role of plant HXKs in defense. Altogether, our results suggest that targeting StHXK1 by Gr29D09 effectors may impair the positive function of StHXK1 in plant immunity, thereby aiding nematode parasitism. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

General Regulatory Factor7 regulates innate immune signalling to enhance Verticillium wilt resistance in cotton

Sessile growing plants are always vulnerable to microbial pathogen attacks throughout their lives. To fend off pathogen invasion, plants have evolved a sophisticated innate immune system that consists of cell surface receptors and intracellular receptors. Somatic embryogenesis receptor kinases (SERKs) belong to a small group of leucine-rich repeat receptor-like kinases (LRR-RLKs) that function as co-receptors regulating diverse physiological processes. GENRAL REGULATORY FACTOR (GRF) proteins play an important role in physiological signalling transduction. However, the function of GRF proteins in plant innate immune signalling remains elusive. Here, we identified a GRF gene, GauGRF7, that is expressed both constitutively and in response to fungal pathogen infection. Intriguingly, silencing of GRF7 compromised plant innate immunity, resulting in susceptibility to Verticillium dahliae infection. Both transgenic GauGRF7 cotton and transgenic GauGRF7 Arabidopsis lines enhanced the innate immune response to V. dahliae infection, leading to high expression of two helper NLRs (hNLR) genes (ADR1 and NRG1) and pathogenesis-related genes, and increased ROS production and salicylic acid level. Moreover, GauGRF7 interacted with GhSERK1, which positively regulated GRF7-mediated innate immune response in cotton and Arabidopsis. Our findings revealed the molecular mechanism of the GRF protein in plant immune signaling and offer potential opportunities for improving plant resistance to V. dahliae infection.

The CAP superfamily protein PsCAP1 secreted by Phytophthora triggers immune responses in Nicotiana benthamiana through a leucine-rich repeat receptor-like protein

The role of cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 (CAP) superfamily proteins in the innate immune responses of mammals is well characterized. However, the biological function of CAP superfamily proteins in plant-microbe interactions is poorly understood. We used proteomics and transcriptome analyses to dissect the apoplastic effectors secreted by the oomycete Phytophthora sojae during early infection of soybean leaves. By transiently expressing these effectors in Nicotiana benthamiana, we identified PsCAP1, a novel type of secreted CAP protein that triggers immune responses in multiple solanaceous plants including N. benthamiana. This secreted CAP protein is conserved among oomycetes, and multiple PsCAP1 homologs can be recognized by N. benthamiana. PsCAP1-triggered immune responses depend on the N-terminal immunogenic fragment (aa 27-151). Pretreatment of N. benthamiana with PsCAP1 or the immunogenic fragment increases plant resistance against Phytophthora. The recognition of PsCAP1 and different homologs requires the leucine-rich repeat receptor-like protein RCAP1, which associates with two central receptor-like kinases BRI1-associated receptor kinase 1 (BAK1) and suppressor of BIR1-1 (SOBIR1) in planta. These findings suggest that the CAP-type apoplastic effectors act as an important player in plant-microbe interactions that can be perceived by plant membrane-localized receptor to activate plant resistance.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

LysM Proteins TaCEBiP and TaLYK5 are Involved in Immune Responses Mediated by Chitin Coreceptor TaCERK1 in Wheat

Plant lysin motif (LysM) ectodomain receptors interact with pathogen-associated molecular patterns (PAMPs) and have critical functions in plant-microbe interactions. In this study, 65 LysM family genes were identified using the recent version of the reference sequence of bread wheat (Triticum aestivum), in which 23, 16, 20, and 6 members belonged to LysM-containing receptor-like kinases (LYKs), LysM-containing receptor-like proteins (LYPs), extracellular LysM proteins (LysMes), and intracellular nonsecretory LysM proteins (LysMns), respectively. The study found that TaCEBiP, TaLYK5, and TaCERK1 were highly responsive to PAMP elicitors and phytopathogens, with TaCEBiP and TaLYK5 binding directly to chitin. TaCERK1 acted as a coreceptor with TaCEBiP and TaLYK5 at the plasma membrane. Overexpression of TaCEBiP, TaLYK5, and TaCERK1 in Nicotiana benthamiana leaves exhibited enhanced resistance to Sclerotinia sclerotiorum. Subsequently, knocking down TaCEBiP, TaLYK5, and TaCERK1 genes with barley stripe mosaic virus-VIGS compromised the wheat defense response to an avirulent strain of Puccinia striiformis. The study concluded that wheat has two synergistic chitin perception systems for detecting pathogen elicitors, with the activated CERK1 intracellular kinase domain leading to signaling transduction. This research provides valuable insights into the functional roles and regulatory mechanisms of wheat LysM members under biotic stress.

A Phytophthora receptor-like kinase regulates oospore development and can activate pattern-triggered plant immunity

Plant cell-surface leucine-rich repeat receptor-like kinases (LRR-RLKs) and receptor-like proteins (LRR-RLPs) form dynamic complexes to receive a variety of extracellular signals. LRR-RLKs are also widespread in oomycete pathogens, whereas it remains enigmatic whether plant and oomycete LRR-RLKs could mediate cell-to-cell communications between pathogen and host. Here, we report that an LRR-RLK from the soybean root and stem rot pathogen Phytophthora sojae, PsRLK6, can activate typical pattern-triggered immunity in host soybean and nonhost tomato and Nicotiana benthamiana plants. PsRLK6 homologs are conserved in oomycetes and also exhibit immunity-inducing activity. A small region (LRR5-6) in the extracellular domain of PsRLK6 is sufficient to activate BAK1- and SOBIR1-dependent immune responses, suggesting that PsRLK6 is likely recognized by a plant LRR-RLP. Moreover, PsRLK6 is shown to be up-regulated during oospore maturation and essential for the oospore development of P. sojae. Our data provide a novel type of microbe-associated molecular pattern that functions in the sexual reproduction of oomycete, and a scenario in which a pathogen LRR-RLK could be sensed by a plant LRR-RLP to mount plant immunity.

Biochemical and Biotechnological Insights into Fungus-Plant Interactions for Enhanced Sustainable Agricultural and Industrial Processes

The literature is full of studies reporting environmental and health issues related to using traditional pesticides in food production and storage. Fortunately, alternatives have arisen in the last few decades, showing that organic agriculture is possible and economically feasible. And in this scenario, fungi may be helpful. In the natural environment, when associated with plants, these microorganisms offer plant-growth-promoting molecules, facilitate plant nutrient uptake, and antagonize phytopathogens. It is true that fungi can also be phytopathogenic, but even they can benefit agriculture in some way-since pathogenicity is species-specific, these fungi are shown to be useful against weeds (as bioherbicides). Finally, plant-associated yeasts and molds are natural biofactories, and the metabolites they produce while dwelling in leaves, flowers, roots, or the rhizosphere have the potential to be employed in different industrial activities. By addressing all these subjects, this manuscript comprehensively reviews the biotechnological uses of plant-associated fungi and, in addition, aims to sensitize academics, researchers, and investors to new alternatives for healthier and more environmentally friendly production processes.

Plant lysin motif extracellular proteins are required for arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal fungi (AMF) can form a mutually beneficial symbiotic relationship with most land plants. They are known to secrete lysin motif (LysM) effectors into host root cells for successful colonization. Intriguingly, plants secrete similar types of LysM proteins; however, their role in plant-microbe interactions is unknown. Here, we show that Medicago truncatula deploys LysM extracellular (LysMe) proteins to facilitate symbiosis with AMF. Promoter analyses demonstrated that three M. truncatula LysMe genes MtLysMe1/2/3, are expressed in arbuscule-containing cells and those adjacent to intercellular hyphae. Localization studies showed that these proteins are targeted to the periarbuscular space between the periarbuscular membrane and the fungal cell wall of the branched arbuscule. M. truncatula mutants in which MtLysMe2 was knocked out via CRISPR/Cas9-targeted mutagenesis exhibited a significant reduction in AMF colonization and arbuscule formation, whereas genetically complemented transgenic plants restored wild-type level AMF colonization. In addition, knocking out the ortholog of MtLysMe2 in tomato resulted in a similar defect in AMF colonization. In vitro binding affinity precipitation assays suggested binding of MtLysMe1/2/3 with chitin and chitosan, while microscale thermophoresis (MST) assays revealed weak binding of these proteins with chitooligosaccharides. Moreover, application of purified MtLysMe proteins to root segments could suppress chitooctaose (CO8)-induced reactive oxygen species production and expression of reporter genes of the immune response without impairing chitotetraose (CO4)-triggered symbiotic responses. Taken together, our results reveal that plants, like their fungal partners, also secrete LysM proteins to facilitate symbiosis establishment.

Structural mechanism of heavy metal-associated integrated domain engineering of paired nucleotide-binding and leucine-rich repeat proteins in rice

Plant nucleotide-binding and leucine-rich repeat (NLR) proteins are immune sensors that detect pathogen effectors and initiate a strong immune response. In many cases, single NLR proteins are sufficient for both effector recognition and signaling activation. These proteins possess a conserved architecture, including a C-terminal leucine-rich repeat (LRR) domain, a central nucleotide-binding (NB) domain, and a variable N-terminal domain. Nevertheless, many paired NLRs linked in a head-to-head configuration have now been identified. The ones carrying integrated domains (IDs) can recognize pathogen effector proteins by various modes; these are known as sensor NLR (sNLR) proteins. Structural and biochemical studies have provided insights into the molecular basis of heavy metal-associated IDs (HMA IDs) from paired NLRs in rice and revealed the co-evolution between pathogens and hosts by combining naturally occurring favorable interactions across diverse interfaces. Focusing on structural and molecular models, here we highlight advances in structure-guided engineering to expand and enhance the response profile of paired NLR-HMA IDs in rice to variants of the rice blast pathogen MAX-effectors (Magnaporthe oryzae AVRs and ToxB-like). These results demonstrate that the HMA IDs-based design of rice materials with broad and enhanced resistance profiles possesses great application potential but also face considerable challenges.

Pan-genome of Citrullus genus highlights the extent of presence/absence variation during domestication and selection

The rich genetic diversity in Citrullus lanatus and the other six species in the Citrullus genus provides important sources in watermelon breeding. Here, we present the Citrullus genus pan-genome based on the 400 Citrullus genus resequencing data, showing that 477 Mb contigs and 6249 protein-coding genes were absent in the Citrullus lanatus reference genome. In the Citrullus genus pan-genome, there are a total of 8795 (30.5%) genes that exhibit presence/absence variations (PAVs). Presence/absence variation (PAV) analysis showed that a lot of gene PAV were selected during the domestication and improvement, such as 53 favorable genes and 40 unfavorable genes were identified during the C. mucosospermus to C. lanatus landrace domestication. We also identified 661 resistance gene analogs (RGAs) in the Citrullus genus pan-genome, which contains 90 RGAs (89 variable and 1 core gene) located on the pangenome additional contigs. By gene PAV-based GWAS, 8 gene presence/absence variations were found associated with flesh color. Finally, based on the results of gene PAV selection analysis between watermelon populations with different fruit colors, we identified four non-reference candidate genes associated with carotenoid accumulation, which had a significantly higher frequency in the white flesh. These results will provide an important source for watermelon breeding.

Identification of a damage-associated molecular pattern (DAMP) receptor and its cognate peptide ligand in sweet potato (Ipomoea batatas)

Sweet potato (Ipomoea batatas) is an important tuber crop, but also target of numerous insect pests. Intriguingly, the abundant storage protein in tubers, sporamin, has intrinsic trypsin protease inhibitory activity. In leaves, sporamin is induced by wounding or a volatile homoterpene and enhances insect resistance. While the signalling pathway leading to sporamin synthesis is partially established, the initial event, perception of a stress-related signal is still unknown. Here, we identified an IbLRR-RK1 that is induced upon wounding and herbivory, and related to peptide-elicitor receptors (PEPRs) from tomato and Arabidopsis. We also identified a gene encoding a precursor protein comprising a peptide ligand (IbPep1) for IbLRR-RK1. IbPep1 represents a distinct signal in sweet potato, which might work in a complementary and/or parallel pathway to the previously described hydroxyproline-rich systemin (HypSys) peptides to strengthen insect resistance. Notably, an interfamily compatibility in the Pep/PEPR system from Convolvulaceae and Solanaceae was identified.

Peptidoglycan recognition in Drosophila is mediated by LysMD3/4

Microbial recognition is a key step in regulating the immune signaling pathways of multicellular organisms. Peptidoglycan, a component of the bacterial cell wall, exhibits immune stimulating activity in both plants and animals. Lysin motif domain (LysMD) family proteins are ancient peptidoglycan receptors that function in bacteriophage and plants. This report focuses on defining the role of LysMD-containing proteins in animals. Here, we characterize a novel transmembrane LysMD family protein. Loss-of-function mutations at the lysMD3/4 locus in Drosophila are associated with systemic innate immune activation following challenge, so we refer to this gene as immune active (ima). We show that Ima selectively binds peptidoglycan, is enriched in cell membranes, and is necessary to regulate terminal innate immune effectors through an NF-kB-dependent pathway. Hence, Ima fulfills the key criteria of a peptidoglycan pattern recognition receptor. The human Ima ortholog, hLysMD3, exhibits similar biochemical properties. Together, these findings establish LysMD3/4 as the founding member of a novel family of animal peptidoglycan recognition proteins.

Cellobiose elicits immunity in lettuce conferring resistance to Botrytis cinerea

Cellobiose is the primary product of cellulose hydrolysis and is expected to function as a type of pathogen/damage-associated molecular pattern in evoking plant innate immunity. In this study, cellobiose was demonstrated to be a positive regulator in the immune response of lettuce, but halted autoimmunity when lettuce was exposed to concentrations of cellobiose >60 mg l-1. When lettuce plants were infected by Botrytis cinerea, cellobiose endowed plants with enhanced pre-invasion resistance by activating high β-1,3-glucanase and antioxidative enzyme activities at the initial stage of pathogen infection. Cellobiose-activated core regulatory factors such as EDS1, PTI6, and WRKY70, as well as salicylic acid signaling, played an indispensable role in modulating plant growth-defense trade-offs. Transcriptomics data further suggested that the cellobiose-activated plant-pathogen pathways are involved in microbe/pathogen-associated molecular pattern-triggered immune responses. Genes encoding receptor-like kinases, transcription factors, and redox homeostasis, phytohormone signal transduction, and pathogenesis-related proteins were also up- or down-regulated by cellobiose. Taken together, the findings of this study demonstrated that cellobiose serves as an elicitor to directly activate disease-resistance-related cellular functions. In addition, multiple genes have been identified as potential modulators of the cellobiose-induced immune response, which could aid understanding of underlying molecular events.

Disruption of plant plasma membrane by Nep1-like proteins in pathogen-plant interactions

Lipid membrane destruction by microbial pore-forming toxins (PFTs) is a ubiquitous mechanism of damage to animal cells, but is less prominent in plants. Nep1-like proteins (NLPs) secreted by phytopathogens that cause devastating crop diseases, such as potato late blight, represent the only family of microbial PFTs that effectively damage plant cells by disrupting the integrity of the plant plasma membrane. Recent research has elucidated the molecular mechanism of NLP-mediated membrane damage, which is unique among microbial PFTs and highly adapted to the plant membrane environment. In this review, we cover recent insight into how NLP cytolysins damage plant membranes and cause cell death.

Transcriptomic alterations in the sweet orange vasculature correlate with growth repression induced by a variant of citrus tristeza virus

Citrus tristeza virus (CTV, family Closteroviridae) is an economically important pathogen of citrus. CTV resides in the phloem of the infected plants and induces a range of disease phenotypes, including stem pitting and quick decline as well as a number of other deleterious syndromes. To uncover the biological processes underlying the poorly understood damaging symptoms of CTV, we profiled the transcriptome of sweet orange (Citrus sinensis) phloem-rich bark tissues of non-infected, mock-inoculated trees and trees singly infected with two distinct variants of CTV, T36 or T68-1. The T36 and T68-1 variants accumulated in the infected plants at similar titers. With that, young trees infected with T68-1 were markedly repressed in growth, while the growth rate of the trees infected with T36 was comparable to the mock-inoculated trees. Only a small number of differentially expressed genes (DEGs) were identified in the nearly asymptomatic T36-infected trees, whereas almost fourfold the number of DEGs were identified with the growth-restricting T68-1 infection. DEGs were validated using quantitative reverse transcription-PCR. While T36 did not induce many noteworthy changes, T68-1 altered the expression of numerous host mRNAs encoding proteins within significant biological pathways, including immunity and stress response proteins, papain-like cysteine proteases (PLCPs), cell-wall modifying enzymes, vascular development proteins and others. The transcriptomic alterations in the T68-1-infected trees, in particular, the strong and persistent increase in the expression levels of PLCPs, appear to contribute to the observed stem growth repression. On the other hand, analysis of the viral small interfering RNAs revealed that the host RNA silencing-based response to the infection by T36 and that by T68-1 was comparable, and thus, the induction of this antiviral mechanism may not contribute to the difference in the observed symptoms. The DEGs identified in this study promote our understanding of the underlying mechanisms of the yet unexplained growth repression induced by severe CTV isolates in sweet orange trees.

Exploring the role of plant lysin motif receptor-like kinases in regulating plant-microbe interactions in the bioenergy crop Populus

For plants, distinguishing between mutualistic and pathogenic microbes is a matter of survival. All microbes contain microbe-associated molecular patterns (MAMPs) that are perceived by plant pattern recognition receptors (PRRs). Lysin motif receptor-like kinases (LysM-RLKs) are PRRs attuned for binding and triggering a response to specific MAMPs, including chitin oligomers (COs) in fungi, lipo-chitooligosaccharides (LCOs), which are produced by mycorrhizal fungi and nitrogen-fixing rhizobial bacteria, and peptidoglycan in bacteria. The identification and characterization of LysM-RLKs in candidate bioenergy crops including Populus are limited compared to other model plant species, thus inhibiting our ability to both understand and engineer microbe-mediated gains in plant productivity. As such, we performed a sequence analysis of LysM-RLKs in the Populus genome and predicted their function based on phylogenetic analysis with known LysM-RLKs. Then, using predictive models, molecular dynamics simulations, and comparative structural analysis with previously characterized CO and LCO plant receptors, we identified probable ligand-binding sites in Populus LysM-RLKs. Using several machine learning models, we predicted remarkably consistent binding affinity rankings of Populus proteins to CO. In addition, we used a modified Random Walk with Restart network-topology based approach to identify a subset of Populus LysM-RLKs that are functionally related and propose a corresponding signal transduction cascade. Our findings provide the first look into the role of LysM-RLKs in Populus-microbe interactions and establish a crucial jumping-off point for future research efforts to understand specificity and redundancy in microbial perception mechanisms.

The Small Ras Superfamily GTPase Rho4 of the Maize Anthracnose Fungus Colletotrichum graminicola Is Required for β-1,3-glucan Synthesis, Cell Wall Integrity, and Full Virulence

Small Ras superfamily GTPases are highly conserved regulatory factors of fungal cell wall biosynthesis and morphogenesis. Previous experiments have shown that the Rho4-like protein of the maize anthracnose fungus Colletotrichum graminicola, formerly erroneously annotated as a Rho1 protein, physically interacts with the β-1,3-glucan synthase Gls1 (Lange et al., 2014; Curr. Genet. 60:343–350). Here, we show that Rho4 is required for β-1,3-glucan synthesis. Accordingly, Δrho4 strains formed distorted vegetative hyphae with swellings, and exhibited strongly reduced rates of hyphal growth and defects in asexual sporulation. Moreover, on host cuticles, conidia of Δrho4 strains formed long hyphae with hyphopodia, rather than short germ tubes with appressoria. Hyphopodia of Δrho4 strains exhibited penetration defects and often germinated laterally, indicative of cell wall weaknesses. In planta differentiated infection hyphae of Δrho4 strains were fringy, and anthracnose disease symptoms caused by these strains on intact and wounded maize leaf segments were significantly weaker than those caused by the WT strain. A retarded disease symptom development was confirmed by qPCR analyses. Collectively, we identified the Ras GTPase Rho4 as a new virulence factor of C. graminicola.

Membrane Localized GbTMEM214s Participate in Modulating Cotton Resistance to Verticillium Wilt

Verticillium wilt (VW) is a soil-borne fungal disease caused by Verticillium dahliae Kleb, which leads to serious damage to cotton production annually in the world. In our previous study, a transmembrane protein 214 protein (TMEM214) gene associated with VW resistance was map-based cloned from Gossypium barbadense (G. barbadense). TMEM214 proteins are a kind of transmembrane protein, but their function in plants is rarely studied. To reveal the function of TMEM214s in VW resistance, all six TMEM214s were cloned from G. barbadense in this study. These genes were named as GbTMEM214-1_A/D, GbTMEM214-4_A/D and GbTMEM214-7_A/D, according to their location on the chromosomes. The encoded proteins are all located on the cell membrane. TMEM214 genes were all induced with Verticillium dahliae inoculation and showed significant differences between resistant and susceptible varieties, but the expression patterns of GbTMEM214s under different hormone treatments were significantly different. Virus-induced gene silencing analysis showed the resistance to VW of GbTMEM214s-silenced lines decreased significantly, which further proves the important role of GbTMEM214s in the resistance to Verticillium dahliae. Our study provides an insight into the involvement of GbTMEM214s in VW resistance, which was helpful to better understand the disease-resistance mechanism of plants.

The RECEPTOR-LIKE PROTEIN53 immune complex associates with LLG1 to positively regulate plant immunity

Pattern recognition receptors (PRRs) sense ligands in pattern-triggered immunity (PTI). Plant PRRs include numerous receptor-like proteins (RLPs), but many RLPs remain functionally uncharacterized. Here, we examine an Arabidopsis thaliana RLP, RLP53, which positively regulates immune signaling. Our forward genetic screen for suppressors of enhanced disease resistance1 (edr1) identified a point mutation in RLP53 that fully suppresses disease resistance and mildew-induced cell death in edr1 mutants. The rlp53 mutants showed enhanced susceptibility to virulent pathogens, including fungi, oomycetes, and bacteria, indicating that RLP53 is important for plant immunity. The ectodomain of RLP53 contains leucine-rich repeat (LRR) motifs. RLP53 constitutively associates with the LRR receptor-like kinase SUPPRESSOR OF BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE (BAK1)-INTERACTING RECEPTOR KINASE1 (SOBIR1) and interacts with the co-receptor BAK1 in a pathogen-induced manner. The double mutation sobir1-12 bak1-5 suppresses edr1-mediated disease resistance, suggesting that EDR1 negatively regulates PTI modulated by the RLP53-SOBIR1-BAK1 complex. Moreover, the glycosylphosphatidylinositol (GPI)-anchored protein LORELEI-LIKE GPI-ANCHORED PROTEIN1 (LLG1) interacts with RLP53 and mediates RLP53 accumulation in the plasma membrane. We thus uncovered the role of a novel RLP and its associated immune complex in plant defense responses and revealed a potential new mechanism underlying regulation of RLP immune function by a GPI-anchored protein.

Differing Responses to Phytophthora cinnamomi Infection in Susceptible and Partially Resistant Persea americana (Mill.) Rootstocks: A Case for the Role of Receptor-Like Kinases and Apoplastic Proteases

The hemibiotrophic plant pathogen Phytophthora cinnamomi Rands is the most devastating pathogen of avocado (Persea americana Mill.) and, as such, causes significant annual losses in the industry. Although the molecular basis of P. cinnamomi resistance in avocado and P. cinnamomi virulence determinants have been the subject of recent research, none have yet attempted to compare the transcriptomic responses of both pathogen and host during their interaction. In the current study, the transcriptomes of both avocado and P. cinnamomi were explored by dual RNA sequencing. The basis for partial resistance was sought by the inclusion of both susceptible (R0.12) and partially resistant (Dusa®) rootstocks sampled at early (6, 12 and 24 hours post-inoculation, hpi) and late time-points (120 hpi). Substantial differences were noted in the number of differentially expressed genes found in Dusa® and R0.12, specifically at 12 and 24 hpi. Here, the partially resistant rootstock perpetuated defense responses initiated at 6 hpi, while the susceptible rootstock abruptly reversed course. Instead, gene ontology enrichment confirmed that R0.12 activated pathways related to growth and development, essentially rendering its response at 12 and 24 hpi no different from that of the mock-inoculated controls. As expected, several classes of P. cinnamomi effector genes were differentially expressed in both Dusa® and R0.12. However, their expression differed between rootstocks, indicating that P. cinnamomi might alter the expression of its effector arsenal based on the rootstock. Based on some of the observed differences, several P. cinnamomi effectors were highlighted as potential candidates for further research. Similarly, the receptor-like kinase (RLK) and apoplastic protease coding genes in avocado were investigated, focusing on their potential role in differing rootstock responses. This study suggests that the basis of partial resistance in Dusa® is predicated on its ability to respond appropriately during the early stages following P. cinnamomi inoculation, and that important components of the first line of inducible defense, apoplastic proteases and RLKs, are likely to be important to the observed outcome.

PeTGA1 enhances disease resistance against Colletotrichum gloeosporioides through directly regulating PeSARD1 in poplar

Basic leucine zipper (bZIP) proteins play important roles in responding to biotic and abiotic stresses in plants. However, the molecular mechanisms of plant resistance to pathogens remain largely unclear in poplar. The present study isolated a TGACG-binding (TGA) transcription factor, PeTGA1, from Populus euphratica. PeTGA1 belongs to subgroup D of the bZIP family and was localized to the nucleus. To study the role PeTGA1 plays in response to Colletotrichum gloeosporioides, transgenic triploid white poplars overexpressing PeTGA1 were generated. Results showed that poplars with overexpressed PeTGA1 showed a higher effective defense response to C. gloeosporioides than the wild-type plants. A yeast one-hybrid assay and an electrophoretic mobility shift assay revealed that PeTGA1 could directly bind to the PeSARD1 (P. euphratica SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1) promoter, an important regulator for salicylic acid biosynthesis. The transactivation assays indicated that PeTGA1 activated the expression of PeSARD1, and PR1 (PATHOGENESIS-RELATED 1), a SA marker gene involved in SA signaling. Subsequently, we observed that the PeTGA1 overexpression lines showed elevated SA levels, thereby resulting in the increased resistance to C. gloeosporioides. Taken together, our results indicated that PeTGA1 may exert a key role in plant immunity not only by targeting PeSARD1 thus participating in the SA biosynthesis pathway but also by involving in SA signaling via activating the expression of PR1.

Parasitic Plants: An Overview of Mechanisms by Which Plants Perceive and Respond to Parasites

In contrast to most autotrophic plants, which produce carbohydrates from carbon dioxide using photosynthesis, parasitic plants obtain water and nutrients by parasitizing host plants. Many important crop plants are infested by these heterotrophic plants, leading to severe agricultural loss and reduced food security. Understanding how host plants perceive and resist parasitic plants provides insight into underlying defense mechanisms and the potential for agricultural applications. In this review, we offer a comprehensive overview of the current understanding of host perception of parasitic plants and the pre-attachment and post-attachment defense responses mounted by the host. Since most current research overlooks the role of organ specificity in resistance responses, we also summarize the current understanding and cases of cross-organ parasitism, which indicates nonconventional haustorial connections on other host organs, for example, when stem parasitic plants form haustoria on their host roots. Understanding how different tissue types respond to parasitic plants could provide the potential for developing a universal resistance mechanism in crops against both root and stem parasitic plants.

CONSTITUTIVE EXPRESSER OF PATHOGENESIS-RELATED GENES 5 is an RNA-binding protein controlling plant immunity via an RNA processing complex

Plant innate immunity is capable of combating diverse and ever evolving pathogens. The plasticity of innate immunity could be boosted by RNA processing. Arabidopsis thaliana CONSTITUTIVE EXPRESSER OF PATHOGENESIS-RELATED GENES 5 (CPR5), a key negative immune regulator, is a component of the nuclear pore complex. Here we further identified CPR5 as a component of RNA processing complexes. Through genetic screening, we found that RNA splicing activator NineTeen Complex and RNA polyadenylation factor CLEAVAGE AND POLYADENYLATION SPECIFICITY FACTOR, coordinately function downstream of CPR5 to activate plant immunity. CPR5 and these two regulators form a complex that is localized in nuclear speckles, an RNA processing organelle. Intriguingly, we found that CPR5 is an RNA-binding protein belonging to the Transformer 2 (Tra2) subfamily of the serine/arginine-rich family. The RNA recognition motif of CPR5 protein binds the Tra2-targeted RNA sequence in vitro and is functionally replaceable by those of Tra2 subfamily proteins. In planta, it binds RNAs of CPR5-regulated alternatively spliced genes (ASGs) identified by RNA-seq. ARGONAUTE 1 (AGO1) is one of the ASGs and, consistent with this, the ago1 mutant suppresses the cpr5 phenotype. These findings reveal that CPR5 is an RNA-binding protein linking RNA processing with plant immunity.

Pan-Genome Analysis Reveals the Abundant Gene Presence/Absence Variations Among Different Varieties of Melon and Their Influence on Traits

Melon (Cucumismelo L.) is an important vegetable crop that has been subjected to domestication and improvement. Several varieties of melons with diverse phenotypes have been produced. In this study, we constructed a melon pan-genome based on 297 accessions comprising 168 Mb novel sequences and 4,325 novel genes. Based on the results, there were abundant genetic variations among different melon groups, including 364 unfavorable genes in the IMP_A vs. LDR_A group, 46 favorable genes, and 295 unfavorable genes in the IMP_M vs. LDR_M group. The distribution of 709 resistance gene analogs (RGAs) was also characterized across 297 melon lines, of which 603 were core genes. Further, 106 genes were found to be variable, 55 of which were absent in the reference melon genome. Using gene presence/absence variation (PAV)-based genome-wide association analysis (GWAS), 13 gene PAVs associated with fruit length, fruit shape, and fruit width were identified, four of which were located in pan-genome additional contigs.

Plant immunity suppression via PHR1-RALF-FERONIA shapes the root microbiome to alleviate phosphate starvation

The microbiome plays an important role in shaping plant growth and immunity, but few plant genes and pathways impacting plant microbiome composition have been reported. In Arabidopsis thaliana, the phosphate starvation response (PSR) was recently found to modulate the root microbiome upon phosphate (Pi) starvation through the transcriptional regulator PHR1. Here, we report that A. thaliana PHR1 directly binds to the promoters of rapid alkalinization factor (RALF) genes, and activates their expression under phosphate-starvation conditions. RALFs in turn suppress complex formation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) receptor through FERONIA, a previously-identified PTI modulator that increases resistance to certain detrimental microorganisms. Suppression of immunity via the PHR1-RALF-FERONIA axis allows colonization by specialized root microbiota that help to alleviate phosphate starvation by upregulating the expression of PSR genes. These findings provide a new paradigm for coordination of host-microbe homeostasis through modulating plant innate immunity after environmental perturbations.

Genotyping-by-sequencing-based identification of Arabidopsis pattern recognition receptor RLP32 recognizing proteobacterial translation initiation factor IF1

Activation of plant pattern-triggered immunity (PTI) relies on the recognition of microbe-derived structures, termed patterns, through plant-encoded surface-resident pattern recognition receptors (PRRs). We show that proteobacterial translation initiation factor 1 (IF1) triggers PTI in Arabidopsis thaliana and related Brassicaceae species. Unlike for most other immunogenic patterns, IF1 elicitor activity cannot be assigned to a small peptide epitope, suggesting that tertiary fold features are required for IF1 receptor activation. We have deployed natural variation in IF1 sensitivity to identify Arabidopsis leucine-rich repeat (LRR) receptor-like protein 32 (RLP32) as IF1 receptor using a restriction site-associated DNA sequencing approach. RLP32 confers IF1 sensitivity to rlp32 mutants, IF1-insensitive Arabidopsis accessions and IF1-insensitive Nicotiana benthamiana, binds IF1 specifically and forms complexes with LRR receptor kinases SOBIR1 and BAK1 to mediate signaling. Similar to other PRRs, RLP32 confers resistance to Pseudomonas syringae, highlighting an unexpectedly complex array of bacterial pattern sensors within a single plant species.

Pattern-triggered immunity is activated by recognition of microbe-derived structures by host pattern recognition receptors. Here the authors use a genotype-by sequencing approach to show that bacterial translation initiation factor 1 triggers PTI in Arabidopsis thaliana after recognition by the RLP32 receptor.

LysM receptors in Coffea arabica: Identification, characterization, and gene expression in response to Hemileia vastatrix

Pathogen‐associated molecular patterns (PAMPs) are recognized by pattern recognition receptors (PRRs) localized on the host plasma membrane. These receptors activate a broad-spectrum and durable defense, which are desired characteristics for disease resistance in plant breeding programs. In this study, candidate sequences for PRRs with lysin motifs (LysM) were investigated in the Coffea arabica genome. For this, approaches based on the principle of sequence similarity, conservation of motifs and domains, phylogenetic analysis, and modulation of gene expression in response to Hemileia vastatrix were used. The candidate sequences for PRRs in C. arabica (Ca1-LYP, Ca2-LYP, Ca1-CERK1, Ca2-CERK1, Ca-LYK4, Ca1-LYK5 and Ca2-LYK5) showed high similarity with the reference PRRs used: Os-CEBiP, At-CERK1, At-LYK4 and At-LYK5. Moreover, the ectodomains of these sequences showed high identity or similarity with the reference sequences, indicating structural and functional conservation. The studied sequences are also phylogenetically related to the reference PRRs described in Arabidopsis, rice, and other plant species. All candidates for receptors had their expression induced after the inoculation with H. vastatrix, since the first time of sampling at 6 hours post‐inoculation (hpi). At 24 hpi, there was a significant increase in expression, for most of the receptors evaluated, and at 48 hpi, a suppression. The results showed that the candidate sequences for PRRs in the C. arabica genome display high homology with fungal PRRs already described in the literature. Besides, they respond to pathogen inoculation and seem to be involved in the perception or signaling of fungal chitin, acting as receptors or co-receptors of this molecule. These findings represent an advance in the understanding of the basal immunity of this species.

Investigating Host and Parasitic Plant Interaction by Tissue-Specific Gene Analyses on Tomato and Cuscuta campestris Interface at Three Haustorial Developmental Stages

Parasitic weeds cause billions of dollars in agricultural losses each year worldwide. Cuscuta campestris (C. campestris), one of the most widespread and destructive parasitic plants in the United States, severely reduces yield in tomato plants. Reducing the spread of parasitic weeds requires understanding the interaction between parasites and hosts. Several studies have identified factors needed for parasitic plant germination and haustorium induction, and genes involved in host defense responses. However, knowledge of the mechanisms underlying the interactions between host and parasitic plants, specifically at the interface between the two organisms, is relatively limited. A detailed investigation of the crosstalk between the host and parasite at the tissue-specific level would enable development of effective parasite control strategies. To focus on the haustorial interface, we used laser-capture microdissection (LCM) with RNA-seq on early, intermediate and mature haustorial stages. In addition, the tomato host tissue that immediately surround the haustoria was collected to obtain tissue- resolution RNA-Seq profiles for C. campestris and tomato at the parasitism interface. After conducting RNA-Seq analysis and constructing gene coexpression networks (GCNs), we identified CcHB7, CcPMEI, and CcERF1 as putative key regulators involved in C. campestris haustorium organogenesis, and three potential regulators, SlPR1, SlCuRe1-like, and SlNLR, in tomatoes that are involved in perceiving signals from the parasite. We used host-induced gene silencing (HIGS) transgenic tomatoes to knock-down the candidate genes in C. campestris and produced CRISPR transgenic tomatoes to knock out candidate genes in tomatoes. The interactions of C. campestris with these transgenic lines were tested and compared with that in wild-type tomatoes. The results of this study reveal the tissue-resolution gene regulatory mechanisms at the parasitic plant-host interface and provide the potential of developing a parasite-resistant system in tomatoes.

Bacterial type III effector-induced plant C8 volatiles elicit antibacterial immunity in heterospecific neighbouring plants via airborne signalling

Upon sensing attack by pathogens and insect herbivores, plants release complex mixtures of volatile compounds. Here, we show that the infection of lima bean (Phaseolus lunatus L.) plants with the non-host bacterial pathogen Pseudomonas syringae pv. tomato led to the production of microbe-induced plant volatiles (MIPVs). Surprisingly, the bacterial type III secretion system, which injects effector proteins directly into the plant cytosol to subvert host functions, was found to prime both intra- and inter-specific defense responses in neighbouring wild tobacco (Nicotiana benthamiana) plants. Screening of each of 16 effectors using the Pseudomonas fluorescens effector-to-host analyser revealed that an effector, HopP1, was responsible for immune activation in receiver tobacco plants. Further study demonstrated that 1-octen-3-ol, 3-octanone and 3-octanol are novel MIPVs emitted by the lima bean plant in a HopP1-dependent manner. Exposure to synthetic 1-octen-3-ol activated immunity in tobacco plants against a virulent pathogen Pseudomonas syringae pv. tabaci. Our results show for the first time that a bacterial type III effector can trigger the emission of C8 plant volatiles that mediate defense priming via plant-plant interactions. These results provide novel insights into the role of airborne chemicals in bacterial pathogen-induced inter-specific plant-plant interactions.

A G-type lectin receptor-like kinase regulates the perception of oomycete apoplastic expansin-like proteins

Phytophthora capsici is one of the most harmful pathogens in agriculture, which threatens the safe production of multiple crops and causes serious economic losses worldwide. Here, we identified a P. capsici expansin-like protein, PcEXLX1, by liquid chromatography-tandem mass spectrometry from Nicotiana benthamiana apoplastic fluid infected with P. capsici. Clustered regularly interspaced short palindromic repeats/crispr associated protein 9 (CRISPR/Cas9)-mediated PcEXLX1 knockout mutants exhibited significantly enhanced virulence, while the overexpression of PcEXLX1 impaired the virulence. Prokaryotically expressed PcEXLX1 activated multiple plant immune responses, which were BRI1-associated kinase 1 (BAK1)- and suppressor of BIR1-1 (SOBIR1)-dependent. Furthermore, overexpression of PcEXLX1 homologs in N. benthamiana could also increase plant resistance to P. capsici. A G-type lectin receptor-like kinase from N. benthamiana, expansin-regulating kinase 1 (ERK1), was shown to regulate the perception of PcEXLX1 and positively mediate the plant resistance to P. capsici. These results reveal that the expansin-like protein, PcEXLX1, is a novel apoplastic effector with plant immunity-inducing activity of oomycetes, perception of which is regulated by the receptor-like kinase, ERK1.

The RLK protein TaCRK10 activates wheat high-temperature seedling-plant resistance to stripe rust through interacting with TaH2A.1

Plants sense various pathogens and activate immunity responses through receptor-like kinases (RLKs). Cysteine-rich receptor-like kinases (CRKs) are involved in massive transduction pathways upon perception of a pathogen. However, the roles of CRKs in response to stripe rust are unclear. In the present study, we identified a CRK gene (designated TaCRK10) from wheat variety Xiaoyan 6 (XY6) that harbors high-temperature seedling-plant (HTSP) resistance to stripe rust caused by fungal pathogen Puccinia striiformis f. sp. tritici (Pst). The expression level of TaCRK10 was induced by Pst inoculation and high temperature treatment. Knockdown of TaCRK10 by virus-induced gene silencing resulted in attenuated wheat HTSP resistance to Pst, whereas there is no effect on Pst development and host responses under normal temperatures. Notably, overexpression of TaCRK10 in susceptible variety Fielder provided resistance only under normal temperatures at 14 days with reactive oxygen species accumulation and defense-related gene expression of the salicylic acid pathway. Moreover, TaCRK10 physically interacted with and phosphorylated a histone variant TaH2A.1, which belongs to the H2A.W group. Silencing of TaH2A.1 suppressed wheat resistance to Pst, indicating that TaH2A.1 plays a positive role in wheat resistance to Pst. Thus, TaCRK10 serves as an important sensor of Pst infection and high temperatures, and it activates wheat resistance to Pst through regulating nuclear processes. This knowledge helps elucidate the molecular mechanism of wheat HTSP resistance to Pst and promotes efforts in developing wheat varieties with resistance to stripe rust.

Cyclic nucleotide gated channel genes (CNGCs) in Rosaceae: genome-wide annotation, evolution and the roles on Valsa canker resistance

In Rosaceae, tandem duplication caused the drastic expansion of CNGC gene family Group I. The members MdCN11 and MdCN19 negatively regulate Valsa canker resistance. Apple (Malus domestica) and pear (Pyrus bretschneideri and P. communis) are important fruit crops in Rosaceae family but are suffering from threats of Valsa canker. Cyclic nucleotide-gated ion channels (CNGCs) take crucial roles in plant immune responses. In the present study, a total of 355 CNGCs was identified from 8 Rosaceae plants. Based on phylogenetic analysis, 540 CNGCs from 18 plants (8 in Rosaceae and 10 others) could be divided into four groups. Group I was greatly expanded in Rosaceae resulted from tandem duplications. A large number of cis-acting regulatory elements (cis-elements) responsive to signals from multiple stresses and hormones were identified in the promoter regions of CNGCs in Malus spp. and Pyrus spp. Expressions of most Group I members were obviously up-regulated in Valsa canker susceptible varieties but not in the resistant ones. Furthermore, overexpression of the MdCN11 and MdCN19 in both apple fruits and 'Duli' (P. betulifolia) suspension cells compromised Valsa canker resistance. Overexpression of MdCN11 induced expression of hypersensitive response (HR)-related genes. In conclusion, tandem duplication resulted in a drastic expansion of CNGC Group I members in Rosaceae. Among these, MdCN11 and MdCN19 negatively regulate the Valsa canker resistance via inducting HR.

QTL dissection and mining of candidate genes for Ascochyta fabae and Orobanche crenata resistance in faba bean (Vicia faba L.)

BACKGROUND

Ascochyta blight caused by Ascochyta fabae Speg. and broomrape (Orobanche crenata) are among the economically most significant pathogens of faba bean. Several QTLs conferring resistance against the two pathogens have been identified and validated in different genetic backgrounds. The aim of this study was to saturate the most stable QTLs for ascochyta and broomrape resistance in two Recombinant Inbred Line (RIL) populations, 29H x Vf136 and Vf6 x Vf136, to identify candidate genes conferring resistance against these two pathogens.

RESULTS

We exploited the synteny between faba bean and the model species Medicago truncatula by selecting a set of 219 genes encoding putative WRKY transcription factors and defense related proteins falling within the target QTL intervals, for genotyping and marker saturation in the two RIL populations. Seventy and 50 of the candidate genes could be mapped in 29H x Vf136 and Vf6 x Vf136, respectively. Besides the strong reduction of the QTL intervals, the mapping process allowed replacing previous dominant and pedigree-specific RAPD flanking markers with robust and transferrable SNP markers, revealing promising candidates for resistance against the two pathogens.

CONCLUSIONS

Although further efforts in association mapping and expression studies will be required to corroborate the candidate genes for resistance, the fine-mapping approach proposed here increases the genetic resolution of relevant QTL regions and paves the way for an efficient deployment of useful alleles for faba bean ascochyta and broomrape resistance through marker-assisted breeding.

A rice LRR receptor-like protein associates with its adaptor kinase OsSOBIR1 to mediate plant immunity against viral infection

Plants sense pathogen attacks using a variety of receptors at the cell surface. The LRR receptor-like proteins (RLP) and receptor-like kinases (RLK) are widely reported to participate in plant defence against bacterial and fungal pathogen invasion. However, the role of RLP and RLK in plant antiviral defence has rarely been reported. We employed a high-throughput-sequencing approach, transgenic rice plants and viral inoculation assays to investigate the role of OsRLP1 and OsSOBIR1 proteins in rice immunity against virus infection. The transcript of a rice LRR receptor-like protein, OsRLP1, was markedly up-regulated following infection by RBSDV, a devastating pathogen of rice and maize. Viral inoculation on various OsRLP1 mutants demonstrated that OsRLP1 modulates rice resistance against RBSDV infection. It was also shown that OsRLP1 is involved in the RBSDV-induced defence response by positively regulating the activation of MAPKs and PTI-related gene expression. OsRLP1 interacted with a receptor-like kinase OsSOBIR1, which was shown to regulate the PTI response and rice antiviral defence. Our results offer a novel insight into how a virus-induced receptor-like protein and its adaptor kinase activate the PTI response and antiviral defence in rice.

Pathogen- and plant-derived peptides trigger plant immunity

Plants are constantly exposed to pathogens in their immediate environment. Plants sense the invasion of pathogens by recognizing the components including peptide fragments derived from pathogens, known as pathogen-associated molecular patterns (PAMPs). Plants also produce immunogenic peptides called phytocytokines that regulate immune responses. These molecules are recognized by pattern recognition receptors (PRRs) at plasma membrane. Activated PRRs induce a variety of immune responses including production of reactive oxygen species (ROS), induction of Ca²⁺ influx and activation of mitogen activated protein kinases (MAPKs). Pattern-triggered immunity (PTI) wards off microbes and pests. In this review, we summarize recent our advances in understanding how the peptide fragments are generated and perceived by plant PRRs at cell surface, and the activated PRRs transduce the downstream immune signaling.

Impact of Wheat Streak Mosaic Virus on Peroxisome Proliferation, Redox Reactions, and Resistance Responses in Wheat

Although peroxisomes play an essential role in viral pathogenesis, and viruses are known to change peroxisome morphology, the role of genotype in the peroxisomal response to viruses remains poorly understood. Here, we analyzed the impact of wheat streak mosaic virus (WSMV) on the peroxisome proliferation in the context of pathogen response, redox homeostasis, and yield in two wheat cultivars, Patras and Pamir, in the field trials. We observed greater virus content and yield losses in Pamir than in Patras. Leaf chlorophyll and protein content measured at the beginning of flowering were also more sensitive to WSMV infection in Pamir. Patras responded to the WSMV infection by transcriptional up-regulation of the peroxisome fission genes PEROXIN 11C (PEX11C), DYNAMIN RELATED PROTEIN 5B (DRP5B), and FISSION1A (FIS1A), greater peroxisome abundance, and activation of pathogenesis-related proteins chitinase, and β-1,3-glucanase. Oppositely, in Pamir, WMSV infection suppressed transcription of peroxisome biogenesis genes and activity of chitinase and β-1,3-glucanase, and did not affect peroxisome abundance. Activity of ROS scavenging enzymes was higher in Patras than in Pamir. Thus, the impact of WMSV on peroxisome proliferation is genotype-specific and peroxisome abundance can be used as a proxy for the magnitude of plant immune response.

The CrRLK1L subfamily: One of the keys to versatility in plants

Catharanthus roseous kinase 1L receptors (CrRLK1Ls) are a subfamily of membrane receptors unique to plant cells that perceive internal and external signals, integrate metabolic, physiological, and molecular processes, and regulate plant development. Recent genomic studies have suggested that this receptor subfamily arose during the emergence of terrestrial plants and has since diversified, preserving its essential functions. Participation of some of these CrRLK1Ls in different processes is presented and discussed herein, as well as the increasing number of interactors necessary for their function. At least five different responses have been detected after activating these receptors, such as physiological changes, formation or disassembly of protein complexes, metabolic responses, modification of gene expression, and modulation of phytohormone activity. To date, a common response mechanism for all processes involving CrRLK1Ls has not been described. In this review, the information available on the different functions of CrRLK1Ls was compiled. Additionally, the physiological and/or molecular mechanisms involved in the signaling processes triggered by these receptors are also discussed. In this review, we propose a possible common signaling mechanism for all processes regulated by CrRLK1Ls and pose questions to be answered in the future.

A cysteine-rich receptor-like protein kinase CaCKR5 modulates immune response against Ralstonia solanacearum infection in pepper

BACKGROUND

Cysteine-rich receptor-like kinases (CRKs) represent a large subfamily of receptor-like kinases and play vital roles in diverse physiological processes in regulating plant growth and development.

RESULTS

CaCRK5 transcripts were induced in pepper upon the infection of Ralstonia solanacearum and treatment with salicylic acid. The fusions between CaCRK5 and green fluorescence protein were targeted to the plasma membrane. Suppression of CaCRK5 via virus-induced gene silencing (VIGS) made pepper plants significantly susceptible to R. solanacearum infection, which was accompanied with decreased expression of defense related genes CaPR1, CaSAR8.2, CaDEF1 and CaACO1. Overexpression of CaCRK5 increased resistance against R. solanacearum in Nicotiana benthamiana. Furthermore, electrophoretic mobility shift assay and chromatin immunoprecipitation coupled with quantitative real-time PCR analysis revealed that a homeodomain zipper I protein CaHDZ27 can active the expression of CaCRK5 through directly binding to its promoter. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses suggested that CaCRK5 heterodimerized with the homologous member CaCRK6 on the plasma membrane.

CONCLUSIONS

Our data revealed that CaCRK5 played a positive role in regulating immune responses against R. solanacearum infection in pepper.

Overexpression of LYK4, a lysin motif receptor with non-functional kinase domain, enhances tolerance to Alternaria brassicicola and increases trichome density in Brassica juncea

Lysin motif receptor-like kinases (LYKs) are involved in the recognition of chitin and activation of plant immune response. In this study, we found LYK4 to be strongly induced in resistant Sinapis alba compared with susceptible Brassica juncea on challenge with Alternaria brassicicola. In silico analysis and in vitro kinase assay revealed that despite the presence of canonical protein kinase fold, B.juncea LYK4 (BjLYK4) lacks several key residues of a prototype protein kinase which renders it catalytically inactive. Transient expression analysis confirmed that fluorescently tagged BjLYK4 localizes specifically to the plasma membrane. Overexpression (OE) of BjLYK4 in B. juncea enhanced tolerance against A. brassicicola. Interestingly, the OE lines also exhibited a novel trichome dense phenotype and increased jasmonic acid (JA) responsiveness. We further showed that many chitin responsive WRKY transcription factors and JA biosynthetic genes were strongly induced in the OE lines on challenge with the pathogen. Moreover, several JA inducible trichome developmental genes constituting the WD-repeat/bHLH/MYB activator complex were also upregulated in the OE lines compared with vector control and RNA interference line. These results suggest that BjLYK4 plays an essential role in chitin-dependent activation of defense response and chitin independent trichome development likely by influencing the JA signaling pathway.