The cysteine-rich receptor-like kinase TaCRK3 contributes to defense against Rhizoctonia cerealis in wheat

TaRLK2.4, a transgressive expression receptor like kinase, improves powdery mildew resistance in wheat

Plants form two immune systems, pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI), to combat Blumeria graminis f. sp. tritici (Bgt) infection during the evolutionary process. In PTI, receptor-like kinases (RLKs) play important roles during pathogen infections. Based on our previous reports, there were 280 TaRLKs identified in early response to powdery mildew infection, which were divided into 34 subfamilies in this study. Differences in gene structures, cis-acting elements, and expression levels implied the function diversity of TaRLKs. TaRLK2.4, a member of LRK10L-RLKs subfamily, contained 665 amino acids, and located on the cell membrane. The main objective of this study was to investigate the role of the receptor-like kinase gene TaRLK2.4 in conferring powdery mildew resistance in wheat. Real-time quantitative PCR results indicated that TaRLK2.4 expressed during Bgt infection process, and exhibited a transgressive expression characteristic in disease resistance NILs (BJ-1). To elucidate the function of TaRLK2.4 during Bgt infection, the comprehensive analysis of virus induced gene silence and over-expression demonstrated that TaRLK2.4 promoted powdery mildew resistance positively. In summary, these results contribute to a deeper understanding of the complex and diverse biological functions of RLKs, and provide new genetic resources for wheat molecular breeding.

A haplotype-resolved chromosome-level assembly and annotation of European hazelnut (C. avellana cv. Jefferson) provides insight into mechanisms of eastern filbert blight resistance

European hazelnut (Corylus avellana L.) is an important tree nut crop. Hazelnut production in North America is currently limited in scalability due to Anisogramma anomala, a fungal pathogen that causes Eastern Filbert Blight (EFB) disease in hazelnut. Successful deployment of EFB resistant cultivars has been limited to the state of Oregon, where the breeding program at Oregon State University (OSU) has released cultivars with a dominant allele at a single resistance locus identified by classical breeding, linkage mapping, and molecular markers. C. avellana cultivar "Jefferson" is resistant to the predominant EFB biotype in Oregon and has been selected by the OSU breeding program as a model for hazelnut genetic and genomic research. Here, we present a near complete, haplotype-resolved chromosome-level hazelnut genome assembly for "Jefferson". This new assembly is a significant improvement over a previously published genome draft. Analysis of genomic regions linked to EFB resistance and self-incompatibility confirmed haplotype splitting and identified new gene candidates that are essential for downstream molecular marker development, thereby facilitating breeding efforts.

The cysteine-rich receptor-like kinase CRK10 targeted by Coniella diplodiella effector CdE1 contributes to white rot resistance in grapevine

Grape white rot is a devastating fungal disease caused by Coniella diplodiella. The pathogen delivers effectors into the host cell that target crucial immune components to facilitate its infection. Here, we examined a secreted effector of C. diplodiella, known as CdE1, which has been found to inhibit Bax-triggered cell death in Nicotiana benthamiana plants. The expression of CdE1 was induced at 12-48 h after inoculation with C. diplodiella, and the transient overexpression of CdE1 led to increased susceptibility of grapevine to the fungus. Subsequent experiments revealed an interaction between CdE1 and Vitis davidii cysteine-rich receptor-like kinase 10 (VdCRK10) and suppression of VdCRK10-mediated immunity against C. diplodiella, partially by decreasing the accumulation of VdCRK10 protein. Furthermore, our investigation revealed that CRK10 expression was significantly higher and was up-regulated in the resistant wild grapevine V. davidii during C. diplodiella infection. The activity of the VdCRK10 promoter is induced by C. diplodiella and is higher than that of Vitis vitifera VvCRK10, indicating the involvement of transcriptional regulation in CRK10 gene expression. Taken together, our results highlight the potential of VdCRK10 as a resistant gene for enhancing white rot resistance in grapevine.

Domain of unknown function (DUF) proteins in plants: function and perspective

Domains of unknown function (DUFs), which are deposited in the protein family database (Pfam), are protein domains with conserved amino acid sequences and uncharacterized functions. Proteins with the same DUF were classified as DUF families. Although DUF families are generally not essential for the survival of plants, they play roles in plant development and adaptation. Characterizing the functions of DUFs is important for deciphering biological puzzles. DUFs were generally studied through forward and reverse genetics. Some novelty approaches, especially the determination of crystal structures and interaction partners of the DUFs, should attract more attention. This review described the identification of DUF genes by genome-wide and transcriptome-wide analyses, summarized the function of DUF-containing proteins, and addressed the prospects for future studies in DUFs in plants.

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.

The cysteine-rich receptor-like kinase CRK4 contributes to the different drought stress response between Columbia and Landsberg erecta

The natural variation between Arabidopsis (Arabidopsis thaliana) ecotypes Columbia (Col) and Landsberg erecta (Ler) strongly affects abscisic acid (ABA) signalling and drought tolerance. Here, we report that the cysteine-rich receptor-like protein kinase CRK4 is involved in regulating ABA signalling, which contributes to the differences in drought stress tolerance between Col-0 and Ler-0. Loss-of-function crk4 mutants in the Col-0 background were less drought tolerant than Col-0, whereas overexpressing CRK4 in the Ler-0 background partially to completely restored the drought-sensitive phenotype of Ler-0. F1 plants derived from a cross between the crk4 mutant and Ler-0 showed an ABA-insensitive phenotype with respect to stomatal movement, along with reduced drought tolerance like Ler-0. We demonstrate that CRK4 interacts with the U-box E3 ligase PUB13 and enhances its abundance, thus promoting the degradation of ABA-INSENSITIVE 1 (ABI1), a negative regulator of ABA signalling. Together, these findings reveal an important regulatory mechanism for modulating ABI1 levels by the CRK4-PUB13 module to fine-tune drought tolerance in Arabidopsis.

The Expression of Triticum aestivum Cysteine-Rich Receptor-like Protein Kinase Genes during Leaf Rust Fungal Infection

Understanding the role of cysteine-rich receptor-like kinases (CRKs) in plant defense mechanisms is crucial for enhancing wheat resistance to leaf rust fungus infection. Here, we identified and verified 164 members of the CRK gene family using the Triticum aestivum reference version 2 collected from the international wheat genome sequencing consortium (IWGSC). The proteins exhibited characteristic features of CRKs, including the presence of signal peptides, cysteine-rich/stress antifungal/DUF26 domains, transmembrane domains, and Pkinase domains. Phylogenetic analysis revealed extensive diversification within the wheat CRK gene family, indicating the development of distinct specific functional roles to wheat plants. When studying the expression of the CRK gene family in near-isogenic lines (NILs) carrying Lr57- and Lr14a-resistant genes, Puccinia triticina, the causal agent of leaf rust fungus, triggered temporal gene expression dynamics. The upregulation of specific CRK genes in the resistant interaction indicated their potential role in enhancing wheat resistance to leaf rust, while contrasting gene expression patterns in the susceptible interaction highlighted potential susceptibility associated CRK genes. The study uncovered certain CRK genes that exhibited expression upregulation upon leaf rust infection and the Lr14a-resistant gene. The findings suggest that targeting CRKs may present a promising strategy for improving wheat resistance to rust diseases.

Cysteine-rich receptor-like protein kinases: emerging regulators of plant stress responses

Cysteine-rich receptor-like kinases (CRKs) belong to a large DUF26-containing receptor-like kinase (RLK) family. They play key roles in immunity, abiotic stress response, and growth and development. How CRKs regulate diverse processes is a long-standing question. Recent studies have advanced our understanding of the molecular mechanisms underlying CRK functions in Ca2+ influx, reactive oxygen species (ROS) production, mitogen-activated protein kinase (MAPK) cascade activation, callose deposition, stomatal immunity, and programmed cell death (PCD). We review the CRK structure-function relationship with a focus on the roles of CRKs in immunity, the abiotic stress response, and the growth-stress tolerance tradeoff. We provide a critical analysis and synthesis of how CRKs control sophisticated regulatory networks that determine diverse plant phenotypic outputs.

Assessment of Tunisian Trichoderma Isolates on Wheat Seed Germination, Seedling Growth and Fusarium Seedling Blight Suppression

Beneficial microorganisms, including members of the Trichoderma genus, are known for their ability to promote plant growth and disease resistance, as well as being alternatives to synthetic inputs in agriculture. In this study, 111 Trichoderma strains were isolated from the rhizospheric soil of Florence Aurore, an ancient wheat variety that was cultivated in an organic farming system in Tunisia. A preliminary ITS analysis allowed us to cluster these 111 isolates into three main groups, T. harzianum (74 isolates), T. lixii (16 isolates) and T. sp. (21 isolates), represented by six different species. Their multi-locus analysis (tef1, translation elongation factor 1; rpb2, RNA polymerase B) identified three T. afroharzianum, one T. lixii, one T. atrobrunneum and one T. lentinulae species. These six new strains were selected to determine their suitability as plant growth promoters (PGP) and biocontrol agents (BCA) against Fusarium seedling blight disease (FSB) in wheat caused by Fusarium culmorum. All of the strains exhibited PGP abilities correlated to ammonia and indole-like compound production. In terms of biocontrol activity, all of the strains inhibited the development of F. culmorum in vitro, which is linked to the production of lytic enzymes, as well as diffusible and volatile organic compounds. An in planta assay was carried out on the seeds of a Tunisian modern wheat variety (Khiar) by coating them with Trichoderma. A significant increase in biomass was observed, which is associated with increased chlorophyll and nitrogen. An FSB bioprotective effect was confirmed for all strains (with Th01 being the most effective) by suppressing morbid symptoms in germinated seeds and seedlings, as well as by limiting F. culmorum aggressiveness on overall plant growth. Plant transcriptome analysis revealed that the isolates triggered several SA- and JA-dependent defense-encoding genes involved in F. culmorum resistance in the roots and leaves of three-week-old seedlings. This finding makes these strains very promising in promoting growth and controlling FSB disease in modern wheat varieties.

Phylogeny of the plant receptor-like kinase (RLK) gene family and expression analysis of wheat RLK genes in response to biotic and abiotic stresses

BACKGROUND

The receptor-like kinase (RLK) gene families in plants contains a large number of members. They are membrane proteins with an extracellular receptor domain and participate in biotic and abiotic stress responses.

RESULTS

In this study, we identified RLKs in 15 representative plant genomes, including wheat, and classified them into 64 subfamilies by using four types of phylogenetic trees and HMM models. Conserved exon‒intron structures with conserved exon phases in the kinase domain were found in many RLK subfamilies from Physcomitrella patens to Triticum aestivum. Domain distributions of RLKs were also diagrammed. Collinearity events and tandem gene clusters suggested that polyploidization and tandem duplication events contributed to the member expansions of T. aestivum RLKs. Global expression pattern analysis was performed by using public transcriptome data. These analyses were involved in T. aestivum, Aegilops tauschii and Brachypodium distachyon RLKs under biotic and abiotic stresses. We also selected 9 RLKs to validate the transcriptome prediction by using qRT‒PCR under drought treatment and with Fusarium graminearum infection. The expression trends of these 9 wheat RLKs from public transcriptome data were consistent with the results of qRT‒PCR, indicating that they might be stress response genes under drought or F. graminearum treatments.

CONCLUSION

In this study, we identified, classified, evolved, and expressed RLKs in wheat and related plants. Thus, our results will provide insights into the evolutionary history and molecular mechanisms of wheat RLKs.

Genome-wide identification and molecular characterization of CRK gene family in cucumber (Cucumis sativus L.) under cold stress and sclerotium rolfsii infection

BACKGROUND

The plant cysteine-rich receptor-like kinases (CRKs) are a large family having multiple roles, including defense responses under both biotic and abiotic stress. However, the CRK family in cucumbers (Cucumis sativus L.) has been explored to a limited extent. In this study, a genome-wide characterization of the CRK family has been performed to investigate the structural and functional attributes of the cucumber CRKs under cold and fungal pathogen stress.

RESULTS

A total of 15 C. sativus CRKs (CsCRKs) have been characterized in the cucumber genome. Chromosome mapping of the CsCRKs revealed that 15 genes are distributed in cucumber chromosomes. Additionally, the gene duplication analysis of the CsCRKs yielded information on their divergence and expansion in cucumbers. Phylogenetic analysis divided the CsCRKs into two clades along with other plant CRKs. Functional predictions of the CsCRKs suggested their role in signaling and defense response in cucumbers. The expression analysis of the CsCRKs by using transcriptome data and via qRT-PCR indicated their involvement in both biotic and abiotic stress responses. Under the cucumber neck rot pathogen, Sclerotium rolfsii infection, multiple CsCRKs exhibited induced expressions at early, late, and both stages. Finally, the protein interaction network prediction results identified some key possible interacting partners of the CsCRKs in regulating cucumber physiological processes.

CONCLUSIONS

The results of this study identified and characterized the CRK gene family in cucumbers. Functional predictions and validation via expression analysis confirmed the involvement of the CsCRKs in cucumber defense response, especially against S. rolfsii. Moreover, current findings provide better insights into the cucumber CRKs and their involvement in defense responses.

The Cytosolic Acetoacetyl-CoA Thiolase TaAACT1 Is Required for Defense against Fusarium pseudograminearum in Wheat

Fusarium pseudograminearum is a major pathogen for the destructive disease Fusarium crown rot (FCR) of wheat (Triticum aestivum). The cytosolic Acetoacetyl-CoA thiolase II (AACT) is the first catalytic enzyme in the mevalonate pathway that biosynthesizes isoprenoids in plants. However, there has been no investigation of wheat cytosolic AACT genes in defense against pathogens including Fusarium pseudograminearum. Herein, we identified a cytosolic AACT-encoding gene from wheat, named TaAACT1, and demonstrated its positively regulatory role in the wheat defense response to F. pseudograminearum. One haplotype of TaAACT1 in analyzed wheat genotypes was associated with wheat resistance to FCR. The TaAACT1 transcript level was elevated after F. pseudograminearum infection, and was higher in FCR-resistant wheat genotypes than in susceptible wheat genotypes. Functional analysis indicated that knock down of TaAACT1 impaired resistance against F. pseudograminearum and reduced the expression of downstream defense genes in wheat. TaAACT1 protein was verified to localize in the cytosol of wheat cells. TaAACT1 and its modulated defense genes were rapidly responsive to exogenous jasmonate treatment. Collectively, TaAACT1 contributes to resistance to F. pseudograminearum through upregulating the expression of defense genes in wheat. This study sheds new light on the molecular mechanisms underlying wheat defense against FCR.

The somatic embryogenesis receptor kinase TaSERK1 participates in the immune response to Rhizoctonia cerealis infection by interacting and phosphorylating the receptor-like cytoplasmic kinase TaRLCK1B in wheat

The sharp eyespot, caused by necrotrophic pathogen Rhizoctonia cerealis, often causes serious yield loss in wheat (Triticum aestivum). However, the mechanisms underlying wheat resistant responses to the pathogen are still limited. In this study, we performed a genome-wide analysis of somatic embryogenesis receptor kinase (SERK) family in wheat. As a result, a total of 26 TaSERK candidate genes were identified from the wheat genome. Only 6 TaSERK genes on the chromosomes 2A, 2B, 2D, 3A, 3B, and 3D showed obvious heightening expression patterns in resistant wheat infected with R. cerealis compared than those un-infected wheat. Of them, the transcripts of 3 TaSERK1 homoeologs on the chromosomes 2A, 2B, and 2D were significantly up-regulated in the highest level compared to other TaSERKs. Importantly, silencing of TaSERK1 significantly impaired wheat resistance to sharp eyespot. Further bio-molecular assays showed that TaSERK1 could interact with the defence-associated receptor-like cytoplasmic kinase TaRLCK1B, and phosphorylated TaRLCK1B. Together, the results suggest that TaSERK1 mediated resistance responses to R. cerealis infection by interacting and phosphorylating TaRLCK1B in wheat. This study sheds light on the understanding of the wheat SERKs in the innate immunity against R. cerealis, and provided a theoretical fulcrum to identify candidate resistant genes for improving wheat resistance against sharp eyespot in wheat.

Evolutionary and functional analysis reveals the crucial roles of receptor-like proteins in resistance to Valsa canker in Rosaceae

Rosaceae is an economically important plant family that can be affected by a multitude of pathogenic microbes, some of which can cause dramatic losses in production. As a type of pattern-recognition receptor, receptor-like proteins (RLPs) are considered vital regulators of plant immunity. Based on genome-wide identification, bioinformatic analysis, and functional determination, we investigated the evolutionary characteristics of RLPs, and specifically those that regulate Valsa canker, a devastating fungal disease affecting apple and pear production. A total of 3028 RLPs from the genomes of 19 species, including nine Rosaceae, were divided into 24 subfamilies. Five subfamilies and seven co-expression modules were found to be involved in the responses to Valsa canker signals of the resistant pear rootstock Pyrus betulifolia 'Duli-G03'. Fourteen RLPs were subsequently screened as candidate genes for regulation of resistance. Among these, PbeRP23 (Chr13.g24394) and PbeRP27 (Chr16.g31400) were identified as key resistance genes that rapidly enhance the resistance of 'Duli-G03' and strongly initiate immune responses, and hence they have potential for further functional exploration and breeding applications for resistance to Valsa canker. In addition, as a consequence of this work we have established optimal methods for the classification and screening of disease-resistant RLPs.

WGCNA Identifies a Comprehensive and Dynamic Gene Co-Expression Network That Associates with Smut Resistance in Sugarcane

Sugarcane smut is a major fungal disease caused by Sporisorium scitamineum, which seriously reduces the yield and quality of sugarcane. In this study, 36 transcriptome data were collected from two sugarcane genotypes, YT93-159 (resistant) and ROC22 (susceptible) upon S. scitamineum infection. Data analysis revealed 20,273 (12,659 up-regulated and 7614 down-regulated) and 11,897 (7806 up-regulated and 4091 down-regulated) differentially expressed genes (DEGs) in YT93-159 and ROC22, respectively. A co-expression network was then constructed by weighted gene co-expression network analysis (WGCNA), which identified 5010 DEGs in 15 co-expressed gene modules. Four of the 15 modules, namely, Skyblue, Salmon, Darkorange, and Grey60, were significantly associated with smut resistance. The GO and KEGG enrichment analyses indicated that the DEGs involving in these four modules could be enriched in stress-related metabolic pathways, such as MAPK and hormone signal transduction, plant-pathogen interaction, amino acid metabolism, glutathione metabolism, and flavonoid, and phenylpropanoid biosynthesis. In total, 38 hub genes, including six from the Skyblue module, four from the Salmon module, 12 from the Darkorange module, and 16 from the Grey60 module, were screened as candidate hub genes by calculating gene connectivity in the corresponding network. Only 30 hub genes were amplifiable with RT-qPCR, of which 27 were up-regulated upon S. scitamineum infection. The results were consistent with the trend of gene expression in RNA-Seq, suggesting their positive roles in smut resistance. Interestingly, the expression levels of AOX, Cyb5, and LAC were higher in ROC22 than in YT93-159, indicating these three genes may act as negative regulators in response to S. scitamineum infection. This study revealed the transcriptome dynamics in sugarcane challenged by S. scitamineum infection and provided gene targets for smut resistance breeding in sugarcane.

Whole-Genome Metalloproteases in the Wheat Sharp Eyespot Pathogen Rhizoctonia cerealis and a Role in Fungal Virulence

Rhizoctonia cerealis is the causal agent of sharp eyespot, a devastating disease of cereal crops including wheat. Several metalloproteases have been implicated in pathogenic virulence, but little is known about whole-genome metalloproteases in R. cerealis. In this study, a total of 116 metalloproteases-encoding genes were identified and characterized from the R. cerealis Rc207 genome. The gene expression profiles and phylogenetic relationship of 11 MEP36/fungalysin metalloproteases were examined during the fungal infection to wheat, and function of an upregulated secretory MEP36 named RcFL1 was validated. Of 11 MEP36 family metalloproteases, ten, except RcFL5, were predicted to be secreted proteins and nine encoding genes, but not RcFL5 and RcFL2, were expressed during the R. cerealis infection process. Phylogenetic analysis suggested that MEP36 metalloproteases in R. cerealis were closely related to those of Rhizoctonia solani but were remote to those of Bipolaris sorokiniana, Fusarium graminearum, F. pseudograminearum, and Pyricularia oryzae. Expression of RcFL1 was significantly upregulated during the infection process and induced plant cell death in wheat to promote the virulence of the pathogen. The MEP36 domain was necessary for the activities of RcFL1. Furthermore, RcFL1 could repress the expression of wheat genes coding for the chitin elicitor receptor kinase TaCERK1 and chitinases. These results suggest that this MEP36 metalloprotease RcFL1 may function as a virulence factor of R. cerealis through inhibiting host chitin-triggered immunity and chitinases. This study provides insights on pathogenic mechanisms of R. cerealis. RcFL1 likely is an important gene resource for improving resistance of wheat to R. cerealis through host-induced gene silencing strategy.

The Pathogen-Induced MATE Gene TaPIMA1 Is Required for Defense Responses to Rhizoctonia cerealis in Wheat

The sharp eyespot, mainly caused by the soil-borne fungus Rhizoctonia cerealis, is a devastating disease endangering production of wheat (Triticum aestivum). Multi-Antimicrobial Extrusion (MATE) family genes are widely distributed in plant species, but little is known about MATE functions in wheat disease resistance. In this study, we identified TaPIMA1, a pathogen-induced MATE gene in wheat, from RNA-seq data. TaPIMA1 expression was induced by Rhizoctonia cerealis and was higher in sharp eyespot-resistant wheat genotypes than in susceptible wheat genotypes. Molecular biology assays showed that TaPIMA1 belonged to the MATE family, and the expressed protein could distribute in the cytoplasm and plasma membrane. Virus-Induced Gene Silencing plus disease assessment indicated that knock-down of TaPIMA1 impaired resistance of wheat to sharp eyespot and down-regulated the expression of defense genes (Defensin, PR10, PR1.2, and Chitinase3). Furthermore, TaPIMA1 was rapidly induced by exogenous H₂O₂ and jasmonate (JA) treatments, which also promoted the expression of pathogenesis-related genes. These results suggested that TaPIMA1 might positively regulate the defense against R. cerealis by up-regulating the expression of defense-associated genes in H₂O₂ and JA signal pathways. This study sheds light on the role of MATE transporter in wheat defense to Rhizoctonia cerealis and provides a potential gene for improving wheat resistance against sharp eyespot.

A Novel Guanine Elicitor Stimulates Immunity in Arabidopsis and Rice by Ethylene and Jasmonic Acid Signaling Pathways

Rice sheath blight (ShB) caused by Rhizoctonia solani is one of the most destructive diseases in rice. Fungicides are widely used to control ShB in agriculture. However, decades of excessive traditional fungicide use have led to environmental pollution and increased pathogen resistance. Generally, plant elicitors are regarded as environmentally friendly biological pesticides that enhance plant disease resistance by triggering plant immunity. Previously, we identified that the plant immune inducer ZhiNengCong (ZNC), a crude extract of the endophyte, has high activity and a strong ability to protect plants against pathogens. Here, we further found that guanine, which had a significant effect on inducing plant resistance to pathogens, might be an active component of ZNC. In our study, guanine activated bursts of reactive oxygen species, callose deposition and mitogen-activated protein kinase phosphorylation. Moreover, guanine-induced plant resistance to pathogens depends on ethylene and jasmonic acid but is independent of the salicylic acid signaling pathway. Most importantly, guanine functions as a new plant elicitor with broad-spectrum resistance to activate plant immunity, providing an efficient and environmentally friendly biological elicitor for bacterial and fungal disease biocontrol.

Blocked at the Stomatal Gate, a Key Step of Wheat Stb16q-Mediated Resistance to Zymoseptoria tritici

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is among the most threatening wheat diseases in Europe. Genetic resistance remains one of the main environmentally sustainable strategies to efficiently control STB. However, the molecular and physiological mechanisms underlying resistance are still unknown, limiting the implementation of knowledge-driven management strategies. Among the 22 known major resistance genes (Stb), the recently cloned Stb16q gene encodes a cysteine-rich receptor-like kinase conferring a full broad-spectrum resistance against Z. tritici. Here, we showed that an avirulent Z. tritici inoculated on Stb16q quasi near isogenic lines (NILs) either by infiltration into leaf tissues or by brush inoculation of wounded tissues partially bypasses Stb16q-mediated resistance. To understand this bypass, we monitored the infection of GFP-labeled avirulent and virulent isolates on Stb16q NILs, from germination to pycnidia formation. This quantitative cytological analysis revealed that 95% of the penetration attempts were unsuccessful in the Stb16q incompatible interaction, while almost all succeeded in compatible interactions. Infectious hyphae resulting from the few successful penetration events in the Stb16q incompatible interaction were arrested in the sub-stomatal cavity of the primary-infected stomata. These results indicate that Stb16q-mediated resistance mainly blocks the avirulent isolate during its stomatal penetration into wheat tissue. Analyses of stomatal aperture of the Stb16q NILs during infection revealed that Stb16q triggers a temporary stomatal closure in response to an avirulent isolate. Finally, we showed that infiltrating avirulent isolates into leaves of the Stb6 and Stb9 NILs also partially bypasses resistances, suggesting that arrest during stomatal penetration might be a common major mechanism for Stb-mediated resistances.

The Receptor-like Kinase TaCRK-7A Inhibits Fusarium pseudograminearum Growth and Mediates Resistance to Fusarium Crown Rot in Wheat

The fungus F. pseudograminearum can cause the destructive disease Fusarium crown rot (FCR) of wheat, an important staple crop. Functional roles of FCR resistance genes in wheat are largely unknown. In the current research, we characterized the antifungal activity and positive-regulatory function of the cysteine-rich repeat receptor-like kinase TaCRK-7A in the defense against F. pseudograminearum in wheat. Antifungal assays showed that the purified TaCRK-7A protein inhibited the growth of F. pseudograminearum. TaCRK-7A transcript abundance was elevated after F. pseudograminearum attack and was positively related to FCR-resistance levels of wheat cultivars. Intriguingly, knocking down of TaCRK-7A transcript increased susceptibility of wheat to FCR and decreased transcript levels of defense-marker genes in wheat. Furthermore, the transcript abundances of TaCRK-7A and its modulated-defense genes were responsive to exogenous jasmonate treatment. Taken together, these results suggest that TaCRK-7A can directly inhibit F. pseudograminearum growth and mediates FCR-resistance by promoting the expression of wheat defense genes in the jasmonate pathway. Thus, TaCRK-7A is a potential gene resource in FCR-resistant wheat breeding program.

TaWAK2A-800, a Wall-Associated Kinase, Participates Positively in Resistance to Fusarium Head Blight and Sharp Eyespot in Wheat

Fusarium head blight (FHB) and sharp eyespot are important diseases of the cereal plants, including bread wheat (Triticum aestivum) and barley. Both diseases are predominately caused by the pathogenic fungi, Fusarium graminearum and Rhizoctonia cerealis. The roles of the wheat-wall-associated kinases (WAKs) in defense against both F. graminearum and R. cerealis have remained largely unknown. This research reports the identification of TaWAK2A-800, a wheat WAK-coding gene located on chromosome 2A, and its functional roles in wheat resistance responses to FHB and sharp eyespot. TaWAK2A-800 transcript abundance was elevated by the early infection of R. cerealis and F. graminearum, or treatment with exogenous chitin. The gene transcript seemed to correspond to the resistance of wheat. Further functional analyses showed that silencing TaWAK2A-800 compromised the resistance of wheat to both FHB (F. graminearum) and sharp eyespot (R. cerealis). Moreover, the silencing reduced the expression levels of six defense-related genes, including the chitin-triggering immune pathway-marker genes, TaCERK1, TaRLCK1B, and TaMPK3. Summarily, TaWAK2A-800 participates positively in the resistance responses to both F. graminearum and R. cerealis, possibly through a chitin-induced pathway in wheat. TaWAK2A-800 will be useful for breeding wheat varieties with resistance to both FHB and sharp eyespot.

The Wheat Wall-Associated Receptor-Like Kinase TaWAK-6D Mediates Broad Resistance to Two Fungal Pathogens Fusarium pseudograminearum and Rhizoctonia cerealis

The soil-borne fungi Fusarium pseudograminearum and Rhizoctonia cerealis are the major pathogens for the economically important diseases Fusarium crown rot (FCR) and sharp eyespot of common wheat (Triticum aestivum), respectively. However, there has been no report on the broad resistance of wheat genes against both F. pseudograminearum and R. cerealis. In the current study, we identified TaWAK-6D, a wall-associated kinase (WAK) which is an encoding gene located on chromosome 6D, and demonstrated its broad resistance role in the wheat responses to both F. pseudograminearum and R. cerealis infection. TaWAK-6D transcript induction by F. pseudograminearum and R. cerealis was related to the resistance degree of wheat and the gene expression was significantly induced by exogenous pectin treatment. Silencing of TaWAK-6D compromised wheat resistance to F. pseudograminearum and R. cerealis, and repressed the expression of a serial of wheat defense-related genes. Ectopic expression of TaWAK-6D in Nicotiana benthamiana positively modulated the expression of several defense-related genes. TaWAK-6D protein was determined to localize to the plasma membrane in wheat and N. benthamiana. Collectively, the TaWAK-6D at the plasma membrane mediated the broad resistance responses to both F. pseudograminearum and R. cerealis in wheat at the seedling stage. This study, therefore, concludes that TaWAK-6D is a promising gene for improving wheat broad resistance to FCR and sharp eyespot.