TaWAK6 encoding wall-associated kinase is involved in wheat resistance to leaf rust similar to adult plant resistance

Plant cell wall-mediated disease resistance: Current understanding and future perspectives

Beyond their function as structural barriers, plant cell walls are essential elements for the adaptation of plants to environmental conditions. Cell walls are dynamic structures whose composition and integrity can be altered in response to environmental challenges and developmental cues. These wall changes are perceived by plant sensors/receptors to trigger adaptative responses during development and upon stress perception. Plant cell wall damage caused by pathogen infection, wounding, or other stresses leads to the release of wall molecules, such as carbohydrates (glycans), that function as damage-associated molecular patterns (DAMPs). DAMPs are perceived by the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) and disease resistance. Similarly, glycans released from the walls and extracellular layers of microorganisms interacting with plants are recognized as microbe-associated molecular patterns (MAMPs) by specific ECD-PRRs triggering PTI responses. The number of oligosaccharides DAMPs/MAMPs identified that are perceived by plants has increased in recent years. However, the structural mechanisms underlying glycan recognition by plant PRRs remain limited. Currently, this knowledge is mainly focused on receptors of the LysM-PRR family, which are involved in the perception of various molecules, such as chitooligosaccharides from fungi and lipo-chitooligosaccharides (i.e., Nod/MYC factors from bacteria and mycorrhiza, respectively) that trigger differential physiological responses. Nevertheless, additional families of plant PRRs have recently been implicated in oligosaccharide/polysaccharide recognition. These include receptor kinases (RKs) with leucine-rich repeat and Malectin domains in their ECDs (LRR-MAL RKs), Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE group (CrRLK1L) with Malectin-like domains in their ECDs, as well as wall-associated kinases, lectin-RKs, and LRR-extensins. The characterization of structural basis of glycans recognition by these new plant receptors will shed light on their similarities with those of mammalians involved in glycan perception. The gained knowledge holds the potential to facilitate the development of sustainable, glycan-based crop protection solutions.

Phylogenetic Analysis of Wall-Associated Kinase Genes in Triticum Species and Characterization of TaWAK7 Involved in Wheat Powdery Mildew Resistance

Wall-associated kinases (WAKs), a group of receptor-like kinases, have been found to play important roles in defending against pathogens and in various developmental processes. However, the importance of this family in wheat remains largely unknown. Wheat powdery mildew is caused by Blumeria graminis f. sp. tritici (Bgt), which initiates infection on the cell surface and forms haustoria inside the cell; therefore, the defense to Bgt involves extracellular and subsequently intracellular signals. In this study, WAKs were identified genome-wide and analyzed phylogenetically, and then a transmembrane WAK gene that putatively participated in pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity to Bgt was functionally and evolutionarily investigated. In total, 1,193 WAKs were identified from wheat and its Gramineae relatives. Phylogenetic analysis indicated that WAKs expanded through tandem duplication or segment duplication. TaWAK7, from chromosome 2A, was identified as a Bgt-inducible gene both in susceptible and resistant materials, but it showed distinct responsive patterns. Functional analysis showed that TaWAK7 was involved in both the basal and resistance gene-mediated resistances. The specific gene structures and protein characteristics of TaWAK7, along with its orthologs, were characterized both in subgenomes of Triticum spp. and in the A genome of multiple wheat accessions, which revealed that TaWAK7 orthologs underwent complex evolution with frequent gene fusion and domain deletion. In addition, three cytoplasmic proteins interacting with TaWAK7 were indicated by yeast two-hybrid and bimolecular fluorescence complementation assays. Binding of TaWAK7 with these proteins could change its subcellular localization from the plasma membrane to the cytoplasm. This study provides a better understanding of the evolution of WAKs at the genomic level and TaWAK7 at the gene level and provides useful clues for further investigation of how WAKs transmit the extracellular signals to the cytoplasm to activate defense responses.

Identification of quantitative trait loci associated with leaf rust resistance in rye by precision mapping

BACKGROUND

Leaf rust (LR) is among the most destructive fungal diseases of rye (Secale cereale L.). Despite intensive research using various analytical and methodological approaches, such as quantitative trait locus (QTL) mapping, candidate gene expression analysis, and transcriptome sequencing, the genetic basis of the rye immune response to LR remains unclear.

RESULTS

A genome-wide association study was employed to detect QTLs controlling the immune response to LR of rye. A mapping population, G38A, was constructed by crossing two inbred lines: 723 (susceptible to LR) and JKI-NIL-Pr3 (a donor of the LR resistance gene Pr3). For genotyping, SNP-DArT and silico-DArT markers were used. Resistance phenotyping was conducted by visual assessment of the infection severity in detached leaf segments inoculated with two isolates of Puccinia recondita f. sp. secalis, namely, 60/17/2.1 (isolate S) in the main experiment and 86/n/2.1_5x (isolate N) in the validation experiment, at 10 and 17 days post-infection (dpi), respectively. In total, 42,773 SNP-DArT and 105,866 silico-DArT markers were included in the main analysis including isolate S, of which 129 and 140 SNP-DArTs and 767 and 776 silico-DArTs were significantly associated (p ≤ 0.001; - log10(p) ≥ 3.0) with the immune response to LR at 10 and 17 dpi, respectively. Most significant markers were mapped to chromosome 1R. The number of common markers from both systems and at both time points occupying common chromosomal positions was 37, of which 21 were positioned in genes, comprising 18 markers located in exons and three in introns. This gene pool included genes encoding proteins with a known function in response to LR (e.g., a NBS-LRR disease resistance protein-like protein and carboxyl-terminal peptidase).

CONCLUSION

This study has expanded and supplemented existing knowledge of the genetic basis of rye resistance to LR by (1) detecting two QTLs associated with the LR immune response of rye, of which one located on the long arm of chromosome 1R is newly detected, (2) assigning hundreds of markers significantly associated with the immune response to LR to genes in the 'Lo7' genome, and (3) predicting the potential translational effects of polymorphisms of SNP-DArT markers located within protein-coding genes.

The ZmWAKL-ZmWIK-ZmBLK1-ZmRBOH4 module provides quantitative resistance to gray leaf spot in maize

Gray leaf spot (GLS), caused by the fungal pathogens Cercospora zeae-maydis and Cercospora zeina, is a major foliar disease of maize worldwide (Zea mays L.). Here we demonstrate that ZmWAKL encoding cell-wall-associated receptor kinase-like protein is the causative gene at the major quantitative disease resistance locus against GLS. The ZmWAKLY protein, encoded by the resistance allele, can self-associate and interact with a leucine-rich repeat immune-related kinase ZmWIK on the plasma membrane. The ZmWAKLY/ZmWIK receptor complex interacts with and phosphorylates the receptor-like cytoplasmic kinase (RLCK) ZmBLK1, which in turn phosphorylates its downstream NADPH oxidase ZmRBOH4. Upon pathogen infection, ZmWAKLY phosphorylation activity is transiently increased, initiating immune signaling from ZmWAKLY, ZmWIK, ZmBLK1 to ZmRBOH4, ultimately triggering a reactive oxygen species burst. Our study thus uncovers the role of the maize ZmWAKL-ZmWIK-ZmBLK1-ZmRBOH4 receptor/signaling/executor module in perceiving the pathogen invasion, transducing immune signals, activating defense responses and conferring increased resistance to GLS.

Genome-wide characterization of the wall-associated kinase-like (WAKL) family in sesame (Sesamum indicum) identifies a SiWAKL6 gene involved in resistance to Macrophomina Phaseolina

BACKGROUND

Sesame charcoal rot caused by Macrophomina phaseolina is one of the most serious fungal diseases in sesame production, and threatens the yield and quality of sesame. WAKL genes are important in the plant response to biotic stresses by sensing and transmitting external signals to the intracellular receptor. However, there is still a lack about the WAKL gene family and its function in sesame resistance to M. phaseolina. The aim of this study was to interpret the roles of WAKL genes in sesame resistance to M. phaseolina.

RESULTS

In this study, a comprehensive study of the WAKL gene family was conducted and 31 WAKL genes were identified in the sesame genome. Tandem duplication events were the main factor in expansion of the SiWAKL gene family. Phylogenetic analysis showed that the sesame SiWAKL gene family was divided into 4 groups. SiWAKL genes exhibited different expression patterns in diverse tissues. Under M. phaseolina stress, most SiWAKL genes were significantly induced. Notably, SiWAKL6 was strongly induced in the resistant variety "Zhengzhi 13". Functional analysis showed that SiWAKL6 was induced by salicylic acid but not methyl jasmonate in sesame. Overexpression of SiWAKL6 in transgenic Arabidopsis thaliana plants enhanced their resistance to M. phaseolina by inducing the expression of genes involved in the salicylic acid signaling pathway and reconstructing reactive oxygen species homeostasis.

CONCLUSIONS

Taken together, the results provide a better understanding of functions about SiWAKL gene family and suggest that manipulation of these SiWAKL genes can improve plant resistance to M. phaseolina. The findings contributed to further understanding of functions of SiWAKL genes in plant immunity.

Genome-Wide Association Study Reveals Novel Powdery Mildew Resistance Loci in Bread Wheat

Powdery mildew (PM), caused by the fungal pathogen Blumeria graminis f. sp. tritici (Bgt), significantly threatens global bread wheat production. Although the use of resistant cultivars is an effective strategy for managing PM, currently available wheat cultivars lack sufficient levels of resistance. To tackle this challenge, we conducted a comprehensive genome-wide association study (GWAS) using a diverse panel of 286 bread wheat genotypes. Over three consecutive years (2020-2021, 2021-2022, and 2022-2023), these genotypes were extensively evaluated for PM severity under field conditions following inoculation with virulent Bgt isolates. The panel was previously genotyped using the Illumina 90K Infinium iSelect assay to obtain genome-wide single-nucleotide polymorphism (SNP) marker coverage. By applying FarmCPU, a multilocus mixed model, we identified a total of 113 marker-trait associations (MTAs) located on chromosomes 1A, 1B, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6B, 7A, and 7B at a significance level of p ≤ 0.001. Notably, four novel MTAs on chromosome 6B were consistently detected in 2020-2021 and 2021-2022. Furthermore, within the confidence intervals of the identified SNPs, we identified 96 candidate genes belonging to different proteins including 12 disease resistance/host-pathogen interaction-related protein families. Among these, protein kinases, leucine-rich repeats, and zinc finger proteins were of particular interest due to their potential roles in PM resistance. These identified loci can serve as targets for breeding programs aimed at developing disease-resistant wheat cultivars.

Wall-associated kinase BrWAK1 confers resistance to downy mildew in Brassica rapa

The plant cell wall is the first line of defence against physical damage and pathogen attack. Wall-associated kinase (WAK) has the ability to perceive the changes in the cell wall matrix and transform signals into the cytoplasm, being involved in plant development and the defence response. Downy mildew, caused by Hyaloperonospora brassicae, can result in a massive loss in Chinese cabbage (Brassica rapa L. ssp. pekinensis) production. Herein, we identified a candidate resistant WAK gene, BrWAK1, in a major resistant quantitative trait locus, using a double haploid population derived from resistant inbred line T12-19 and the susceptible line 91-112. The expression of BrWAK1 could be induced by salicylic acid and pathogen inoculation. Expression of BrWAK1 in 91-112 could significantly enhance resistance to the pathogen, while truncating BrWAK1 in T12-19 increased disease susceptibility. Variation in the extracellular galacturonan binding (GUB) domain of BrWAK1 was found to mainly confer resistance to downy mildew in T12-19. Moreover, BrWAK1 was proved to interact with BrBAK1 (brassinosteroid insensitive 1 associated kinase), resulting in the activation of the downstream mitogen-activated protein kinase (MAPK) cascade to trigger the defence response. BrWAK1 is the first identified and thoroughly characterized WAK gene conferring disease resistance in Chinese cabbage, and the plant biomass is not significantly influenced by BrWAK1, which will greatly accelerate Chinese cabbage breeding for downy mildew resistance.

Comparative genomics analysis of WAK/WAKL family in Rosaceae identify candidate WAKs involved in the resistance to Botrytis cinerea

BACKGROUND

Wall associated kinase (WAK) and WAK-like (WAKL) are typical pattern recognition receptors act as the first sentry of plant defense. But little of WAK/WAKL family is known in Rosaceae.

RESULTS

In this study, 131 WAK/WAKL genes from apple, peach and strawberry were identified using a bioinformatics approach. Together with 68 RcWAK/RcWAKL in rose, we performed a comparative analysis of 199 WAK/WAKL in four Rosaceae crops. The phylogenetic analysis divided all the WAK/WAKL into five clades. Among them, the cis-elements of Clade II and Clade V promoters were enriched in jasmonic acid (JA) signaling and abiotic stress, respectively. And this can also be verified by the rose transcriptome responding to different hormone treatments. WAK/WAKL families have experienced a considerable proportion of purifying selection during evolution, but still 26 amino acid sites evolved under positive selection, which focused on extracellular conserved domains. WAK/WAKL genes presented collinearity relationship within and between crops, throughout four crops we mined four orthologous groups (OGs). The WAK/WAKL genes in OG1 and OG4 were speculated to involve in plant-Botrytis cinerea interaction, which were validated in rose via VIGS as well as strawberry by qRT-PCR.

CONCLUSIONS

These results not only provide genetic resources and valuable information for the evolutionary relationship of WAK/WAKL gene family, but also offer a reference for future in-depth studies of Rosaceae WAK/WAKL genes.

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.

The WAK-like protein RFO1 acts as a sensor of the pectin methylation status in Arabidopsis cell walls to modulate root growth and defense

Most organisms adjust their development according to the environmental conditions. For the majority, this implies the sensing of alterations to cell walls caused by different cues. Despite the relevance of this process, few molecular players involved in cell wall sensing are known and characterized. Here, we show that the wall-associated kinase-like protein RESISTANCE TO FUSARIUM OXYSPORUM 1 (RFO1) is required for plant growth and early defense against Fusarium oxysporum and functions by sensing changes in the pectin methylation levels in the cell wall. The RFO1 dwell time at the plasma membrane is affected by the pectin methylation status at the cell wall, regulating MITOGEN-ACTIVATED PROTEIN KINASE and gene expression. We show that the extracellular domain of RFO1 binds de-methylated pectin in vitro, whose distribution in the cell wall is altered during F. oxysporum infection. Further analyses also indicate that RFO1 is required for the BR-dependent plant growth alteration in response to inhibition of pectin de-methyl-esterase activity at the cell wall. Collectively, our work demonstrates that RFO1 is a sensor of the pectin methylation status that plays a unique dual role in plant growth and defense against vascular pathogens.

Genome-Wide Analysis of the Wall-Associated Kinase (WAK) Genes in Medicago truncatula and Functional Characterization of MtWAK24 in Response to Pathogen Infection

The wall-associated kinases (WAKs) can perceive and transmit extracellular signals as one kind of unique receptor-like kinases (RLKs) involved in the regulation of cell expansion, pathogen resistance and abiotic stress tolerance. To understand their potential roles and screen some key candidates in Medicago truncatula (M. truncatula), genome-wide identification and characterization of MtWAKs were conducted in this study. A total of 54 MtWAK genes were identified and classified into four groups based on their protein domains. They were distributed on all chromosomes, while most of them were clustered on chromosome 1 and 3. The synteny analysis showed that 11 orthologous pairs were identified between M. truncatula and Arabidopsis thaliana (A. thaliana) and 31 pairs between M. truncatula and Glycine max (G. max). The phylogenetic analysis showed that WAK-RLKs were classified into five clades, and they exhibited a species-specific expansion. Most MtWAK-RLKs had similar exon-intron organization and motif distribution. Multiple cis-acting elements responsive to phytohormones, stresses, growth and development were observed in the promoter regions of MtWAK-RLKs. In addition, the expression patterns of MtWAK-RLKs varied with different plant tissues, developmental stages and biotic and abiotic stresses. Interestingly, plasm membrane localized MtWAK24 significantly inhibited Phytophthora infection in tobacco. The study provides valuable information for characterizing the molecular functions of MtWAKs in regulation of plant growth, development and stress tolerance in legume plants.

A Novel Wall-Associated Kinase TaWAK-5D600 Positively Participates in Defense against Sharp Eyespot and Fusarium Crown Rot in Wheat

Sharp eyespot and Fusarium crown rot, mainly caused by soil-borne fungi Rhizoctonia cerealis and Fusarium pseudograminearum, are destructive diseases of major cereal crops including wheat (Triticum aestivum). However, the mechanisms underlying wheat-resistant responses to the two pathogens are largely elusive. In this study, we performed a genome-wide analysis of wall-associated kinase (WAK) family in wheat. As a result, a total of 140 TaWAK (not TaWAKL) candidate genes were identified from the wheat genome, each of which contains an N-terminal signal peptide, a galacturonan binding domain, an EGF-like domain, a calcium binding EGF domain (EGF-Ca), a transmembrane domain, and an intracellular Serine/Threonine protein kinase domain. By analyzing the RNA-sequencing data of wheat inoculated with R. cerealis and F. pseudograminearum, we found that transcript abundance of TaWAK-5D600 (TraesCS5D02G268600) on chromosome 5D was significantly upregulated, and that its upregulated transcript levels in response to both pathogens were higher compared with other TaWAK genes. Importantly, knock-down of TaWAK-5D600 transcript impaired wheat resistance against the fungal pathogens R. cerealis and F. pseudograminearum, and significantly repressed expression of defense-related genes in wheat, TaSERK1, TaMPK3, TaPR1, TaChitinase3, and TaChitinase4. Thus, this study proposes TaWAK-5D600 as a promising gene for improving wheat broad resistance to sharp eyespot and Fusarium crown rot (FCR) in wheat.

Whole-genome sequencing uncovers the structural and transcriptomic landscape of hexaploid wheat/Ambylopyrum muticum introgression lines

Wheat is a globally vital crop, but its limited genetic variation creates a challenge for breeders aiming to maintain or accelerate agricultural improvements over time. Introducing novel genes and alleles from wheat's wild relatives into the wheat breeding pool via introgression lines is an important component of overcoming this low variation but is constrained by poor genomic resolution and limited understanding of the genomic impact of introgression breeding programmes. By sequencing 17 hexaploid wheat/Ambylopyrum muticum introgression lines and the parent lines, we have precisely pinpointed the borders of introgressed segments, most of which occur within genes. We report a genome assembly and annotation of Am. muticum that has facilitated the identification of Am. muticum resistance genes commonly introgressed in lines resistant to stripe rust. Our analysis has identified an abundance of structural disruption and homoeologous pairing across the introgression lines, likely caused by the suppressed Ph1 locus. mRNAseq analysis of six of these introgression lines revealed that novel introgressed genes are rarely expressed and those that directly replace a wheat orthologue have a tendency towards downregulation, with no discernible compensation in the expression of homoeologous copies. This study explores the genomic impact of introgression breeding and provides a schematic that can be followed to characterize introgression lines and identify segments and candidate genes underlying the phenotype. This will facilitate more effective utilization of introgression pre-breeding material in wheat breeding programmes.

Harnessing genetic resistance to rusts in wheat and integrated rust management methods to develop more durable resistant cultivars

Wheat is one of the most important staple foods on earth. Leaf rust, stem rust and stripe rust, caused by Puccini triticina, Puccinia f. sp. graminis and Puccinia f. sp. striiformis, respectively, continue to threaten wheat production worldwide. Utilization of resistant cultivars is the most effective and chemical-free strategy to control rust diseases. Convectional and molecular biology techniques identified more than 200 resistance genes and their associated markers from common wheat and wheat wild relatives, which can be used by breeders in resistance breeding programmes. However, there is continuous emergence of new races of rust pathogens with novel degrees of virulence, thus rendering wheat resistance genes ineffective. An integration of genomic selection, genome editing, molecular breeding and marker-assisted selection, and phenotypic evaluations is required in developing high quality wheat varieties with resistance to multiple pathogens. Although host genotype resistance and application of fungicides are the most generally utilized approaches for controlling wheat rusts, effective agronomic methods are required to reduce disease management costs and increase wheat production sustainability. This review gives a critical overview of the current knowledge of rust resistance, particularly race-specific and non-race specific resistance, the role of pathogenesis-related proteins, non-coding RNAs, and transcription factors in rust resistance, and the molecular basis of interactions between wheat and rust pathogens. It will also discuss the new advances on how integrated rust management methods can assist in developing more durable resistant cultivars in these pathosystems.

Current understanding of atypical resistance against fungal pathogens in wheat

Pathogens and pests are a major challenge to global food security. Around one hundred different pests and pathogens challenge wheat, one of the most important food crops in the world. Traditional worldwide use of a few key resistance genes in wheat cultivars has necessitated a diversification of the toolbox of resistance genes in wheat varieties over the coming decades to meet the global production demands. Recent advances in gene discovery and functional characterization of genetic resistance mechanisms in wheat reveal great diversity in the types and effectiveness of the underlying resistance genes. This article summarizes the recent developments in the discovery of non-traditional "atypical" resistance genes in wheat against diverse fungal pathogens.

Characterization of the WAK Gene Family Reveals Genes for FHB Resistance in Bread Wheat (Triticum aestivum L.)

Wall-associated kinases (WAKs) are important receptor-like proteins that play major roles in plant defense against pathogens. Fusarium head blight (FHB), one of the most widespread and devastating crop diseases, reduces wheat yield and leads to quality deterioration. Although WAK gene families have been studied in many plants, systematic research on bread wheat (Triticum aestivum) and its role in FHB resistance, in particular, is lacking. In this study, we identified and characterized 320 genes of the TaWAK family in wheat distributed across all chromosomes except 4B and divided them into three phylogenetic groups. Duplication and synteny analyses provided valuable information on the evolutionary characteristics of the TaWAK genes. The gene expression pattern analysis suggested that TaWAK genes play diverse roles in plant biological processes and that at least 30 genes may be involved in the response to Fusarium infection in wheat spikes, with most of the genes contributing to pectin- and chitin-induced defense pathways. Furthermore, 45 TaWAK genes were identified within 17 hcmQTLs that are related to wheat FHB resistance. Our findings provide potential candidate genes for improving FHB resistance and insights into the future functional analysis of TaWAK genes in wheat.

WAKsing plant immunity, waning diseases

With the requirement to breed more productive crop plants in order to feed a growing global population, compounded by increasingly widespread resistance to pesticides exhibited by pathogens, plant immunity is becoming an increasingly important area of research. Of the genes that contribute to disease resistance, the wall-associated receptor-like kinases (WAKs) are increasingly shown to play a major role, in addition to their contribution to plant growth and development or tolerance to abiotic stresses. Being transmembrane proteins, WAKs form a central pillar of a plant cell's ability to monitor and interact with the extracellular environment. Found in both dicots and monocots, WAKs have been implicated in defence against pathogens with diverse lifestyles and contribute to plant immunity in a variety of ways. Whilst some act as cell surface-localized immune receptors recognizing either pathogen- or plant-derived invasion molecules (e.g. effectors or damage-associated molecular patterns, respectively), others promote innate immunity through cell wall modification and strengthening, thus limiting pathogen intrusion. The ability of some WAKs to provide both durable resistance against pathogens and other agronomic benefits makes this gene family important targets in the development of future crop ideotypes and important to a greater understanding of the complexity and robustness of plant immunity.

Resistance of the Wheat Cultivar ‘Renan’ to Septoria Leaf Blotch Explained by a Combination of Strain Specific and Strain Non-Specific QTL Mapped on an Ultra-Dense Genetic Map

Quantitative resistance is considered more durable than qualitative resistance as it does not involve major resistance genes that can be easily overcome by pathogen populations, but rather a combination of genes with a lower individual effect. This durability means that quantitative resistance could be an interesting tool for breeding crops that would not systematically require phytosanitary products. Quantitative resistance has yet to reveal all of its intricacies. Here, we delve into the case of the wheat/Septoria tritici blotch (STB) pathosystem. Using a population resulting from a cross between French cultivar Renan, generally resistant to STB, and Chinese Spring, a cultivar susceptible to the disease, we built an ultra-dense genetic map that carries 148,820 single nucleotide polymorphism (SNP) markers. Phenotyping the interaction was done with two different Zymoseptoria tritici strains with contrasted pathogenicities on Renan. A linkage analysis led to the detection of three quantitative trait loci (QTL) related to resistance in Renan. These QTL, on chromosomes 7B, 1D, and 5D, present with an interesting diversity as that on 7B was detected with both fungal strains, while those on 1D and 5D were strain-specific. The resistance on 7B was located in the region of Stb8 and the resistance on 1D colocalized with Stb19. However, the resistance on 5D was new, so further designated Stb20q. Several wall-associated kinases (WAK), nucleotide-binding and leucine-rich repeats (NB-LRR) type, and kinase domain carrying genes were present in the QTL regions, and some of them were expressed during the infection. These results advocate for a role of Stb genes in quantitative resistance and for resistance in the wheat/STB pathosystem being as a whole quantitative and polygenic.

Characterization of wall-associated kinase/wall-associated kinase-like (WAK/WAKL) family in rose (Rosa chinensis) reveals the role of RcWAK4 in Botrytis resistance

BACKGROUND

Wall-associated kinase (WAK)/WAK-like (WAKL) is one of the subfamily of receptor like kinases (RLK). Although previous studies reported that WAK/WAKL played an important role in plant cell elongation, response to biotic and abiotic stresses, there are no systematic studies on RcWAK/RcWAKL in rose.

RESULTS

In this study, we identified a total of 68 RcWAK/RcWAKL gene family members within rose (Rosa chinensis) genome. The RcWAKs contained the extracellular galacturonan-binding domain and calcium-binding epidermal growth factor (EGF)-like domain, as well as an intracellular kinase domains. The RcWAKLs are missing either calcium-binding EGF-like domain or the galacturonan-binding domain in their extracellular region. The phylogenetic analysis showed the RcWAK/RcWAKL gene family has been divided into five groups, and these RcWAK/RcWAKL genes were unevenly distributed on the 7 chromosomes of rose. 12 of RcWAK/RcWAKL genes were significantly up-regulated by Botrytis cinerea-inoculated rose petals, where RcWAK4 was the most strongly expressed. Virus induced gene silencing of RcWAK4 increased the rose petal sensitivity to B. cinerea. The results indicated RcWAK4 is involved in the resistance of rose petal against B. cinerea.

CONCLUSION

Our study provides useful information to further investigate the function of the RcWAK/RcWAKL gene family and breeding research for resistance to B. cinerea in rose.

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.

Tandem Protein Kinases Emerge as New Regulators of Plant Immunity

Plant-pathogen interactions result in disease development in a susceptible host. Plants actively resist pathogens via a complex immune system comprising both surface-localized receptors that sense the extracellular space as well as intracellular receptors recognizing pathogen effectors. To date, the majority of cloned resistance genes encode intracellular nucleotide-binding leucine-rich repeat receptor proteins. Recent discoveries have revealed tandem kinase proteins (TKPs) as another important family of intracellular proteins involved in plant immune responses. Five TKP genes-barley Rpg1 and wheat WTK1 (Yr15), WTK2 (Sr60), WTK3 (Pm24), and WTK4-protect against devastating fungal diseases. Moreover, a large diversity and numerous putative TKPs exist across the plant kingdom. This review explores our current knowledge of TKPs and serves as a basis for future studies that aim to develop and exploit a deeper understanding of innate plant immunity receptor proteins.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Wall-associated Receptor Kinase and The Expression Profiles in Wheat Responding to Fungal Stress.. [PMID: 0 DOI: 10.1101/2021.07.11.451968]

Cell wall-associated kinases (WAKs), which are encoded by conserved gene families in plants, are crucial for development and responses to diverse stresses. However, the wheat (Triticum aestivum L.) WAKs have not been systematically classified, especially those involved in protecting plants from disease. Here, we classified 129 WAK proteins (encoded by 232 genes) and 75 WAK-Like proteins (WAKLs; encoded by 109 genes) into four groups, via a phylogenetic analysis. An examination of protein sequence alignment revealed diversity in the GUB-domain of WAKs structural organization, but it was usually characterized by a PYPFG motif followed by CxGxGCC motifs, while the EGF-domain was usually initiated with a YAC motif, and eight cysteine residues were spliced by GNPY motif. The expression profiles of WAK-encoding homologous genes varied in response to Blumeria graminis f. sp. tritici (Bgt), Puccinia striiformis f. sp. tritici (Pst) and Puccinia triticina (Pt) stress. A quantitative real-time polymerase chain reaction (qRT-PCR) analysis proved that TaWAK75 and TaWAK76b were involved in wheat resistance to Bgt. This study revealed the structure of the WAK-encoding genes in wheat, which may be useful for future functional elucidation of wheat WAKs responses to fungal infections.

The Wall-Associated Receptor-Like Kinase TaWAK7D Is Required for Defense Responses to Rhizoctonia cerealis in Wheat

Sharp eyespot, caused by necrotrophic fungus Rhizoctonia cerealis, is a serious fungal disease in wheat (Triticum aestivum). Certain wall-associated receptor kinases (WAK) mediate resistance to diseases caused by biotrophic/hemibiotrophic pathogens in several plant species. Yet, none of wheat WAK genes with positive effect on the innate immune responses to R. cerealis has been reported. In this study, we identified a WAK gene TaWAK7D, located on chromosome 7D, and showed its positive regulatory role in the defense response to R. cerealis infection in wheat. RNA-seq and qRT-PCR analyses showed that TaWAK7D transcript abundance was elevated in wheat after R. cerealis inoculation and the induction in the stem was the highest among the tested organs. Additionally, TaWAK7D transcript levels were significantly elevated by pectin and chitin treatments. The knock-down of TaWAK7D transcript impaired resistance to R. cerealis and repressed the expression of five pathogenesis-related genes in wheat. The green fluorescent protein signal distribution assays indicated that TaWAK7D localized on the plasma membrane in wheat protoplasts. Thus, TaWAK7D, which is induced by R. cerealis, pectin and chitin stimuli, positively participates in defense responses to R. cerealis through modulating the expression of several pathogenesis-related genes in wheat.

Identification of cell wall-associated kinases as important regulators involved in Gossypium hirsutum resistance to Verticillium dahliae

BACKGROUND

Verticillium wilt, caused by the soil borne fungus Verticillium dahliae, is a major threat to cotton production worldwide. An increasing number of findings indicate that WAK genes participate in plant-pathogen interactions, but their roles in cotton resistance to V. dahliae remain largely unclear.

RESULTS

Here, we carried out a genome-wide analysis of WAK gene family in Gossypium hirsutum that resulted in the identification of 81 putative GhWAKs, which were all predicated to be localized on plasma membrane. In which, GhWAK77 as a representative was further located in tobacco epidermal cells using transient expression of fluorescent fusion proteins. All GhWAKs could be classified into seven groups according to their diverse protein domains, indicating that they might sense different outside signals to trigger intracellular signaling pathways that were response to various environmental stresses. A lot of cis-regulatory elements were predicted in the upstream region of GhWAKs and classified into four main groups including hormones, biotic, abiotic and light. As many as 28 GhWAKs, playing a potential role in the interaction between cotton and V. dahliae, were screened out by RNA-seq and qRT-PCR. To further study the function of GhWAKs in cotton resistance to V. dahliae, VIGS technology was used to silence GhWAKs. At 20 dpi, VIGSed plants exhibited more chlorosis and wilting than the control plants. The disease indices of VIGSed plants were also significantly higher than those of the control. Furthermore, silencing of GhWAKs significantly affected the expression of JA- and SA-related marker genes, increased the spread of V. dahliae in the cotton stems, dramatically compromised V. dahliae-induced accumulation of lignin, H2O2 and NO, but enhanced POD activity.

CONCLUSION

Our study presents a comprehensive analysis on cotton WAK gene family for the first time. Expression analysis and VIGS assay provided direct evidences on GhWAKs participation in the cotton resistance to V. dahliae.

Alleles of a wall-associated kinase gene account for three of the major northern corn leaf blight resistance loci in maize

Northern corn leaf blight, caused by the fungal pathogen Setosphaeria turcica (anamorph Exserohilum turcicum), is one of the most devastating foliar diseases of maize (Zea mays). Four genes Ht1, Ht2, Ht3 and Htn1 represent the major sources of genetic resistance against the hemibiotrophic fungus S. turcica. Differential maize lines containing these genes also form the basis to classify S. turcica races. Here, we show that Ht2 and Ht3 are identical and allelic to the previously cloned Htn1 gene. Using a map-based cloning approach and Targeting Induced Local Lesions in Genomes (TILLING), we demonstrate that Ht2/Ht3 is an allele of the wall-associated receptor-like kinase gene ZmWAK-RLK1. The ZmWAK-RLK1 variants encoded by Htn1 and Ht2/Ht3 differ by multiple amino acid polymorphisms that particularly affect the putative extracellular domain. A diversity analysis in maize revealed the presence of dozens of ZmWAK-RLK1 alleles. Ht2, Ht3 and Htn1 have been described over decades as independent resistance loci with different race spectra and resistance responses. Our work demonstrates that these three genes are allelic, which has major implications for northern corn leaf blight resistance breeding and nomenclature of S. turcica pathotypes. We hypothesize that genetic background effects have confounded the classical description of these disease resistance genes in the past.

High-resolution mapping of Rym14Hb, a wild relative resistance gene to barley yellow mosaic disease

We mapped the Rym14^Hb resistance locus to barley yellow mosaic disease in a 2Mbp interval. The co-segregating markers will be instrumental for marker-assisted selection in barley breeding. Barley yellow mosaic disease is caused by Barley yellow mosaic virus and Barley mild mosaic virus and leads to severe yield losses in barley (Hordeum vulgare) in Central Europe and East-Asia. Several resistance loci are used in barley breeding. However, cases of resistance-breaking viral strains are known, raising concerns about the durability of those genes. Rym14^Hb is a dominant major resistance gene on chromosome 6HS, originating from barley's secondary genepool wild relative Hordeum bulbosum. As such, the resistance mechanism may represent a case of non-host resistance, which could enhance its durability. A susceptible barley variety and a resistant H. bulbosum introgression line were crossed to produce a large F₂ mapping population (n = 7500), to compensate for a ten-fold reduction in recombination rate compared to intraspecific barley crosses. After high-throughput genotyping, the Rym14^Hb locus was assigned to a 2Mbp telomeric interval on chromosome 6HS. The co-segregating markers developed in this study can be used for marker-assisted introgression of this locus into barley elite germplasm with a minimum of linkage drag.

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.

Genome-Wide Identification, Classification, Characterization, and Expression Analysis of the Wall-Associated Kinase Family during Fruit Development and under Wound Stress in Tomato (Solanum lycopersicum L.)

The wall-associated kinase (WAK) and wall-associated kinase like (WAKL) is a subfamily of receptor-like kinases associated with the cell wall, which have been suggested as sensors of the extracellular environment and triggers of intracellular signals. However, these proteins have not yet been comprehensively analyzed in tomato (Solanum lycopersicum L.). In this study, 11 SlWAK and 18 SlWAKL genes were identified in an uneven distribution in 9 of 12 chromosomes. GUB-WAK-bind (wall-associated receptor kinase galacturonan-binding) and epidermal growth factor (EGF) domains appear more often in SlWAK proteins. However, more SlWAKLs (wall-associated kinase like) have a WAK-assoc (wall-associated receptor kinase C-terminal) domain. Based on their phylogenetic relationships, 29 SlWAK-RLKs (wall associated kinase-receptor like kinases) were clustered into three distinct categories analogous to those in Arabidopsis thaliana. High similarities were found in conserved motifs of the genes within each group. Cis-elements in the promoter region of these 29 genes were found mainly in response to methyl jasmonate (MeJA), abscisic acid (ABA), salicylic acid (SA), anaerobic, light, wound, and MYB transcription factors. Public tomato genome RNA-seq data indicates that multiple SlWAK-RLKs showed different expression patterns under developmental and ripening stages of fruits, such as SlWAK4, SlWAKL11, SlWAKL9, SlWAKL15, SlWAKL14, and SlWAKL1, their RPKM (Reads Per Kilo bases per Million reads) value constantly increases during the fruit expansion period, and decreases as the fruit matures. In tomato leaves, our RNA-seq data showed that nine SlWAK-RLKs transcripts (SlWAK3, SlWAK4, SlWAK10,SlWAKL1, SlWAKL2, SlWAKL3, SlWAKL5, SlWAKL14, and SlWAKL18) were significantly induced (p < 0.001), and three transcripts (SlWAK2, SlWAK5, and SlWAKL15) were significantly inhibited (p < 0.001) under mechanical wounding. The qRT-PCR (Quantitative reverse transcription polymerase chain reaction) of SlWAKL1 and SlWAKL6 verify these results.

Understanding Host-Pathogen Interactions in Brassica napus in the Omics Era

Brassica napus (canola/oilseed rape/rapeseed) is an economically important crop, mostly found in temperate and sub-tropical regions, that is cultivated widely for its edible oil. Major diseases of Brassica crops such as Blackleg, Clubroot, Sclerotinia Stem Rot, Downy Mildew, Alternaria Leaf Spot and White Rust have caused significant yield and economic losses in rapeseed-producing countries worldwide, exacerbated by global climate change, and, if not remedied effectively, will threaten global food security. To gain further insights into the host-pathogen interactions in relation to Brassica diseases, it is critical that we review current knowledge in this area and discuss how omics technologies can offer promising results and help to push boundaries in our understanding of the resistance mechanisms. Omics technologies, such as genomics, proteomics, transcriptomics and metabolomics approaches, allow us to understand the host and pathogen, as well as the interaction between the two species at a deeper level. With these integrated data in multi-omics and systems biology, we are able to breed high-quality disease-resistant Brassica crops in a more holistic, targeted and accurate way.

Tomato Wall-Associated Kinase SlWak1 Depends on Fls2/Fls3 to Promote Apoplastic Immune Responses to Pseudomonas syringae

Wall-associated kinases (Waks) are important components of plant immunity against various pathogens, including the bacterium Pseudomonas syringae pv. tomato (Pst). However, the molecular mechanisms of their role(s) in plant immunity are largely unknown. In tomato (Solanum lycopersicum), wall-associated kinase 1 (SlWak1), has been implicated in pattern recognition receptor (PRR)-triggered immunity (PTI) because its transcript abundance increases significantly after treatment with the flagellin-derived, microbe-associated molecular patterns flg22 and flgII-28, which activate the PRRs Fls2 and Fls3, respectively. We generated two SlWak1 tomato mutants (Δwak1) using CRISPR/Cas9 gene editing technology and investigated the role of SlWak1 in tomato-Pst interactions. Late PTI responses activated in the apoplast by flg22 or flgII-28 were compromised in Δwak1 plants, but PTI at the leaf surface was unaffected. The Δwak1 plants developed fewer callose deposits than wild-type plants, but retained early PTI responses such as generation of reactive oxygen species and activation of mitogen-activated protein kinases upon exposure to flg22 and flgII-28. Induction of Wak1 gene expression by flg22 and flgII-28 was greatly reduced in a tomato mutant lacking Fls2 and Fls3, but induction of Fls3 gene expression by flgII-28 was unaffected in Δwak1 plants. After Pst inoculation, Δwak1 plants developed disease symptoms more slowly than Δfls2.1/2.2/3 mutant plants, although ultimately, both plants were similarly susceptible. SlWak1 coimmunoprecipitated with both Fls2 and Fls3, independently of flg22/flgII-28 or of BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1. These observations suggest that SlWak1 acts in a complex with Fls2/Fls3 and is important at later stages of PTI in the apoplast.