TaVQ22 Interacts with TaWRKY19-2B to Negatively Regulate Wheat Resistance to Sheath Blight

Wheat sheath blight caused by the necrotic fungal pathogen Rhizoctonia cerealis is responsible for severe damage to bread wheat. Reactive oxygen species (ROS) are vital for stress resistance by plants and their homeostasis plays an important role in wheat resistance to sheath blight. Valine-glutamine (VQ) proteins play important roles in plant growth and development, and responses to biotic and abiotic stresses. However, the functional mechanism mediated by wheat VQ protein in response to sheath blight via ROS homeostasis regulation is unclear. In this study, we identified TaVQ22 protein containing the VQ motif and clarified the functional mechanisms involved in the defense of wheat against R. cerealis. TaVQ22 silencing reduced the accumulation of ROS and enhanced the resistance of wheat to R. cerealis. In addition, we showed that TaVQ22 regulated ROS generation by interacting with the WRKY transcription factor TaWRKY19-2B, thereby indicating that TaVQ22 and TaWRKY19-2B formed complexes in the plant cell nucleus. Yeast two-hybrid analysis showed that the VQ motif in TaVQ22 is crucial for the interaction, where it inhibits the transcriptional activation function of TaWRKY19-2B. In summary, TaVQ22 interacts with TaWRKY19-2B to regulate ROS homeostasis and negatively regulate the defense response to R. cerealis infection. This study provides novel insights into the mechanism that allows VQ protein to mediate the immune response in plants.

Extreme Diversity of Mycoviruses Present in Single Strains of Rhizoctonia cerealis, the Pathogen of Wheat Sharp Eyespot

Rhizoctonia cerealis is the pathogen of wheat sharp eyespot, which occurs throughout temperate wheat-growing regions of the world. In this project, the genomes of viruses from four strains of R. cerealis were analyzed based on Illumina high-throughput transcriptome sequencing (RNA-Seq) data. After filtering out reads that mapped to the fungal genome, viral genomes were assembled. In total, 131 virus-like sequences containing complete open reading frames (ORFs), belonging to 117 viruses, were obtained. Based on phylogenetic analysis, some of them were identified as novel members of the families Curvulaviridae, Endornaviridae, Hypoviridae, Mitoviridae, Mymonaviridae, and Phenuiviridae, while others were unclassified viruses. Most of these viruses from R. cerealis were significantly different from the viruses already reported. We propose the establishment of a new family, Rhizoctobunyaviridae, and two new genera, Rhizoctobunyavirus and Iotahypovirus. We further clarified the distribution and coinfection of these viruses in the four strains. Surprisingly, 39 viral genomes of up to 12 genera were found in strain R1084. Strain R0942, containing the fewest viruses, also contained 21 viral genomes belonging to 10 genera. Based on the RNA-Seq data, we estimated the accumulation level of some viruses in host cells and found that the mitoviruses in R. cerealis generally have very high accumulation. In conclusion, in the culturable phytopathogenic fungus R. cerealis, we discovered a considerable diversity of mycoviruses and a series of novel viruses. This study expands our understanding of the mycoviral diversity in R. cerealis and provides a rich resource for the further use of mycoviruses to control wheat sharp eyespot. IMPORTANCE Rhizoctonia cerealis is a binucleate fungus that is widely distributed worldwide and can cause sharp eyespot disease in cereal crops. In this study, 131 virus-like sequences belonging to 117 viruses were obtained based on analysis of high-throughput RNA-Seq data from four strains of R. cerealis. Many of these viruses were novel members of various virus families, while others were unclassified viruses. As a result, a new family named Rhizoctobunyaviridae and two new genera, Rhizoctobunyavirus and Iotahypovirus, were proposed. Moreover, the discovery of multiple viruses coinfecting a single host and the high accumulation levels of mitoviruses have shed light on the complex interactions between different viruses in a single host. In conclusion, a significant diversity of mycoviruses was discovered in the culturable phytopathogenic fungus R. cerealis. This study expands our understanding of mycoviral diversity, and provides a valuable resource for the further utilization of mycoviruses to control wheat diseases.

Trehalase Inhibitor Validamycin May Have Additional Mechanisms of Toxicology against Rhizoctonia cerealis

Sharp eyespot is a crucial disease affecting cereal plants, such as bread wheat (Triticum aestivum) and barley (Hordeum vulgare), and is primarily caused by the pathogenic fungus Rhizoctonia cerealis. As disease severity has increased, it has become imperative to find an effective and reasonable control strategy. One such strategy is the use of the trehalose analog, validamycin, which has been shown to have a potent inhibitory effect on several trehalases found in both insects and fungi, and is widely used as a fungicide in agriculture. In this study, we demonstrated that 0.5 μg/mL validamycin on PDA plates had an inhibitory effect on R. cerealis strain R0301, but had no significant impact on Fusarium graminearum strain PH-1. Except for its inhibiting the trehalase activity of pathogenic fungi, little is known about its mechanism of action. Six trehalase genes were identified in the genome of R. cerealis, including one neutral trehalase and five acidic trehalase genes. Enzyme activity assays indicated that treatment with 5 μg/mL validamycin significantly reduces trehalase activity, providing evidence that validamycin treatment does indeed affect trehalase, even though the expression levels of most trehalase genes, except Rc17406, were not obviously affected. Transcriptome analysis revealed that treatment with validamycin downregulated genes involved in metabolic processes, ribosome biogenesis, and pathogenicity in the R. cerealis. KEGG pathway analysis further showed that validamycin affected genes related to the MAPK signaling pathway, with a significant decrease in ribosome synthesis and assembly. In conclusion, our results indicated that validamycin not only inhibits trehalose activity, but also affects the ribosome synthesis and MAPK pathways of R. cerealis, leading to the suppression of fungal growth and pesticidal effects. This study provides novel insights into the mechanism of action of validamycin.

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.

Identification of Quantitative Trait Loci for Resistance to Wheat Sharp Eyespot in the Einstein × Tubbs Recombinant Inbred Line Population

Wheat sharp eyespot (SES), caused by the soilborne pathogen Rhizoctonia cerealis Van der Hoeven (teleomorph: Ceratobasidium cereale), is a common stem disease of wheat globally. The disease caused a severe and extensive epidemic throughout the Willamette Valley of Oregon in 2014 and has remained one of the most important wheat diseases in this region. However, little was known about the genetics of host resistance to this disease. A recombinant inbred line (RIL) population with 257 lines developed from a cross of Einstein × Tubbs was used to study SES resistance of wheat. The phenotyping was conducted at two locations and in 3 years. Genotyping by sequencing was done by using Illumina HiSeq 3000. Low broad-sense heritability across four environments was obtained. The results of analysis of variance demonstrated that disease severity was significantly different among RILs for the data combined over environments and for one of the individual environments. Four SES resistance quantitative trait loci (QTL) were detected, including QSES-1A, QSES-2B, QSES-6A, and QSES-7A, and explained 5.9, 5.9, 8.8, and 8.3%, respectively, of the phenotypic variance. All four QTL overlapped or are in close proximity with one or more plant defense genes, and could lay the foundation for marker-assisted breeding.

Antifungal Activity and Plant Growth-Promoting Properties of Bacillus mojovensis B1302 against Rhizoctonia Cerealis

Rhizoctonia cerealis is a worldwide soil-borne pathogenic fungus that significantly infects wheat and causes sharp eyespot in China. However, the biocontrol strains used for the control of Rhizoctonia cerealis are insufficient. In the present study, antagonistic strain B1302 from the rhizosphere of wheat were isolated and identified as Bacillus mojovensis based on their morphological, physiological, and biochemical characteristics, and their 16S rDNA sequence. Culture filtrate of strain B1302 had a broad antifungal spectrum. In order to improve the antifungal activity of B1302, response surface methodology (RSM) was used to optimize the culture conditions. The final medium composition and culture conditions were 13.2 g/L of wheat bran, 14.1 g/L of soybean meal, 224 r/min of rotation speed, 7.50 of initial pH, and 1.5 × 10⁸ CFU/mL of inoculation amount at 35 °C for a culture duration of 72 h. B. mojavensis B1302 inhibited the hyphae growth of R.cerealis and produced hydrolytic enzymes (protease, chitinase, and glucanase), IAA, and had N-fixing potentiality and P-solubilisation capacity. It can also promote wheat seedling growth in potted plants. The disease incidence and index of wheat seedlings were consistent with the effect of commercial pesticides under treatment with culture filtrate. The biocontrol efficacy of culture filtrate was significant—up to 65.25%. An animal toxicological safety analysis suggested that culture filtrate was safe for use and could be developed into an effective microbial fungicide to control wheat sharp eyespot.

Comparative transcriptome analysis of resistant and susceptible wheat in response to Rhizoctonia cerealis

BACKGROUND

Sheath blight is an important disease caused by Rhizoctonia cerealis that affects wheat yields worldwide. No wheat varieties have been identified with high resistance or immunity to sheath blight. Understanding the sheath blight resistance mechanism is essential for controlling this disease. In this study, we investigated the response of wheat to Rhizoctonia cerealis infection by analyzing the cytological changes and transcriptomes of common wheat 7182 with moderate sensitivity to sheath blight and H83 with moderate resistance.

RESULTS

The cytological observation showed that the growth of Rhizoctonia cerealis on the surface and its expansion inside the leaf sheath tissue were more rapid in the susceptible material. According to the transcriptome sequencing results, a total of 88685 genes were identified in both materials, including 20156 differentially expressed genes (DEGs) of which 12087 was upregulated genes and 8069 was downregulated genes. At 36 h post-inoculation, compared with the uninfected control, 11498 DEGs were identified in resistant materials, with 5064 downregulated genes and 6434 upregulated genes, and 13058 genes were detected in susceptible materials, with 6759 downregulated genes and 6299 upregulated genes. At 72 h post-inoculation, compared with the uninfected control, 6578 DEGs were detected in resistant materials, with 2991 downregulated genes and 3587 upregulated genes, and 7324 genes were detected in susceptible materials, with 4119 downregulated genes and 3205 upregulated genes. Functional annotation and enrichment analysis showed that the main pathways enriched for the DEGs included biosynthesis of secondary metabolites, carbon metabolism, plant hormone signal transduction, and plant-pathogen interaction. In particular, phenylpropane biosynthesis pathway is specifically activated in resistant variety H83 after infection. Many DEGs also belonged to the MYB, AP2, NAC, and WRKY transcription factor families.

CONCLUSIONS

Thus, we suggest that the normal functioning of plant signaling pathways and differences in the expression of key genes and transcription factors in some important metabolic pathways may be important for defending wheat against sheath blight. These findings may facilitate further exploration of the sheath blight resistance mechanism in wheat and the cloning of related genes.

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.

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.

Genetic Dissection of Adult Plant Resistance to Sharp Eyespot Using an Updated Genetic Map of Niavt14 × Xuzhou25 Winter Wheat Recombinant Inbred Line Population

Wheat sharp eyespot, a disease mainly caused by soilborne fungus Rhizoctonia cerealis, is a threat to world wheat production. Wheat's genetic resistance to sharp eyespot is a potential approach to reducing the application of fungicides and farming practice inputs. To identify the genetic basis of sharp eyespot resistance in Niavt14, a recombinant inbred line population comprising 215 F₈ lines from Niavt14 × Xuzhou25, was developed. An earlier linkage map (148 simple sequence repeat markers) was updated with 5,792 polymorphic Affymetrix Axiom 55K single-nucleotide polymorphisms to a new map of 5,684.2 centimorgans with 1,406 nonredundant markers. The new linkage map covered all 21 chromosomes of common wheat and showed a good collinearity with the IWGSC RefSeq v1.0 genome. We conducted quantitative trait locus (QTL) mapping for sharp eyespot resistance using the adult plant response data from the field of five consecutive growing seasons and one greenhouse test. Two stable QTL on chromosomes 2B and 7D that were identified in the previous study were confirmed, and three novel, stable QTL, explaining 4.0 to 17.5% phenotypic variation, were mapped on 1D, 6D, and 7A, which were independent of QTL for phenology and plant height. The QTL on 1D, 2B, 6D, and 7A showed low frequencies in 384 landraces (0 to 10%) and 269 elite cultivars (5 to 23%) from the southern winter wheat region and the Yellow and Huai River Valley facultative wheat region in China, respectively. These identified QTL could be used in wheat breeding programs for improving sharp eyespot resistance through marker-assisted selection.

Genome structure and diversity of novel endornaviruses from wheat sharp eyespot pathogen Rhizoctonia cerealis

Rhizoctonia cerealis (teleomorph Ceratobasidium cereale) is a soil-borne plant pathogenic fungus that can cause sharp eyespot in wheat or yellow patch in grasses. In this study, 21 new endornavirus genomes were obtained from five R. cerealis strains through the high-throughput sequencing of viral double-stranded RNA. Eighteen viruses were identified as Alphaendornavirus, and three viruses were identified as new species of Betaendornavirus on the basis of the phylogenetic analysis of the deduced amino acid sequences of RNA-dependent RNA polymerase. Notably, 12 of the new alphaendornaviruses could encode two open reading frames (ORFs), which were a rare feature of Endornaviridae. The amino acid sequences encoded by ORF2 from different endornaviruses had very low identity, and their functions and evolution origins remained unclear. Different endornavirus species with remarkably different genome structures could be found in the same R. cerealis strain. This study indicated that endornaviruses are common in R. cerealis and display wide diversity. Betaendornaviruses were found in R. cerealis, and a new species was proposed. This study is the first to report that the endornaviruses from R. cerealis can encode two ORFs and enhances our understanding of the viruses in the Endornaviridae family.

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.

Methods for Screening Wheat Genotypes for Resistance to Sharp Eyespot in the Field and Greenhouse

Screening methodology of wheat genotypes for resistance to sharp eyespot (caused by Rhizoctonia cerealis) was developed. Disease severity differed among cultivars and between field and greenhouse trials. However, the cultivars Bobtail and Rosalyn had consistently lower severity in field experiments with high sharp eyespot disease pressure. Artificial inoculation was crucial to achieving adequate disease levels for effective screening but planting date had very little effect. Greenhouse inoculation of adult wheat plants was much less successful in categorizing resistance to sharp eyespot. Seedling inoculations in the greenhouse were highly inadequate as a screening method. Selection for resistance to sharp eyespot by artificial inoculation in field trials is feasible in wheat breeding programs.

Isolation, heterologous expression, and purification of a novel antifungal protein from Bacillus subtilis strain Z-14

BACKGROUND

Wheat sheath blight, a soil borne fungal disease caused by Rhizoctonia cerealis, is considered as one of the most serious threats to wheat worldwide. Bacillus subtilis Z-14 was isolated from soil sampled from a wheat rhizosphere and was confirmed to have strong antifungal activity against R. cerealis.

RESULTS

An antifungal protein, termed F2, was isolated from the culture supernatant of Z-14 strain using precipitation with ammonium sulfate, anion exchange chromatography, and reverse phase chromatography. Purified F2 had a molecular mass of approximately 8 kDa, as assessed using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Edman degradation was used to determine the amino acid sequence of the N-terminus, which was NH₂ASGGTVGIYGANMRS. This sequence is identical to a hypothetical protein RBAM_004680 (YP_001420098.1) synthesized by B. amyloliquefaciens FZB42. The recombinant F2 protein (rF2) was heterologously expressed in the yeast host Pichia pastoris, purified using a Niaffinity column, and demonstrated significant antifungal activity against R. cerealis. The purified rF2 demonstrated broad spectrum antifungal activity against different varieties of fungi such as Fusarium oxysporum, Verticillium dahliae, Bipolaris papendorfii, and Fusarium proliferatum. rF2 was thermostable, retaining 91.5% of its activity when incubated for 30 min at 100 °C. Meanwhile, rF2 maintained its activity under treatment by proteinase K and trypsin and over a wide pH range from 5 to 10.

CONCLUSIONS

A novel antifungal protein, F2, was purified from biocontrol Bacillus subtilis Z-14 strain fermentation supernatant and heterologously expressed in Pichia pastoris to verify its antifungal activity against R. cerealis and the validity of the gene encoding F2. Considering its significant antifungal activity and stable characteristics, protein F2 presents an alternative compound to resist fungal infections caused by R. cerealis.

The Cysteine-Rich Repeat Protein TaCRR1 Participates in Defense against Both Rhizoctonia cerealis and Bipolaris sorokiniana in Wheat

The domain of unknown function 26 (DUF26), harboring a conserved cysteine-rich motif (C-X8-C-X2-C), is unique to land plants. Several cysteine-rich repeat proteins (CRRs), belonging to DUF26-containing proteins, have been implicated in the defense against fungal pathogens in ginkgo, cotton, and maize. However, little is known about the functional roles of CRRs in the important staple crop wheat (Triticum aestivum). In this study, we identified a wheat CRR-encoding gene TaCRR1 through transcriptomic analysis, and dissected the defense role of TaCRR1 against the soil-borne fungi Rhizoctonia cerealis and Bipolaris sorokiniana, causal pathogens of destructive wheat diseases. TaCRR1 transcription was up-regulated in wheat towards B. Sorokiniana or R. cerealis infection. The deduced TaCRR1 protein contained a signal peptide and two DUF26 domains. Heterologously-expressed TaCRR1 protein markedly inhibited the mycelia growth of B. sorokiniana and R. cerealis. Furthermore, the silencing of TaCRR1 both impaired host resistance to B. sorokiniana and R. cerealis and repressed the expression of several pathogenesis-related genes in wheat. These results suggest that the TaCRR1 positively participated in wheat defense against both B. sorokiniana and R. cerealis through its antifungal activity and modulating expression of pathogenesis-related genes. Thus, TaCRR1 is a candidate gene for improving wheat resistance to B. sorokiniana and R. cerealis.

Use of resistant Rhizoctonia cerealis strains to control wheat sharp eyespot using organically developed pig manure fertilizer

The effects of Rhizoctonia cerealis resistant strains to control wheat sharp eyespot in pig manure developed organic fertilizer were investigated. Organic fertilizer was produced in a self-built aerated static composting box (0.37 m³) and later assessed using values of temperature, moisture, pH, C/N ratio, nitrogen transformation, fluvic- and humic acid composition and germination index. Together, these assessments indicated that disposed pig manure could be considered as a non-hazardous fertilizer. By adding R. cerealis resistant strains (Bacillus flexus, B. amyloliquefaciens, B. amyloliquefaciens, B. licheniformis and Paenibacillus sp.), we were successfully able to develop wheat sharp eyespot inhibiting bio-organic fertilizer. Our results showed that soil applied with 10 t/hm² of the developed bio-organic fertilizer significantly improved the dry weight of the wheat, promoted chlorophylls and soluble protein composition and effectively prevented the wheat from sharp eyespot. The protection rate (77.1%) of our developed bio-organic fertilizer is 42.3% higher than the tested commercially available organic fertilizer (34.8%), and the disease index is significantly lower (P < 0.001) than both the control and commercial organic fertilizer. In conclusion, the prevention and control effects of our developed bio-organic fertilizer on wheat sharp eyespot were both significant and promising.

Genome-Wide Identification of M35 Family Metalloproteases in Rhizoctonia cerealis and Functional Analysis of RcMEP2 as a Virulence Factor during the Fungal Infection to Wheat

Rhizoctonia cerealis is the causal pathogen of the devastating disease, sharp eyespot, of the important crop wheat (Triticum aestivum L.). In phytopathogenic fungi, several M36 metalloproteases have been implicated in virulence, but pathogenesis roles of M35 family metalloproteases are largely unknown. Here, we identified four M35 family metalloproteases from R. cerealis genome, designated RcMEP2–RcMEP5, measured their transcriptional profiles, and investigated RcMEP2 function. RcMEP2-RcMEP5 are predicted as secreted metalloproteases since each protein sequence contains a signal peptide and an M35 domain that includes two characteristic motifs HEXXE and GTXDXXYG. Transcription levels of RcMEP2-RcMEP5 markedly elevated during the fungus infection to wheat, among which RcMEP2 expressed with the highest level. Functional dissection indicated that RcMEP2 and its M35 domain could trigger H₂O₂ rapidly-excessive accumulation, induce cell death, and inhibit expression of host chitinases. This consequently enhanced the susceptibility of wheat to R. cerealis and the predicated signal peptide of RcMEP2 functions required for secretion and cell death-induction. These results demonstrate that RcMEP2 is a virulence factor and that its M35 domain and signal peptide are necessary for the virulence role of RcMEP2. This study facilitates a better understanding of the pathogenesis mechanism of metalloproteases in phytopathogens including R. cerealis.

Global Characterization of GH10 Family Xylanase Genes in Rhizoctonia cerealis and Functional Analysis of Xylanase RcXYN1 During Fungus Infection in Wheat

Wheat (Triticum aestivum L.) is an important staple crop. Rhizoctonia cerealis is the causal agent of diseases that are devastating to cereal crops, including wheat. Xylanases play an important role in pathogenic infection, but little is known about xylanases in R. cerealis. Herein, we identified nine xylanase-encoding genes from the R. cerealis genome, named RcXYN1–RcXYN9, examined their expression patterns, and investigated the pathogenicity role of RcXYN1. RcXYN1–RcXYN9 proteins contain two conserved glutamate residues within the active motif in the glycoside hydrolase 10 (GH10) domain. Of them, RcXYN1–RcXYN4 are predicted to be secreted proteins. RcXYN1–RcXYN9 displayed different expression patterns during the infection process of wheat, and RcXYN1, RcXYN2, RcXYN5, and RcXYN9 were expressed highly across all the tested inoculation points. Functional dissection indicated that the RcXYN1 protein was able to induce necrosis/cell-death and H₂O₂ generation when infiltrated into wheat and Nicotiana benthamiana leaves. Furthermore, application of RcXYN1 protein followed by R. cerealis led to significantly higher levels of the disease in wheat leaves than application of the fungus alone. These results demonstrate that RcXYN1 acts as a pathogenicity factor during R. cerealis infection in wheat. This is the first investigation of xylanase genes in R. cerealis, providing novel insights into the pathogenesis mechanisms of R. cerealis.

Ecotopic Expression of the Antimicrobial Peptide DmAMP1W Improves Resistance of Transgenic Wheat to Two Diseases: Sharp Eyespot and Common Root Rot

Wheat (Triticum aestivum L.) is an important staple crop. Sharp eyespot and common root rot are destructive diseases of wheat. Antimicrobial peptides (AMPs) are small peptides with broad-spectrum antimicrobial activity. In this study, we synthesized the DmAMP1W gene, encoding Dahlia merckii DmAMP1, and investigated the antifungal role of DmAMP1W in vitro and in transgenic wheat. Protein electrophoresis analysis and in vitro inhibition results demonstrated that the synthesized DmAMP1W correctly translated to the expected peptide DmAMP1W, and the purified peptide inhibited growths of the fungi Rhizoctonia cerealis and Bipolaris sorokiniana, the pathogenic causes of wheat sharp eyespot and common root rot. DmAMP1W was introduced into a wheat variety Zhoumai18 via Agrobacterium-mediated transformation. The molecular characteristics indicated that DmAMP1W could be heritable and expressed in five transgenic wheat lines in T₁–T₂ generations. Average sharp eyespot infection types of these five DmAMP1W transgenic wheat lines in T₁–T₂ generations decreased 0.69–1.54 and 0.40–0.82 compared with non-transformed Zhoumai18, respectively. Average common root rot infection types of these transgenic lines and non-transformed Zhoumai18 were 1.23–1.48 and 2.27, respectively. These results indicated that DmAMP1W-expressing transgenic wheat lines displayed enhanced-resistance to both sharp eyespot and common root rot. This study provides new broad-spectrum antifungal resources for wheat breeding.

Genome-Wide Identification and Expression Analysis of Cutinase Gene Family in Rhizoctonia cerealis and Functional Study of an Active Cutinase RcCUT1 in the Fungal-Wheat Interaction

Wheat (Triticum aestivum L.) is a staple food of more than 50% of global population. Rhizoctonia cerealis is the causal agent of sharp eyespot, a devastating disease of cereal crops including wheat. Cutinases produced by fungal pathogens play important roles in host-pathogen compatible interactions, but little is known about cutinases in R. cerealis. In this study, we identified a total of six cutinase encoding genes from R. cerealis genome, designated as RcCUT1-RcCUT6, analyzed their expression patterns during the infection, and determined virulence role for RcCUT1. All the proteins, RcCUT1-RcCUT6, contain a highly conserved GYSKG motif and another conserved C-x(3)-D-x(2)-C-x(2)-[GS]-[GSD]-x(4)-[AP]-H motif in the carbohydrate esterase 5 domain. The RcCUT1, RcCUT2, RcCUT4, and RcCUT5 are predicted to be secreted proteins containing four cysteine residues. These six cutinase genes had different expression patterns during the fungal infection process to wheat, among which RcCUT1 was highly expressed across all the infection time points but RcCUT6 was not expressed at all and the others were expressed only at certain time points. Further, RcCUT1 was heterologously expressed in Escherichia coli to obtain a purified protein. The purified RcCUT1 was shown to possess the cutinase activity and be able to induce necrosis, H2O2 accumulation, and expression of defense-related genes when infiltrated into wheat and Nicotiana benthamiana leaves. In contrast, RcCUT1 protein with serine mutation at the first motif had no cutinase activity, consequently lost the ability to induce necrosis. Noticeably, application of the purified RcCUT1 with R. cerealis led to significantly higher levels of the disease in wheat leaves than application of the fungus alone. These results strongly suggest that RcCUT1 serves as a virulence factor for the fungus. This is the first investigation of the cutinase genes in R. cerealis and the findings provide an important insight into pathogenesis mechanisms of R. cerealis on wheat.

Spatial Distribution of Root and Crown Rot Fungi Associated With Winter Wheat in the North China Plain and Its Relationship With Climate Variables

The distribution frequency of pathogenic fungi associated with root and crown rot of winter wheat (Triticum aestivum) from 104 fields in the North China Plain was determined during the period from 2013 to 2016. The four most important species identified were Bipolaris sorokiniana (24.0% from roots; 33.7% from stems), Fusarium pseudograminearum (14.9% from roots; 27.8% from stems), Rhizoctonia cerealis (1.7% from roots; 4.4% from stems), and Gaeumannomyces graminis var. tritici (9.8% from roots; 4.4% from stems). We observed that the recovered species varied with the agronomic zone. Fusarium pseudograminearum was predominant in regions 1 and 3, whereas F. graminearum, F. acuminatum, and R. cerealis were predominant in regions 2 and 4. The incidence of F. pseudograminearum and R. cerealis was significantly different between regions 1 and 4, while no significant association was found in the distribution of the other species and the agronomic zones. A negative correlation between the frequency of occurrence of F. pseudograminearum and mean annual precipitation during 2013-2016 (r = -0.71; P < 0.01) in the North China Plain and a positive correlation between the mean annual precipitation during 2013-2016 and the frequency of occurrence of F. asiaticum (r = 0.74; P < 0.01) were observed. Several Fusarium species were also found with low frequencies of ~2.1%-3.4 % (F. graminearum, F. acuminatum, and F. sinensis) and ~0.1%-1.3% (F. equiseti, F. oxysporum, F. proliferatum, F. culmorum, F. avenaceum, and F. asiaticum). In more than 93% of the fields, from the root and crown tissues of wheat, two or more root and crown rot species were isolated. The coexistence of Fusarium spp. and B. sorokiniana in one field (65.4%) or in individual plants (11.6%) was more common than for the other species combinations. Moreover, this is the first report on the association between F. sinensis and root and crown rot of wheat. Our results would be useful in the framing guidelines for the management of root and crown rot fungi in wheat in different agronomic zones of the North China Plain.

Silencing of the Wheat Protein Phosphatase 2A Catalytic Subunit TaPP2Ac Enhances Host Resistance to the Necrotrophic Pathogen Rhizoctonia cerealis

Eukaryotic type 2A protein phosphatases (protein phosphatase 2A, PP2A) consist of a scaffold subunit A, a regulatory subunit B, and a catalytic subunit C. Little is known about the roles of PP2Ac proteins that are involved in plant responses to necrotrophic fungal pathogens. Sharp eyespot, caused by the necrotrophic fungus Rhizoctonia cerealis, is a destructive disease of wheat (Triticum aestivum), an important staple food crop. Here, we isolated TaPP2Ac-4D from wheat, which encodes a catalytic subunit of the heterotrimeric PP2A, and characterized its properties and role in plant defense response to R. cerealis. Based on the sequence alignment of TaPP2Ac-4D with the draft sequences of wheat chromosomes from the International Wheat Genome Sequencing Consortium (IWGSC), it was found that TaPP2Ac-4D gene is located on the long arm of the wheat chromosome 4D and has two homologs assigned on wheat chromosomes 4A and 4B. Sequence and phylogenetic tree analyses revealed that the TaPP2Ac protein is a typical member of the PP2Ac family and belongs to the subfamily II. TaPP2Ac-4B and TaPP2Ac-4D displayed higher transcriptional levels in the R. cerealis-susceptible wheat cultivar Wenmai 6 than those seen in the resistant wheat line CI12633. The transcriptional levels of TaPP2Ac-4B and TaPP2Ac-4D were significantly elevated in wheat R. cerealis after infection and upon H₂O₂ treatment. Virus-induced gene silencing results revealed that the transcriptional knockdown of TaPP2Ac-4D and TaPP2Ac-4B significantly increased wheat resistance to R. cerealis infection. Meanwhile, the transcriptional levels of certain pathogenesis-related (PR) and reactive oxygen species (ROS)-scavenging enzyme encoding genes were increased in TaPP2Ac-silenced wheat plants. These results suggest that TaPP2Ac-4B and TaPP2Ac-4D negatively regulate defense response to R. cerealis infection possibly through modulation of the expression of certain PR and ROS-scavenging enzyme genes in wheat. This study reveals a novel function of the plant PP2Ac genes in plant immune responses.

Variation of rDNA Internal Transcribed Spacer Sequences in Rhizoctonia cerealis

Fifty-four single protoplast isolates (SPIs) were regenerated from three Rhizoctonia cerealis strains. A total of 169 rDNA-ITS regions were cloned and sequenced from these 54 SPIs. Variations in the ITS1 and ITS2 regions that flank the 5.8S gene were found within clones from the same strain, as well as within clones from the same SPI. These include variations in GC content and ITS length, and single-nucleotide polymorphisms (SNPs). The different strains and SPIs GC contents range from 40.25 to 41.74% and from 42.40 to 45.02%, in the ITS1 and ITS2 regions, respectively. All SNPs occur in the ITS1 and ITS2 regions, with 3-6 and 4-6 polymorphic sites in each region, respectively, in the different strains. SNP variation is relatively stable within the same strain. For example, the 89 ITS sequences generated from isolate WK-207, regardless of SPI or clone, predominantly cluster into two separate clades on a phylogenetic tree, suggesting that nuclei genetic heterogeneity is related to ITS variation in R. cerealis. Although rDNA-ITS sequences from the three strains and different SPIs are somewhat variable, all of our ITS sequences cluster together in anastomosis subgroup AG-DI during phylogenetic analysis. The ITS variation we observed does not negatively influence R. cerealis anastomosis group or subgroup classification.

Homozygous and heterozygous point mutations in succinate dehydrogenase subunits b, c and d of Rhizoctonia cerealis conferring resistance to thifluzamide

BACKGROUND

Thifluzamide, a succinate dehydrogenase inhibitor (SDHI) fungicide, is a promising fungicide for controlling wheat sharp eyespot (WSE). WSE is caused by Rhizoctonia cerealis. Information on the resistance mechanism of this pathogen to thifluzamide remains unavailable.

RESULTS

We used selective reculturing and UV mutagenesis to generate thifluzamide-resistant mutants. Thifluzamide-resistant mutants were only generated through UV mutagenesis. Sequence analysis of succinate dehydrogenase (Sdh) genes revealed that two mutants had no mutation in RCSdhB, RCSdhC and RCSdhD, and the other 18 mutants all had at least one mutation in RCSdhB, RCSdhC or RCSdhD, either in a homozygous or heterozygous state. The majority of mutants included either RCSdhD-H116Y or RCSdhC-H139Y. They showed slight resistance to boscalid, bixafen and penflufen. Only one mutant possessed RCSdhB-H246Y, and it showed medium resistance to boscalid and penflufen and a slight resistance to bixafen. All the thifluzamide mutants were sensitive to flutolanil. Compared with their parental isolates, these mutants present no or minor fitness penalties.

CONCLUSION

Homozygous and heterozygous point mutations in the succinate dehydrogenase subunits b, c and d of R. cerealis may be involved in thifluzamide resistance. © 2016 Society of Chemical Industry.

A Wheat Cinnamyl Alcohol Dehydrogenase TaCAD12 Contributes to Host Resistance to the Sharp Eyespot Disease

Sharp eyespot, caused mainly by the necrotrophic fungus Rhizoctonia cerealis, is a destructive disease in hexaploid wheat (Triticum aestivum L.). In Arabidopsis, certain cinnamyl alcohol dehydrogenases (CADs) have been implicated in monolignol biosynthesis and in defense response to bacterial pathogen infection. However, little is known about CADs in wheat defense responses to necrotrophic or soil-borne pathogens. In this study, we isolate a wheat CAD gene TaCAD12 in response to R. cerealis infection through microarray-based comparative transcriptomics, and study the enzyme activity and defense role of TaCAD12 in wheat. The transcriptional levels of TaCAD12 in sharp eyespot-resistant wheat lines were significantly higher compared with those in susceptible wheat lines. The sequence and phylogenetic analyses revealed that TaCAD12 belongs to IV group in CAD family. The biochemical assay proved that TaCAD12 protein is an authentic CAD enzyme and possesses catalytic efficiencies toward both coniferyl aldehyde and sinapyl aldehyde. Knock-down of TaCAD12 transcript significantly repressed resistance of the gene-silenced wheat plants to sharp eyespot caused by R. cerealis, whereas TaCAD12 overexpression markedly enhanced resistance of the transgenic wheat lines to sharp eyespot. Furthermore, certain defense genes (Defensin, PR10, PR17c, and Chitinase1) and monolignol biosynthesis-related genes (TaCAD1, TaCCR, and TaCOMT1) were up-regulated in the TaCAD12-overexpressing wheat plants but down-regulated in TaCAD12-silencing plants. These results suggest that TaCAD12 positively contributes to resistance against sharp eyespot through regulation of the expression of certain defense genes and monolignol biosynthesis-related genes in wheat.

The wheat AGC kinase TaAGC1 is a positive contributor to host resistance to the necrotrophic pathogen Rhizoctonia cerealis

Considerable progress has been made in understanding the roles of AGC kinases in mammalian systems. However, very little is known about the roles of AGC kinases in wheat (Triticum aestivum). The necrotrophic fungus Rhizoctonia cerealis is the major pathogen of the destructive disease sharp eyespot of wheat. In this study, the wheat AGC kinase gene TaAGC1, responding to R. cerealis infection, was isolated, and its properties and role in wheat defence were characterized. R. cerealis-resistant wheat lines expressed TaAGC1 at higher levels than susceptible wheat lines. Sequence and phylogenetic analyses showed that the TaAGC1 protein is a serine/threonine kinase belonging to the NDR (nuclear Dbf2-related) subgroup of AGC kinases. Kinase activity assays proved that TaAGC1 is a functional kinase and the Asp-239 residue located in the conserved serine/threonine kinase domain of TaAGC1 is required for the kinase activity. Subcellular localization assays indicated that TaAGC1 localized in the cytoplasm and nucleus. Virus-induced TaAGC1 silencing revealed that the down-regulation of TaAGC1 transcripts significantly impaired wheat resistance to R. cerealis. The molecular characterization and responses of TaAGC1 overexpressing transgenic wheat plants indicated that TaAGC1 overexpression significantly enhanced resistance to sharp eyespot and reduced the accumulation of reactive oxygen species (ROS) in wheat plants challenged with R. cerealis. Furthermore, ROS-scavenging and certain defence-associated genes were up-regulated in resistant plants overexpressing TaAGC1 but down-regulated in susceptible knock-down plants. These results suggested that the kinase TaAGC1 positively contributes to wheat immunity to R. cerealis through regulating expression of ROS-related and defence-associated genes.

Baseline sensitivity and resistance risk assessmemt of Rhizoctonia cerealis to thifluzamide, a succinate dehydrogenase inhibitor

During 2010-2012, a total of 120 isolates of Rhizoctonia cerealis were collected from wheat with symptoms of sharp eyespot in four provinces (Henan, Shandong, Anhui and Jiangsu) in China. All the isolates were determined for baseline sensitivity to thifluzamide, a succinate dehydrogenase inhibitor (SDHI) with strong antifungal activity. The sampled pathogenic populations, never exposed to SDHIs, had similar sensitivity to trifluzamide (0.025-0.359 µg/ml) in the four regions and over the two years. The baseline sensitivity was distributed as a skewed unimodal curve with a mean EC50 value (effective concentrations for 50% inhibiting mycelial growth) of 0.064 ± 0.013 µg/ml. The resistance risk of R. cerealis to thifluzamide was further evaluated in vitro. Two thifluzamide-resistant mutants of R. cerealis were obtained by culturing on thifluzamide-amended plates. The resistance factors (RF = EC50 value of a mutant/EC50 value of the wild type progenitor of the mutant) were 120 and 40 for two R. cerealis mutants, respectively. All the mutants exhibited similar fitness after 10 successive transfers when compared to their wild-type parents in mycelial growth, sclerotia production, and virulence. However, the two thifluzamide-resistant mutants differed significantly in sensitivity to boscalid and flutolanil. Therefore, a low-to-moderate risk of resistance development was recommended for thifluzamide.

Various effects of the photosystem II--inhibiting herbicides on 5-n-alkylresorcinol accumulation in rye seedlings

The effect of three PSII-inhibiting herbicides, lenacil, linuron, and pyrazon, on the accumulation of 5-n-alkylresorcinols in rye seedlings (Secale cereale L.) grown under various light and thermal conditions was studied. All used chemicals increased resorcinolic lipid content in both green and etiolated plants grown at 29 °C. At 22 °C pyrazon and lenacil decreased the content of alkylresorcinols in plants kept in the darkness and increased their amount in the light-grown seedlings. In turn, level of resorcinolic lipids was decreased by linuron in both etiolated and green plants. At the lowest tested temperature lenacil enhanced production of alkylresorcinols only in etiolated rye seedlings, whereas the light-independent stimulatory action of pyrazon on alkylresorcinol accumulation in rye grown at 15 °C was observed. Additionally, only the latter did not exert a negative effect on rye seedling growth under any of tested conditions. Compared with respective controls, the herbicides used also markedly modified the qualitative pattern of resorcinolic homologs. Interestingly, the observed changes generally favored the enhanced antifungal activity of these compounds. Our study provides novel information on the influence of PSII inhibitors on alkylresorcinol metabolism in rye seedlings. The unquestionable achievement of this work is the observation that low dose of pyrazon mainly stimulated both growth and alkylresorcinol synthesis in rye seedlings, a non-target plant. Moreover, our experimental work showed unambiguously that the observed pyrazon-driven accumulation and homolog pattern modification of alkylresorcinols dramatically improved the resistance of winter rye to infections caused by Rhizoctonia cerealis.

The phosphotransferase system gene ptsI in the endophytic bacterium Bacillus cereus is required for biofilm formation, colonization, and biocontrol against wheat sharp eyespot

Natural resistance of wheat plants to wheat sharp eyespot is inadequate, and new strategies for controlling the disease are required. Biological control is an alternative and attractive way of reducing the use of chemicals in agriculture. In this study, we investigated the biocontrol properties of endophytic bacterium Bacillus cereus strain 0-9, which was isolated from the root systems of healthy wheat varieties. The phosphotransferase system is a major regulator of carbohydrate metabolism in bacteria. Enzyme I is one of the protein components of this system. Specific disruption and complementation of the enzyme I-coding gene ptsI from B. cereus was achieved through homologous recombination. Disruption of ptsI in B. cereus caused a 70% reduction in biofilm formation, a 30.4% decrease in biocontrol efficacy, and a 1000-fold reduction in colonization. The growth of ΔptsI mutant strain on G-tris synthetic medium containing glucose as the exclusive carbon source was also reduced. Wild-type properties could be restored to the ΔptsI mutant strain by ptsI complementation. These results suggested that ptsI may be one of the key genes involved in biofilm formation, colonization, and biocontrol of B. cereus and that B. cereus wild-type strain 0-9 may be an ideal biocontrol agent for controlling wheat sharp eyespot.

Metabolism of fungicide 2-allylphenol in Rhizoctonia cerealis

2-Allylphenol is a biomimetic synthetic fungicide that mimics the compound ginkgol found in gingko fruit (Gingko biloba L.). This systemic fungicide can effectively suppress a wide range of plant diseases, including wheat sharp eyespot (Rhizoctonia cerealis). However, its degradation in environment after application is still unknown. To understand this fungicide degradation, major metabolites of 2-allylphenol in R. cerealis were examined. The parent and metabolites of 2-allylphenol were detected and quantified in the mycelia and liquid medium. Results showed that 2-allylphenol was metabolized and bio-transformed by R. cerealis, and four metabolites were found, including 2-(2-hydroxyphenyl) acetic acid (M1), 2-(2, 3-dihydroxypropyl) phenol (M2), 2-(2-hydroxypropyl)-phenol (M3) and 2-(3-hydroxypropyl)-phenol (M4). Based on the results, we propose that the biodegradation pathway is that 2-allylphenol is rapidly oxidized into metabolite M2 and hydrolyzed into M3 and M4, which formed M2, and carboxylation of M2 to 2-hydroxy-3-(2׳-hydroxyphenyl) propionic acid which undergo hydrolyzation and decarboxylation to form M1. 2-Allylphenol can be bio-transformed to new compounds by R. cerealis, suggesting the existence of microbe metabolic pathways for 2-allylphenol.