Identification of the toxin components of Rhizoctonia solani AG1-IA and its destructive effect on plant cell membrane structure

Rice sheath blight is a fungal disease caused mainly by Rhizoctonia solani AG1-IA. Toxins are a major pathogenic factor of R. solani, and some studies have reported their toxin components; however, there is no unified conclusion. In this study, we reported the toxin components and their targets that play a role in R. solani AG1-IA. First, toxins produced by R. solani AG1-IA were examined. Several important phytotoxins, including benzoic acid (BZA), 5-hydroxymethyl-2-furanic aid (HFA), and catechol (CAT), were identified by comparative analysis of secondary metabolites from AG1-IA, AG1-IB, and healthy rice. Follow-up studies have shown that the toxin components of this fungus can rapidly disintegrate the biofilm structure while maintaining the content of host plant membrane components, thereby affecting the organelles, which may also explain the lack of varieties highly resistant to sheath blight.

Mycoviruses: Antagonistic Potential, Fungal Pathogenesis, and Their Interaction with Rhizoctonia solani

Mycoviruses, or fungal viruses, are prevalent in all significant fungal kingdoms and genera. These low-virulence viruses can be used as biocontrol agents to manage fungal diseases. These viruses are divided into 19 officially recognized families and 1 unclassified genus. Mycoviruses alter sexual reproduction, pigmentation, and development. Spores and fungal hypha spread mycoviruses. Isometric particles mostly encapsulate dsRNA mycoviruses. The widespread plant-pathogenic fungus Rhizoctonia solani, which has caused a rice sheath blight, has hosted many viruses with different morphologies. It causes significant crop diseases that adversely affect agriculture and the economy. Rice sheath blight threatens the 40% of the global population that relies on rice for food and nutrition. This article reviews mycovirology research on Rhizoctonia solani to demonstrate scientific advances. Mycoviruses control rice sheath blight. Hypovirulence-associated mycoviruses are needed to control R. solani since no cultivars are resistant. Mycoviruses are usually cryptic, but they can benefit the host fungus. Phytopathologists may use hypovirulent viruses as biological control agents. New tools are being developed based on host genome studies to overcome the intellectual challenge of comprehending the interactions between viruses and fungi and the practical challenge of influencing these interactions to develop biocontrol agents against significant plant pathogens.

Comparison of Transcriptome between Tolerant and Susceptible Rice Cultivar Reveals Positive and Negative Regulators of Response to Rhizoctonia solani in Rice

Rice (Oryza sativa L.) is one of the world's most crucial food crops, as it currently supports more than half of the world's population. However, the presence of sheath blight (SB) caused by Rhizoctonia solani has become a significant issue for rice agriculture. This disease is responsible for causing severe yield losses each year and is a threat to global food security. The breeding of SB-resistant rice varieties requires a thorough understanding of the molecular mechanisms involved and the exploration of immune genes in rice. To this end, we conducted a screening of rice cultivars for resistance to SB and compared the transcriptome based on RNA-seq between the most tolerant and susceptible cultivars. Our study revealed significant transcriptomic differences between the tolerant cultivar ZhengDao 22 (ZD) and the most susceptible cultivar XinZhi No.1 (XZ) in response to R. solani invasion. Specifically, the tolerant cultivar showed 7066 differentially expressed genes (DEGs), while the susceptible cultivar showed only 60 DEGs. In further analysis, we observed clear differences in gene category between up- and down-regulated expression of genes (uDEGs and dDEGs) based on Gene Ontology (GO) classes in response to infection in the tolerant cultivar ZD, and then identified uDEGs related to cell surface pattern recognition receptors, the Ca2+ ion signaling pathway, and the Mitogen-Activated Protein Kinase (MAPK) cascade that play a positive role against R. solani. In addition, DEGs of the jasmonic acid and ethylene signaling pathways were mainly positively regulated, whereas DEGs of the auxin signaling pathway were mainly negatively regulated. Transcription factors were involved in the immune response as either positive or negative regulators of the response to this pathogen. Furthermore, our results showed that chloroplasts play a crucial role and that reduced photosynthetic capacity is a critical feature of this response. The results of this research have important implications for better characterization of the molecular mechanism of SB resistance and for the development of resistant cultivars through molecular breeding methods.

Corrigendum: Progress in rice sheath blight resistance research

[This corrects the article DOI: 10.3389/fpls.2023.1141697.].

Design, Synthesis, and Fungicidal Activity against Rice Sheath Blight of Novel N-Acyl-1,2,3,4-tetrahydroquinoline Derivatives

To discover fungicides with novel targets, a series of N-acyl-1,2,3,4-tetrahydroquinoline (NATHQ) derivatives were designed and synthesized by linking the active substructure NATHQ moiety in aspernigerin with the O-benzyl oxime-ether scaffold in commercial agrochemicals. Target compound structures were identified using proton and carbon-13 nuclear magnetic resonance spectroscopies and high-resolution mass spectrometry. Preliminary bioassays indicated that at 40 mg/L, some target compounds exhibited moderate to considerable in vitro fungicidal activities against Rhizoctonia solani and Botrytis cinerea. In particular, compound 3j exhibited higher fungicidal activities both in vitro (EC₅₀ = 0.733 mg/L) and in vivo (EC₅₀ = 15.2 mg/L) against R. solani than the commercial fungicide prochloraz; therefore, it should be a promising fungicide candidate against rice sheath blight. Additionally, compound 3j exhibited good laccase inhibitory activity (73.2% at 200 mg/L). Molecular docking revealed that the bis-cyano-oxime-ether moiety of compound 3j exhibited an excellent binding mode with the laccase target protein and could be used as a lead compound for developing laccase inhibitors. The structural features of these NATHQ derivatives will provide inspiration for developing laccase inhibitors and discovering more effective fungicides to control agricultural diseases.

β-Glucanase Family Genes Promote Resistance to Sheath Blight in Rice by Inhibiting the Permeability of Plasmodesmata

Rice sheath blight (ShB) caused by Rhizoctonia solani is one of the most serious diseases that threatens rice (Oryza sativa) production. However, the mechanisms of defense against ShB in rice remain largely unknown. In this study, we identified that the expression levels of β-glucanase (OsBGL) family genes sensitively respond to infection by R. solani, and OsBGLs positively regulate rice resistance to ShB. In addition, OsBGL2 colocalized with AtPDCB1 at the plasmodesmata (PD) and limited the PD permeability. The level of callose accumulation in osbgls mutants and overexpressors was examined, and OsBGLs were found contribute to callose accumulation. Taken together, these data suggest that OsBGLs can regulate the deposition of callose at the PD to reduce its permeability to defend itself against ShB. Through the identification of these genes and the elucidation of their functions, this research fills the gap in the mechanism of PD permeability in rice ShB resistance.

Kurstakin Triggers Multicellular Behaviors in Bacillus cereus AR156 and Enhances Disease Control Efficacy Against Rice Sheath Blight

Kurstakin is the latest discovered family of lipopeptides secreted by Bacillus spp. In this study, the effects of kurstakin on the direct antagonism, multicellularity, and disease control ability of Bacillus cereus AR156 were explored. An insertion mutation in the nonribosomal peptide synthase responsible for kurstakin synthesis led to a significant reduction of antagonistic ability of AR156 against the plant-pathogenic fungi Rhizoctonia solani, Ascochyta citrullina, Fusarium graminearum, and F. oxysporum f. sp. cubense. The loss of kurstakin synthesis ability significantly impaired the swarming motility of AR156 and reduced biofilm formation and amyloid protein accumulation. Although the loss of kurstakin synthesis ability did not reduce the competitiveness of AR156 under laboratory conditions, the colonization and environmental adaptability of the mutant was significantly weaker than that of wild-type AR156 on rice leaves. The cell surface of wild-type AR156 colonizing the leaf surface was covered by a thick biofilm matrix under a scanning electron microscope, but not the mutant. The colonization ability on rice roots and control efficacy against rice sheath blight disease of the mutant were also impaired. Thus, kurstakin participates in the control of plant diseases by B. cereus AR156 through directly inhibiting the growth of pathogenic fungi and improving long-term environmental adaptability and colonization of AR156 on the host surface by triggering multicellularity. This study explored the multiple functions of kurstakin in plant disease control by B. cereus.

Red-light receptor phytochrome B inhibits BZR1-NAC028-CAD8B signaling to negatively regulate rice resistance to sheath blight

Phytochrome (Phy)-regulated light signalling plays important roles in plant growth, development, and stress responses. However, its function in rice defence against sheath blight disease (ShB) remains unclear. Here, we found that PhyB mutation or shade treatment promoted rice resistance to ShB, while resistance was reduced by PhyB overexpression. Further analysis showed that PhyB interacts with phytochrome-interacting factor-like 15 (PIL15), brassinazole resistant 1 (BZR1), and vascular plant one-zinc-finger 2 (VOZ2). Plants overexpressing PIL15 were more susceptible to ShB in contrast to bzr1-D-overexpressing plants compared with the wild-type, suggesting that PhyB may inhibit BZR1 to negatively regulate rice resistance to ShB. Although BZR1 is known to regulate brassinosteroid (BR) signalling, the observation that BR signalling negatively regulated resistance to ShB indicated an independent role for BZR1 in controlling rice resistance. It was also found that the BZR1 ligand NAC028 positively regulated resistance to ShB. RNA sequencing showed that cinnamyl alcohol dehydrogenase 8B (CAD8B), involved in lignin biosynthesis was upregulated in both bzr1-D- and NAC028-overexpressing plants compared with the wild-type. Yeast-one hybrid, ChIP, and transactivation assays demonstrated that BZR1 and NAC028 activate CAD8B directly. Taken together, the analyses demonstrated that PhyB-mediated light signalling inhibits the BZR1-NAC028-CAD8B pathway to regulate rice resistance to ShB.

Progress in rice sheath blight resistance research

Rice sheath blight (ShB) disease poses a major threat to rice yield throughout the world. However, the defense mechanisms against ShB in rice remain largely unknown. ShB resistance is a typical quantitative trait controlled by multiple genes. With the rapid development of molecular methods, many quantitative trait loci (QTLs) related to agronomic traits, biotic and abiotic stresses, and yield have been identified by genome-wide association studies. The interactions between plants and pathogens are controlled by various plant hormone signaling pathways, and the pathways synergistically or antagonistically interact with each other, regulating plant growth and development as well as the defense response. This review summarizes the regulatory effects of hormones including auxin, ethylene, salicylic acid, jasmonic acid, brassinosteroids, gibberellin, abscisic acid, strigolactone, and cytokinin on ShB and the crosstalk between the various hormones. Furthermore, the effects of sugar and nitrogen on rice ShB resistance, as well as information on genes related to ShB resistance in rice and their effects on ShB are also discussed. In summary, this review is a comprehensive description of the QTLs, hormones, nutrition, and other defense-related genes related to ShB in rice. The prospects of targeting the resistance mechanism as a strategy for controlling ShB in rice are also discussed.

Introgression of the QTL qSB11-1TT conferring sheath blight resistance in rice (Oryza sativa) into an elite variety, UKMRC 2, and evaluation of its backcross-derived plants

INTRODUCTION

Sheath blight (SB) is the most damaging fungal disease in rice caused by a soil-borne pathogenic fungus, Rhizoctonia solani Kuhn (R. solani). The disease resistance in rice is a complex quantitative trait controlled by a few major genes. UKMRC2 is a newly developed elite rice variety that possesses high yield potential but is susceptible to sheath blight disease indicating a huge risk of varietal promotion, mass cultivation, and large-scale adoption. The aim of our present study was the development of varietal resistance against R. solani in UKMRC2 to enhance its stability and durability in a wide range of environments and to validate the effects of an SB-resistance QTL on the new genetic background.

METHODS

In our study, we developed 290 BC1F1 backcross progenies from a cross between UKMRC2 and Tetep to introgress the QTL qSBR11-1_TT into the UKMRC2 genetic background. Validation of the introgressed QTL region was performed via QTL analysis based on QTL-linked SSR marker genotyping and phenotyping against R. solani artificial field inoculation techniques.

RESULTS AND DISCUSSION

The QTL qSBR11-1_TT was then authenticated with the results of LOD score (3.25) derived from composite interval mapping, percent phenotypic variance explained (14.6%), and additive effect (1.1) of the QTLs. The QTL region was accurately defined by a pair of flanking markers K39512 and RM7443 with a peak marker RM27360. We found that the presence of combination of alleles, RM224, RM27360 and K39512 demonstrate an improved resistance against the disease rather than any of the single allele. Thus, the presence of the QTL qSBR11-1_TT has been validated and confirmed in the URMRC2 genetic background which reveals an opportunity to use the QTL linked with these resistance alleles opens an avenue to resume sheath blight resistance breeding in the future with marker-assisted selection program to boost up resistance in rice varieties.

Osa-miR160a confers broad-spectrum resistance to fungal and bacterial pathogens in rice

Rice production is threatened by multiple pathogens. Breeding cultivars with broad-spectrum disease resistance is necessary to maintain and improve crop production. Previously we found that overexpression of miR160a enhanced rice blast disease resistance. However, it is unclear whether miR160a also regulates resistance against other pathogens, and what the downstream signaling pathways are. Here, we demonstrate that miR160a positively regulates broad-spectrum resistance against the causative agents of blast, leaf blight and sheath blight in rice. Mutations of miR160a-targeted Auxin Response Factors result in different alteration of resistance conferred by miR160a. miR160a enhances disease resistance partially by suppressing ARF8, as mutation of ARF8 in MIM160 background partially restores the compromised resistance resulting from MIM160. ARF8 protein binds directly to the promoter and suppresses the expression of WRKY45, which acts as a positive regulator of rice immunity. Mutation of WRKY45 compromises the enhanced blast resistance and bacterial leaf blight resistance conferred by arf8 mutant. Overall, our results reveal that a microRNA coordinates rice broad-spectrum disease resistance by suppressing multiple target genes that play different roles in disease resistance, and uncover a new regulatory pathway mediated by the miR160a-ARF8 module. These findings provide new resources to potentially improve disease resistance for breeding in rice.

Status of Fungicide Resistance and Physiological Characterization of Tebuconazole Resistance in Rhizocotonia solani in Sichuan Province, China

The resistance prevalence of chemical fungicides has caused increasingly serious agro-ecological environmental problems. However, there are few previous reports about resistance to succinate dehydrogenase (SDHI) or sterol demethylation inhibitor (DMI) in Rhizoctonia solani, one of the main agro-diseases. In this study, the fungicide resistance of 122 R. solani isolates in Sichuan Province was monitored by the mycelial growth rate method. Results showed that all isolates were susceptible to hexaconazole and most isolates were susceptible to thifluzamide, except for the field isolate MSRS-2-7 due to a moderate resistance to thifluzamide (16.43-fold resistance ratio, RR), compared to the sensitivity baseline of thifluzamide (0.042 μg/mL EC₅₀ values). On the contrary, many isolates showed moderate or high resistance to tebuconazole (10.59- to 60.78-fold RR), reaching EC₅₀ values of 0.54~3.10 μg/mL, especially for a highly resistant isolate LZHJ-1-8 displaying moderate resistance to epoxiconazole (35.40-fold RR due to a 3.54 μg/mL EC₅₀ value). The fitness determination found that the tebuconazole-resistant isolates showed higher fitness cost with these characteristics, including a lower growth rate, higher relative electric conductivity, an increased ability to tolerate tebuconazole, and high osmotic pressure. Four new mutations of cytochrome P450 sterol 14α-demethylase (CYP51), namely, S94A, N406S, H793R, and L750P, which is the target for DMI fungicides, was found in the tebuconazole-resistant isolates. Furthermore, the lowest binding energy with tebuconazole was also found in the LZHJ-1-8 isolate possessing all the mutations through analyses with Discovery Studio software. Therefore, these new mutation sites of CYP51 may be linked to the resistance against tebuconazole, and its application for controlling R. solani should be restricted in some areas.

Functional validation of pathogenicity genes in rice sheath blight pathogen Rhizoctonia solani by a novel host-induced gene silencing system

Rice sheath blight, caused by the soilborne fungus Rhizoctonia solani, causes severe yield losses worldwide. Elucidation of the pathogenic mechanism of R. solani is highly desired. However, the lack of a stable genetic transformation system has made it challenging to examine genes' functions in this fungus. Here, we present functional validation of pathogenicity genes in the rice sheath blight pathogen R. solani by a newly established tobacco rattle virus (TRV)-host-induced gene silencing (HIGS) system using the virulent R. solani AG-1 IA strain GD-118. RNA interference constructs of 33 candidate pathogenicity genes were infiltrated into Nicotiana benthamiana leaves with the TRV-HIGS system. Of these constructs, 29 resulted in a significant reduction in necrosis caused by GD-118 infection. For further validation of one of the positive genes, trehalose-6-phosphate phosphatase (Rstps2), stable rice transformants harbouring the double-stranded RNA (dsRNA) construct for Rstps2 were created. The transformants exhibited reduced gene expression of Rstps2, virulence, and trehalose accumulation in GD-118. We showed that the dsRNA for Rstps2 was taken up by GD-118 mycelia and sclerotial differentiation of GD-118 was inhibited. These findings offer gene identification opportunities for the rice sheath blight pathogen and a theoretical basis for controlling this disease by spray-induced gene silencing.

Enhanced Fungicidal Efficacy by Co-Delivery of Azoxystrobin and Diniconazole with Cauliflower-Like Metal-Organic Frameworks NH2-Al-MIL-101

Long-term use of a single fungicide increases the resistance risk and causes adverse effects on natural ecosystems. Controlled release formulations of dual fungicides with different modes of action can afford a new dimension for addressing the current issues. Based on adjustable aperture and superhigh surface area, metal-organic frameworks (MOFs) are ideal candidates as pesticide release carriers. This study used Al3+ as the metal node and 2-aminoterephthalic acid as the organic chain to prepare aluminum-based metal-organic framework material (NH2-Al-MIL-101) with "cauliflower-like" structure and high surface area of 2359.0 m2/g. Fungicides of azoxystrobin (AZOX) and diniconazole (Dini) were simultaneously encapsulated into NH2-Al-MIL-101 with the loading content of 6.71% and 29.72%, respectively. Dual fungicide delivery system of AZOX@Dini@NH2-Al-MIL-101 demonstrated sustained and pH responsive release profiles. When the maximum cumulative release rate of AZOX and Dini both reached about 90%, the release time was 46 and 136 h, respectively. Furthermore, EC50 values as well as the percentage of inhibition revealed that AZOX@Dini@NH2-Al-MIL-101 had enhanced germicidal efficacy against rice sheath blight (Rhizoctonia solani), evidenced by the synergistic ratio of 1.83. The present study demonstrates a potential application prospect in sustainable plant protection through co-delivery fungicides with MOFs as a platform.

Identification of rice (Oryza sativa L.) genes involved in sheath blight resistance via a genome-wide association study

Rice sheath blight (RSB) is an economically significant disease affecting rice yield worldwide. Genetic resistance to RSB is associated with multiple minor genes, with each providing a minor phenotypic effect, but the underlying dominant resistance genes remain unknown. A genome-wide association study (GWAS) of 259 diverse rice varieties, with genotypes based on a single nucleotide polymorphism (SNP) and haplotype, was conducted to assess their sheath blight reactions at three developmental stages (seedlings, tillering and booting). A total of 653 genes were correlated with sheath blight resistance, of which the disease resistance protein RPM1 (OsRSR1) and protein kinase domain-containing protein (OsRLCK5) were validated by overexpression and knockdown assays. We further found that the coiled-coil (CC) domain of OsRSR1 (OsRSR1-CC) and full-length OsRLCK5 interacted with serine hydroxymethyltransferase 1 (OsSHM1) and glutaredoxin (OsGRX20), respectively. It was found that OsSHM1, which has a role in the reactive oxygen species (ROS) burst, and OsGRX20 enhanced the antioxidation ability of plants. A regulation model of the new RSB resistance though the glutathione (GSH)-ascorbic acid (AsA) antioxidant system was therefore revealed. These results enhance our understanding of RSB resistance mechanisms and provide better gene resources for the breeding of disease resistance in rice.

Growth Restriction of Rhizoctonia solani via Breakage of Intracellular Organelles Using Crude Extracts of Gallnut and Clove

Plant diseases reduce crop yield and quality, hampering the development of agriculture. Fungicides, which restrict chemical synthesis in fungi, are the strongest controls for plant diseases. However, the harmful effects on the environment due to continued and uncontrolled utilization of fungicides have become a major challenge in recent years. Plant-sourced fungicides are a class of plant antibacterial substances or compounds that induce plant defenses. They can kill or inhibit the growth of target pathogens efficiently with no or low toxicity, they degrade readily, and do not prompt development of resistance, which has led to their widespread use. In this study, the growth inhibition effect of 24 plant-sourced ethanol extracts on rice sprigs was studied. Ethanol extract of gallnuts and cloves inhibited the growth of bacteria by up to 100%. Indoor toxicity measurement results showed that the gallnut and glove constituents inhibition reached 39.23 μg/mL and 18.82 μg/mL, respectively. Extract treated rice sprigs were dry and wrinkled. Gallnut caused intracellular swelling and breakage of mitochondria, disintegration of nuclei, aggregation of protoplasts, and complete degradation of organelles in hyphae and aggregation of cellular contents. Protection of Rhizoctonia solani viability reached 46.8% for gallnut and 37.88% for clove in water emulsions of 1000 μg/mL gallnut and clove in the presence of 0.1% Tween 80. The protection by gallnut was significantly stronger than that of clove. The data could inform the choice of plant-sourced fungicides for the comprehensive treatment of rice sprig disease. The studied extract effectively protected rice sprigs and could be a suitable alternative to commercially available chemical fungicides. Further optimized field trials are needed to effectively sterilize rice paddies.

Field Sanitation and Foliar Application of Streptomyces padanus PMS-702 for the Control of Rice Sheath Blight

Rice sheath blight (ShB), caused by Rhizoctonia solani Kühn AG1-IA, is one of the destructive rice diseases worldwide. The aims of this study were to develop biocontrol strategies focusing on field sanitation and foliar application with a biocontrol agent for ShB management. Streptomyces padanus PMS-702 showed a great antagonistic activity against R. solani. Fungichromin produced by S. padanus PMS-702, at 3.07 mg/l inhibited 50% mycelial growth, caused leakage of cytoplasm, and inhibited the formation of infection structures of R. solani. Fungichromin could reach to 802 mg/l when S. padanus PMS-702 was cultured in MACC broth for 6 days. Addition of 0.5% S. padanus PMS-702 broth into soil decreased the survival rate of the pathogen compared to the control. Soil amended with 0.5% S. padanus broth and 0.5% tea seed pomace resulted in the death of R. solani mycelia in the infested rice straws, and the germination of sclerotia was inhibited 21 days after treatment. Greenhouse trials revealed that S. padanus cultured in soybean meal-glucose (SMGC-2) medium after mixing with different surfactants could enhance its efficacy for inhibiting the pathogen. Of six surfactants tested, the addition of 2% tea saponin was the most effective in suppressing the pathogen. S. padanus broth after being fermented in SMGC-2, mixed with 2% tea saponin, diluted 100 fold, and sprayed onto rice plants significantly reduced ShB disease severity. Thus, S. padanus PMS-702 is an effective biocontrol agent. The efficacy of S. padanus PMS-702 for disease control could be improved through formulation.

Identification of Rice Sheath Blight through Spectral Responses Using Hyperspectral Images

Sheath blight (ShB), caused by Rhizoctonia solani AG1-I, is one of the most important diseases in rice worldwide. The symptoms of ShB primarily develop on leaf sheaths and leaf blades. Hyperspectral remote sensing technology has the potential of rapid, efficient and accurate detection and monitoring of the occurrence and development of rice ShB and other crop diseases. This study evaluated the spectral responses of leaf blade fractions with different development stages of ShB symptoms to construct the spectral feature library of rice ShB based on “three-edge” parameters and narrow-band vegetation indices to identify the disease on the leaves. The spectral curves of leaf blade lesions have significant changes in the blue edge, green peak, yellow edge, red valley, red edge and near-infrared regions. The variables of the normalized index between green peak amplitude and red valley amplitude (Rg − Ro)/(Rg + Ro), the normalized index between the yellow edge area and blue edge area (SDy − SDb)/(SDy + SDb), the ratio index of green peak amplitude and red valley amplitude (Rg/Ro) and the nitrogen reflectance index (NRI) had high relevance to the disease. At the leaf scale, the importance weights of all attributes decreased with the effect of non-infected areas in a leaf by the ReliefF algorithm, with Rg/Ro being the indicator having the highest importance weight. Estimation rate of 95.5% was achieved in the decision tree classifier with the parameter of Rg/Ro. In addition, it was found that the variety degree of absorptive valley, reflection peak and reflecting steep slope was different in the blue edge, green and red edge regions, although there were similar spectral curve shapes between leaf sheath lesions and leaf blade lesions. The significant difference characteristic was the ratio index of the red edge area and green peak area (SDr/SDg) between them. These results can provide the basis for the development of a specific sensor or sensors system for detecting the ShB disease in rice.

Mechanism of Action of Novel Pyrazole Carboxamide Containing a Diarylamine Scaffold against Rhizoctonia solani

In the last few decades, Rhizoctonia solani causing rice sheath blight has resulted in a lot of economic losses in the world. Therefore, many novel pyrazole carboxamide fungicides have been intensively researched and employed to fight against it. In this regard, in recent years, our group reported a novel pyrazole carboxamide containing a diarylamine scaffold with good antifungal activity against rice sheath blight in the pot test and field trial. Following this project, the antifungal mechanism of action of the pyrazole carboxamide has been elucidated in this work. The antifungal result showed that compound SCU2028, N-[2-[(3-chlorophenyl)amino]-phenyl]-3-(difluoromethyl)-1-methyl-1H-pyrazole-4-carboxamide, was equivalent to the commercial fungicide thifluzamide and its EC₅₀ value was 0.022 mg/L against R. solani. Also, the observation results by scanning electron microscopy and transmission electron microscopy showed that it could destroy the fungus' cell walls or membranes and result in the leakage of contents and increase of the number of mitochondria and abnormal morphology. Meanwhile, the result on the mitochondrial membrane potential (MMP) showed that it could decrease R. solani's MMP. Furthermore, the results by label-free quantitative proteomic analysis showed that 1153 proteins were found after R. solani was treated with compound SCU2028, including 212 proteins in the control group and 257 proteins in the treatment group. A total of 142 differential proteins were obtained, of which 92 proteins were upregulated and 50 proteins were downregulated. The differentially expressed proteins affected a series of physiological and biochemical pathways in the mitochondria, endoplasmic reticulum, ribosome, and other related GO and KEGG pathways. In particular, the inhibition of the respiratory chain caused by the TCA cycle and oxidative phosphorylation KEGG pathway indicated that complex II (succinate dehydrogenase) and complex IV (cytochrome oxidase) might be compound SCU2028's main action targets. In addition, multiple experiments of qRT-PCR, enzyme activity detection, and molecular docking confirmed complex II and complex IV as targets. It could be seen that these findings provided a theoretical support for further research and development of the pyrazole carboxamide fungicides.

Identification of Genes Involved in Antifungal Activity of Burkholderia seminalis Against Rhizoctonia solani Using Tn5 Transposon Mutation Method

Rhizoctonia solani is the causative agent of rice sheath blight disease. In a previous study, we found that the growth of R. solani was inhibited by Burkholderia seminalis strain R456. Therefore, the present study was conducted to identify the genes involved in the antifungal activity of B. seminalis strain R456 by using a Tn5 transposon mutation method. Firstly, we constructed a random insertion transposon library of 997 mutants, out of which 11 mutants showed the defective antifungal activity against R. solani. Furthermore, the 10 antagonism-related genes were successfully identified based on analysis of the Tn5 transposon insertion site. Indeed, this result indicated that three mutants were inserted on an indigenous plasmid in which the same insertion site was observed in two mutants. In addition, the remaining eight mutants were inserted on different genes encoding glycosyl transferase, histone H1, nonribosomal peptide synthetase, methyltransferase, MnmG, sulfate export transporter, catalase/peroxidase HPI and CysD, respectively. Compared to the wild type, the 11 mutants showed a differential effect in bacteriological characteristics such as cell growth, biofilm formation and response to H₂O₂ stress, revealing the complexity of action mode of these antagonism-related genes. However, a significant reduction of cell motility was observed in the 11 mutants compared to the wild type. Therefore, it can be inferred that the antifungal mechanism of the 10 above-mentioned genes may be, at least partially, due to the weakness of cell motility. Overall, the result of this study will be helpful for us to understand the biocontrol mechanism of this bacterium.

Rice siR109944 suppresses plant immunity to sheath blight and impacts multiple agronomic traits by affecting auxin homeostasis

Plant small RNAs (sRNAs) play significant roles in regulating various developmental processes and hormone signalling pathways involved in plant responses to a wide range of biotic and abiotic stresses. However, the functions of sRNAs in response to rice sheath blight remain unclear. We screened rice (Oryza sativa) sRNA expression patterns against Rhizoctonia solani and found that Tourist-miniature inverted-repeat transposable element (MITE)-derived small interfering RNA (siRNA) (here referred to as siR109944) expression was clearly suppressed upon R. solani infection. One potential target of siR109944 is the F-Box domain and LRR-containing protein 55 (FBL55), which encode the transport inhibitor response 1 (TIR1)-like protein. We found that rice had significantly enhanced susceptibility when siR109944 was overexpressed, while FBL55 OE plants showed resistance to R. solani challenge. Additionally, multiple agronomic traits of rice, including root length and flag leaf inclination, were affected by siR109944 expression. Auxin metabolism-related and signalling pathway-related genes were differentially expressed in the siR109944 OE and FBL55 OE plants. Importantly, pre-treatment with auxin enhanced sheath blight resistance by affecting endogenous auxin homeostasis in rice. Furthermore, transgenic Arabidopsis overexpressing siR109944 exhibited early flowering, increased tiller numbers, and increased susceptibility to R. solani. Our results demonstrate that siR109944 has a conserved function in interfering with plant immunity, growth, and development by affecting auxin homeostasis in planta. Thus, siR109944 provides a genetic target for plant breeding in the future. Furthermore, exogenous application of indole-3-acetic acid (IAA) or auxin analogues might effectively protect field crops against diseases.

Yeast Associated with Rice Phylloplane and Their Contribution to Control of Rice Sheath Blight Disease

The phylloplane is an important habitat for yeasts and these yeasts may have antagonistic activities against pathogens and could be used as biocontrol agents. To investigate rice phylloplane yeasts, 282 strains were isolated from 89 rice leaf samples and identified as 15 known yeast species in the phylum Ascomycota and 35 known and two potential new species in the phylum Basidiomycota. The majority of rice phylloplane yeasts belonged to the phylum Basidiomycota. The evaluation of antagonistic activities of 83 yeast strains against rice pathogenic fungi Pyricularia oryzae, Rhizoctonia solani, Fusarium moniliforme, Helminthosporium oryzae and Curvularia lunata revealed that 14 strains inhibited these pathogens. Among the antagonistic strains, Torulaspora indica DMKU-RP31, T. indica DMKU-RP35 and Wickerhamomyces anomalus DMKU-RP25 inhibited all rice pathogens tested, and the production of volatile organic compounds, fungal cell wall degrading enzymes and biofilm were the possible antagonistic mechanisms against all rice pathogens tested in vitro. These yeast strains were evaluated for controlling rice sheath blight caused by R. solani in rice plants in the greenhouse and were found to suppress the disease by 60.0-70.3%, whereas 3% validamycin suppressed by 83.8%. Therefore, they have potential for being developed to be used as biocontrol agents for rice sheath blight.

Silicon-Solubilizing Media and Its Implication for Characterization of Bacteria to Mitigate Biotic Stress

Silicon (Si), the second most abundant element on earth, remains unavailable for plants' uptake due to its poor solubility. Microbial interventions to convert it in soluble forms are well documented. However, studies on discrimination of Si and P solubilizing microbes due to common estimation method and sharing of solubilization mechanism are still obscure. A defined differential media, i.e. silicon-solubilizing media (NBRISSM) is developed to screen Si solubilizers. NBRISN13 (Bacillus amyloliquefaciens), a Si solubilizer, exhibiting antagonistic property against Rhizoctonia solani, was further validated for disease resistance. The key finding of the work is that NBRISSM is a novel differential media for screening Si solubilizers, distinct from P solubilizers. Dominance of Pseudomonas and Bacillus spp. for the function of Si solubilization was observed during diversity analysis of Si solubilizers isolated from different rhizospheres. Sphingobacterium sp., a different strain has been identified for silicon solubilization other than Pseudomonas and Bacillus sp. Role of acidic phosphatase during Si solubilization has been firstly reported in our study in addition to other pH dependent phenomenon. Study also showed the combinatorial effect of feldspar and NBRISN13 on elicited immune response through (i) increased Si uptake, (ii) reduced disease severity, (iii) modulation of cell wall degrading and antioxidative enzyme activities, and (iv) induced defense responsive gene expression.

Genome-Wide Associations and Transcriptional Profiling Reveal ROS Regulation as One Underlying Mechanism of Sheath Blight Resistance in Rice

Rice sheath blight, caused by the necrotrophic fungus Rhizoctonia solani Kühn, continues to be an important and challenging rice disease worldwide. Here, we used genome-wide association studies over a high-density rice array to facilitate the identification of potential novel genes and quantitative trait loci related to sheath blight resistance. We identified multiple regions that significantly associated with independent disease components in chromosomes 1, 4, and 11 under controlled condition. In particular, we investigated qLN11²⁸, a quantitative trait locus enriched with defense-related genes that reduce disease lesions in a near-isogenic line. RNA profiling of the line carrying qLN11²⁸ showed a number of differentially expressed genes related to the reactive oxygen species (ROS)-redox pathway. Histochemical staining revealed less ROS accumulation on the resistant line, suggesting efficient ROS deregulation that delays pathogen colonization. The detection of genomic regions controlling multiple mechanisms of resistance to sheath blight will provide tools to design effective breeding interventions in rice.

Molecular Characterization of a Novel Endornavirus Conferring Hypovirulence in Rice Sheath Blight Fungus Rhizoctonia solani AG-1 IA Strain GD-2

The complete sequence and genome organization of a novel Endornavirus from the hypovirulent strain GD-2 of Rhizoctonia solani AG-1 IA, the causal agent of rice sheath blight, were identified using a deep sequencing approach and it was tentatively named as Rhizoctonia solani endornavirus 1 (RsEV1). It was composed of only one segment that was 19,936 bp in length and was found to be the longest endornavirus genome that has been reported so far. The RsEV1 genome contained two open reading frames (ORFs): ORF1 and ORF2. ORF1 contained a glycosyltransferase 1 domain and a conserved RNA-dependent RNA polymerase domain, whereas ORF2 encoded a conserved hypothetical protein. Phylogenetic analysis revealed that RsEV1 was phylogenetically a new endogenous RNA virus. A horizontal transmission experiment indicated that RsEV1 could be transmitted from the host fungal strain GD-2 to a virulent strain GD-118P and resulted in hypovirulence in the derivative isogenic strain GD-118P-V1. Metabolomic analysis showed that 32 metabolites were differentially expressed between GD-118P and its isogenic hypovirulent strain GD-118P-V1. The differential metabolites were mainly classified as organic acids, amino acids, carbohydrates, and the intermediate products of energy metabolism. Pathway annotation revealed that these 32 metabolites were mainly involved in pentose and glucuronate interconversions and glyoxylate, dicarboxylate, starch, and sucrose metabolism, and so on. Taken together, our results showed that RsEV1 is a novel Endornavirus, and the infection of virulent strain GD-118P by RsEV1 caused metabolic disorders and resulted in hypovirulence. The results of this study lay a foundation for the biocontrol of rice sheath blight caused by R. solani AG1-IA.

Discovery of β-Carboline Oxadiazole Derivatives as Fungicidal Agents against Rice Sheath Blight

A series of β-carboline oxadiazoles were synthesized, and their fungicidal activities and mechanism of action against rice sheath blight caused by Rhizoctonia solani was evaluated. The results showed that all of these compounds exhibited significant in vitro fungicidal activity. Significantly, compound 5i (EC₅₀ = 4.2 μg/mL) displayed the best efficacy and superior fungicidal activity compared to validamycin A (EC₅₀ = 197.6 μg/mL). Moreover, the in vivo test also demonstrated that compound 5i could effectively control rice sheath blight and showed higher in vivo protective and curative activities against R. solani than validamycin A. Preliminary mechanism studies revealed that compound 5i caused the loss of mitochondrial membrane potential, reactive oxygen species accumulation, cell membrane destruction, and DNA synthesis interference. These findings indicated that compound 5i displayed superior fungicidal activities against R. solani and could be a potential fungicidal candidate against rice sheath blight.

Design, synthesis, fungicidal property and QSAR studies of novel β-carbolines containing urea, benzoylthiourea and benzoylurea for the control of rice sheath blight

BACKGROUND

Rice sheath blight is a globally important rice disease. Unfortunately, this critical disease has not been effectively controlled, and the intensive and continuous use of the same fungicide might increase the risk of resistance development in the pathogen. To discover new active agents against rice sheath blight, in this study, three series of β-carboline urea, benzoylurea and benzoylthiourea derivatives were designed, synthesized and evaluated for in vitro and in vivo fungicidal activity against Rhizoctonia solani.

RESULTS

All these compounds (EC₅₀ : 0.131-1.227 mmol L^-1 ) exhibited better fungicidal activity than harmine itself (EC₅₀ : 2.453 mmol L^-1 ). Significantly, compound 17c (EC₅₀ : 0.131 mmol L^-1 ) displayed the best efficacy in vitro and superior fungicidal activity compared with validamycin A (EC₅₀ : 0.397 mmol L^-1 ). Moreover, the in vivo bioassay also indicated that compound 17c could be effective for the control of rice sheath blight.

CONCLUSION

Based on the bioassay result and quantitative structure-activity relationship (QSAR) information, structure modification in β-carboline warrants further investigation and its benzoylurea derivative 17c, which showed the best fungicidal activities, could emerge as a potential fungicide against rice sheath blight. © 2018 Society of Chemical Industry.

Integrated biological and chemical control of rice sheath blight by Bacillus subtilis NJ-18 and jinggangmycin

BACKGROUND

Sheath blight caused by Rhizoctonia solani Kühn is a major disease of rice that greatly reduces yield and grain quality and jinggangmycin is the most widely used fungicide to control this disease in China. Bacillus subtilis NJ-18 has broad antimicrobial activity to many phytopathogenic bacteria and fungi; it is especially effective against Rhizoctonia solani. Laboratory, greenhouse and field tests were conducted to determine the effect of combining the biological control agent Bacillus subtilis NJ-18 with the fungicide jinggangmycin for control of rice sheath blight.

RESULTS

Growth of NJ-18 in vitro was not affected by jinggangmycin. In a greenhouse experiment, disease control was greater with a mixture of NJ-18 and jinggangmycin than with either alone; a mixture of NJ-18 at 10(8)  cfu mL(-1) and jinggangmycin at 50 or 100 mg L(-1) reduced lesion length by 35% and 20%, respectively, and the combinations showed a synergistic action. In three field trials, disease control was significantly greater with a mixture of NJ-18 at 10(8)  cfu mL(-1) and jinggangmycin at 75 or 150 g a.i. ha(-1) than with either component alone.

CONCLUSION

The results of the study indicate that, when Bacillus subtilis NJ-18 strain was combined with jinggangmycin, there was an increased suppression of rice sheath blight, and thus could provide an alternative disease control option.