Pectin induced transcriptome of a Rhizoctonia solani strain causing sheath blight disease in rice reveals insights on key genes and RNAi machinery for development of pathogen derived resistance

Comparative Field Evaluation and Transcriptome Analysis Reveals that Chromosome Doubling Enhances Sheath Blight Resistance in Rice

Rice sheath blight, caused by Rhizoctonia solani Kihn (R. solani), poses a significant threat to rice production and quality. Autotetraploid rice, developed through chromosome doubling of diploid rice, holds great potential for enhancing biological and yield traits. However, its resistance to sheath blight in the field has remained unclear. In this study, the field resistance of 35 autotetraploid genotypes and corresponding diploids was evaluated across three environments from 2020 to 2021. The booting stage was optimal for inoculating period based on the inoculation and analysis of R. solani at five rice growth stages. We found autotetraploids generally exhibited lower disease scores than diploids, indicating enhanced resistance after chromosome doubling. Among the 35 genotypes, 16 (45.71%) displayed increased resistance, 2 (5.71%) showed decreased resistance, and 17 (48.57%) displayed unstable resistance in different sowing dates. All combinations of the genotype, environment and ploidy, including the genotype-environment-ploidy interaction, contributed significantly to field resistance. Chromosome doubling increased sheath blight resistance in most genotypes, but was also dependent on the genotype-environment interaction. To elucidate the enhanced resistance mechanism, RNA-seq revealed autotetraploid recruited more down-regulated differentially expressed genes (DEGs), additionally, more resistance-related DEGs, were down-regulated at 24 h post inoculation in autotetraploid versus diploid. The ubiquinone/terpenoid quinone and diterpenoid biosynthesis pathways may play key roles in ploidy-specific resistance mechanisms. In summary, our findings shed light on the understanding of sheath blight resistance mechanisms in autotetraploid rice.

Oschib1 gene encoding a GH18 chitinase confers resistance against sheath blight disease of rice caused by Rhizoctonia solani AG1-IA

Sheath blight disease of rice caused by Rhizoctonia solani AG1-IA, is a major fungal disease responsible for huge loss to grain yield and quality. The major limitation of achieving persistent and reliable resistance against R. solani is the governance of disease resistance trait by many genes. Therefore, functional characterization of new genes involved in sheath blight resistance is necessary to understand the mechanism of resistance as well as evolving effective strategies to manage the disease through host-plant resistance. In this study, we performed RNA sequencing of six diverse rice genotypes (TN1, BPT5204, Vandana, N22, Tetep, and Pankaj) from sheath and leaf tissue of control and fungal infected samples. The approach for identification of candidate resistant genes led to identification of 352 differentially expressed genes commonly present in all the six genotypes. 23 genes were analyzed for RT-qPCR expression which helped identification of Oschib1 showing differences in expression level in a time-course manner between susceptible and resistant genotypes. The Oschib1 encoding classIII chitinase was cloned from resistant variety Tetep and over-expressed in susceptible variety Taipei 309. The over-expression lines showed resistance against R. solani, as analyzed by detached leaf and whole plant assays. Interestingly, the resistance response was correlated with the level of transgene expression suggesting that the enzyme functions in a dose dependent manner. We report here the classIIIb chitinase from chromosome10 of rice showing anti-R. solani activity to combat the dreaded sheath blight disease.

Genome sequencing and comparative genome analysis of Rhizoctonia solani AG-3

Rhizoctonia solani AG-3 is a plant pathogenic fungus that belongs to the group of multinucleate Rhizoctonia. According to its internal transcribed spacer (ITS) cluster analysis and host range, it is divided into TB, PT, and TM subgroups. AG-3 TB mainly causes tobacco target spots, AG-3 PT mainly causes potato black scurf, and AG-3 TM mainly causes tomato leaf blight. In our previous study, we found that all 36 tobacco target spot strains isolated from Yunnan (Southwest China) were classified into AG-3 TB subgroup, while only two of the six tobacco target spot strains isolated from Liaoning (Northeast China) were classified into AG-3 TB subgroup, and the remaining four strains were classified into AG-3 TM subgroup, which had a unique taxonomic status, and there was no previous report on the whole genome information of AG-3 TM subgroup. In this study, the whole genomes of R. solani AG-3 strains 3T-1 (AG-3 TM isolated from Liaoning) and MJ-102 (AG-3 TB isolated from Yunnan) isolated from tobacco target spot in Liaoning and Yunnan were sequenced by IIumina and PacBio sequencing platforms. Comparative genomic analysis was performed with the previously reported AG-3 PT strain Rhs1AP, revealing their differences in genomes and virulence factors. The results indicated that the genome size of 3T-1 was 42,103,597 bp with 11,290 coding genes and 49.74% GC content, and the genome size of MJ-102 was 41,908,281 bp with 10,592 coding genes and 48.91% GC content. Through comparative genomic analysis with the previously reported strain Rhs1AP (AG-3 PT), it was found that the GC content between the genomes was similar, but the strains 3T-1 and MJ-102 contained more repetitive sequences. Similarly, there are similarities between their virulence factors, but there are also some differences. In addition, the results of collinearity analysis showed that 3T-1 and MJ-102 had lower similarity and longer evolutionary distance with Rhs1AP, but the genetic relationship between 3T-1 and MJ-102 was closer. This study can lay a foundation for studying the molecular pathogenesis and virulence factors of R. solani AG-3, and revealing its genomic composition will also help to develop more effective disease control strategies.

Inhibitory role of copper and silver nanocomposite on important bacterial and fungal pathogens in rice (Oryza sativa)

Rice (Oryza sativa) being among the most important food crops in the world is also susceptible to various bacterial and fungal diseases that are the major stumbling blocks in the way of increased production and productivity. The bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae and the sheath blight disease caused by Rhizoctonia solani are among the most devastating diseases of the rice crop. In spite of the availability of array of chemical control, there are chances of development of resistance. Thus, there is a need for the nanotechnological intervention for management of disease in the form of copper and silver nano-composites. The copper (CuNPs) and silver nanoparticles (AgNPs) were synthesized using green route and characterized using different high throughput techniques, i.e., UV-Vis, FT-IR, DLS, XRD, FE-SEM, TEM. The particle size and zeta potential of synthesized CuNPs and AgNPs were found 273 nm and - 24.2 mV; 95.19 nm and - 25.5 mV respectively. The nanocomposite of CuNPs and AgNPs were prepared having particle size in the range of 375-306 nm with improved stability (zeta potential - 54.7 to - 39.4 mV). The copper and silver nanoparticle composites evaluated against Xanthomonas oryzae pv. oryzae and Rhizoctonia solani were found to have higher antibacterial (inhibition zone 13 mm) and antifungal activities (77%) compared to only the copper nanoparticle (8 mm; 62% respectively). Net house trials of nano-composite formulations against the bacterial blight of rice also corroborated the potential of nanocomposite formulation. In silico studies were carried out selecting two disease-causing proteins, peptide deformylase (Xanthomonas oryzae) and pectate lyase (Rhizoctonia solani) to perform the molecular docking. Interaction studies indicatedthat both of these proteins generated better complex with CuNPs than AgNPs. The study suggested that the copper and silver nano-composites could be used for developing formulations to control these devastating rice diseases.

The Resistance of Maize to Ustilago maydis Infection Is Correlated with the Degree of Methyl Esterification of Pectin in the Cell Wall

Common smut caused by Ustilago maydis is one of the dominant fungal diseases in plants. The resistance mechanism to U. maydis infection involving alterations in the cell wall is poorly studied. In this study, the resistant single segment substitution line (SSSL) R445 and its susceptible recurrent parent line Ye478 of maize were infected with U. maydis, and the changes in cell wall components and structure were studied at 0, 2, 4, 8, and 12 days postinfection. In R445 and Ye478, the contents of cellulose, hemicellulose, pectin, and lignin increased by varying degrees, and pectin methylesterase (PME) activity increased. The changes in hemicellulose and pectin in the cell wall after U. maydis infection were analyzed via immunolabeling using monoclonal antibodies against hemicellulsic xylans and high/low-methylated pectin. U. maydis infection altered methyl esterification of pectin, and the degree of methyl esterification was correlated with the resistance of maize to U. maydis. Furthermore, the relationship between methyl esterification of pectin and host resistance was validated using 15 maize inbred lines with different resistance levels. The results revealed that cell wall components, particularly pectin, were important factors affecting the colonization and propagation of U. maydis in maize, and methyl esterification of pectin played a role in the resistance of maize to U. maydis infection.

Double-stranded RNA (dsRNA) technology to control forest insect pests and fungal pathogens: challenges and opportunities

Climate change alters the seasonal synchronization between plants and respective pests plus pathogens. The geographical infiltration helps to shift their hosts, resulting in novel outbreaks that damage forests and ecology. Traditional management schemes are unable to control such outbreaks, therefore unconventional and competitive governance is needed to manage forest pests and pathogens. RNA interference (RNAi) mediated double-stranded RNA (dsRNA) treatment method can be implemented to protect forest trees. Exogenous dsRNA triggers the RNAi-mediated gene silencing of a vital gene, and suspends protein production, resulting in the death of targeted pathogens and pests. The dsRNA treatment method is successful for many crop insects and fungi, however, studies of dsRNA against forest pests and pathogens are depleting. Pesticides and fungicides based on dsRNA could be used to combat pathogens that caused outbreaks in different parts of the world. Although the dsRNA has proved its potential, the crucial dilemma and risks including species-specific gene selection, and dsRNA delivery methods cannot be overlooked. Here, we summarized the major fungi pathogens and insect pests that have caused outbreaks, their genomic information, and studies on dsRNA fungi-and pesticides. Current challenges and opportunities in dsRNA target decision, delivery using nanoparticles, direct applications, and a new method using mycorrhiza for forest tree protection are discussed. The importance of affordable next-generation sequencing to minimize the impact on non-target species is discussed. We suggest that collaborative research among forest genomics and pathology institutes could develop necessary dsRNA strategies to protect forest tree species.

Metabolomic analysis of sheath blight disease of rice (Oryza sativa L.) induced by Rhizoctonia solani phytotoxin

AIM

To understand the mechanism of necrosis incited by a host-selective phytotoxin designated as Rhizoctonia solani toxin (RST) identified to be a potential pathogenic factor of Rhizoctonia solani AG1 IA, causing sheath blight (ShB) of rice.

METHODS AND RESULTS

The metabolomic changes induced by the phytotoxic metabolite in a ShB susceptible rice cultivar were elucidated by Gas Chromatography-Mass Spectrometry (GC-MS) analysis and compared with that of the pathogen to identify rice metabolites targeted by the phytotoxin. The profiles of about 29 metabolites with various physiological roles in rice plants have been identified worldwide. Unsupervised and supervised multivariate chemometrics (Principal Component Analysis, PCA and Partial Least Squares-Discriminant Analysis, PLS-DA) and cluster (Heat maps) analyses were used to compare the metabolites obtained from chemical profiles of the treatments with sterile distilled water (SDW) control. The results indicated that the rice plant expressed more metabolites in response to the pathogen than the phytotoxin and was lowest in SDW control. The key metabolites expressed in rice in response to the treatments were investigated by the Variable Importance in Projection (VIP) analysis using P< 0.05 VIP >15. The analysis identified 7 and 11 upregulating metabolites in the phytotoxin and the pathogen treatments, respectively, compared to the untreated control. Among the phytotoxin-treated and the pathogen inoculated samples, the phytotoxin treated sample recorded upregulation of 6 metabolites, whereas 9 metabolites were upregulated in the pathogen inoculated samples. These upregulating metabolites are speculated for the necrotic symptoms characteristic to both the phytotoxin and pathogen. In this analysis, hexadecanoic acid and dotriacontane were highly expressed metabolites specific to the phytotoxin and pathogen-treated samples, respectively. Besides upregulation, the metabolites also have a VIP score of >1.5 and hence fulfilled the criteria of classifying them as reliable potential biomarkers. In the pathway analysis, hexadecanoic acid and dotriacontane were identified to be involved in several important biosynthetic pathways of rice, such as the biosynthesis of saturated fatty acid and unsaturated fatty acids cutin, suberin, and wax.

CONCLUSIONS

The study concludes that though certain metabolites induced by the phytotoxin in the susceptible variety during necrosis shares with that of the pathogen, the identification of metabolites specific to the phytotoxin in comparison to the pathogenic and SDW controls indicated that the phytotoxin modulates the host metabolism differently and hence can be a potential pathogenicity factor of the ShB fungus.

SIGNIFICANCE AND IMPACT OF THE STUDY

Due to lack of knowledge on the pathway genes of RST and in the absence of an ShB resistant variety, understanding differentially expressed metabolic changes induced in the susceptible variety by the phytotoxin in comparison to that of the pathogenic and uninoculated controls enables us to identify the key metabolite changes during the ShB infection. Such metabolomic changes can further be used to infer gene functions for exploitation in ShB control.

Rhizoctonia solani Kühn Pathophysiology: Status and Prospects of Sheath Blight Disease Management in Rice

Sheath blight caused by necrotrophic fungus Rhizoctonia solani Kühn is one of the most serious diseases of rice. Use of high yielding semi dwarf cultivars with dense planting and high dose of nitrogenous fertilizers accentuates the incidence of sheath blight in rice. Its diverse host range and ability to remain dormant under unfavorable conditions make the pathogen more difficult to manage. As there are no sources of complete resistance, management through chemical control has been the most adopted method for sheath blight management. In this review, we provide an up-to-date comprehensive description of host-pathogen interactions, various control measures such as cultural, chemical, and biological as well as utilizing host plant resistance. The section on utilizing host plant resistance includes identification of resistant sources, mapping QTLs and their validation, identification of candidate gene(s) and their introgression through marker-assisted selection. Advances and prospects of sheath blight management through biotechnological approaches such as overexpression of genes and gene silencing for transgenic development against R. solani are also discussed.

Gene expression analysis of resistant and susceptible rice cultivars to sheath blight after inoculation with Rhizoctonia solani

BACKGROUND

Rice sheath blight, caused by Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris), is one of the most severe diseases in rice (Oryza sativa L.) worldwide. Studies on resistance genes and resistance mechanisms of rice sheath blight have mainly focused on indica rice. Rice sheath blight is a growing threat to rice production with the increasing planting area of japonica rice in Northeast China, and it is therefore essential to explore the mechanism of sheath blight resistance in this rice subspecies.

RESULTS

In this study, RNA-seq technology was used to analyse the gene expression changes of leaf sheath at 12, 24, 36, 48, and 72 h after inoculation of the resistant cultivar 'Shennong 9819' and susceptible cultivar 'Koshihikari' with R. solani. In the early stage of R. solani infection of rice leaf sheaths, the number of differentially expressed genes (DEGs) in the inoculated leaf sheaths of resistant and susceptible cultivars showed different regularity. After inoculation, the number of DEGs in the resistant cultivar fluctuated, while the number of DEGs in the susceptible cultivar increased first and then decreased. In addition, the number of DEGs in the susceptible cultivar was always higher than that in the resistant cultivar. After inoculation with R. solani, the overall transcriptome changes corresponding to multiple biological processes, molecular functions, and cell components were observed in both resistant and susceptible cultivars. These included metabolic process, stimulus response, biological regulation, catalytic activity, binding and membrane, and they were differentially regulated. The phenylalanine metabolic pathway; tropane, piperidine, and pyridine alkaloid biosynthesis pathways; and plant hormone signal transduction were significantly enriched in the early stage of inoculation of the resistant cultivar Shennong 9819, but not in the susceptible cultivar Koshihikari. This indicates that the response of the resistant cultivar Shennong 9819 to pathogen stress was faster than that of the susceptible cultivar. The expression of plant defense response marker PR1b gene, transcription factor OsWRKY30 and OsPAL1 and OsPAL6 genes that induce plant resistance were upregulated in the resistant cultivar. These data suggest that in the early stage of rice infection by R. solani, there is a pathogen-induced defence system in resistant rice cultivars, involving the expression of PR genes, key transcription factors, PAL genes, and the enrichment of defence-related pathways.

CONCLUSION

The transcriptome data revealed the molecular and biochemical differences between resistant and susceptible cultivars of rice after inoculation with R. solani, indicating that resistant cultivars have an immune response mechanism in the early stage of pathogen infection. Disease resistance is related to the overexpression of PR genes, key transcriptome factors, and PAL genes, which are potential targets for crop improvement.

Comparative Proteomic Analysis of Rhizoctonia solani Isolates Identifies the Differentially Expressed Proteins with Roles in Virulence

Sheath blight of rice is a destructive disease that could be calamitous to rice cultivation. The significant objective of this study is to contemplate the proteomic analysis of the high virulent and less virulent isolate of Rhizoctonia solani using a quantitative LC-MS/MS-based proteomic approach to identify the differentially expressed proteins promoting higher virulence. Across several rice-growing regions in Odisha, Eastern India, 58 Rhizoctonia isolates were obtained. All the isolates varied in their pathogenicity. The isolate RS15 was found to be the most virulent and RS22 was identified as the least virulent. The PCR amplification confirmed that the RS15 and RS22 belonged to the Rhizoctonia subgroup of AG1-IA with a specific primer. The proteomic information generated has been deposited in the PRIDE database with PXD023430. The virulent isolate consisted of 48 differentially abundant proteins, out of which 27 proteins had higher abundance, while 21 proteins had lower abundance. The analyzed proteins acquired functionality in fungal development, sporulation, morphology, pathogenicity, detoxification, antifungal activity, essential metabolism and transcriptional activities, protein biosynthesis, glycolysis, phosphorylation and catalytic activities in fungi. A Quantitative Real-Time PCR (qRT-PCR) was used to validate changes in differentially expressed proteins at the mRNA level for selected genes. The abundances of proteins and transcripts were positively correlated. This study provides the role of the proteome in the pathogenicity of R. solani AG1-IA in rice and underpins the mechanism behind the pathogen’s virulence in causing sheath blight disease.

Efficiency of mesoporous silica/carboxymethyl β-glucan as a fungicide nano-delivery system for improving chlorothalonil bioactivity and reduce biotoxicity

Understanding the lethal effects of pesticides nano formulations on the targeted organisms (pathogens) and the non-targeted organisms (fish, earthworms, etc) is essential in assessing the probable impact of new technologies on agriculture and environment. Here we evaluated the bioactivity and the biotoxicity of new type of fungicide smart-delivery formulation based on conjugating carboxymethylated-β-glucans on the mesoporous silica nanoparticles (MSNs) surface after loading chlorothalonil (CHT) fungicide in the MSNs pores. The obtained formulation has been characterized with FE-SEM, and HR-TEM. The CHT loading efficiency has been measured with TGA. The bioactivity of the obtained formulation (CHT@MSNs-β-glucans) has been tested against four pathogens, fusarium head blight (Fusarium graminearum), sheath rot (Sarocladium oryzae), rice sheath blight (Rhizoctonia solani), and soyabean anthracnose (Colletotrichum truncatum) compared with CHT WP 75% commercial formulation (CHT-WP) and technical CHT. The environmental biotoxicity of CHT@MSNs-β-glucans compared with CHT-WP has been tested toward earthworm (Eisenia fetida) and zebra fish (Danio rerio). The results showed that CHT@MSNs-β-glucans has an excellent bioactivity against the subjected pathogens with better inhabiting effects than CHT-WP. CHT@MSNs-β-glucans toxicity to Eisenia fetida was found 2.25 times lower than CHT-WP toxicity. The LC₅₀ of CHT@MSNs-β-glucans to zebra fish after the first 24h was 2.93 times higher than CHT-WP. After 96h of treatment, the LC₅₀ of CHT@MSNs-β-glucans was 2.66 times higher than CHT-WP. This work highlighted the necessity to increase the mandatory bioassays of nano formulations with the major non-target organisms in the environmental risk assessment of new pesticide formulations.

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.

Spray-induced gene silencing for disease control is dependent on the efficiency of pathogen RNA uptake

Recent discoveries show that fungi can take up environmental RNA, which can then silence fungal genes through environmental RNA interference. This discovery prompted the development of Spray-Induced Gene Silencing (SIGS) for plant disease management. In this study, we aimed to determine the efficacy of SIGS across a variety of eukaryotic microbes. We first examined the efficiency of RNA uptake in multiple pathogenic and non-pathogenic fungi, and an oomycete pathogen. We observed efficient double-stranded RNA (dsRNA) uptake in the fungal plant pathogens Botrytis cinerea, Sclerotinia sclerotiorum, Rhizoctonia solani, Aspergillus niger and Verticillium dahliae, but no uptake in Colletotrichum gloeosporioides, and weak uptake in a beneficial fungus, Trichoderma virens. For the oomycete plant pathogen, Phytophthora infestans, RNA uptake was limited and varied across different cell types and developmental stages. Topical application of dsRNA targeting virulence-related genes in pathogens with high RNA uptake efficiency significantly inhibited plant disease symptoms, whereas the application of dsRNA in pathogens with low RNA uptake efficiency did not suppress infection. Our results have revealed that dsRNA uptake efficiencies vary across eukaryotic microbe species and cell types. The success of SIGS for plant disease management can largely be determined by the pathogen's RNA uptake efficiency.

Spray-induced gene silencing for disease control is dependent on the efficiency of pathogen RNA uptake

Recent discoveries show that fungi can take up environmental RNA, which can then silence fungal genes through environmental RNA interference. This discovery prompted the development of Spray-Induced Gene Silencing (SIGS) for plant disease management. In this study, we aimed to determine the efficacy of SIGS across a variety of eukaryotic microbes. We first examined the efficiency of RNA uptake in multiple pathogenic and non-pathogenic fungi, and an oomycete pathogen. We observed efficient double-stranded RNA (dsRNA) uptake in the fungal plant pathogens Botrytis cinerea, Sclerotinia sclerotiorum, Rhizoctonia solani, Aspergillus niger and Verticillium dahliae, but no uptake in Colletotrichum gloeosporioides, and weak uptake in a beneficial fungus, Trichoderma virens. For the oomycete plant pathogen, Phytophthora infestans, RNA uptake was limited and varied across different cell types and developmental stages. Topical application of dsRNA targeting virulence-related genes in pathogens with high RNA uptake efficiency significantly inhibited plant disease symptoms, whereas the application of dsRNA in pathogens with low RNA uptake efficiency did not suppress infection. Our results have revealed that dsRNA uptake efficiencies vary across eukaryotic microbe species and cell types. The success of SIGS for plant disease management can largely be determined by the pathogen's RNA uptake efficiency.

Genome-Wide Expression Profiling of Small RNAs in Indian Strain of Rhizoctonia solani AG1-1A Reveals Differential Regulation of milRNAs during Pathogenesis and Crosstalk of Gene Regulation

Rhizoctonia solani AG1-1A is a necrotrophic fungus that causes sheath blight disease in rice. The reliable resistant source against this phytopathogenic fungus is not available in the gene pool of rice. Better understanding of pathogen genomics and gene regulatory networks are critical to devise alternate strategies for developing resistance against this noxious pathogen. In this study, miRNA-like RNAs (milRNAs) of an Indian strain of R. solani were identified by deep sequencing of small RNAs. We identified 128 known and 22 novel milRNAs from 20,963,123 sequence reads. These milRNAs showed 1725 target genes in the fungal genome which include genes associated with growth, development, pathogenesis and virulence of R. solani. Notably, these fungal milRNAs showed their target genes in host (rice) genome also which were later verified by qRT-PCR. The host target genes are associated with auxin metabolism, hypersensitive response, defense genes, and genes related to growth and development of rice. Osa-vacuolar-sorting receptor precursor: Rhi-milR-13, Osa-KANADI1:Rhi-milR-124, Osa-isoflavone reductase: Rhi-milR-135, Osa-nuclear transcription factor Y:Rhi-milR-131, Osa-NB-ARC domain containing protein: Rhi-milR-18, and Osa-OsFBX438: Rhi-milR-142 are notable potential regulons of host target genes: fungal milRNAs that need to be investigated for better understanding of the crosstalk of RNAi pathways between R. solani and rice. The detailed expression analysis of 17 milRNAs by qRT-PCR was analysed during infection at different time points of inoculation, at different growth stages of the host, in four different genotypes of the host, and also in four different strains of fungi which revealed differential regulation of milRNAs associated with pathogenesis and virulence. This study highlights several important findings on fungal milRNAs which need to be further studied and characterized to decipher the gene expression and regulation of this economically important phytopathogen.

Genome Sequence of Rhizoctonia solani Anastomosis Group 4 Strain Rhs4ca, a Widespread Pathomycete in Field Crops

Rhizoctonia solani is an important soil-borne fungal pathogen that causes serious diseases on many agricultural crops and vegetables. Here, we report a complete genome assembly of R. solani AG4 (assembly: 45.47 Mb; contig N₅₀: 1.56 Mb), using a combination of Illumina paired-end and PacBio long-read sequencing data. A total of 267 noncoding RNAs and 11,592 genes were predicted, including 109 genes associated with carbohydrate-active enzymes and 2,488 genes involved in host-pathogen interactions. The complete genome of R. solani AG4 represents a valuable base for studying interactions between host plants and pathogenic fungi and to search for potential antimicrobial targets.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Strategies to Manage Rice Sheath Blight: Lessons from Interactions between Rice and Rhizoctonia solani

Rhizoctonia solani is an important phytopathogenic fungus with a wide host range and worldwide distribution. The anastomosis group AG1 IA of R. solani has been identified as the predominant causal agent of rice sheath blight, one of the most devastating diseases of crop plants. As a necrotrophic pathogen, R. solani exhibits many characteristics different from biotrophic and hemi-biotrophic pathogens during co-evolutionary interaction with host plants. Various types of secondary metabolites, carbohydrate-active enzymes, secreted proteins and effectors have been revealed to be essential pathogenicity factors in R. solani. Meanwhile, reactive oxygen species, phytohormone signaling, transcription factors and many other defense-associated genes have been identified to contribute to sheath blight resistance in rice. Here, we summarize the recent advances in studies on molecular interactions between rice and R. solani. Based on knowledge of rice-R. solani interactions and sheath blight resistance QTLs, multiple effective strategies have been developed to generate rice cultivars with enhanced sheath blight resistance.

Evolutionary and genomic comparisons of hybrid uninucleate and nonhybrid Rhizoctonia fungi

The basidiomycetous fungal genus, Rhizoctonia, can cause severe damage to many plants and is composed of multinucleate, binucleate, and uninucleate species differing in pathogenicity. Here we generated chromosome-scale genome assemblies of the three nuclear types of Rhizoctonia isolates. The genomic comparisons revealed that the uninucleate JN strain likely arose by somatic hybridization of two binucleate isolates, and maintained a diploid nucleus. Homeolog gene pairs in the JN genome have experienced both decelerated or accelerated evolution. Homeolog expression dominance occurred between JN subgenomes, in which differentially expressed genes show potentially less evolutionary constraint than the genes without. Analysis of mating-type genes suggested that Rhizoctonia maintains the ancestral tetrapolarity of the Basidiomycota. Long terminal repeat-retrotransposons displayed a reciprocal correlation with the chromosomal GC content in the three chromosome-scale genomes. The more aggressive multinucleate XN strain had more genes encoding enzymes for host cell wall decomposition. These findings demonstrate some evolutionary changes of a recently derived hybrid and in multiple nuclear types of Rhizoctonia.

A rapid colorimetric LAMP assay for detection of Rhizoctonia solani AG-1 IA causing sheath blight of rice

Rhizoctonia solani is one of the most devastating pathogens. R. solani AG-1 IA causes sheath blight in rice, maize, and other Gramineous plants. Accurate identification is essential for the effective management of this pathogen. In the present study, a set of four primers were designed viz. RSPG1, RSPG2, RSPG4, and RSPG5 for polygalacturonase (PG) gene, an important virulence factor in phytopathogenic fungi. All four primer sets showed specific amplification of 300 bp (RSPG1F/R), 375 bp (RSPG2F/R), 500 bp (RSPG4F/R) and 336 bp (RSPG5F/R) amplicons. q-PCR detection using each primer sets could detect up to 10 pg of DNA. We also designed six primers (RS_pg_F3_1/RS_pg_B3_1, RS_pg_FIP_1.1/RS-pg_BIP_1.1, and RS_pg_LF_1/RS_pg_LB_1) for PG gene. Further, a colorimetric LAMP assay developed yielded visual confirmation of the pathogen within 45 min of sample collection when coupled with rapid high throughput template preparation method (rHTTP) from infected samples. The sensitivity of the LAMP assay was as low as 1.65 fg/µl of template DNA and could effectively detect R. solani AG-1 IA from diseased plant tissues and soil samples. The LAMP assay was highly specific for R. solani as it did not show any amplification with other AG groups of R. solani and closely related fungal and bacterial outgroups. This study will help in designing an effective point of care diagnostic method for early monitoring of R. solani and thereby planning timely preventive measures against the pathogen.

A Comprehensive Analysis of MicroRNAs Expressed in Susceptible and Resistant Rice Cultivars during Rhizoctonia solani AG1-IA Infection Causing Sheath Blight Disease

MicroRNAs regulate plant responses to fungal infections and immunity. In this study, miRNAs were identified in six rice cultivars during a Rhizoctonia solani Kühn AG1-IA infection using a deep sequencing approach. Known and novel miRNAs were analyzed in these rice cultivars, and a set of fungal infection/immunity-associated miRNAs and target genes were quantified by reverse transcription (RT)-qPCR in six rice cultivars. Additionally, the relative expression of these miRNAs was analyzed in different time points of the infection, wild species of rice, and in response to different strains of R. solani. Osa-miR1320-5p showed preferential expression during the fungal infection in all the six rice genotypes, while Osa-miR156d, Osa-miR159b, Osa-miR820c, and Osa-miR1876 were differentially regulated in susceptible and resistant genotypes. A greater degree of downregulation of miRNAs was observed during the initial time points of infection (24-72 h), suggesting a maximum molecular activity of rice-R. solani interaction and resistance response of the host during the early phase of infection. After R. solani infection, the expression of Osa-miR820c and Osa-miR156d was downregulated in Oryza rufipogon, O. alta, O. latifolia, and O. minuta, while Osa-miR397b was downregulated in all the wild rice species except O. officinalis. This study provided comprehensive information on the repertoire of miRNAs expressed in six sheath blight disease-susceptible and resistant indica and aus rice cultivars.

A Comprehensive Gene Expression Profile of Pectin Degradation Enzymes Reveals the Molecular Events during Cell Wall Degradation and Pathogenesis of Rice Sheath Blight Pathogen Rhizoctonia solani AG1-IA

Sheath blight disease of rice caused by Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris) remains a global challenge due to the absence of reliable resistance genes and poor understanding of pathogen biology. Pectin, one of the most vital constituents of the plant cell wall, is targeted by pectin methylesterases, polygalacturonases, and few other enzymes of fungal pathogens. In this study, we catalogued the expressed genes of the fungal genome from RNAseq of R. solani infected four rice genotypes. Analysis of RNAseq revealed 3325 pathogen genes commonly expressed in all rice genotypes, in which 49, 490, and 83 genes were specific to BPT5204, Tetep, and Pankaj genotypes, respectively. To identify the early and late responding genes of R. solani during plant cell wall degradation, a real-time PCR analysis of 30 pectinolytic enzymes was done at six different time points after inoculation. The majority of these genes showed maximum induction at the 72 h time point, suggesting that it is the most crucial stage of infection. Pankaj showed lesser induction of these genes as compared to other genotypes. Leaf-blade tissue and 45 days old-growth stage are more favorable for the expression of pectin degradation genes of R. solani. Additionally, the expression analysis of these genes from four different strains of R. solani suggested differential regulation of genes but no distinct expression pattern between highly virulent and mild strains. The implications of the differential regulation of these genes in disease development have been discussed. This study provides the first such comprehensive analysis of R. solani genes encoding pectin degrading enzymes, which would help to decipher the pathogen biology and sheath blight disease development.

Understanding sheath blight resistance in rice: the road behind and the road ahead

Rice sheath blight disease, caused by the basidiomycetous necrotroph Rhizoctonia solani, became one of the major threats to the rice cultivation worldwide, especially after the adoption of high-yielding varieties. The pathogen is challenging to manage because of its extensively broad host range and high genetic variability and also due to the inability to find any satisfactory level of natural resistance from the available rice germplasm. It is high time to find remedies to combat the pathogen for reducing rice yield losses and subsequently to minimize the threat to global food security. The development of genetic resistance is one of the alternative means to avoid the use of hazardous chemical fungicides. This review mainly focuses on the effort of better understanding the host-pathogen relationship, finding the gene loci/markers imparting resistance response and modifying the host genome through transgenic development. The latest development and trend in the R. solani-rice pathosystem research with gap analysis are provided.

Comparison of leaf transcriptome in response to Rhizoctonia solani infection between resistant and susceptible rice cultivars

Background

Sheath blight (SB), caused by Rhizoctonia solani, is a common rice disease worldwide. Currently, rice cultivars with robust resistance to R. solani are still lacking. To provide theoretic basis for molecular breeding of R. solani-resistant rice cultivars, the changes of transcriptome profiles in response to R. solani infection were compared between a moderate resistant cultivar (Yanhui-888, YH) and a susceptible cultivar (Jingang-30, JG).

Results

In the present study, 3085 differentially express genes (DEGs) were detected between the infected leaves and the control in JG, with 2853 DEGs in YH. A total of 4091 unigenes were significantly upregulated in YH than in JG before infection, while 3192 were significantly upregulated after infection. Further analysis revealed that YH and JG showed similar molecular responses to R. solani infection, but the responses were earlier in JG than in YH. Expression levels of trans-cinnamate 4-monooxygenase (C4H), ethylene-insensitive protein 2 (EIN2), transcriptome factor WRKY33 and the KEGG pathway plant-pathogen interaction were significantly affected by R. solani infection. More importantly, these components were all over-represented in YH cultivar than in JG cultivar before and/or after infection.

Conclusions

These genes possibly contribute to the higher resistance of YH to R. solani than JG and were potential target genes to molecularly breed R. solani-resistant rice cultivar.

Sheath blight of rice: a review and identification of priorities for future research

Rice sheath blight research should prioritise optimising biological control approaches, identification of resistance gene mechanisms and application in genetic improvement and smart farming for early disease detection. Rice sheath blight, caused by Rhizoctonia solani AG1-1A, is one of the most devasting diseases of the crop. To move forward with effective crop protection against sheath blight, it is important to review the published information related to pathogenicity and disease management and to determine areas of research that require deeper study. While progress has been made in the identification of pathogenesis-related genes both in rice and in the pathogen, the mechanisms remain unclear. Research related to disease management practices has addressed the use of agronomic practices, chemical control, biological control and genetic improvement: Optimising nitrogen fertiliser use in conjunction with plant spacing can reduce spread of infection while smart agriculture technologies such as crop monitoring with Unmanned Aerial Systems assist in early detection and management of sheath blight disease. Replacing older fungicides with natural fungicides and use of biological agents can provide effective sheath blight control, also minimising environmental impact. Genetic approaches that show promise for the control of sheath blight include treatment with exogenous dsRNA to silence pathogen gene expression, genome editing to develop rice lines with lower susceptibility to sheath blight and development of transgenic rice lines overexpressing or silencing pathogenesis related genes. The main challenges that were identified for effective crop protection against sheath blight are the adaptive flexibility of the pathogen, lack of resistant rice varieties, abscence of single resistance genes for use in breeding and low access of farmers to awareness programmes for optimal management practices.