No abstract

Rhizoctonia solani AG1 IA is a soil-borne fungal phytopathogen that can significantly harm crops resulting in economic loss. This species overwinters in grass roots and diseased plants, and produces sclerotia that infect future crops. R. solani AG1 IA does not produce spores; therefore, understanding the molecular mechanism of sclerotia formation is important for crop disease control. To identify the genes involved in this process for the development of disease control targets, the transcriptomes of this species were determined at three important developmental stages (mycelium, sclerotial initiation, and sclerotial maturation) using an RNA-sequencing approach. A total of 5,016, 6,433, and 5,004 differentially expressed genes (DEGs) were identified in the sclerotial initiation vs. mycelial, sclerotial maturation vs. mycelial, and sclerotial maturation vs. sclerotial initiation stages, respectively. Moreover, gene ontology (GO) and kyoto encyclopedia of genes and genomes (KEGG) analyses showed that these DEGs were enriched in diverse categories, including oxidoreductase activity, carbohydrate metabolic process, and oxidation-reduction processes. A total of 12 DEGs were further verified using reverse transcription quantitative PCR. Among the genes examined, NADPH oxidase 1 (NOX1) and superoxide dismutase (SOD) were highly induced in the stages of sclerotial initiation and maturation. In addition, the highest reactive oxygen species (ROS) production levels were detected during sclerotial initiation, and enzyme activities of NOX1, SOD, and catalase (CAT) matched with the gene expression profiles. To further evaluate the role of ROS in sclerotial formation, R. solani AG1 IA was treated with the CAT inhibitor aminotriazole and H2O2, resulting in the early differentiation of sclerotia. Taken together, this study provides useful information toward understanding the molecular basis of R. solani AG1 IA sclerotial formation and maturation, and identified the important role of ROS in these processes.

Rhizoctonia solani AG1-IA (R. solani AG1-IA) is a major soil-borne fungal pathogen of maize that causes significant yield losses in all maize-growing regions worldwide. The sclerotium produced by R. solani AG1-IA can overwinter in grass roots or diseased plants and infect crops the following year. The molecular mechanism underlying sclerotium formation in R. solani is poorly understood. In this study, we constructed the cDNA library of the R. solani AG1-IA pathogenic strain WF-9, from which 329 high-quality expressed sequence tags (ESTs) were obtained. Of the 250 clustered unigenes, 12 genes were selected for further expression analysis during the three stages of sclerotial growth (mycelium, initiation of sclerotium, and maturation of sclerotium). The results of expression analysis support the previously suggested roles of chitin synthase D and exo-beta-1,3-glucanase in facilitating sclerotial growth through preservation of water content and energy. In addition, cytochrome P450, NADPH oxidase, catalase (CAT), acyl-CoA oxidase 1 (ACOX1), mitogen-activated protein kinase (MAPK), mitogen-activated protein kinase HOG1 (HOG 1), and the G-protein α subunit play important roles in balancing reactive oxygen species (ROS) levels during sclerotial development. The findings of this study can help understand the molecular mechanism of sclerotial development further.

During the plant infection process, pathogens can secrete several effectors. Some of the effectors are well-known for their roles in regulating plant immunity and promoting successful pathogen colonization. However, there are few studies on the ribonuclease (RNase) effectors secreted by fungi. In the present study, we discovered a secretable RNase (SRE1) in the secretome of Setosphaeria turcica that was significantly upregulated during the early stages of S. turcica infection in maize. Knockdown of SRE1 significantly reduced the virulence of S. turcica. SRE1 can induce cell death in maize and Nicotiana benthamiana. However, unlike the conventional hypersensitive response (HR) caused by other effectors, SRE1 is not dependent on its signal peptide (SP) or plant receptor kinases (such as BAK1 and SOBIR1). SRE1-induced cell death depends upon its enzymatic activity and the N-terminal β-hairpin structure. SRE1 relies on its N-terminal β-hairpin structure to enter cells, and then degrades plant's RNA through its catalytic activity causing cytotoxic effects. Additionally, SRE1 enhances N. benthamiana's resistance to pathogenic fungi and oomycetes. In summary, SRE1 promotes the virulence of S. turcica, inducing plant cell death and activating plant immune responses.

Northern corn leaf blight (NCLB), caused by Setosphaeria turcica, is an important foliar disease in corn. Since 2005, the damage from NCLB has increased in Northeast China, probably due to the emergence of new physiological races. In this study, 883 single conidial isolates of S. turcica were obtained from 12 sites across three provinces of Northeast China between 2007 and 2017. The virulence of the isolates was evaluated in five corn lines (B37, B37Ht1, B37Ht2, B37Ht3, B37HtN). Sixteen physiological races (0, 1, 2, 3, N, 12, 13, 1N, 23, 2N, 3N, 123, 12N, 13N, 23N, and 123N) were obtained, depending on their resistance or susceptibility. Three races (0, 1, and 2) were most prevalent, with frequencies of 40.5%, 19.6%, and 11.3% in all isolates, respectively. Races varied across provinces and years. Virulence to more than one Ht resistance genes occurred in 21.5% of isolates, with 8.5% virulent to three or more genes. Overall, 41% of isolates were avirulent to all Ht genes, 36% were virulent to Ht1, 28% to Ht2, 11% to Ht3, and 16% to HtN. Isolates from Heilongjiang had a greater frequency of virulence to Ht2 and Ht3, whereas isolates from Jilin and Liaoning were more frequently virulent to Ht1 and HtN, respectively. The frequency of isolate virulence to Ht2 ranged from 8% in 2009 to a maximum of 29% in 2015, and in 2015, isolates were more virulent to Ht2 than Ht1. This study will help growers to purposefully select commercial hybrids with multiple effective Ht resistance genes, and reduce the utilization of Ht1 and Ht2 genes in the process of corn production to strengthen NCLB control.