Genome Sequencing and Comparative Genomics of the Broad Host-Range Pathogen Rhizoctonia solani AG8

Abstract

Rhizoctonia solani is a soil-borne basidiomycete fungus with a necrotrophic lifestyle which is classified into fourteen reproductively incompatible anastomosis groups (AGs). One of these, AG8, is a devastating pathogen causing bare patch of cereals, brassicas and legumes. R. solani is a multinucleate heterokaryon containing significant heterozygosity within a single cell. This complexity posed significant challenges for the assembly of its genome. We present a high quality genome assembly of R. solani AG8 and a manually curated set of 13,964 genes supported by RNA-seq. The AG8 genome assembly used novel methods to produce a haploid representation of its heterokaryotic state. The whole-genomes of AG8, the rice pathogen AG1-IA and the potato pathogen AG3 were observed to be syntenic and co-linear. Genes and functions putatively relevant to pathogenicity were highlighted by comparing AG8 to known pathogenicity genes, orthology databases spanning 197 phytopathogenic taxa and AG1-IA. We also observed SNP-level “hypermutation” of CpG dinucleotides to TpG between AG8 nuclei, with similarities to repeat-induced point mutation (RIP). Interestingly, gene-coding regions were widely affected along with repetitive DNA, which has not been previously observed for RIP in mononuclear fungi of the Pezizomycotina. The rate of heterozygous SNP mutations within this single isolate of AG8 was observed to be higher than SNP mutation rates observed across populations of most fungal species compared. Comparative analyses were combined to predict biological processes relevant to AG8 and 308 proteins with effector-like characteristics, forming a valuable resource for further study of this pathosystem. Predicted effector-like proteins had elevated levels of non-synonymous point mutations relative to synonymous mutations (dN/dS), suggesting that they may be under diversifying selection pressures. In addition, the distant relationship to sequenced necrotrophs of the Ascomycota suggests the R. solani genome sequence may prove to be a useful resource in future comparative analysis of plant pathogens.

Author Summary

The fungus Rhizoctonia solani is divided into several sub-species which cause disease in a range of plant species that includes most major agriculture, forestry and bioenergy species. This study focuses on sub-species AG8 which causes disease of cereals, canola and legumes, and compares its genome to other R. solani sub-species and a wide range of fungal and non-fungal species. R. solani is unusual in that it can possess more than one nucleus per cell. The multiple nuclei and sequence mutations between them made assembly of its genome challenging, and required novel techniques. We observed signs that DNA sequences originating from multiple nuclei in AG8 exhibit a high frequency of single nucleotide polymorphisms (SNPs) and more SNP diversity than most fungal populations. These SNP mutations also have similarities to repeat-induced point mutations (RIP). Moreover in AG8, RIP-like SNPs are not restricted to intergenic regions but are also widely observed in gene-coding regions. This is novel as RIP has previously only been reported in repetitive DNA of distantly-related fungi that have only a single nucleus per cell. We generated a list of 308 genes with similar properties to known plant-disease proteins, in which we found higher rates of non-synonymous mutations than normal.