Phylogenetic placement and host specificity of mycorrhizal isolates belonging to AG-6 and AG-12 in the Rhizoctonia solani species complex

Abstract

Rhizoctonia solani isolates belonging to anastomosis group 6 (AG-6) and AG-12 are mycorrhizal with the Australian orchid Pterostylis acuminata. In this study, phylogenetic analyses were conducted using sequences of the ITS (internal transcribed spacer) region of the rRNA gene from all known anastomosis groups of R. solani. The mycorrhizal isolates of AG-6 and AG-12 were placed on independent branches, despite their functional and geographic similarities. The mycorrhizal AG-6 isolates were positioned most closely to the pathogenic AG-6 isolates from Southern Africa. The remaining AG-6 isolates from Japan and the USA were positioned in a distinct and independent cluster, suggesting that various subgroups of AG-6 have evolved independently within R. solani. Germination tests were conducted using AG-6 isolates obtained from pathogenic and saprophytic interactions, as well as isolates belonging to the pathogenic AG 3, 4, 5, 7, and 8. All AG-6 isolates promoted germination of P. acuminata seed, regardless of their origin, although they did so more slowly than the AG-12 isolates. All other AGs either parasitized or disregarded the orchid seed, indicating that the ability to form mycorrhizal interactions is restricted in R. solani. This is the first phylogenetic analysis to include all known AG with a range of host associations and geographic origins including the newly described mycorrhizal AG, and will assist in understanding the evolutionary history and host specificity of this fungus.