Stable Isotope Signatures Illuminate Diverse Nutritional Strategies in Rhizoctonias and Their Orchid Partners.

Understanding the nutritional ecology of orchids, particularly those in symbiosis with rhizoctonias, presents a complex challenge. This complexity arises partly from the absence of macroscopic fruit bodies in rhizoctonias, which impedes the acquisition of their stable isotope data. In this study, we investigated the fungal associations and isotopic signatures in the pelotons of Stigmatodactylus sikokianus (associated with non-ectomycorrhizal [non-ECM] rhizoctonias) and Chamaegastrodia shikokiana (associated with ECM rhizoctonias). Our research reveals elevated levels of C enrichment in S. sikokianus plants and their pelotons, similar to those found in fully mycoheterotrophic orchids and their mycobionts. Interestingly, C. shikokiana plants and their pelotons exhibited even higher levels of C and N enrichment than many other fully mycoheterotrophic species. Our findings imply that both ECM and saprotrophic mycobionts, including certain rhizoctonias, can fulfill the carbon needs of highly mycoheterotrophic orchids. This finding also indicates that C enrichment can be an indicator of mycoheterotrophy in at least some rhizoctonia-associated orchids, despite the typically low C enrichment in non-ECM rhizoctonias. Our demonstration of partial mycoheterotrophy in S. sikokianus suggests a broader prevalence of this nutritional strategy among orchids, given that almost all orchids are associated with non-ECM rhizoctonias.