Subterranean morphology underpins the degree of mycoheterotrophy, mycorrhizal associations, and plant vigor in a green orchid <i>Oreorchis patens</i>

The evolution of full heterotrophy is a fascinating topic in plant evolution, with recent studies suggesting that partial mycoheterotrophy (mixotrophy) serves as a transitional stage toward full mycoheterotrophy in orchids. However, the adaptive significance of fungal-derived carbon in mixotrophic plants remains largely unexplored. In this study, we investigated the photosynthetic orchid Oreorchis patens, a species related to the leafless genus Corallorhiza within the subtribe Calypsoinae. Using high-throughput DNA sequencing, ¹³C and ¹⁵N isotopic analyses, and phenotypic evaluations, we explored the role of coralloid rhizomes - a feature common in fully mycoheterotrophic orchids - in fungal partnerships, the degree of mycoheterotrophy, and plant vigor. Our findings reveal that O. patens plants with coralloid rhizomes predominantly associate with saprotrophic Psathyrellaceae fungi, whereas those without coralloid rhizomes also partner with rhizoctonias and other potentially orchid mycorrhizal fungi. Notably, plants with coralloid rhizomes exhibited enriched ¹³C signatures, indicating a greater reliance on fungal-derived carbon. These plants also demonstrated more vigorous flowering scapes and produced a higher number of flowers, suggesting that mycoheterotrophy significantly enhances plant vigor. This study provides rare insights into the adaptive significance of mycoheterotrophy. Recent research suggests that some partially mycoheterotrophic orchids can adjust their heterotrophic status to optimize carbon resource use under specific conditions, such as low-light environments. However, an increased proportion of fungal-derived carbon may sometimes merely reflect reduced photosynthesis in such conditions, thereby amplifying the apparent contribution of fungal-derived carbon. Our findings offer more direct evidence that carbon acquisition via mycoheterotrophy is beneficial for partially mycoheterotrophic orchids.