Pan-genome and multi-omics analyses reveal the genomic basis of symbiotic evolution in the mycoheterotrophic medicinal orchid Gastrodia elata

Mycoheterotrophy represents an extreme evolutionary strategy in which plants abandon photosynthesis and become obligately dependent on fungal partners for carbon and nutrients. Gastrodia elata, a medicinal orchid forming long-term symbioses with Armillaria and Mycena, provides a compelling system to investigate the genomic basis of obligate plant-fungus mutualism. Here, we generate a chromosome-level genome assembly (1.09 Gb) of a dark-red G. elata accession and construct a pan-genome from 12 Gastrodia accessions to dissect the evolutionary consequences of mycoheterotrophy. Phylogenomic analyses reveal pervasive hybridization across cultivated germplasm and demonstrate that tuber morphology more accurately reflects intraspecific relationships than stem color, challenging the traditional color-based taxonomy. Comparative genomic analyses reveal a pronounced degeneration of photosynthetic capacity. We detect no intact nuclear-encoded rbcS loci, which would preclude assembly of the Rubisco holoenzyme and thus canonical Calvin-Benson carbon fixation. Notably, a subset of photosynthesis-related genes is retained and transcriptionally active, suggesting their co-option into non-photosynthetic regulatory roles. Despite millions of years of intimate association with fungal partners, we detect no evidence of fungal-derived horizontal gene transfer based on a multi-step phylogenomic validation pipeline, indicating that G. elata-fungus mutualism is sustained through metabolic exchange rather than genetic integration. We further identify the Gastrodia antifungal protein (GAFP) gene family as a key molecular innovation underlying symbiotic homeostasis. The recently derived Class 1 GAFP lineage exhibits distinct domain architecture, tandem expansion, and elevated expression, consistent with enhanced regulation of fungal overgrowth. Finally, we delineate a specialized nutrient-acquisition framework adapted to obligate heterotrophy, in which fungal trehalose hydrolysis supplies hexose carbon via an expanded sugar transporter protein (STP) repertoire, while organic nitrogen is assimilated through amino acid and oligopeptide transporters coupled with urease-mediated conversion, compensating for the loss of nitrate uptake and reduction pathways. Together, these results establish a pan-genomic blueprint for symbiotic adaptation and reductive genome evolution in G. elata.