To improve our understanding of the origin and evolution of mycoheterotrophic plants, we here present the chromosome-scale genome assemblies of two sibling orchid species: partially mycoheterotrophic Platanthera zijinensis and holomycoheterotrophic Platanthera guangdongensis. Comparative analysis shows that mycoheterotrophy is associated with increased substitution rates and gene loss, and the deletion of most photoreceptor genes and auxin transporter genes might be linked to the unique phenotypes of fully mycoheterotrophic orchids. Conversely, trehalase genes that catalyse the conversion of trehalose into glucose have expanded in most sequenced orchids, in line with the fact that the germination of orchid non-endosperm seeds needs carbohydrates from fungi during the protocorm stage. We further show that the mature plant of P. guangdongensis, different from photosynthetic orchids, keeps expressing trehalase genes to hijack trehalose from fungi. Therefore, we propose that mycoheterotrophy in mature orchids is a continuation of the protocorm stage by sustaining the expression of trehalase genes. Our results shed light on the molecular mechanism underlying initial, partial and full mycoheterotrophy.

The phylogenetic positions of two little-known species of Cymbidium (Cymbidiinae) with creeping rhizomatous habit, C. lii and C. repens, has previously been unclear. Here, we present a comprehensive molecular phylogenetic study of the genus based on nuclear ribosomal ITS and seven plastid regions. All analyses showed that C. lii and C. repens are sister to C. dayanum, an epiphytic species, with high support. However, many topological conflicts in inter- and intra-sectional relationships were observed in other parts of the trees, supporting existence of hybridization in Cymbidium but not affecting the taxa of interest in this study.

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