Sequencing of the plastome in the leafless green mycoheterotroph Cymbidium macrorhizon helps us to understand an early stage of fully mycoheterotrophic plastome structure

To date, plastome studies of mycoheterotrophic orchids have focused on nongreen mycoheterotrophic or partially mycoheterotrophic species. Cymbidium macrorhizon is a fully mycoheterotrophic orchid that lacks leaves and roots, although its inflorescence rachis is pale green. It has degraded stomata, specific fungal partners, and high concentrations of heavy stable nitrogen and carbon isotopes. Therefore, the plastome of this species is expected to represent an early stage of a fully mycoheterotrophic plastome. In this study, we sequenced the plastomes of C. macrorhizon and closely related species (C. ensifolium, C. kanran, and C. lancifolium). Plastomes of the four Cymbidium species were almost identical structurally, but differed somewhat from those of previously studied species. The genes for the photosynthetic subunits of NADH dehydrogenase, ndhF and ndhH, were absent from all four newly sequenced plastomes, whereas only ndhJ was absent from C. ensifolium. In section Pachyrhizanthe (C. lancifolium and C. macrorhizon), ndhE, ndhI, and ndhJ were pseudogenized. With the exception of ndh and ycf, 64 protein-coding genes in C. macrorhizon were apparently functional. Most of them were highly conserved and under purifying selection. Therefore, no direct evidence is available to suggest that genes related to photosynthesis have lost their functions in C. macrorhizon. This discordance between molecular and physiological features for the trophic status of C. macrorhizon might result from a lag between photosynthetic function loss and relaxed purifying selection.