Basal orchid cheats on its fungal partner

The orchid family, which comprises more than 25,000 species including over 40% of all monocots, has a number of shared characteristics that make most members easily recognizable, such as a zygomorphic perianth, pollen grouped into pollinia, and a column made up of a fused style and androecium. However, basal orchids of the subfamily Apostasioideae share few of these traits, appearing instead similar to ancestral orchids: only weakly zygomorphic or actinomorphic perianths, with loosely aggregated pollen and an under- or undeveloped column.

Surprisingly, the only trait shared by all members of the orchid family is that of an initial dependence on a fungal symbiont to complete germination. Typically, orchid seedlings utilize fungal carbon during germination before establishing a more balanced mutualism wherein the fungus becomes a carbon sink for the mature plant, requiring up to a third of its photosynthates. Some orchids, termed mycoheterotrophs, cheat the symbiosis, continuing to use fungal carbon in addition to photosynthates.

In a recent article published in New Phytologist, authors Kenji Suetsugu and Jun Matsubayashi used molecular barcoding and isotopic measurements, respectively, to identify the fungal symbionts of the basal photosynthetic orchid Apostasia nipponica and determine whether it is mycoheterotrophic. If so, it will be the first member of the earliest-diverging lineage of Orchidaceae determined to be using this strategy.

The authors found that the orchid consistently associates with ectomycorrhizal (ECM) fungi of the Ceratobasidiaceae. Ectomycorrhizal fungi are often found to be present in mycoheterotrophic orchids, and some suggest that the ectomycorrhiza is the source of the carbon being taken up by the cheating orchid via the fungus. Isotopic analysis strongly pointed toward mycoheterotrophy for A. nipponica, with ¹³C values comparable to other mycoheterotrophs exploiting ECM fungi and ¹⁵N values even higher than average for those plants.

More research will need to be done to determine whether mycoheterotrophy is widespread in the Apostasioideae, but two other species in the genus have been found to associate with related fungal partners. “There is mounting evidence that full mycoheterotrophy is an end point on a continuous gradient that starts with autotrophy, and that initial and partial mycoheterotrophy are intermediate steps in the gradual transition from one to the other,” write the authors. “According to this hypothesis, the evolution of initial mycoheterotrophy is a critical step in the departure from full autotrophy. Therefore, given that initial mycoheterotrophy is thought to have evolved in the common ancestor of the Orchidaceae, partial mycoheterotrophy could occur widely in the Apostasioideae.”