A stunning diversity
Flowering plants include over 250 000 known species and are thus, by far, the most diverse group of photosynthetic organisms on Earth. For over 100 million years, the flowering plants have dominated the land and, through evolution, they have adapted tenaciously to the most extreme conditions. Flowering plants grow in deserts and seas, on mountaintops, on each other, and some remain their entire life underground. One of the most remarkable adaptations of some flowering plants is their ability to grow in absence of sunlight. These include parasitic plants, which steal their nutrients directly from other plants. Less-known but perhaps even more intriguing are mycoheterotrophic plants (‘saprophytes’).
Adapting to darkness
In deeply shaded forest understorys, plants compete intensively for light. Mycoheterotrophic plants have evolved an ingenious mode of life to escape from this competition. Mycoheterotrophic plants obtain all of their carbon requirements through symbiotic associations with fungi, and, while achlorophyllous, they are not directly parasitic on other plants. Mycoheterotrophy evolved over 40 times independently in flowering plants, and occurs both in monocots and eudicots. There are over 400 fully achlorophyllous mycoheterotrophic species and nearly 20 000 species that are partly mycoheterotrophic (most are initial mycoheterotrophs in the Orchidaceae). Except for a few orchids that grow on saprotrophic fungi, all mycoheterotrophic plants exploit mycorrhizal fungi and are thus epiparasitic on green mycorrhizal plants.
Rare but significant
Mycoheterotrophic plants have been considered botanical curiosities and, although they arose repeatedly in plant evolution, fully mycoheterotrophic plants are quite rare. Research on these fascinating organisms may, at first glance, appear to represent a disproportionate focus on the minutiae. However, recent studies on mycoheterotrophs have revealed they are far from mere botanical curiosities, and led, for example, to the discovery of partially mycoheterotrophic plants. Mycoheterotrophs offer powerful models to study the interaction between plants and mycorrhizal fungi, and the evolution of mycorrhizal specificity.