Partial mycoheterotrophy: a common strategy in arbuscular mycorrhizal plants?

The vast majority of land plants transfer part of the organic carbon they produce by photosynthesis to arbuscular mycorrhizal (AM) fungi inside their root cells; the fungi in turn help plants to take up nutrients and water from the soil. This carbon can subsequently be acquired from mycorrhizal fungi by rare nonphotosynthetic 'mycoheterotrophic' plants that tap into the same fungal network. However, recent findings suggest that carbon uptake from AM fungi may exist among some green plants too. If so, this would qualify them as partial mycoheterotrophs (mixotrophs) rather than as pure autotrophs. Here, we discuss the evolutionary, ecophysiological, morphological, genetic, and environmental evidence for the existence and prevalence of this trait. We conclude that there is strong, albeit indirect, evidence for its existence, although its taxonomic distribution remains to be determined. This knowledge gap currently prevents us from inferring the magnitude of AM partial mycoheterotrophy in terrestrial ecosystems, which in turn severely limits our understanding of its role in plant establishment and survival and in ecosystem structure and function.