Abstract
Most plants are autotrophic and interact with soil fungi, forming mycorrhizal symbioses (van der Heijden et al., 2015) where plants gain mineral nutrients and provide photosynthates to fungi. Yet, plants repeatedly evolve heterotrophy (Tĕšitel et al., 2018), and several lineages, especially in orchids, import carbon from their mycorrhizal fungi, a strategy called mycoheterotrophy (Merckx, 2013).
Green plants that are photosynthetic but also import carbon from their mycorrhizal fungi have raised considerable interest over the past two decades (Julou et al., 2005; Selosse & Roy, 2009). These plants with two carbon sources are termed mixotrophic, and pave the evolutionary way to full mycoheterotrophy (Selosse & Roy, 2009).
Mixotrophy enables them to adapt to shaded conditions (Julou et al., 2005; Preiss et al., 2010; with some exceptions: Girlanda et al., 2011; Schiebold et al., 2017) and sometimes drives a reduction of their photosynthetic abilities (Girlanda et al., 2006). Their study is therefore of crucial interest to an understanding of the evolution to full mycoheterotrophy.