Discussion

Sucrose allocation may exist in G. elata tuberFungal symbiosis plays a crucial role in nutrient exchange that supports plant communities (Smith & Read 2008). The Gastrodia-Armillariainteraction represents an
unusual plant adaptation to fully heterotrophy, where the symbiotic fungus provides all the carbon to sustain the plant’s growth. How this carbon allocation mechanism developed and functions in this heterotrophic interface is an interesting question in plant biology. Here we show that high levels of sucrose (up to 1% of dry weight) accumulated in all stages of G. elata tubers (Fig. 1), similar to autotrophic tuber plants such as potato (Schwimmer, Bevenue, Weston & Potter 1954), indicating that sucrose may be the predominant form of carbohydrate transported into G. elata at the symbiotic interface. A similar sucrose flow functions in the heterotrophic relationship between autotrophic- and parasitic plant species (Péronet al. 2017), and at the symbiotic interface between plants and AM fungi (Boldt et al. 2011; Doidy et al. 2012b; Garciaet al. 2016; Gutjahr et al. 2011). Some of this sucrose is likely used for starch biosynthesis given the numerous starch grains observed in infected cortical cells and neighboring large cells (Fig. 2b) (Yeh et al. 2017). Expression of genes involved in starch and sucrose metabolism is also observed in G. elata vegetative tubers (Tsai et al. 2016)(Fig. 3c).