GeSUT4 mediates inter- and intra-cellular sucrose allocation

Distinct accumulation of GeSUT4 transcripts in infected cells (Fig. 5c) and the localization of GeSUT4-GFP fusion proteins provide evidence that GeSUT4 physiologically functions at the plasma membrane in symbiotic cells, the primary site for apoplastic sucrose exchange at the mycoheterotrophic interface. GeSUT4transcripts also accumulated in nucleus of large cells (Fig. 5c), which exhibited a distinct apoplastic barrier at the interface facing infected cortical cells (Fig. 2d, e). These observations support the hypothesis that GeSUT4 mediates apoplastic sucrose import between the symbiotic interface as well as during intercellular sugar translocation from the infected cortical cells to the inner large cells.GeSUT4 was also localized on the tonoplast (Fig. 5b, S7b). Similar results have been reported for several other SUT4 proteins, such as AtSUT4,HvSUT2 (Endler et al. 2006), OsSUT2 (Eom et al. 2011) and LjSUT4 (Reinders, Sivitz, Starker, Gantt & Ward 2008). Such dual-targeting to both the plasma membrane and the endomembrane system has also been observed for NtSUT4 andStSUT4 (Chincinska et al. 2013; Okubo-Kurihara et al. 2010). GeSUT4 may also mediate subcellular sucrose distribution depending on its localization. The involvement ofGeSUT4 in multiple transport mechanisms may be related to a marked condensation of the G. elata genome (Yuan et al.2018). Considering that GeSUT4 may also function as a proton-symporter, GeSUT4 may function on the tonoplast to export sucrose from vacuolar lumen to the cytoplasm for intracellular metabolism or further allocation.