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.