The role of GeSUT4 in sucrose
allocation
It has
been long believed that the primary source of carbon forGastrodia is from degradation of fungal cells (Guo & Xu 1990;
Kusano 1911). Based on the high expression of β-glucosidase and xylanase
in symbiotic cortex cells, fungal glycans could be hydrolyzed to derive
D-glucose and xylose at the interface of Gastrodia tubers (Yuan et
al. 2018) or inside large cells (Guo & Xu 1990). To prevent their
metabolism, the extracellular D-glucose or xylose would be converted to
sucrose via plasma membrane localized sucrose synthase (Amor, Haigler,
Johnson, Wainscott & Delmer 1995; Noël & Pontis 2000), which and is
also highly expressed in Gastrodia (Fig. 3c) and has been
proposed to play a primary role in sucrose translocation(Stein & Granot
2019; Yao, Gonzales-Vigil & Mansfield 2019). Based on the results
presented here, we propose that plasma-membrane localized Ge SUT4
can actively import apoplastic sucrose to retrieve carbon source when
establishing symbiosis with the fungus Armillaria (Fig. 7). This
sucrose can also be translocated to the inner large cells along the
apoplastic pathway, where Ge SUT4 also functions to import sucrose
released from infected cells. When needed, the tonoplast localizedGe SUT4 can also export stored sucrose for cytosolic demand.