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.