Ectopic expression of GeSUT4 affects growth and symbiotic capacity

The import activity of Ge SUT4 at the plasma membrane in plantawas supported by experiments showing that overexpression ofGeSUT4 in Arabidopsis caused sucrose toxicity in roots and increased leaf expansion due to efficient sucrose import (Fig. 6b, c). The activity of Ge SUT4 also allowed efficient competition for apoplastic sucrose in the symbiotic interface in planta (Fig. 6d), likely by retrieving more apoplastic sucrose that, otherwise, originates from plant roots and serves for Bacillus propagation (Allard- Massicotte et al. 2016; Beauregard, Chai, Vlamakis, Losick & Kolter 2013). Sucrose
import mediated by GeSUT4 parallels the sucrose import system observed in AM colonized autotrophic plants. Reduced expression of theSlSUT2 active importer, also a SUT4 type transporter, at the symbiotic interface promotes AM mycorrhization in roots (Bitterlichet al. 2014). All these observations highlight the critical involvement of sugar import activity in plant-fungal interactions to fine tune carbon fluxes, both for autotrophic plants and for fully heterotrophic Gastrodia orchids.