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.