Skip to main content
. 2016 Jun 27;7:912. doi: 10.3389/fpls.2016.00912

Figure 11.

Figure 11

The futile cycling of sugar. (A) Transporter network that allows the efflux of electroneutral sugar molecules from the plant to the periarbuscular space via proton-coupled transporters (H/C) and sugar channels (SWEET). The red circle indicates the futile sugar cycle that results from the co-existence of active H/C and SWEET transporters. (B) Dependency of the sugar/phosphate exchange between plant and fungus on the activity of SWEET channels. Without active SWEET channels, there is a constant sugar flux from the plant cytosol to the periarbuscular space and from the periarbuscular space to the fungus. All displayed fluxes are normalized to this H/C-flux (black circle). In return there is a constant flux of phosphate from the fungus via the periarbuscular space to the plant (white square). Upon increasing the activity of the SWEET channel, this phosphate flux is not affected while the sugar flux from the plant to the fungus rises. The dotted line indicates the maximal flux that can be achieved at infinitely high SWEET activity levels. (C) Activation of SWEET channels floods the apoplast with sugar. Equilibrium apoplastic sugar concentration as a function of the activity of SWEET channels. (D) Sugar flux through H/C and SWEET from the plant to the apoplast and from the apoplast to the plant as a function of the activity of SWEET channels. The right panel is an enlargement of the black rectangle in the left panel. Without active SWEET channels the transmembrane sugar gradient drives H+/sugar efflux via H/C-transporters. Here, each sugar molecule transports a proton from the plant cytosol to the apoplast and assists the H+-ATPase in charging the proton gradient that can be harvested by the H/P transporter for phosphate uptake. With increasing SWEET activity more and more sugar flows without piggybacked proton from the cytosol to the apoplast and is re-taken up by H/C-transporters. Activity of SWEET channels was normalized, so that at a relative activity of 1 the flux through H/C-transporters reverts its direction.