Sugar allocation from source to sink (young) leaves, critical for plant development, relies on activities of plasma-membrane sugar transporters. However, the key sugar unloading mechanism to sink leaves remains elusive. SWEET transporters mediate sugar efflux into reproductive sinks; therefore, they are promising candidates for sugar unloading during leaf growth.
In a new study, it was found that transcripts of SlSWEET1a, belonging to clade I of the SWEET family, were markedly more abundant than all other 30 SlSWEET genes in young leaves of tomatoes.
High expression of SlSWEET1a was also detected in reproductive sinks, e.g. flowers. SlSWEET1a was dominantly expressed in leaf unloading veins and the GFP fusion protein was localized to the plasma membrane using Arabidopsis protoplasts, further implicating this carrier in sugar unloading. In addition, yeast growth assays and radiotracer uptake analyses further demonstrated that SlSWEET1a acted as a low affinity (Km ~100 mM) glucose-specific carrier with a passive diffusion manner. Finally, virus-induced gene silencing of SlSWEET1a expression reduced hexose accumulation to ~50% in young leaves, with a parallel twofold increase in mature leaves.
Thus, the study authors propose a novel function for SlSWEET1a in the uptake of glucose into unloading cells as part of the sugar unloading mechanism in sink leaves of tomato.