Humboldt review: Function and characterization of sugar transport across plant membranes: History and perspectives

Sugars are indispensable to life, acting as metabolic substrates, signalling molecules, and structural components. Their controlled movement across membranes is essential for cellular function and development. In this Humboldt Review, we trace the historical and scientific trajectory of plant sugar transporter research, beginning with the discovery of proton-coupled glucose uptake in Chlorella kessleri. This finding led to the molecular identification of HUP1, the first cloned plant sugar transporter and a founding member of the plant Monosaccharide Transporter (MST) family. We then follow the translation of these insights to vascular plants, focusing on Arabidopsis thaliana, where a large and functionally diverse MST superfamily, including seven subfamilies that differ in characteristics such as localization, transport substrate, and transport energization, was uncovered. We also provide insights into the evolution, structure, and multifunctionality of the newest described SWEET family of sugar transport proteins with cross-kingdom conservation. By integrating early discoveries with recent advances, this review offers a comprehensive perspective on the critical roles of sugar transporters in plant physiology and environmental adaptation. We conclude by highlighting current knowledge gaps and proposing directions for future research to deepen our understanding of plant sugar transport mechanisms.