Plants, like all organisms, require energy to grow. They obtain this energy by converting light into a usable chemical form through photosynthesis, producing carbohydrates such as sucrose. This sucrose, the primary product of photosynthesis, is then transported to non-photosynthetic sink organs (e.g., flowers, seeds, and roots) to support their growth and development.

Efficient transport of sucrose from leaves to energy-demanding sink tissues is mediated by the phloem, the vascular tissue in plants. This process involves different sugar transporters from two families: SWEET and SUC/SUT. While SWEET transporters facilitate sugar efflux, SUC transporters handle the import of sucrose from photosynthetic tissues into the phloem, creating a high pressure in the phloem, driving the movement of the sugar-rich sap to various parts of the plant. Such a process is particularly essential for organs like fruits and seeds that store large amounts of sugar.

Despite their pivotal role, the precise working mechanism of SUC transporters had remained a mystery. A recent study by researchers from Aarhus University has shed light on this mystery. The revealed structure of the SUC1 from the Arabidopsis plant, combined with molecular dynamics simulations, SSM-based electrophysiology, and biochemical data, has provided key insights. Essentially, the researchers presented a model for how sucrose is recognized and transported by SUC transporters.

The study identified the key acidic residue, Asp152, required for proton-driven sucrose uptake and described how protonation and sucrose binding are strongly coupled. In SUC1, sucrose binding is directly linked to proton binding (with a tight 1:1 coupling ratio between proton and sucrose), allowing sugar release into an environment with a very high sucrose concentration, as long as the proton dissociates.

By pinpointing Asp152 as the central proton-binding site in the SUC transporters and explaining how it couples with sucrose binding, we now have a clearer picture of a process that plays a pivotal role in plant growth and response to stress.

Understanding the pathways through which sucrose travels from leaves to seeds becomes crucial when we think about the larger picture: food security. As the global population surges, ensuring that every person has access to nutritious food is paramount. A deeper understanding of the nuances of sucrose transport could pave the way for innovative methods to boost crop yields, ultimately benefiting millions worldwide.

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