Sub-cellular sugar partitioning Dr. Michael Büttner

In higher plants, sugars play important roles as nutrients as well as signal molecules during the entire life cycle. While the green parts (source) synthesize sugars during the day, non-green parts (sink) depend on a constant import of carbohydrates delivered via the phloem. In addition, source and sink cells allocate sugars between different cellular compartments, depending on the actual requirements. During the day, many plant species use up to half of their assimilated carbon for starch synthesis and transient storage in chloroplasts of source leaves. At night, transitory starch is degraded to glucose or maltose, which can then be exported from the chloroplast. Furthermore, hexoses can be imported into the vacuole for transient or long-term storage. The distribution of sugars on the sub-cellular level but also for long-distance transport requires several essential transport steps across membranes. Experiments on isolated vacuoles provided biochemical evidence for the uptake of hexoses into these organelles, and both mechanisms, passive diffusion as well as active transport have been suggested. However, very little is known about the corresponding transport proteins. My group has started to identify and characterize transporter proteins which mediate sub-cellular sugar partitioning in order to develop a clearer picture of how plants exploit different types of monosaccharide transporters during development and stress responses. Our molecular tools comprise biochemical as well as molecular and cell biological methods to elucidate the functional role of these sugar transporters in plants. We further use baker's yeast as heterologous system to study the properties of sugar transporters.