Sucrose Metabolism

Abstract Sucrose is one of the main products of photosynthesis in plants, and the most common form of carbohydrate transported from source to sink organs. It also functions as a storage reserve, compatible solute and signal metabolite in plants. Sucrose is synthesized via the phosphorylated intermediate sucrose‐6′‐phosphate, by sucrose‐phosphate synthase (SPS) and sucrose‐phosphatase (SPP). In sink organs, sucrose is broken down by invertase or sucrose synthase to provide carbon and energy for growth and accumulation of storage reserves, such as starch, oil and fructans. Sucrose and trehalose are the only common nonreducing disaccharides found in nature, and their metabolism is inextricably linked in plants. Trehalose‐6‐phosphate, the intermediate of trehalose synthesis, is thought to act as a signal of sucrose availability in plant cells, and regulates the partitioning of photoassimilate between sucrose and starch in leaves and the utilization of sucrose in sink organs. Key concepts Sucrose is one of the main products of photosynthesis and the most common transport sugar in plants. Sucrose is a nonreducing disaccharide, and is synthesized in the cytosol via the phosphorylated intermediate, sucrose‐6′‐phosphate. In leaves, the rate of sucrose synthesis is tightly co‐ordinated with the rates of photosynthetic carbon dioxide fixation and starch synthesis in the chloroplasts. Sucrose is transported from leaves via the phloem, to provide the rest of the plant with carbon and energy for growth and storage product synthesis. Sucrose is unloaded from the phloem in sink organs. It can be hydrolyzed by cell wall invertases and imported into the cells as hexose sugars, or taken up intact and metabolized by intracellular invertases or sucrose synthase. Fructans are soluble polymers of fructose found in about 15% of all flowering plants. Dicotyledonous plants generally make inulin‐type fructans, whereas monocotyledonous plants also make levan, inulin neoseries, levan neoseries and mixed levan (graminan)‐type fructans. Fructans are synthesized from sucrose via trisaccharide intermediates – 1‐kestose, 6‐kestose or 6G‐kestose – by various fructosyltransferases in the vacuole. Trehalose is a nonreducing disaccharide found in many bacteria, archaea, invertebrates and fungi, and is often used as a stress protectant, compatible solute, storage reserve or transport sugar. Trehalose metabolism is ubiquitous in plants, and essential for their normal growth and development, but most flowering plants accumulate only trace amounts of trehalose. Trehalose‐6‐phosphate, the intermediate of trehalose synthesis, is a signal metabolite that reflects sucrose status in plants, and is involved in regulation of photoassimilate partitioning in leaves and the utilization of sucrose in sink organs.