Regulation and function of extracellular invertase from higher plants in relation to assimilate partitioning, stress responses and sugar signalling

Carbohydrates are synthesised in photosynthetically active source tissues a nd exported, in most species in the form of sucrose, to photosynthetically less active or inactive sink tissues. Sucrose hydrolysis at the site of uti lisation contributes to phloem unloading. This phenomenon links sink metabo lism with phloem transport to, and partitioning between, sinks. Invertases catalyse the irreversible hydrolysis of sucrose and thus are expected to co ntribute to carbohydrate partitioning. Different invertase isoenzymes may b e distinguished based on their intracellular location, their isoelectric po ints and pH optima. Extracellular, cell-wall-bound invertase is uniquely po sitioned to supply carbohydrates to sink tissues via an apoplasmic pathway, and links the transport sugar sucrose to hexose transporters. A number of studies demonstrate an essential function of this invertase isoenzyme for p hloem unloading, carbohydrate partitioning and growth of sink tissues. Extr acellular invertases were shown to be specifically expressed under conditio ns that require a high carbohydrate supply to sink tissues. Further, their expression is upregulated by a number of stimuli that affect source-sink re lations. Substrate and reaction products of invertases are not only nutrien ts, but also signal molecules. Like hormones and in combination with hormon es and other stimuli, they can regulate many aspects of plant development f rom gene expression to long-distance nutrient allocation. Based on studies in Chenopodium rubrum, tomato (Lycopersicon esculentum) and tobacco (Nicoti ana tabacum), the regulation of extracellular invertase and its function in assimilate partitioning, defence reactions and sugar signal transduction p athways are discussed.