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 and exported, in most species in the form of sucrose, to photosynthetically less active or inactive sink tissues. Sucrose hydrolysis at the site of utili- sation contributes to phloem unloading. This phenomenon links sink metabolism with phloem transport to, and par- titioning between, sinks. Invertases catalyse the irreversible hydrolysis of sucrose and thus are expected to contribute to carbohydrate partitioning. Different invertase isoenzymes may be distinguished based on their intracellular loca- tion, their isoelectric points and pH optima. Extracellular, cell-wall-bound invertase is uniquely positioned 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 phloem unloading, carbo- hydrate partitioning and growth of sink tissues. Extracellular invertases were shown to be specifically expressed under conditions that require a high carbohydrate supply to sink tissues. Further, their expression is upregulated by a number of stimuli that affect source-sink relations. Substrate and reaction products of invertases are not only nutri- ents, but also signal molecules. Like hormones and in combination with hormones and other stimuli, they can regu- late many aspects of plant development from gene expression to long-distance nutrient allocation. Based on studies in Chenopodium rubrum, tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum), the regulation of extracellular invertase and its function in assimilate partitioning, defence reactions and sugar signal transduction pathways are discussed.