Sugars as signal molecules in plant seed development

Higher plants as sessile organisms react very flexible to environmental cha nges and stresses and use metabolites like glucose, sucrose acid nitrate no t only as nutrients but also as signals as part of their life strategies. T he role of metabolites as signal molecules has attracted considerable inter est during recent years. Data reviewed here for developing plant seeds sugg est a trigger function of especially sugars also in development in that met abolic regulatory control can override developmental regulation, i.e., the developmental programme only continues normally if a certain metabolic stat e is sensed at a given time point in a given cell or tissue. Several experimental strategies have provided mainly correlative evidence t hat certain sugar levels and/or the resulting changes in osmotic values are necessary within defined tissues or cells to maintain a distinct stage of differentiation or to proceed with the developmental programme, In young le gume seeds, but certainly also in other tissues, a high hexose (probably ma inly glucose) level seems to maintain the capacity of cells to divide where as - later in seed development - a certain sucrose level is necessary to in duce storage-associated cell differentiation, A major determinant of embryo hexose levels in young legume seeds is an apoplastic invertase preferentia lly expressed in the inner cell layers of the seed coat. The enzyme cleaves the incoming photoassimilate sucrose into glucose and fructose, During dev elopment the tissue harbouring the invertase is degraded in a very specific spatial and temporal pattern as part of the developmental programme and is thus creating steep glucose gradients within the cotyledons, These gradien ts can be measured at nearly cellular resolution and were found to be corre lated positively with cell division rate and negatively with cell different iation and storage activities. A hexose and a sucrose transporter accumulat ing only in the epidermal cell layer of the cotyledons seem to be essential in creating and maintaining these gradients, To gain further insights into the role of metabolites, especially sugars, a s triggers of developmental processes we foremost have to identify receptor molecules already characterised in yeast, and to describe and understand t he signal transduction networks involved.