The past 10 years have seen a marked progress in our understanding of the process of sucrose mobilisation and starch synthesis in potato tubers. The pathways have been elucidated and the key processes have been studied in detail. Molecular tools for the manipulation of starch synthesis and the production of modified starches in planta have been identified and tested. Given the current effort aimed at the dissection of starch granule biogenesis, a rapid progress in this area is predicted. Focus is also being placed on the elucidation of transport properties of amyloplasts, which appear to have a strong impact on starch yield. On the other hand, whole plant processes such as the regulation of assimilate partitioning and sink-source relationship remains poorly understood. There has been a remarkable lack of success in the attempts at improving potato productivity (e.g. by increasing harvest index). Indeed, the controversy as to whether productivity in potato is "sink" or "source" limited is still unresolved (83, 169). It has become increasingly evident that no obvious or "easy" targets for increased crop performance exist due to the complexity and integration of the plant system at all levels of organization. For example, attempts at manipulating carbon fluxes have often resulted in unexpected physiological responses such as modification of developmental processes or induction of biochemical pathways due to localized sugar accumulation and substrate channeling. In order to deal with the complexity of the plant system we urgently require more integrated investigative approaches which can unveil, for example, the pathways, signals and transduction mechanisms of the cross-talk communication between different tissue types and organs. Only then we will be able to elucidate complex processes such as tuberisation, the control of assimilate unloading and allocation (i.e. sink-source relationships).
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