Because sugar producing and sugar consuming and/or storage organs are spatially separated, plants have developed ways to understand their "sugar status" in order to adapt carbohydrate synthesis and transport. In a manner resembling the well-known glycaemia regulation in mammals, plants sense their sugar levels, and modulate them according to the general "feast and famine" pattern of responses: (a) carbohydrate excess favours the expression of enzymes connected with biosynthesis and storage of reserves and (b) represses transcripts encoding enzymes involved in photosynthesis and reserve mobilisation (Koch 1996).
Additionally, sugars provide signals able to control many aspects of seed and plant development. The mechanisms of sugar perception and the subsequent transduction pathways regulated are referred to as sugar sensing and signalling, respectively (Smeekens 2000). These terms, however, qualify for a complex scheme regarding (a) the diversity of perception mechanisms and levels involved, (b) the molecular pathways induced, (c) the multilevel control of gene and protein expressed and, (d) the interactions with pathways mediating the transduction of hormonal and environmental stimuli. The study of the molecular mechanisms of sugar sensing and signalling pathways in plants takes advantage of the well-known pathways described in yeasts (Rolland et al. 2006, Santan-gelo 2006). The following paragraphs give an overview of the sugar control over sugar transport and polyphenol metabolism in grapevine.
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