Plant cells are highly compartmentalized and so is their metabolism. Most metabolic pathways are distributed across several cellular compartments, which requires the activities of membrane transporters to catalyze the flux of precursors, intermediates, and end products between compartments. Metabolites such as sucrose and amino acids have to be transported between cells and tissues to supply, for example, metabolism in developing seeds or fruits with precursors and energy. Thus, rational engineering of plant primary metabolism requires a detailed and molecular understanding of the membrane transporters. This knowledge however still lags behind that of soluble enzymes. Recent advances include the molecular identification of pyruvate transporters at the chloroplast and mitochondrial membranes and of a new class of transporters called SWEET that are involved in the release of sugars to the apoplast.
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