The question whether sucrose (Suc) is present inside plastids has been long debated. Low Suc levels were reported to be present inside isolated chloroplasts, but these were argued to be artifacts of the isolation procedures used. We have introduced Suc-metabolizing enzymes in plastids and our experiments suggest substantial Suc entry into plastids. The enzyme levansucrase from Bacillus subtilis efficiently synthesizes fructan from Suc. Targeting of this enzyme to the plastids of tobacco (Nicotiana tabacum) and potato (Solanum tuberosum) plants leads to high-level fructan accumulation in chloroplasts and amyloplasts, respectively. Moreover, introduction of this enzyme in amyloplasts leads to an altered starch structure. Expression of the yeast invertase in potato tuber amyloplasts results in an 80% reduction of total Suc content, showing efficient hydrolysis of Suc by the plastidic invertase. These observations suggest that Suc can enter plastids efficiently and they raise questions as to its function and metabolism in this organelle.Plastids are of tremendous metabolic importance. Next to photosynthesis they are involved in the synthesis of fatty acids, amino acids, starch, and many compounds of secondary metabolism. This diverse metabolic capacity of plastids requires an extensive array of selective transporting systems for interaction with other cellular compartments. Plastids are surrounded by two membranes, the inner and the outer membrane. In the inner membrane of the plastid envelope, many metabolite specific transporters are present, whereas the outer membrane contains nonspecific porin-like channels. The envelope outer membrane was proposed to be non-selective and permeable for many small molecules (Heldt and Sauer, 1971). However, recent data suggest that outer membranes can also act as selective and regulated molecular sieves (Flü gge, 2000; Neuhaus and Wagner, 2000; Soll et al., 2000).Several metabolite transporters in plastids have now been identified (Emes and Neuhaus, 1997; Flü gge, 1998; Neuhaus and Wagner, 2000). The wellknown triose phosphate/phosphate translocator exports the triose phosphates generated by photosynthetic CO 2 fixation into the cytosol. The phosphoenolpyruvate/phosphate translocator is responsible for the import of phosphoenolpyruvate into plastids for several plastidic metabolic pathways, like the shikimate pathway or amino acid synthesis (Streatfield et al., 1999). Another phosphate antiporter is the Glc-6-P/phosphate translocator (Naeem et al., 1997;Wischmann et al., 1999). The imported Glc-6-P in amyloplasts can be used for starch biosynthesis or in the oxidative pentose phosphate pathway (Naeem et al., 1997). Next to sugar-phosphates, unphosphorylated carbohydrates like Glc and maltose can be transported (Schleucher et al., 1998) and recently a gene encoding plastidic Glc translocator was identified (Weber et al., 2000). Furthermore, plastids contain transporters involved in ammonia and nitrogen assimilation, transporting Glu, Gln, and oxaloacetate in exchange for malat...