Sucrose metabolism in plastids

The question whether sucrose (Suc) is present inside plastids has been long debated. Low Suc levels were reported to be present inside isolated chloro plasts, but these were argued to be artifacts of the isolation procedures u sed. We have introduced Sue-metabolizing enzymes in plastids and our experi ments suggest substantial Suc entry into plastids. The enzyme levansucrase from Bacillus subtilis efficiently synthesizes fructan from Sue. Targeting of this enzyme to the plastids of tobacco (Nicotiana tabacum) and potato (S olanum tuberosum) plants leads to high-level fructan accumulation in chloro plasts and amyloplasts, respectively. Moreover, introduction of this enzyme in amyloplasts leads to an altered starch structure. Expression of the yea st invertase in potato tuber amyloplasts results in an 80% reduction of tot al Suc content, showing efficient hydrolysis of Suc by the plastidic invert ase. These observations suggest that Suc can enter plastids efficiently and they raise questions as to its function and metabolism in this organelle.