Tissue-specific distribution of glutamine synthetase in potato tubers

Cytosolic isoforms of the enzyme glutamine synthetase (GS) located in the phloem have been implicated in the mobilization of nitrogen for intracellular transport in higher plants. The potato tuber represents an important reservoir of nitrogen and an approach was made to the characterization of GS in this organ, particularly at the stages of sprouting and of new tuber formation. By immunoblotting after SDS-PAGE, and by immunological tissue printing, it was possible to conclude that a cytosolic GS is present in tubers and sprouts, and that it is mainly expressed in the internal phloem, in a very precise tissue-specific pattern of distribution. These data provide additional clues to the interpretation of the functional role of GS in the mobilization of nitrogen and its utilization in growing parts of the plant. The importance of morphological data and localization studies in complementing molecular and biochemical work is emphasized. The proposed functional importance of the internal phloem in Solanum tuberosum organs is also reinforced. (C) 1996 Annals of Botany Company