Immunolocalization of glutamine synthetase in senescing tobacco (Nicotianatabacum L.) leaves suggests that ammonia assimilation is progressively shifted to the mesophyll cytosol

Glutamine synthetase (GS) catalyses the formation of glutamine (a major for m of nitrogen transport in giants) in an ATP-dependent reaction using ammon ium and glutamate, This enzyme is present in the plastids and/or in th cyto sol depending on the plant or the organ examined. In order to understand th e role of GS isoforms in the remobilization of leaf nitrogen, we studied th e localization of GS isoenzymes during natural senescence of tobacco (Nicot iana tabacum L.) leaves. Parallel to the progression of leaf senescence, an increase in cytosolic GS polypeptides was detected in the mesophyll cytoso l of senescing leaves while a significant decrease in GS protein content wa s observed in the phloem companion cells. The presence of GS polypeptides i n the leaf cytosol of senescing leaves appears to be the result of an induc tion of the Gln1-3 gene, the transcripts of which are not detected in matur e leaves but are abundant in senescing leaves. Alltogether, our results sug gest that during senescence, ammonia assimilation is progressively shifted from the chloroplasts to the cytosol of leaf mesophyll cells.