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

• Published in: Planta 2000-09, Vol.211 (4), p.519-527
• Main Authors: Brugière, Norbert, Dubois, Frédéric, Masclaux, Céline, Sangwan, Rajbir S., Hirel, Bertrand
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Ammonia - metabolism; Biological and medical sciences; Cellular senescence; Chloroplasts; Cytoplasm; Cytosol; Cytosol - metabolism; Economic plant physiology; Fundamental and applied biological sciences. Psychology; Genes; Genetics; Glutamate-Ammonia Ligase - metabolism; Growth and development; Leaves; Life Sciences; Mesophyll cells; Microscopy, Electron; Morphogenesis, differentiation, rhizogenesis, tuberization. Senescence; Nicotiana - enzymology; Nicotiana - ultrastructure; Nitrogen; Plant cells; Plant Leaves - enzymology; Plant Leaves - ultrastructure; Plant physiology and development; Plants; Plants genetics; Plants, Toxic; Senescence and abscission; Subcellular Fractions - enzymology; Vegetative apparatus, growth and morphogenesis. Senescence
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s004250000309

Glutamine synthetase (GS) catalyses the formation of glutamine (a major form of nitrogen transport in plants) in an ATP-dependent reaction using ammonium and glutamate. This enzyme is present in the plastids and/or in the cytosol depending on the plant or the organ examined. In order to understand the role of GS isoforms in the remobilization of leaf nitrogen, we studied the localization of GS isoenzymes during natural senescence of tobacco (Nicotiana tabacum L.) leaves. Parallel to the progression of leaf senescence, an increase in cytosolic GS polypeptides was detected in the mesophyll cytosol of senescing leaves while a significant decrease in GS protein content was observed in the phloem companion cells. The presence of GS polypeptides in the leaf cytosol of senescing leaves appears to be the result of an induction of the Gln1-3 gene, the transcripts of which are not detected in mature leaves but are abundant in senescing leaves. Alltogether, our results suggest that during senescence, ammonia assimilation is progressively shifted from the chloroplasts to the cytosol of leaf mesophyll cells.