Expression of glutamine synthetase genes during natural senescence of tomato leaves

Changes in the expression of cytosolic glutamine synthetase (GS1; EC 6.3.1.2) and chloroplastic glutamine synthetase (GS2) were examined in tomato (Lycopersicon esculentum Mill.) leaves during natural senescence. Total GS activity during this process declined to 4% of the highest value reached al the time of maximum leaf growth rate. Changes of activity were probably caused by a decrease in the amount of GS2 polypeptides. The presence of GS1 polypeptides was evident during the late senescence stage of the leaf despite the low content of total soluble protein. The differential expression of GS genes during leaf senescence in tomato plants was followed using gene-specific homologous probes synthesized from cloned cDNAs encoding cytosolic and chloroplastic GS polypeptides. Northern blot analysis showed that the level of GS2 transcripts decreased during leaf senescence as did the amounts of GS2 polypeptide, but messengers for GS2 were undetectable in the very late senescence stage when a small amount of polypeptides was still present. The content of mRNA detected with the 3' non-coding region of the GS1 probe dramatically increased with leaf senescence, thus explaining the presence of GS1 polypeptides at this stage, although there was no correspondence between the amount of GS protein and the high abundancy of GS1 mRNA. The presence of both GS1 polypeptide and messenger in the leaf, and their increase during natural leaf senescence, support a role for GS1 in this process.