Senescence-specific Increase in Cytosolic Glutamine Synthetase and Its MRNA in Radish Cotyledons

Changes in the levels of cytosolic and chloroplastic isoforms of glutamine synthetase were examined in senescing radish (Raphanus sativus L. cv Comet) cotyledons by immunoblotting analysis using antibodies raised separately against maize glutamine synthetase isoforms. Translatable mRNAs for these isoforms were also examined by analyzing translation products from poly(A)(+) RNA in a wheat germ system with the antibodies. The relative content of cytosolic isoform (GS(1)) increased twofold in the cotyledons that were placed in the dark for 72 hours to accelerate senescence, while that of chloroplastic isoform (GS(2)) declined to half of its initial level. The dark-treatment also increased the relative level of translatable mRNA for GS(1) sevenfold after 72 hours, and decreased rapidly that for GS(2) and for other nuclear-coded chloroplast proteins as well. Cotyledons also accumulated GS(1) mRNA when they became senescent after a lengthy growth period under continuous light. These observations suggested that GS(1) genes were activated, while those for GS(2) were repressed, and eventually the population of the enzyme was altered in senescent cotyledonary cells. The role of increased cytosolic enzyme is discussed in relation to the nitrogen metabolism in senescent leaves.