Activity and immunocytochemical localization of glutamine synthetase inLathraea clandestins L.

Summary The activity of glutamate synthetase (GS) was determined in the different organs ofLathraea clandeslina L., a holoparasitic Scrophulariaceae. It was very low throughout the plant but levels were slightly higher in the scale leaves. Immunoprecipitation reactions carried out with immune serums raised against the isoforms GS1 or GS2 of the enzyme showed that, in the scale leaves, isoenzyme GS1 was present, but the existence of small amounts of GS2 remained in question on account of possible cross reactions. On the other hand, the study of intracellular localization of GS in the scale leaves by indirect immunofluorescence, using the same antibodies anti-GS1 and anti-GS2, clearly demonstrated the occurrence of two GS forms: a GS1 isoenzyme located in the cytoplasm of glandular and parenchymatous cells and a GS2-type isoenzyme only detected in the stroma of the large amyloplasts present in the outer parenchyma. This amyloplastidial isoenzyme seems to be a peculiar GS form, distinct from both GS1 and GS2.Abbreviations GS glutamate synthetase - GS₁, GS₂, GSR glutamate synthetase isoforms - PBS phosphate buffered saline - PEG poly ethylene glycol - GP peltate glands - GB shield glands - P amyloplasts