Identification of multiple forms of glutamine synthetase in field bean ( Vicia faba L.)

Abstract

Glutamine synthetase (GS) activity was detected after electrophoresis on starch gels using the glutamyl transfer reaction. Two bands of GS activity (designated GS 1 and GS 2) were observed in a chloroplast fraction and four bands (designated GS 1, GS 2, GS 4 and GS 5) in a total leaf extract from field beans. GS 4 and GS 5 were assumed to be cytoplasmic GS. Differences in the chloroplast GS bands (GS 1 and GS 2) but not in the cytoplasmic bands (GS 4 and GS 5) were observed between cultivars and between lines of the same cultivar. No differences in GS banding patterns were noted between extracts from pods or subtended leaves sampled at different stages of development. Other field bean plants were grown from seed in perlite moistened with distilled water and then pretreated with solutions of NaNO 3, NH 4NO 3, NH 4Cl or urea for either 1 or 7 days. Electrophoresis of root extracts from plants pretreated for 1 day with NH 4NO 3, NH 4Cl or urea revealed the presence of 1 band of GS activity, designated GS 3, clearly distinguished from the 2 forms in the leaf cytoplasm. An additional band with a mobility similar to one of those in the chloroplast, GS 1, was found in root extracts of plants given NaNO 3 or NH 4NO 3 for 1 week, but not in those pretreated with NH 4Cl or urea. Incubation of the starch gel with methionine sulphoximine (MSO) in the presence of ATP and Mg 2+ prior to the detection of GS activity prevented any bands being observed. The possible roles of these different forms of GS are discussed in the light of previously reported evidence.