Woodall, Janet; (1994) Studies of glutamine synthetase in lotus species and the occurrence of root GS2 in other legumes. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The main body of this thesis is an analysis of the octameric enzyme of ammonia assimilation, glutamine synthetase (GS), in the model legume Lotus japonicus and its close relative L. corniculatus. Both species were shown to contain GS1 (cytosolic) and plastidic GS2 isoforms in differing amounts in all tissues. Two-dimensional electrophoresis revealed four non-glycosylated, isoelectric variants of GS1 and four variants of GS2, and evidence for the existence of four distinct GS genes in L. corniculatus was obtained using the polymerase chain reaction. The root GS2 isoform was present in plants grown with NO3 as the sole N source and absent in plants grown on NH4+. To test the hypothesis that there may be a correlation between the presence of GS2 in roots and root nitrate assimilation, 55 temperate and tropical legume species (representing 38 genera from 21 different tribes) were surveyed for their site of nitrate assimilation and for the presence or absence of root GS2. Root GS2 was found to be present in each of the 31 species with temperate origins, but absent from all of the tropical papilionoid legumes (17 species) and from four of the seven tropical non-papilionoid species. Based on in vivo and in vitro NR activities, some of the temperate legumes were probably shoot nitrate assimilators, and some of the tropical legumes were capable of assimilating nitrate in their roots. However, there was a positive correlation between the total GS activities and NR activities in the roots of the temperate species indicating a relationship between the two functions. The identification of some tropical species that can assimilate NO3 in their roots, but which lack GS2, indicates that in these species at least, GS1 can assimilate the NH4+ derived from NO3 reduction, providing evidence that the functions of the two isoforms can overlap.

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