GLUTAMINE-SYNTHETASE POLYPEPTIDES IN THE ROOTS OF 55 LEGUME SPECIES IN RELATION TO THEIR CLIMATIC ORIGIN AND THE PARTITIONING OF NITRATE ASSIMILATION

Glutamine synthetase (GS) exists as two main isoforms in plants, a cyt osolic form (GS1) and a chloroplast or plastidic form (GS2). Fifty-fiv e species of legume, representing a phylogenetically diverse group of tropical and temperate species, were screened by western blotting for the presence of GS2 in their roots. A remarkably strong correlation wa s found between the climatic origin of the species and the presence or absence of a GS2-like polypeptide in the root. Root GS2 was found in all 31 temperate species examined (30 papilionoids, one caesalpinoid), but was not detected in any of the 17 tropical papilionoid species. I t was also absent in the roots of four out of seven tropical non-papil ionoid species. The 'in vivo' NR activities of roots, stems and leaves of 46 of the legume species were analysed to establish their major si te of nitrate reduction, and the ratio of nitrate:reduced N in the xyl em sap was determined for some species, but no clear correlation betwe en possession of a root GS2 and a preference for root nitrate assimila tion was found. We discuss the possibility that expression of GS2 in t he root was part of a more extensive physiological adaptation to root nitrate assimilation that evolved in temperate species to suit the alk aline, nitrate-rich soils found in the centres of origin in temperate latitudes.