Ammonia assimilation in lupin nodules

Abstract

ROBERTSON et al.^1,2 proposed that plant enzymes rather than bacteroid enzymes are physiologically important in the assimilation of the ammonia produced after nitrogen reduction in the bacteroids of leguminous nodules. They demonstrated that during the development of the nodule, glutamine synthetase (EC 6.3.1.2, equation (1)) and glutamate synthase(EC 2.6.1.53, equation (2)) activity increased considerably in the plant fraction but not in the bacteroid fraction of the nodule, parallel with the induction of nitrogenase and leghaemoglobin.But, the predominant amino acid transported in the xylem of lupins and many other legumes is asparagine^2–4. Robertson et al.² suggested therefore that asparagine was most probably synthesised in the plant fraction of the nodule by a glutamine-dependent asparagine synthetase^5,6 (EC 6.3.5.4, equation (3)) rather than in the bacteroids, which might utilise for asparagine biosynthesis an ammonium-dependent asparagine synthetase (EC 6.3.1.1, equation (4)), which has been reported to occur in other prokaryotes⁷.