The pathway of nitrogen assimilation in plants

Publisher Summary This chapter discusses enzymes of glutamate formation—glutamate dehydrogenase (GDH), glutamine synthetase, and glutamate synthase. The amino acid, glutamate, and its amide called glutamine are the primary products of inorganic nitrogen assimilation and as such occupy central positions in intermediary nitrogen metabolism. The chapter describes the characteristics, properties, and behavior of glutamate dehydrogenase, glutamine synthetase, and glutamate synthase. Mitochondrial GDH enzymes are isolated and characterized from a number of plant tissues including pea roots, lettuce leaves, mung bean seedlings, and fronds of Lemna minor. These enzymes react with both nicotinamide adenine dinucleotide hydride (NADH) and nicotinamide adenine dinucleotide phosphate in the aminating direction but show greater activity with NADH. Glutamine synthetase is found throughout the plant and animal kingdoms and is characterized from a number of sources including bacteria, algae, fungi, and higher plants and animals. Angiosperm glutamine synthetase exhibits an absolute requirement for divalent metal cations, which can be satisfied by Mg2+ and, to a lesser degree, Mn2+ and Co2+. The catalytic activity of glutamate synthases is unstable.

Enzymes of Glutamate Formation: Glutamate Dehydrogenase, Glutamine Synthetase, and Glutamate Synthase

Carbon and nitrogen metabolism in legume root nodules

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2875%2980950-1

Induction of glutamate synthase during nodule development in lupin.

Aspartate aminotransferase (AAT) is a key enzyme in the synthesis of amino acids. It plays an important role in regulating carbon and nitrogen metabolism in almost all organisms. In this study, we over-expressed in rice separately all three AAT genes from rice (OsAAT1~3) and one AAT gene from Escherichia coli (EcAAT). Over-expression was driven by the CaMV 35S promoter and constructs were introduced into rice by Agrobacterium tumefaciens-mediated transformation. Compared with control plants, the transformants showed significantly increased leaf AAT activity and greater seed amino acid and protein contents. No other phenotypic changes were observed. The total leaf AAT activities in plants over-expressing OsAAT1, OsAAT2, and EcAAT were 26.6, 23.6, and 19.6 A min−1 mg−1 FW (A: units of activity, defined as increase of absorbency per min per mg; FW: fresh weight), which were significantly higher than that in the wild-type control (17.7 A min−1 mg−1 FW). The amino acid content in seeds of transgenic plants over-expressing OsAAT1, OsAAT2, and EcAAT was 119.36, 115.36, and 113.72 mg g−1, respectively, which were 16.1, 12.0, and 5.4% higher, respectively, than that in the control plants. The transgenic plants over-expressing OsAAT1, OsAAT2, and EcAAT had significantly higher protein contents (increased 22.2, 21.1, and 11.1%, respectively) than wild-type plants. No significant changes were found in leaf AAT activity, seed amino acid content or protein content in OsAAT3 over-expressed plants. The expression patterns of the three OsAAT genes and their different functions are also discussed.

Eamon Ryan

Papers

Subcellular localization of enzymes involved in the assimilation of Ammonia by soybean root nodules