Amino Acids (2000) 18: 207–217 Anaerobic accumulation of amino acids in rice roots: role of the glutamine synthetase/glutamate synthase cycle R. Reggiani, M. Nebuloni, M. Mattana, and I. Brambilla Istituto Biosintesi Vegetali, CNR, Milano, Italy Accepted September 20, 1999 Summary. Accumulation of amino acids was studied in rice roots of 3-day-old seedlings subjected for 48 h to anaerobic conditions. Alanine and Gaba were the main amino acids accumulated under anoxia. Their synthesis was strongly inhibited by MSX and AZA, inhibitors of glutamine synthetase and glutamate synthase. These activities increased after 8 h of anaerobic treatment and, by immunoprecipitation of 35 S-labeled proteins, it was shown that glutamine synthetase and ferredoxin-dependent glutamate synthase were synthesized during the treatment. These findings indicate that the glutamine synthetase/ glutamate synthase cycle play an important role in anaerobic amino acid accumulation. Keywords: Amino acids – Anoxia – Glutamate synthase – Glutamine synthetase – Rice – Root Introduction In anaerobiosis, several compounds like alanine, γ-aminobutyric acid (Gaba), proline and putrescine are accumulated in plant tissues (Streeter and Thompson, 1971; Reggiani et al., 1988; Reggiani et al., 1989). Alanine and Gaba may represent about 50% of the amino acid pool in anoxic tissues (Reggiani et al., 1988). Alanine is anaerobically synthesized by the increase in alanine aminotransferase activity that involves the expression of the AlaAT- 2 mRNA (Good and Crosby, 1989; Muench and Good, 1989). The synthesis of Gaba through glutamate decarboxylase in reduced oxygen supply occurred by effect of decreasing cytoplasmic pH (Carroll et al., 1994). Accumulation of this amino acid took place without changes in the quantity of glutamate decarboxylase protein (Serraj et al., 1998), but by an activation of glutamate decarboxylase, probably via a signal transduction pathway involving calcium and calmodulin (Arazi et al., 1995). Alanine, Gaba, proline and putrescine are synthesized, directly or indirectly, from glutamic acid. In plant tissues, this amino acid may derive: i) from proteolytic processes (Aurisano et al.,