The aluminum (Al)-induced secretion of citrate has been regarded as an important mechanism for Al resistance in soybean (Glycine max). However, the mechanism of how Al induces citrate secretion remains unclear. In this study, we investigated the regulatory role of plasma membrane H 1 -ATPase activity and expression were higher in an Al-resistant cultivar than in an Al-sensitive cultivar. Al activated the threonine-oriented phosphorylation of plasma membrane H 1 -ATPase in a dose-and time-dependent manner. Taken together, our results demonstrated that up-regulation of plasma membrane H 1 -ATPase activity was associated with the secretion of citrate from soybean roots.

The uptake of nitrate and ammonium by rice (Oryza sativa) seedlings was compared under various conditions. Nitrate uptake showed a 1-hour lag phase and then a rapid absorption phase, whereas ammonium uptake showed passive absorption during the first hour, then a shoulder of absorption, followed by a rapid metabolism-dependent absorption phase. Light did not affect the uptake of nitrate or ammonium. The uptake of nitrate and ammonium was markedly suppressed by removal of the endosperm. After removal of the endosperm, the uptake was restored by exogenous supply of 30 mm sucrose. No appreciable nitrate uptake was detected at temperatures below 15 C, whereas appreciable ammonium uptake occurred at 15 C, although lowered passive absorption and complete inhibition of the rapid metabolism-dependent absorption phase were observed at 5 C.Nitrate uptake was decreased by aeration and also by bubbling the mixture with O(2). It was greatly depressed by bubbling the mixture with N(2). These results suggest that there is a low optimal O(2) concentration for nitrate uptake. In contrast, ammonium uptake was increased by aeration, not influenced by bubbling with O(2), and slightly decreased by bubbling the mixture with N(2).Nitrate uptake was almost entirely inhibited by addition of cycloheximide, KCN, or arsenate. These compounds had scarcely any effect on the passive absorption of ammonium, but entirely depressed its successive metabolism-dependent absorption. Ammonium uptake occurred before nitrate uptake in solution containing both nitrate and ammonium.

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The cytosolic and mitochondrial isozymes of aspartate aminotransferase (AspAT) function in the C4 dicarboxylate cycle of photosynthesis. We constructed a cDNA library from leaf tissues of Panicum miliuceum, an NAD-malic-enzyme-type C4 plant and screened the library for AspAT isozymes. A full-length cDNA clone for cytosolic AspAT was isolated. This clone contains an open reading frame that encodes 409 amino acids. We also isolated two cDNA clones for different precursors of mitochondria1 AspAT. Comparing these two sequences in the coding regions, we found 12 amino acid substitutions out of 28 base substitutions. The encoded amino acid sequences predict that mitochondrial AspAT arc synthesized as precursor proteins of 428 amino acid residues, which each consist of a mature enzyme of 400 amino acid residues and a 28-amino-acid presequence. This prediction coincides with the observation that the in vitro translation product of the mRNA for mitochondrial AspAT was substantially larger than the mature form. A comparison of the amino acid sequences of the AspAT isozymes from P . miliaceum with the published sequences for the enzymes from various animals and microorganisms reveals that functionally and/or structurally important residues are almost entirely conserved in all AspAT species.Aspartate aminotransferase (AspAT) is a pyridoxal-phosphate-dependent enzyme which is distributed widely among plants, animals and microorganisms. This enzyme exists as isozymes in higher organisms. In animal cells, two distinct isozymes of AspAT can be found; one in the cytosol (c) and the other in the mitochondria (m). These isozymes function in amino acid metabolism, in the tricarboxylic acid cycle and in the malate-aspartate shuttle. The nucleotide sequences of the cDNA for both isozymes are known for the chicken [l, 21, pig [3,4], rat [5,6], mouse [7] and human [8,9]. The similarities of the amino acid sequences between these isozymes are close to 45% for all species studied. Apparently, the isozymes have evolved from a common ancestral AspAT by gene duplication. Studies of their three-dimcnsional structures have revealed some functionally and structurally important residues [lo -131. These residues are conserved among many AspAT isozymes.The leaf tissues of NAD-malic-enzyme-type C4 plants also contain cAspAT and mAspAT. These AspAT isozymes are important participants in the photosynthetic carbon assimilation pathway, being essential components in the movement of photosynthetic intermediates between mesophyll and Correspondence to M. Taniguchi, Department of Agricultural Chemistry, School of Agriculturc, Nagoya University, Nagoya, Japan 464-01Ahhrei~iations. AspAT, aspartate aminotransferase; c, cytosolic; m, mitochondrial.Erizyn7e. Aspartate aminotransferase (EC 2.6.1.1). Note. The novel nucleotide sequence data published here have been submitted to the EMBL sequence data bank(s) and are available under accession number(s) X63428 for pmAATl, X63429 for pcAAT2 and X63430 for pmAAT3.bundle sheath cells [14, 151. Both isozymes exhibi...