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...

The fresh weight of Casuarina equisetifolia seedlings decreased slightly with elevation of NaCl concentration in the culture solution. Na + concentration in the shoots gradually increased with increasing NaCl concentration in the culture solution and with increasing duration of the treatment. The Na + level reached 150 mmol L −1 in seedlings treated with 500 mmol L −1 NaCl for 6 weeks. The proline content of the shoots increased with increasing NaCl concentration in the culture solution and reached 13.5 µmol g −1 fresh weight in the seedlings treated with 500 mmol L −1 NaCl for 6 weeks. The Na + concentration and proline content in the roots also increased with NaCl treatment, but the level was considerably lower than that recorded in the shoots. A highly positive correlation between Na + concentration and proline content was observed in the shoots. These results strongly suggest that C. equisetifolia plants, which are highly tolerant to salt stress, primarily synthesize proline as a major compatible solute to adjust the osmotic pressure when Na + accumulates in the cells, and maintain cell homeostasis under salt-stress conditions.

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The effects of red (R), far red (FR), or blue light (B) on the enhancement of nitrate reductase (NR) activity and on nitrate uptake in etiolated rice seedlings were examined. On 5-minute ilumination foUowed by 12-hour dark, R caused marked increase of NR activity, but FR and B caused only slight increase. Illumination with 560 ergs per square centimeter per second of R for 5 minutes caused maximal increase. The effect of R was almost completely counteracted by subsequent illumination with 2,000 ergs per square centimeter per second of FR for 10 minutes, indicating that NR induction was mediated by phytochrome. Exogenous supply of inducer nitrate was not required during the 5-minute illumination and the R-FR cycles, if the seedlings were transferred to nitrate solution at the beginning of the dark incubation. NR activity in the shoots was found high when shoots were illuminated but was low when only roots were illuminated. On continuous illumination for 12 hours, B had more effect on NR increase than R.Nitrate uptake during 6-hour dark was not increased by exposure to R, FR, or B for 5 minutes at the beginning. On continuous illumination for 6 hours, R slightly increased nitrate uptake, whereas FR and B had no effect.NR' activity in green leaves is known to be increased by light (3,6,18,21); the light energy transformed to chemical energy by photosynthesis is used in induction of NR (1,5,15,16 MATERIALS AND METHODSRice seeds (Oryza saliva var. Nihonbare) were sterilized by treatment with 70o alcohol for 5 min and then with saturated bleaching powder solution for 30 min. The seeds were rinsed with running tap water and allowed to germinate for 48 h in the dark at 30 C. Seedlings of nearly uniform size were selected and transplanted onto a Saran net floating on tap water in a pot. The pot was placed in a zinc box with a lid. The box was covered with light-proof cloth and placed in a dark room at 28 C for 6 days.Six-day-old seedlings were collected in a beaker and washed thoroughly with several changes of distilled H20 under dim green light. Root portions of groups of 20 seedlings were then soaked in distilled H20 in 125-ml Erlenmeyer flasks. The flasks were covered with aluminum foil and only the shoots were exposed to R, FR, B, or R-FR cycles. For exposing only the roots to light (Table II), the shoots were covered with aluminum foil. After illumination as indicated, seedlings were transferred to other Erlenmeyer flasks containing 120 ml of 10 mm KNO3. The flasks were placed in a zinc box covered with light-proof cloth and incubated for 12 h at 28 C. When the seedlings were exposed to light continuously or intermittently, they were soaked in 120 ml of KNO3 before illumination and left in the irradiation box for 12 h at 28 C.A dim green safelight was obtained from one 10-w fluorescent

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