Molecular characterization of a gene for alanine aminotransferase from rice (Oryza sativa)

Candidate Gene Analysis for Nitrogen Absorption and Utilization in Japonica Rice at the Seedling Stage Based on a Genome-Wide Association Study

Over-application of nitrogen (N) fertilizer in fields has had a negative impact on both environment and human health. Domesticated rice varieties with high N use efficiency (NUE) reduce fertilizer requirements, enabling sustainable agriculture. Genome-wide association study (GWAS) analysis of N absorption and utilization traits under low and high N conditions was performed to obtain 12 quantitative trait loci (QTLs) based on genotypic data including 151,202 single-nucleotide polymorphisms (SNPs) developed by re-sequencing 267 japonica rice varieties. Eighteen candidate genes were obtained by integrating GWAS and transcriptome analyses; among them, the functions of OsNRT2.4, OsAMT1.2, and OsAlaAT genes in N transport and assimilation have been identified, and OsJAZ12 and OsJAZ13 also play important roles in rice adaptation to abiotic stresses. A NUE-related candidate gene, OsNAC68, was identified by quantitative real-time PCR (qRT-PCR) analyses. OsNAC68 encodes a NAC transcription factor and has been shown to be a positive regulator of the drought stress response in rice. Overexpression of OsNAC68 significantly increased rice NUE and grain yield under deficient N conditions, but the difference was not significant under sufficient N conditions. NUE and grain yield significantly decreased under both N supply conditions in the osbnac68 mutant. This study provides crucial insights into the genetic basis of N absorption and utilization in rice, and a NUE-related gene, OsNAC68, was cloned to provide important resources for rice breeding with high NUE and grain yield.

Activity-based protein profiling of rice (Oryza sativa L.) bran serine hydrolases

Rice bran is an underutilized agricultural by-product with economic importance. The unique phytochemicals and fatty acid compositions of bran have been targeted for nutraceutical development. The endogenous lipases and hydrolases are responsible for the rapid deterioration of rice bran. Hence, we attempted to provide the first comprehensive profiling of active serine hydrolases (SHs) present in rice bran proteome by activity-based protein profiling (ABPP) strategy. The active site-directed fluorophosphonate probe (rhodamine and biotin-conjugated) was used for the detection and identification of active SHs. ABPP revealed 55 uncharacterized active-SHs and are representing five different known enzyme families. Based on motif and domain analyses, one of the uncharacterized and miss annotated SHs (Os12Ssp, storage protein) was selected for biochemical characterization by overexpressing in yeast. The purified recombinant protein authenticated the serine protease activity in time and protein-dependent studies. Os12Ssp exhibited the maximum activity at a pH between 7.0 and 8.0. The protease activity was inhibited by the covalent serine protease inhibitor, which suggests that the ABPP approach is indeed reliable than the sequence-based annotations. Collectively, the comprehensive knowledge generated from this study would be useful in expanding the current understanding of rice bran SHs and paves the way for better utilization/stabilization of rice bran.

Review: Crucial role of inorganic pyrophosphate in integrating carbon metabolism from sucrose breakdown to starch synthesis in rice endosperm

The endosperm is a primary constituent of mature seeds in rice as well as in other cereal crops, serving as the major storage reserve of starch. Observations indicate that the central part of the endosperm is subject to hypoxic conditions, which require a switch of energy metabolism owing to limited mitochondrial respiration. Uniquely, this endosperm generates a large source of inorganic pyrophosphate (PPi) as a byproduct of the reaction of ADP glucose pyrophosphorylase in the cytosol. Recent results derived from examination of the mutants of cereal crops, especially rice, for PPi-utilizing enzymes clearly suggest an important role of PPi as an alternative energy currency for integrating carbon metabolism from sucrose breakdown to starch synthesis in the endosperm. Thus, the present review provides an outline of the interlaced PPi-dependent metabolic pathways, which are critical for starch synthesis in the endosperm in terms of energy metabolism, along with its application to enhance yield potential.

Season Affects Yield and Metabolic Profiles of Rice (Oryza sativa) under High Night Temperature Stress in the Field

Rice (Oryza sativa) is the main food source for more than 3.5 billion people in the world. Global climate change is having a strong negative effect on rice production. One of the climatic factors impacting rice yield is asymmetric warming, i.e., the stronger increase in nighttime as compared to daytime temperatures. Little is known of the metabolic responses of rice to high night temperature (HNT) in the field. Eight rice cultivars with contrasting HNT sensitivity were grown in the field during the wet (WS) and dry season (DS) in the Philippines. Plant height, 1000-grain weight and harvest index were influenced by HNT in both seasons, while total grain yield was only consistently reduced in the WS. Metabolite composition was analysed by gas chromatography-mass spectrometry (GC-MS). HNT effects were more pronounced in panicles than in flag leaves. A decreased abundance of sugar phosphates and sucrose, and a higher abundance of monosaccharides in panicles indicated impaired glycolysis and higher respiration-driven carbon losses in response to HNT in the WS. Higher amounts of alanine and cyano-alanine in panicles grown in the DS compared to in those grown in the WS point to an improved N-assimilation and more effective detoxification of cyanide, contributing to the smaller impact of HNT on grain yield in the DS.

An Overview of Important Enzymes Involved in Nitrogen Assimilation of Plants

Nitrogen (N) is a macro-nutrient that is essential for growth development and resistance against biotic and abiotic stresses of plants. Nitrogen is a constituent of amino acids, proteins, nucleic acids, chlorophyll, and various primary and secondary metabolites. The atmosphere contains huge amounts of nitrogen but it cannot be taken up directly by plants. Plants can take up nitrogen in the form of nitrate, ammonium, urea, nitrite, or a combination of all these forms. In addition, in various leguminous rhizobia, bacteria can convert atmospheric nitrogen to ammonia and supply it to the plants. The form of nitrogen nutrition is also important in plant growth and resistance against pathogens. Nitrogen content has an important function in crop yield. Nitrogen deficiency can cause reduced root growth, change in root architecture, reduced plant biomass, and reduced photosynthesis. Hence, understanding the function and regulation of N metabolism is important. Several enzymes and intermediates are involved in nitrogen assimilation. Here we provide an overview of the important enzymes such as nitrate reductase, nitrite reductase, glutamine synthase, GOGAT, glutamate dehydrogenase, and alanine aminotransferase that are involved in nitrogen metabolism.

The Zea mays mutants opaque2 and opaque16 disclose lysine change in waxy maize as revealed by RNA-Seq

In maize, opaque2 (o2) and opaque16 (o16) alleles can increase lysine content, while the waxy (wx) gene can enhance the amylopectin content of grains. In our study, o2 and o16 alleles were backcrossed into waxy maize line (wxwx). The o2o2o16o16wxwx lines had amylopectin contents similar to those of waxy line. Their nutritional value was better than waxy line, but the mechanism by which the o2 and o16 alleles increased the lysine content of waxy maize remained unclear. The o2o2o16o16wxwx lines and their parents on kernels (18th day after pollination) were subjected to RNA sequencing (RNA-Seq). The RNA-Seq analysis revealed 272 differentially expressed genes (DEGs). Functional analyses revealed that these DEGs were mainly related to biomass metabolism. Among them, in o2o2o16o16wxwx lines, 15 genes encoding α-zein were down-regulated, which resulted in the reduction of α-zein synthesis and increased lysine content; lkr/sdh1 and Zm00001d020984.1 genes involved in the lysine degradation pathway were down-regulated, thereby inhibited lysine degradation; sh2, bt2 and ae1 genes involved in starch metabolism were upregulated, leaded to wrinkling kernel and farinaceous endosperm. Our transcriptional-level identification of key genes responsible for increased grain lysine content and farinaceous endosperm formation following introgression of o2 and o16 alleles should promote molecular breeding for maize quality.

Identification and expression analyses of the alanine aminotransferase (AlaAT) gene family in poplar seedlings

Alanine aminotransferase (AlaAT, E.C.2.6.1.2) catalyzes the reversible conversion of pyruvate and glutamate to alanine and α-oxoglutarate. The AlaAT gene family has been well studied in some herbaceous plants, but has not been well characterized in woody plants. In this study, we identified four alanine aminotransferase homologues in Populus trichocarpa, which could be classified into two subgroups, A and B. AlaAT3 and AlaAT4 in subgroup A encode AlaAT, while AlaAT1 and AlaAT2 in subgroup B encode glutamate:glyoxylate aminotransferase (GGAT), which catalyzes the reaction of glutamate and glyoxylate to α-oxoglutarate and glycine. Four AlaAT genes were cloned from P. simonii × P. nigra. PnAlaAT1 and PnAlaAT2 were expressed predominantly in leaves and induced by exogenous nitrogen and exhibited a diurnal fluctuation in leaves, but was inhibited in roots. PnAlaAT3 and PnAlaAT4 were mainly expressed in roots, stems and leaves, and was induced by exogenous nitrogen. The expression of PnAlaAT3 gene could be regulated by glutamine or its related metabolites in roots. Our results suggest that PnAlaAT3 gene may play an important role in nitrogen metabolism and is regulated by glutamine or its related metabolites in the roots of P. simonii × P. nigra.

Proteins associated with heat-induced leaf senescence in creeping bentgrass as affected by foliar application of nitrogen, cytokinins, and an ethylene inhibitor

Heat stress causes premature leaf senescence in cool-season grass species. The objective of this study was to identify proteins regulated by nitrogen, cytokinins, and ethylene inhibitor in relation to heat-induced leaf senescence in creeping bentgrass (Agrostis stolonifera). Plants (cv. Penncross) were foliar sprayed with 18 mM carbonyldiamide (N source), 25 μM aminoethoxyvinylglycine (AVG, ethylene inhibitor), 25 μM zeatin riboside (ZR, cytokinin), or a water control, and then exposed to 20/15°C (day/night) or 35/30°C (heat stress) in growth chambers. All treatments suppressed heat-induced leaf senescence, as shown by higher turf quality and chlorophyll content, and lower electrolyte leakage in treated plants compared to the untreated control. A total of 49 proteins were responsive to N, AVG, or ZR under heat stress. The abundance of proteins in photosynthesis increased, with ribulose-1,5-bisphosphate carboxylase/oxygenase affected by all three treatments, chlorophyll a/b-binding protein by AVG and N or Rubisco activase by AVG. Proteins for amino acid metabolism were upregulated, including alanine aminotransferase by three treatments and ferredoxin-dependent glutamate synthase by AVG and N. Upregulated proteins also included catalase by AVG and N and heat shock protein by ZR. Exogenous applications of AVG, ZR, or N downregulated proteins in respiration (enolase, glyceraldehyde 3-phosphate dehydrogenase, and succinate dehygrogenase) under heat stress. Alleviation of heat-induced senescence by N, AVG, or ZR was associated with enhanced protein abundance in photosynthesis and amino acid metabolism and stress defense systems (heat shock protection and antioxidants), as well as suppression of those imparting respiration metabolism.

Alanine aminotransferase 1 (OsAlaAT1) plays an essential role in the regulation of starch storage in rice endosperm

Alteration of storage substances, in particular the major storage form starch, leads to floury endosperm. Because floury mutants have physical attributes for milling processes, identification and characterization of those mutants are valuable. In this study we identified a floury endosperm mutant caused by a T-DNA insertion in Oryza sativa alanine-aminotransferase1 (OsAlaAT1). OsAlaAT1 is localized in the cytosol and has aminotransferase enzyme activity. The osalaat1 mutant has less amylose and its amylopectin is structurally altered. OsAlaAT1 is predominantly expressed in developing seeds during active starch synthesis. AlaAT catalyzes the interconversion of pyruvate to alanine, and this pathway is activated under low-oxygen conditions. Consistently, OsAlaAT1 is induced by such conditions. Expression of the starch synthesis genes AGPases, OsSSI, OsSSIIa, and OsPPDKB is decreased in the mutant. Thus, our observations suggest that OsAlaAT1 plays an essential role in starch synthesis in developing seeds that are exposed to low concentrations of oxygen.

A proteomic study on molecular mechanism of poor grain-filling of rice (Oryza sativa L.) inferior spikelets

Cultivars of rice (Oryza sativa L.), especially of the type with large spikelets, often fail to reach the yield potential as expected due to the poor grain-filling on the later flowering inferior spikelets (in contrast to the earlier-flowering superior spikelets). The present study showed that the size and grain weight of superior spikelets (SS) was greater than those of inferior spikelets (IS), and the carbohydrate supply should not be the major problem for the poor grain-filling because there was adequate amount of sucrose in IS at the initial grain-filling stage. High resolution two-dimensional gel electrophoresis (2-DE) in combination with Coomassie-brilliant blue (CBB) and Pro-Q Diamond phosphoprotein fluorescence stain revealed that 123 proteins in abundance and 43 phosphoproteins generated from phosphorylation were significantly different between SS and IS. These proteins and phosphoproteins were involved in different cellular and metabolic processes with a prominently functional skew toward metabolism and protein synthesis/destination. Expression analyses of the proteins and phosphoproteins associated with different functional categories/subcategories indicated that the starch synthesis, central carbon metabolism, N metabolism and cell growth/division were closely related to the poor grain-filling of IS. Functional and expression pattern studies also suggested that 14-3-3 proteins played important roles in IS poor grain-filling by regulating the activity of starch synthesis enzymes. The proteome and phosphoproteome obtained from this study provided a better understanding of the molecular mechanism of the IS poor grain-filling. They were also expected to be highly useful for improving the grain filling of rice.

Analysis of the enzymatic properties of a broad family of alanine aminotransferases

Alanine aminotransferase (AlaAT) has been studied in a variety of organisms due to the involvement of this enzyme in mammalian processes such as non-alcoholic hepatocellular damage, and in plant processes such as C₄ photosynthesis, post-hypoxic stress response and nitrogen use efficiency. To date, very few studies have made direct comparisons of AlaAT enzymes and fewer still have made direct comparisons of this enzyme across a broad spectrum of organisms. In this study we present a direct kinetic comparison of glutamate:pyruvate aminotransferase (GPAT) activity for seven AlaATs and two glutamate:glyoxylate aminotransferases (GGAT), measuring the K_M values for the enzymes analyzed. We also demonstrate that recombinant expression of AlaAT enzymes in Eschericia coli results in differences in bacterial growth inhibition, supporting previous reports of AlaAT possessing bactericidal properties, attributed to lipopolysaccharide endotoxin recognition and binding. A probable lipopolysaccharide binding region within the AlaAT enzymes, homologous to a region of a lipopolysaccharide binding protein (LBP) in humans, was also identified in this study. The AlaAT enzyme differences identified here indicate that AlaAT homologues have differentiated significantly and the roles these homologues play in vivo may also have diverged significantly. Specifically, the differing kinetics of AlaAT enzymes and how this may alter the nitrogen use efficiency in plants is discussed.

Arabidopsis thaliana ASN2 encoding asparagine synthetase is involved in the control of nitrogen assimilation and export during vegetative growth

We investigated the function of ASN2, one of the three genes encoding asparagine synthetase (EC 6.3.5.4), which is the most highly expressed in vegetative leaves of Arabidopsis thaliana. Expression of ASN2 and parallel higher asparagine content in darkness suggest that leaf metabolism involves ASN2 for asparagine synthesis. In asn2-1 knockout and asn2-2 knockdown lines, ASN2 disruption caused a defective growth phenotype and ammonium accumulation. The asn2 mutant leaves displayed a depleted asparagine and an accumulation of alanine, GABA, pyruvate and fumarate, indicating an alanine formation from pyruvate through the GABA shunt to consume excess ammonium in the absence of asparagine synthesis. By contrast, asparagine did not contribute to photorespiratory nitrogen recycle as photosynthetic net CO(2) assimilation was not significantly different between lines under both 21 and 2% O(2). ASN2 was found in phloem companion cells by in situ hybridization and immunolocalization. Moreover, lack of asparagine in asn2 phloem sap and lowered (15) N flux to sinks, accompanied by the delayed yellowing (senescence) of asn2 leaves, in the absence of asparagine support a specific role of asparagine in phloem loading and nitrogen reallocation. We conclude that ASN2 is essential for nitrogen assimilation, distribution and remobilization (via the phloem) within the plant.

Biochemical characterization and kinetic properties of alanine aminotransferase homologues partially purified from wheat (Triticum aestivum L.)

Four homologues of alanine aminotransferase have been isolated from shoots of wheat seedlings and purified by saline precipitation, gel filtration, preparative electrophoresis and anion exchange chromatography on Protein-Pak Q 8HR column attached to HPLC. Alanine aminotransferase 1 (AlaAT1) and 2 (AlaAT2) were purified 303- and 452-fold, respectively, whereas l-glutamate: glyoxylate aminotransferase 1 (GGAT1) and 2 (GGAT2) were purified 485- and 440-fold, respectively. Consistent inhibition of AlaAT (EC 2.6.1.2) and GGAT (EC 2.6.1.4) activities by p-hydroxymercuribenzoate points on participation of cysteine residues in the enzyme activity. The molecular weight of AlaAT1 and AlaAT2 was estimated to be 65kDa and both of them are monomers in native state. Nonsignificant differences between K(m) using alanine as substrate and catalytic efficiency (k(cat)/K(m)) for l-alanine in reaction with 2-oxoglutarate indicate comparable kinetic constants for AlaAT1 and AlaAT2. Similar kinetic constants for l-alanine in reaction with 2-oxoglutarate and for l-glutamate in reaction with pyruvate for all four homologues suggest equally efficient reaction in both forward and reverse directions. GGAT1 and GGAT2 were able to catalyze transamination between l-glutamate and glyoxylate, l-alanine and glyoxylate and reverse reactions between glycine and 2-oxoglutarate or pyruvate. Both GGATs also consisted of a single subunit with molecular weight of about 50kDa. The estimated K(m) for GGAT1 (3.22M) and GGAT2 (1.27M) using l-glutamate as substrate was lower in transamination with glyoxylate than with pyruvate (9.52 and 9.09mM, respectively). Moreover, distinctively higher values of catalytic efficiency for l-glutamate in reaction with glyoxylate than for l-glutamate in reaction with pyruvate confirm involvement of these homologues into photorespiratory metabolism.

Differential regulation of alanine aminotransferase homologues by abiotic stresses in wheat (Triticum aestivum L.) seedlings

Wheat (Triticum aestivum L.) seedlings contain four alanine aminotransferase (AlaAT) homologues. Two of them encode AlaAT enzymes, whereas two homologues act as glumate:glyoxylate aminotransferase (GGAT). To address the function of the distinct AlaAT homologues a comparative examination of the changes in transcript level together with the enzyme activity and alanine and glutamate content in wheat seedlings subjected to low oxygen availability, nitrogen and light deficiency has been studied. Shoots of wheat seedlings were more tolerant to hypoxia than the roots as judging on the basis of enzyme activity and transcript level. Hypoxia induced AlaAT1 earlier in roots than in shoots, while AlaAT2 and GGAT were unaffected. The increase in AlaAT activity lagged behind the increase in alanine content. Nitrogen deficiency has little effect on the activity of GGAT. In contrast, lower activity of AlaAT and the level of mRNA for AlaAT1 and AlaAT2 in wheat seedlings growing on a nitrogen-free medium seems to indicate that AlaAT is regulated by the availability of nitrogen. Both AlaAT and GGAT activities were present in etiolated wheat seedlings but their activity was half of that observed in light-grown seedlings. Exposure of etiolated seedlings to light caused an increase in enzyme activities and up-regulated GGAT1. It is proposed that hypoxia-induced AlaAT1 and light-induced peroxisomal GGAT1 appears to be crucial for the regulation of energy availability in plants grown under unfavourable environmental conditions. Key message In young wheat seedlings, both AlaAT and GGAT are down-regulated by nitrogen deficiency, whereas AlaAT1 is upregulated by hypoxia and GGAT1 by light.

Combined noninvasive imaging and modeling approaches reveal metabolic compartmentation in the barley endosperm

The starchy endosperm of cereals is a priori taken as a metabolically uniform tissue. By applying a noninvasive assay based on (13)C/(1)H-magnetic resonance imaging (MRI) to barley (Hordeum vulgare) grains, we uncovered metabolic compartmentation in the endosperm. (13)C-Suc feeding during grain filling showed that the primary site of Ala synthesis was the central region of the endosperm, the part of the caryopsis experiencing the highest level of hypoxia. Region-specific metabolism in the endosperm was characterized by flux balance analysis (FBA) and metabolite profiling. FBA predicts that in the central region of the endosperm, the tricarboxylic acid cycle shifts to a noncyclic mode, accompanied by elevated glycolytic flux and the accumulation of Ala. The metabolic compartmentation within the endosperm is advantageous for the grain's carbon and energy economy, with a prominent role being played by Ala aminotransferase. An investigation of caryopses with a genetically perturbed tissue pattern demonstrated that Ala accumulation is a consequence of oxygen status, rather than being either tissue specific or dependent on the supply of Suc. Hence the (13)C-Ala gradient can be used as an in vivo marker for hypoxia. The combination of MRI and metabolic modeling offers opportunities for the noninvasive analysis of metabolic compartmentation in plants.

Analysis of alanine aminotransferase in various organs of soybean (Glycine max) and in dependence of different nitrogen fertilisers during hypoxic stress

Alanine aminotransferase (AlaAT) catalyses the reversible conversion of pyruvate and glutamate into alanine and oxoglutarate. In soybean, two subclasses were identified, each represented by two highly similar members. To investigate the role of AlaAT during hypoxic stress in soybean, changes in transcript level of both subclasses were analysed together with the enzyme activity and alanine content of the tissue. Moreover, the dependency of AlaAT activity and gene expression was investigated in relation to the source of nitrogen supplied to the plants. Using semi-quantitative PCR, GmAlaAT genes were determined to be highest expressed in roots and nodules. Under normal growth conditions, enzyme activity of AlaAT was detected in all organs tested, with lowest activity in the roots. Upon waterlogging-induced hypoxia, AlaAT activity increased strongly. Concomitantly, alanine accumulated. During re-oxygenation, AlaAT activity remained high, but the transcript level and the alanine content decreased. Our results show a role for AlaAT in the catabolism of alanine during the initial period of re-oxygenation following hypoxia. GmAlaAT also responded to nitrogen availability in the solution during waterlogging. Ammonium as nitrogen source induced both gene expression and enzyme activity of AlaAT more than when nitrate was supplied in the nutrient solution. The work presented here indicates that AlaAT might not only be important during hypoxia, but also during the recovery phase after waterlogging, when oxygen is available to the tissue again.

Alanine aminotransferase catalyses the breakdown of alanine after hypoxia in Arabidopsis thaliana

Alanine aminotransferase (AlaAT) catalyses the reversible transfer of an amino group from glutamate to pyruvate to form 2-oxoglutarate and alanine. The regulation of AlaAT in several plant species has been studied in response to low-oxygen stress, light and nitrogen application. In this study, induction of Arabidopsis AlaAT1 and AlaAT2 during hypoxia was observed at the transcriptional level, and an increase in enzyme activity was detected in hypoxically treated roots. In addition, the tissue-specific expression of AlaAT1 and AlaAT2 was analysed using promoter:GUS fusions. The GUS staining patterns indicated that both AlaAT genes are expressed predominantly in vascular tissues. We manipulated AlaAT expression to determine the relative importance of this enzyme in low-oxygen stress tolerance and nitrogen metabolism. This was done by analysing T-DNA mutants and over-expressing barley AlaAT in Arabidopsis. The AlaAT1 knockout mutant (alaat1-1) showed a dramatic reduction in AlaAT activity, suggesting that AlaAT1 is the major AlaAT isozyme in Arabidopsis. Over-expression of barley AlaAT significantly increased the AlaAT activity in the transgenic plants. These plants were analysed for metabolic changes over a period of hypoxic stress and during their subsequent recovery. The results showed that alaat1-1 plants accumulate more alanine than wild-type plants during the early phase of hypoxia, and the decline in accumulated alanine was delayed in the alaat1-1 line during the post-hypoxia recovery period. When alanine was supplied as the nitrogen source, alaat1-1 plants utilized alanine less efficiently than wild-type plants did. These results indicate that the primary role of AlaAT1 is to break down alanine when it is in excess. Therefore, AlaAT appears to be crucial for the rapid conversion of alanine to pyruvate during recovery from low-oxygen stress.

Synergism between RPBF Dof and RISBZ1 bZIP activators in the regulation of rice seed expression genes

The Dof (DNA binding with one finger) transcriptional activator rice (Oryza sativa) prolamin box binding factor (RPBF), which is involved in gene regulation of rice seed storage proteins, has been isolated from rice cDNA expressed sequence tag clones containing the conserved Dof. RPBF is found as a single gene per haploid genome. Comparison of RPBF genomic and cDNA sequences revealed that the genomic copy is interrupted by one long intron of 1,892 bp in the 5' noncoding region. We demonstrated by transient expression in rice callus protoplasts that the isolated RPBF trans-activated several storage protein genes via an AAAG target sequence located within their promoters, and with methylation interference experiments the additional AAAG-like sequences in promoters of genes expressed in maturing seeds were recognized by the RPBF protein. Binding was sequence specific, since mutation of the AAAG motif or its derivatives decreased both binding and trans-activation by RPBF. Synergism between RPBF and RISBZ1 recognizing the GCN4 motif [TGA(G/C)TCA] was observed in the expression of many storage protein genes. Overexpression of both transcription factors gave rise to much higher levels of expression than the sum of individual activities elicited by either RPBF or RISBZ1 alone. Furthermore, mutation of recognition sites suppressed reciprocal trans-activation ability, indicating that there are mutual interactions between RISBZ1 and RPBF. The RPBF gene is predominantly expressed in maturing endosperm and coordinately expressed with seed storage protein genes, and is involved in the quantitative regulation of genes expressed in the endosperm in cooperation with RISBZ1.

Tissue expression map of a large number of expressed sequence tags and its application to in silico screening of stress response genes in common wheat

In order to assess global changes in gene expression patterns in stress-induced tissues, we conducted large-scale analysis of expressed sequence tags (ESTs) in common wheat. Twenty-one cDNA libraries derived from stress-induced tissues, such as callus, as well as liquid cultures and abiotic stress conditions (temperature treatment, desiccation, photoperiod, moisture and ABA) were constructed. Several thousand colonies were randomly selected from each of these 21 cDNA libraries and sequenced from both the 5' and 3' ends. By computing abundantly expressed ESTs, correlated expression patterns of genes across the tissues were monitored. Furthermore, the relationships between gene expression profiles among the stress-induced tissues were inferred from the gene expression patterns. Multi-dimensional analysis of EST data is analogous to microarray experiments. As an example, genes specifically induced and/or suppressed by cold acclimation and heat-shock treatments were selected in silico. Four hundred and ninety genes showing fivefold induction or 218 genes for suppression in comparison to the control expression level were selected. These selected genes were annotated with the BLAST search. Furthermore, gene ontology was conducted for these genes with the InterPro search. Because genes regulated in response to temperature treatment were successfully selected, this method can be applied to other stress-treated tissues. Then, the method was applied to screen genes in response to abiotic stresses such as drought and ABA treatments. In silico selection of screened genes from virtual display should provide a powerful tool for functional plant genomics.

Expression of a ferredoxin-dependent glutamate synthase gene in mesophyll and vascular cells and functions of the enzyme in ammonium assimilation in Nicotiana tabacum (L.)

GLU1 encodes the major ferredoxin-dependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) in Arabidopsis thaliana (ecotype Columbia). With the aim of providing clues on the role of Fd-GOGAT, we analyzed the expression of Fd-GOGAT in tobacco (Nicotiana tabacum L. cv. Xanthi). The 5' flanking element of GLU1 directed the expression of the uidA reporter gene in the palisade and spongy parenchyma of mesophyll, in the phloem cells of vascular tissue and in the roots of tobacco. White light, red light or sucrose induced GUS expression in the dark-grown seedlings in a pattern similar to the GLU1 mRNA accumulation in Arabidopsis. The levels of GLU2 mRNA encoding the second Fd-GOGAT and NADH-glutamate synthase (NADH-GOGAT, EC 1.4.1.14) were not affected by light. Both in the light and in darkness, (15)NH4(+) was incorporated into [5-(15)N]glutamine and [2-(15)N]glutamate by glutamine synthetase (GS, EC 6.3.1.2) and Fd-GOGAT in leaf disks of transgenic tobacco expressing antisense Fd-GOGAT mRNA and in wild-type tobacco. In the light, low level of Fd-glutamate synthase limited the [2-(15)N]glutamate synthesis in transgenic leaf disks. The efficient dark labeling of [2-(15)N]glutamate in the antisense transgenic tobacco leaves indicates that the remaining Fd-GOGAT (15-20% of the wild-type activity) was not the main limiting factor in the dark ammonium assimilation. The antisense tobacco under high CO2 contained glutamine, glutamate, asparagine and aspartate as the bulk of the nitrogen carriers in leaves (62.5%), roots (69.9%) and phloem exudates (53.2%). The levels of glutamate, asparagine and aspartate in the transgenic phloem exudates were similar to the wild-type levels while the glutamine level increased. The proportion of these amino acids remained unchanged in the roots of the transgenic plants. Expression of GLU1 in mesophyll cells implies that Fd-GOGAT assimilates photorespiratory and primary ammonium. GLU1 expression in vascular cells indicates that Fd-GOGAT provides amino acids for nitrogen translocation.

CaAlaAT1 catalyzes the alanine: 2-oxoglutarate aminotransferase reaction during the resistance response against Tobacco mosaic virus in hot pepper

Hot pepper (Capsicum annuum L. cv. Bugang) plants exhibit a hypersensitive response (HR) upon infection by Tobacco mosaic virus (TMV) pathotype P0. To elucidate molecular mechanism that underlies this resistance, hot pepper cv. Bugang leaves were inoculated with TMV-P0 and genes specifically up-regulated during the HR were isolated by differential screening. One of the clones, CaAlaAT1 encoding a putative alanine aminotransferase (EC 2.6.1.2) exhibited organ-specific expression pattern and the transcript accumulated abundantly in red (ripe) fruit tissues. CaAlaAT1 transcript was also induced in older leaves during senescence. The expression of CaAlaAT1 gene was increased in the incompatible interaction with TMV-P0 but was not in the compatible interaction with TMV-P1.2. When a strain of Xanthomonas campestris pv. vesicatoria (Xcv) carrying an AvrBs2 gene was infiltrated into the leaves of a pepper cv. ECW 20R carrying Bs2 resistance gene, a marked induction and maintenance of CaAlaAT1 gene expression was observed. The expression of CaAlaAT1 gene was triggered by salicylic acid (SA) and ethylene but not by methyl jasmonate (MeJA). CaAlaAT1 seemed to be localized mostly at the cytosol from the polyethylene glycol (PEG)-mediated transformation experiment. CaAlaAT1 seemed to catalyze alanine: 2-oxoglutarate aminotransferase (AKT) reaction, which was a main activity among the four activities in vitro, during the resistance response against TMV in hot pepper. These results suggest that CaAlaAT1, a protein known to be involved in metabolic reactions, might be one of the components in the plant's defense signal pathway against pathogens.

Amino acid metabolism in maize earshoots. Implications for assimilate preconditioning and nitrogen signaling

Nitrogen (N) is an essential requirement for kernel growth in maize (Zea mays); however, little is known about how N assimilates are metabolized in young earshoots during seed development. The objective of this study was to assess amino acid metabolism in cob and spikelet tissues during the critical 2 weeks following silking. Two maize hybrids were grown in the field for 2 years at two levels of supplemental N fertilizer (0 and 168 kg N/ha). The effects of the reproductive sink on cob N metabolism were examined by comparing pollinated to unpollinated earshoots. Earshoots were sampled at 2, 8, 14, and 18 d after silking; dissected into cob, spikelet, and/or pedicel and kernel fractions; then analyzed for amino acid profiles and key enzyme activities associated with amino acid metabolism. Major amino acids in the cob were glutamine (Gln), aspartic acid (Asp), asparagine (Asn), glutamate, and alanine. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. N supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. We found that the cob performs significant enzymatic interconversions among Gln, alanine, Asp, and Asn during early reproductive development, which may precondition the N assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn to Gln ratio in cobs may be part of a signal transduction pathway involving aspartate aminotransferase, Gln synthetase, and Asn synthetase to indicate plant N status for kernel development.

Evaluation of tissue specificity and expression strength of rice seed component gene promoters in transgenic rice

Using stable transgenic rice plants, the promoters of 15 genes expressed in rice seed were analysed for their spatial and temporal expression pattern and their potential to promote the expression of recombinant proteins in seeds. The 15 genes included 10 seed storage protein genes and five genes for enzymes involved in carbohydrate and nitrogen metabolism. The promoters for the glutelins and the 13 kDa and 16 kDa prolamins directed endosperm-specific expression, especially in the outer portion (peripheral region) of the endosperm, whilst the embryo globulin and 18 kDa oleosin promoters directed expression in the embryo and aleurone layer. Fusion of the GUS gene to the 26 kDa globulin promoter resulted in expression in the inner starchy endosperm tissue. It should be noted that the 10 kDa prolamin gene was the only one tested that required both the 5' and 3' flanking regions for intrinsic endosperm-specific expression. The promoters from the pyruvate orthophosphate dikinase (PPDK) and ADP-glucose pyrophosphorylase (AGPase) small subunit genes were active not only in the seed, but also in the phloem of vegetative tissues. Within the seed, the expression from these two promoters differed in that the PPDK gene was only expressed in the endosperm, whereas the AGPase small subunit gene was expressed throughout the seed. The GUS reporter gene fused to the alanine aminotransferase (AlaAT) promoter was expressed in the inner portion of the starchy endosperm, whilst the starch branching enzyme (SBE1) and the glutamate synthase (GOGAT) genes were mainly expressed in the scutellum (between the endosperm and embryo). When promoter activities were examined during seed maturation, the glutelin GluB-4, 26 kDa globulin and 10 kDa and 16 kDa prolamin promoters exhibited much higher activities than the others. The seed promoters analysed here exhibited a wide variety of activities and expression patterns, thus providing many choices suitable for various applications in plant biotechnology.

Alanine aminotransferase homologs catalyze the glutamate:glyoxylate aminotransferase reaction in peroxisomes of Arabidopsis

Plant peroxisomal glyoxylate aminotransferases play central roles within the photorespiratory pathway. Genes encoding glyoxylate aminotransferases have been isolated from several animals and microbes, but only recently have plant homologs been identified. Three Arabidopsis homologs of alanine (Ala):glyoxylate aminotransferase 2 (AGT2) contain a putative type 1 peroxisomal targeting signal (PTS1), but the metabolic significance of these AGT2 homologs is unknown. GGT1 and GGT2 are Ala aminotransferase (AlaAT) homologs from Arabidopsis that represent another type of glyoxylate aminotransferase. These proteins are class I aminotransferases, each containing a putative PTS1. GGT1 and GGT2 are members of a small family of AlaATs in Arabidopsis. When expressed as recombinant proteins in Escherichia coli, GGT1 and GGT2 displayed biochemical characteristics very similar to one another, and to the Arabidopsis protein purified from leaves. Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. GGT1 and GGT2 may have partially redundant functions; transcripts of both genes were detected in many of the same tissues. Although Glu:glyoxylate aminotransferase (GGT) activity has been observed in several locations in different plants and algae, including the cytoplasm and mitochondria, our subcellular fractionation data indicate that GGT activity was exclusively peroxisomal in Arabidopsis. Thus, glyoxylate aminotransferase reactions in plant peroxisomes appear to be catalyzed by at least two distinct types of aminotransferases: an AGT1 homolog with serine:glyoxylate aminotransferase activity (A.H. Liepman, L.J. Olsen [2001] Plant J 25: 487-498), and a pair of closely related, potentially redundant AlaAT homologs with GGT activity.

Rice (Oryza sativa) contains a novel isoform of glutamate decarboxylase that lacks an authentic calmodulin-binding domain at the C-terminus

We have isolated full-length cDNAs for two distinct isoforms of glutamate decarboxylase (GAD), designated OsGAD1 and OsGAD2 from a rice shoot cDNA library. Open reading frames found in OsGAD1 and OsGAD2 cDNAs encode putative proteins of 501 (56.7 kDa) and 500 amino acids (55.6 kDa), respectively. They show 69% identity to each other and 67-78% identity to dicotyledonous counterpart sequences determined so far. Comparative analysis of relevant genomic clones obtained from the rice genomic library with these cDNAs as probes demonstrated that the number and sizes of introns deduced for these two genes differ considerably. Interestingly, in the regions in the putative gene products corresponding to the C-terminal 30-amino-acid peptide known as the calmodulin-binding domain of plant GADs, OsGAD1 possesses a typical motif, while OsGAD2 contains several substitutions of amino acids that contribute strongly to the binding of calmodulin (CaM). An in vitro CaM-binding assay of these proteins over-expressed in Escherichia coli revealed that OsGAD1 can in fact bind specifically to bovine CaM but OsGAD2 cannot. RNA analysis showed that transcripts of OsGAD1 and OsGAD2 were present in all tissues examined, but their expression was differentially regulated, at least in roots and maturing seeds.