Three genes showing distinct regulatory patterns encode the asparagine synthetase of sunflower (Helianthus annuus)

Genome-wide expression analysis reveals involvement of asparagine synthetase family in cotton development and nitrogen metabolism

Asparagine synthetase (ASN) is one of the key enzymes of nitrogen (N) metabolism in plants. The product of ASN is asparagine, which is one of the key compounds involved in N transport and storage in plants. Complete genome-wide analysis and classifications of the ASN gene family have recently been reported in different plants. However, little is known about the systematic analysis and expression profiling of ASN proteins in cotton development and N metabolism. Here, various bioinformatics analysis was performed to identify ASN gene family in cotton. In the cotton genome, forty-three proteins were found that determined ASN genes, comprising of 20 genes in Gossypium hirsutum (Gh), 13 genes in Gossypium arboreum, and 10 genes in Gossypium raimondii. The ASN encoded genes unequally distributed on various chromosomes with conserved glutamine amidotransferases and ASN domains. Expression analysis indicated that the majority of GhASNs were upregulated in vegetative and reproductive organs, fiber development, and N metabolism. Overall, the results provide proof of the possible role of the ASN genes in improving cotton growth, fiber development, and especially N metabolism in cotton. The identified hub genes will help to functionally elucidate the ASN genes in cotton development and N metabolism.

Identification of Differentially Expressed Genes and Pathways Involved in Growth and Development of Mesona chinensis Benth Under Red- and Blue-Light Conditions

Mesona chinensis Benth (MCB) is an important Chinese herbal medicine. The plant factories might be one of the ways to solve the shortage of MCB supply. In this study, the MCB seedlings were treated under the red (R) and blue (B) lights in the plant factory. Results showed that the red light promoted the growth and development of MCB in comparison with the blue light. Under the red-light condition, the biomass, plant height, and root characteristics were significantly higher than those under blue-light condition, while the soil and plant analyzer development (SPAD) under the red-light treatment was significantly lower than that under the blue-light treatment. Red light also significantly promoted the content of soluble sugar and pectin of MCB compared with blue light. Transcriptome analysis showed that a total of 4,165 differentially expressed genes (DEGs) were detected including 2,034 upregulated and 2,131 downregulated. Of these, 1,112 DEGs including 410 upregulated and 702 downregulated genes were associated with 111 pathways. Moreover, a total of 8,723 differentially expressed transcription factors (TFs) were identified in R vs. B, and these TFs were distributed in 56 gene families. Metabonomic results revealed that a total of 184 metabolites and 99 differentially expressed metabolites (DEMs) (42 upregulated and 57 downregulated) were identified in the red- and blue-light treatments. Integrative analysis of transcriptome and metabolome unveiled that a total of 24 pathways included 70 compounds (metabolites) and were associated with 28 unigenes. In particular, these pathways included starch and sucrose metabolism, phenylpropanoid biosynthesis, cysteine and methionine metabolism, glycolysis/gluconeogenesis, and pentose and glucuronate interconversions. The unigenes included asparagine synthetase (AS), thymidine kinase (TK), alpha, alpha-trehalose-phosphate synthase (TPS), phosphatase IMPL1 (IMPL1), dihydroflavonol 4-reductase (D4R), and 4-coumarate-CoA ligase-like 6 (4CL6), bifunctional aspartokinase-homoserine dehydrogenase 1 (thrA), and abscisic acid 8'-hydroxylase 2 isoform X1 (ABA8). It was indicated that these pathways and genes might play important roles in the growth and development of MCB. This study laid a foundation for the future research of MCB.

Exogenous Carbon Compounds Modulate Tomato Root Development

NO3- is not only a nutrient, but also a signaling compound that plays an important role in several plant processes, like root development. The present study aimed to investigate the effect of three different exogenous C compounds (sucrose, glucose, 2-oxoglutarate) added to NO3- nutrition on C/N, auxin and antioxidant metabolisms in 10-day-old tomato seedlings. Sucrose and glucose supplementation enhanced primary root (PR) length, lateral root number and root density, while 2-oxoglutarate negatively affected them. This phenomenon was accompanied by a slight increase in NRT2.1 and GS1 gene expression, together with an increase in LAX2 and LAX3 and a decrease in LAX4 in the roots growing under sucrose and glucose sources. The addition of 2-oxoglutarate enhanced the expression of NiR, GDH, PEPC1, LAX1, LAX3 and the antioxidant gene SOD Cl. Taken together, these findings contribute to a better understanding of how these C sources can modulate N uptake and C/N, auxin and antioxidant gene expression, which could be useful for improving nitrogen use efficiency.

Tomato root development and N assimilation depend on C and ABA content under different N sources

Root plasticity is controlled by hormonal homeostasis and nutrient availability. In this work, we have determined the influence of different N regimens on growth parameters and on the expression of genes involved in auxin transport and N-assimilation in tomato seedlings. NH₄⁺ nutrition led to an inhibitory effect on root fresh weight (FW), lateral root (LR) number and root density, while an increase in the primary root (PR) length was observed. The expression of N assimilation genes GS2 and ASN1, is affected by NH₄⁺ nutrition. Moreover, in order to relieve the toxic effect of NH₄⁺ on root development, glucose or 2-oxoglutarate was supplied as a C source during NH₄⁺ treatment. The addition of 2-oxoglutarate improved root parameters compared to the NH₄⁺ regimen. N-assimilation gene analysis showed that NH₄⁺-fed tomato plants try to alleviate the toxic effect by concurrently upregulating ASN1 and anaplerotic PEPC2 expression, whereas when 2-oxoglutarate is supplied, ASN1 induction was not observed. The addition of both C skeletons induced the expression of the ROS-scavenging genes GSH and SOD. In addition, since ABA plays a role in root development, the ABA-synthesis-defective mutant flacca was studied under NO₃^- and NH₄⁺ regimens. It displayed a decrease in LR number under NO₃^- conditions, whereas, the NH₄⁺-fed seedlings showed a decrease solely in PR length that was reverted when ABA was exogenously supplied. Moreover, flacca seedlings displayed a reprogramming of the N/C assimilation genes. Altogether, these results reflect the importance of N and C sources and ABA homeostasis in root development of tomato seedlings.

Functional Research on Three Presumed Asparagine Synthetase Family Members in Poplar

Asparagine synthetase (AS), a key enzyme in plant nitrogen metabolism, plays an important role in plant nitrogen assimilation and distribution. Asparagine (Asn), the product of asparagine synthetase, is one of the main compounds responsible for organic nitrogen transport and storage in plants. In this study, we performed complementation experiments using an Asn-deficient Escherichia coli strain to demonstrate that three putative asparagine synthetase family members in poplar (Populussimonii× P.nigra) function in Asn synthesis. Quantitative real-time PCR revealed that the three members had high expression levels in different tissues of poplar and were regulated by exogenous nitrogen. PnAS1 and PnAS2 were also affected by diurnal rhythm. Long-term dark treatment resulted in a significant increase in PnAS1 and PnAS3 expression levels. Under long-term light conditions, however, PnAS2 expression decreased significantly in the intermediate region of leaves. Exogenous application of ammonium nitrogen, glutamine, and a glutamine synthetase inhibitor revealed that PnAS3 was more sensitive to exogenous glutamine, while PnAS1 and PnAS2 were more susceptible to exogenous ammonium nitrogen. Our results suggest that the various members of the PnAS gene family have distinct roles in different tissues and are regulated in different ways.

Establishing gene models from the Pinus pinaster genome using gene capture and BAC sequencing

BACKGROUND

In the era of DNA throughput sequencing, assembling and understanding gymnosperm mega-genomes remains a challenge. Although drafts of three conifer genomes have recently been published, this number is too low to understand the full complexity of conifer genomes. Using techniques focused on specific genes, gene models can be established that can aid in the assembly of gene-rich regions, and this information can be used to compare genomes and understand functional evolution.

RESULTS

In this study, gene capture technology combined with BAC isolation and sequencing was used as an experimental approach to establish de novo gene structures without a reference genome. Probes were designed for 866 maritime pine transcripts to sequence genes captured from genomic DNA. The gene models were constructed using GeneAssembler, a new bioinformatic pipeline, which reconstructed over 82% of the gene structures, and a high proportion (85%) of the captured gene models contained sequences from the promoter regulatory region. In a parallel experiment, the P. pinaster BAC library was screened to isolate clones containing genes whose cDNA sequence were already available. BAC clones containing the asparagine synthetase, sucrose synthase and xyloglucan endotransglycosylase gene sequences were isolated and used in this study. The gene models derived from the gene capture approach were compared with the genomic sequences derived from the BAC clones. This combined approach is a particularly efficient way to capture the genomic structures of gene families with a small number of members.

CONCLUSIONS

The experimental approach used in this study is a valuable combined technique to study genomic gene structures in species for which a reference genome is unavailable. It can be used to establish exon/intron boundaries in unknown gene structures, to reconstruct incomplete genes and to obtain promoter sequences that can be used for transcriptional studies. A bioinformatics algorithm (GeneAssembler) is also provided as a Ruby gem for this class of analyses.

Silencing of vacuolar invertase and asparagine synthetase genes and its impact on acrylamide formation of fried potato products

Acrylamide is produced in a wide variety of carbohydrate-rich foods during high-temperature cooking. Dietary acrylamide is a suspected human carcinogen, and health concerns related to dietary acrylamide have been raised worldwide. French fries and potato chips contribute a significant proportion to the average daily intake of acrylamide, especially in developed countries. One way to mitigate health concerns related to acrylamide is to develop potato cultivars that have reduced contents of the acrylamide precursors asparagine, glucose and fructose in tubers. We generated a large number of silencing lines of potato cultivar Russet Burbank by targeting the vacuolar invertase gene VInv and the asparagine synthetase genes StAS1 and StAS2 with a single RNA interference construct. The transcription levels of these three genes were correlated with reducing sugar (glucose and fructose) and asparagine content in tubers. Fried potato products from the best VInv/StAS1/StAS2-triple silencing lines contained only one-fifteenth of the acrylamide content of the controls. Interestingly, the extent of acrylamide reduction of the best triple silencing lines was similar to that of the best VInv-single silencing lines developed previously from the same potato cultivar Russet Burbank. These results show that an acrylamide mitigation strategy focused on developing potato cultivars with low reducing sugars is likely to be an effective and sufficient approach for minimizing the acrylamide-forming potential of French fry processing potatoes.

Asparagine synthetase1, but not asparagine synthetase2, is responsible for the biosynthesis of asparagine following the supply of ammonium to rice roots

Asparagine is synthesized from glutamine by the reaction of asparagine synthetase (AS) and is the major nitrogen form in both xylem and phloem sap in rice (Oryza sativa L.). There are two genes encoding AS, OsAS1 and OsAS2, in rice, but the functions of individual AS isoenzymes are largely unknown. Cell type- and NH4(+)-inducible expression of OsAS1 as well as analyses of knockout mutants were carried out in this study to characterize AS1. OsAS1 was mainly expressed in the roots, with in situ hybridization showing that the corresponding mRNA was specifically accumulated in the three cell layers of the root surface (epidermis, exodermis and sclerenchyma) in an NH4(+)-dependent manner. Conversely, OsAS2 mRNA was abundant in leaf blades and sheathes of rice. Although OsAS2 mRNA was detectable in the roots, its content decreased when NH4(+) was supplied. Retrotransposon-mediated knockout mutants lacking AS1 showed slight stimulation of shoot length and slight reduction in root length at the seedling stage. On the other hand, the mutation caused an approximately 80-90% reduction in free asparagine content in both roots and xylem sap. These results suggest that AS1 is responsible for the synthesis of asparagine in rice roots following the supply of NH4(+). Characteristics of the NH4(+)-dependent increase and the root surface cell-specific expression of OsAS1 gene are very similar to our previous results on cytosolic glutamine synthetase1;2 and NADH-glutamate synthase1 in rice roots. Thus, AS1 is apparently coupled with the primary assimilation of NH4(+) in rice roots.

High irradiance improves ammonium tolerance in wheat plants by increasing N assimilation

Ammonium is a paradoxical nutrient ion. Despite being a common intermediate in plant metabolism whose oxidation state eliminates the need for its reduction in the plant cell, as occurs with nitrate, it can also result in toxicity symptoms. Several authors have reported that carbon enrichment in the root zone enhances the synthesis of carbon skeletons and, accordingly, increases the capacity for ammonium assimilation. In this work, we examined the hypothesis that increasing the photosynthetic photon flux density is a way to increase plant ammonium tolerance. Wheat plants were grown in a hydroponic system with two different N sources (10mM nitrate or 10mM ammonium) and with two different light intensity conditions (300 μmol photon m(-2)s(-1) and 700 μmol photon m(-2)s(-1)). The results show that, with respect to biomass yield, photosynthetic rate, shoot:root ratio and the root N isotopic signature, wheat behaves as a sensitive species to ammonium nutrition at the low light intensity, while at the high intensity, its tolerance is improved. This improvement is a consequence of a higher ammonium assimilation rate, as reflected by the higher amounts of amino acids and protein accumulated mainly in the roots, which was supported by higher tricarboxylic acid cycle activity. Glutamate dehydrogenase was a key root enzyme involved in the tolerance to ammonium, while glutamine synthetase activity was low and might not be enough for its assimilation.

Response to nitrate/ammonium nutrition of tomato (Solanum lycopersicum L.) plants overexpressing a prokaryotic NH4(+)-dependent asparagine synthetase

Nitrogen availability is an important limiting factor for plant growth. Although NH4(+) assimilation is energetically more favorable than NO3(-), it is usually toxic for plants. In order to study if an improved ammonium assimilatory metabolism could increase the plant tolerance to ammonium nutrition, tomato (Solanum lycopersicum L. cv P-73) plants were transformed with an NH4(+)-dependent asparagine synthetase (AS-A) gene from Escherichia coli (asnA) under the control of a PCpea promoter (pea isolated constitutive promotor). Homozygous (Hom), azygous (Az) asnA and wild type (WT) plants were grown hydroponically for 6 weeks with normal Hoagland nutrition (NO3(-)/NH4(+)=6/0.5) and high ammonium nutrition (NO3(-)/NH4(+)=3.5/3). Under Hoagland's conditions, Hom plants produced 40-50% less biomass than WT and Az plants. However, under NO3(-)/NH4(+)=3.5/3 the biomass of Hom was not affected while it was reduced by 40-70% in WT and Az plants compared to Hoagland, respectively. The Hom plants accumulated 1.5-4 times more asparagine, glycine, serine and soluble proteins and registered higher glutamine synthetase (GS) and glutamate synthase (GOGAT) activities in the light-adapted leaves than the other genotypes, but had similar NH4(+) and NO3(-) levels in all conditions. In the dark-adapted leaves, a protein catabolism occurred in the Hom plants with a concomitant 25-40% increase in organic acid concentration, while asparagine accumulation registered the highest values. The aforementioned processes might be responsible for a positive energetic balance as regards the futile cycle of the transgenic protein synthesis and catabolism. This explains growth penalty under standard nutrition and growth stability under NO3(-)/NH4(+)=3.5/3, respectively.

Tuber-specific silencing of asparagine synthetase-1 reduces the acrylamide-forming potential of potatoes grown in the field without affecting tuber shape and yield

Simultaneous silencing of asparagine synthetase (Ast)-1 and -2 limits asparagine (ASN) formation and, consequently, reduces the acrylamide-forming potential of tubers. The phenotype of silenced lines appears normal in the greenhouse, but field-grown tubers are small and cracked. Assessing the effects of silencing StAst1 and StAst2 individually, we found that yield drag was mainly linked to down-regulation of StAst2. Interestingly, tubers from untransformed scions grafted onto intragenic StAst1/2-silenced rootstock contained almost the same low ASN levels as those in the original silenced lines, indicating that ASN is mainly formed in tubers rather than being transported from leaves. This conclusion was further supported by the finding that overexpression of StAst2 caused ASN to accumulate in leaves but not tubers. Thus, ASN does not appear to be the main form of organic nitrogen transported from leaves to tubers. Because reduced ASN levels coincided with increased levels of glutamine, it appears likely that this alternative amide amino acid is mobilized to tubers, where it is converted into ASN by StAst1. Indeed, tuber-specific silencing of StAst1, but not of StAst2, was sufficient to substantially lower ASN formation in tubers. Extensive field studies demonstrated that the reduced acrylamide-forming potential achieved by tuber-specific StAst1 silencing did not affect the yield or quality of field-harvested tubers.

A kinetic comparison of asparagine synthetase isozymes from higher plants

Four previously identified maize asparagine synthetase (AsnS) genes and a soy AsnS gene have been cloned and expressed in Escherichia coli. The enzymes have been purified and kinetically characterized. The plant AsnS proteins were expressed mainly in the inclusion bodies although small amounts of one form (ZmAsnS2) were recovered in the soluble fraction. In order to measure the kinetic properties of these enzymes a sensitive assay based on the detection of Asn by HPLC has been developed. In addition a method to refold the recombinant plant AsnS to produce active enzyme has been developed. The plant AsnS enzymes are kinetically distinct with substantial differences in K(m) (Gln) and V(max) values when compared to each other. These differences may be important factors for transgenic studies using AsnS genes for crop improvement.

Nitrogen metabolism in the developing ear of maize (Zea mays): analysis of two lines contrasting in their mode of nitrogen management

*The main steps of nitrogen (N) metabolism were characterized in the developing ear of the two maize (Zea mays) lines F2 and Io, which were previously used to investigate the genetic basis of nitrogen use efficiency (NUE) in relation to yield. *During the grain-filling period, we monitored changes in metabolite content, enzyme activities and steady-state levels of transcripts for marker genes of amino acid synthesis and interconversion in the cob and the kernels. *Under low N fertilization conditions, line Io accumulated glutamine, asparagine and alanine preferentially in the developing kernels, whereas in line F2, glutamine and proline were the predominant amino acids. Quantification of the mRNA-encoding enzymes involved in asparagine, alanine and proline biosynthesis confirmed that the differences observed between the two lines at the physiological level are likely to be attributable to enhanced expression of the cognate genes. *Integrative analysis of physiological and gene expression data indicated that the developing ear of line Io had higher N use and transport capacities than line F2. Thus, in maize there is genetic and environmental control of N metabolism not only in vegetative source organs but also in reproductive sink organs.

PVAS3, a class-II ubiquitous asparagine synthetase from the common bean (Phaseolus vulgaris)

A gene encoding a putative asparagine synthetase (AS; EC 6.3.5.4) has been isolated from common bean (Phaseolus vulgaris). A 2.4 kb cDNA clone of this gene (PVAS3) encodes a protein of 570 amino acids with a predicted molecular mass of 64,678 Da, an isoelectric point of 6.45, and a net charge of -5.9 at pH 7.0. The PVAS3 protein sequence conserves all the amino acid residues that are essential for glutamine-dependent AS, and PVAS3 complemented an E. coli asparagine auxotroph, that demonstrates that it encodes a glutamine-dependent AS. PVAS3 displayed significant similarity to other AS. It showed the highest similarity to soybean SAS3 (92.9% identity), rice AS (73.7% identity), Arabidopsis ASN2 (73.2%) and sunflower HAS2 (72.9%). A phylogenetic analysis revealed that PVAS3 belongs to class-II asparagine synthetases. Expression analysis by real-time RT-PCR revealed that PVAS3 is expressed ubiquitously and is not repressed by light.

Nitrogen stress and the expression of asparagine synthetase in roots and nodules of soybean (Glycine max)

The difficulty of assaying asparagine synthetase (AS) (EC 6.3.5.4) activity in roots of soybean has been circumvented by measuring expression of the AS genes. Expression of three soybean asparagine synthetase (SAS) genes (SAS1, SAS2 and SAS3) was observed in roots of non-nodulated soybean plants cultivated on nitrate. Expression of these genes was reduced to very low levels within days after submitting the plants to a N-free medium. The subsequent return to a complete medium (containing nitrate) restored expression of all three AS genes. Roots of nodulated plants, where symbiotic nitrogen fixation was the exclusive source of N (no nitrate present), showed very weak expression of all three AS genes, but on transfer to a nitrate-containing medium, strong expression of these genes was observed within 24 h. In nodules, all three genes were expressed in the absence of nitrate. Under conditions that impair nitrogen fixation (nodules submerged in aerated hydroponics), only SAS1 expression was reduced. However, in the presence of nitrate, an inhibitor of N(2) fixation, SAS1 expression was maintained. High and low expressions of AS genes in the roots were associated with high and low ratios of Asn/Asp transported to the shoot through xylem. It is concluded that nitrate (or one of its assimilatory products) leads to the induction of AS in roots of soybean and that this underlies the variations found in xylem sap Asn/Asp ratios. Regulation of nodule AS expression is quite different from that of the root, but nodule SAS1, at least, appears to involve a product of N assimilation rather than nitrate itself.

Evidence for sugar signalling in the regulation of asparagine synthetase gene expressed in Phaseolus vulgaris roots and nodules

A cDNA clone, designated as PvNAS2, encoding asparagine amidotransferase (asparagine synthetase) was isolated from nodule tissue of common bean (Phaseolus vulgaris cv. Negro Jamapa). Southern blot analysis indicated that asparagine synthetase in bean is encoded by a small gene family. Northern analysis of RNAs from various plant organs demonstrated that PvNAS2 is highly expressed in roots, followed by nodules in which it is mainly induced during the early days of nitrogen fixation. Investigations with the PvNAS2 promoter gusA fusion revealed that the expression of PvNAS2 in roots is confined to vascular bundles and meristematic tissues, while in root nodules its expression is solely localized to vascular traces and outer cortical cells encompassing the central nitrogen-fixing zone, but never detected in either infected or non-infected cells located in the central region of the nodule. PvNAS2 is down-regulated when carbon availability is reduced in nodules, and the addition of sugars to the plants, mainly glucose, boosted its induction, leading to the increased asparagine production. In contrast to PvNAS2 expression and the concomitant asparagine synthesis, glucose supplement resulted in the reduction of ureide content in nodules. Studies with glucose analogues as well as hexokinase inhibitors suggested a role for hexokinase in the sugar-sensing mechanism that regulates PvNAS2 expression in roots. In light of the above results, it is proposed that, in bean, low carbon availability in nodules prompts the down-regulation of the asparagine synthetase enzyme and concomitantly asparagine production. Thereby a favourable environment is created for the efficient transfer of the amido group of glutamine for the synthesis of purines, and then ureide generation.

Expression of asparagine synthetase genes in sunflower (Helianthus annuus) under various environmental stresses

In sunflower, asparagine synthetase (AS; EC 6.3.5.4) is encoded by a small family of three genes (HAS1, HAS1.1 and HAS2) that are differentially regulated by light, carbon and nitrogen availability. In this study, the response of each gene to various stress conditions was examined by Northern analysis with gene-specific probes in leaves and roots. The expression of HAS1 and HAS1.1 genes was induced by osmotic stress (300 mM mannitol), salt stress (150 mM NaCl), and heavy-metal stress (20 microM CuSO(4)), more in roots than in leaves. The expression of HAS2 was not significantly altered by stress treatments. The positive response of HAS1 and HAS1.1 genes to osmotic and salt stresses occurred in the light, in contrast to that previously found in unstressed plants. Measurements of sucrose and total free amino acid contents in leaves and roots indicate that the expression of root HAS1 and HAS1.1 genes in stressed plants is not under metabolic control by the intracellular C/N ratio, suggesting the involvement of some specific stress factor(s). Growth of plants at 40 degrees C for 12h negatively affected the expression of HAS1 and HAS1.1 but not that of HAS2.

Role of asparagine and asparagine synthetase genes in sunflower (Helianthus annuus) germination and natural senescence

Sunflower (Helianthus annuus) contains three active asparagine synthetase (EC 6.3.5.4, AS) genes: HAS1, HAS1.1 and HAS2. Asparagine content and AS gene expression were determined during germination and leaf and cotyledon natural senescence to assess the role of asparagine as well as the extent of participation of each AS gene in different nitrogen mobilizing processes. Asparagine accumulated in the dry seed and was the predominant amide throughout germination. During cotyledon senescence, the asparagine level was slightly higher than that of glutamine. The opposite was true for leaf senescence. According to transcript accumulation data, most of the asparagine newly synthesized for germination and cotyledon expansion was due to HAS2 activity, with little contribution of the other AS genes. However, all three genes work together to synthesize asparagine for leaf senescence. The absence of significant AS gene expression in cotyledon senescence differentiates leaf and cotyledon senescence, and suggests a cotyledon-specific regulation.

Asparagine synthetase gene TaASN1 from wheat is up-regulated by salt stress, osmotic stress and ABA

Differences in gene expression between salinity stressed and normally grown wheat seedlings were compared by the differential display (DD) technique. One DD-derived cDNA clone was characterized as a partial sequence of the wheat asparagine ynthetase (AS) gene by sequence analysis and homology search of GenBank databases. Two AS genes of wheat, TaASN1 and TaASN2, were further isolated by the RT-PCR approach. Comparison of the deduced polypeptide of TaASN1 and TaASN2 with AS proteins from other organisms revealed several homologous regions, in particular, the conserved glutamine binding sites and Class-II Glutamine amidotransferases domain. The functionality of TaASN1 was demonstrated by complementing an Escherichia coli asparagine auxotroph. TaASN1 transcripts were detected in roots, shoots, anthers and young spikes by RT-PCR analysis. Abundance of TaASN1 mRNA in young spikes and anthers was higher than that in shoots and roots under normal growth conditions. TaASN1 was dramatically induced by salinity, osmotic stress and exogenous abscisic acid (ABA) in wheat seedlings. TaASN2 transcripts were very low in all detected tissues and conditions and were only slightly induced by ABA in roots.

Light and metabolic regulation of HAS1, HAS1.1 and HAS2, three asparagine synthetase genes in Helianthus annuus

The role of light, carbon and nitrogen availability on the regulation of three asparagine synthetase (AS, EC 6.3.5.4)-coding genes, HAS1, HAS1.1 and HAS2, has been investigated in sunflower (Helianthus annuus). The response of each gene to different illumination conditions and to treatments that modify the carbon and nitrogen status of the plant was evaluated by Northern analysis with gene-specific probes. Light represses the expression of HAS1 and HAS1.1. Phytochrome and photosynthesis-derived carbohydrates mediate this repression. On the contrary, maintained HAS2 expression requires light and is positively affected by sucrose. HAS1 and HAS1.1 expression is dependent on nitrogen availability, while HAS2 transcripts are still found in N-starved plants. High ammonium level induces all three AS genes and partially reverts sucrose repression of HAS1 and HAS1.1. In summary, light, carbon and nitrogen availability control asparagine synthesis in sunflower by regulating three AS-coding genes. Illumination and carbon sufficiency maintain HAS2 active to supply asparagine that can be used for growth. Darkness and low C/N ratio conditions trigger the response of the specialized HAS1 and HAS1.1 genes which contribute to store the excess nitrogen as asparagine. Ammonium induces all three AS-genes which may favor its detoxification.

Molecular cloning and characterisation of two genes encoding asparagine synthetase in barley (Hordeum vulgare L.)

Two different cDNA clones encoding asperagine synthetase (AS: EC 6.3.5.4.) were cloned from barley (Hordeum vulgare L. cv. Alexis). The corresponding genes were designated HvAS1 (GenBank no AF307145) and HvAS2 (GenBank no AY193714). Chromosomal mapping using wheat-barley addition lines revealed that the HvAS1 gene is located on the long arm of barley chromosome 5, while the HvAS2 gene maps to the short arm of chromosome 3. Both genes are expressed in barley leaves according to RT-PCR analysis but only the HvAS1 gene expression can be detected in roots. Northern blots show no expression of HvAS1 in plants grown under a normal 16 h light/8 h dark cycle but after 10 h of continuous darkness, transcript appears and mRNA accumulates over a 48-h period of dark treatment. In roots, low-level expression of HvAS1 could be detected and the expression level appears to be unaffected by light. A polyclonal antibody was raised against the HvAS1 protein and used in Western blot analysis. The AS protein accumulated during a 48-h period of dark treatment, following the increase in HvAS1 transcript.