Cereal asparagine synthetase genes

Abstract

Asparagine synthetase catalyses the transfer of an amino group from glutamine to aspartate to form glutamate and asparagine. The accumulation of free (nonprotein) asparagine in crops has implications for food safety because free asparagine is the precursor for acrylamide, a carcinogenic contaminant that forms during hightemperature cooking and processing. Here we review publicly available genome data for asparagine synthetase genes from species of the Pooideae subfamily, including bread wheat and related wheat species (Triticum and Aegilops spp.), barley (Hordeum vulgare) and rye (Secale cereale) of the Triticeae tribe. Also from the Pooideae subfamily: brachypodium (Brachypodium dIstachyon) of the Brachypodiae tribe. More diverse species are also included, comprising sorghum (Sorghum bicolor) and maize (Zea mays) of the Panicoideae subfamily and rice (Oryza sativa) of the Ehrhartoideae subfamily. The asparagine synthetase gene families of the Triticeae species each comprise five genes per genome, with the genes assigned to four groups: 1, 2, 3 (subdivided into 3.1 and 3.2) and 4. Each species has a single gene per genome in each group, except that some bread wheat varieties (genomes AABBDD) and emmer wheat (Triticum dicoccoides; genomes AABB) lack a group 2 gene in the B genome. This raises questions about the ancestry of cultivated pasta wheat and the B genome donor of bread wheat, suggesting that the hybridisation event that gave rise to hexaploid bread wheat occurred more than once. In phylogenetic analyses, genes from the other species cluster with the Triticeae genes, but brachypodium, sorghum and maize lack a group 2 gene, while rice has only two genes, one group 3 and one group 4. This means that TaASN2, the most highly expressed asparagine synthetase gene in wheat grain, has no equivalent in maize, rice, sorghum or brachypodium. An evolutionary pathway is proposed in which a series of gene duplications gave rise to the five genes found in modern Triticeae species.

Reduced free asparagine in wheat grain resulting from a natural deletion of TaASN-B2: investigating and exploiting diversity in the asparagine synthetase gene family to improve wheat quality

“…spelta) to characterise natural allelic variation in the wheat asparagine synthetase gene family. The results are shown in Table 1, ordered by gene name [14,15] and the corresponding annotated gene model ID from the Chinese Spring RefSeq v1.1 genome assembly [20]. For each orthologous gene, Sorting Intolerant From Tolerant (SIFT) analysis was performed on the translated protein to predict whether the variation in amino acid sequences was likely to disrupt protein function (highlighted in yellow in Table 1) or to be tolerated (highlighted in green).…”

“…Asparagine is synthesised by the transfer of an amino group from glutamine to aspartate to make asparagine and glutamate in a reaction catalysed by asparagine synthetase. The cereal asparagine synthetase gene family comprises between two and ve genes per diploid genome [14], with members of the Triticeae tribe all having ve genes per genome, assigned to four groups: 1, 2, 3 (subdivided into 3.1 and 3.2) and 4. The one documented exception is that some hexaploid common wheat (Triticum aestivum L.; genomes AABBDD) and tetraploid emmer wheat (T. turgidum; genomes AABB) genotypes lack a group 2 gene in the B genome (TaASN-B2/TdASN-B2) [14,15].…”

“…The cereal asparagine synthetase gene family comprises between two and ve genes per diploid genome [14], with members of the Triticeae tribe all having ve genes per genome, assigned to four groups: 1, 2, 3 (subdivided into 3.1 and 3.2) and 4. The one documented exception is that some hexaploid common wheat (Triticum aestivum L.; genomes AABBDD) and tetraploid emmer wheat (T. turgidum; genomes AABB) genotypes lack a group 2 gene in the B genome (TaASN-B2/TdASN-B2) [14,15]. This gene is absent from the IWGSC RefSeq v1.1 genome assembly of the common wheat landrace Chinese Spring but is present in the cultivar Cadenza [14].…”

“…Husn.) [14]. Since wild emmer wheat is believed to be the B genome donor for both tetraploid durum and hexaploid common wheat [16], the most likely explanation for the presence of ASN-B2 in some cultivars but not others is that the hybridisation event that produced hexaploid wheat occurred more than once, involving emmer wheats with and without ASN-B2 [14].…”

“…Natural variation in ASN proteins in 14 wheat varieties. Each gene is annotated by name[14,15] and genome (hence TaASN-A1, for example, is the A genome TaASN1 gene), and the gene ID from the Chinese Spring RefSeq v1.1 gene models[18] (note that TaASN-B2 is absent in Chinese Spring). Green shading indicates no predicted impact of variety-specific mutations; yellow shading indicates one or more mutations that are predicted to disrupt protein function; red shading indicates that the gene is not present in that genome.…”