Contrasting gene expression patterns in grain of high and low asparagine wheat genotypes in response to sulphur supply

Abstract

Background Free asparagine is the precursor for acrylamide formation during cooking and processing of grains, tubers, beans and other crop products. In wheat grain, free asparagine, free glutamine and total free amino acids accumulate to high levels in response to sulphur deficiency. In this study, RNA-seq data were acquired for the embryo and endosperm of two genotypes of bread wheat, Spark and SR3, growing under conditions of sulphur sufficiency and deficiency, and sampled at 14 and 21 days post anthesis (dpa). The aim was to provide new knowledge and understanding of the genetic control of asparagine accumulation and breakdown in wheat grain. Results There were clear differences in gene expression patterns between the genotypes. Sulphur responses were greater at 21 dpa than 14 dpa, and more evident in SR3 than Spark. TaASN2 was the most highly expressed asparagine synthetase gene in the grain, with expression in the embryo much higher than in the endosperm, and higher in Spark than SR3 during early development. There was a trend for genes encoding enzymes of nitrogen assimilation to be more highly expressed in Spark than SR3 when sulphur was supplied. TaASN2 expression in the embryo of SR3 increased in response to sulphur deficiency at 21 dpa, although this was not observed in Spark. This increase in TaASN2 expression was accompanied by an increase in glutamine synthetase gene expression and a decrease in asparaginase gene expression. Asparagine synthetase and asparaginase gene expression in the endosperm responded in the opposite way. Genes encoding regulatory protein kinases, SnRK1 and GCN2, both implicated in regulating asparagine synthetase gene expression, also responded to sulphur deficiency. Genes encoding bZIP transcription factors, including Opaque2/bZIP9, SPA/bZIP25 and BLZ1/OHP1/bZIP63, all of which contain SnRK1 target sites, were also expressed. Homeologues of many genes showed differential expression patterns and responses, including TaASN2 . Conclusions Data on the genetic control of free asparagine accumulation in wheat grain and its response to sulphur supply showed grain asparagine levels to be determined in the embryo, and identified genes encoding signalling and metabolic proteins involved in asparagine metabolism that respond to sulphur availability. Electronic supplementary material The online version of this article (10.1186/s12864-019-5991-8) contains supplementary material, which is available to authorized users.

Cereal asparagine synthetase genes

“…BdASN1 the most highly expressed in the embryo by mid-development, with expression in the embryo much higher than in the endosperm. This pattern of expression is similar to that of bread wheat (Curtis et al, 2019;Gao et al, 2016) McCormick et al, 2018). Three asparagine synthetase genes were identified, one on chromosome 1, SORBI_3001G406800; one on chromosome 5, SORBI_3005G003200; and one on chromosome 10, SORBI_3010G110000 (Table 4).…”

“…The analysis confirmed a previous observation (Xu et al, 2018) that variety Chinese Spring lacks a B genome TaASN2 gene. Analysis of variety Spark and a doubled haploid line, SR3, from a Spark × Rialto mapping population also found no evidence of expression of a B genome TaASN2 gene (Curtis et al, 2019), suggesting that the gene might be missing in those genotypes as well. However, the variety Cadenza genome data, also available from EnsemblPlants, do contain a B genome TaASN2 gene, so the absence of the B genome TaASN2 gene is not universal in modern bread wheat genotypes.…”

Genome-wide identification of sucrose nonfermenting-1-related protein kinase (SnRK) genes in barley and RNA-seq analyses of their expression in response to ABA treatment

“…SnRK1 genes of cereals have been subdivided into SnRK1a and SnRK1b according to their expression patterns, with SnRK1a expressed in multiple tissues and SnRK1b expressed predominantly in the endosperm [1,18,19]. Curtis et al [16] then identi ed a subclass similar to SnRK1b that was expressed in the embryo as well as the endosperm and named it SnRK1b*. These three subclasses of HvSnRK1s could be readily distinguished in the phylogenetic analysis in this study.…”

“…A second route is via asparagine synthetase gene expression, which has been shown to be regulated by SnRK1 in Arabidopsis (Arabidopsis thaliana) [6]. In wheat (Triticum aestivum), asparagine synthetase gene expression in the developing embryo increases in response to sulphur de ciency, and SnRK1 has been implicated in regulating this response [16,17].…”

“…Cereals contain a class of SnRK1 genes that are expressed endosperm-speci cally [18,19] and these have been called SnRK1b [1]. A third class that is closely related to SnRK1b but is expressed in the embryo as well as the endosperm has recently been identi ed and called SnRK1b* [16]. It was this class that was suggested to be regulating asparagine synthetase gene expression in the wheat embryo [16].…”