Transcriptomics in grain of high and low asparagine wheat genotypes in response to sulphur availability

RNA-seq data was acquired for the embryo and endosperm of two related 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 of the study was to investigate further the genetic control of asparagine accumulation and turnover in wheat grain, and genes previously shown to be part of the network of asparagine metabolism and its regulation were the focus of the study. There were clear and significant differences in gene expression patterns between the genotypes. Sulphur responses were greater at 21 dpa than 14 dpa, and much more evident in SR3 than Spark. TaASN2 was confirmed to be 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 (14 dpa). There was also a trend for genes encoding enzymes of nitrogen assimilation to be more highly expressed in Spark than SR3 when sulphur was supplied. Genes encoding both nitrate reductase and nitrite reductase were expressed, showing that nitrate must be imported into the grain. 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, falling and rising, respectively, in response to sulphur deficiency. The expression of genes encoding regulatory protein kinases, SnRK1 and GCN2, both of which have been 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 present in the dataset, providing a possible mechanism through which SnRK1 could regulate TaASN1 and TaASN2 expression. The homeologues of many genes showed differential expression patterns and responses, including TaASN2, the A genome homeologue of which was expressed at much higher levels than the D genome homeologue, while the 3B homeologue was missing altogether .