Cyanogenic glucosides are amino-acid derived specialised metabolites with roles in defence and may be involved in nitrogen management. The cyanogenic glucoside dhurrin is produced in the genus Sorghum, and is an issue in the crop Sorghum bicolor when grown as feed for livestock as it can accumulate dhurrin to toxic levels in young plants, under drought stress and after application of nitrogen fertiliser. While the genes involved in the dhurrin biosynthesis and turnover pathways that enable recovery of the nitrogen in dhurrin are well-studied, regulatory genes controlling these pathways are yet to be characterised.
We explored the transcriptional response of nitrogen-limited S. bicolor to application of high nitrogen-containing solution across a 60-hour timeframe in leaf, stem and root tissues using transcriptomics. We found upregulation of key dhurrin biosynthesis genes, CYP79A1, CYP71E1 and UGT85B1, from 24-hours after the nitrogen surge in leaf tissue, alongside changes in genes involved in dhurrin turnover. Construction of gene regulatory networks for leaf tissue linked dhurrin metabolism genes with transcription factors with known roles in nitrogen responses. The wild species Sorghum macrospermum accumulates low levels of dhurrin compared to the domestic crop. We grew S. macrospermum under similar conditions and compared changes in expression of key genes involved in regulation of dhurrin to S. bicolor.
Our findings provide insight into the regulation of dhurrin and links control of dhurrin metabolism to gene networks involved in nitrogen responses in sorghum. Understanding how crops manage nitrogen and toxic metabolites will help develop of safer and more productive crops.