Plants produce a wealth of molecules known as specialized metabolites. They are important for plants in e.g. defense and climate adaptation, and for humans as medicines and food ingredients, and understanding specialized metabolism is therefore important for plant utilization. Cyanogenic glucosides (CNglcs) are well-known defense compounds. However, growing evidence suggests that CNglcs undergo endogenous conversions into compounds with other important biological functions, but little is known about these biochemical processes. Young seedlings of the important cereal crop sorghum produces high amounts of the CNglc dhurrin, but it is absent from the grain. To study the onset and significance of dhurrin biosynthesis and conversions we analyzed germinating sorghum grain by LC-MS (extracts) and by Matrix Assisted Laser Desorption Ionization-Mass Spectrometry Imaging (MALDI-MSI) to determine the spatial distribution of the compounds. Dhurrin concentration increased rapidly during germination where it was localized in the embryonic axis and scutellum. Subsequently, total dhurrin concentration continued to increase, but its localization was restricted to the developing seedling, consistent with its primary role in defense of young tissue. On the other hand, the dhurrin conversion products were accumulated in the scutellum and/or aleurone layer during germination, suggesting different physiological functions for these compounds, e.g. in signaling and nutrient transport. The results were used to guide a spatio-temporal investigation of gene expression by RNAseq. The transcriptome results confirmed that genes encoding the dhurrin biosynthetic pathway were highly expressed in the embryonic axis, whereas genes known to be involved in dhurrin conversions, such as specific glutathione transferases, were expressed in other tissues. Co-expression analyses are used to mine the data for clues towards identifying other genes involved in dhurrin conversions, to provide targets for further scrutiny that will help us understand the significance of CNglc derived compounds in germination.
Coauthors: Christian Janfelt – University of Copenhagen;Huijun Liu – University of Copenhagen;Sara Miller – University of Copenhagen;Lucia Montini – Maastricht University