Abstract
Dhurrin is the most abundant cyanogenic glucoside found in sorghum (Sorghum bicolor) where it plays a key role in chemical defense by releasing toxic hydrogen cyanide upon tissue disruption. Besides this well-established function, there is strong evidence that dhurrin plays additional roles, e.g. as a transport and storage form of nitrogen, released via endogenous recycling pathways. However, knowledge about how, when and why dhurrin is endogenously metabolized is limited. We combined targeted metabolite profiling with matrix-assisted laser desorption/ionization-mass spectrometry imaging to investigate accumulation of dhurrin, its recycling products and key general metabolites in four different sorghum lines during 72 h of grain imbibition, germination and early seedling development, as well as the spatial distribution of these metabolites in two of the lines. Little or no dhurrin or recycling products were present in the dry grain, but their de novo biosynthesis started immediately after water uptake. Dhurrin accumulation increased rapidly within the first 24 h in parallel with an increase in free amino acids, a key event in seed germination. The trajectories and final concentrations of dhurrin, the recycling products and free amino acids reached within the experimental period were dependent on genotype. Matrix-assisted laser desorption/ionization-mass spectrometry imaging demonstrated that dhurrin primarily accumulated in the germinating embryo, confirming its function in protecting the emerging tissue against herbivory. The dhurrin recycling products, however, were mainly located in the scutellum and/or pericarp/seed coat region, suggesting unknown key functions in germination.
Highlights
Dhurrin is the most abundant cyanogenic glucoside found in sorghum (Sorghum bicolor) where it plays a key role in chemical defense by releasing toxic hydrogen cyanide upon tissue disruption
We investigate the spatial distribution of dhurrin and its derivatives in the germinating sorghum grain using mass spectrometry imaging (MSI)
This was included as a unique reference system to confirm the identities of dhurrin and its derivatives detected in the wild type, and to explore their metabolic relationship and possible significance in the germination process
Summary
Dhurrin is the most abundant cyanogenic glucoside found in sorghum (Sorghum bicolor) where it plays a key role in chemical defense by releasing toxic hydrogen cyanide upon tissue disruption. The HCN release is caused by b-glycosidase catalyzed hydrolysis of the CNglcs, initiated by tissue disruption, as for example caused by chewing herbivores (Morant et al, 2008) Besides this well-established function, CNglcs serve roles as storage and transport forms of reduced nitrogen to be reincorporated in general metabolism upon demand (Selmar, 1993; Swain and Poulton, 1994; Picmanová et al, 2015; Nielsen et al, 2016; Del Cueto et al, 2017; Bjarnholt et al, 2018; Blomstedt et al, 2018; Schmidt et al, 2018). Tissue-wise localization of compounds during early germination and seedling development may, lend clues to their significance and biological activity
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