Abstract
Cyanogenic glucosides are nitrogen-containing specialized metabolites that provide chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide. It has been suggested that cyanogenic glucosides are also a store of nitrogen that can be remobilized for general metabolism via a previously unknown pathway. Here we reveal a recycling pathway for the cyanogenic glucoside dhurrin in sorghum (Sorghum bicolor) that avoids hydrogen cyanide formation. As demonstrated invitro, the pathway proceeds via spontaneous formation of a dhurrin-derived glutathione conjugate, which undergoes reductive cleavage by glutathione transferases of the plant-specific lambda class (GSTLs) to produce p-hydroxyphenyl acetonitrile. This is further metabolized to p-hydroxyphenylacetic acid and free ammonia by nitrilases, and then glucosylated to form p-glucosyloxyphenylacetic acid. Two of the four GSTLs in sorghum exhibited high stereospecific catalytic activity towards the glutathione conjugate, and form a subclade in a phylogenetic tree of GSTLs in higher plants. The expression of the corresponding two GSTLs co-localized with expression of the genes encoding the p-hydroxyphenyl acetonitrile-metabolizing nitrilases at the cellular level. The elucidation of this pathway places GSTs as key players in a remarkable scheme for metabolic plasticity allowing plants to reverse the resource flow between general and specialized metabolism in actively growing tissue.
Highlights
Plants produce a range of specialized metabolites to fend off herbivores and pests, to attract pollinators and adapt to a changing environment
While the specific number of days that elapsed between sowing and maximal dhurrin content varied between experiments, in all cases the data confirmed that maximal
This variation in dhurrin content indicates the onset of a change in the relative rates of synthesis vs. metabolism of dhurrin, with the differences
Summary
Glutathione transferase enzymes (GSTs) are highly abundant in plants, but only very few have been assigned a physiological function. We demonstrate that GSTs are key players in a pathway for endogenous recycling of a cyanogenic glucoside defense compound in sorghum, revealing that plants continuously optimize, and can even reverse, their allocation of resources between general and specialized metabolism
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call