ABSTRACT
Of the known plant specialized metabolites, we understand the mechanism of action (i.e., specic receptor-ligand interaction) in only a few cases. For example, on tissue damage by herbivores, the sugar moiety of cyanogenic glucoside is cleaved in sorghum and cassava to release hydrogen cyanide (HCN) (Zagrobelny and Moller 2011). HCN blocks cellular respiration by acting as an inhibitor of cytochrome c oxidase. In another instance, the terpenoid natural product, thapsigargin, from Thapsia garganica binds to sarco/endoplasmic reticulum (ER) Ca2+ ATPase to release calcium ions from the ER lumen to the cytosol, which leads to cell death (Quynh Doan and Christensen 2015). However, in a vast majority of cases, specic physiological roles of specialized metabolites remain unknown. It has been a long-standing debate over whether the majority of plant secondary metabolites have undetermined functions or are mere metabolic wastes without function.
