Old poisons, new signaling molecules: the case of HCN.

The high phenotypic plasticity developed by plants includes rapid responses and adaptations to aggressive or changing environments. To achieve this, they evolved extremely efficient mechanisms of signaling mediated by a wide range of molecules, including small signal molecules. Among them, hydrogen cyanide has been largely ignored due to its toxic characteristics. However, not only is it present in living organisms, but it has been shown that it serves several functions in all kingdoms of life. Research using model plants has changed the traditional point of view, and it has been demonstrated that hydrogen cyanide plays a positive role in the plant response to pathogens independently of its toxicity. Indeed, hydrogen cyanide induces a response aimed at protecting the plant from pathogen attack, and the hydrogen cyanide is provided either exogenously (in vitro or by some cyanogenic bacteria species present in the rhizosphere), or endogenously (in reactions involving ethylene, camalexin or other cyanide-containing compounds). The contribution of different mechanisms to HCN function, including a new post-translational modification of cysteines in proteins, S-cyanylation, is discussed here. This work opens an expanding "HCN field" of research related to plants and other organisms.