LAUSR

A new concept has arisen regarding two cysteine metabolism-related molecules, hydrogen sulfide and hydrogen cyanide, which are considered toxic but have now been established as signaling molecules. Hydrogen sulfide is produced in chloroplasts through the sulfite reductase activity and in the cytosol and mitochondria by the action of sulfide-generating enzymes and regulates/affects essential plant processes such as plant adaptation, development, photosynthesis, autophagy and stomatal movement, where interplay with other signaling molecules occurs. The mechanism of action of sulfide, which modifies protein cysteine thiols to form persulfides, is related to its chemical features. This posttranslational modification, called persulfidation, could play a protective function for thiols against oxidative damage. Hydrogen cyanide is produced during the biosynthesis of ethylene and camalexin in noncyanogenic plants and is detoxified by the action of sulfur-related enzymes. Cyanide functions include the breaking of seed dormancy, modifying the plant responses to biotic stress, and inhibition of root hair elongation. The mode of action of cyanide is under investigation, although it has recently been demonstrated to perform posttranslational modification of protein cysteine thiols to form thiocyanate, a process called S-cyanylation. Therefore, the signaling roles of sulfide and most probably of cyanide are performed through the modification of specific cysteine residues, thus altering protein functions.