<i>S</i> ‐Methyl‐ <scp>l</scp> ‐Cysteine Sulfoxide: A Hidden Layer of Defences Against Herbivorous Insects in Brassicaceae

Plants defend themselves against herbivorous insects through diverse morphological and biochemical traits. Non-protein amino acids (NPAAs) are an important component of the plant metabolome, although their defensive function remains largely unexplored. Here, we investigated the role of S-methyl-l-cysteine sulfoxide (SMCSO), a sulphur-containing NPAA accumulated in Brassicaceae, in mediating plant defence against herbivorous insects. SMCSO was quantified in inflorescences, leaves (young and old) and roots (primary and secondary) of 14 Brassicaceae species. Additionally, feeding tests on artificial substrates supplemented with physiological SMCSO levels were conducted with both generalist and specialist herbivorous insects feeding on one of the different plant parts studied. In line with the optimal defence theory, we found higher SMCSO levels in reproductive than vegetative tissues, and in young leaves and primary roots compared to old leaves and secondary roots, respectively. SMCSO also exerted a consistent phagodeterrent effect on generalist herbivores, while specialists showed variable responses from deterrence to stimulation. This study provides the first evidence of the influence of this compound on herbivore feeding behaviour, with effects depending on the diet breadth. These findings broaden our understanding of plant chemical defence complexity and highlight the ecological role of NPAAs in plant-insect interactions.