Biosynthesis of pathogen-induced hydroxylated tryptamine derivatives in barley

Barley (Hordeum vulgare), a major cereal crop, suffers significant yield losses due to pathogen attack. Understanding the chemical defense responses that follow pathogen infection is essential to uncover new resistance targets in barley. In this study, we employed an untargeted metabolomics and mass spectrometry imaging approach to profile the chemical landscape of barley leaves inoculated with the hemibiotrophic fungal pathogen Pyrenophora teres f. teres, the causal agent of net blotch. Pathogen infection triggered a significant chemical defense response, including the induction of many phenylpropanoids and alkaloids. Specifically, a strong induction of tryptophan-derived metabolites was observed, including tryptamine, serotonin, and the novel 2-oxo-tryptamine, which accumulated at infection sites. Integration of transcriptomic-driven pathway discovery with biochemical characterization identified a flavin-containing monooxygenase and a cytochrome P450 (CYP71P10) involved in 2-oxo-tryptamine and serotonin biosynthesis, respectively. Our results show increased metabolic investment toward indolic compounds in barley leaves following infection, and activation of these pathways across different pathogens and cultivars suggests a conserved resistance mechanism. This study provides new insights into the barley defense response, offering new metabolic targets for the development of disease-resistant cereal crops.