Untargeted metabolomics reveals PTI-associated metabolites in cultivated tomato

Plants employ a multi-layered innate immune system to detect and fend off invading fungal pathogens. In one such layer, recognition of Pathogen- or Microbe-Associated Molecular Patterns or elicitors, triggers a signaling cascade that leads to defence against the pathogen and ultimately Pattern-Triggered Immunity (PTI). Secondary Metabolites (SMs) are expected to play an important role in this kind of resistance, because they are potentially mycotoxic compounds. Tomato plants inoculated with Alternaria solani show clear symptoms of infection 5 days after inoculation. Whereas plants inoculated with Alternaria alternata remain symptomless. We hypothesized that pattern-triggered induction of resistance-related metabolites in Solanum lycopersicum contribute to the resistance against A. alternata, yet such SMs are suppressed in a compatible interaction. We compared the metabolomic profile (metabolome) of S. lycopersicum at two time points (3 and 24 hours) after treatments with A. alternata, A. solani and the fungal elicitor chitin and identified SMs that are involved in the early defence response of tomato plants. Our study revealed differential metabolome fingerprints and shows that the molecular composition of A. alternata and chitin-induced indeed show larger overlap with each other than with the A. solani-induced metabolome. We identify 65 candidate metabolites possibly associated with pattern-triggered resistance in tomato plants, including the alkaloid, trigonelline, for which we can confirm that it inhibits fungal growth in vitro when supplied at physiological concentrations. Our findings show that a true, pattern-triggered, chemical defence is mounted against A. alternata and that it contains mycotoxin compounds previously unidentified in tomato, that could be interesting for future crop protection strategies.