Tomato Systemin induces resistance against Plectosphaerella cucumerina in Arabidopsis through the induction of phenolic compounds and priming of tryptophan derivatives.

Phytocytokines are endogenous danger peptides that are actively released after a pest or pathogen attack, triggering an amplification of plant immune responses. Here, we found that Systemin, a peptide from tomato, has a substantial impact at the molecular level in Arabidopsis plants that leads to induced resistance against Plectosphaerella cucumerina. Using transcriptional and metabolomics approaches, and loss-of-function mutants to analyse the molecular mechanisms underlying induced resistance against the necrotroph, we decipher the enhanced molecular responses in Systemin-treated plants following infection. Some protein complexes involved in the response to other damage signals, including the BAK1-BIK1 protein complex and heterotrimeric G proteins, as well as MPK activation, were among the early signalling events triggered by Systemin in Arabidopsis upon infection. Non-targeted analysis of the late responses underlying Systemin-Induced Resistance1 (Sys-IR) showed that phenolic and indolic compounds were the most representative groups in the Systemin metabolic fingerprint. Lack of flavonoids resulted in the impairment of Sys-IR. On the other hand, some indolic compounds showed a priming profile and were also essential for functional Sys-IR. Evidence presented here shows that plants can sense heterologous peptides from other species as danger signals driving the participation of common protein cascades activated in the PTI and promoting enhanced resistance against necrotrophic fungus.