Identification of defence metabolites in tomato plants infected by the bacterial pathogen Pseudomonas syringae

In the present work, we aimed to define the chemical nature of the phenylpropanoids induced during the interaction of the tomato plant, cultivar Rutgers, with the bacterial pathogen Pseudomonas syringae pv. tomato. Using LC–MS and NMR spectroscopy, we determined the chemical structures of this class of metabolites, which accumulated in response to exposure to the pathogen. A rapid accumulation of phytoalexin hydroxycinnamic acid amides (HCAA) of noradrenaline (cis/trans N-p-coumaroylnoradrenaline and cis/trans N-feruloylnoradrenline) and octopamine (cis/trans N-p-coumaroyloctopamine and cis/trans N-feruloyloctopamine), chlorogenic acid (CGA), and the flavonoid rutin was detected after bacterial infiltration. Among them, cis N-p-coumaroylnoradrenaline, cis N-p-coumaroyloctopamine, and isomers cis/trans of N-feruloylnoradrenaline have never been described in the literature. Moreover, cis/trans isomers of N-feruloyloctopamine are identified for the first time as being associated with the tomato/P. syringae interaction. Inhibition of the P. syringae-induced ethylene by 2-aminoethoxyvinylglycine, markedly impaired the accumulation of HCAA in inoculated tomato leaves, but it had no effect on CGA or rutin synthesis. On the other hand, the lack of accumulation of SA in NahG transgenic tomato plants, overexpressing a bacterial salicylic hydroxylase, did not prevent the enhancing of HCAA produced by P. syringae infection. Taken together, our results indicate that ethylene, but not SA, is essential for the synthesis of HCAA in response to bacterial infection of tomato leaves. Antibiotic activities of the induced metabolites were also studied. CGA induced a notable expression of the defence-related genes PR1 and P23. Moreover, trans-HCAA of noradrenaline and octopamine showed a potent free radical scavenging competence. In particular, trans N-feruloylnoradrenaline presented a very outstanding antioxidant activity, thus indicating that these compounds may play a role in the defence response of tomato plants against bacterial infection.

• Hydroxycinnamic acid amides, chlorogenic acid and rutin accumulate in tomato leaves upon Pseudomonas syringae infection.
• Ethylene, but not salicylic acid, is essential for the synthesis of hydroxycinnamic acid amides in response to bacterial infection.
• Chlorogenic acid treatment induces the expression of tomato defence-related genes.
• Increased expression of genes from the phenylpropanoid pathway is associated to bacterial infection.
• The novel hydroxycinnamic amide trans N-feruloylnoradrenaline shows an extremely high antioxidant activity.