Chitin Oligomers from Alternaria alternata Induce Activation of Signal Transduction Pathways by Ethylene, Jasmonic Acid, and Salicylic Acid in Solanum lycopersicum Fruits

Tomato is among the most widely traded and consumed vegetables throughout the world; however, it is highly vulnerable to infection by the fungus Alternaria alternata. Fungal elicitors such as chitin oligomers have been shown to trigger the plant’s immune response, protecting the plant against pathogen attacks. Signaling molecules such as ethylene (Et), jasmonic acid (JA), and salicylic acid (SA) are key players in this immune response; however, it is unknown whether fungal chitin oligomers induce the production of these molecules. This study aimed to assess the effect of chitin oligomers isolated from the biomass of the A. alternata on the production of Et, JA, and SA in tomato fruits, as well as the expression of genes encoding transcription factors related with the signaling of Et (SlERF1), JA (SlMYC2), and SA (SlWRKY31). Low-molecular weight chitin oligomers were obtained from A. alternata. The results showed that SlMYC2 involved in JA signaling and production was the first gene induced by chitin oligomers 0.5 h post treatment. Furthermore, after 6 h, a second increase in gene expression was observed. However, SlERF1 involved in Et signaling increased 1 h post treatment and was highly correlated with high expression levels of the SlMYC2 gene, suggesting a strong relationship between Et and JA signaling. The most significant increase in gene expression was observed in SlWRKY31 involved in SA signaling 6 h post treatment with chitin oligomers, which showed a high correlation with Et production. It is concluded that the chitin oligomers of A. alternata elicit an early response in the production of Et, JA, and SA in tomato fruit, which play an important role as signaling molecules in the activation of plant defense mechanisms.