Silencing of the SAMDC gene decreases resistance of tomato to Cladosporium fulvum

S-adenosylmethionine decarboxylase (SAMDC) is a key enzyme in polyamine (PA) biosynthesis, and overexpression of SAMDC in plants can increase the PA level and enhance tolerance to multiple abiotic and biotic stresses. In our previous study, this gene was significantly up-regulated during Cladosporium fulvum resistance gene (Cf)/Avirulence (Avr) interaction in tomato. In this study, to further understand the role that the SAMDC gene plays in Cf/Avr interaction, the virus-induced gene silencing (VIGS) method was applied to down-regulate SAMDC expression in CGN18423, a line carrying the Cf-19 gene for resistance to C. fulvum. Then, these plants were infected with C. fulvum to analyze changes in resistance. The silencing of SAMDC was confirmed by quantitative real-time PCR (qRT-PCR) analysis and PA quantification. The resistance levels of SAMDC-silenced plants were lower than those of untreated CGN18423 plants. No visible signs of infection were observed on the leaves of untreated CGN18423 plants, while some small chlorotic spots were observed on parts of the leaf margins of SAMDC-silenced plants, and hyphae inside the chlorotic spots were observed late in the hypersensitive response (HR) process. These changes suggested that silencing of the SAMDC gene affected the process of Cf/Avr interaction and decreased the resistance of CGN18423 to C. fulvum.