Involvement of SA, ET and JA signaling pathways in susceptibility of tomato for Fusarium oxysporum

Phytohormones such as salicylic acid (SA), ethylene (ET) and jasmonic acid (JA) play key roles in plant defense following pathogen attack. Involvement of these hormones in plant susceptibility following Fusarium oxysporum (Fo) infection has mostly been studied in Arabidopsis thaliana. However, Fo causes vascular wilt disease in a broad range of crops, including tomato (Solanum lycopersicum). Surprisingly little is known about the involvement of these phytohormones in tomato toward Fusarium oxysporum f. sp. lycopersici (Fol) susceptibility. Here we investigate their involvement by analyzing the expression of ET and SA marker genes, following Fol infection and using bioassays on tomato mutants affected in either hormone production or -perception. Fol inoculation triggered expression of SA and ET marker genes showing activation of these pathways. NahG tomato in which SA is degraded, was hyper-susceptible to Fol infection and exerted stronger disease symptoms than wild-type. In contrast ACD and Never ripe (Nr) mutants in which respectively ET biosynthesis or -perception is impaired, showed decreased disease symptom development and a reduced fungal colonization upon infection. Susceptibility of the def1 tomato mutant, which is deficient in JA biosynthesis, was unaltered, while a prosystemin over-expressing line, showed slightly more disease symptoms concomitantly with increased fungal colonization. The latter is surprising as prosystemin indirectly regulates JA biosynthesis. Our results show that SA is a negative and ET a positive regulator of susceptibility. The SA and ET signaling pathways appear to act synergistically as an intact ET pathway is required for induction of the SA reporter gene and vice versa.