Pathogens secrete small proteins, called effectors, to alter the environment in their host to facilitate infection. The causal agent of Fusarium wilt on tomato, Fusarium oxysporum f. sp. lycopersici (Fol), secretes these proteins in the xylem sap of infected plants and hence they have been called Six (Secreted In Xylem) proteins. Many effectors of diverse pathogens bind to plant proteins, also called effector targets, in order to alter plant processes to their own benefit. The main focus of this thesis is the identification of (putative) effector targets of Six proteins. I show that Fol Six8 interacts with the plant protein Topless and that silencing of this effector target reduces tomato susceptibility to Fol infection. Furthermore, I identify several putative plant targets of Fol Six1 by pull-down experiments and yeast two-hybrid screens. I find that only small Heat Shock Proteins interact with Six1 in both systems. I also show that Six4 interacts with plant Glutamate Decarboxylases (GADs) and that a C-terminal deletion mutant of a tomato GAD (which supposedly is auto-active) suppresses an I-2-mediated hypersensitive response in N. benthamiana, similar to Six4. Finally, I describe the generation of tomato plants expressing Fol SIX1 and Fol SIX4 and show that effector-expressing transgenic tomato lines can be used to investigate resistance responses.
