Pathogens use effector proteins to manipulate their hosts.During infection of tomato,the fungus Fusarium oxysporum secretes the effectors Avr2 and Six5.Whereas Avr2 suffices to trigger I-2-mediated cell death in heterologous systems,both effectors are required for I-2-mediated disease resistance in tomato.How Six5 participates in triggering resistance is unknown.Using bimolecular fluorescence complementation assays we found that Avr2 and Six5 interact at plasmodesmata.Single-cell transformation revealed that a 2xRFP marker protein and Avr2-GFP only move to neighboring cells in the presence of Six5.Six5 alone does not alter plasmodesmatal transduction as 2xRFP was only translocated in the presence of both effectors.In SIX5-expressing transgenic plants,the distribution of virally expressed Avr2-GFP,and subsequent onset of I-2-mediated cell death,differed from that in wild-type tomato.Taken together,our data show that in the presence of Six5,Avr2 moves from cell to cell,which in susceptible plants contributes to virulence,but in I-2 containing plants induces resistance.