The AVR2–SIX5 gene pair is required to activate I-2-mediated immunity in tomato

Plant-invading microbes betray their presence to a plant by exposure of antigenic molecules such as small, secreted proteins called ‘effectors’. In Fusarium oxysporum f. sp. lycopersici (Fol) we identified a pair of effector gene candidates, AVR2-SIX5, whose expression is controlled by a shared promoter. The pathogenicity of AVR2 and SIX5 Fol knockouts was assessed on susceptible and resistant tomato (Solanum lycopersicum) plants carrying I-2. The I-2 NB-LRR protein confers resistance to Fol races carrying AVR2. Like Avr2, Six5 was found to be required for full virulence on susceptible plants. Unexpectedly, each knockout could breach I-2-mediated disease resistance. So whereas Avr2 is sufficient to induce I-2-mediated cell death, Avr2 and Six5 are both required for resistance. Avr2 and Six5 interact in yeast two-hybrid assays as well as in planta. Six5 and Avr2 accumulate in xylem sap of plants infected with the reciprocal knockouts, showing that lack of I-2 activation is not due to a lack of Avr2 accumulation in the SIX5 mutant. The effector repertoire of a pathogen determines its host specificity and its ability to manipulate plant immunity. Our findings challenge an oversimplified interpretation of the gene-for-gene model by showing requirement of two fungal genes for immunity conferred by one resistance gene.