Allele-specific recognition of the MAX effector AvrRvi6 by Rvi6 resistance protein in the apple–Venturia inaequalis pathosystem

Apple scab caused by the pathogenic fungi Venturia inaequalis is the most common disease in apple orchards. Resistance genes such as Rvi6 have been introduced in apple varieties to reduce the use of fungicides in the management strategies of this disease. However, in the last 20 years, we observed a breakdown of Rvi6 resistance and the apparition of V. inaequalis virulent stains on trees carrying Rvi6. Using comparative genomic analysis, we identified AvrRvi6, the first fungal avirulent gene responsible for a gene-for-gene interaction in the Apple–V. inaequalis pathosystem. AvrRvi6 codes for a small secreted protein with structure similar to the MAX effector family. On the plant side, Rvi6 is a receptor-like protein (RLP), similar to RXEG1 and predicted to localize at the plasma membrane. We observed that fungal virulent strains have mutations in the coding or regulatory sequences of AvrRvi6 which we hypothesize are responsible for the escape of Rvi6­ immunity. Using transient expression in Nicotiana benthamiana we were able to replicate the recognition of AvrRvi6 by Rvi6 resulting in a strong cell death phenotype. This N. benthamiana assay allowed us to test the AvrRvi6 alleles and identify essential residues for Rvi6 recognition. We also investigated the protein-protein interactions between AvrRvi6 and Rvi6 and the subcellular localization of this allele-specific immune recognition.