The Phytophthora sojae Avirulence Locus Avr3c Encodes a Multi-Copy RXLR Effector with Sequence Polymorphisms among Pathogen Strains

Root and stem rot disease of soybean is caused by the oomycete Phytophthora sojae. The avirulence (Avr) genes of P. sojae control race-cultivar compatibility. In this study, we identify the P. sojae Avr3c gene and show that it encodes a predicted RXLR effector protein of 220 amino acids. Sequence and transcriptional data were compared for predicted RXLR effectors occurring in the vicinity of Avr4/6, as genetic linkage of Avr3c and Avr4/6 was previously suggested. Mapping of DNA markers in a F(2) population was performed to determine whether selected RXLR effector genes co-segregate with the Avr3c phenotype. The results pointed to one RXLR candidate gene as likely to encode Avr3c. This was verified by testing selected genes by a co-bombardment assay on soybean plants with Rps3c, thus demonstrating functionality and confirming the identity of Avr3c. The Avr3c gene together with eight other predicted genes are part of a repetitive segment of 33.7 kb. Three near-identical copies of this segment occur in a tandem array. In P. sojae strain P6497, two identical copies of Avr3c occur within the repeated segments whereas the third copy of this RXLR effector has diverged in sequence. The Avr3c gene is expressed during the early stages of infection in all P. sojae strains examined. Virulent alleles of Avr3c that differ in amino acid sequence were identified in other strains of P. sojae. Gain of virulence was acquired through mutation and subsequent sequence exchanges between the two copies of Avr3c. The results illustrate the importance of segmental duplications and RXLR effector evolution in the control of race-cultivar compatibility in the P. sojae and soybean interaction.