RXLR effector gene Avr3a from Phytophthora sojae is recognized by Rps8 in soybean

The use of resistance genes in elite soybean cultivars is one of the most widely used methods to manage Phytophthora sojae. This method relies on effector-triggered immunity, where a Resistant to P. sojae (Rps) gene product from the plant recognizes a specific effector from the pathogen, encoded by an avirulence (Avr) gene. Many Avr genes from P. sojae have been identified in the last decade, allowing a better exploitation of this type of resistance. The objective of the present study was to identify the Avr gene triggering immunity derived from the soybean resistance gene Rps8. The analysis of a segregating F₂ progeny coupled with a genotyping-by-sequencing approach led to the identification of a putative Avr8 locus. The investigation of this locus using whole-genome sequencing data from 31 isolates of P. sojae identified Avr3a as the likely candidate for Avr8. Long-read sequencing also revealed that P. sojae isolates can carry up to five copies of the Avr3a gene, compared to the four previously reported. Haplotype and transcriptional analyses showed that amino acid changes and absence of Avr3a transcripts from P. sojae isolates caused changes in virulence towards Rps8. Functional analyses using CRISPR/Cas9 knockout and constitutive expression demonstrated that Rps8 interacted with Avr3a. We also showed that a specific allele of Avr3a is recognized by Rps3a but not Rps8. While Rps3a and Rps8 have been previously described as closely linked, this is the first report of a clear distinction hitherto undefined between these two resistance genes.