Natural allelic variations provide insights into host adaptation of Phytophthora avirulence effector PsAvr3c

Filamentous pathogens, such as fungi and oomycetes, secrete avirulence (AVR) effectors that trigger plant immune responses and provide striking examples of host adaptations. Avr effector genes display different types of allelic variations, including deletions, epigenetic silencing and sequence polymorphisms, to avoid detection. However, how effector sequence polymorphisms enable pathogens to dodge host immune surveillance remains largely unknown. PsAvr3c is a Phytophthora AVR gene that is recognized by soybean carrying Rps3c. PsAvr3c natural alleles display a rich diversity of single nucleotide polymorphisms in field isolates. We combined both site-directed mutagenesis and population sequence surveys to identify a serine substitution of glycine at position 174 in PsAvr3c that resulted in evasion of Rps3c-mediated soybean immunity. The S174G substitution did not affect the nuclear localization of PsAvr3c in planta, which is required to activate Rps3c, but it significantly impaired the binding affinity of PsAvr3c with a previously identified spliceosome-associated protein GmSKRPs. Silencing GmSKRPs specifically impaired PsAvr3c-triggered cell death in Rps3c soybean. This study uncovered a plant Phytophthora pathogen that adapted to a resistant plant through a key amino acid mutation and subsequently reduced the binding affinity with a plant immune regulator to evade host resistance.