Enzymatic oxidation of galacturonides from pectin breakdown contributes to stealth infection by Oomycota phytopathogens

Phytophthora phytopathogens from Oomycota cause devastating crop losses and threaten food security. However, Phytophthora secreted proteins that interact with plant-hosts remain underexplored. Here, auxiliary activity family 7 (AA7) enzymes from Ascomycota and Oomycota phytopathogens were shown to oxidise pectin-derived galacturonic acid and/or oligogalacturonides (OGs). Unique mono-cysteinyl-FAD oxidases with positively-charged active sites, suited to oxidise OGs, were discovered in Phytophthora sojae. The P. sojae OG oxidase genes, prevalent in this genus, were co-transcribed with pectin-degradation counterparts during early infection of soybean. Single OG oxidase knockouts significantly decreased P. sojae biomass in planta, potentially linking OG oxidases to virulence. We propose that oxidation by AA7 enzymes impairs the elicitor activity of OGs, potentially contributing to stealth Oomycota infection. Oxidation of OGs unravels a previously unknown microbial mechanism that contributes to evade plant immune-response against pathogens. Our findings highlight a unique oxidase architecture and hitherto unexplored targets for bioprotection from major plant pathogens.