Oomycetes manipulate plant innate immunity through galacturonide oxidases

Phytophthora infestans is a damaging crop pathogen and a model oomycete for studying plant-pathogen interactions. We report the functional characterisation of a group of P. infestans berberine bridge enzyme-like proteins (BBEs) and their role in plant infection. We demonstrate that BBE-encoding genes are upregulated early during infection and that the secreted enzymes specifically oxidise fragments of pectin, the most abundant charged polysaccharide in the plant cell wall. We further show that these enzymes preferentially oxidise longer pectin fragments, which evade detection by the plant and fail to trigger reactive oxygen species (ROS) signalling. Microscopy revealed that the most abundant P. infestans BBE localises at germ tube tips prior to leaf penetration, and at haustoria during early infection. Combined with the reduced infection observed upon silencing of the encoding genes, these findings point to a key role for this enzyme class in host penetration and colonisation by microbial pathogens. The identification of BBEs as oomycete pathogenicity factors opens new opportunities for crop protection and food security.