The olive pathogen Pseudomonas savastanoi pv. savastanoi NCPPB3335 suppresses the plant defense response through the function of two proteins from the HopAO effector family

P. savastanoi pv. savastanoi NCPPB 3335 is the causal agent of the olive knot disease. The effector repertoire of this olive pathogen includes two members of the HopAO effector family, one of the most diverse T3E (Type III Effector) families of the P. syringae complex. The study described here explores the phylogeny of these dissimilar members, HopAO1 and HopAO2, among the complex and reveals their activities as immune defense suppressors. Although HopAO1 is predominantly encoded by strains isolated from woody organs of woody hosts, both HopAO1 and HopAO2 are phylogenetically clustered according to the woody/herbaceous nature of their host of isolation, suggesting host specialization of the HopAO family across the P. syringae complex. Our data also show that HopAO1 and HopAO2 possess phosphatase activity, a hallmark of the members of this family. Both of them exert an inhibitory effect on early plant defense responses, such as ROS (Reactive Oxygen Species) production and callose deposition, and are able to suppress Effector Trigered Inmune Responses (ETI). Moreover, we demonstrate that a hopAO1 mutant of P. savastanoi NCPBB 3335 exhibits a reduced fitness and virulence in olive plants, which supports the relevance of this effector during the interaction of this strain with its host plants. This work contributes to the field with the first report regarding functional analysis of HopAO homologs encoded by P. syringae or P. savastanoi strains isolated from woody hosts.