Allelic variations in a serine protease effector withinClavibacter michiganensispopulations determine pathogen host range

Abstract

Plant pathogenic bacteria often have a narrow host range, which can vary among different isolates within a population. Here we investigated the host range of the tomato pathogenClavibacter michiganensis(Cm). We determined the genome sequences of 40 tomato Cm isolates and screened them for pathogenicity on tomato and eggplant. Our screen revealed that out of the tested isolates, five were unable to cause disease on any of the hosts, 33 were exclusively pathogenic on tomato, and two were capable of infecting both tomato and eggplant. Through comparative genomic analyses, we identified that the five non-pathogenic isolates lacked thechp/tomApathogenicity island, which has previously been associated with virulence in tomato. In addition, we found that the two eggplant-pathogenic isolates encode a unique allelic variant of the serine proteasechpG(chpGC), an effector that is recognized in eggplant. Introduction ofchpGCinto achpGinactivation mutant in the eggplant-non-pathogenic strain Cm101, failed to complement the mutant, which retained its ability to cause disease in eggplant and failed to elicit hypersensitive response (HR). Conversely, introduction of thechpGvariant from Cm101 into an eggplant pathogenic isolate (CmC48), eliminated its pathogenicity on eggplant, and enabled CmC48 to elicit HR. Our study demonstrates that allelic variation in thechpGeffector gene is a key determinant of host range plasticity within Cm populations.