Host-Driven Selection, Revealed by Comparative Analysis of <i>Xanthomonas</i> Type III Secretion Effectoromes, Unveils Novel Recognized Effectors

<i>Xanthomonas</i> species are specialized plant pathogens, often exhibiting a narrow host range. They rely on the translocation of effector proteins through the type III secretion system to colonize their respective hosts. The effector arsenal varies among <i>Xanthomonas</i> spp., typically displaying species-specific compositions. This species-specific effector composition, collectively termed the effectorome, is thought to influence host specialization. We determined the plant host-derived effectoromes of more than 300 deposited genomes of <i>Xanthomonas</i> species associated with either <i>Solanaceae</i> or <i>Brassicaceae</i> hosts. Comparative analyses revealed clear species-specific effectorome signatures. However, <i>Solanaceae</i> or <i>Brassicaceae</i> host-associated effectorome signatures were not detected. Nevertheless, host biases in the presence or absence of specific effector classes were observed. To assess whether host-associated effector absence results from selective pressures, we introduced effectors unique to <i>Solanaceae</i> pathogens to <i>X. campestris</i> pv. <i>campestris</i> and effectors unique to <i>Brassicaceae</i> pathogens to <i>X. euvesicatoria</i> pv. <i>euvesicatoria</i> (Xeue) and evaluated if these introductions hindered virulence on their respective hosts. Introducing the effector XopI into <i>X. campestris</i> pv. <i>campestris</i> reduced virulence on white cabbage leaves without affecting localized or systemic colonization. Introducing the XopAC or XopJ5 effectors into Xeue reduced virulence and colonization on tomato but not on pepper. Additionally, XopAC and XopJ5 induced a hypersensitive response on tomato leaves when delivered by Xeue or through <i>Agrobacterium</i>-mediated transient expression, confirming recognition in tomato. This study demonstrates the role of host-derived selection in establishing species-specific effectoromes, identifying XopAC and XopJ5 as recognized effectors in tomato.