Pto evolved from malectin-like RLKs phosphorylates AvrPtoB to promote Prf-mediated immunity in Solanum pimpinellifolium

Bacterial speck, caused by Pseudomonas syringae pv. tomato (Pst), is a devastating disease of tomato that severely limits global tomato productivity. Understanding the molecular mechanisms underlying Pst and tomato is essential for developing disease resistant varieties. Here, we demonstrate that Pto, the first disease-resistance gene conferring recognition of a specific pathogen, phosphorylates Pst type III effector AvrPtoB at serine 335 site. This post-translational modification triggers the dissociation of the Prf immune complex, enhancing immune signaling and reducing bacterial pathogenicity. Furthermore, evolutionary analyses indicate that Pto-associated proteins originated from malectin-like receptor kinases (MLRs) through loss of the extracellular domain. Crucially, we identified two key amino acid substitutions, Arg158 and Glu258 in Pto, which replace the ancestral lysine residues in MLRs (SpHREK1-1, SpHERK1-2 and SpHERK1-3). These substitutions stabilize Pto by preventing degradation mediated by AvrPtoB's E3 ubiquitin ligase activity. Our findings reveal a novel mechanism, by which Pto phosphorylates a bacterial effector to trigger enhanced immunity and elucidate the key evolutionary adaptations that have shaped Pto into a stable resistance protein.