PRB4 and infection: Here, we demonstrate that infection of <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las) in psyllid vectors activates the peptidoglycan recognition protein (PGRP)-CLIP1-CLIP4-PPO-PO signaling cascade to induce a mild melanization response, ensuring persistent <i>C</i>Las infection without causing significant insect fitness costs to the insect.