Unculturable bacteria exploit a secretory protein to antagonize insect melanization for persistent infection

Phloem-inhabiting unculturable bacterial pathogens are persistently transmitted by insect vectors. However, how they evade insect immune responses to ensure persistent transmission remains unknown. The important melanization immune response in insects is triggered by cleavage of prophenoloxidase (PPO) into active phenoloxidase (PO) via clip-domain serine proteases (CLIPs). 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. A <i>C</i>Las-encoded secretory protein, SDE3230, directly interacts with PGRP and suppresses its activity in transducing this signaling cascade. CLIP4 cleaves PPO between arginine 125 and methionine 126 residues to active PO to induce melanization, and this cleavage pattern in psyllid is distinct from other insects. However, SDE3230 competitively binds to this cleavage site of PPO with CLIP4, thereby suppressing PPO effective cleavage. Collectively, these findings reveal the dual role of SDE3230 in facilitating the mild melanization response, benefitting persistent <i>C</i>Las infection and insect fitness.IMPORTANCEPsyllid-borne huanglongbing is the most destructive citrus disease worldwide, causing billions of dollars in annual production losses and threatening the entire citrus industry. Currently, the mechanism by which the causal agent <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las) antagonizes psyllid innate immune responses to facilitate its coexistence with psyllid vectors is still unknown. Here, we report that <i>C</i>Las exploits the highly expressed secretory protein SDE3230 in psyllids to suppress the important melanization immune response in hemolymph via inhibiting the pattern recognition receptor PGRP activity and the cleavage of prophenoloxidase into active phenoloxidase by clip-domain serine proteases. The pattern of PPO cleavage is novel, and this process ultimately ensures persistent <i>C</i>Las infection and insect fitness. Our findings provide insights into how <i>C</i>Las has evolved novel strategies to evade the insect melanization response, thereby facilitating persistent <i>C</i>Las transmission.