Immune response and survival of Circulifer haematoceps to Spiroplasma citri infection requires expression of the gene hexamerin

Spiroplasma citri is a cell wall-less bacterium that infects plants. It is transmitted by the leafhopper Circulifer haematoceps, which hosts this bacterium in the haemocel and insect tissues. Bacterial factors involved in spiroplasma colonization of the insect host have been identified, but the immune response of the leafhopper to S. citri infection remains unknown. In this study, we showed that C. haematoceps activates both humoral and cellular immune responses when challenged with bacteria. When infected by S. citri, C. haematoceps displayed a specific immune response, evidenced by activation of phagocytosis and upregulation of a gene encoding the protein hexamerin. S. citri infection also resulted in decreased phenoloxidase-like activity. Inhibition of hexamerin by RNA interference resulted in a significant reduction in phenoloxidase-like activity and increased mortality of infected leafhoppers. Therefore, the gene hexamerin is involved in S. citri control by interfering with insect phenoloxidase activity. •Spiroplasma citri induced a specific immune response compared to Escherichia coli and Micrococcus luteus.•Activation of phagocytosis is observed in Circulifer haematoceps after S. citri infection.•S. citri promotes a continuous phenoloxydase activity decrease in insect hemolymph.•We succeed in using RNA interference to inhibit the transcription of hexamerin gene.•Hexamerin is required for full PO activity and insect surviving to S. citri infection.