SRP gene is required for Helicoverpa armigera prophenoloxidase activation and nodulation response

• Knockdown of SRP repressed the transcriptions of PPO genes in larval hemocytes.
• Knockdown of SRP decreased PO activity in bacteria-challenged larval hemolymph.
• Knockdown of SRP led to reduced nodule formation in bacteria-injected larvae.
• RNAi-treated larvae showed increased bacterial growth in larval hemolymph.
• SRP gene might involve in PPO activation and nodulation response.

SRP gene was first identified from the fall webworm, Hyphantria cunea as one of genes up-regulated after bacteria injection. A rent study in Spodoptera litura showed that stress-induced elevation of SRP expression highly correlates with reduced feeding activities and growth retardation of larvae. In this study, we identified a SRP gene from the cotton bollworm, Helicoverpa armigera, namely Ha-SRP, and studied its precise roles in insect immunity. Expressions of Ha-SRP were upregulated in H. armigera larval hemocytes after injection of Escherichia coli. When the expression of Ha-SRP in H. armigera larval hemocytes was inhibited by dsHa-SRP injection, the transcription of prophenoloxidase genes in hemocytes was repressed, phenoloxidase activity in bacteria-challenged larval hemolymph was significantly decreased, and nodule formation in bacteria-injected larvae was reduced. More importantly, RNAi-treated insects infected with E. coli showed higher bacterial growth in hemolymph compared with infected controls. These results suggest that Ha-SRP gene plays importance roles in H. armigera innate immunity, possibly by mediating prophenoloxidase activation and nodulation response.