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.