Methionine-Rich Storage Protein 1 Is Essential for Methionine Homeostasis and Larval Growth in Helicoverpa armigera

Proteins are essential for insect development, with methionine (Met) being crucial for protein synthesis. However, the roles of methionine and the methionine-rich storage protein 1 (HaSP1) in Helicoverpa armigera larvae remain unclear. In this study, casein removal (-casein) from the larval diet was employed to simulate protein deficiency. Compared with the normal diet control, the -casein group displayed significant reductions in larval body weight and pupation rate, along with markedly increased mortality. Additionally, the methionine content in the fat body and the expression of HaSP1 were both significantly reduced. During larval development, both methionine levels and HaSP1 expression peaked in the late final instar stage. Both 20-hydroxyecdysone (20E) and juvenile hormone (JH) upregulated HaSP1 in midgut. Further RNA interference (RNAi) experiments indicated that knockdown of HaSP1 resulted in decreased larval body weight, increased mortality, and reduced Met content than the Green Fluorescent Protein (dsGFP) control. Furthermore, Methoprene-tolerant 1 (HaMet1) and Hormone Receptor 3 (HaHR3) expression was significantly downregulated upon HaSP1 knockdown. This study revealed that Met was indispensable for larval development and that HaSP1 governed systemic Met levels, thereby playing a pivotal role in larval growth. The findings provide experimental evidence and a theoretical basis for developing green pest-control pesticides that target key genes in nutrient metabolism.