Mechanisms regulating threonine-induced silk protein synthesis in Bombyx mori (Lepidoptera: Bombycidae) on artificial diet

The economic value of the silkworm (Bombyx mori) is largely determined by the capacity of its silk gland (SG) to synthesize silk proteins. Therefore, a comprehensive understanding of the nutritional regulatory mechanisms governing this process is essential for improving silk yield and quality. Dietary glycine and threonine enhance silk yield in silkworms fed an artificial diet, but threonine has a stronger effect at equimolar concentrations, and the underlying mechanisms of this difference remain unclear. This study demonstrated that dietary supplementation with 0.6% threonine significantly improved silkworm feed efficiency, silk yield, and pupal weight without affecting food consumption or digestibility. Threonine supplementation significantly increased SG DNA content in mid-to-late fifth instars and raised hemolymph levels of glycine and serine, which accounted for 41% and 13% of silk protein, respectively, by 278.32% and 56.81%, respectively. Furthermore, threonine supplementation enhanced the digestion and absorption efficiency of carbohydrates, leading to significant increases in hemolymph glucose, fructose, sorbitol, trehalose, and maltose by 112.57%, 96.45%, 47.76%, 19.69%, and 17.95%, respectively. In summary, dietary threonine supplementation upregulated silk protein synthesis through three synergistic pathways: stimulating DNA replication to elevate SG gene copy number; replenishing precursor amino acids for silk protein synthesis; and enhancing carbohydrate digestion and absorption to provide energy support. These findings provide a scientific basis for optimizing artificial diet formulations.