Protease Dynamics: The Diversity and Plasticity in the Proteases of Insect Pest <i>Chilo partellus</i> in Response to <i>Sorghum bicolor</i> Genotypes

The insect pest Chilo partellus (C. Swinhoe 1885), commonly known as the spotted stem borer, poses a significant threat to Sorghum bicolor (L. Moench 1794), a major cereal crop. This study investigates the complex interaction between C. partellus and different sorghum genotypes, with a focus on the contrasting effects on larval mass and mortality when reared on resistant versus susceptible sorghum varieties. Notably, larvae fed on susceptible sorghum exhibited enhanced growth performance. Our analysis revealed qualitative and quantitative differences in digestive protease expression in response to host plant genotype, with the highest overall protease activity observed in larvae fed on resistant sorghum. Furthermore, protease isoforms demonstrated distinct responses to synthetic non-host proteinaceous protease inhibitors, indicating variable inhibitor sensitivities. Transcriptomic analysis revealed a diverse array of hydrolase and protease genes expressed in C. partellus larvae reared on natural sorghum. The expression profiles of the newly identified chymotrypsins (CpaChy1-4) varied according to the resistance status of the host sorghum genotype. Phylogenetic analysis positioned these novel CpaChys within a clade closely related to chymotrypsins identified in Ostrinia nubilalis. In silico binding studies suggested that CpaChy1 and CpaChy2 are less likely to interact with sorghum-derived serine protease inhibitors compared to the other identified chymotrypsins. Unraveling the molecular mechanisms underlying these diet-specific protease adaptations in C. partellus is critical for advancing our understanding of the insect's ability to adapt to different host plant defenses.