Non-host Armor Against Insect: Characterization and Application of Capsicum annuum Protease Inhibitors in Developing Insect Tolerant Plants

The non-host plant shows elevated defense response against herbivory attack. Among several defense molecules, protease inhibitors (PIs) are vital and lead to antibiosis effect on the insects upon ingestion. Out of several PI families, Potato Type-II (Pin-II) serves as the first line of defense upon wounding due to insect attack. Several in vivo and transgenic assessments have shown that PIs are effective in imparting insect tolerance. Similar to other Solanaceous plants, Capsicum annuum Pin-II type PIs (CanPIs) exist as a polypeptide, with beads on the string-like arrangement. Upon differential expression across tissues in response to insect attack, these premature CanPIs are processed proteolytically in plants or inside insect gut to produce active inhibitory repeat domains (IRDs). These IRDs might be single and/or multidomain with conserved structural features, whereas the functional variability is attributed to the sequence variation in the reactive center loops of the IRDs. Single- and/or multi-domain CanPIs interact with target proteases through their reactive center loops and inhibit their activity competitively. Inhibition of insect digestive proteases by the CanPI-IRDs leads to impairment in the digestive process, thereby depleting a pool of free amino acids required for insect growth and development. This negative impact of CanPIs/IRDs on insect growth and development has been explored at different levels using multiple approaches. In this chapter, we have provided an overview of the structure–function characteristics of the CanPIs and their utility in combating insect infestation that is being accomplished for the development of insect-tolerant plants.