Protease Inhibitors: an Induced Plant Defense Mechanism Against Herbivores

Plants have evolved with various effective mechanisms to defend themselves against herbivores, and one such mechanism is the production of protease inhibitors that disrupt digestion of the insects. Plant protease inhibitors (PPIs) interact with the target proteases of the insect gut, forming an enzyme–inhibitor complex that impairs normal enzymatic activity, leading to indigestion, reduced food intake, retarded growth, reproduction abnormalities, and sometimes mortality of the insect. Insects responses to these anti-nutritional compounds through remarkable physiological adaptations rendering this vast array of PPIs ineffective. Several molecular and biotechnological tools enable us to modify the expression of desired genes. CRISPR-Cas tools could cause complete knock-out of the gene, and RNA interference can cause gene knock-down. Transgenic plants overexpressing highly efficient PI genes are being developed for the management of crop pests. Although few, the reports of marginal detrimental effects of PPI-transgenic plants on non-target beneficial arthropods should be taken into consideration. Research suggests that insect-gut-associated microbes can affect insect protease levels or elicit plant defense responses and play a significant role in changing the dynamics of plant–insect interaction. Knowledge of protease gene regulation in herbivore insects can be used to deploy PPIs for the development of effective and alternate pest management strategies.