Action of plant defensive enzymes in the insect midgut

Abstract

Insect feeding activates the expression of host plant defensive proteins that exert direct effects on the attacker. In addition to the well-studied proteinase inhibitors, the plant's defensive protein arsenal includes enzymes that disrupt various aspects of insect digestive physiology. In this chapter, we summarize recent studies on isoforms of arginase and threonine deaminase (TD) that degrade the essential amino acids arginine and threonine, respectively, in the alkaline environment of the lepidopteran gut.We also discuss a vegetative storage protein (VSP2) whose phosphatase activity is responsible for potent insecticidal effects on dipteran and coleopteran insects. A common feature of VSP2, TD, and arginase is their wound-inducible expression via the jasmonate signaling pathway. Jasmonate-regulated defensive enzymes may have evolved from pre-existing housekeeping enzymes that catabolize essential nutrients during normal plant development. The application of proteomics-based approaches to identify plant proteins in the digestive tract of phytophagous insects is facilitating the discovery of novel plant proteins with insecticidal activity.