Plant and fungal cell wall fragments activate expression of proteinase inhibitor genes for plant defense

Plant and fungal cell wall fragments produced by enzymic degradation during pest attacks are hypothesized to be activators of a universal recognition system for locally and systemically activating genes which control the synthesis of plant defense chemicals such as the antibiotic phytoalexins and antinutritive proteinase inhibitors. Proteinase inhibitor cDNAs have been prepared from wound-induced mRNAs, isolated, and characterized. The cDNAs have been utilized to quantify specific proteinase inhibitor mRNAs in leaves following wounding or simulated insect attacks. The cDNAs have also been utilized as hybridization probes to isolate and characterize proteinase inhibitor genes from tomato and potato genomic DNA. Proteinase inhibitor proteins have been induced in tomato leaves by chewing insects and shown to be highly correlated with a systemically mediated reduction in the nutritional quality of the leaves toward the larval noctuidSpodoptera exigua. Thus, the wound-induced proteinase inhibitors, whose genes in tomato leaves can be activated by wounding, insect attacks, and plant and fungal cell wall fragments, can significantly decrease the quality of the leaves for such herbivorous insects. This inducible set of biochemical reactions leading to the de novo biosynthesis of proteinase inhibitors is, therefore, considered to be a potentially important defense of plant leaves that should be considered both in developing general theories on insect-plant interactions and in selecting insect-resistant crop varieties.