Fatty acid-derived signals that induce or regulate plant defenses against herbivory

Jasmonic acid and other derivatives of linolenic acid produced in plants by the octadecanoid pathway, as well as the six-carbon fatty acid derivatives called green leaf compounds, play major roles in regulating plant defenses against herbivores. So do also conjugates of linolenic acid with glutamine and glutamate, found in the regurgitant of several lepidopteran larvae, as well as in crickets and Drosophila larvae. In all these cases, the linolenic acid precursor of the regulating compound is produced by the plant on which the herbivore feeds. More recently elicitors of plant volatiles containing a 16-carbon fatty acid moiety with either a saturated chain or a double bond at carbon six in the chain have been isolated and identified from the spit of grasshoppers. The origin of the grasshopper elicitors is not yet known. Also not known is how broadly active the herbivore-produced elicitors are across a range of plant species. Further, the mechanisms by which the green leaf compounds and the elicitors induce defensive reactions in plants are not understood, although it appears that they affect the octadecanoid pathway in different ways.