Regulation of insect-induced plant defenses by competition signals in the light environment

Plants have evolved sophisticated mechanisms to deal with competitors and consumers. Plants use specific photoreceptors to obtain information about the proximity of neighboring plants and the intensity of future competition. The phytochromes can detect far-red (FR) radiation reflected by neighbors and elicit rapid shade-avoidance response before the plants are actually shaded by competitors. Plants can also detect mechanical damage and specific herbivore elicitors, and respond with an array of defensive mechanisms (induced defenses). We have shown recently that exposure of non-shaded plants to reflected FR can inhibit the expression of induced defenses. We have now carried out a series of experiments using an Argentine native species of the genus Nicotiana and the model plant Arabidopsis thaliana to elucidate the mechanistic basis of this inhibition. Plants responded to insect herbivory by inducing a variety of direct defenses (phenolic compounds and proteinase inhibors). This induction was the result of a clear, rapid up-regulation of several defense-related genes in response to insect attack. Plant exposed to reflected FR failed to show this induction of defense genes and were more vulnerable to attack by a variety of insects. Analysis of gene expression data suggests that FR down-regulates the jasmonic-acid signaling pathway, thereby weakening the expression of a broad spectrum of anti-herbivore responses. Our results indicate that the light quality shifts that signal the proximity of neighboring individuals induce functional changes in the plants that affect their ability to deal with herbivorous insects, and that these changes take place well before there is real competition for resources. These results have important implications for the design of cropping strategies that take advantage of natural plant defenses and are less dependent of the use of synthetic pesticides.