Extrafloral nectar production of the ant-associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid

Plant species in at least 66 families produce extrafloral nectar (EFN) on t heir leaves or shoots and therewith attract predators and parasitoids, such as ants and wasps, which in turn defend them against herbivores. We invest igated whether EFN secretion is induced by herbivory and/or artificial dama ge, and thus can be regarded as an induced defensive response. In addition, we studied the underlying signaling pathway. EFN secretion by field-grown Macaranga tanarius increased after herbivory, artificial leaf damage, and e xogenous jasmonic acid (JA) application. Artificial damage strongly enhance d endogenous JA concentrations. The response in EFN production to artificia l damage was much less pronounced in those leaves that were treated with ph enidone to inhibit endogenous JA synthesis. Quantitative dose-response rela tions were found between the increase in nectar production and both the int ensity of leaf damage and the amounts of exogenously applied JA. The amount of endogenously produced JA was positively correlated with the intensity o f leaf damage. Increased numbers of defending insects and decreased numbers of herbivores were observed on leaves after inducing EFN production by exo genous JA treatment. Over 6 weeks, repeatedly applied JA or artificial dama ge resulted in a ten-fold reduction in herbivory. These results demonstrate that EFN production represents an alternative mechanism for induced, indir ect defensive plant responses that are mediated via the octadecanoid signal transduction cascade.