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

Abstract

Plant species in at least 66 families produce extrafloral nectar (EFN) on their leaves or shoots and therewith attract predators and parasitoids, such as ants and wasps, which in turn defend them against herbivores. We investigated whether EFN secretion is induced by herbivory and/or artificial damage, 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 exogenous jasmonic acid (JA) application. Artificial damage strongly enhanced endogenous JA concentrations. The response in EFN production to artificial damage was much less pronounced in those leaves that were treated with phenidone to inhibit endogenous JA synthesis. Quantitative dose-response relations were found between the increase in nectar production and both the intensity of leaf damage and the amounts of exogenously applied JA. The amount of endogenously produced JA was positively correlated with the intensity of leaf damage. Increased numbers of defending insects and decreased numbers of herbivores were observed on leaves after inducing EFN production by exogenous JA treatment. Over 6 weeks, repeatedly applied JA or artificial damage resulted in a ten-fold reduction in herbivory. These results demonstrate that EFN production represents an alternative mechanism for induced, indirect defensive plant responses that are mediated via the octadecanoid signal transduction cascade.

Figure 1

Changes in daily EFN production (relative difference day 1/day 0) are given separately for different treatments (mean + SD). Different letters indicate significant differences between single treatments (for all significant differences: P < 0.05, post hoc test following repeated measures ANOVA with Fisher's least significant difference (LSD); see Tables 1 and 2 for ANOVA results; n = 10 plants for each treatment with four leaves per plant perceiving identical treatment). (A) Experiment on induction of EFN production. (B) Experiment on effects of phenidone treatment.

Figure 2

Induction of endogenous JA after artificial leaf damage. Changes in JA levels (mean ± SD) in leaves that were either wounded artificially by 300 punctures with a needle (diameter 1 mm—this damage corresponded to about 4% missing leaf area and was spread regularly over the entire leaf surface) once at time 0 (filled circles) or remained untreated (open circles). Leaves were harvested after 10, 30, 45, 60, 120, 180, 360, and 600 min (three leaves of different plants per harvest). (Inset) Dependence of JA production on damage intensity measured 30 min after artificial damage. The regression line indicates a saturated dose–response relation (r² = 0.64).

Figure 3

Numbers of insects (mean of 10 plants + SD) counted per day on two census dates (22 September and 1 October 1999: untreated controls, open bars; JA-induced, dark bars). Numbers of defenders (a), visitors (b), herbivores (c), and others (d) were summed for the twelve single counts conducted on each census date for each plant. See Materials and Methods (Effect of JA Treatment on Visiting Insects) for definition of functional insect groups and a description of JA treatment.

Figure 4

Herbivory on leaves of control plants, plants treated with JA, and artificially damaged plants 6 weeks after the beginning of treatment (mean + SD). The percentage of leaf area consumed by chewing herbivores was measured on the youngest six leaves (number 1 representing the youngest leaf) of 15 plants per treatment. Treatments were conducted every 4 days by puncturing leaves (Holes) 100 times with a needle (diameter 1 mm; punctures spread regularly over the entire leaf surface) and spraying 2.5 ml of an aqueous 1-mM solution of JA on the JA leaves. Control plants remained untreated during the entire study period.