An antifungal terpenoid defends a neotropical tree (Hymenaea) against attack by fungus-growing ants (Atta)

Abstract

Foragers of the leafcutting ant, Atta cephalotes L. (Formicidae, Attini) seldom or never attack many of the plant species available to them in nature. In the semideciduous forests of lowland Guanacaste Province, Costa Rica, one of the tree species seldom cut is Hymenaea courbaril L. (Leguminosae, Caesalpinioideae). We tested the hypothesis that this species is avoided by the ants because of the presence of ant-repellent secondary compounds in the leaves. A bioassay to test repellency of leaf extracts was developed to guide the chemical isolation of ant repellents, using a laboratory colony of Atta cephalotes.The presence of one or more extractable ant repellents was quickly demonstrated. Subsequent chemical isolation and identification revealed that there was essentially only one terpenoid responsible for the repellency: caryophyllene epoxide. Tests with a concentration series of the pure compound demonstrated that the natural concentration of this terpenoid in Hymenaea could fully account for the observed repellency of intact leaves. Field bioassays of the terpenoid in Costa Rica confirmed this result; leaves of a preferred species, Spondias purpurea L. (Anacardiaceae), became as repellent as Hymenaea leaves when treated with caryophyllene epoxide at natural Hymenaea leaf concentrations. Repellency of the epoxide was 20 times greater than that of caryophyllene, its sesquiterpene hydrocarbon precursor, which is also found in Hymenaea leaves.Attine ants cut leaves to serve as substrate for culturing a specific fungus for food, principally for their larvae. A reasonable hypothesis is that these ants selectively avoid plant species whose leaves contain compounds which are toxic to their fungus. We tested caryophyllene epoxide for antifungal activity and found that it is an extremely potent compound, not only against the attine fungus, but other fungi as well. We speculate that many of the other plant species avoided by these ants in nature may be similarly protected from ant attack by antifungal compounds in their leaves. We further suggest that plant defense against leafcutting ants may be largely an incidental byproduct of selection for fungal resistance in plants.

Identification and comparison of the volatile constituents of fresh and dried leaves ofSpondias mombinfound in North-central Nigeria:in vitroevaluation of their cytotoxic and antioxidant activities

“…Owolabi et al (2013) reported in vitro cytotoxicity activity of (E)-b-caryophellene, caryophyllene oxide and a-humulene obtained from A. muricana leaf oil against MCF-7 cells with LC 50 values of 38.8, 23.0 and 22.1 lg/mL. Also, the role of (E)-b-caryophellene has been demonstrated in flora defence against pathogens and repellent activities against leaf-cutting ants and termites (Hubbell et al 1983;Messer et al 1990;Huang et al 2012). The fresh leaves oils (LC 50 ¼ 0.01 lg/mL) showed more potency than the standardized pesticidal extracts obtained from small twigs of pawpaw tree (LC 50 ¼ 0.04 lg/mL) reported by McLaughlin and Rogers (1998).…”

Volatile sesquiterpenes from fungi: what are they good for?

“…Repellency of the epoxide was 20 times greater than that of caryophyllene, its sesquiterpene hydrocarbon precursor. Caryophyllene epoxide was also tested for antifungal activity and found to be an extremely potent compound against many fungi (Hubbell et al 1983). …”

Field colonies of leaf‐cutting ants select plant materials containing low abundances of endophytic fungi

“…Leaf-cutting ant foraging is complex and a variety of factors other than endophyte content have also been found to influence the foraging preferences of leaf-cutting ants including: leaf toughness (Nichols-Orians and Schultz 1989), secondary compounds (Hubbell et al 1983, Howard 1988, nutrient content (Berish 1986, Howard 1988, Farji-Brener 2001, and epiphylls (Mueller and Wolf-Mueller 1991). To further complicate the matter, many of these variables in leaf tissue are covariates.…”