Fungal Alkaloids Mediate Defense Against Bruchid Beetles in Field Populations of an Arborescent Ipomoea

Abstract

Many Convolvulaceae species harbor heritable fungal endophytes from which alkaloids are translocated to reproductive tissues of the plant host. Evidence for the distribution and ecological role of these fungal alkaloids, however, is lacking or incomplete for many host species and growth forms. Here we report on the quantities of alkaloids present in the leaves and seeds of the arborescent morning glory, Ipomoea murucoides (Convolvulaceae). Young leaf samples taken from the wild harbored one of two fungal taxa. Seeds had higher concentrations of the indolizidine alkaloid swainsonine than leaves. Additionally, seeds from trees harboring Ceramothyrium (Chaetothyriales) fungi exhibited less bruchid damage and had higher concentrations of swainsonine than seeds from trees harboring Truncatella (Xylariales) fungi. Five sesquiterpenes were detected in the leaf trichomes of both types of trees. The seed content of the nortropane alkaloids, tropine and tropinone, did not differ significantly among the two fungal symbionts. Overall, our field data support the defensive-symbiosis hypothesis for swainsonine as proposed by Clay (2014) where the fungal partner supplies chemical defenses to the host. It is likely that the host allocates the defensive chemicals from leaves to seeds, protecting them from seed predators such as bruchid beetles.