Many species of morning glories (Convolvulaceae) form symbioses with seed-transmitted Periglandula fungal endosymbionts, which produce ergot alkaloids and may contribute to defensive mutualism. Allocation of seed-borne ergot alkaloids to various tissues of several Ipomoea species has been demonstrated, including roots of I. tricolor. The goal of this study was to determine if infection of I. tricolor by the Periglandula sp. endosymbiont affects Southern root-knot nematode (Meloidogyne incognita) gall formation and host plant biomass. We hypothesized that I. tricolor plants infected by Periglandula (E+) would develop fewer nematode-induced galls compared to non-symbiotic plants (E-). E+ or E- status of plant lines was confirmed by testing methanol extracts from individual seeds for endosymbiont-produced ergot alkaloids. To test the effects of Periglandula on nematode colonization, E+ and E- I. tricolor seedlings were grown in soil infested with high densities of M. incognita nematodes (N+) or no nematodes (N-) for four weeks in the greenhouse before harvesting. After harvest, nematode colonization of roots was visualized microscopically, and total gall number and plant biomass were quantified. Four ergot alkaloids were detected in roots of E+ plants, but no alkaloids were found in E- plants. Gall formation was reduced by 50% in E+ plants compared to E- plants, independent of root biomass. Both N+ plants and E+ plants had significantly reduced biomass compared to N- and E- plants, respectively. These results demonstrate Periglandula's defensive role against biotic enemies, albeit with a potential trade-off with host plant growth.
Keywords: Defensive mutualism; Endosymbiont; Ipomoea tricolor; Meloidogyne incognita; Morning glory; Phytobiome; Plant-fungal interactions; Symbiosis.