A unique mode of parasitism in the conifer coral tree Parasitaxus ustus (Podocarpaceae)

Abstract

Almost all parasitic plants, including more than 3000 species, are angiosperms. The only suggested gymnosperm exception is the New Caledonian conifer, Parasitaxus ustus , which forms a bizarre graft-like attachment to the roots of another conifer Falcatifolium taxoides . Yet, the degree of resource dependence of Parasitaxus on Falcatifolium has remained speculative. Here we show that Parasitaxus is definitively parasitic, but it displays a physiological habit unlike any known angiosperm parasite. Despite possessing chloroplasts, it was found that the burgundy red shoots of Parasitaxus lack significant photosynthetic electron transport. However unlike nonphotosynthetic angiosperm parasites (holoparasites), tissues of Parasitaxus are considerably enriched in 13 carbon relative to its host. In line with anatomical observations of fungal hyphae embedded in the parasite/host union, stable carbon isotopic measurements indicate that carbon transport from the host to Parasitaxus most likely involves a fungal partner. Therefore, Parasitaxus parallels fungusfeeding angiosperms (mycoheterotrophs) that steal carbon from soil mycorrhizal fungi. Yet with its tree-like habit, association with fungi residing within the host union, high stomatal conductance, and low water potential, it is demonstrated that Parasitaxus functions unlike any known angiosperm mycoheterotroph or holoparasite. Parasitaxus appears to present a unique physiological chimera of mistletoe- like water relations and fungal-mediated carbon trafficking from the host.