Fungal colonization of the rooting system of the early land plantAsteroxylon mackieifrom the 407-Myr-old Rhynie Chert (Scotland, UK)

Abstract

Associations between plants and fungi were an important and varied feature of early terrestrial ecosystems, but in most instances their biological functions remain poorly understood. We document a new species of fungus colonizing the rooting system of the early lycopod Asteroxylon mackiei, based on exceptionally well-preserved fossils from the Rhynie Chert. We investigated historical petrographic thin sections using standard optical microscopy and confocal laser scanning microscopy. Palaeozoosporites renaultii gen. nov., sp. nov. colonized the inner cortex of the plant rooting system. The fungus had an aseptate thallus with isotomous or sympodial branching. The mycelium bore distinctive porate, globose to elongated structures that we interpret as zoosporangia and resting sporangia. Doubts remain over the precise systematic affinity of P. renaultii, which closely resembles chytrids. Whereas most of the Rhynie Chert plants developed symbiotic associations of the mycorrhizal type, it seems that this was not the case for Asteroxylon mackiei, which possessed the most evolved rooting system among the Rhynie Chert plants. We argue that the new root-borne fungus was probably parasitic. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179, 201–213.