Sebacinales everywhere: previously overlooked ubiquitous fungal endophytes

Abstract

Inconspicuous basidiomycetes from the order Sebacinales are known to be involved in a puzzling variety of mutualistic plant-fungal symbioses (mycorrhizae), which presumably involve transport of mineral nutrients. Recently a few members of this fungal order not fitting this definition and commonly referred to as 'endophytes' have raised considerable interest by their ability to enhance plant growth and to increase resistance of their host plants against abiotic stress factors and fungal pathogens. Using DNA-based detection and electron microscopy, we show that Sebacinales are not only extremely versatile in their mycorrhizal associations, but are also almost universally present as symptomless endophytes. They occurred in field specimens of bryophytes, pteridophytes and all families of herbaceous angiosperms we investigated, including liverworts, wheat, maize, and the non-mycorrhizal model plant Arabidopsis thaliana. They were present in all habitats we studied on four continents. We even detected these fungi in herbarium specimens originating from pioneering field trips to North Africa in the 1830s/40s. No geographical or host patterns were detected. Our data suggest that the multitude of mycorrhizal interactions in Sebacinales may have arisen from an ancestral endophytic habit by specialization. Considering their proven beneficial influence on plant growth and their ubiquity, endophytic Sebacinales may be a previously unrecognized universal hidden force in plant ecosystems.

Figure 1. Phylogenetic relationships of Sebacinales based on maximum likelihood analysis of partial nuclear-encoded ribosomal large subunit sequences.

Sequences are color-coded by type of symbiosis or origin. Highlighted clades 1-14, including zoomed-in lineages 2, 6, 7 and 11, are explained in the text. Sequences marked with an asterisk (clades 7, 11) are from herbarium specimens collected by G.W. Schimper and T. Kotschy in pioneering field trips in the 1830/40s. The full tree, including all host plants, places of origin, bootstrap values, accession numbers and clades/sequences discussed in the text is shown in Fig. S2, using the same colors and three-letter symbiosis-identifying codes. Red: sequences of endophytes (END), magenta: cavendishioid mycorrhiza (CAV), blue: ericoid mycorrhiza (ERM), dark green: orchid mycorrhiza (ORM), turquoise: jungermannoid mycorrhiza (JMM), bright green: ectomycorrhiza (ECM), brown: soil samples, black: sequences from fruitbodies or cultures. Country codes used here: AUT, Austria; ECU, Ecuador; EGY, Egypt; ETH, Ethiopia; FRA, France; GER, Germany; GBR, Great Britain; ITA, Italy; NAM, Namibia; USA, United States of America.

Figure 2. Anatomy and ultrastructure of a field-collected root sample of wheat (Triticum aestivum) infested with a sebacinalean endophyte.

(A) cross section through the root as seen in the light microscope; singular rhizodermal cells are heavily colonized by fungal hyphae (arrowheads). Bar  = 100 µm. (B) transmission electron micrograph showing that the colonized rhizodermal cell is dead: intracellular fungal hyphae are not surrounded by host plasma membrane; arrowheads point to hyphal septa in cross section showing septal pores. Bar  = 3 µm. (C) dolipore with continuous parenthesome as typical for members of the Sebacinales (arrowhead). Bar  = 200 nm.