Abstract
Roots of terrestrial plants host a wide spectrum of soil fungi that form various parasitic, neutral and mutualistic associations. A similar trend is evident in freshwater aquatic plants and plants inhabiting salt marshes or mangroves. Marine vascular plants (seagrasses), by contrast, seem to lack specific root–fungus symbioses. We examined roots of two Mediterranean seagrasses, Posidonia oceanica and Cymodocea nodosa, in the northwestern Mediterranean Sea for fungal colonization using light and scanning and transmission electron microscopy. We found that P. oceanica, but not C. nodosa, is regularly associated with melanized septate hyphae in a manner resembling colonization by the ubiquitous dark septate endophytes (DSE) in roots of most terrestrial plants. P. oceanica roots were found to be colonized by sparse dematiaceous running hyphae as well as dense parenchymatous nets/hyphal sheaths on the root surface, intracellular melanized microsclerotia and occasionally also intra- and intercellular hyphae. The colonization was most prominent in the thick-walled hypodermis of the thinnest healthy looking roots, and the mycobiont seemed to colonize both living and dead host cells. Dark septate hyphae infrequently occurred also inside rhizodermal cells, but never colonized vascular tissues. The biological significance of this overlooked marine symbiosis remains unknown, but its morphology, extent, distribution across the NW Mediterranean Sea and absence in C. nodosa indicate an intriguing relationship between the dominant Mediterranean seagrass and its dark septate root mycobionts.
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Acknowledgments
Financial support was provided by the Czech Science Foundation (GACR P504/10/0781) and the Grant Agency of Charles University in Prague (GAUK 68313/PrF/B-BIO). This study is a part of the long-term research projects of the Institute of Botany ASCR (RVO 67985939) and Faculty of Science, Charles University in Prague (MŠMT LO1417). Lukáš Kalous organized a workshop on marine biology in Borak, Croatia which substantially stimulated our research at its beginning, Kateřina Rylková helped with collection and processing of some Croatian root samples and Jiří Machač took SEM microphotographs and assembled the figures; all these contributions are greatly acknowledged. In Croatia, root samples were collected in accordance with a permit issued by the Croatian Ministry of Environmental and Nature Protection (Klasa UP/I-612-07/13-48/48, Urbroj 517-07-1-1-1-13-2).
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Vohník, M., Borovec, O., Župan, I. et al. Anatomically and morphologically unique dark septate endophytic association in the roots of the Mediterranean endemic seagrass Posidonia oceanica . Mycorrhiza 25, 663–672 (2015). https://doi.org/10.1007/s00572-015-0642-7
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DOI: https://doi.org/10.1007/s00572-015-0642-7