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Seagrasses form an important part of the marine vegetation and its growth create a habitat for many other organisms. Equal to the terrestrial plants, seagrasses can associate with various fungal species ranging from the parasitic to the endophytic ones. Posidonia oceanica and Thalassodendron ciliatum, whose ontogenetic development are very similar, are considered to be the ones of the best studied seagrass. Seedlings of both species have a primary root densely covered with root hairs, which is later replaced by secondary roots. In the vast majority of cases, the adult P. oceanica and T. ciliatum do not form root hairs on the secondary roots. In the case of P. oceanica, this transition from roots with root hairs to roots lacking root hairs appears to be associated with interaction with endophytic fungi. The incidence of interaction with endophytic fungi is still unclear for T. ciliatum. A summary of existing knowledge of the root hairs and fungal root symbionts of P. oceanica and T. ciliatum has revealed an incomplete knowledge of the effect of endophytic fungi on roots and root hairs formation in P. oceanica and a complete absence of studies of T. ciliatum mycobionts. The summary has further led to the hypothesis of possible coevolution of endophytic fungi and seagrass P. oceanica.

Plant roots host a wide spectrum of endophytic fungi ranging from parasites through neutralistic fungi to mutualistic mycorrhizal fungi. In most of terrestrial ecosystems, these groups of symbiotic fungi are well documented. However, much less is known about fungal endophytes of aquatic plants, especially the only group of submerged marine plants, seagrasses. We focused on roots of the seagrass species Posidonia oceanica which is a Mediterranean endemite. Its roots hosted an abundant presence of endophytic fungi. We examined the roots using optical and electron microscopy in order to study its morphology. We isolated the mycobionts and determined their taxonomic classification. A specific symbiosis of P. oceanica roots and dark coloured septate fungus has been recorded all over a vast area of the Mediterranean spreading from southern Spain to south Turkey. The fungus forms distinctive and typical structures: superficial hyphae occasionally forming hyphal sheaths or dense hyphal nets, intraradical hyphae colonizing extracellular space and, occasionally, primary cortex cells, and finally intracellular microsclerotia. The colonization pattern of the fungi resembles dark septate endophytes (DSE), group of fungi commonly found in roots of both terrestrial and freshwater plants. In our following studies, we...

Seagrasses are the only group of submerged plants that are permanently growing in marine environment. They play an important role in the sea bottom ecosystem. Seagrasses are primary producers capable of accumulation and deposition of carbon. They influence water flow at the sea bottom and form symbioses with variety of organisms. Our knowledge of symbiotic interactions of seagrasses is still limited even though several studies of the topic have been carried out in recent years. Unlike most of terrestrial plants, seagrasses are generally considered as plants that do not form any specific associations with mycorrhizal or endophytic fungi. Surprisingly, we have discovered a novel fungal endophytic association in roots of Mediterranean endemic seagrass species Posidonia oceanica (L.) Delile. Morphology of this symbiotic fungi strongly resembles common symbiotic fungi of terrestrial plants, dark septate endophytes (DSE). We sampled roots of P. oceanica in large area of the Mediterranean from southeastern Spain to Albania and described range and taxonomical classification of the endophyte using microscopy, in vitro cultivation and molecular determination. Roots of P. oceanica in whole area of study are colonized by mere two endophytic fungal species. Over 90 % of the fungal symbionts belong to a single...

Seagrasses are narrow ecological and taxonomic group of plants which evolved from terrestrial ancestors some 100 million years ago. Unlike most terrestrial plants they are traditionally considered as a group not forming mycorrhizal symbioses or specific root endophytic associations. However, this opinion is likely biased by a very low number of studies targeting this group of vascular plants. For example, an anatomically and morphologically unique endophytic fungal symbiosis was recently discovered in the roots of the Mediterranean endemic species Posidonia oceanica, a species that is considered to be one of the best studied. It thus seems that more specialized research can yield similar results, i.e. discoveries of new fungal association in seagrasses. This could have a big impact on our opinions regarding not only mineral nutrition of these fascinating plants. The aims of this thesis therefore are 1) to summarize published information about fungal associations in the roots of seagrasses, 2) to summarize basic knowledge of anatomy and morphology of the roots of individual groups of seagrasses and 3) an attempt to predict seagrass groups where we may more likely assume presence of fungal associations. Special emphasis will be placed on the genus Posidonia which has an evolutionary origin in the...