Ontogenetic transition from specialized root hairs to specific root-fungus symbiosis in the dominant Mediterranean seagrass Posidonia oceanica

Ondřej Borovec,Martin Vohník

doi:10.1038/s41598-018-28989-4

Ondřej Borovec, Martin Vohník

Open Access

https://doi.org/10.1038/s41598-018-28989-4

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Abstract

Terrestrial plants typically take up nutrients through roots or mycorrhizae while freshwater plants additionally utilize leaves. Their nutrient uptake may be enhanced by root hairs whose occurrence is often negatively correlated with mycorrhizal colonization. Seagrasses utilize both leaves and roots and often form root hairs, but seem to be devoid of mycorrhizae. The Mediterranean seagrass Posidonia oceanica is an exception: its adults commonly lack root hairs and regularly form a specific association with a single pleosporalean fungus. Here we show that at two sites in the southern Adriatic, all its seedlings possessed abundant root hairs with peculiar morphology (swollen terminal parts) and anatomy (spirally formed cell walls) as apparent adaptations for better attachment to the substrate and increase of breaking strain. Later on, their roots became colonized by dark septate mycelium while root hairs were reduced. In adults, most of terminal fine roots possessed the specific fungal association while root hairs were absent. These observations indicate for the first time that processes regulating transition from root hairs to root fungal colonization exist also in some seagrasses. This ontogenetic shift in root traits may suggests an involvement of the specific root symbiosis in the nutrient uptake by the dominant Mediterranean seagrass.

Highlights

Mineral nutrition of vascular terrestrial plants typically depends on roots that transport dissolved elements from the rhizosphere, a relatively small volume of the soil surrounding the root
Submerged freshwater macrophytes have adapted a rather different strategy – nutrient uptake through roots may still play an important role in their nutrition, especially in water with low nutrient content[9], most of them acquire a significant part of mineral nutrients from the water column through their leaves[10]
To our knowledge, such a transition from young seedlings possessing dense root hair cover but lacking fungal root colonization to adult plants lacking root hairs but possessing abundant fungal root colonization has never been reported before in any seagrass species. While this shift may resemble negative correlation between root hair production and mycorrhizal colonization typical in terrestrial as well as some aquatic plants[8,15,16], it rather seems to be connected with the seagrass ontogeny and likely reflects different evolutionary pressures/environmental constraints typical for the three studied ontogenetic phases

Summary

Introduction

Mineral nutrition of vascular terrestrial plants typically depends on roots that transport dissolved elements from the rhizosphere, a relatively small volume of the soil surrounding the root (i.e., the zone of ca. 1–2 mm around the root). Nutrient uptake through root hairs may be further increased by secretion of root exudates[3] which play an important role in mediating interactions between roots and symbiotic soil organisms such as the mutualistic mycorrhizal fungi or nitrogen-fixing bacteria[4]. In addition to their role in plant nutrition, roots play an important role in anchoring plants in the substrate, storage of nutrients, clonal growth/vegetative propagation, aeration of the rhizosphere, etc.[5]. Many of the so far reported seagrass fungal endophytes represent facultative marine fungi which readily occur in terrestrial ecosystems, but some are strictly marine fungal genera[27] which can partly act as endophytes but mostly grow as saprobes and decompose seagrass tissues

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