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western Sicily and appeared to be responsible for the strong anchor-
age strength displayed by seedlings settled on bare cobbles.
Two- to three-month-old seedlings collected at Ustica, Favigana
and Capo Feto exhibited similar morphological features and were
comparable to specimens of similar age analysed in previous stud-
ies 27,28,30 . Earlier studies did not, however, document the presence of
sticky root hairs. Belzunce et al. extensively studied the structure
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and development of P. oceanica seedling root systems of individuals
cultured in the laboratory. These authors observed scattered hairs on
primary and adventitious roots with a maximum length of 300 mm,
less than half the mean length observed in this study, with no sign of
adhesive substances.
In our study, seedlings settled on volcanic cobbles were firmly
anchored to the substrate, as the force needed to dislodge the plant-
lets exceeded 2 N on average, which is up to 100-fold higher than that
required to uproot seedlings of other seagrasses growing on sand or
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matte . Seedling root systems conformed to substrate morphology;
after detachment, it was possible to observe that roots were adhering
directly to the substrate surface rather than via ‘‘facilitator’’ species
Figure 3 | Posidonia oceanica seedling root section observed under SEM that provide a settlement surface.
showing conspicuous root hair coverage originating from epidermal Root hairs have been described for many seagrasses in adult indi-
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cells, with attached sand grains. (a), (b) details of the root section and of viduals . The density and length of root hairs vary greatly among and
the root hairs stuck to sand grains; (c), (d) close-up of the proximal part of within genera, with no clear relationship with features of substrates
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the root hair close to the epidermal cells. inhabited by each species . Massive and long root hairs are found in
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the genera Thalassia, Halophila, Zostera and Heterozostera .In
enlarged, lobed, foot-like shape (Fig. 4). This structure had a mean Posidonia, Cymodocea, Halodule and Thalassodendron root hairs
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width of 52.70 6 4.32 mm and a maximum width of 78.33 mm, and it are reported to be rare ; however, root hairs have not been recorded
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extended the contact area between the hair tip and the substrate in Amphibolis . In general, root hairs are transient; the presence of
(Fig. 4). numerous wall ingrowths and plasmodesmata in cells abutting root
hairs suggests that they play an important role in nutrient uptake .
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Among seagrasses, adhesive root hairs have been reported only for
Anchorage strength. The number of standing leaves plus leaf 6,7 37
sheaths indicated that seedlings collected at Ustica in 2009 were the species P. scouleri . Gibbs described the emergence of a dense,
approximately 5 months old. The force needed to detach seedlings woolly covering of root hairs near the tip of developing roots in
settled on volcanic cobbles ranged from 0.78 to 5.23 N, with a mean Phyllospadix spp. seedlings, similar to that reported in this study
value of 2.97 6 0.42 (N61 SE). Adhesive root hairs were observed on for P. oceanica seedlings. Adult P. scouleri adhesive root hairs are
primary and adventitious roots and on the hypocotyl region of the short, thick-walled and lobed and adhere directly to the substrate,
seeds in all specimens. allowing the plant to form mat-like colonies in surf-exposed habi-
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tats . Histochemical analysis revealed that roots were covered by a
PAS-positive, mucilaginous-like substance that is most likely pro-
Discussion duced by roots and acts as an adhesive .
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This study documents for the first time the presence and morphology Among freshwater plants, the presence of adhesive root hairs has
of adhesive root hairs in P. oceanica seedlings. Adhesive root hairs been widely described in the riverweed Podostemaceae Rich. ex
were recorded in all specimens collected from different substrates Kunth 38,39 . This family inhabits the extreme habitats of waterfalls
along both the mainland and island coasts of northwestern and and has a highly polymorphic vegetative body. The lower part of
the seed, roots and vegetative body present persistent unicellular
hairs produced by epidermal cells that secrete a sticky cement once
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in contact with the substrate . These hairs elongate and become
tube-like holdfasts at maturity with an enlarged and branched tip
that has a foot-like shape , which is very similar to that documented
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here for P. oceanica seedlings. More recently, the adhesive material
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was found to be a bacterial biofilm that sticks to the rocky substrate .
Podostemaceae ‘‘adhesive hairs’’ are therefore mechanical elements,
growing into the upper layer of the biofilm matrix; the broadened
hair tips are important for expanding the contact area between the
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plant and the biofilm .
Typical root hairs in terrestrial and aquatic plants exist for a lim-
ited developmental period on a root and function primarily in water
and nutrient uptake. The root hairs of P. oceanica seedlings described
in this study appear quite distinct as they are persistent, at least for
some months, and build up an enduring adhesive coat that covers the
greater part of the root length, indicating a mechanical, anchoring
function. These structures appear more similar to those observed
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macroscopically in Phyllospadix spp. seedlings and microscopically
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Figure 4 | Structural details of Posidonia oceanica root hair tips observed in species belonging to the Podostemaceae family . The polysac-
under SEM. (a–d) branching of the distal part and lobed root hairs tips; charidic nature of the adhesive substance can either be associated
(e) detail of the root hair tip. with plant cell secretions 7,40 or with extracellular polymeric sub-
SCIENTIFIC REPORTS | 5 : 8804 | DOI: 10.1038/srep08804 3