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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
                                                              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
                                                              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-
          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
          the root hair close to the epidermal cells.         inhabited by each species . Massive and long root hairs are found in
                                                              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
          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
          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 .
                                                                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-
                                                              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 .
          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
                                                              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
                                                              here for P. oceanica seedlings. More recently, the adhesive material
                                                              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
                                                              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
                                                              macroscopically in Phyllospadix spp. seedlings and microscopically
          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
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