Silicon (Si) is not considered an essential element, however, its tissue concentration can exceed that of many essential elements in several evolutionary distant plant species. Roots take up Si using Si transporters and then translocate it to aboveground organs. In some plant species, root tissues are also places where a high accumulation of Si can be found. Three basic modes of Si deposition in roots have been identified so far: (1) impregnation of endodermal cell walls (e.g., in cereals, such as Triticum (wheat)); (2) formation of Si-aggregates associated with endodermal cell walls (in the Andropogoneae family, which includes Sorghum and Saccharum (sugarcane)); (3) formation of Si aggregates in “stegmata” cells, which form a sheath around sclerenchyma fibers e.g., in some palm species (Phoenix (date palm)). In addition to these three major and most studied modes of Si deposition in roots, there are also less-known locations, such as deposits in xylem cells and intercellular deposits. In our research, the ontogenesis of individual root cells that accumulate Si is discussed. The documented and expected roles of Si deposition in the root is outlined mostly as a reaction of plants to abiotic and biotic stresses.

Date palm (Phoenix dactylifera) can accumulate as much as 1% silicon (Si), but not much is known about the mechanisms inherent to this process. Here, we investigated in detail the uptake, accumulation and distribution of Si in date palms, and the phylogeny of Si transporter genes in plants. We characterized the PdNIP2 transporter following heterologous expression in Xenopus oocytes and used qPCR to determine the relative expression of Si transporter genes. Silicon accumulation and distribution was investigated by light microscopy, scanning electron microscopy coupled with X-ray microanalysis and Raman microspectroscopy. We proved that PdNIP2-1 codes for a functional Si-permeable protein and demonstrated that PdNIP2 transporter genes were constitutively expressed in date palm. Silicon aggregates/phytoliths were found in specific stegmata cells present in roots, stems and leaves and their surfaces were composed of pure silica. Stegmata were organized on the outer surface of the sclerenchyma bundles or associated with the sclerenchyma of the vascular bundles. Phylogenetic analysis clustered NIP2 transporters of the Arecaceae in a sister position to those of the Poaceae. It is suggested, that Si uptake in date palm is mediated by a constitutively expressed Si influx transporter and accumulated as Si aggregates in stegmata cells abundant in the outer surface of the sclerenchyma bundles (fibers).

Root hairs increase the contact area of roots with soil and thereby enhance the capacity for solute uptake. The strict hair/non-hair pattern of Arabidopsis thaliana can change with nutrient deficiency or exposure to toxic elements, which modify root hair density. The effects of root hair density on cadmium (Cd) accumulation in shoots of arabidopsis genotypes with altered root hair development and patterning were studied. Arabidopsis mutants that are unable to develop root hairs (rhd6-1 and cpc/try) or produce hairy roots (wer/myb23) were compared with the ecotype Columbia (Col-0). Plants were cultivated on nutrient agar for 2 weeks with or without Cd. Cadmium was applied as Cd(NO₃)₂ at two concentrations, 10 and 100 µm. Shoot biomass, root characteristics (primary root length, lateral root number, lateral root length and root hair density) and Cd concentrations in shoots were assessed. Anatomical features (suberization of the endodermis and development of the xylem) that might influence Cd uptake and translocation were also examined. Cadmium inhibited plant growth and reduced root length and the number of lateral roots and root hairs per plant. Suberin lamellae in the root endodermis and xylem differentiation developed closer to the root apex in plants exposed to 100 µm Cd. The latter effect was genotype dependent. Shoot Cd accumulation was correlated with root hair abundance when plants were grown in the presence of 10 µm Cd, but not when grown in the presence of 100 µm Cd, in which treatment the development of suberin lamellae closer to the root tip appeared to restrict Cd accumulation in shoots. Root hair density can have a large effect on Cd accumulation in shoots, suggesting that the symplasmic pathway might play a significant role in the uptake and accumulation of this toxic element.