Phytolith Formation in Plants: From Soil to Cell

Abstract

Silica is deposited extra- and intracellularly in plants in solid form, as phytoliths. Phytoliths have emerged as accepted taxonomic tools and proxies for reconstructing ancient flora, agricultural economies, environment, and climate. The discovery of silicon transporter genes has aided in the understanding of the mechanism of silicon transport and deposition within the plant body and reconstructing plant phylogeny that is based on the ability of plants to accumulate silica. However, a precise understanding of the process of silica deposition and the formation of phytoliths is still an enigma and the information regarding the proteins that are involved in plant biosilicification is still scarce. With the observation of various shapes and morphologies of phytoliths, it is essential to understand which factors control this mechanism. During the last two decades, significant research has been done in this regard and silicon research has expanded as an Earth-life science superdiscipline. We review and integrate the recent knowledge and concepts on the uptake and transport of silica and its deposition as phytoliths in plants. We also discuss how different factors define the shape, size, and chemistry of the phytoliths and how biosilicification evolved in plants. The role of channel-type and efflux silicon transporters, proline-rich proteins, and siliplant1 protein in transport and deposition of silica is presented. The role of phytoliths against biotic and abiotic stress, as mechanical barriers, and their use as taxonomic tools and proxies, is highlighted.

Keywords: Lsi; biomineralization; biosilicification; palaeoenvironment reconstruction; phytoliths; silicon; siliplant1 (Sip1) protein.

Figure 1

Biogenic silica deposition in hemp leaves after acid extraction and fluorescence imaging with (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole) (PDMPO). (A) Isolated trichomes, some of which with basal cells; (B) Isolated trichomes and two adjacent trichomes with basal and epidermal cells; (C) Trichomes with epidermal cells showing replicas of waxy “wrinkles” (boxed region and bottom magnified image thereof); (D) Compound trichome base (arrow) interspersed with single-celled trichome bases; (E) Reticulated structure possibly representing a developmental stage of trichomes with compound bases; and,(F) strongly labelled structures inside epidermal cells possibly representing vacuoles/large vesicles. The scale bars in panels A–C are 200μm, in D–E 100μm and in F 50μm (with permissions from [57]).

Figure 2

(a) Gene structure of four rice low-silicon genes Lsi1, Lsi2, Lsi3, and Lsi6 and a sorghum SIp1 gene. Gene sequences were retrieved from Phytozome and visualized in Gene structure display server. (b) A generalized structure of silicon transporter gene (modified from [100]) and (c) sorghum SIp1 gene (modified from [69]).

Figure 3

(a) Scalloped phytoliths of Cucurbita moschata with different rotations showing scalloped impression created by hypodermal (hh) and mesodermal (mh) cells, (b) SEM micrograph of C. moschata showing interlocked stone cells (st) and phytoliths (phy) (with permissions from [103]).

Figure 4

Large sized phytoliths. (a) Cymbopogon schoenanthus vine, (b–d) Melanoleuca grammopodia (figure source [10]).