Phytoliths from some grasses (Poaceae) in arid lands of Xinjiang, China

Opal phytoliths, as silicon dioxide inclusions, are abundant in different parts of a plant. It is known that grasses are the most representative in this respect. The research of phytoliths, removed from 25 most common grass species in the arid and semiarid lands of the Junggar Basin and adjacent areas, has been undertaken. The visual estimation of diversity and variability of silica cells and identification of their morphological types (patterns) were also the aim of our research. Since the work is preliminary, we have emphasized on the visual estimation of silica cell variability and involved only the leaf blades in the analysis. Drawings of the revealed silica cells, characteristic of 25 species, are provided. The sig-nificant morphological diversity of phytoliths has been revealed, as well as their taxonomic similarity at the level of subfamilies. These data can be used for the identification of phytoliths from sediments.

Shedding light on biosilica morphogenesis by comparative analysis of the silica-associated proteomes from three diatom species

Morphogenesis of the intricate patterns of diatom silica cell walls is a protein-guided process, yet to date only very few such silica morphogenetic proteins have been identified. Therefore, it is unknown whether all diatoms share conserved proteins of a basal silica forming machinery, and whether unique proteins are responsible for the morphogenesis of species specific silica patterns. To answer these questions, we extracted proteins from the silica of three diatom species (Thalassiosira pseudonana, Thalassiosira oceanica and Cyclotella cryptica) by complete demineralization of the cell walls. LC-MS/MS analysis of the extracts identified 92 proteins that we name 'Soluble Silicome Proteins' (SSPs). Surprisingly, no SSPs are common to all three species, and most SSPs showed very low similarity to one another in sequence alignments. In depth bioinformatics analyses revealed that SSPs can be grouped into distinct classes bases on short unconventional sequence motifs whose functions are yet unknown. The results from in vivo localization of selected SSPs indicates that proteins, which lack sequence homology but share unconventional sequence motifs may exert similar functions in the morphogenesis of the diatom silica cell wall.

Exocytosis of the silicified cell wall of diatoms involves extensive membrane disintegration

Diatoms are unicellular algae that are characterized by their silica cell walls. The silica elements form intracellularly in a membrane-bound organelle, and are exocytosed after completion. How diatoms maintain membrane homeostasis during the exocytosis of these large and rigid silica elements is a long-standing enigma. We studied membrane dynamics during cell wall formation and exocytosis in the diatom Stephanopyxis turris, using live-cell confocal microscopy and advanced electron microscopy. Our results provide detailed information on the ultrastructure and dynamics of the silicification process, showing that during cell wall formation, the organelle membranes tightly enclose the mineral phase, creating a precise mold of the delicate geometrical patterns. Surprisingly, during exocytosis of the mature silica elements, the proximal organelle membrane becomes the new plasma membrane, and the distal membranes gradually disintegrate into the extracellular space without any noticeable endocytic retrieval or extracellular repurposing. These observations suggest that diatoms evolved an extraordinary exocytosis mechanism in order to secrete their cell wall elements.

Structural evidence for extracellular silica formation by diatoms

The silica cell wall of diatoms, a widespread group of unicellular microalgae, is an exquisite example for the ability of organisms to finely sculpt minerals under strict biological control. The prevailing paradigm for diatom silicification is that this is invariably an intracellular process, occurring inside specialized silica deposition vesicles that are responsible for silica precipitation and morphogenesis. Here, we study the formation of long silicified extensions that characterize many diatom species. We use cryo-electron tomography to image silica formation in situ, in 3D, and at a nanometer-scale resolution. Remarkably, our data suggest that, contradictory to the ruling paradigm, these intricate structures form outside the cytoplasm. In addition, the formation of these silica extensions is halted at low silicon concentrations that still support the formation of other cell wall elements, further alluding to a different silicification mechanism. The identification of this unconventional strategy expands the suite of mechanisms that diatoms use for silicification.

Mini-Review: Potential of Diatom-Derived Silica for Biomedical Applications

Diatoms are unicellular eukaryotic microalgae widely distributed in aquatic environments, possessing a porous silica cell wall known as frustule. Diatom frustules are considered as a sustainable source for several industrial applications because of their high biocompatibility and the easiness of surface functionalisation, which make frustules suitable for regenerative medicine and as drug carriers. Frustules are made of hydrated silica, and can be extracted and purified both from living and fossil diatoms using acid treatments or high temperatures. Biosilica frustules have proved to be suitable for biomedical applications, but, unfortunately, they are not officially recognised as safe by governmental food and medical agencies yet. In the present review, we highlight the frustule formation process, the most common purification techniques, as well as advantages and bottlenecks related to the employment of diatom-derived silica for medical purposes, suggesting possible solutions for a large-scale biosilica production.

Anti-herbivore silicon defences in a model grass are greatest under Miocene levels of atmospheric CO2

<p>Silicon (Si) has important role in mitigating diverse biotic and abiotic stresses, mainly via silicification of plant tissues. However, environmental changes such as reduced atmospheric CO<sub>2</sub> concentrations may affect grass Si concentration which, in turn, can alter herbivore performance. Recently, we demonstrated that pre-industrial atmospheric CO<sub>2</sub> increased Si accumulation in a grass, however, how Si is deposited and whether this affects insect herbivores performance is unknown. We, therefore, investigated how pre-industrial (reduced) (rCO<sub>2</sub>, 200 ppm), ambient (aCO<sub>2</sub>, 410 ppm) and elevated (eCO<sub>2</sub>, 640 ppm) CO<sub>2</sub> concentrations and Si-treatments (Si+ or Si-) affect Si accumulation in the model grass, <em>Brachypodium distachyon</em> and its subsequent effects on the performance of the global insect, <em>Helicoverpa armigera</em>. rCO<sub>2</sub> caused Si concentrations to increase by 29% and 36% compared to aCO<sub>2</sub> and eCO<sub>2</sub>, respectively. Furthermore, increased Si accumulation under rCO<sub>2</sub> decreased herbivore relative growth rate (RGR) by 120% relative to eCO<sub>2, </sub>whereas<sub></sub> rCO<sub>2</sub> caused herbivore RGR to decrease by 26% compared to eCO<sub>2</sub>. Moreover, Si supplementation increased the density of trichomes, silica and prickle cells, and these changes in leaf surface morphology reduced larval feeding performance. The observed negative correlation between macrohair density, silica cell density, prickle cell density and herbivore RGR supports this. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO<sub>2</sub> environment reduced the performance of this generalist insect herbivore performance.<strong> </strong>Contrastingly, we found  reduced Si accumulation under higher CO<sub>2</sub>, which suggests  that some grasses might become more susceptible to insect herbivore under the projected climate change scenarios.</p>