Abstract Silicon is absorbed by plant roots as silicic acid. The acid moves with the transpiration stream to the shoot, and mineralizes as silica. In grasses, leaf epidermal cells called silica cells deposit silica in most of their volume using an unknown biological factor. Using bioinformatics tools, we identified a previously uncharacterized protein in Sorghum bicolor, which we named Siliplant1 (Slp1). Slp1 is a basic protein with seven repeat units rich in proline, lysine, and glutamic acid. We found Slp1 RNA in sorghum immature leaf and immature inflorescence. In leaves, transcription was highest just before the active silicification zone (ASZ). There, Slp1 was localized specifically to developing silica cells, packed inside vesicles and scattered throughout the cytoplasm or near the cell boundary. These vesicles fused with the membrane, releasing their content in the apoplastic space. A short peptide that is repeated five times in Slp1 precipitated silica in vitro at a biologically relevant silicic acid concentration. Transient overexpression of Slp1 in sorghum resulted in ectopic silica deposition in all leaf epidermal cell types. Our results show that Slp1 precipitates silica in sorghum silica cells.

This research was supported by the COST Action FA1201 (GEMINI) by granting a Short Term Scientific Mission and the Instituto de Salud Carlos III by granting Grant PI11/01645. The authors have no conflict of interest to declare.

28• Silicon is absorbed by plant roots as silicic acid. The acid moves with the transpiration 29 stream to the shoot, and mineralizes as silica. In grasses, leaf epidermal cells called 30 silica cells deposit silica in most of their volume by unknown mechanism. 31• Using bioinformatics tools, we identified a previously uncharacterized protein in 32 sorghum (Sorghum bicolor), which we named Siliplant1 (Slp1). Silica precipitation 33 activity in vitro, expression profile, and activity in precipitating biosilica in vivo were 34 characterized. 35• Slp1 is a basic protein with seven repeat units rich in proline, lysine, and glutamic acid. 36 A short peptide, repeating five times in the protein precipitated silica in vitro at a 37 biologically relevant silicic acid concentration. Raman and NMR spectroscopies 38 showed that the peptide attached the silica through lysine amine groups, forming a 39 mineral-peptide open structure. We found Slp1 expression in immature leaf and 40 inflorescence tissues. In the immature leaf active silicification zone, Slp1 was localized 41 to the cytoplasm or near cell boundaries of silica cells. It was packed in vesicles and 42 secreted to the paramural space. Transient overexpression of Slp1 in sorghum resulted 43 in ectopic silica deposition in all leaf epidermal cell types. 44 • Our results show that Slp1 precipitates silica in sorghum silica cells. 45 46 47

BackgroundImplantation is a complex process that requires a delicate cooperation between the immune and reproductive system. Any interference in the fine balance could result in embryo loss and infertility. We have recently shown that Toll-like receptor 5 activation results in a decrease of trophoblast cells binding to endometrial cells in an in vitro model of human implantation. However, little is known about the downstream signalling leading to the observed failure in implantation and the factors that modulate this immune response.Methods and Principal FindingsAn in vitro model of embryo implantation was used to evaluate the effect of trophoblasts and flagellin on the activation of NF-κB in endometrial cells and whether TLR5-related in vitro implantation failure is signalled through NF-κB. We generated two different NF-κB reporting cell lines by transfecting either an immortalized endometrial epithelial cell line (hTERT-EECs) or a human endometrial carcinoma cell line (Ishikawa 3-H-12) with a plasmid containing the secreted alkaline phosphatase (SEAP) under the control of five NF-κB sites. The presence of trophoblast cells as well as flagellin increased NF-κB activity when compared to controls. The NF-κB activation induced by flagellin was further increased by the addition of trophoblast cells. Moreover, blocking NF-κB signalling with a specific inhibitor (BAY11-7082) was able to restore the binding ability of our trophoblast cell line to the endometrial monolayer.ConclusionsThese are the first results showing a local effect of the trophoblasts on the innate immune response of the endometrial epithelium. Moreover, we show that implantation failure caused by intrauterine infections could be associated with abnormal levels of NF-κB activation. Further studies are needed to evaluate the target genes through which NF-κB activation after TLR5 stimulation lead to failure in implantation and the effect of the embryo on those genes. Understanding these pathways could help in the diagnosis and treatment of implantation failure cases.