Silica deposition by a unique protein in sorghum

Plants produce large quantities of biominerals, such as calcium oxalate, calcium carbonate, and silica. These minerals form the majority of plants’ ash and are typically involved in defence strategies. Most astonishingly, very little is known about the control over mineral formation in plants. Recently we described for the first time the ontogenesis of leaf epidermal silica cells in sorghum (Sorghum bicolor) and explained on cellular scales how silica formation is controlled (Kumar et al, 2017, Kumar and Elbaum, 2018, Kumar et al, 2019). Silicic acid, the precursor of silica, is pumped into the plant roots and distributed throughout the plant body together with water. The acid is present in the extracellular solution at super-saturation. Nonetheless, in sorghum seedlings the mineral is forming only in specific cells. We identified a protein we called Siliplant1 (SLP1, previously GSM) that induces silica formation in vitro. We then found that this protein is expressed right before silica deposition in silica cells of sorghum seedlings. The protein is packed in vesicles. Our results suggest that the biomineralization is initiated by the fusion of the vesicles to the cell membrane and the secretion of SLP1 to the space between the cell plasma membrane and wall. Silica forms wherever silicic acid meets with the protein matrix. The forming mineral thickens the cell wall and constricts the cell lumen. The silicification process ends when the shrunk cell is dead, and the supply of SLP1 stops. Over expression of SLP1 in sorghum causes ectopic silica formation, demonstrating its activity in planta. Our results open the way to synthesize silica in vitro at room conditions using environmentally friendly precursors.