Interplay between silica deposition and viability during the life span of sorghum silica cells

Silica cells are specialized epidermal cells found on both surfaces of grass leaves, with almost the entire lumen filled with solid silica. The mechanism precipitating silicic acid into silica is not known. Here we investigate this process in sorghum (Sorghum bicolor) leaves. Using fluorescent confocal microscopy, we followed silica cells' ontogeny, aiming to understand the fate of vacuoles and nuclei. Correlating the confocal and scanning electron microscopy, we timed the initiation of silica deposition in relation to cell's viability. Contrary to earlier reports, silica cells did not lose their nucleus before silica deposition. Vacuoles in silica cells did not concentrate silicic acid. Instead, postmaturation silicification initiated at the cell periphery in live cells. Less than 1% silica cells showed characteristics of programmed cell death in the cell maturation zone. In fully elongated mature leaves, 2.4% of silica cells were nonsilicified and 1.6% were partially silicified. Silica deposition occurs in the paramural space of live silica cells. The mineral does not kill the cells. Instead, silica cells are genetically programmed to undergo cell death independent of silicification. Fully silicified cells seem to have nonsilicified voids containing membrane remains after the completion of the cell death processes.