Silicification in Leaves of Sorghum Mutant with Low Silicon Accumulation

Purpose: Silicon improves plants’ ability to tolerate stresses. It is taken up by roots as silicic acid, transported via the transpiration stream, and is unloaded in the shoot by specific silicon transporters. In grasses, silicon deposits may reach 10 % of the leaf dry weight. However, no molecular mechanism is known to control the deposition. Our purpose thus was to identify a sorghum mutant unable to absorb silicic acid and use it to study leaf silicification. Methods: We generated and characterized a Sorghum bicolor knockout mutant in a root silicon transporter, SbLsi1, and followed leaf epidermal silicification, using an airSEM (air-scanning electron microscope). Results: The mutant contained 40 times less silica than the wild type (about 0.01 % per dry weight, compared to 3.7 %). The base of wild type leaf blades contained very few, partially silicified dumbbell-shaped silica cells. The silicification intensified towards the leaf tip. Contrary to this, the mutant leaf epidermis displayed empty and probably non-turgid dumbbells. Mature mutant leaves supplied with silicic acid through their base, accumulated silica in the cell walls along the vasculature. No specific dumbbell silicification was detected. Conclusions: The loss of turgor may indicate that cell death is part of the development of dumbbell-shaped silica cells. These cells do not accumulate silica after turgor loss, suggesting that a biological process may be involved in their silicification.