How is silicic acid transported in plants?

Abstract

Plants accumulate silicon in their tissues as amorphous silica. The form of silicon taken up by plants is silicic acid, a neutral molecule that passes through membrane channels with water. After seminal work on rice identified an aquaporin that appeared to mediate the passage of silicic acid, several papers followed and classified similar channels (referred to as “transporters”) in a number of plant species. These channels have been described as essential for silicon uptake and specific for the metalloid. Herein, we critically review the published data on the characterisation of one channel in particular, Lsi1, and identify possible caveats in results and limitations in methods used. Our analysis does not support the suggestion that the identified channels are specific for silicic acid. Computational analyses of the size of the Lsi1 pore additionally suggest that it may not play a significant role in mediating the movement of silicic acid in planta. We suggest that to avoid further confusion, channels currently implicated in the transport of silicic acid in planta are not referred to as silicon-specific transporters. Future research including the use of molecular dynamics simulations will enable the unequivocal identification of channels involved in silicon transport in plants.