How is silicic acid transported in plants?
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ORIGINAL PAPER
How is silicic acid transported in plants? Christopher Exley 1
&
Gea Guerriero 2 & Xabier Lopez 3
Received: 29 August 2019 / Accepted: 15 December 2019 # The Author(s) 2020
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. Keywords Silicic acid . Silica . Plants . Aquaporin . Oocytes
1 Introduction Silicon is biologically available in soil solution as neutral, monomeric silicic acid. The vehicle for its uptake and distribution in plants is water. Silicic acid is a substantially larger molecule than water [1, 2] (Fig. 1). However, like water, hydroxyl groups dominate its chemistry and while it has no known organic chemistry and extremely limited inorganic chemistry, it is capable of hydrogen bonding [3]. In following water via the symplastic route,
silicic acid encounters enumerable water channels, such as aquaporins, and these size-restrictive pores will either allow or deny the passage of the larger silicic acid molecule [4]. Ostensibly, silicic acid has access to all areas in all plants, but in practice the myriad different water channels that characterise plant species limits the movement of silicic acid in some, while allowing it in others.
2 Mechanism of Transport of Silicic Acid * Christopher Exley [email protected] Gea Guerriero [email protected] Xabier Lopez [email protected] 1
The Birchall Centre, Lennard-Jones Laboratories, Keele University, Keele, Staffordshire, UK
2
Environmental Research and Innovation Department, Luxembourg Institute of Science and Technology, Esch/Alzette, Luxembourg, Luxembourg
3
Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU), and Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain
In a landmark paper, published in Nature in 2006, Ma et al. described a putative sili
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