Molecular evolution of aquaporins and silicon influx in plants

Silicon (Si), although mostly ignored by plant nutritionists and ecologists, is now gaining more attention because of its beneficial role in plant fitness under stress environment imposed by a diverse range of biotic and abiotic factors. Si appears to systematically confer benefits to plants as long as a given species can absorb the element. Here, we review recent developments regarding the molecular mechanisms, evolution, regulation and structural specificity of influx transporter proteins involved in Si uptake by plants. Si absorption is facilitated by specific nodulin 26-like intrinsic proteins (NIPs). The Si transporter NIPs have evolved a unique amino acid selective filter (SF), which is one of the required features to regulate the influx of Si. While Si accumulation in plants requires the dual action of both an influx transporter and an efflux transporter, it appears that the presence of the former is the indispensable key for a plant to be able to absorb Si. Based on sequence analyses and comparisons, influx transporters appear to have conserved features across all species that allow to discriminate between plants that are Si competent or not. While it is unclear how and why plants have acquired or lost this trait, genomic data now offer a reliable tool to predict with accuracy which plant species are predisposed to benefit from Si. This will undoubtedly result in a better understanding of Si role in many fundamental aspects of ecology regarding plant fitness under stress.