Understanding Differences in Silicon Uptake between High and Low Foliar Accumulators: Concentration May Not Predict Protection.

Silicon (Si) fertilization protects a wide variety of plants against environmental stress and pathogenic attack. A misconception when it comes to Si nutrition in plants is that only varieties containing macronutrient concentrations of the element will benefit from Si fertilization. However, the literature is inundated with findings of protective roles against stress following Si application in plants that accumulate less than 0.1% Si. Historically, plants are classified as high or low Si accumulators based on foliar concentrations above or below roughly 0.3% Si. Recently, we have shown that the low foliar accumulator, Nicotiana tabacum, contains a functional Si transporter, NtNIP2;1, homologous to those identified in high accumulators. Two other transport proteins, NtPIP1;1 and NtRT-TIP1, were also shown to transport Si in this study, providing evidence for Si movement into cells and organelles in this species. Si distribution in non-stressed N. tabacum is typically 1,000 ppm and 300 ppm in roots and shoots, respectively, but the foliar Si concentration can be significantly increased by viral infection or heavy metal toxicity. Copper toxicity studies indicate a soluble Si pool is needed at the time of stress for additional uptake opposed to the release of stored Si from root tissue. These data support the theory that low foliar accumulators regulate root-to-shoot movement of Si and beneficial responses cannot be predicted by foliar concentrations alone. Understanding how the conserved uptake mechanism is differentially regulated between high and low foliar accumulators, in addition to differences in Si partitioning within plant structures are important avenues of research to explore in order to facilitate the acceptance of Si as a plant nutrient.