| While Silicon (Si) is not a panacea for stress resistance in plants, the element has a number of beneficial effects against both abiotic and biotic stress. Most research to date has been inundated with salt, cold, and fungal resistance induced by Si in high accumulators, with little work performed on viral infections and low accumulators. The low accumulator, N. tabacum infected with Tobacco ringspot virus and supplemented with Si showed a reduction in viral systemic symptoms and a increase in foliar Si levels compared to controls. Si supplementation did not influence the systemic symptoms induced by the unrelated pathogen, Tobacco mosaic virus, nor did infections influence foliar Si levels. Si accumulation in the TRSV-N. tabacum pathosystem was quite variable, so to study stress induced Si accumulation (SISA), a more consistent system was developed. Copper (Cu) toxicity increases foliar Si accumulation in a low accumulator and in N. tabacum. The consistency of this response indicates that it is a good model to study SISA. In addition to Cu, hormones involved in defense and environmental perception were tested to determine if they too influenced Si accumulation in leaves. N. tabacum were exposed to exogenous abscisic acid, methyl jasmonate, or salicylic acid and foliar Si content was determined. Plants treated with ABA showed a significant reduction in foliar Si levels, while the other two hormones had no effect. This suggests the ABA has an antagonistic effect on SISA. Along with physiological work, we examined factors involved in the transport of Si into the plant. Aquaporins, belonging to the major intrinsic protein (MIP) family, are involved in water and/or solute transport into and throughout the plant. We identified putative MIPs in N. tabacum, including a novel ntNIP3;1. This protein is likely a Si transporter based on sequence homology to documented transporters. In addition, Si caused a reduction in expression of ntNIP3;1as well as a root-specific tonoplast intrinsic protein (TIP), ntRT-TIP1, which correlates with the expression of other Si transporters. This leads us to believe that in addition to the classical NIP family members involved in Si transport, TIPs may also play a role. Taken together, these data suggest that characterizing plants as low-, intermediate- or high-accumulators can be somewhat misleading, since the ability to accumulate the element is dependent on not just the tissue assayed, but can also be influenced by the environmental conditions in which the plants are grown. In summary, Cu toxicity is a reliable system to begin studying the molecular aspects of SISA in the inducible-accumulator N. tabacum. This process may be under hormonal regulation, since ABA reduces Si levels. With the heavy use of hormone regulators (such as ABA), especially in the floriculture industry, could lead to lowered resistance against a number of abiotic and biotic stressors, because of reduced foliar Si levels. |
