| Popis: |
Silicon (Si) supplementation can provide plants with several benefits: increased photosynthetic activity, crop yield, and resistance to abiotic and/or biotic stress. While most research examining these benefits mainly focused on high Si accumulators such as certain monocotyledonous plants, low Si accumulators are thought not to benefit from Si supplementation. In the present study, I show that the low Si accumulator, Nicotiana tabacum, benefits from Si supplementation, although these effects were time dependent. Si supplementation reduces metal stress in N. tabacum propagated hydroponically under copper (Cu) toxicity after 3 weeks of treatment. Plants displayed increased shoot and root biomass almost to that of control. Furthermore, Si supplementation reversed the reduction in maximum quantum yield of photosystem II caused by Cu toxicity. However, Si supplementation showed no beneficial effect on growth after 1 week in Cu-treated plants but did affect other plant processes. The foliar tissue of Si-supplemented plants under Cu toxicity accumulated more Si than plants provided with Si alone after 1 week. In this work, I have termed this phenomenon stress-induced Si accumulation (SISA), and similar responses in N. tabacum infected with Tobacco ringspot virus have been reported. These data suggest that foliar SISA may be part of the N. tabacum stress response. I observed SISA in roots but only after three weeks of treatment, which corresponded with recovery from Cu toxicity. Silicon might influence Cu toxicity by affecting metal uptake, chelator expression, redox homeostasis, and phytohormone signaling. Therefore, I investigated these processes via a transcriptomic analysis. I found that Si-mediated alleviation of Cu toxicity corresponded to reduced mRNA expression of COPT1, the major Cu transporter in roots. This could, in part, play a role reducing root Cu concentrations as observed in Si-supplemented plants under Cu toxicity conditions after 3 weeks of treatment. Silicon supplementation did not affect Cu chelator gene mRNA expression. However, fluctuations in the mRNA expression of redox homeostatic genes suggest that the antioxidative capacity of N. tabacum roots was increased by Si supplementation in plants under Cu toxicity after 1 week of treatment, but the mRNA expression returned to control or near-control levels by three weeks. A similar pattern of mRNA expression was observed for genes involved in biosynthesis/signaling of phytohormones such as abscisic acid, salicylic acid, and possibly gibberellic acid. In contrast, the mRNA expression of genes involved in ethylene and jasmonic acid signaling pathways appeared to not be influenced by Si supplementation of N. tabacum roots under Cu toxic conditions after 1 week; however, the mRNA expression of these genes increased after 3 weeks. Overall, these studies suggest that Si alleviates Cu toxicity by altering the balance of Cu uptake, redox homeostasis, and phytohormone signaling. The balance of these processes varies over time and illustrates the complexity of Si-mediated alleviation of Cu toxicity. Furthermore, these data demonstrate that a low Si accumulator does benefit from Si supplementation, and that Si-mediated alleviation of Cu toxicity corresponds with changes in many plant processes. |
