FIGURE 8.

The proposed model of Si-mediated alleviation of Fe deficiency in barley. In the early phase (5 h after Fe withdrawal) Si stimulates the expression of Strategy II genes in root, enhancing that way Fe uptake from the root apoplast and its remobilization toward shoot. In the mid-phase response (2 and 3 days after Fe withdrawal) the expression of Strategy II genes in root starts to increase under the influence of Fe deficiency. In this phase, Si enhances the expression of HvTOM1 and HvNAS1 in the leaf. As a consequence, after prolonged period of Fe deficiency, the status of Fe in the youngest leaf of Si treated plants is improved compared to the low Fe concentration in the plants grown without Si. Low Fe status in leaf of -Si plants is a signal for further increase in the gene expression in the root. On the contrary, improved Fe status in Si treated plants inhibits the expression of Strategy II genes. Another consequence of the reduced leaf Fe concentration is an increased uptake of other metals, which leads to elevated concentrations of Zn, Mn, and Cu in shoot. Si treatment increases concentrations and contents of Mn and Zn in root, resulting in decreased translocation toward shoot and lowered accumulation of these metals in the youngest leaves. The left halves of the schemes represent plants grown in the absence of Si, the right halves represent plants supplied with Si. Purple arrows – positive influence, purple T-end lines – inhibitory influence, brown open arrows – flow of the metal ions. Metal ions are represented by circles; the size of the circles in the youngest leaf corresponds to the concentration of the element.