Although plants are fixed to the soil, yet they show some kind of movements. no alspam
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Fe is essential for plant growth. At the same time, Fe is highly reactive and toxic via the Fenton reaction. Consequently, plants tightly control Fe homeostasis and react to Fe deficiency as well as Fe overload. The ability of plants to respond to Fe availability ultimately affects human nutrition, both in terms of crop yield and the Fe concentration of edible tissues. Thus, elucidating the mechanisms of Fe uptake and transport is essential for the breeding of crops that are more nutrient rich and more tolerant of Fe-limited soils.
This review covers Fe transport and homeostasis in plants, focusing on the research published in the past five years. Because Fe transporters often have a broad range of substrates, we also examine the relationship between Fe and the toxic metals that often accompany Fe uptake, namely Cd, Co, and Ni. We begin by discussing Fe uptake into the root, then long-distance transport to the shoot, and finally, the loading of Fe into seeds. And, as Fe is essential to the metabolism of the mitochondria and chloroplast, we also look at the recent discoveries in Fe transport and homeostasis at the intracellular level. We do not cover the regulation of these transporters as this topic has been recently reviewed.1
2. Fe UPTAKE
Plants mainly acquire Fe from the rhizosphere. Although Fe is one of the most abundant metals in the earth's crust, its availability to plant roots is very low. Fe availability is dictated by the soil redox potential and pH. In soils that are aerobic or of higher pH, Fe is readily oxidized, and is predominately in the form of insoluble ferric oxides. At lower pH, the ferric Fe is freed from the oxide, and becomes more available for uptake by roots. Because 30% of the world's cropland is too alkaline for optimal plant growth,2 and some staple crops, like rice, are especially susceptible to Fe deficiency,3 much research has focused on how plants cope with Fe limitation.
The responses to Fe deficiency include changes in root morphology,2 and up-regulation of genes involved in Fe uptake. 4, 5 In fact, in Arabidopsis thaliana, up to 85% of the genes expressed in particular regions of the root are differentially regulated by Fe.4 This transcriptome analysis was made possible by the isolation, via fluorescence activated cell sorting analysis, of cells from specific root layers that were expressing GFP under the control of cell-specific promoters.6 The transcript levels within each layer were then measured via microarray analysis. This allows detection of differential expression profiles among specific cell types that cannot be seen when the root as a whole is examined. Large transcriptional differences between layers in response to Fe deficiency were identified, indicating layer-specific roles (Figure 1). The expression of genes related to metal transport and chelation was increased in the epidermis, while genes related to root hair morphogenesis were downregulated; in the stele, genes associated with signaling and stress responses were upregulated. These results suggest that sensing of Fe levels and control of the Fe deficiency response occurs in the vasculature, while regulation of Fe levels in the root is facilitated by modulating uptake in the epidermis.
When these Fe deficiency-induced changes were compared with the response to salt stress, it was found that the vast majority of the transcriptome is altered by environmental stress, and that these changes are most dramatic in the root epidermis. Interestingly, there is also a small set of genes unaffected by stress; this core may define the essential features of each cell type, and mediate the appropriate transcriptional responses to environmental stresses. Of the changes in the epidermis, two specific strategies of Fe uptake have been identified in plants. Non-graminaceous plants reduce Fe3+ via a membrane-bound reductase to make it accessible for uptake by a Fe2+ transporter, while grasses secrete phytosiderophores (PS) that readily bind Fe3+, and the Fe-PS complexes are then transported back into the roots.
This review covers Fe transport and homeostasis in plants, focusing on the research published in the past five years. Because Fe transporters often have a broad range of substrates, we also examine the relationship between Fe and the toxic metals that often accompany Fe uptake, namely Cd, Co, and Ni. We begin by discussing Fe uptake into the root, then long-distance transport to the shoot, and finally, the loading of Fe into seeds. And, as Fe is essential to the metabolism of the mitochondria and chloroplast, we also look at the recent discoveries in Fe transport and homeostasis at the intracellular level. We do not cover the regulation of these transporters as this topic has been recently reviewed.1
2. Fe UPTAKE
Plants mainly acquire Fe from the rhizosphere. Although Fe is one of the most abundant metals in the earth's crust, its availability to plant roots is very low. Fe availability is dictated by the soil redox potential and pH. In soils that are aerobic or of higher pH, Fe is readily oxidized, and is predominately in the form of insoluble ferric oxides. At lower pH, the ferric Fe is freed from the oxide, and becomes more available for uptake by roots. Because 30% of the world's cropland is too alkaline for optimal plant growth,2 and some staple crops, like rice, are especially susceptible to Fe deficiency,3 much research has focused on how plants cope with Fe limitation.
The responses to Fe deficiency include changes in root morphology,2 and up-regulation of genes involved in Fe uptake. 4, 5 In fact, in Arabidopsis thaliana, up to 85% of the genes expressed in particular regions of the root are differentially regulated by Fe.4 This transcriptome analysis was made possible by the isolation, via fluorescence activated cell sorting analysis, of cells from specific root layers that were expressing GFP under the control of cell-specific promoters.6 The transcript levels within each layer were then measured via microarray analysis. This allows detection of differential expression profiles among specific cell types that cannot be seen when the root as a whole is examined. Large transcriptional differences between layers in response to Fe deficiency were identified, indicating layer-specific roles (Figure 1). The expression of genes related to metal transport and chelation was increased in the epidermis, while genes related to root hair morphogenesis were downregulated; in the stele, genes associated with signaling and stress responses were upregulated. These results suggest that sensing of Fe levels and control of the Fe deficiency response occurs in the vasculature, while regulation of Fe levels in the root is facilitated by modulating uptake in the epidermis.
When these Fe deficiency-induced changes were compared with the response to salt stress, it was found that the vast majority of the transcriptome is altered by environmental stress, and that these changes are most dramatic in the root epidermis. Interestingly, there is also a small set of genes unaffected by stress; this core may define the essential features of each cell type, and mediate the appropriate transcriptional responses to environmental stresses. Of the changes in the epidermis, two specific strategies of Fe uptake have been identified in plants. Non-graminaceous plants reduce Fe3+ via a membrane-bound reductase to make it accessible for uptake by a Fe2+ transporter, while grasses secrete phytosiderophores (PS) that readily bind Fe3+, and the Fe-PS complexes are then transported back into the roots.
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