Iron Acquisition by Plants

Although iron (Fe) is one of the major soil constituents (0.5-5%), where it is usually present in the oxidized state (Felll), plant availability is severely limited by the low solubility of Fe-(hydr)oxides at pH levels favorable for plant growth. Therefore, plants need special mechanisms foraquiring Fe from sparingly soluble Fe forms to fit the requirements for growth, especially in neutral and alkaline soils, where the availability of Fe is particularly low (151). Mechanisms involved in iron acquisition by plants are also discussed in Chap. 8. 

G. Scholz, R. Becker, A. Pich, and U. W. Stephan, Nicotianamine—a common constituent of strategy 1 and II of iron acquisition by plants a review. J. Plant Nutr. 75 1647 (1992). 

Iron deficiency is a serious nutritional disorder in many crops on neutral and alkaline aerobic soils because of the low availability of iron. However, some gramineous plants such as barley and wheat have developed a specific strategy to acquire sparingly soluble iron in soil they produce MAs which, as already mentioned, are involved in the iron acquisition process as phytosiderophores. The process of iron acquisition by plants has been divided into four steps, namely Fe-deficiency-induced biosynthesis of MAs inside the roots, secretion of MAs to the rhizosphere, solubilization of sparingly soluble inorganic Fe(III) in soils by chelation with MAs, and specific uptake of MA-Fe(III) complexes by the roots [32],... 

S. Mori, Mechanisms of iron acquisition by graminaceous (strategy ID plants. Biochemistry of Meta Micronutrients in the Rhizosphere (J. A. Manthey, D. E. Crowley, and D. G. Luster, eds.), Lewis Publishers, Boca Raton, Florida, 1994, p. 225. 

Mori, S., 1994. Mechanisms of iron acquisition by graminaceous (Strategy II) plants. In Crowley,... 

Figure 7 Mixld for iron (Fe) deficiency induced changes in root physiology and rhizo-sphere chemistry associated with Fc acquisition in strategy I plants. (Modified froin Ref. 1.) A. Stimulation of proton extru.sion by enhanced activity of the plasnialemma ATPase —> Felll solubilization in the rhizospherc. B. Enhanced exudation of reductanls and chela-tors (carhoxylates. phenolics) mediated by diffusion or anion channels Pe solubilization by Fein complexation and Felll reduction. C. Enhanced activity of plasma membrane (PM)-bound Felll reductase further stimulated by rhizosphere acidificalion (A). Reduction of FolII chelates, liberation of Fell. D. Uptake of Fell by a PM-bound Fell transporter.

The acquisition of iron, copper, and zinc in plant roots has been described in Chapter 7. Once within the root epidermal cell, the iron must be transported through the roots to the xylem and thence to the leaves, and this intercellular metal transport is illustrated for dicots in Fig. 8.8 and for monocots in Fig. 8.9. In dicots, Fe, Zn, and Cu are taken up into the symplast by transporters in the epidermis. Reduction of Fe and possibly of Cu by FR02 and acidification of the soil by an Arabidopsis ATPase contribute to increased metal uptake. Metals can then travel through the symplastic space to the vasculature. Transport into the xylem is still not fully characterised. In the case of Fe, it is probably as citrate, and the citrate transporter FRD3 has been shown to efflux citrate into the xylem and is required for Fe transport to the shoot. Zn and Cu are thought to be effluxed into the xylem by... 

Siderophores are highly specific Fe(III) ligands (formation constants often > 10 ) and are excreted by a wide variety of fungi and bacteria to aid iron assimilation because of the low solubility of Fe + at pH values where most life exists. Siderophores are the most common means of acquisition of iron by bacteria and fungi (Crichton, 1991), and in soil, many different species, including plants, will compete for Fe. Although produced primarily as a means of obtaining iron. 

---