Chelation root exudates

In contrast to strategy 1 plants, grasses are characterized by a diffeient mechani.sm for Fe acquisition, with Fe-mobilizing root exudates as main feature. In response to Fe deficiency, graminaceous plants (strategy II plants) (39) are able to release considerable amounts of non-proteinaceous amino acids (Fig. 8B), so called phytosiderophores (PS), which are highly effective chelators for Felll (Fig. 8)... 

The major soil properties that contribute to the changes in copper availability are pH, redox potential, DOC (including root exudates as chelates) and microbial activity (Horak, 1982 Hamon et al., 1995 Marschner and Rdmheld, 1996). To monitor the change in these properties during cultivation, the differences in levels of pH, Eh, DOC and MBM in rhizosphere and bulk soils were measured, and are depicted in Fig. 5. 

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 individual reactions affected by iron stress can be considered as regulated biochemical pathways, although regulation by iron is not understood. The mechanism of iron absorption and transport involves the release of hydrogen ions by the root, which lowers the pH of the root zone. This favors Fe3+ solubility and reduction of Fe3 to Fe2+. Reductants are released by roots or accumulate in roots of plants that are under iron stress. These "reductants, along with Fe3+ reduction by the root, reduce Fe3+ to Fe2+, and Fe2+ can enter the root. Ferrous iron has been detected throughout the protoxylem of the young lateral roots. The Fe2+ is probably kept reduced by the reductant in the root, and it may or may not have entered the root by a carrier mechanism. The root-absorbed Fe2+ is believed to be oxidized to Fe3, chelated by citrate, and transported in the metaxylem to the tops of the plant for use. We assume Fe2+ is oxidized as it enters the metaxylem because there is no measureable Fe2+ there (13), and Fe3+ citrate is transported in the xylem exudate (30, 31,32). 

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