Strategy I plants

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.

Based on the above observation, it can conceivably be concluded that the presence of Fe-WEHS-type complexes can be of relevance to the Fe nutrition of both strategy I and strategy II plants (Fig. 1). In the case of strategy I plants, an effect on the mechanisms of active proton extrusion should also be considered (see Sect. V). 

In iron-deficient conditions Strategy I plants (Figure 4.1) acidify the soil, through the activation of a specific H+-ATPase (Guerinot and Yi, 1994), increase their reducing capacity at the root plasma membrane, and possibly also release reduc-tants into the rhizosphere/I 11 They also release iron chelators, such as caffeic acid... 

Figure 8.9 Model for the regulation of iron deficiency responses in Strategy I plants. (From Schmidt, 2003. Copyright 2003, with permission from Elsevier.)...

The results of this work support the view that Fe-WEHS complex may serve as a natural substrate for the inducible plasma-membrane-bound Felll-chelate reductase in Strategy I plants (Pinton et al., 1999b). In Strategy II plants, an indirect mechanism, conceivably operating via ligand exchange between the humic fraction and the phytosiderophores released, appears to be involved in the use of Fe bound to WEHS. 

Plant iron uptake can be divided into two distinct families, with quite distinct strategies. Strategy I plants reduce Fe + to Fe + outside of the roots, and then take up the Fe +. In contrast. Strategy II plants solubilise Fe + by excreting Fe +phytosiderophores, which are taken up by specihc transporters and the iron is then reduced to Fe " " in the symplasm of the root cell (Fig. 7.14). In Strategy I plants, (dicotyledons such as Arabidopsis pea. 

A model for the regulation of iron-deficiency responses in strategy I plants (Fig. 8.12) involves a regulatory circuit consisting of two parts (a) a shoot—root loop and (b) a root—shoot loop. In the root—shoot part of the... 

Under Fe-deficient conditions, Strategy I plant species (i.e. all plant species but grasses) show a typical enhanced release of protons (and thus rhizosphere acidification) behind the apical root zone (Marschner et al, 1982 Romheld,... 

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