Phytosiderophore complexes

Dissolved organic molecules have many acidic functions (hydroxol and carbonic groups) to complex trace elements and their compounds to form soluble chelates. This is one of the reasons why solubility and bioavailability of trace elements in the rhizosphere are higher than bulk soils. At the same time, many organic acids also directly dissolve trace elements and their compounds in soils. Plant-produced phytosiderophores facilitate elements, such as Fe and Zn, uptake by plants (Zhang et al., 1991 Romheld, 1991 Hopkins et al., 1998). However, Shenker et al. (2001) did not find significant uptake of the Cd-phytosiderophores complex by plant roots. 

Grasses, cereals, and rice secrete phytosiderophores into the soil when grown under iron-deficient conditions, for instance, alkaline soils. The iron-phytosiderophore complex is then transported via the root cells to the phloem, where the iron is transferred to nicotianamine 31. a molecule closely related to mugineic acid 6 and translocated to the rest of the plant tissues. Many phytosiderophores are hydroxylated, and this results in a lowering of the pK values of the amino functions and, hence, reduces the ability of protons to compete with the binding of iron. 

Phytosiderophores structures and properties of tnugineic acids and their metal complexes. Y. Sugiura and K. Nomoto, Struct. Bonding (Berlin), 1984,58,107 (50). 

Strohmeier W (1968) Problem und Modell der homogenen Katalyse. 5 96-117 Sugiura Y, Nomoto K (1984) Phytosiderophores - Structures and Properties of Mugineic Acids and Their Metal Complexes. 58 107-135 Sun H, Cox MC, Li H, Sadler PJ (1997) Rationalisation of Binding to Transferrin Prediction of Metal-Protein Stability Constants. 88 71-102 Swann JC, see Bray RC (1972) II 107-144... 

Sugiura, Y., Nomoto, K. Phytosiderophores - Structures and Properties of Mugineic Acids and Their Metal Complexes. Vol. 58, pp. 107-135. 

Dissolved organic carbon and plant- or microbe- produced phytosiderophores increase the solubility of most trace elements in arid and semi-arid soils. This is especially important in arid regions. High pH increases the solubility of organic matter as dissolved organic carbon in arid soils. Copper, lead and nickel have a strong tendency to form complexes, while Cd complexes are weaker. Zinc, cobalt and manganese are... 

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. 

Some metals may need to be mobilized from the environment to make them bioavailable. Iron in particular must be rendered more soluble to be accessible for uptake. Microorganisms and some plants have evolved with secreted ligands known as siderophores (or phytosiderophores). These ligands bind Fe + with extraordinary affinity. For example, a complex of the siderophore enterobactin with ferric iron has a formal stability constant of 10 (19). Once siderophores compete with other environmental ligands for iron, the ferric iron-siderophore complex then binds to specific transport proteins at the microbial... 

Fig. 3a. Structures of phytosiderophores b dreiding model of the ferric nicotianamine complex...

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