Phytosiderophores uptake

N. von Wiren, S. Mori, H. Marschner, and V. Romheld, Iron inefficiency in maize mutant ysl Zea mays L. cv. Yellow-stripe) is caused by a defect in uptake of iron phytosiderophores. Plant Physiol. 106 1 (1994). 

Table 3 Effect of Various Antibiotics on Phytosiderophore (PS) Concentrations in Root Exudates and PS Uptake in Fe-deficient Barley and Sorghum...

Treatment/Plant species Phytosiderophore concentration in root exudates [relative values] Fe-PS uptake rate... 

H. Marschner, V. Romheld, and M. Kissel, Localization of phytosiderophore re-lea.se and iron uptake along intact barley roots. Physiol. Plant. 71 51 (1987). 

V. Romheld and H. Marschner, Evidence for a specific uptake system for iron phytosiderophores in roots of grasses. Plant Physiol. 80 15 (1986). 

Nutrient availability also plays a major role in exudation, with deficiencies in N, P, or K often increasing the rate of exudation (218). It is believed that nutrient deficiency may trigger the release of substances such as organic acids or nonproteinogenic amino acids (phytosiderophores), which may enhance the acquisition of the limiting nutrient (219,220). An example here might be the release of phenolic acids such as caffeic acid in response to iron deficiency, which results in an increase in uptake of the cation (221). 

In the first case the mechanisms are based on an increased reducing capacity of Fe(lll)-chelates, a necessary step in the uptake process, with a concurrent increase in acidification and release of organic acids into the rhizosphere in the latter case molecules having high affinity for Fe (phytosiderophores) are synthesized and released into the rhizosphere when Fe is lacking. 

IV. IRON UPTAKE MECHANISMS FROM SIDEROPHORES AND PHYTOSIDEROPHORES... 

In addition to iron, phytosiderophores also function for the uptake of zinc... 

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

D. E. Crowley. V. Romheld, H. Marschner, and P. J. Szaniszlo. Root-microbial effects on plant iron uptake from siderophores and phytosiderophores. Plant Soil 142 1 (1992). 

H. Marschner, V. Romheld, and M. Kissel, Localization of phytosiderophore release and of iron uptake along intact barley roots. Physiol. Planta. 77 157 (1987). V. Romheld, The role of phytosiderophores for acquisition of iron and other micronutrients in graminaceous species An Ecological Approach Iron Nutrition and Interactions in Plants (Y Chen and Y Hadar, eds.), Kluwer Academic Publishers, Boston, 1991, pp. 159-166. 

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. 

Shenker M., Fan T.W.M., Crowley D.E. Phytosiderophores influence on cadmium mobilization and uptake by wheat and barley plants. J Environ Qual 2001 30 2091-2098. 

Zhang F., Romheld V., Marschner H. Diurnal rhythm of release of phytosiderophores and uptake rate of zinc in iron-deficient wheat. Soil Sci Plant Nutri 1991 37 671-678. 

Using the PETIS, real-time [ CJmethionine translocation was studied for barley. For the mechanism of Fe uptake in an Fe-delicient barley, it was found that leaf methionine does not participate in the reaction of mugineic acid synthesis, but the methionine produced in barley roots is used in the biosynthesis of mugineic acid phytosiderophores [131,132]. 

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... 

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. 

In strategy II monocots, the key transcriptional regulator of genes involved in phytosiderophore synthesis and iron uptake is the BHLH protein, IR02. A model of the regulatory network has been established. In response... 

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