Phytosiderophores exudation

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... 

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)... 

Formation of stable chelates with phytosiderophores occurs with Fe but also with Zn, Cu, Co, and Mn (Fig. 8) (39,207,208) and can mediate the extraction of considerable amounts of Zn, Mn, Cu, and even Cd in calcareous soils (204,209). There is increasing evidence that PS release in graminaceous plants is also stimulated in response to Zn deficiency (210-212), but possibly also under Mn and Cu deficiency (213). Similar to Fe deficiency, the tolerance of different graminaceous plant species to Zn deficiency was found to be related to the amount of released PS (211,212), but correlation within cultivars of the same species seems to be low (214). It is, however, still a matter of debate as to what extent PS release is a specific response to deficiencies of the various inicronutrients. Cries et al. (213) reported that exudation of PS in Fe-deficient barley was about 15-30 times greater than PS release in response to Zn, Mn, and Cu deficiency. In contrast, PS exudation in Zn-deficient bread wheat was in a similar range as PS... 

Diffusion-mediated release of root exudates is likely to be affected by root zone temperature due to temperature-dependent changes in the speed of diffusion processes and modifications of membrane permeability (259,260). This might explain the stimulation of root exudation in tomato and clover at high temperatures, reported by Rovira (261), and also the increase in exudation of. sugars and amino acids in maize, cucumber, and strawberry exposed to low-temperature treatments (5-10°C), which was mainly attributed to a disturbance in membrane permeability (259,262). A decrease of exudation rates at low temperatures may be predicted for exudation processes that depend on metabolic energy. This assumption is supported by the continuous decrease of phytosiderophore release in Fe-deficient barley by decreasing the temperature from 30 to 5°C (67). 

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). 

Figure 7.11. DPASV (differential pulse anodic stripping voltametry) polarograms obtained for the exudate solutions obtamed from either Fe-deficient (-Fe) or Fe-sufficient (4- Fe) ryegrass Lolium perenne ). at two levels of addition of Cu (0.14 and 0.71 xM). The dashed line indicates the electric potential corresponding to the labile Cu species (presumably dominated by Cu- ) that was obtained in the Cu solution without exudates (0.011 V). The shift of the peak toward more negative electric potential values and lower intensities is indicative of increasing complexahon of Cu with decreasing Cu concentrations and increasing phytosiderophore secretion as a result of Fe deficiency. (Adapted from Thomas et al., 2005.)...

The application of pig slurry in agricultural land is a potential source of Cu and Zn contamination of soils. This study was conducted at the Solepur experimental site in Brittany, northwestern France, which received massive, controlled applications of pig slurry over 5 consecutive years. A first objective was to evaluate the effect of pig slurry apphcation on the bioavailabihty and chemical extractability of soil Cu and Zn. The bioavailabihty was assessed (i) either in situ via the analysis of ryegrass shoots (ii) or ex situ via a biotest with two grasses, ryegrass and wheat, which enabled easy access to plant roots and to the rhizosphere. A second objective was to examine precisely how rhizosphere processes could affect the bioavailabihty and chemical extractabihty of soil Cu and Zn, with a particular emphasis on the exudation of phytosiderophores as related to Fe deficiency. It was found that amounts of extractable Cu and Zn significantly increased as a consequence of heavy apphcations of pig slurry in the topsoil of the field site, regardless of the chemical extractant. However, the bioavailabihty of soil Cu and Zn, as assessed by plant analysis, was not always affected by pig slurry apphcation. When assessed in situ, Cu and Zn concentrahons... 

Methods for the collection and determination of the complexing properties of root exudates (phytosiderophores)... 

The ability of root exudates, and especially phytosiderophores, to modify the speciation of Cu and Zn via complexafion processes was assessed by voltametry (DPASV). The collecting solution, i.e. milliQ deioiuzed water (with or without bacteriostatic), was placed in contact with plant roots for 3 h. After collection of the exudates, the collecting solution was spiked with known amounts of both Cu and Zn. A control (exudate-free) solution followed the same protocol, except the contact period with plant roots. In the control solution, DPASV polarograms showed that all the added Cu and Zn were recovered as labile species with an electrical potential (Ep) of, respectively, 0.011 and —1.011 V (Fig. 3). A slight peak was also observed, which was attributed to trace contamination of Pb (Ep = — 0.402 V). When Micropur was added, an extra peak was found at Ep = 0.278 V that could not be attributed to any known metal species. Apart from this, very similar results were observed with or without... 

Analytical results were consistent with the observed effects of Fe deficiency on the bioavailability of soil Cu and the hypothesized involvement of the exudation of phytosiderophores in this process. Two possible pathways might be invoked (i) the exudation of phytosiderophores would complex Fe, result in an increased dissolution of Fe-oxide and thereby increase the release of Cu bound to this fraction or (ii) the exudation of phytosiderophores would complex Cu and thereby result in a greater release of Cu bound to any soil constituent. The present results favor the first pathway, but demonstration was inconclusive. It should be remembered that Cu jy and not only represent metals bound to Fe-oxide, but also Mn-oxides, which are also expected to be dissolved by this extractant (Ponthieu, 2003). Only the second pathway can be advocated for in the case of metals bound to Mn-oxides. 

The role of root exudates in the solubilization of nutrients has often been discussed (Bhat et al., 1976 Claassen, 1990). Integrating root exudation into mechanistic rhizosphere models is quite complex owing to the difficulties in quantifying the nature and role of the exudates. The effect of soluble exudates such as phytosiderophores, phosphatases, mucilage and polycarboxylic acids on the availability of P, Fe and heavy metals has been recognized (Tinker and Nye,... 

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