Interaction in the rhizosphere

Organic ligands bind metal ions with different stability constants. In the soil solution in the rhizosphere, metals can either form this kind of complexes or precipitate with inorganic anions. Knowing the stability constants of complexes (Table 6-1) and the pH of the nutrient solution, it is possible to theoretically predict which metals and ligands will form complexes and which will precipitate, using specialized software developed for this purpose (MinteqA2, Allison et al., 1991; Geochem-PC, Parker et al., 1995). Any software, however, may fail to give a reliable prediction in soils, where many different metals and ligands are present in the rhizosphere and physico-chemical conditions (e.g. adsorptive surfaces) are very diverse.

Undoubtedly, heavy metals compete with Fe for organic ligands. Although Fe3+ forms the most stable chelates as compared with divalent heavy metal ions, after reduction of Fe3+-chelates at the outer surface of the root cell plasmalemma in dicotyledonous plants the released free chelating agents may chelate other metals such as Pb2+ or Cd2+ if no more free Fe3+ ions are available (Srivastava and Appenroth, 1995). The competition would also depend on the stoichiometry of competing ions.

Table 6-1. Stability of some metal chelate complexes presented as logK for the equation Men+ + Lm- = MeL(m-n)-. (L: EDTA, ethylenediamintetraacetic acid; citrate; MA, mugineic acid; DMA, deoxymugineic acid; NA, nicotianamine).

Metal

EDTA1

Citrate1

MA3

DMA

NA

Fe3+

25.1

11.5

17.7

18.12

20.62

Fe2+

14.3

4.4

10.1

10.43

12.84

Zn2+

16.5

5.0

12.7

12.83

15.44

Cu2+

18.8

5.9

18.1

18.73

Mn2+

13.9

4.2

8.3

8.33

Ni2+

18.6

5.4

14.9

14.83

Cd2+

16.5

3.8

Pb2+

18.0

4.1

1Smith and Martell, 1989, 2von Wiren et al., 1999, 3Murakami et al., 1989, 4Anderegg and Ripperger, 1989.

1Smith and Martell, 1989, 2von Wiren et al., 1999, 3Murakami et al., 1989, 4Anderegg and Ripperger, 1989.

Metal ions are first adsorbed by the root/cell wall carbohydrate network. Its pectin constituents, glucuronic and galacturonic acids, have diffusible protons that can be easily exchanged by metal ions. The adsorption of metals on these binding sites is of the same nature as complexation with soluble organic acids. However, no specific stability constant has been determined for metal binding at the cell wall, due to difficulties originated from their complex nature. In cucumber treated with 10 ^M Pb(NO3)2 and supplied with the same concentration of Fe-citrate or Fe-EDTA, the relative stability of cell wall-Pb bonds was determined by bleaching experiments with Na2EDTA and citric acid, respectively. It was found that the stability order was Pb-citrate < Pb-cell wall < Pb-EDTA (Varga et al., 1997). Iron chelates having similar or higher stability as Pb-EDTA (Table 6-1) usually tend to stay in the solution instead of being adsorbed at the cell wall, resulting in very low Fe concentrations in the apoplast. In cucumber roots, Fe-accumulation was approximately 3 times higher with Fe-citrate than with Fe-EDTA, both in control and Cd-treated plants (Fodor et al., 1996). Iron concentrations may increase in the root surfaces in unbuffered solutions even when chelates such as Fe-EDTA are applied, as a consequence of Fe precipitation due to low chelate stability above pH 6.5 (Strasser et al., 1999).

The real amount of metals adsorbed either specifically or unspecifically in the root apoplast is not easy (if at all possible) to be determined. So far, the only apoplastic Fe determination methods generally accepted are those based on the reduction of Fe3+ in N2 atmosphere by sodium dithionite and subsequent complexation with a,a-bipyridyl (first proposed by Bienfait et al., 1985; Becker et al., 1992; Nikolic et al., 2000). This procedure is believed to remove reliably all apoplastic Fe. Besides, strongly and weakly bound Fe fractions can be distinguished by using separately a cell wall isolation protocol and an infiltration centrifugation technique (Nikolic and Romheld, 2003). Although the procedure of Bienfait et al. (1985) was initially designed for roots, it may be generalized that this method solubilizes only a weakly bound fraction of Fe, which is about twice as large as the strongly bound fraction. However, it is unclear if the strongly bound fraction of Fe, and perhaps those of other heavy metals, has any significance concerning their metabolic utilization.

There is a lack of information on the apoplastic concentrations of other heavy metals, although some attempts have been made to have an estimate. An example is the use of sodium EDTA for the removal of metals from extracellular spaces (Varga et al., 2000, 2002). Studies with Cd-, Ni- and Pb-treated plants revealed that the majority of Pb and Fe accumulated in the apoplasm (more than 70% and 76%, respectively), whereas more than 65% of Cd, 85% of Ni and 67% of Mn (the latter only in Pb treated plants) were not removable by EDTA. Heavy metals applied at the same concentration as Fe (10 ^.M) increased the total and apoplasmic concentration of Fe in the roots. Results, however, depended on the form of Fe used. Both apoplasmic Pb and Fe increased in Cd-, Ni- and Pb-treated plants when Fe-citrate was supplied as compared to Fe-EDTA (Varga et al., 2002). Although the authors did not carry out a validation for the removal procedure and the scavenging of metals from the apoplasm may not be complete, the results can be well interpreted. Comparing the stability of complexes formed by EDTA and the above metals (see Table 6-1) it is clear that Fe can be removed most efficiently, followed by Cu, Ni and Pb. However, Ni was retained in the roots, and may be strongly bound or absorbed into the cytosol. These data suggest that chelating agents may substantially influence metal adsorption patterns in root cell walls, but for their accumulation it may be more important how they are taken up into the cytosol.

Complexation with chelates not only improves solubility of metal ions, but also ensures their mobility. However, the complexes must reach the root cell plasmalemma transport proteins as a prerequisite to uptake, and the cell wall matrix is a natural filter. Apart from the cation exchange function of pectin chains, that reduces the intrusion of cations, the network of filaments contains pores, and the interfibrillar and intermicellar spaces spatially limit the free movement of particles larger than hydrated ions or low molecular weight organic solutes. The size of metal chelates is beyond this limit (Marschner, 1995). However, there may be substantial differences in pore size in plant species. How do the complexes reach the plasmalemma and the transporter proteins? Despite this open question, Fe-chelates are very efficient in supplying plants with Fe, and complexes of other heavy metals have been also shown to facilitate their accumulation in the shoot.

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