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II. Strategies of Plant Iron Acquisition

II. Strategies of Plant Iron Acquisition

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PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION



5



Figure 1 Calculated pH and total concentrations of dissolved inorganic Ca and Fe species as a

function of the CO2 partial pressure in equilibrium with calcite and ‘‘soil iron oxide’’ (Lindsay and

Schwab, 1982). All solubility coeYcients and equilibrium constants listed in Tables IV and V.



Various strategy‐I plant species of the Proteaceae, Casuarinaceae, Mimosadeae, Fabaceae (e.g., Lupinus albus), Myricaceae, and Moraceae families

respond to nutrient limitations by forming cluster (i.e., proteoid) roots that

are particularly eYcient in modifying rhizosphere pH and organic acid concentrations (Dinkelaker et al., 1995; Neumann and Martinoia, 2002). Cluster

root formation has been observed as a response to phosphorus (Dinkelaker

et al., 1995) and iron (Arahou and Diem, 1996; Gardner et al., 1982; Waters

and Blevins, 2000; White and Robson, 1989) limitations.

Strategy I plants promote the release of iron from organic complexes by

enzymatic reduction via membrane‐bound chelate reductases of rhizodermal

cells (Bienfait, 1985; Chaney et al., 1972; Robinson et al., 1999; Waters et al.,

2002). The reduction of iron in soluble complexes leads to the kinetic and

thermodynamic labilization of iron and facilitates its uptake (Marschner,

1995). For in‐depth discussions of the role of membrane‐bound reductases in

root iron uptake, the reader is referred to a number of reviews and textbooks

(Berczi and Moller, 2000; Curie and Briat, 2003; Hell and Stephan, 2003;

Marschner, 1995; Schmidt, 1999, 2003).



B.



STRATEGY II



Graminaceous plant species, including agriculturally important crops,

such as barley, wheat, and corn, respond to iron deficiency by exudation of

iron‐specific organic ligands, the so‐called phytosiderophores (Takagi, 1976;



6



S. M. KRAEMER ET AL.



Figure 2 Schematic representation of important processes in strategy II iron acquisition (not

to scale). PS, phytosiderophores; OrgAc, low‐molecular weight organic acids; MicrSid, bacterial

and fungal siderophores; HS, soluble particulate, or sorbed humic and fulvic substance.



Takagi et al., 1984). The resistance of graminaceous species to iron deficiency is

correlated to their phytosiderophore release rates (Roămheld and Marschner,

1990). In the apoplastic space and the rhizosphere, phytosiderophores can

scavenge iron from a range of iron‐bearing compounds including iron oxides

(Fig. 2).

The iron deficiency induced synthesis and exudation of phytosiderophores,

and the subsequent uptake of iron–siderophore complexes has been described

as the ‘‘strategy II’’ iron acquisition mechanism (Marschner et al., 1986b). This

strategy resembles bacterial and fungal iron acquisition systems involving

microbial siderophores. A large body of work has been devoted to the regulation and molecular level understanding of the plant physiological responses to

iron deficiency. We refer the reader to a number of excellent review and textbooks for detailed information on these subjects (Curie and Briat, 2003; Grotz

and Guerinot, 2003; Reid and Hayes, 2003; Schmidt, 2003). This chapter

focuses on the geochemical aspects of strategy II iron acquisition.



PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION



7



III. HOW MUCH IS ENOUGH? PLANT

IRON REQUIREMENTS

Iron is a constituent of a number of plant enzymes such as heme proteins

(including cytochromes, catalases, and peroxidases), iron–sulfur proteins (including ferredoxin, superoxide dismutase, and aconitase), lipoxygenase, and

so on (Marschner, 1995). Considering the array of functions of iron‐bearing

enzymes, it is not surprising that iron limitation has a range of consequences

including the impairment of various metabolic and biosynthetic functions and

of photosynthesis (Abadia, 1992; Marschner, 1995). The resulting deficiency

syndrome is iron‐deficiency chlorosis. The minimum total iron content of

iron‐suYcient plant leaves is in the range of 50–150 mg kgÀ1 dry weight

(Marschner, 1995).

The speciation of iron in the rhizosphere has an important eVect on iron‐

uptake rates. Hydroponic culture experiments have been very important to

investigate this eVect and to establish free ion activity models (FIAM) in

which uptake rates (V ) can be related to the activity of a rate‐controlling

metal species {Mi} (Hudson, 1998):

V ẳ f organism; chemical environment; physical environment; fMi gị

The rate‐controlling iron species is understood as the species, that is

directly taken up by the plant root with the highest uptake rate compared

to other species in the following reaction:

Mi ỵ X $ MX



ð1Þ



MX ! Mcellular



ð2Þ



where X is the receptor for Mi of the uptake system at the cell surface.

Generally, it is assumed that Mi is the metal aquo complex (the ‘‘free ion’’)

(Chaney et al., 1992; Morel and Hering, 1993; Parker and Norvell, 1999).

The activity of Mi is usually calculated by equilibrium models rather than

measured, due to the inherent diYculty to measure individual species. The

FIAM is based on the assumption that an equilibrium exists between Mi, all

other species in solution, and the binding sites of the transporter at the cell

surface (Hudson and Morel, 1990). The application of the FIAM to total plant

uptake requires that there is no indiscriminate uptake of complexes via breaks

of the Casparian strip etc. However, evidence of such direct uptake pathways

of natural and synthetic ligand complexes exists (Bell et al., 2003, 2005a;

Wang et al., 1993). Further complications arise if chelators of the metal ion

are toxic to plants (Rengel, 1999, 2002). Some excellent reviews and textbooks



S. M. KRAEMER ET AL.



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Table I

Iron Requirements of Various Plant Species Grown in Hydroponic Culture

Plant species



Ligand



[Fe]tota [M] Assumed Mib {Mi}c [M]



HEDTAd 6 Â 10À6

EDTAe

9 Â 106

f

Barley

HBED

10 106

Tomato, soybean DTPAg

?

Soybean



0.1 106

Barley

HEDTA 7.5 106

Barley



Fe3ỵ

Fe3ỵ

Fe3ỵ

Fe2ỵ

Fe3ỵ



1018

1019

1014.1

1028

107

1017.5



References

Bell et al., 1991

Bell et al., 2005b

Chaney et al., 1988

Lindsay and Schwab, 1982

Gries et al., 1995



a



[Fe]tot: Total soluble iron concentration in the nutrient solution.

Mi: The rate controlling metal species, here free aquocomplexes of Fe(III) or Fe(II).

c

{Mi}: Activity of a rate‐controlling metal species Mi.

d

HEDTA: N‐(2‐Hydroxyethyl)ethylenediamine‐N,N0 ,N0 ‐triacetic acid.

e

EDTA: Ethylenediaminetetraacetic acid.

f

HBED: N,N‐di(2‐hydroxybenzoyl)‐ethylenediamine‐N,N‐diacetic acid.

g

DTPA: Diethylenetriaminepentaacetic acid.

b



discussing the FIAM are available (Campbell, 1995; Campbell et al., 2002;

Hudson, 1998; Morel and Hering, 1993; Parker and Norvell, 1999; Parker and

Pedler, 1997).

The uptake of iron in strategy II plants proceeds via a high‐aYnity and a

low‐aYnity uptake system (von Wire´n et al., 1995). The high‐aYnity system

involves the enzymatic transport of intact Fe–phytosiderophore complexes

through the plasma membrane by a transporter (Roămheld and Marschner,

1986). This uptake system controls the rate of iron uptake under iron‐

limiting conditions. Therefore, it seems appropriate to assume that in this

case {Mi} is the activity of the Fe–phytosiderophore complex. Unless phytosiderophore concentrations are measured or added to the nutrient solution

to a known level, their concentration is not known and {Mi} cannot be

calculated in equilibrium models. Their concentration in hydroponic culture

experiments will be a function of exudation rates, degradation rates, and the

volume of nutrient solution relative to the root biomass among other factors. In the rhizosphere, siderophore concentrations are influenced by diVusional transport away from the root and advective transport to the root.

During maximum exudation periods, local siderophore concentrations can

reach very high levels as discussed later (Roămheld, 1991). Almost certainly,

local rhizosphere concentrations will be very diVerent from siderophore

concentrations in well‐mixed hydroponic culture experiments. Nevertheless,

iron‐limiting conditions are usually defined as the maximum activity or

concentration of the iron hexaquo complex (i.e., Fe3ỵ) at which iron

deficiency chlorosis occurs (see Table I).



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