5.2.3. Iron

Under iron deficiency plants can enhance their production of organic acids in order to lower pH and increase Fe solubility. Based on their response to Fe deficiency, plants are classified into two categories (Strategy I and Strategy II). Note that for both strategies the responses are limited to the apical regions of growing roots. Once the roots are supplied with iron, within one day these mechanisms are fully repressed.

Strategy I (Figure 5.5) is found in dicotyledonous and non-graminaceous monocotyledonous plants. Under iron deficiency these plants increase their ferric (Fe³⁺) reduction capacity, increase the acidification of the rhizosphere by ATP-ase H⁺ excretion and the release of organic acid anions and phenolic compounds into the soil solution. Root morphology and anatomy change, in particular the formation of transfer structures in rhizodermal cells. If iron is resupplied, both the physiological root responses disappear, and the transfer cells degenerate within 1 to 2 days. The formation of cluster roots is also enhanced in response to Fe deficiency. Cluster roots are hairy like structures that have a high capacity to reduce Fe(III) and excrete protons. Currently various iron transporters have been identified and described for root uptake, shoot transport and seed deposition of Fe. For example, over-expression of the nicotianamine synthase gene in rice grains resulted in an about three-fold increase in grain Fe concentration (Lee et al. 2009).

Figure 5.5: Model for root responses to iron deficiency in dicots and non-graminaceous monocots (Strategy I): increased acidification of the rhizosphere by H+-ATPases, induction of ferric reductase activity, reduction of Fe(III)-chelates to Fe2+, uptake of Fe2+ across the plasma membrane by Fe deficiency-inducible, high-affinity Fe2+ transporters. (Copied from Marschner 2012)

Strategy II is only found in graminaceous plant species (cereals and grasses). These plants release non-proteinogenic amino acids called phytosiderophores in to the rhizosphere when grown under iron deficiency. The chemistry of phytosiderophores is species specific and together with the excreted amount explains the different abilities of different grasses and cereals to acquire Fe (5.2.3. Iron:Figure 5.6).

Figure 5.6: Model for root responses to iron deficiency in graminaceous species (Strategy II): enhanced synthesis and release of phytosiderophores into the rhizosphere, chelation of Fe³⁺, Fe²⁺, Cu²⁺ and Mn²⁺, and transport of metal-phytosiderophore chelates across the plasma membrane by transport proteins. The structures of the phytosiderophore mucigenic acid and its corresponding Fe(III) chelate are also shown. (Copied from Marschner 2012)

Iron deficiency becomes apparent when leave Fe concentration comes in the range of 50–150 mg Fe kg^-1 dw. This refers to total Fe and because Fe is protein or cell structure bound this number is only of limited value for characterization of the Fe nutritional status of field-grown plants. Although C4 species require a higher Fe supply than C3 species, their critical deficiency concentrations are similar, i.e. 72 mg Fe kg^-1 in C3 species and 66 mg Fe kg^-1 in C4 species) (Smith et al. 1984). Note that in fast growing meristematic and expanding tissues, for example shoot apices, the critical deficiency concentrations are higher, in the range of 200 mg Fe kg^-1 dw of total Fe (Hurrell and Egli 2010). Moreover, Fe demand in legumes is high because of the role of iron in nodule development.

Especially on alkaline calcareous soils iron deficiency is a global problem in crop production. The so-called “lime-induced chlorosis” is caused by iron deficiency. Iron toxicity also known as ‘bronzing’ is a severe and yield limiting problem in crop production on waterlogged soils. Critical toxicity concentrations of iron depend on several factors such as concentration of other nutrients but are generally above 500 mg Fe kg^-1 leaf dw (Yamauchi 1989). Iron toxicity is associated with formation of ROS. For example, drought-induced damage in photosynthetic tissue is caused by Fe-catalysed formation ROS in the chloroplasts (Price and Hendry 1991).

Bioavailability of iron in seeds or grains is important in relation to human nutrition. Ferritin Fe is a valuable dietary source as ferritin from legumes is well taken up by the human gut. Ferritin production could, therefore, be a target compound for Fe biofortification of food crops. Phytate is the antinutrient of iron as it hinders iron uptake in the human gut by binding Fe and forming insoluble complexes (Marie Minihane and Rimbach 2002). Therefore, phytate rich diets (e.g., cereal-based foods) may be a key factor in high prevalence of Fe deficiency in humans (Hurrell and Egli 2010).