Iron (Fe) is abundantly present in most soils in its oxidized form (Fe3+), it is however hard to solubilise, especially under alkaline conditions. Iron solubilisation is required for plant uptake. Even in hydroculture where pH can be controlled, aeration can lead to Fe oxidation, making it unavailable for plant uptake. Therefore chelates, i.e. compounds that can bind Fe3+, are often used. Although chelates can significantly increase iron availability they are easily broken down under UV light from the sun or disinfection systems, which makes iron shortage a frequent problem in protected horticulture. As Fe is hardly mobile in plants, iron deficiency will therefore lead to chlorosis of the youngest leaves.

Once solubilized, Fe3+ can be actively reduced by the plant roots into Fe2+ (Figure 5.5) which can then be taken up and transported to other plants parts. However, Fe2+ is able to reduce oxygen, producing reactive oxygen species (ROS), i.e. oxidizing Fe2+ back into Fe3+. As ROS is very toxic to plants the reduction-oxidation (redox) of Fe needs to be well controlled to prevent oxidative damage in the cells. Fe has to be bound or incorporated into structures such as proteins which allow control of its oxidation-reduction process. An important example of a Fe oxidation regulating protein are cytochromes which constituents of the redox systems in chloroplasts, in mitochondria and also form a component in the redox chain in nitrate reductase. Enzyme activity can be used as biological indicator for Fe status of a plant. For example, the activity of both peroxidases (including ascorbate peroxidase) and aconitase (a key enzyme in the citric acid cycle of mitochondria) decrease as iron status of the plant decreases.

Iron is not part of but is required for the biosynthesis of chlorophyll. But is also plays a directly active role in photosynthesis as about 20 Fe atoms are present in the thylakoid membranes. These atoms are directly involved in the electron transport chain. Photosystem (PS) I contain around 12 Fe atoms, PSII 3 atoms of Fe per complex and the Cyt bf complex 5 atoms of Fe per complex. Additionally, iron is required for the number of ribosomes and protein synthesis.

An example of an iron containing enzymes are lipoxygenases, which is a group of enzyme that contain one Fe atom per enzyme molecule. Lipoxygenases catalyse the peroxidation of long chain polyunsaturated fatty acids, a crucial process as these fatty acids are components of cell membranes. Hence, high lipoxygenase activity is typical for fast growing tissue and may be critical for membrane stability. Ferredoxin is an important iron containing protein which acts as an electron transmitter in several metabolic processes, e.g. low Ferredoxin concentration is coincide with reduced nitrate reductase activity.

Phytoferritin (plant ferritin) is a hollow protein shell that serves as the “iron storage container” of plants. It can store up to 5000 atoms of iron as Fe(III) (Fe content 12–23% dw). When plants are grown in the dark, phytoferritin concentrations are high, up to 50% of the total Fe, but in light grown plants concentration are generally low. Ferritin-bound Fe is an important Fe source for protein synthesis of Fe-containing proteins used during photosynthesis (Briat et al. 2010). Ferritin is an important compound in Fe homeostasis and facilitates the protection against oxidative damage caused by ROS. Chloroplasts are not the only organs where ferritin can be found, also the xylem and phloem contain ferritin. Ferritin is moreover abundant in seeds it serves as a major form of Fe storage. In pea plants, for example, ferritin-bound Fe represents 92% of the total Fe in seed embryos (Marentes and Grusak 1998).


Iron uptake

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 (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).


Iron biofortification

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).


References

Briat JF, Duc C, Ravet K, Gaymard F. 2010. Ferritins and iron storage in plants. Biochimica et Biophysica Acta - General Subjects 1800: 806–814. DOI: 10.1016/j.bbagen.2009.12.003.

Hurrell R, Egli I. 2010. Iron bioavailability and dietary reference values. The American Journal of Clinical Nutrition 91: 1461S–1467S. DOI: 10.3945/ajcn.2010.28674F.

Lee S, Jeon US, Lee SJ, et al. 2009. Iron fortification of rice seeds through activation of the nicotianamine synthase gene. Proceedings of the National Academy of Sciences 106: 22014–22019. DOI: 10.1073/pnas.0910950106.

Marentes E, Grusak MA. 1998. Iron transport and storage within the seed coat and embryo of developing seeds of pea (Pisum sativum L.). Physiology & Biochemistry 8: 367–375. DOI: 10.1017/S0960258500004293.

Marie Minihane A, Rimbach G. 2002. Iron absorption and the iron binding and anti-oxidant properties of phytic acid. International Journal of Food Science and Technology 37: 741–748. DOI: 10.1046/j.1365-2621.2002.00619.x.

Marschner P. 2012. Marschner ’ s Mineral Nutrition of Higher Plants (Third Edition). Universitat of Hohenheim, Germany: Academic Press. DOI: 10.1016/B978-0-12-384905-2.X0001-5

Price AH, Hendry GAF. 1991. Iron‐catalysed oxygen radical formation and its possible contribution to drought damage in nine native grasses and three cereals. Plant, Cell & Environment 14: 477–484.

Smith GS, Cornforth S, Henderson H V. 1984. IRON REQUIREMENTS OF C3 AND C4 PLANTS. New Phytologist 97: 543–556. DOI: 10.1111/j.1469-8137.1984.tb03618.x.

Yamauchi M. 1989. Rice bronzing in Nigeria caused by nutrient imbalances and its control by potassium sulfate application. Plant and Soil 117: 275–286. DOI: 10.1007/BF02220722.