As sessile organisms, plants are dependent on a
balanced nutrient soil composition. This includes the presence of macronutrients,
such as nitrogen, phosphorous, magnesium, potassium and others, as well as micronutrients
which are considerably less abundant. Most essential micronutrients are metal
elements, such as Fe, manganese, nickel, copper, zinc and molybdenum. These metal
ions are much needed cofactors for enzymes, which are crucial for enzyme
activity. In addition, metal ions have the ability to change redox-states,
which renders them indispensable for metabolic pathways. Fe, for instance, is involved
in the mitochondrial electron transport chain. In chloroplasts, a major sink
for Fe, it is involved in photosynthesis and present in all electron transfer
complexes (Blaby-Haas and Merchant, 2013). Fe limitation causes a
re-structuring of the photosynthetic apparatus as well as interveinal leaf
chlorosis, since chlorophyll production is dependent on a Fe-cofactor
containing enzyme (López-Millán et al., 2016). Fe as co-factor is also present in
DNA polymerases and helicases and is hence involved in DNA replication and
repair (Zhang, 2014).

Two forms of Fe can be utilized by the plant, ferric
iron (Fe3+) and ferrous iron (Fe2+). Not only the
concentration of both forms in the soil, but also other factors such as soil composition
and pH affect the accessibility of Fe and hence plant nutrition. Although, with
4.32% being the fourth most abundant element to be found in the continental
crust (Wedepohl, 1995), Fe is mostly present as Fe3+ oxides
such as ferric hydroxide Fe(OH)3, which displays a limiting factor
for plants due to its low solubility. Neutral or alkaline soil pH, as is the
case for one-third of the soils (Chen and Barak, 1982), as well as the presence of oxygen,
leads to increased proportion of those insoluble Fe forms (Guerinot and Yi, 1994). Under such conditions, the plant’s
demand of approximately 10-9 to 10-4 M Fe for optimal
development cannot even be nearly met. In addition to the developing leaf
chlorosis, this often accounts for reduced crop yield or entire crop failure (Guerinot and Yi, 1994; Briat et al., 1995). As a
response to low amounts of Fe, plants employ different strategies to buffer and
minimize these effects. The synthesis of Fe-containing proteins or metabolic
pathways can be reduced in favor of Fe sparing and nutrient recycling. This is
executed in a hierarchical manner, where the transcript abundance of individual
genes is preferentially reduced over others or the degradation of certain
Fe-containing proteins is favored upon Fe limitations (Page et al., 2012; Blaby-Haas and Merchant, 2013;
Hantzis et al., 2017). In
order to elevate the Fe uptake capacity, the root architecture is altered, e.g. by increasing the amount to root
hairs, varying the length of the primary root as well as the lateral roots (Li et al., 2016). Ultimately, Fe deficiency results
in the expression of Fe uptake and homeostasis genes, which include respective
transcription factors, Fe transporters as well as chelators (Kim and Guerinot, 2007; Brumbarova et al., 2015). External
foliage application of Fe or fertilization with chelates or bio-available Fe is
possible but inefficient for agricultural application partly due to its
tremendous costs (Chen and Barak, 1982).

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Fe overaccumulation frequently occurs in acidic soils
or when plants are grown submerged in water. This can lead to severe problems
because Fe is highly reactive when freely present in the cell. In aerobic conditions,
Fe can participate in the Fenton reaction which leads to the formation of
highly reactive oxygen species and oxidative damage of cells (Ravet et al., 2009; Naranjo?Arcos and Bauer, 2016). A
first indication of Fe overload is the appearance of necrotic spots and bronzing
of leaves, which can ultimately lead to yield loss (Sperotto et al., 2010). Hence, plants quickly have to
react to high amounts of Fe, especially if unbound, by applying different chelation,
transportation and storage mechanisms.

Due to the crucial function of Fe in plastids and
mitochondria, both organelles display major Fe sinks. There, Fe3+ is
sequestered by the Fe storage protein ferritin (Zancani et al., 2004; Morrissey and Guerinot, 2009). In
both organelles, the synthesis of prosthetic groups, such as heme and Fe-S clusters,
aids a second way of sequestering free Fe. Heme and Fe-S clusters are either
directly involved in the respective metabolic pathways or used for the assembly
of holoproteins (Morrissey and Guerinot, 2009). Fraxatin displays another Fe storage
protein, which is suggested to be involved in the assembly of Fe-S clusters in
mitochondria (Busi et al., 2004; Bencze et al., 2006). The
vacuole is associated Fe storage, which is of special importance in seeds (Kim et al., 2006). Besides, Fe can also be stored in
the apoplast (Krämer and Clemens, 2006). Long-distance transport is
mediated through Fe2+-NA chelates which are transported via the
phloem as well as Fe3+-citrate complexes in the xylem (Curie and Briat, 2003; Briat et al., 2007; Morrissey
and Guerinot, 2009). This
displays that a well-balanced Fe uptake and homeostasis system is needed,
serving the need for Fe in plant metabolism on one site, and to avoid excessive
Fe accumulation which exceeds the cell sequestering capacities on the other
site.

It is of high importance to notice that decreased
nutrient availability in soils not only affects the well-being of plants or
agricultural economy. Affected plants do not provide high quality nutrition,
which consequently leads to a malnourished human population.