Iron and Stress

Iron is a metal that is essential for all living things as it is heavily involved in cell redox reactions and the electron transport chain (a major part of aerobic respiration). However it is strongly reactive with oxygen - outside of living organisms this leads to rust, inside it can lead to the production of dangerous reactive oxygen species - and therefore needs to be controlled and contained within the cell. In order to provide this control, all living organisms (apart from yeast, weirdly enough) use a protein called ferritin. Multiple subunits of ferritin proteins (usually 24, although occasionally only 12) form an outer shell, with a central cavity that can contain around 2000-400 individual ferric ions (iron ions) keeping them safely out of harms way.

Diagram shows the shell created by ferritin (iron, or ferrous, ions help inside). Image from wikimedia commons.

In plants, the ferritin proteins are found in non-chlorophyll containing plastids, and occasionally in mitochondria (although no one is quite sure why). Many plants contain a number of different genes coding for ferritin proteins which, due to a high similarity in sequence identity and functional redundancy (i.e. most of them do similar things) makes individual ferritins difficult to study. The small weed Arabidopsis thaliana is a good model organism in this case as it contains only four ferritins, imaginatively names AtFer1-4.

These four ferritins are expressed differently at different stages of the cell lifecycle, and in responses to different materials. The diagram below shows which genes are upregulated in response to the oxidative compound H2O2, free iron (Fe) and the plant hormone abscisic acid (ABA):

Upstream of the AtFer1 gene is a 15 base pair sequence named IDRS (iron-dependent regulatory sequence) which is used to repress the gene under iron deficient conditions. This is thought to be upregulated by a phosphatase (which would remove phosphate from a DNA binding protein bound to the IDRS) which in turn is upregulated by the plant hormone NO (nitric oxide). The kinetics of AtFer3 are very similar to AtFer1 and it is therefore thought to be regulated by a similar system. AtFer2 may be activated in a different manner, and it shows very different kinetics to the other three genes. It does contain an IDRS sequence upstream of the gene, but it is not certain whether this is functional.

Despite containing a large source of iron, ferritins are likely to function more to prevent the damage caused by reactive oxygen species (caused by reactions of free iron) rather than as an iron store. Mutant plants containing no ferritin do not have an immediately obvious phenotype (outward appearance) although if extra iron is added they have to produce a large number of energetically wasteful detoxifying enzymes, in order to combat the dangers of oxidative stress. Ferritins therefore seem to have evolved not as an iron storage system, but as a buffering mechanism, to allow increases in iron within a plant to have a beneficial, rather than damaging, effect.

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Briat JF, Ravet K, Arnaud N, Duc C, Boucherez J, Touraine B, Cellier F, & Gaymard F (2010). New insights into ferritin synthesis and function highlight a link between iron homeostasis and oxidative stress in plants. Annals of botany, 105 (5), 811-22 PMID: 19482877