Agriculture Reference
In-Depth Information
approach provided more evidence that ferritins are not actually required for the
proper formation of the photosynthetic chloroplast or for the functioning of the
photosynthetic apparatus. Indeed, ferritins seem to play a fundamental role in the
protection against oxidative stress (Ravet et al.
2009
).
Other studies have attempted to further elucidate the localisation of ferritins in
plant cells.
According to Zancani et al. (
2004
) ferritins can also localise to the mitochondria.
Indeed, according to bioinformatics analyses, the
Arabidopsis
AtFER4 is the
isoform most likely to be targeted to the mitochondria. A study was conducted
based on the knock-out mutant
atfer4
. An antibody against FER was applied to
protein fractions of isolated mitochondria and a ferritin signal was found in
mitochondria isolated from wild type plants subjected to high Fe supply. The signal
was not present in the fraction isolated from the mutant plants. This mitochondrial
isoform seems to be of great importance for balancing Fe homeostasis in hetero-
trophic tissues, as shown by work on suspension cell cultures (Tarantino
et al.
2010
). Petit et al. (
2001
) identified a
cis
-element in the region of maize ferritin
gene
ZmFER1
and in its orthologue from
Arabidopsis AtFER1
. This regulator
named iron-dependent regulatory sequence (IDRS) is able to repress the transcrip-
tion of the gene under low Fe conditions. IDRS also has additional functions in
Arabidopsis
, where it triggers the expression of
AtFER1
under dark-induced senes-
cence but not in age-dependent senescence and in seedlings (Tarantino et al.
2003
).
This suggests that more regulatory elements must be involved in the regulation of
AtFER1
expression under such conditions.
Time for coffee (TIC) has been found in a luciferase-based genetic screen of the
AtFER1
promoter (Duc et al.
2009
). TIC has been previously described as a nuclear
component of the circadian clock. Mutants of TIC are chlorotic unless supplied
with exogenous iron and are hypersensitive to iron during the early stages of
development. Thus TIC is a central regulator of
AtFER1
as it represses its expres-
sion in low Fe conditions, in a way that is independent from IDRS. The
tic
mutants
also fail to repress other genes induced by Fe overload under low Fe, pointing out
that TIC-dependent pathways are fundamental for the response to Fe overload.
Another Fe binding protein, which has been reported to be involved in the
protection against photo-oxidative damage is frataxin. This protein has been
hypothesised to participate in the mitochondrial biosynthesis of Fe-S cluster acting
as Fe donor. Its importance for plant cell has been demonstrated by analysis of
T-DNA insertion mutants in
Arabidopsis
. Indeed frataxin knock-out mutants are
lethal, while the knockdown ones are viable but accumulate high levels of ROS and
induce the expression of genes encoding for ROS scavenging proteins (Busi
et al.
2006
).
The import of Fe into the chloroplast is linked to ferritin function. The permease
in chloroplast 1 (PIC1) was first identified as member of the chloroplast inner
membrane translocon complex TIC (Teng et al.
2006
). This transporter emerged
as a possible candidate in a bioinformatic screening for the Fe importer in the
chloroplast proteome, due to its biochemical characteristics such as hydrophobicity,
basic isoelectric point and predicted transmembrane domains, and was therefore
