Agriculture Reference
In-Depth Information
chain and aconitase. Other Fe-S proteins are found in the nucleus and function in
DNA replication (Balk and Pilon
2011
) and damage repair (Liu et al.
2003
).
The coordination of the plant
s demand for Fe-S clusters supports the hypothesis
of a co-evolution between the Fe and the S metabolisms and the development of
interaction traits between them.
Chelated Fe and reduced S in form of cysteine represent the substrates of the
biosynthetic pathway. The Fe donor molecule is not known yet, whereas it is known
that sulfur is mobilised from cysteine by a cysteine desulfurase. The protein frataxin
has received attention for its putative role as Fe donor in the mitochondria assembly
pathway (Busi et al.
2006
).
As sulfur is present as acid-labile sulfide (S
2
) in the Fe-S cluster, two additional
electrons are needed to reduce elemental sulfur S
0
to S
2
(Lill
2009
). In the first step
of the pathway, the Fe-S cluster is assembled on scaffold proteins. In the second
step, the cluster is transferred to the specific apoprotein, which provides free amino
acidic residues to bind it. Additional carrier proteins are involved in this step. The
assembly machineries have been characterised in plants: chloroplasts contain the
ISC (iron-sulfur cluster) biosynthetic pathway, whilst mitochondria contain the
SUF-like (sulfur mobilization) pathway and in the cytoplasm the CIA (cytosolic
iron-sulfur cluster assembly) pathway (reviewed by Balk and Pilon
2011
; Couturier
et al.
2013
). The CIA is dependent on the mitochondria SUF assembly machinery,
which provided the sulfide-containing compound that is used for the biosynthesis of
the cluster. The mitochondrial ABC transporter, STARIK/ATM3, is thought to be
involved in this process. Indeed the mutant
atm3
shows severely impaired activity
of cytosolic aconitase while the mitochondria and plastidic isoforms are unaffected.
Furthermore,
atm3
does not accumulate Fe in the mitochondria and the general Fe
homeostasis is not affected (Bernard et al.
2009
).
Upon nutrient deficiency, a complex reprogramming of cell metabolism occurs,
in order to maintain viability. The strong requirement of Fe and S for the biosyn-
thesis of Fe-S clusters in the organelles might constitute a feedback signal for the
co-regulation of the assimilation pathways. Indeed the existence of such signals has
been recently proposed for Fe and S metabolism (Vigani et al.
2013
; Chan
et al.
2013
). The plant responses triggered by Fe or S deficiency have been well
characterised. The consequences of the combined shortage of these nutrients
however have only seldom been investigated, and might impact particularly on
Fe-S cluster assembly. Significant interactions between Fe uptake mechanisms and
external sulfate supply have been reported. It has been shown that S deficiency
limits the capacity to cope with Fe shortage in tomato plants, preventing the
expression of Fe chelate reductase
FRO1
and reducing the activity of Fe
2+
trans-
porter (Zuchi et al.
2009
). Other studies using barley plants reported a positive
correlation between S supply in the growth media and the plant capability of coping
with Fe deficiency. Indeed phytosiderophores represent another important junction
between Fe and S metabolism, as they are derived from the S-containing amino acid
methionine. The release rate of phytosiderophores was diminished in barley plants
upon sulfate deficiency, due to a decrease of the methionine level. After sulfate
resupply, plants increased the release of phytosiderophores when exposed to Fe
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