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shortage (Astolfi et al.
2010
,
2012
). Recently the effect of Fe deficiency on sulfur
metabolism has been analyzed in durum wheat (Ciaffi et al.
2013
). Wheat plants
grown under sufficient S supply showed an up-regulation of certain S deficiency
responses when exposed to Fe deficiency. The expression of the high affinity sulfate
transporters was increased in the root, as well as of several genes of the S metabolic
pathway.
Recently we found that also in
Arabidopsis thaliana
the expression of the two
key genes for the uptake of Fe (
IRT1
) and of S (
SULTR1
;
1
) correlates with the
supply of both Fe and S in the growth media. The expression is differentially
regulated in case of double nutrient shortage (Forieri et al.
2013
). We suggested
that Fe-S cluster availability might function in sensing and signalling of combined
Fe and S deficiencies. Altogether, these analyses strongly support the existence of a
co-regulation between the metabolic pathways, as the limitation of one nutrient
influences the uptake of the other one. Such a co-regulation is very likely to be the
outcome of a complex remodelling of the whole plant metabolism upon nutrient
limitation as known for the prolonged deficiency of the single nutrients (Schuler
et al.
2011
; Nikiforova et al.
2003
). Hence, we propose different signals that might
contribute to this co-regulation, such as the sensing of the Fe and S concentrations
in the root rhizosphere or within root cells, ROS, metabolism intermediates, Fe-S
cluster assembly machineries or Fe-S proteins.
Conclusions
Fe is one of the most fascinating elements for life functions due to its redox
properties and is of great importance for human nutrition. Crop plants are the
direct or indirect source of Fe in our food, and research on the dicot model
plant
Arabidopsis thaliana
and also on rice has provided tremendous
advances in our understanding of plant Fe homeostasis in the past years. In
particular, the primary uptake processes into the root are now based on
molecular evidence for the genes involved in Strategy I (reduction based)
and Strategy II (chelation based). The allocation of Fe from the rhizodermis
to xylem and phloem for supply of young and growing tissues and
recirculation to roots is, however, much less well understood. Finally, trans-
port processes inside cells are beginning to be unraveled, explaining how iron
homeostasis is mediated between cytosol, plastids, mitochondria and the
vacuole. The key genes involved in these processes also represent possible
candidates in the search for Fe use efficient plants.
A crucial process in homeostatic control is the chelation of the almost
insoluble and redox active free Fe ions. Fe chelated by nicotianamine is
carried across plasmalemma and endomembranes and also for long distance
transport. Future work needs to address the mechanisms of donation of Fe
from chelators to acceptor molecules such as heme, Fe-S clusters and pro-
teins. In addition,
the regulatory networks of transcription factors that
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