Agriculture Reference
In-Depth Information
several Fe-responsive genes. In contrast, cytokinin and jasmonic acid can nega-
tively regulate the expression of different Fe genes such as
IRT1
,
FRO2
and
FIT
(reviewed by Kobayashi and Nishizawa
2012
).
While the regulation of the Fe deficiency responses has been elucidated, the Fe
sensing mechanism in the root remains unidentified. Recently the IDEF1 transcrip-
tion factor has been found to bind directly to Fe and other divalent metals via its
proline-rich domains and histidine-asparagine residues (Kobayashi et al.
2012
).
Thus, this transcription factor might be one factor for the sensing of the actual Fe
situation in the cell and thus the Fe availability.
Fe Homeostasis in the Cell
After uptake in the root, further steps are required in order to allocate Fe in the rest
of the plant. Fe must first be transported from the root epidermis through the root
tissues to be loaded into the xylem. Due to its low solubility, a symplastic transport
is assumed, but little is known about the mechanism and possible carrier (Morrissey
and Guerinot
2009
). Once it reaches shoot tissues, other mechanisms must be
involved for
the unloading and the transport
into the different cellular
compartments.
The solubility problem requires that Fe must always be in a chelated form during
transport within the plant. Another important reason for the chelation is the poten-
tial toxicity of free ionic Fe that can catalyse the formation of reactive oxygen
species (ROS) causing cell damage. Citrate, NA and MAs are the known predom-
inant Fe chelators. In particular, citrate plays a fundamental role in chelating Fe in
the xylem. Indeed, citrate-Fe(III) complexes have been found in the xylem sap of
tomato plants (Rellan-Alvarez et al.
2010
). FERRIC REDUCTASE DEFECTIVE
3 (FRD3) is an
Arabidopsis
multidrug and toxin efflux (MATE) transporter that
plays a fundamental role in balancing Fe homeostasis. Its orthologue, OsFRDL1,
has been found in rice. Both transporters mediate the efflux of citrate into the
xylem. The
frd3
mutant displays chlorotic and dwarf phenotype, constitutive
up-regulation of the Fe deficiency genes and of FRO activity. A high level of Fe
is found in the roots of the mutant, due to inefficient Fe translocation to the shoot,
emphasising the importance of citrate for Fe transport from root to shoot (Durrett
et al.
2007
; Rogers and Guerinot
2002
). As FRD3 and FRDL1 efflux citrate in the
Fe-free form, other transporters must be involved in the transport of Fe into the
xylem. The
Arabidopsis
ferroportin 1/iron regulated 1 (AtFPN/AtREG1) could be
involved in this process. Although direct evidence is still lacking, the localisation,
the promoter activity and the mutant phenotype make this transporter a promising
candidate (Morrissey et al.
2009
).
Other transporters are involved in unloading the xylem into phloem. Members of
the YSL family are widely expressed in different tissues also of non-graminaceous
plants, suggesting a role in Fe translocation from xylem to phloem besides the
uptake of MA-Fe complexes from the soil. Indeed YSL transporters have been
