Agriculture Reference
In-Depth Information
After reduction, the uptake of Fe
2+
in the root epidermal cells is performed by
the iron-regulated transporter 1 (IRT1) in
Arabidopsis
(Vert et al.
2003
). Orthologs
of IRT1 were cloned in both pea and tomato (Cohen et al.
1998
; Eckhardt
et al.
2001
). The
Arabidopsis
knock-out mutant
irt1
is lethal unless plants are
watered with an excess of Fe (Vert et al.
2002
). Another transporter, IRT2, is also
induced in roots when exposed to Fe shortage (Vert et al.
2001
) but its knock-out
mutant is not affected under normal Fe conditions. The attempt to complement the
irt1
mutant with
IRT2
driven by a constitutive promoter did not rescue the pheno-
type, showing that the two transporters have different roles in Fe uptake (Varotto
et al.
2002
). Both transporters belong to the zinc-regulated transporter iron-
regulated transporter like
pr
otein family (ZIP) that takes its name from the first
transporter that was identified, the zinc regulated transporter ZRT. Indeed IRT1 and
IRT2 are not highly specific for Fe and can mediate the import of a broad spectrum
of metal species including zinc Zn
2+
, manganese Mn
2+
and cadmium Cd
2+
. The
Arabidopsis thaliana
genome encodes for 16 ZIP proteins (M¨ser et al.
2001
) and
they function in the uptake of different bivalent metal ions.
IRT1
is regulated at different levels. Its expression promptly responded after the
plants were transferred to Fe deficiency (Connolly et al.
2002
). In particular, the
mRNA accumulated after 24 h and the protein level peaked after 72 h. After Fe
resupply the IRT1 protein was already almost undetectable after 12 h, indicating
that its expression is tightly regulated. An over-expressing line of
IRT1
showed
accumulation of the protein only under Fe deficiency, indicating a fine regulation of
Fe homeostasis at the uptake level. Indeed IRT1 protein is rapidly degraded in
response to changing Fe conditions and this degradation is mediated by
ubiquitination (Kerkeb et al.
2008
; Barberon et al.
2011
). Recently the specific
E3 ubiquitin ligase IRT1 DEGRADATION FACTOR 1 (IDF1) was identified in a
screen of insertional mutants (Shin et al.
2013
).
The main feature of Strategy II is the excretion of chelating compounds such as
mugineic acids (MAs) in the root rhizosphere that chelate Fe
3+
, to enhance solu-
bility and allow for mobilisation of Fe
3+
(Fig.
5.1
). The name mugineic acid is
derived from the Japanese word komugi for wheat from which these compounds
had first been isolated. The MAs biosynthetic pathway is conserved among the
Poaceae family, which comprises many of the most important food plants: rice,
wheat and maize, and starts from
S
-adenosyl-L-methionine (SAM). Three mole-
cules of SAM are converted into nicotianamine (NA) in one reaction by
nicotianamine synthase (NAS). NA is a precursor for MAs in strategy II plants
but additionally functions as Fe chelator in different plant organs, and seems to be
essential not only for the uptake of Fe from the soil, but also for the cell-to-cell and
long distance transport within the plant (Schuler et al.
2012
). Studies on NA started
with the analysis of the tomato mutant
chloronerva
. This mutant is NA-free and
shows retarded growth and intercostal chlorosis of young leaves. Map-based clon-
ing revealed that chloronerva is a single copy gene in tomato and encodes for NAS
(Ling et al.
1999
). The NAS genes have been cloned from other plant species such
as barley, rice and
Arabidopsis
, showing that NA carries out functions in strategy I
and II species (reviewed by Hell and Stephan
2003
; Klatte et al.
2009
). The
