Agriculture Reference
In-Depth Information
expression of most NAS genes is strongly induced upon Fe deficiency. NA is then
further processed by NA aminotransferase (NAAT) and deoxymugineic acid
synthase (DMAS) to form 2
0
-deoxymugineic acid (DMA). DMA is the starting
point for the synthesis of all the other chemical forms of MAs (Nakanishi
et al.
2000
). The secretion of MAs is diurnally regulated, with a peak in the morning
(Cakmak et al.
1998
). The transporter of mugineic acid family phytosiderophores
1 (TOM) has been identified in rice and barley as responsible for the secretion of
MAs (Nozoye et al.
2011
). After secretion, MAs can bind to Fe
3+
and the MA-Fe
3+
complexes are taken up by the root YELLOW STRIPE 1 (YS1) and YELLOW
STRIPE 1-like transporters (YSL1) (Curie et al.
2009
; Inoue et al.
2009
). The study
of these transporters started with the analysis of the maize mutant
yellow stripe 1
.
This mutant shows leaf chlorosis and fails to take up phytosiderophores from the
soil (Von Wiren et al.
1994
). The
YELLOW STRIPE 1
gene was then mapped and
cloned (Curie et al.
2001
) and found to encode for a membrane transporter that
mediates the uptake of phytosiderophores bound to Fe
3+
. The biochemical function
is directly shown by the ability of YS1 to restore the growth of a yeast strain
deficient in Fe uptake, when phytosiderophores are present in the media. Its mRNA
accumulates significantly under Fe deficiency in both root and shoot. The latter was
surprising as phytosiderophores were not expected to be transported in green tissues
as they are only present in the root (Curie et al.
2001
). Indeed, although the release
of phytosiderophores is a prerogative of graminaceous plants, YSL transporters are
found also in non-graminaceous taxa (Chu et al.
2010
,
2013
). In particular,
Arabidopsis thaliana
has eight YSL genes (M¨ser et al.
2001
). In addition, YSL
transporters have been found to transport NA-Fe
3+
complexes (reviewed by Chu
et al.
2013
), adding to the fundamental role of NA as Fe chelator in both strategy I
and strategy II plants.
Interestingly, rice additionally possesses an iron transporter, OsIRT1, an
ortholog of AtIRT1. However, rice roots show a very low ferric-chelate reductase
activity, suggesting a role of OsIRT1 in the direct uptake of Fe
2+
in anaerobic
growth conditions that is typical for this crop (Ishimaru et al.
2006
).
Both graminaceous and non-graminaceous plants possess other divalent metal
transporters, which can facilitate Fe assimilation. The NRAMP (natural resistance-
associated macrophage
p
rotein) family transporters have been first found in mam-
mals and have then been cloned from
Arabidopsis thaliana
(Curie et al.
2000
) There
are six genes encoding for NRAMP proteins in
Arabidopsis
(M¨ser et al.
2001
).
They can mediate the uptake of several divalent metals, including Fe
2+
, zinc Zn
2+
,
manganese Mn
2+
, nickel Ni
2+
and cadmium Cd
2+
. The expression of three of these
transporters, NRAMP1, NRAMP2 and NRAMP4, is induced by Fe deficiency in
roots and leaves. In particular, NRAMP1 is thought to mediate the uptake of Fe and
other essential nutrients, such as manganese from the soil (Curie et al.
2000
;
Cailliatte et al.
2010
). The other two transporters NRAMP3 and NRAMP4 are
involved in the Fe distribution to developing seeds in low Fe conditions (Lanquar
et al.
2005
).
