Agriculture Reference
In-Depth Information
The study of the regulation of Fe responses by using these pea mutants
(and other approaches) is not solely important in relation to Fe nutrition but
also in relation to other aspects related to plant nutrition and nodulation. As
examples, it should be mentioned that these mutants are also affected in the
accumulation of Al [21] and other micronutrients besides Fe [17], and also in
nodulation [20, 22].
F
E
A
CQUISITION IN
D
ICOT
P
LANTS
The main characteristic of Strategy I plants, that include dicot plants such
as pea and other legume crops, is the necessity of reducing Fe
3+
, the most
abundant in soils, to Fe
2+
, which is the one preferentially absorbed. The Fe
3+
reduction is mediated by a ferric reductase while the Fe
2+
uptake is mediated
by a Fe
2+
transporter, both of them located in the plasma membrane of root
epidermal cells [1]. The genes encoding the ferric reductase and the Fe
2+
transporter were first cloned in Arabidopsis (
AtFRO2
, Ferric Reductase
Oxidase [23];
AtIRT1
, Iron Regulated Transporter [24]), and subsequent
homologs have been cloned in other plant species, such as tomato and
cucumber [25-27]. In pea, the gene encoding the ferric reductase (
PsFRO1
)
was cloned by Waters et al [28] and the gene encoding the Fe
2+
transporter
(
PsRIT1
, Root Iron Transporter) by Cohen et al [29]. The ferric reductase has
also been cloned in other legume crops, such as peanut (
AhFRO1
[30]) and
Medicago truncatula
(
MtFRO1
[31]). The enhanced expression of
PsRIT1
(and
other
IRT1
-like genes) under Fe deficiency is associated with increased uptake
capacity of several divalent metals besides Fe
2+
, like Cd
2+
, Zn
2+
and Mn
2+
,
which suggests that the PsRIT1 transporter (and other IRT1-like transporters)
has a broad substrate range [29,32-33]. Under Fe-deficient conditions, Strategy
I plants induce morphological and physiological changes in their roots, known
as Fe responses, which favor Fe acquisition. Among the physiological
responses are: enhanced ferric reductase activity (due to higher expression of
the
FRO
gene); enhanced Fe
2+
uptake capacity (due to higher expression of the
IRT
gene); acidification of the rhizosphere [due to higher expression of
HA
(H
+
-ATPase) genes]; release of riboflavin and phenolic compounds; and
enhanced production of organic acids, like citric acid [1,14]. Some H
+
-ATPase
genes up-regulated by Fe deficiency are already known in dicot plants, such as
the
CsHA1
in cucumber [27,34]. To our knowledge, no H
+
-ATPase gene
related to Fe deficiency has been identified in pea yet although it has been
cloned in the legume
Medicago truncatula
(
MtHA1
[31]). Among the
