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direct interaction between PYE and BTS has not been reported, but interestingly
BTS interacts with the PYE interactors ILR3 and bHLH115. It is therefore specu-
lated that this interaction participates in the regulation of Fe deficiency responses in
the root. The induction of PYE under Fe limiting conditions might serve to regulate
Fe homeostasis in the plant. Additionally BTS, the antagonist of PYE, might help in
this regulation controlling PYE activity (Long et al.
2010
).
Other regulatory elements have been identified in Strategy II plants (Fig.
5.2b
).
The analysis was based on stepwise promoter analysis of the barley
IDS2
gene in
tobacco and allowed the identification of two key regulators of Fe deficiency
responses, the
cis
-acting iron deficiency responsive element 1 (IDE1) and IDE2
(Kobayashi et al.
2003
). IDE1 and IDE2 were the first discovered
cis
-acting
elements related to nutrient deficiency. From sequence alignment of the promoters
of several Fe responsive genes it emerged that these
cis
-elements are quite con-
served among different plant species. Indeed they have been found in several genes,
e.g. HvNAAT
,
HvNAS
,
OsNAS2
,
OsNAS3
,
OsIRT1
,
AtIRT1
and
AtFRO2
. IDE1 and
IDE2 can interact with two rice transcription factors IDE-binding factor 1 (IDEF1)
and IDEF2 (Kobayashi et al.
2007
; Ogo et al.
2008
). These two factors are members
of the ABI3/VP1 (ABSCISIC ACID INSENSITIVE 3/VIVIPAROUS 1) family and
NAC (NO APICAL MERISTEM,
Arabidopsis
transcription activation factor and
CUP SHAPED COTYLEDON) family, respectively. IDEF1 and IDEF2 are con-
stitutively expressed in vegetative tissues and can regulate two different sets of
genes (Kobayashi et al.
2009
). IDE1 regulates most of the Fe related genes in
normal Fe conditions and during the early responses to Fe deficiency. Interestingly,
IDEF1 can switch its target genes in the late stages of Fe deficiency. IDEF2 instead
maintains the same target genes during the responses to Fe deficiency and it is
known to positively regulate the expression of
OsYSL2
(Kobayashi et al.
2010
).
Therefore, IDEF2 is also involved in the correct partitioning of Fe between roots
and shoot.
Many regulators from graminaceous plants have been identified by a microarray
analysis approach. The most extensively studied candidate is
OsIRO2
, which
encodes a bHLH transcription factor (Ogo et al.
2011
). Its expression is positively
regulated by IDEF1 in Fe deficiency. OsIRO2 can in turn positively regulate
different Strategy II genes, such as
OsNAS1
,
OsNAS2
,
OsNAAT1
,
TOM1
and
OsYSL15
. Another bHLH transcription factor,
OsIRO3
, is present in rice and its
expression is induced by Fe deficiency (Zheng et al.
2010
). It seems to be a negative
regulator of several genes related to Fe deficiency responses.
Intriguingly, sequence comparison with
Arabidopsis
transcription factors has
shown that OsIRO2 is similar to AtbHLH038, 039, 100 and 101, whereas IRO3 is
similar to PYE (Ogo et al.
2006
). Thus far, no correspondent of FIT has been found
in graminaceous plants and no orthologue of IDEF1 and IDEF2 has been found in
non-graminaceous plants. Therefore, it seems that the regulatory mechanisms are
only partially conserved between Strategy I and Strategy II plants.
Apart from the positive regulator ethylene, other signaling molecules and plant
hormones participate in the regulation of Fe deficiency responses. Among these
nitric oxide (NO), carbon dioxide and auxin also contribute to the induction of
