Agriculture Reference
In-Depth Information
Regulation of the Strategies
The regulation of Fe deficiency responses is very complex and requires the coor-
dination of several regulatory elements. The presence of different pathway and
feedback signals constitute an important aspect in the regulation.
Several transcription factors that are involved in the regulation of the Fe uptake
machinery have been already identified in different Strategy I plants (Fig.
5.2a
).
The key element/regulator in this respect was first identified in the tomato
fer
mutant. Map-based cloning revealed that
FER
encodes for a basic helix-loop-
helix (bHLH) transcription factor (Ling et al.
2002
).
Arabidopsis
possesses an
ortholog of
FER
, which has been named
FIT
(
FER
-
like iron deficiency
-
induced
transcription factor
, also named before
FIT1
/
FRU
/
AtbHLH029
; Bauer et al.
2007
).
FIT
expression is repressed upon full Fe supply, whereas the expression is highly
induced in Fe deficiency. FIT positively regulates the expression of different
Fe-responsive genes, including
IRT1
and
FRO2
. The
fit
mutant shows leaf chlorosis
and decreased Fe content and fails to induce the typical Strategy I responses
(Colangelo and Guerinot
2004
). Moreover, the mutant dies at the seedling stage
unless watered with additional Fe.
FRO2
mRNA level is severely downregulated in
the mutant and the FRO activity cannot be detected. The transcript level of
IRT1
is
decreased but still detectable in
fit
plants whereas the protein IRT1 is not present.
These results suggest that FIT can control the Strategy I responses at different level,
regulating the gene expression but also the turnover of IRT1 protein.
FIT can also interact directly with other bHLH factors, such as bHLH038 and
bHLH039. This interaction is thought to serve in modulating the plant response to
Fe deficiency (Yuan et al.
2008
). These two factors belong together with bHLH100
and bHLH101 to a specific sub-group of bHLH and their expression is strongly
induced upon Fe deficiency (Wang et al.
2007
). They are also functioning inde-
pendently from FIT to mediate Fe deficiency responses (Sivitz et al.
2012
).
FIT can also directly interact with ETHYLENE INSENSITIVE 3 and ETHYL-
ENE INSENSITIVE 3-LIKE1 (Lingam et al.
2011
). This interaction provides the
molecular link between ethylene and the responses to Fe starvation, which was
elusive before. Ethylene is known to be a positive regulator of the induction of
different Fe responsive genes. The ethylene downstream transcription factors EIN3
and EIL1 are required for FIT accumulation and therefore thought to inhibit its
proteasomal degradation, thus enhancing the plant responses to Fe deficiency
(Lingam et al.
2011
).
Microarray analysis aimed at finding new regulatory candidates identified the
bHLH transcription factor POPEYE (PYE) (Long et al.
2010
). PYE is upregulated
specifically in the cells of the root perycicle upon Fe deficiency. The mutant
pye
displays severely impaired growth under - Fe condition; therefore PYE seems to
play a fundamental role in the roots of plants exposed to Fe deficiency. Moreover,
PYE is proposed to negatively regulate a cluster of Fe-responsive genes, amongst
these
NAS4
and
FRO3
. PYE can directly interact with PYE homologues, such as
IAA-Leu Resistant3 (ILR3) and bHLH115. ILR3 in turn interacts with another
