Agriculture Reference
In-Depth Information
morphological responses are: development of root hairs and transfer cells
[1,14]. Both physiological and morphological responses are located in the
subapical regions of the roots [14,34]. The morphological responses are rarely
mentioned in the studies about Fe in pea because root hairs are not as
prominent as in other plant species and transfer cells have not been studied yet
in this plant species (at least, to our knowledge).
The Fe responses are induced under Fe deficiency and are shut down once
plants acquire sufficient Fe, to avoid excess Fe accumulation and energy lost.
Their regulation is not totally known but in the last years evidence has been
accumulating suggesting that ethylene [and other hormones such as auxin and
nitric oxide (NO)] could be involved in their activation while some other Fe-
related signals, probably coming from shoots to roots through the phloem,
could act as inhibitors ([35]; see Regulation of Fe responses…).
R
EGULATION OF
F
E
R
ESPONSES IN
D
ICOT
P
LANTS
The responses of dicot plants to Fe deficiency are switched on or off
depending on the Fe necessities of the plant. Their regulation is crucial for
plants for, at least, two reasons. First, intracellular Fe excess can cause
oxidative damage to cells because free ionic Fe catalyses the generation of
hydroxyl radicals trough the Fenton reaction [36-37]. Second, the activation of
Fe responses requires a lot of energy and, consequently, plants do not activate
them at least the responses are strictly necessary. In the last years, the
knowledge of the mechanisms involved in Fe acquisition by Strategy I plants,
as well as their regulation, has increased considerably. Now, it is known that
many Fe acquisition genes, that facilitate Fe uptake, are induced by Fe
deficiency in roots. Moreover, some of them [such as
FRO
(reductase),
IRT
(Fe transporter) and
HA
(acidification) genes] are activated by bHLH TFs
(Transcription Factors), induced themselves by Fe deficiency too [1,38]. From
these bHLH TFs, FER (
F
e-
E
fficiency
R
esponse) in tomato and its homolog
FIT in Arabidopsis (bHLH29,
F
ER-LIKE
I
RON DEFICIENCY INDUCED
T
RANSCRIPTION FACTOR; formerly named FRU or FIT1 [39]) are
considered the master regulators of Fe acquisition from the medium, since
their knock-out mutants are very chlorotic and lack the ability to activate most
Fe responses in roots [40-43]. Other bHLH TFs involved in Fe acquisition in
Arabidopsis are bHLH38, bHLH39, bHLH100 and bHLH101, which seem to
be redundant and need to interact with FIT for most of their functions [44-45].
All these bHLH TFs are induced in both shoots and roots under Fe deficiency
