REGULATION OF Fe DEFICIENCY RESPONSES IN WT PEA AND SOME OF ITS MUTANTS (BRZ AND DGL) - Pisum Sativum - page 7

mutants (Arabidopsis ctr1 ) and ethylene overproducer mutants (Arabidopsis eto )
[5,14,34,47,50-52]. The use of ethylene insensitive mutants has served to
demonstrate that ethylene is involved in the development of subapical root hairs
(reviewed in [14]) and other Fe responses [52].
The mutants altered in the regulation of Fe responses can be divided in two
types: mutants that can not activate Fe responses under Fe deficiency and mutants
that constitutively activate them even when grown under Fe sufficiency. Among
the first ones, we can mention the tomato fer mutant and the Arabidopsis fit
mutant, affected in the FER (or its homolog FIT) TF. These knock-out mutants
are very chlorotic and lack the ability to activate most Fe responses in roots
[40-43]. Other mutant of similar characteristics is the melon fefe mutant, that is
probably affected in some step upstream of the melon FER homolog [53]. The
study of the fer mutant has allowed the identification of the master regulator
FER [41] and, later on, of its homolog FIT [42-43].
Among the mutants that show constitutive activation of Fe responses,
even when grown under Fe-sufficient conditions, we can mention the
Arabidopsis frd3 , opt3-2 and nas4x mutants; the tomato chloronerva mutant;
and the pea brz (also named E107) and dgl mutants [15-20,49,54-57]. Most of
these mutants are related, either directly or indirectly, with the movement of Fe in
the phloem. Arabidopsis frd3 ( ferric reductase defective ) is impaired in the
movement of Fe in the xylem but, because this, less Fe goes to leaves (this
mutant is chlorotic) and, consequently, less Fe can recirculate back from
leaves to roots through the phloem [34]. In supporting this view, it should be
mentioned that Fe responses are repressed in this mutant when is sprayed with
Fe in its leaves [34, 49]. Arabidopsis opt3-2 ( oligopeptide transporter ) is
affected in phloem Fe because OPT3 is presumably a transporter involved in
the loading of Fe into the phloem [56,58]. Arabidopsis nas4x ( nicotianamine
synthase ) and tomato chloronerva mutants are also related to phloem Fe since
these mutants are defective in the synthesis of NA (nicotianamine: non
proteinogenic aminoacid), a chelating agent implied in the movement of Fe
(and other metals) within the plant and in the unloading of Fe out of the
phloem [54,57,59]. For the pea dgl mutant there are also some evidence
suggesting that it is impaired in the movement of a Fe-related signal in the
phloem [18,49,60; see below].
To date, it is not known which genes are affected in the pea brz and dgl
mutants although there have been several studies about them that we are going
to comment in the following paragraphs.
The pea brz mutant (also named E107) was the result of mutagenesis with
ethylmethane sulfonic acid of the Pisum sativum (L.) cv „Sparkle‟ [20]. The