REGULATION OF Fe DEFICIENCY RESPONSES IN WT PEA AND SOME OF ITS MUTANTS (BRZ AND DGL) - Pisum Sativum - page 8

brz mutant phenotype is under monogenic semidominant control and has
several pleiotropic effects: decreased nodulation, altered Fe responses and
increased Al uptake [15-17,20-22]. The pea dgl mutant is an X-ray induced
mutant line of Pisum sativum (L.) cv „Dippes Gelbe Viktoria‟ or „DGV‟ [61].
There is also a near isogenic line homozygous for the dgl mutation from the wt
cv „Sparkle‟ [Sparkle ( dgl , dgl )] [60], that allow better comparative studies
among the wt cv „Sparkle‟ and its mutants: dgl [Sparkle ( dgl , dgl )] and brz
[Sparkle ( brz , brz )]. The dgl mutant phenotype is also under monogenic
semidominant control and has similar pleiotropic effects to the brz mutant:
decreased nodulation and altered Fe responses [18,20]. The two genes partially
complement one another, indicating that brz and dgl are not allelic but that can
interact physiologically [20].
Both pea mutants ( brz and dgl ) present constitutive activation of Fe
responses when grown under Fe-sufficient conditions: enhanced ferric
reductase activity, enhanced capacity for Fe 2+ uptake and acidification of the
medium [15-19]. The two first responses can be explained by the constitutive
activation of the PsFRO1 and PsRIT1 genes under Fe-sufficient conditions
(Figure 1;[28]). As a consequence of the constitutive activation of Fe
responses, both mutants hyperaccumulate Fe in their vegetative tissues when
grown under Fe-sufficient conditions and can become intoxicated (Figure
2;[17,19-20]). In fact, the name brz ( br on z e ) derives from the bronze color of
the necrotic leaf spots caused by Fe excess in the leaves of this mutant [20].
Similarly, the name dgl comes from d e g enerated l eaflets (Figure 2;[36,61]).
Besides Fe, the brz mutant also accumulates other metals in shoots, such as
Zn, Mn and Al [17,21]. As a consequence of the Fe hyperaccumulation in the
brz and dgl mutants, both of them also accumulate ferritin in leaves [36] and
NA in leaves and roots [37] to very high levels. Ferritins are proteins with high
capacity for Fe storage and act as a buffer for this element, protecting cells
from harmful concentration of free Fe [62]. NA is a chelating agent for Fe and
other metals, that has been involved in their movement within the plant [57,59]
and also in Fe detoxification [37].
Reciprocal grafting experiments between wt cultivars and each of these
mutants have been done to know whether the BRZ and DGL genes are
associated with either roots or shoots. In the case of brz , the results obtained
with this approach have been contradictory. Welch and LaRue [17] grafted the
wt cv „Sparkle‟(Sp) onto brz roots (Sp/ brz ) and vice versa ( brz /Sp), and found
that Fe accumulation in leaves only occurred when the brz genotype acted as
roots (Sp/ brz ). This clearly indicates that the brz mutation is related to roots
[17]. However, some years later Grusak and Pezeshgi [18] did similar