Perhaps as a consequence of the lower levels of intracellular iron present in its leaves, the frd3 mutant constitutively expresses its Strategy I iron uptake responses. This includes the acidification response, ferric chelate reductase activity and the FRO2 mRNA and IRT1 mRNA and protein (Rogers and Guerinot, 2002). It is likely that this constitutive expression of IRT1 and accumulation of IRT1 protein is the cause of the overaccumulation of zinc, manganese and cadmium in the frd3 mutant. The IRT1 protein has been shown to transport all three of these metals in addition to iron (Eide et al., 1996; Korshunova et al., 1999; Rogers et al., 2000). Additionally, irt1 mutants, unlike wild type Arabidopsis and most other plants, do not overaccumulate zinc, manganese or cadmium under iron deficiency (Vert et al., 2002).
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