Iron nutrition in plants and yeast: Studies on the FRO1 gene of Pisum sativum and the FET4 gene of Sacharomyces cerevisiae

To acquire iron, many plant species reduce soil Fe(III) to Fe(II) by Fe(III)-chelate reductases embedded in the plasma membrane of root epidermal cells. The reduced product is then taken up by Fe(II) transporter proteins. These activities are induced under Fe deficiency. This report describes the characterization of a gene from Pisum sativum L., designated FRO1, that encodes an Fe(III)-chelate reductase. FRO1 shows similarity to other oxidoreductase proteins and expression of FRO1 in yeast conferred increased Fe(III)-chelate reductase activity. FRO1 mRNA levels corresponded with ferric reductase activity. FRO1 mRNA was detected throughout the root but was most abundant in the outer epidermal cells. Expression was detected in mesophyll cells in leaves, and in root nodules, was detected in the infection zone and nitrogen-fixing region. These results indicate that FRO1 plays roles in root Fe uptake and Fe distribution throughout the plant. FRO1 expression and reductase activity was detected only in Fe-deficient roots of Sparkle, while both were constitutive in brz and dgl, two mutants with incorrectly regulated Fe accumulation. In contrast, FRO1 expression was responsive to Fe status in shoots of all three plant lines. These results indicate differential regulation of FRO1 in roots and shoots and improper FRO1 regulation in response to a shoot-derived signal of iron status in the roots of the brz and dgl mutants.; Acquisition of metals such as Fe, Cu, and Zn by the yeast Saccharomyces cerevisiae is tightly regulated. High affinity uptake systems are induced under metal-limiting conditions to maintain an adequate supply of these essential nutrients. Low affinity uptake systems function when their substrates are in greater supply. The FET4 gene encodes a low affinity Fe and Cu uptake transporter. FET4 expression is regulated by several environmental factors. This report describes the molecular mechanisms underlying this regulation. FET4 expression is induced in Fe-limited cells by the Aft1 Fe-responsive transcriptional activator. FET4 is regulated by Zn status via the Zap1 transcription factor. FET4 is a physiologically relevant Zn transporter and this provides a rationale for its regulation by Zap1. FET4 expression is regulated in response to oxygen by the Rox1 repressor. Rox1 attenuates activation by Aft1 and Zap1 in aerobic cells. Derepression of FET4 may allow the Fet4 transporter to play an even greater role in metal acquisition under anaerobic conditions. Thus, Fet4 is a multi-substrate metal ion transporter under combinatorial control by Fe, Zn and oxygen.