Arabidopsis cpFtsY mutants exhibit pleiotropic defects including an inability to increase iron deficiency-inducible root Fe(III) chelate reductase activity

All plants, except for the grasses, must reduce Fe(III) to Fe(II) in order to acquire iron. In Arabidopsis, the enzyme responsible for this reductase activity in the roots is encoded by FRO2. Two Arabidopsis mutants, frd4-1 and frd4-2, were isolated in a screen for plants that do not induce Fe(III) chelate reductase activity in their roots in response to iron deficiency. frd4 mutant plants are chlorotic and grow more slowly than wild-type Col-0 plants. Additionally, frd4 chloroplasts are smaller in size and possess dramatically fewer thylakoid membranes and grana stacks when compared with wild-type chloroplasts. frd4 mutant plants express both FRO2 and IRT1 mRNA normally in their roots under iron deficiency, arguing against any defects in systemic iron-deficiency signaling. Further, transgenic frd4 plants accumulate FRO2-dHA fusion protein under iron-deficient conditions, suggesting that the frd4 mutation acts post-translationally in reducing Fe(III) chelate reductase activity. FRO2-dHA appears to localize to the plasma membrane of root epidermal cells in both Col-0 and frd4-1 transgenic plants when grown under iron-deficient conditions. Map-based cloning revealed that the frd4 mutations reside in cpFtsY, which encodes a component of one of the pathways responsible for the insertion of proteins into the thylakoid membranes of the chloroplast. The presence of cpFtsY mRNA and protein in the roots of wild-type plants suggests additional roles for this protein, in addition to its known function in targeting proteins to the thylakoid membrane in chloroplasts.