The frd3 mutant of Arabidopsis exhibits constitutive expression of its iron uptake responses and is chlorotic. These phenotypes are consistent with defects either in iron deficiency signaling or in iron translocation and localization. Here we present several experiments demonstrating that a functional FRD3 gene is necessary for correct iron localization in both the root and shoot of Arabidopsis plants. Reciprocal grafting experiments with frd3 and wild-type Arabidopsis plants reveal that the phenotype of a grafted plant is determined by the genotype of the root, not by the genotype of the shoot. This indicates that FRD3 function is root-specific and points to a role for FRD3 in delivering iron to the shoot in a usable form. When grown under certain conditions, frd3 mutant plants overaccumulate iron in their shoot tissues. However, we demonstrate by direct measurement of iron levels in shoot protoplasts that intracellular iron levels in frd3 are only about one-half the levels in wild type. Histochemical staining for iron reveals that frd3 mutants accumulate high levels of ferric iron in their root vascular cylinder, the same tissues in which the FRD3 gene is expressed. Taken together, these results clearly indicate a role for FRD3 in iron localization in Arabidopsis. Specifically, FRD3 is likely to function in root xylem loading of an iron chelator or other factor necessary for efficient iron uptake out of the xylem or apoplastic space and into leaf cells.Iron is both necessary for plant growth and toxic in excess. It participates as a redox cofactor in a number of metalloenzymes involved in respiration and photosynthesis. These same redox properties allow iron to catalyze the formation of damaging oxygen radicals (Halliwell and Gutteridge, 1992). Although iron is plentiful in the earth's crust, it exists primarily in the insoluble ferric, Fe(III), form. Therefore, plants need specific mechanisms to obtain sufficient amounts of this important nutrient. Dicots rely on acidification of the rhizosphere to solubilize ferric iron, reduction of ferric iron to the more soluble ferrous form, and transport of the ferrous iron into the root epidermal cells. These activities are collectively termed iron uptake responses and are maximally expressed under conditions of iron deficiency. The genes responsible for the root iron-deficiency inducible ferric chelate reductase activity and the major ferrous uptake transporter have been identified as FRO2 and IRT1, respectively, in the model plant Arabidopsis (Eide et al., 1996;Robinson et al., 1999;Vert et al., 2002).It is well known that iron deficiency causes chlorosis in plants. On a molecular level, this chlorosis is caused by a reduction in the amount of chlorophyll synthesized and an accumulation of both Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester that are chlorophyll precursors (Spiller et al., 1982). These data imply there is an iron-requiring step between Mg-protoporphyrin IX monomethyl ester and protochlorophyllide (Spiller et al., 1982). Recently, CHL27...

A complete tricarboxylic acid (TCA) cycle is generally considered necessary for energy production from the dicarboxylic acid substrates malate, succinate, and fumarate. However, a Bradyrhizobium japonicum sucA mutant that is missing ␣-ketoglutarate dehydrogenase is able to grow on malate as its sole source of carbon. This mutant also fixes nitrogen in symbiosis with soybean, where dicarboxylic acids are its principal carbon substrate. Using a flow chamber system to make direct measurements of oxygen consumption and ammonium excretion, we confirmed that bacteroids formed by the sucA mutant displayed wild-type rates of respiration and nitrogen fixation. Despite the absence of ␣-ketoglutarate dehydrogenase activity, whole cells of the mutant were able to decarboxylate ␣-[U-14 C]ketoglutarate and [U-14 C]glutamate at rates similar to those of wild-type B. japonicum, indicating that there was an alternative route for ␣-ketoglutarate catabolism. Because cell extracts from B. japonicum decarboxylated [U-14 C]glutamate very slowly, the ␥-aminobutyrate shunt is unlikely to be the pathway responsible for ␣-ketoglutarate catabolism in the mutant. In contrast, cell extracts from both the wild type and mutant showed a coenzyme A (CoA)-independent ␣-ketoglutarate decarboxylation activity. This activity was independent of pyridine nucleotides and was stimulated by thiamine PP i . Thin-layer chromatography showed that the product of ␣-ketoglutarate decarboxylation was succinic semialdehyde. The CoA-independent ␣-ketoglutarate decarboxylase, along with succinate semialdehyde dehydrogenase, may form an alternative pathway for ␣-ketoglutarate catabolism, and this pathway may enhance TCA cycle function during symbiotic nitrogen fixation.

Ferredoxin and ferredoxin-NADP+ oxidoreductase (FNR) were purified from leaves, roots, and red and green pericarp of tomato (Lycopersicon esculentum, cv VFNT and cv Momotaro). Four different ferredoxins were identified on the basis of N-terminal amino acid sequence and charge. Ferredoxins I and 11 were the most prevalent forms in leaves and green pericarp, and ferredoxin

Using the cysA locus of Salmonella typhimurium as a heterologous probe, we have cloned a region of the Anacystis nidulans R2 (Synechococcus PCC 7942) genome involved in sulfate assimilation. The 8.3-kilobase-pair region encodes at least five transcripts that cannot be detected unless the cells are deprived of sulfur. One of the genes in this region has been sequenced, and the protein that it encodes is homologous to a polypeptide component of other permease systems of Escherichia coli and Salmonella. Insertional inactivation of the putative sulfate permease gene, designated cysA, as well as of other genes within this region, results in cysteine auxotrophy, reduced sulfate uptake, and altered expression of soluble and cytoplasmic-membrane polypeptides associated with sulfur starvation.