Rice, the world's major staple crop, is a poor source of essential micronutrients, including folates (vitamin B9). We report folate biofortification of rice seeds achieved by overexpressing two Arabidopsis thaliana genes of the pterin and para-aminobenzoate branches of the folate biosynthetic pathway from a single locus. We obtained a maximal enhancement as high as 100 times above wild type, with 100 g of polished raw grains containing up to four times the adult daily folate requirement.

A mutation in the Arabidopsis gene STARIK leads to dwarfism and chlorosis of plants with an altered morphology of leaf and cell nuclei. We show that the STARIK gene encodes the mitochondrial ABC transporter Sta1 that belongs to a subfamily of Arabidopsis half-ABC transporters. The severity of the starik phenotype is suppressed by the ectopic expression of the STA2 homolog; thus, Sta1 function is partially redundant. Sta1 supports the maturation of cytosolic Fe/S protein in ⌬ atm1 yeast, substituting for the ABC transporter Atm1p. Similar to Atm1p-deficient yeast, mitochondria of the starik mutant accumulated more nonheme, nonprotein iron than did wild-type organelles. We further show that plant mitochondria contain a putative L -cysteine desulfurase. Taken together, our results suggest that plant mitochondria possess an evolutionarily conserved Fe/S cluster biosynthesis pathway, which is linked to the intracellular iron homeostasis by the function of Atm1p-like ABC transporters. INTRODUCTIONABC transporters constitute one of the largest protein families with diverse functions in membrane transport. The designation of ABC transporter recognizes a highly conserved ATP binding cassette, which is the most characteristic feature of this superfamily. These proteins mediate the relatively specific, active transmembrane transport of molecules that can range in size from small ions to proteins (reviewed in Higgins, 1992) and utilize the energy of ATP hydrolysis to pump substrates across membranes, usually against a concentration gradient. The typical ABC transporter consists of four domains. Two of these domains are hydrophobic, and each comprises six membrane-spanning segments. The transmembrane domains are believed to form a channel and to determine the specificity of the transporter. The other two domains are located at the periphery of the membrane and couple ATP hydrolysis to the transport process.The ABC transporter Atm1p of budding yeast mitochondria represents a "half-transporter" in which one transmembrane and one ATP binding domain are expressed in a single polypeptide (Leighton and Schatz, 1995). Atm1p is localized in the mitochondrial inner membrane and is believed to function as an exporter, because its ABC domains face the mitochondrial matrix. The ATM1 loss-of-function results in respiration-deficient mitochondria that lack cytochromes (Leighton and Schatz, 1995; Kispal et al., 1997) and that tend to loose their DNA (Leighton and Schatz, 1995). These pleiotropic effects of the Atm1p deficiency are generally attributed to the fact that in ⌬ atm1 cells, mitochondria accumulate up to 30-fold higher levels of iron than do wild-type organelles (Kispal et al., 1997;Mitsuhashi et al., 2000). The Atm1p transport function appears to be conserved in eukaryotes, and the human mitochondrial ABC transporters Abc7 and MTABC3 have been shown to be true functional orthologs of the Atm1p (Csere et al., 1998;Mitsuhashi et al., 2000). The human ABC7 gene has been implicated in hereditary X-linked sideroblastic anemia and a...

SummaryOne of the immediate reactions of the mammalian cell to many environmental stresses is a massive synthesis of poly(ADP-ribose), catalyzed by poly(ADP-ribose) polymerase (PARP). Most of the biological functions attributed to PARP are inferred from experimentation with mammalian cells. In plants, the biology of PARP may be more complicated and diverse than was previously thought. Two poly(ADP-ribose) polymerase homologues were found in plants, the classical Zn-finger-containing polymerase (ZAP) and the structurally non-classical PARP proteins (APP and NAP), which lack the characteristic N-terminal Zn-finger domain. By enzymatic and cytological experiments the recombinant APP protein was shown to be located in the nucleus and to possess DNA-dependent poly(ADP-ribose) polymerase activity in yeast. The nuclear localization was further confirmed by the analysis of transgenic tobacco plants that expressed a translational gene fusion between APP and the bacterial β-glucuronidase. The app promoter was transcriptionally up-regulated in cells pre-determined to die because of deficiency in a DNA ligase I.