Folate metabolism in plants: an Arabidopsis homolog of the mammalian mitochondrial folate transporter mediates folate import into chloroplasts.
ABSTRACT The distribution of folates in plant cells suggests a complex traffic of the vitamin between the organelles and the cytosol. The Arabidopsis thaliana protein AtFOLT1 encoded by the At5g66380 gene is the closest homolog of the mitochondrial folate transporters (MFTs) characterized in mammalian cells. AtFOLT1 belongs to the mitochondrial carrier family, but GFP-tagging experiments and Western blot analyses indicated that it is targeted to the envelope of chloroplasts. By using the glycine auxotroph Chinese hamster ovary glyB cell line, which lacks a functional MFT and is deficient in folates transport into mitochondria, we showed by complementation that AtFOLT1 functions as a folate transporter in a hamster background. Indeed, stable transfectants bearing the AtFOLT1 cDNA have enhanced levels of folates in mitochondria and can support growth in glycine-free medium. Also, the expression of AtFOLT1 in Escherichia coli allows bacterial cells to uptake exogenous folate. Disruption of the AtFOLT1 gene in Arabidopsis does not lead to phenotypic alterations in folate-sufficient or folate-deficient plants. Also, the atfolt1 null mutant contains wild-type levels of folates in chloroplasts and preserves the enzymatic capacity to catalyze folate-dependent reactions in this subcellular compartment. These findings suggest strongly that, despite many common features shared by chloroplasts and mitochondria from mammals regarding folate metabolism, the folate import mechanisms in these organelles are not equivalent: folate uptake by mammalian mitochondria is mediated by a unique transporter, whereas there are alternative routes for folate import into chloroplasts.
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ABSTRACT: Background Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes that catalyze a variety of biochemical reactions. Previously we showed that free flavin (riboflavin, FMN, and FAD) concentrations were decreased in leaves of transgenic Arabidopsis plants expressing a turtle riboflavin-binding protein (RfBP). Here, we report that flavin downregulation by RfBP induces the early flowering phenotype and enhances expression of floral promoting photoperiod genes.ResultsEarly flowering was a serendipitous phenomenon and was prudently characterized as a constant phenotype of RfBP-expressing transgenic Arabidopsis plants in both long days and short days. The phenotype was eliminated when leaf free flavins were brought back to the steady-state levels either by the RfBP gene silencing and consequently nullified production of the RfBP protein, or by external riboflavin feeding treatment. RfBP-induced early flowering was correlated with enhanced expression of floral promoting photoperiod genes and the florigen gene FT in leaves but not related to genes assigned to vernalization, autonomous, and gibberellin pathways, which provide flowering regulation mechanisms alternative to the photoperiod. RfBP-induced early flowering was further correlated with increased expression of the FD gene encoding bZIP transcription factor FD essential for flowering time control and the floral meristem identity gene AP1 in the shoot apex. By contrast, the expression of FT and photoperiod genes in leaves and the expression of FD and AP1 in the shoot apex were no longer enhanced when the RfBP gene was silenced, RfBP protein production canceled, and flavin concentrations were elevated to the steady-state levels inside plant leaves.Conclusions Token together, our results provide circumstantial evidence that downregulation of leaf flavin content by RfBP induces early flowering and coincident enhancements of genes that promote flowering through the photoperiod pathway.BMC Plant Biology 09/2014; 14(1):237. · 3.94 Impact Factor
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ABSTRACT: The effect of methotrexate (MTX), a folate analogue and specific competitive inhibitor of dihydrofolate reductase (DHFR), is assessed (concentrations: 0.001, 0.01, 0.1, 1 and 10 μM) on germinating grass pea (Lathyrus sativus L.) seedlings in relation to radicle length, mitotic index, total RNA content and DHFR activity. Response of callus growth of the species is also studied following MTX treatments. Furthermore, the effect of MTX on seedlings treated with colchicine (0.5%, 8 h) and 5-formyl tetrahydrofolate (CF; 10 mM) are also analyzed. The objective of the present study is to evaluate the effectivity of the drug MTX on a plant species with the view to use plant system as a model for screening antifolate drugs. Results suggest that MTX possesses distinct role in inhibiting plant cell division, RNA synthesis and DHFR activity; although, at low concentration (0.001 μM) it shows stimulatory effect.Nucleus 07/2014; 57(2):129-134.
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ABSTRACT: In addition to efficient synthesis of ATP by oxidative phosphorylation, acquisition of the mitochondrial endosymbiont brought a whole range of new metabolic capabilities to the ancestral eukaryotic cell lineage such that the mitochondrion retains an important role in numerous anabolic and catabolic processes. While respiration dominates metabolism of the mitochondrion, this organelle is also important in the catabolism of amino acids and the provision of carbon skeletons for biosynthesis of a wide range of compounds including amino acids, vitamins, lipids, and tetrapyrroles. However, mitochondrial metabolism is best understood in the context of cellular metabolism as a whole; this is particularly true in auxotrophic organisms such as plants. For this reason understanding of the integration of mitochondrial metabolism with associated metabolic pathways in distinct cellular locations is of great importance. The examples of photorespiration, proline, cysteine, branched chain amino acid, ascorbate and folate metabolism all indicate that mitochondrial steps in these pathways are critical to their function and regulation. Moreover, the central metabolic position of the mitochondrion and its key roles in bioenergetics and redox regulation, additionally mean that it is ideally placed to act as a sensor of the biochemical status of the cell. When taken together these observations suggest that the myriad nonrespiratory functions of the mitochondria are of vast importance in the coordination of plant cellular metabolism and function.Critical Reviews in Plant Sciences 01/2007; 26(1):17-43. · 5.29 Impact Factor