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. · 4.35 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;
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ABSTRACT: The photorespiratory metabolism is essential in all oxygenic photosynthetic organisms. In plants, it is a highly compartmentalized pathway that involves chloroplasts, peroxisomes, mitochondria, and the cytoplasm. The metabolic pathway itself is well characterized and the enzymes required for its function have been identified. However, very little information is available on the transport proteins that catalyze the high metabolic flux between the involved compartments. Here we show that the A BOUT DE SOUFFLE (BOU) gene, which encodes a mitochondrial carrier, is involved on photorespiration in Arabidopsis. BOU was found to be co-expressed with photorespiratory genes, in leaf tissues. The knockout mutant bou-2 showed the hallmarks of a photorespiratory growth phenotype, elevated CO(2) compensation point, and excessive accumulation of glycine. Furthermore a degradation of the P-protein, a subunit of the glycine decarboxylase (GDC), was demonstrated for bou-2 and is reflected by strongly reduced GDC activity. Photorespiration defect in bou-2 leads to dramatic consequences early at the seedling stage, which are highlighted by transcriptome studies. In bou-2 seedlings as in shm1, another photorespiratory mutant, the shoot apical meristem organization is severely compromised. Cell divisions are arrested, leading to growth arrest at ambient CO(2) . Although the specific substrate for the BOU transporter protein remains elusive, we show that it is essential for the function of photorespiratory metabolism. We hypothesize that BOU function is linked with GDC activity and is also required for proper function of the apical meristems in seedlings. © 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd.The Plant Journal 11/2012; · 6.58 Impact Factor