title: Vitamin B6 biosynthesis in higher plants.
abstract: Vitamin B6 is an essential metabolite in all organisms. It can act as a coenzyme for numerous metabolic enzymes and has recently been shown to be a potent antioxidant. Plants and microorganisms have a de novo biosynthetic pathway for vitamin B6, but animals must obtain it from dietary sources. In Escherichia coli, it is known that the vitamin is derived from deoxyxylulose 5-phosphate (an intermediate in the nonmevalonate pathway of isoprenoid biosynthesis) and 4-phosphohydroxy-l-threonine. It has been assumed that vitamin B6 is synthesized in the same way in plants, but this hypothesis has never been experimentally proven. Here, we show that, in plants, synthesis of the vitamin takes an entirely different route, which does not involve deoxyxylulose 5-phosphate but instead utilizes intermediates from the pentose phosphate pathway, i.e., ribose 5-phosphate or ribulose 5-phosphate, and from glycolysis, i.e., dihydroxyacetone phosphate or glyceraldehyde 3-phosphate. The revelation is based on the recent discovery that, in bacteria and fungi, a novel pathway is in place that involves two genes (PDX1 and PDX2), neither of which is homologous to any of those involved in the previously doctrined E. coli pathway. We demonstrate that Arabidopsis thaliana has two functional homologs of PDX1 and a single homolog of PDX2. Furthermore, and contrary to what was inferred previously, we show that the pathway appears to be cytosolic and is not localized to the plastid. Last, we report that the single PDX2 homolog is essential for plant viability.
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), Universitätstrasse 2, CH-8092 Zürich, Switzerland.
authors.forename: Marina
authors.initials: M
authors.lastname: Tambasco-Studart
authors.forename: Olca
authors.initials: O
authors.lastname: Titiz
authors.forename: Thomas
authors.initials: T
authors.lastname: Raschle
authors.forename: Gabriela
authors.initials: G
authors.lastname: Forster
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Amrhein
authors.forename: Teresa B
authors.initials: TB
authors.lastname: Fitzpatrick
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D019759:Carbon-Nitrogen Lyases; D003593:Cytoplasm / Q000201:enzymology*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D005658:Fungi / Q000201:enzymology / Q000235:genetics; D006019:Glycolysis / Q000502:physiology; D008969:Molecular Sequence Data; D019884:Nitrogenous Group Transferases / Q000235:genetics / Q000378:metabolism*; D010427:Pentose Phosphate Pathway / Q000502:physiology; D010428:Pentosephosphates / Q000235:genetics / Q000378:metabolism; D025101:Vitamin B 6 / Q000096:biosynthesis* / Q000235:genetics
chemical_list: D029681:Arabidopsis Proteins; D029968:Escherichia coli Proteins; D005656:Fungal Proteins; D010428:Pentosephosphates; D025101:Vitamin B 6; D019884:Nitrogenous Group Transferases; C505162:PDX2 protein, Arabidopsis; D019759:Carbon-Nitrogen Lyases; C505161:At2g38230 protein, Arabidopsis