The serine-glycine-one-carbon metabolic network orchestrates changes in nitrogen and sulfur metabolism and shapes plant development.

L-serine (Ser) and L-glycine (Gly) are critically important for the overall functioning of primary metabolism. We investigated the interaction of the phosphorylated pathway of Ser biosynthesis (PPSB) with the photorespiration-associated glycolate pathway of Ser biosynthesis (GPSB) using Arabidopsis thaliana PPSB-deficient lines, GPSB-deficient mutants, and crosses of PPSB with GPSB mutants. PPSB-deficient lines mainly showed retarded primary root growth. Mutation of the photorespiratory enzyme Ser-hydroxymethyltransferase 1 (SHMT1) in a PPSB-deficient background resumed primary root growth and induced a change in the plant metabolic pattern between roots and shoots. Grafting experiments demonstrated that metabolic changes in shoots were responsible for the changes in double mutant development. PPSB disruption led to a reduction in nitrogen (N) and sulfur (S) contents in shoots and a general transcriptional response to nutrient deficiency. Disruption of SHMT1 boosted the Gly flux out of the photorespiratory cycle, which increased the levels of the one-carbon (1C) metabolite 5,10-methylene-tetrahydrofolate and S-adenosylmethionine. Furthermore, disrupting SHMT1 reverted the transcriptional response to N and S deprivation and increased N and S contents in shoots of PPSB-deficient lines. Our work provides genetic evidence of the biological relevance of the Ser-Gly-1C metabolic network in N and S metabolism and in interorgan metabolic homeostasis.

Formate-tetrahydrofolate ligase: supplying the cytosolic one-carbon network in roots with one-carbon units originating from glycolate.

The metabolism of tetrahydrofolate (H4PteGlun)-bound one-carbon (C1) units (C1 metabolism) is multifaceted and required for plant growth, but it is unclear what of many possible synthesis pathways provide C1 units in specific organelles and tissues. One possible source of C1 units is via formate-tetrahydrofolate ligase, which catalyzes the reversible ATP-driven production of 10-formyltetrahydrofolate (10-formyl-H4PteGlun) from formate and tetrahydrofolate (H4PteGlun). Here, we report biochemical and functional characterization of the enzyme from Arabidopsis thaliana (AtFTHFL). We show that the recombinant AtFTHFL has lower Km and kcat values with pentaglutamyl tetrahydrofolate (H4PteGlu5) as compared to monoglutamyl tetrahydrofolate (H4PteGlu1), resulting in virtually identical catalytic efficiencies for the two substrates. Stable transformation of Arabidopsis plants with the EGFP-tagged AtFTHFL, followed with fluorescence microscopy, demonstrated cytosolic signal. Two independent T-DNA insertion lines with impaired AtFTHFL function had shorter roots compared to the wild type plants, demonstrating the importance of this enzyme for root growth. Overexpressing AtFTHFL led to the accumulation of H4PteGlun + 5,10-methylene-H4PteGlun and serine, accompanied with the depletion of formate and glycolate, in roots of the transgenic Arabidopsis plants. This metabolic adjustment supports the hypothesis that AtFTHFL feeds the cytosolic C1 network in roots with C1 units originating from glycolate, and that these units are then used mainly for biosynthesis of serine, and not as much for the biosynthesis of 5-methyl-H4PteGlun, methionine, and S-adenosylmethionine. This finding has implications for any future attempts to engineer one-carbon unit-requiring products through manipulation of the one-carbon metabolic network in non-photosynthetic organs.

Assaying plant formate-tetrahydrofolate ligase with monoglutamylated and polyglutamylated substrates using a fluorescence-HPLC based method.

Formate-tetrahydrofolate ligase catalyzes reversible, ATP-dependent conversion of tetrahydrofolate and formate to 10-formyltetrahydrofolate, simultaneously releasing ADP and inorganic phosphate. This enzyme has traditionally been assayed in the direction of 10-CHO-tetrahydrofolate formation by lowering pH of the reaction post-incubation, thus converting the product of the reaction to 5,10-methenyltetrahydrofolate, which is then quantified spectrophotometrically. To increase sensitivity of the product detection, which is particularly useful when determining the kinetic parameters of the enzyme with polyglutamylated substrates, we have replaced the spectrophotometric detection with HPLC separation and fluorescence detection. In addition to the modified enzyme assay protocol, we are also providing protocols for producing recombinant formate-tetrahydrofolate ligase from Arabidopsis in Escherichia coli cells, producing crude Arabidopsis leaf and root extracts suitable for assaying this enzyme, and for synthesis of polyglutamylated tetrahydrofolate substrates.

Catalase protects against nonenzymatic decarboxylations during photorespiration in Arabidopsis thaliana.

Photorespiration recovers carbon that would be otherwise lost following the oxygenation reaction of rubisco and production of glycolate. Photorespiration is essential in plants and recycles glycolate into usable metabolic products through reactions spanning the chloroplast, mitochondrion, and peroxisome. Catalase in peroxisomes plays an important role in this process by disproportionating H2O2 resulting from glycolate oxidation into O2 and water. We hypothesize that catalase in the peroxisome also protects against nonenzymatic decarboxylations between hydrogen peroxide and photorespiratory intermediates (glyoxylate and/or hydroxypyruvate). We test this hypothesis by detailed gas exchange and biochemical analysis of Arabidopsis thaliana mutants lacking peroxisomal catalase. Our results strongly support this hypothesis, with catalase mutants showing gas exchange evidence for an increased stoichiometry of CO2 release from photorespiration, specifically an increase in the CO2 compensation point, a photorespiratory-dependent decrease in the quantum efficiency of CO2 assimilation, increase in the 12CO2 released in a 13CO2 background, and an increase in the postillumination CO2 burst. Further metabolic evidence suggests this excess CO2 release occurred via the nonenzymatic decarboxylation of hydroxypyruvate. Specifically, the catalase mutant showed an accumulation of photorespiratory intermediates during a transient increase in rubisco oxygenation consistent with this hypothesis. Additionally, end products of alternative hypotheses explaining this excess release were similar between wild type and catalase mutants. Furthermore, the calculated rate of hydroxypyruvate decarboxylation in catalase mutant is much higher than that of glyoxylate decarboxylation. This work provides evidence that these nonenzymatic decarboxylation reactions, predominately hydroxypyruvate decarboxylation, can occur in vivo when photorespiratory metabolism is genetically disrupted.

Characterization of a non-nudix pyrophosphatase points to interplay between flavin and NAD(H) homeostasis in Saccharomyces cerevisiae.

The flavin cofactors FMN and FAD are required for a wide variety of biological processes, however, little is known about their metabolism. Here, we report the cloning and biochemical characterization of the Saccharomyces cerevisiae pyrophosphatase Fpy1p. Genetic and functional studies suggest that Fpy1p may play a key role in flavin metabolism and is the first-reported non-Nudix superfamily enzyme to display FAD pyrophosphatase activity. Characterization of mutant yeast strains found that deletion of fpy1 counteracts the adverse effects that are caused by deletion of flx1, a known mitochondrial FAD transporter. We show that Fpy1p is capable of hydrolyzing FAD, NAD(H), and ADP-ribose. The enzymatic activity of Fpy1p is dependent upon the presence of K+ and divalent metal cations, with similar kinetic parameters to those that have been reported for Nudix FAD pyrophosphatases. In addition, we report that the deletion of fpy1 intensifies the FMN-dependence of null mutants of the riboflavin kinase Fmn1p, demonstrate that fpy1 mutation abolishes the decreased fitness resulting from the deletion of the flx1 ORF, and offer a possible mechanism for the genetic interplay between fpy1, flx1 and fmn1.