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SUMMARY The metabolism of tetrahydrofolate (H₄PteGlu_n)‐bound one‐carbon (C₁) units (C₁metabolism) is multifaceted and required for plant growth, but it is unclear what of many possible synthesis pathways provide C₁units in specific organelles and tissues. One possible source of C₁units is via formate‐tetrahydrofolate ligase, which catalyzes the reversible ATP‐driven production of 10‐formyltetrahydrofolate (10‐formyl‐H₄PteGlu_n) from formate and tetrahydrofolate (H₄PteGlu_n). Here, we report biochemical and functional characterization of the enzyme fromArabidopsis thaliana(AtFTHFL). We show that the recombinant AtFTHFL has lowerK_mandk_catvalues with pentaglutamyl tetrahydrofolate (H₄PteGlu₅) as compared to monoglutamyl tetrahydrofolate (H₄PteGlu₁), resulting in virtually identical catalytic efficiencies for the two substrates. Stable transformation ofArabidopsisplants 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 H₄PteGlu_n + 5,10‐methylene‐H₄PteGlu_nand serine, accompanied with the depletion of formate and glycolate, in roots of the transgenicArabidopsisplants. This metabolic adjustment supports the hypothesis that AtFTHFL feeds the cytosolic C₁network in roots with C₁units 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‐H₄PteGlu_n, methionine, andS‐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. 

Leaf photosynthesis of perennial grasses usually decreases markedly from early to late summer, even when the canopy remains green and environmental conditions are favorable for photosynthesis. Understanding the physiological basis of this photosynthetic decline reveals the potential for yield improvement. We tested the association of seasonal photosynthetic decline in switchgrass ( Panicum virgatum L.) with water availability by comparing plants experiencing ambient rainfall with plants in a rainfall exclusion experiment in Michigan, USA. For switchgrass exposed to ambient rainfall, daily net CO 2 assimilation ( A n e t ' ) declined from 0.9 mol CO 2 m -2 day -1 in early summer to 0.43 mol CO 2 m -2 day -1 in late summer (53% reduction; P<0.0001). Under rainfall exclusion shelters, soil water content was 73% lower and A n e t ' was 12% and 26% lower in July and September, respectively, compared to those of the rainfed plants. Despite these differences, the seasonal photosynthetic decline was similar in the season-long rainfall exclusion compared to the rainfed plants; A n e t ' in switchgrass under the shelters declined from 0.85 mol CO 2 m -2 day -1 in early summer to 0.39 mol CO 2 m -2 day -1 (54% reduction; P<0.0001) in late summer. These results suggest that while water deficit limited A n e t ' late in the season, abundant late-season rainfalls were not enough to restore A n e t ' in the rainfed plants to early-summer values suggesting water deficit was not the sole driver of the decline. Alongside change in photosynthesis, starch in the rhizomes increased 4-fold (P<0.0001) and stabilized when leaf photosynthesis reached constant low values. Additionally, water limitation under shelters had no negative effects on the timing of rhizome starch accumulation, and rhizome starch content increased ~ 6-fold. These results showed that rhizomes also affect leaf photosynthesis during the growing season. Towards the end of the growing season, when vegetative growth is completed and rhizome reserves are filled, diminishing rhizome sink activity likely explained the observed photosynthetic declines in plants under both ambient and reduced water availability. 

Abstract 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 H₂O₂resulting from glycolate oxidation into O₂and 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 ofArabidopsis thalianamutants lacking peroxisomal catalase. Our results strongly support this hypothesis, with catalase mutants showing gas exchange evidence for an increased stoichiometry of CO₂release from photorespiration, specifically an increase in the CO₂compensation point, a photorespiratory‐dependent decrease in the quantum efficiency of CO₂assimilation, increase in the¹²CO₂released in a¹³CO₂background, and an increase in the postillumination CO₂burst. Further metabolic evidence suggests this excess CO₂release 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.