A high affinity pyruvate decarboxylase is present in cottonwood leaf veins and petioles: a second source of leaf acetaldehyde emission?

PDC1, a pyruvate/α-ketoacid decarboxylase, is involved in acetaldehyde, propanal and pentanal biosynthesis in melon (Cucumis melo L.) fruit

Plant pyruvate decarboxylases (PDC) catalyze the decarboxylation of pyruvate to form acetaldehyde and CO₂ and are well known to play a key role in energy supply via fermentative metabolism in oxygen-limiting conditions. In addition to their role in fermentation, plant PDCs have also been hypothesized to be involved in aroma formation although, to date, there is no direct biochemical evidence for this function. We investigated the role of PDCs in fruit volatile biosynthesis, and identified a melon pyruvate decarboxylase, PDC1, that is highly expressed in ripe fruits. In vitro biochemical characterization of the recombinant PDC1 enzyme showed that it could not only decarboxylate pyruvate, but that it also had significant activity toward other straight- and branched-chain α-ketoacids, greatly expanding the range of substrates previously known to be accepted by the plant enzyme. RNAi-mediated transient and stable silencing of PDC1 expression in melon showed that this gene is involved in acetaldehyde, propanal and pentanal production, while it does not contribute to branched-chain amino acid (BCAA)-derived aldehyde biosynthesis in melon fruit. Importantly, our results not only demonstrate additional functions for the PDC enzyme, but also challenge the long standing hypothesis that PDC is involved in BCAA-derived aldehyde formation in fruit.

Ethylene-responsive transcription factors interact with promoters of ADH and PDC involved in persimmon (Diospyros kaki) fruit de-astringency

The persimmon fruit is a particularly good model for studying fruit response to hypoxia, in particular, the hypoxia-response ERF (HRE) genes. An anaerobic environment reduces fruit astringency by converting soluble condensed tannins (SCTs) into an insoluble form. Although the physiology of de-astringency has been widely studied, its molecular control is poorly understood. Both CO(2) and ethylene treatments efficiently removed the astringency from 'Mopan' persimmon fruit, as indicated by a decrease in SCTs. Acetaldehyde, the putative agent for causing de-astringency, accumulated during these treatments, as did activities of the key enzymes of acetaldehyde synthesis, alcohol dehydrogenase (ADH), and pyruvate decarboxylase (PDC). Eight DkADH and DkPDC genes were isolated, and three candidates for a role in de-astringency, DkADH1, DkPDC1, and DkPDC2, were characterized by transcriptional analysis in different tissues. The significance of these specific isoforms was confirmed by principal component analysis. Transient expression in leaf tissue showed that DkPDC2 decreased SCTs. Interactions of six hypoxia-responsive ERF genes and target promoters were tested in transient assays. The results indicated that two hypoxia-responsive ERF genes, DkERF9 and DkERF10, were involved in separately regulating the DkPDC2 and DkADH1 promoters. It is proposed that a DkERF-DkADH/DkPDC cascade is involved in regulating persimmon de-astringency.

Analysis of 1-deoxy-D-xylulose 5-phosphate synthase activity in Grey poplar leaves using isotope ratio mass spectrometry

Deoxy-xylulose phosphate synthase (DXS) catalyzes the first step of the methylerythritol phosphate (MEP) pathway and it might regulate the metabolic flux in plastidic isoprenoid biosynthesis. We developed a sensitive assay suitable for plant extracts that is based on the decarboxylation of labeled pyruvate (1-(13)C)-PYR and detection of (13)CO(2) by isotope ratio mass spectrometry. We tested our method investigating the DXS activity in poplar leaves. Apparent DXS activity showed Michaelis constants of 111 and 158 microM for glyceraldehyde phosphate and pyruvate, respectively; pH and temperature optima were found at pH 8.6 and 45 degrees C. DXS activity was inhibited when the competitive inhibitor beta-fluoropyruvate was added to the reaction mixture. DXS activity strongly depended on leaf development with higher activity in young leaves and correlated fairly well with leaf isoprene emission potential. In mature poplar leaves, isoprene emission is the main metabolic sink of plastidic isoprenoid intermediates. Consequently, we found lower DXS activity in non-isoprene-emitting lines of poplar than in emitting plants as indicator of a lower demand of metabolic flux within the MEP pathway.

Gas phase measurements of pyruvic acid and its volatile metabolites

Pyruvic acid, central to leaf carbon metabolism, is a precursor of many volatile organic compounds (VOCs) that impact air quality and climate. Although the pathways involved in the production of isoprenoids are well-known, those of several oxygenated VOCs remain uncertain. We present concentration and flux measurements of pyruvic acid and other VOCs within the tropical rainforest (TRF) biome at Biosphere 2. Pyruvic acid concentrations varied diurnally with midday maxima up to 15 ppbv, perhaps due to enhanced production rates and suppression of mitochondrial respiration in the light. Branch fluxes and ambient concentrations of pyruvic acid correlated with those of acetone, acetaldehyde, ethanol, acetic acid, isoprene, monoterpenes, and sesquiterpenes. While pyruvic acid is a known substrate for isoprenoid synthesis, this correlation suggests that the oxygenated VOCs may also derive from pyruvic acid, an idea supported by leaf feeding experiments with sodium pyruvate which resulted in large enhancements in emissions of both isoprenoids and oxygenated VOCs. While feeding with sodium pyruvate-2-(13)C resulted in large emissions of both (13)C-labeled isoprenoids and oxygenated VOCs, feeding with sodium pyruvate-1-(13)C resulted in only (13)C-labeled isoprenoids. This suggests that acetaldehyde, ethanol, and acetic acid are produced from pyruvic acid via the pyruvate dehydrogenase (PDH) bypass system (in which the 1-C carbon of pyruvic acid is lost as CO(2)) and that acetone is also derived from the decarboxylation of pyruvic acid.