title: C acid decarboxylases required for C photosynthesis are active in the mid-vein of the C species Arabidopsis thaliana, and are important in sugar and amino acid metabolism.
abstract: Cells associated with veins of petioles of C(3) tobacco possess high activities of the decarboxylase enzymes required in C(4) photosynthesis. It is not clear whether this is the case in other C(3) species, nor whether these enzymes provide precursors for specific biosynthetic pathways. Here, we investigate the activity of C(4) acid decarboxylases in the mid-vein of Arabidopsis, identify regulatory regions sufficient for this activity, and determine the impact of removing individual isoforms of each protein on mid-vein metabolite profiles. This showed that radiolabelled malate and bicarbonate fed to the xylem stream were incorporated into soluble and insoluble material in the mid-vein of Arabidopsis leaves. Compared with the leaf lamina, mid-veins possessed high activities of NADP-dependent malic enzyme (NADP-ME), NAD-dependent malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PEPCK). Transcripts derived from both NAD-ME, one PCK and two of the four NADP-ME genes were detectable in these veinal cells. The promoters of each decarboxylase gene were sufficient for expression in mid-veins. Analysis of insertional mutants revealed that cytosolic NADP-ME2 is responsible for 80% of NADP-ME activity in mid-veins. Removing individual decarboxylases affected the abundance of amino acids derived from pyruvate and phosphoenolpyruvate. Reducing cytosolic NADP-ME activity preferentially affected the sugar content, whereas abolishing NAD-ME affected both the amino acid and the glucosamine content of mid-veins.
authors.affiliation: Department of Plant Sciences, Downing Street, University of Cambridge, Cambridge, CB2 3EA, UK.
authors.forename: Naomi J
authors.initials: NJ
authors.lastname: Brown
authors.forename: Ben G
authors.initials: BG
authors.lastname: Palmer
authors.forename: Susan
authors.initials: S
authors.lastname: Stanley
authors.forename: Hana
authors.initials: H
authors.lastname: Hajaji
authors.forename: Sophie H
authors.initials: SH
authors.lastname: Janacek
authors.forename: Holly M
authors.initials: HM
authors.lastname: Astley
authors.forename: Kate
authors.initials: K
authors.lastname: Parsley
authors.forename: Kaisa
authors.initials: K
authors.lastname: Kajala
authors.forename: W Paul
authors.initials: WP
authors.lastname: Quick
authors.forename: Sandra
authors.initials: S
authors.lastname: Trenkamp
authors.forename: Alisdair R
authors.initials: AR
authors.lastname: Fernie
authors.forename: Veronica G
authors.initials: VG
authors.lastname: Maurino
authors.forename: Julian M
authors.initials: JM
authors.lastname: Hibberd
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism*; D050260:Carbohydrate Metabolism / Q000235:genetics / Q000502:physiology*; D002250:Carbon Radioisotopes / Q000378:metabolism; D002855:Chromatography, Thin Layer; D008291:Malate Dehydrogenase / Q000235:genetics / Q000502:physiology; D008293:Malates / Q000378:metabolism; D016254:Mutagenesis, Insertional; D010730:Phosphoenolpyruvate Carboxylase / Q000235:genetics / Q000502:physiology; D010788:Photosynthesis / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D052584:Xylem
chemical_list: D000596:Amino Acids; D002250:Carbon Radioisotopes; D008293:Malates; C030298:malic acid; D008291:Malate Dehydrogenase; C046446:malate dehydrogenase-(oxaloacetate-decarboxylating) (NAD+); C031917:malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+); D010730:Phosphoenolpyruvate Carboxylase