title: GDP-D-mannose 3,5-epimerase (GME) plays a key role at the intersection of ascorbate and non-cellulosic cell-wall biosynthesis in tomato.
abstract: The GDP-D-mannose 3,5-epimerase (GME, EC 5.1.3.18), which converts GDP-d-mannose to GDP-l-galactose, is generally considered to be a central enzyme of the major ascorbate biosynthesis pathway in higher plants, but experimental evidence for its role in planta is lacking. Using transgenic tomato lines that were RNAi-silenced for GME, we confirmed that GME does indeed play a key role in the regulation of ascorbate biosynthesis in plants. In addition, the transgenic tomato lines exhibited growth defects affecting both cell division and cell expansion. A further remarkable feature of the transgenic plants was their fragility and loss of fruit firmness. Analysis of the cell-wall composition of leaves and developing fruit revealed that the cell-wall monosaccharide content was altered in the transgenic lines, especially those directly linked to GME activity, such as mannose and galactose. In agreement with this, immunocytochemical analyses showed an increase of mannan labelling in stem and fruit walls and of rhamnogalacturonan labelling in the stem alone. The results of MALDI-TOF fingerprinting of mannanase cleavage products of the cell wall suggested synthesis of specific mannan structures with modified degrees of substitution by acetate in the transgenic lines. When considered together, these findings indicate an intimate linkage between ascorbate and non-cellulosic cell-wall polysaccharide biosynthesis in plants, a fact that helps to explain the common factors in seemingly unrelated traits such as fruit firmness and ascorbate content.
authors.affiliation: Institut National de la Recherche Agronomique (INRA), Université de Bordeaux, Unité Mixte de Recherche 619 sur la Biologie du Fruit, Institut Fédératif de Recherche 103, BP 81, F-33883 Villenave d'Ornon Cedex, France.
authors.forename: Louise
authors.initials: L
authors.lastname: Gilbert
authors.forename: Moftah
authors.initials: M
authors.lastname: Alhagdow
authors.forename: Adriano
authors.initials: A
authors.lastname: Nunes-Nesi
authors.forename: Bernard
authors.initials: B
authors.lastname: Quemener
authors.forename: Fabienne
authors.initials: F
authors.lastname: Guillon
authors.forename: Brigitte
authors.initials: B
authors.lastname: Bouchet
authors.forename: Mireille
authors.initials: M
authors.lastname: Faurobert
authors.forename: Barbara
authors.initials: B
authors.lastname: Gouble
authors.forename: David
authors.initials: D
authors.lastname: Page
authors.forename: Virginie
authors.initials: V
authors.lastname: Garcia
authors.forename: Johann
authors.initials: J
authors.lastname: Petit
authors.forename: Rebecca
authors.initials: R
authors.lastname: Stevens
authors.forename: Mathilde
authors.initials: M
authors.lastname: Causse
authors.forename: Alisdair R
authors.initials: AR
authors.lastname: Fernie
authors.forename: Marc
authors.initials: M
authors.lastname: Lahaye
authors.forename: Christophe
authors.initials: C
authors.lastname: Rothan
authors.forename: Pierre
authors.initials: P
authors.lastname: Baldet
mesh_terms: D001205:Ascorbic Acid / Q000096:biosynthesis*; D002238:Carbohydrate Epimerases / Q000235:genetics / Q000378:metabolism*; D002473:Cell Wall / Q000201:enzymology*; D018506:Gene Expression Regulation, Plant; D018551:Solanum lycopersicum / Q000201:enzymology* / Q000254:growth & development; D018384:Oxidative Stress; D030821:Plants, Genetically Modified; D011134:Polysaccharides / Q000096:biosynthesis; D034622:RNA Interference
chemical_list: D011134:Polysaccharides; D002238:Carbohydrate Epimerases; C019192:GDPmannose 3,5-epimerase; D001205:Ascorbic Acid