Caractérisation moléculaire et biochimique de la formation du bois de coeur chez le noyer noir (Juglans nigra L.) : accumulation des flavonoïdes et expression des gènes contrôlant leur biosynthèse

Abstract : The main objective of this thesis was to identify genes controlling walnut wood quality. The fonnation of heartwood contributes to the natural colour and durability of wood. This physiological process was investigated at the biochemical and molecular levels in mature Juglans nigra trees. The temporal dynamics of heartwood expansion, the accumulation of flavonoid extractives, and the expression of genes encoding enzymes of flavonoid pathway were studied in xylem sarnples from Juglans nigra stems. Protocols for RNA extraction were optimised for very aged xylem sarnples and cDNA fragments corresponding to the genes under study were cloned from Juglans nigra xylem in order to obtain homologous probes. The comparison arnong sarnples collected at different seasonal periods suggested that new heartwood is fonned mainly in the surnrner season. In ail of the trees studied, soluble phenolic extractives accumulated during ageing of xylem growth rings. Flavanols strongly accumulated in the transition zone between sapwood and heartwood, suggesting that their biosynthesis was up-regulated concomitant to heartwood fonnation. The flavanol content in xylem correlated with the transcript levels of genes encoding the enzymes chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), and dihydroflavonol4-reductase (DFR) (flavonoid pathway). On the contrary, the expression patterns of genes encoding the enzymes phenylalanine arnrnonia-lyase (PAL), cinnarnate 4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL) (general phenylpropanoid pathway) did not correlate with the distribution of flavanols, even though PAL genes were strongly expressed in the transition zone. The comparison of transcript levels, protein pools, and catalytic activities of PAL and CHS suggested that the activities of these enzymes might differ in the mode ofregulation. PAL activity would be regulated at the post-translation level, while CHS activity would be mainly regulated at the transcription level. The overall results indicate that in Juglans nigra trees flavanols are de novo synthesized in aged xylem, especially during the transfonnation of sapwood into heartwood. Flavanol accumulation appeared to be regulated mainly at the transcription level by the expression ofgenes encoding CHS, F3H, and DFR.
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Isacco Beritognolo. Caractérisation moléculaire et biochimique de la formation du bois de coeur chez le noyer noir (Juglans nigra L.) : accumulation des flavonoïdes et expression des gènes contrôlant leur biosynthèse. Sciences agricoles. Université Henri Poincaré - Nancy 1, 2001. Français. ⟨NNT : 2001NAN10260⟩. ⟨tel-01754334⟩

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