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Caractérisation des gènes AtNCED impliqués dans la biosynthèse de l'acide abscissique dans la graine d'Arabidopsis thaliana

Abstract : The plant hormone abscisic acid (ABA) is involved in the response and adaptation to stress, as well as seed maturation and germination. ABA is produced from cleaved carotenoids (C40) and the early steps in its biosynthesis are common with those for the formation of pigments such as carotene and lycopene. Identification of the ABA deficient, viviparous mutant (vp14), enabled the cloning of a gene encoding a 9-cis-epoxycarotenoid dioxygenase (NCED) (Schwartz et al., 1997) responsible for the cleavage of carotenoids to form xanthoxin (C15), considered to be the rate limiting reaction of the biosynthesis pathway. The subsequent analysis of these enzymes in other species found that they were encoded by multigene families, and that in Arabidopsis the family was composed of 9 members, termed AtNCED, only 5 of which appeared to be involved in ABA biosynthesis (Iuchi et al., 2001) (Tan et al., 2003). The work presented in this thesis concerns the characterisation of the AtNCED genes implicated in ABA biosynthesis in Arabidopsis seeds. An initial study of the 7 genes most homologous to VP14 was carried out with regard to their expression during seed development and in vegetative tissues. This found that 5 of the genes were expressed in seeds. Of these, AtNCED6 and AtNCED9 showed transcript accumulation that was specific to siliques. The detailed analysis of their expression patterns enabled AtNCED6 transcripts to be localised to the endosperm throughout seed development, with a peak in abundance at 14 DAF. The AtNCED9 gene was expressed slightly earlier that AtNCED6, and transcripts were detectable in both seed embryos and endosperm. The phenotypes observed for Atnced9 and Atnced6 mutants were similar and weaker than those observed for ABA deficient mutants affected in unique genes. ABA biosynthesis was apparently only affected in seeds, as observed by reduced ABA levels and paclobutrazol resistance; seed dormancy was, however, unaltered. Dormancy and the requirement of gibberellins for germination would appear, therefore, to be differentially regulated by ABA. In contrast, seeds of the Atnced6, Atnced9 double mutant were less dormant. This implies that ABA synthesized in the endosperm participates in the process of dormancy imposition during seed development. Furthermore, seed fatty acid composition in the Atnced6 mutant was different from that of wild-type, which indicates that ABA synthesized in the endosperm is also involved in the regulation of seed reserves. Analysis of the expression of two other genes, AtNCED2 and AtNCED3, found that although their temporal expression patterns were similar during seed development, the spatial expression patterns were different, suggesting that each enzyme could participate in the synthesis of ABA pools with distinct physiological roles.
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Contributor : Ecole Agroparistech <>
Submitted on : Friday, February 3, 2006 - 8:00:00 AM
Last modification on : Friday, February 3, 2006 - 8:00:00 AM
Long-term archiving on: : Tuesday, September 18, 2012 - 1:00:44 PM

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Valérie Lefebvre. Caractérisation des gènes AtNCED impliqués dans la biosynthèse de l'acide abscissique dans la graine d'Arabidopsis thaliana. Life Sciences [q-bio]. INAPG (AgroParisTech), 2005. English. ⟨pastel-00001585⟩

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