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Characterization of a candidate gene for drought tolerance in Coffea: the CcDREB1D gene, in contrasting genotypes of Coffea canephora and related species

Costa Alves G.. 2015. Montpellier : Montpellier SupAgro, 190 p.. Thèse de doctorat -- Biologie intégrative.

Coffee is the world's second most valuable traded commodity after crude oil. Like for other crops, drought is the key factor affecting plant development, flowering, productivity, fruits development and their quality. In such a context, the generation of drought-tolerant coffee varieties has now turned into one of the priorities of many research institutes. In coffee, several candidate genes for drought tolerance have been identified. This was the case of CcDR EB1D that showed increased gene expression upon drought stress mainly in leaves of Coffea canephora drought-tolerant clones. In several plant species, the DREBs genes play a key role in plant responses to abiotic stress. Based on these information, an in depth characterization of CcDREB1D was initiated by (i) studying the genetic diversity present in DREB1D loci, (ii) identifying DREB coffee subfamily members and evaluating phylogenetic relations between CcDREB1D and homologs from other plant species, and ( iii) performing a functional characterization of CcDREB1D promoters by homologous genetic transformation of C. arabica. Sequence variability in DREB1D promoter and coding regions was evaluated using 38 Coffea accessions most of them characterized by different phenotypes (tolerance vs. susceptibility) regarding to drought. Our findings show several evidences of association between drought tolerance and the genetic variations on DREB1D promoter region. Further analyses indicated that these promoters are evolving by the rearrangement of cis-regulatory elements, and could influence DREB1D expression. The recent release of the C. canephora genome sequence allowed us to identify the DREB gene family in this species composed of at least 31 canonical DREB genes. Compared to other plants, no specific amplification of the DREB gene family was observed. A reconciled phylogenetic tree constructed with homologs from other species allowed us to identify the coffee members of the DREB subgroups I, II, III and IV, in reference to their classification previously established in Arabidopsis. In addition, CcDREB1D is the nearest C. canephora orthologue of AtDREB1/CBF group, but any functional inference could not be done due to a recent duplication that lead to DREB1/CBF group sub functionalization in Arabidopsis. These results reinforced the necessity of CcDREB1D functional characterization in coffee. The study of genetic variability revealed a major source of nucleic diversity in the promoter region of DREB1D genes from C. canephora, with potential implication in local adaptation to drought tolerance. This is for example the case for the promoters of drought-tolerant clone 14 and drought-sensitive clone 22 of C. canephora known to present different CcDREB1D expression profiles regarding drought. In order to perform the functional analysis of these promoters, full length and truncated version of allelic and homolog forms of CcDREB1D promoters were fused to the GUS reporter gene. Transgenic plants of C. arabica harboring these constructions were regenerated. The spatiotemporal, tissue, cell and allelic-specific regulation of CcDREB1D promoters was analyzed by detecting GUS enzymatic activity. During drought stress, the full-length CcDREB1D promoter drives leaf-specific GUS activity only in epidermic, parenchyma and guard-cells. By comparing the different constructs, the most upstream 700 bp of allelic and homolog CcDREB1D promoters were shown to be necessary for the leaf-specific expression. In addition, allelic-specific expression observed in response to different (PEG and relative humidity) treatments mimicking drought stress showed that nucleic variations present between CcDREB1D promoter allelic forms affect the spatiotemporal regulation of the GUS reporter gene. (Résumé d'auteur)...

Mots-clés : coffea; coffea canephora; résistance à la sécheresse; résistance génétique; gène; stress dû à la sécheresse; variation génétique; transformation génétique; plante transgénique; phylogénie; génotype; séquence nucléotidique; coffea arabica; coffea eugenioides; arabidopsis thaliana; génie génétique; stress abiotique; brésil; Éthiopie; congo; guinée

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