Résumé
We are studying the molecular mechanisms underlying a homeotic flowering abnormality, known as mantled, which is induced by tissue culture in oil palm (Elaeis guineensis Jacq.). It has been demonstrated that the mantled character is epigenetic in nature, as illustrated by the spontaneous reversion to a normal floral phenotype of mantled palms in the field and the weak non-Mendelian transmission of the abnormality through seeds. Given the epigenetic nature of the phenotype, it is unlikely to be caused by altered DNA sequences in the oil palm genome. We thus chose to study the mantled abnormality by differential gene expression analysis as a means of identifying early markers of the phenotype during the micro propagation process. Using differential display, we identified and characterised EGAD1, a gene coding for a putative plant defensin. In whole plants, transcripts of the EGAD1 gene were detected only in inflorescences. The closest characterised relative of the oil palm EGAD1 gene is the Petunia PPT gene, which is expressed principally in the pistil of the flower. The 77 amino acid polypeptide encoded by the EGAD1 gene displays strong similarities with a number of plant defensin proteins, which are thought to play a protective role and which have been shown in some cases to possess antifungal properties. Oil palm tissue cultures exhibit a generally strong induction of accumulation of EGAD1 transcripts, which were detected to differing extents at all stages of the tissue culture regeneration process. At the callus stage of the in vitro regeneration procedure, we observed a differential accumulation of EGAD1 transcripts which correlated with the presence or absence of the mantled flowering abnormality. EGAD1 gene expression may therefore be a marker of epigenetic somaclonal variation events. (Texte intégral)