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Expression of DNA methyltransferases and somaclonal variaion in oil palm

Rival A., Jaligot E., Beulé T., Finnegan J.. 2005. In : CSIRO. Epigenetic regulation in disease and development, a Sir Mark Oliphant Conference "International frontiers of sience and technology", Canberra, Australia, 29th November - 2nd December 2005. s.l. : s.n., 1 p.. Sir Mark Oliphant Conference on International Frontiers of Science and Technology, 2005-11-29/2005-12-02, Canberra (Australie).

Somaclonal variation (Larkin and Scowcroft, 1981) is a phenotypic variation displayed among regenerants; its origin can be genetic (mutations, chromosome rearrangements...) although a growing body of data suggests that epigenetic changes are associated with this phenomenon. In oil palm (Elaeis guineensis Jacq), approximately 5% of somatic embryo-derived palms show homeotic changes in their floral development, involving an apparent feminisation of male parts in flowers of both sexes, called the "mantled" phenotype. This somaclonal variation may result in partial or complete flower sterility, depending on the severity of the abnormality. Studies of gene expression are being carried out in tissue cultures as means of establishing an early clonal conformity testing procedure (Tregear et al, 2001). Parallel studies on genomic DNA methylation changes induced by tissue culture have suggested that epigenetic mechanisms play an important role in the determination of somaclonal variation in oil palm (Rival and Parveez, 2004). DNA methylation patterns are established and maintained by DNA methyltransferases which catalyse the transfer of the methyl group from S-adenosylmethionine (SAM) to the C-5 position in the pyrimidine ring of cytosine. In plants, 5mC residues are found predominantly at symmetric CpG and CpNpG sequences, but also occur at cytosines in non-symmetric sequences (Finnegan and Kovac, 2000). We have recently isolated and characterised two different oil palm DNA methyltransferases, belonging to the METI and CMT3 families. Expression of oil palm METI and CMT3 has been compared in normal and variant calli from the same genotype using semi-quantitative RT-PCR. Variant calli are reported to give rise to more than 95% "mantled" palms and they are characterised by a substantial DNA hypomethylation (a 13% drop in global methylation levels) when compared to their normal counterparts (Jaligot et al, 2000). Our results indicate that differences in global DNA methylation rates between normal and variant callus material cannot be attributed to changes in MET or CMT expression. Calli being the material in which global methylation differences were found to be the greatest and the initial step of cell dedifferentiation, it is likely that no wider differences in MTases expression will be detected in regenerant material. Our observation that the genome-wide hypomethylation previously described in mantled oil palm material (Jaligot et al, 2000, 2002 and 2004) cannot be explained by a decrease in expression levels of METI and CMT3 methyltransferases parallel the major unresolved paradox in DNA methylation and cancer research: DNA global hypomethylation occurs while DNA MTases activity remains stable or even increases and gene-specific global hypermethylation is measured (Laird and Jaenisch, 1994; Robertson, 2005). (Texte intégral)
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