Somaclonal variation in oil palm (Elaeis guineensis Jacq.) : the DNA methylation hypothesis

The occurrence of somaclonal variants (ca 5%) among populations of somatic embryo-derived oil palms (Elaeis guineensis Jacq.) currently hampers the scaling-up of clonal plant production. In order to investigate the relationship between the "mantled" somaclonal variant and possible alterations in genomic DNA methylation rate, two complementary approaches have been used. HPLC quantification of relative amounts of 5-methyl-deoxycytidine has shown that global methylation in leaf DNA of abnormal regenerants is 0.5-2.5% lower than in their normal counterparts (20.8% vs 22%, respectively). When comparing nodular compact calli and fast growing calli, yielding respectively 5% and 100% of "mantled" plantlets, this decrease was up to 4.5% (from 23.2 to 18.7%). An alternative method, the Sssl-methylase accepting assay, based on the enzymatic saturation of CG sites with methyl groups, gave convergent results. This work demonstrates that a correlation exists between DNA hypomethylation and the "mantled" somaclonal variation in oil palm. (Résumé d'auteur

Variations in genomic DNA methylation during the long-term in vitro proliferation of oil palm embryogenic suspension cultures

The long-term proliferation of embryogenic cell suspensions of oil palm is associated with changes in both genomic methylation rates and embryogenic capacities. In the aim of exploring the relationship between epigenetic stability and the long-term in vitro proliferation of plant tissues, we have studied changes in genomic DNA methylation levels in embryogenic suspensions of oil palm (Elaeis guineensis Jacq.). Five embryogenic callus lines were obtained from selected hybrid seeds and then proliferated as suspension cultures. Each clonal line obtained from a single genotype was subdivided into three independent subclonal lines. Once established, cultures proliferated for 12 months and genomic DNA was sampled at 4 months intervals for the estimation of global DNA methylation rates through high performance liquid chromatography (HPLC) quantitation of deoxynucleosides. Our results show that in vitro proliferation induces DNA hypermethylation in a time-dependent fashion. Moreover, this trend is statistically significant in several clonal lines and shared between subclonal lines originating from the same genotype. Interestingly, the only clonal line undergoing loss of genomic methylation in the course of proliferation has been found unable to generate somatic embryos. We discuss the possible implications of genome-wide DNA methylation changes in proliferating cells with a view to the maintenance of genomic and epigenomic stability