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

Specific Nutritional Requirements of Coconut Calli (Cocos nucifera L.) during Somatic Embryogenesis Induction

Summary Coconut calli were cultivated on two somatic embryogenesis induction media (SEIMs), differing in their 2,4-D content. Gain in dry matter weight, composition of soluble sugars within calli, but also pH and contents of glucose and macroelements in media were analysed at O, 15, 28, and 60 days of culture. Relationships between contents of endogenous sugars, on the one hand, and between contents of media macroelements, on the other hand, were analysed. Comparison was made with calli maintained on a control multiplication medium. Traits could be classified into 3 types of response with regard to condition of somatic embryogenesis induction (SEI condition). The first correspond to traits that were modified by the SEI condition and varying over time. Two phases were determined. During the first phase (TO-T15), soluble sugar contents within calli decreased over time. The higher the 2,4-D content in SEIMs, the higher the sugar contents. Consumption of glucose and macroelements in media was negligible. However, strong relationships in the contents of chloride, nitrate, phosphate, and sulfate were modified in the SEI condition. During the second phase (T15-T60, growth became lower in the SEI condition. Requirements for glucose, nitrate and phosphate and acidification .of media were higher. The relationship, determined by changes in nitrate and phosphate (R>0.98), was modified by the SEI conditions, showing a preferential consumption for nitrate in this case. Endogenous sucrose content decreased to become lower in the SEI condition. The higher the 2,4-D content in SEIMs, the higher the requirements for media compounds, the higher the contents of sugars within calli, but the lower the growth. The second type of response corresponded to traits modified by the SEI condition, but constant over time. It concerned relationships between contents of some cations in the media. The third type of response corresponded to traits unchanges by the SEI condition and over time. It concerned the high relationship contents of endogenous glucose and fructose (R = 0.88), and between contents of chloride, ammonium, calcium, magnesium, and potassium. Abbreviations: ANOVA = analysis of variance; ANCOVA = analysis of covariance; BAP = 6-benzylaminopurine; D M W = dry matter weight; FMW = fresh matter weight; % WC = percentage of water content; SEIM = somatic embryogenesis induction medium; TFMW = total fresh matter weight