Roles of lipids in coconut (Cocos nucifera L.) embryogenesis
The coconut palm (Cocos nucifera L.) is an important crop world-wide but to maintain its status, a method of somatic embryogenesis to produce clonal populations is required. Despite recent improvements in callus production the formation of somatic embryos and plantlet regeneration remain problematic. This thesis identified lauric acid accumulation as a potential marker for coconut seedling development and used it as a nutrient in combination with other culture conditions to promote embryogenesis or germination. It was found that lauric acid content was correlated with seedling development. The fatty acid together with naphthaleneacetic acid, 6-benzylaminopurine and 77Z}'o-inositol in the culture medium increased the growth of the organs (shoots, roots and haustorium) of seedlings mimicking, to some extent, the development of intact nuts. Metabolic studies showed that lauric acid is used as a precursor of other fatty acids and complex lipids and that their contents varied with the organ and the developmental stage of the seedling. In somatic embryogenesis, lauric acid combined with ABA promoted callus differentiation and formation of proembryos. Studies of 2,4-D metabolism revealed that deciding upon the optimum concentration of the auxin to use in coconut somatic embryogenesis is complicated by its rate of metabolism and conditions of culture (sucrose and 2,4-D concentrations). Five classes of metabolites (glucose esters, peptide conjugates, basic metabolites, triacylglycerol analogues containing short-chain elongated forms of 2,4-D and triacylglycerol analogues containing long-chain elongated forms of 2,4-D) were formed in the explants and their proportions varied according to the growth of the cultured tissue. The presence of non-polar metabolites was associated with de-differentiation of cxplants. Finally an improved protocol for somatic embryogenesis is suggested but many details remain to be improved.