Regulatory mechanisms underlying oil palm fruit mesocarp maturation, ripening and functional specialization in lipid and carotenoid metabolism

Fruit provide essential nutrients and vitamins for human diets. Not only is the lipid rich fleshy mesocarp tissue of the oil palm (Elaeis guineensis Jacq.) fruit the main source of edible oil for the world, but it is also the richest dietary source of provitamin A. The present study examines the transcriptional basis of these two outstanding metabolic characters in the oil palm mesocarp. Morphological, cellular, biochemical and hormonal features defined key phases of mesocarp development. A 454-pyrosequencing derived transcriptome was then assembled for the developmental phases preceding, and during maturation and ripening when high rates of lipid and carotenoid biosynthesis occur. 2,629 contigs with differential representation revealed coordination of metabolic and regulatory components. Further analysis focused on the fatty acid (FA) and triacylglycerol (TA) assembly pathways, and during carotenogenesis. Notably, a contig similar to the Arabidopsis seed oil transcription factor WRINKLED1 was identified with a transcript profile coordinated with those of several FA biosynthetic genes and the high rates of lipid accumulation, suggesting some common regulatory features between seeds and fruits. We also focused on transcriptional regulatory networks of the fruit, in particular those related to ethylene transcriptional activation and MADs-box proteins. Our results suggest key roles for AGL2/SEPALLATA, AGAMOUS and GLOBOSA/PISTILLATA-like proteins in the mesocarp, and a central role for ethylene coordinated transcriptional regulation of type VII Ethylene Response Factors during ripening. Our results suggest divergence has occurred in the regulatory components in this monocot fruit compared to those identified in the dicot tomato fleshy fruit model.