Oil palm fruit is exceptionally rich in lipids and provitamin A carotenoids, which have essential nutritional properties for humans. What are the molecular mechanisms underlying these qualities and how are they integrated within the fruit ripening process? By using the very latest transcriptome sequencing technologies, scientists from CIRAD/IRD have just revealed the molecular determinants of lipid and carotenoid biosynthesis that occurs during the ripening of this unique fruit. This is a first for the species.
Oil palm (Elaeis guineensis) is the plant species that accumulates the highest amount of oil in its fruit. The fruit also contains exceptional quantities of carotenes, or provitamin A, which play a major role in human health and nutrition. However, compared to dicotyledonous species that have been well studied, such as tomato or grapevine, little research has been done on the molecular bases of the development and ripening of this fleshy monocotyledonous fruit. A team from CIRAD/IRD conducted a study on the biosynthetic pathways that occur in the mesocarp, the fleshy part of the fruit, in order to elucidate the underlying mechanisms. The team used high-throughput transcriptome sequencing, a technique used to quantify all the transcribed products, or transcripts, of a genome in a given tissue and which quantifies gene expression.
Oil biosynthetic pathways
Thanks to this technique, it was possible to annotate and identify 29 034 transcripts in the mesocarp. In total, only 2 629 genes were differentially expressed during mesocarp development. The researchers then studied these genes in order to identify the mechanisms that could explain the exceptional accumulation of oil and carotenoids in the mesocarp.
By conducting a detailed analysis of gene expression patterns, they decoded the biosynthetic pathways for oil and revealed a very high level of transcriptional regulation at the early stages of the de novo formation of fatty acids, which occurs in the plastids. They also found that there was little transcriptional regulation of triglyceride assembly (three fatty acids esterified to a glycerol molecule) that occurs in the endoplasmic reticulum. The transcription factor Wrinkled (WRI1), known to be associated with the regulation of seed lipid biosynthesis, was also identified in the oil palm mesocarp. Interestingly, the WRI1 activators described in oilseeds were not found, which suggests other regulatory factors are involved in this fruit.
An original ripening process
The massive accumulation of carotenoids, followed by that of abscisic acid, are two original characteristics of oil palm fruit ripening. As found for fatty acids, the main transcriptional regulation occurs during the early stages of carotenoid biosynthesis.
The researchers revealed the coordinated expression of genes associated with the production and signalization of ethylene, a key hormone in climacteric fruit ripening. In addition, they identified MADS-box transcription factor regulatory genes, described for some model dicotyledonous species as major regulators of fruit ripening. On the basis of the expression of these genes, the analyses revealed a new group of MADS genes that are potentially associated with ripening. Therefore, there is a divergence between the regulatory mechanisms involved in fleshy fruit ripening in monocotyledons and those identified in model dicotyledonous species.
For the first time with this species, the results reveal the molecular determinants of oil biosynthesis, a key component of agronomic yield. In the future, the oil palm can be considered as an original model for studying fruit ripening in tropical monocotyledonous species.