Biochemical and genetic analyses of Jatropha curcas L. seed composition

Diminishing worldwide fossil fuel reserves coupled with the negative impact of their use on the environment has led to increased research and development of renewable energy sources. Renewable liquid biofuels are in demand for the transport sector, particularly if they can be used directly in existing internal combustion engines. Jatropha curcas L. is a perennial plant which belongs to the Euphorbiaceae family. J. curcas seeds contain about 30% oil which is suitable for biodiesel production, and therefore it has received global interest as a source of biofuel. However, to date J. curcas has not been put through any stringent breeding program for traits improvement and thus has not reached its full potential. Improving seed yield, seed oil quality and quantity is necessary for large scale biodiesel production, and developing other by products will add to the economic value of this crop. Apart from oil, J. curcas seeds also contain a high percentage of proteins, which makes the seed meal potentially useful as animal feed. However, seeds from most of the current global J. curcas plantations in Asia and Africa are characterized as non-edible, due to the existence of a few toxins or antinutrients. Phorbol esters have been considered as the main toxic agent. Edible provenances exist in Mexico which are devoid of phorbol esters. Among the other toxins, curcin (a type I ribosome inactivating protein) levels have not been reported for the edible and non-edible varieties.

To improve our understanding of J. curcas natural variation and biochemical composition, seed samples were collected from a variety of locations in Madagascar, Mexico, and purchased from five other countries. Seed oil content, fatty acid composition and phorbol esters content were measured to establish the diversity in these traits. Seed oil content and fatty acid composition was found to vary in seeds collected from different sites, and oleate and linoleate composition were found to correlate strongly with cultivation site temperature indicating the importance of environmental conditions for the production of an optimal feedstock for biodiesel. Phorbol esters were found to be present in all seed samples originating from outside Mexico, and in the Mexican provenance Rosario Chiapas. All other Mexican samples lacked phorbol esters. This suggests that the presence of phorbol esters is a qualitative trait. AFLP analysis revealed that most genetic variation was present in Mexican samples, with all material originating from outside Mexico showing very limited genetic diversity. Edible samples and non-edible samples were found to be genetically distinct, with the edible samples forming a single cluster. The large amount of variation in oil quantity and quality in Madagascan samples, together with the limited genetic diversity in these samples, implies that J. curcas seed oil is largely influenced by environmental factors.

Seed curcin levels were determined in the edible and non-edible varieties. The results showed that curcin levels are equally abundant in both varieties. This demonstrates that curcin is not playing any significant role in determining seed edibility and is consistent with the predominant role understood to be played by phorbol esters in determining this trait. The spatial and temporal expression of different curcin genes was further examined. Four curcin genes showed different patterns of expression with seed and leaf specific patterns of expression being identified. Further analyses revealed that CURCIN2 is induced in mature leaves in response to various abiotic stresses. Furthermore it appears that the induction of CURCIN2 in response to wounding is regulated via the JA (jasmonic acid) signalling pathway.

Together these results represent a valuable addition to the knowledge base underpinning the development of J. curcas as an industrial crop through molecular breeding.