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Nowadays, Jatropha curcas is attracting much attention as an oilseed crop for biofuel, as it can grow under any climate and soil conditions that are unsuitable for food production. However, little is known about Jatropha, and there are a number of challenges to be overcome. In fact, Jatropha has not really been domesticated; most of the Jatropha accessions are toxic, which renders the seedcake unsuitable for use as animal feed. The seeds of Jatropha contain high levels of polyunsaturated fatty acids, which negatively impact the biofuel quality. Fruiting of Jatropha is fairly continuous, thus increasing costs of harvesting. Therefore, before starting any improvement program using conventional or molecular breeding techniques, understanding gene function and the genome scale of Jatropha are prerequisites. The ultimate breeding objectives are high oil yield such as useful chemical composition in each part of the plant and reduced toxicity, without impairing the natural pathogen resistance, and ensuring the protection of animals. Selection and multiplication of superior germplasm for quality planting material is now the prime aim for achieving domestication and improvement in productivity of the species under adverse climate conditions
Figure 1: Yield, chemical composition in each part of J, curcas for which genetic improvement is essential.
However, using conventional breeding, the process from hybridization to cultivar release can span decades. Since J. curcas is a semi-wild plant, it will require a minimum 15 years of conventional breeding before it reaches a level of domestication. This period could be shortened if plant tissue culture and improvement through transgenesis were used. However, a high level of genetic variation is of crucial importance for breeding programs. The lack of knowledge about the genetic constitution of the plant material limits the success of breeding programs and makes it harder to exploit the full potential of the plant. Knowledge about the degree of genetic diversity among naturally occurring populations within and outside of the accepted “Center of Origin” in the world allows a targeted search for interesting backgrounds and to develop appropriate breeding strategies. Traditional methods using morphological characteristics to determine genetic diversity or proximity of different provenances of J. curcas were only of limited success, primarily because of environmental influences on otherwise very stable hereditary characteristics such as 1000 grain weight, protein and oil content of seeds. Climate changes, e.g. in annual average temperature, minimum temperature, annual precipitation and precipitation seasonality, are most significantly affecting yield responses. Higher levels of chemical, floral and molecular variability were found in different accessions of Jatropha, where as less in others. Over the past few years, molecular markers have been used to genetically characterize the J. curcas germ plasm, yielding contrasting results from high to rather low genetic diversity, which might be explained either by the number of accessions or the techniques used.
Additional studies are required to shed light on DNA polymorphism levels within geographical ecotypes; these levels are important cornerstones for genetic conservation and selection programs in this species. Further, due to the low number of cloned genes and its largely uncharacterized genome, J. curcas is a species requiring major research initiatives in agronomy and biotechnology with the aim of breeding new genetically improved varieties. The adoption of transgenesis for the improvement of biofuel crops, including J. curcas, has been recently recommended, while the exploitation of interspecific crosses among closely related Jatropha species was postulated as a strategy for the development of new varieties. However, for domestication, selection of promising individuals, germplasm collection and Interspecific hybridization are necessary. In 1997 Reddy and his colleagues produced crosses between Ricinus Communis and J. curcas and five related species. Based on pollen germination, J. curcas showed a closer relationship to J. gossypiifolia and J. glandulifera. Later, in 2010, experiments by Karanam, K. R., and Bhavanasi, J., involving interspecific hybridization between J. curcas and related species showed successful progenesis. A cross between J. curcas and J. integrrima resulted in successful seed production and allowed backcrossing with J. curcas. Nevertheless, the lack of high genetic variability in J. curcas hampers selective breeding, which calls for other strategies to increase the genetic diversity through chemical/physical mutations or intra/interspecific crossing programs.
Concerning its future directions of its genetic improvement, genome sequencing and systems biology have revolutionized plant functional genomics. Once expression has been altered, mRNA, protein and/or metabolite levels are quantified through various profiling approaches. Knowledge about the pattern of gene expression in plant tissues under variable culture conditions will help to increase production efficiency. To understand processes such as maturation and seed quality, determined by the production of oil, fatty acids, FA, or toxins, transcriptomics must be performed on successive stages of developing seeds. The identification and characterization of the spatial and temporal expression of genes that are economically important for FA biosynthesis will help us understand their regulation. The information on gene expression levels and patterns could provide the necessary data for breeding and genetic engineering to increase oil content or optimize FA composition in Jatropha seeds. Furthermore, proteomic approaches generate great insight into the plant systems biology in general and can explore the metabolic pathways of Jatropha in particular. Knowledge of the protein content and distribution in developing seeds as the most interesting tissue for oil production deserves special attention, and will provide details of the regulatory mechanisms in Jatropha. Therefore, there is a need for essential technologies to enhance the detection of low abundant proteins, as well as protein annotation such as targeted and nontargeted protein analyses. Contrary to proteomics, for which the analysis can start from genomic sequences, for metabolomics there is no initiation reference. Metabolomics are a challenging endeavor since no single analytical approach addresses all the different chemical structures, and many different techniques are needed to cover different compound classes with their diverse chemical properties. Although 164 compounds and some important metabolic pathways in Jatropha have been recognized by de Sant’Anna and his co-workers in 2013, our understanding about complex pathways and how they are regulated under different stress conditions are far from being complete. Therefore, investigations of functional genomics for important metabolic pathways will support the understanding and the improvement of J. curcas as a source of biofuel, as shown in Figure 2, bellow. Without such deep understanding, J. curcas cannot be improved to meet the criteria required for a valuable feedstock.
Figure 2: Functional genomic analyses were applied by the Plant Biotechnology Unit to determine how and when oil and toxins are produced in developing seeds and to identify possible impacts on the strategies for Jatropha genetic improvement.
Temesgen Bedassa Gudeta
Department of Biology, School of Natural Sciences, Madda Walabu University, Bale-Robe, Ethiopia