Sustainable production of energy and industrial materials from non-food crops can substantially reduce our reliance on non-renewable fossil fuel reserves.
Jatropha curcas is a promising non-food crop with oil-rich seeds (30-48%), an easy propagation system, and high adaptability to a wide range of climatic and soil conditions. These attributes have led to the promotion of Jatropha for low-cost biodiesel production and as part of the solution to the challenges of climate change, energy crisis, and provision of rural income.
However, in 2010, the use of Jatropha as an alternate energy resource was questioned due to inconsistent yield patterns recorded for plantations in marginal and low-nutrient environments. The inconsistent yields were identified to result from the lack of information on the optimal agricultural practices and nutrient requirements needed for large-scale Jatropha cultivation. The low productivity of Jatropha plantations arises from variation in seed quality and quantity, high male-to-female flower ratio, asynchronous flowering, seed toxicity, and vulnerability to various biotic and abiotic stresses.
Our paper reports an analysis of recent studies that address the limitations of Jatropha from the biological aspects of harnessing current knowledge on plant-microbe interactions, gene resources, and biotechnological tools, from which we see renewed promise for this energy crop.
Marginal and low-nutrient soils are generally preferred sites for bioenergy crop production as this can reclaim those lands while avoiding competition with food crops for land use. However, a plantation in poorer soil environments cannot guarantee profitable yield, and soil enrichment is essential to make the crop economically feasible.
The use of bioagents including microbes as biofertilizers (arbuscular mycorrhizal fungi, endophytes and plant growth promoting rhizobacteria) and biopesticides (Hymenopteran egg parasitoids: Trissolcus basalis, Psixstriaticeps and Gryon sp and bacteria: Bacillus sp) has shown promising results for enriching soil fertility and in providing protection against biotic and abiotic stresses, with the additional benefits of reduced input cost, improved environmental safety, and target-specificity. The investigation of the molecular basis of host interactions with useful microbes will further ease the establishment of a steady eco-friendly and sustainable Jatropha plantation, avoiding, or at least minimizing, the use of chemical fertilizers, insecticides, or pesticides.
Another approach to boost seed yield is the manipulation of inflorescence architecture, in particular, to increase the female flower ratio. Spraying of plant growth regulators such as gibberellins, cytokinin, and thidiazuron on immature inflorescences prior to sex differentiation has shown a substantial hike in female flower numbers. However, spraying in large-scale plantations is labor-intensive, and success is also weather-dependent.
Recent advancement in genomics and transcriptomics extends the choices for increasing female flower numbers in Jatropha via manipulation of plant growth regulator biosynthesis pathways or of key sex-related genes responsible for sex differentiation at the developmental biology level. Additionally, the establishment of suitable pollination services in Jatropha plantations to optimize fruit set from cross-pollination by honey bees can further increase seed yields. Several species of honey bee (dwarf honey bee, giant honey bee, European honey bee, stingless bee, and Mexican small eusocial stingless bee) have been recorded as efficient pollinators of Jatropha.
The current status of plant molecular biology and biotechnology offers exciting opportunities for Jatropha improvement: a whole genome sequence is available for Jatropha, as is information on floral development and stress-responsive genes; and an efficient plant transformation system has been reported with positive results from genetically modified lines of Jatropha at the lab-scale. Together, these offer good prospects for engineering elite Jatropha cultivars that combine high yield potential with tolerance to biotic and abiotic stresses. Early studies have shown that both over-expression and gene silencing methods can address the various limitations of this crop. However, since the growing of genetically-modified crops still faces issues of public acceptance in several countries, alternative approaches should also be explored such as gene silencing by direct application of dsRNA and gene editing with site-specific nucleases.
Economical utilization of by-products can further aid the economic value of Jatropha plantations. The seed cake left after oil extraction has high levels of protein (45.3–58.6%) and dietary fiber (5–5.5%), with a gross energy value of 19–48%. The main barrier to use of Jatropha seed cake as a food or animal feed or fertilizer are the presence of toxins such as phorbol esters and curcin, and antinutritional compounds such as phytic acid, tannins, and saponin. Several fungal and bacterial strains have shown promising results for detoxification of Jatropha seed cake: yeast (Candida parapsilosis), oyster mushroom (Pleurotus ostreatus), and the bacteria Psesudomonas aeruginosa are reported as super-efficient bioagents for detoxification. Toxin-free seeds can be developed from genetically modifying Jatropha, aiding economic feasibility by reducing the processing costs for toxin-free seed cake for fertilizers and animal feed.
There are excellent prospects to advance Jatropha as a foster energy crop for use on marginal soils, especially if best growing practices and bioagents are used together with elite cultivars developed to suit poor soils and high productivity. The emergence of precise genome editing techniques which in some legislations is not regulated as a genetic modification, has been effective as a novel targeted approach to crop mutagenesis can also be used in Jatropha. Ideally, an integrated approach using elite lines from conventional breeding programs, complemented by and crossed with engineered lines produced via biotechnological tools can expedite crop improvement. Taken together, biological information and biotechnological approaches can help to bridge the gaps in productivity for Jatropha.
These findings are described in the article entitled, An update on biological advancement of Jatropha curcas L.: New insight and challenges, recently published in the journal Renewable and Sustainable Energy Reviews. This study was conducted by Purabi Mazumdar, Pooja Singh and Professor Jennifer Ann Harikrishna from the University of Malaya and by Professor Subramanian Babu and Professor Ramamoorthy Siva from the VIT University.