Trends in Biotechnology
ReviewPlant-Derived Terpenes: A Feedstock for Specialty Biofuels☆
Section snippets
Energy-Rich Terpenes as Specialty Biofuels
In plants, terpenes (also known as terpenoids or isoprenoids) constitute a naturally occurring, chemically diverse set of metabolites associated with developmental physiology as well as mutualistic and antagonistic plant–herbivore and plant–environment interactions (Box 1) [1]. Terpenes, together with aromatic compounds, constitute the essential oils of plants, with the highest concentration usually being found in the specialized storage cavities of leaves. Terpenes are hydrocarbons that are
Engineering Terpene Biosynthesis in Non-Plant Systems
Thus far, efforts to produce terpenes as a specialty biofuel have been limited to microorganisms. The genetic tractability of microbial systems has led to significant progress in reconstructing metabolic pathways within these organisms for several biofuels, including terpenes [18]. Peralta-Yahya et al. identified bisabolane as a suitable alternative to D2 diesel and showed that its precursor, bisabolene, could be produced at high levels in both Escherichia coli (>0.90 g l−1) and Saccharomyces
Plant Terpenes as a Source for Specialty Biofuels
Considering that plants naturally produce and store thousands of terpenes, including those with a potential as biofuels (Table 1), the question remains: why have plants not been leveraged as a viable source of these compounds? The foliar tissues of many plants, such as Eucalyptus species, contain large natural variations in terpene content and composition [31]. For example, Eucalyptus polybractea showed approximately 20-fold natural variation in foliar terpene content [0.7–13.0% dry weight (DW)
Challenges in Understanding the Plant Terpene Biosynthetic Machinery
Despite the potential economic and environmental benefits of terpenes derived from plants, key biological challenges must be overcome. Not all terpenes are represented in all plant lineages, and individuals of the same species may vary in terpene content [54]. There have also been reports of variations in terpene content in response to biotic stress [55], seasonal changes [56], and endogenous hormone levels [57]. Interestingly, terpene diversity is not paralleled by an equivalent number of
A Systems-Level Understanding of Terpene Biosynthesis
In plants, genetic and environmental factors regulate terpene biosynthesis at multiple levels, including transcription, protein accumulation, and post-transcriptional/translational regulation 29, 61. Understanding the molecular underpinnings of terpene metabolism in plants will benefit from a systems-biology approach where the goal is to quantitatively describe the cellular processes through global modeling of the interactions and dynamics of the molecular components. Systems biology integrates
Commercialization Potential of Terpenes from Plants
To produce terpene-based fuels at commercially competitive scales, gains in terpene yield per hectare could be made by producing elite lines with consistently high foliar terpene content, establishing silviculture techniques, and decreasing terpene loss during harvesting and extraction. A crucial step is to identify species with high foliar content of specific terpenes that grow well on marginal lands with minimal impact on food production and native biodiversity. Padovan et al. (2014) reviewed
Concluding Remarks and Future Directions
Plants are attractive systems for the commercial-scale production of specific terpenes owing to their inherent ability to synthesize, transport, accumulate, and store these compounds. To leverage plant systems for terpenes will require that several challenges are addressed (Box 1). A roadmap for the commercial recovery of terpenes synthesized in plant is proposed in Figure 3. Exploiting the existing variation in plant terpene content and using high-throughput omics profiling technology should
Acknowledgments
This review is based on work supported by the US Department of Energy (DOE) and funding from the Laboratory Directed Research and Development (LDRD) project LOIS ID: 7428. The contents of this review are solely the responsibility of the authors and do not necessarily represent the official views of the DOE. The authors also acknowledge funding from the National Research Foundation (South Africa). The authors thank Lee E. Gunter, Aparna Annamraju, Sai Venkatesh Pingali, Hugh M. O’Neill, Daniel
Glossary
- Metabolic engineering
- targeted rewiring of the metabolic network of an organism, using genetic modifications, to optimally produce target metabolites and/or decrease undesirable products.
- Selective breeding
- a process of developing a desired phenotype by pre-selecting specific individuals that will interbreed. Also referred to as artificial selection.
- Silviculture
- the science of forest management, particularly in controlling the establishment, maintenance, and health of the forest for a desired and
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