Review
Plant-Derived Terpenes: A Feedstock for Specialty Biofuels

https://doi.org/10.1016/j.tibtech.2016.08.003Get rights and content

Trends

Options for alternative non-petroleum-based transportation fuels are limited.

Specific terpenes have the physical and chemical properties required for specialty biofuels capable of blending with or replacing petroleum-derived jet, missile, diesel, and gasoline fuels.

Many plants naturally produce and store these specific terpenes.

The biosynthesis of plant-derived terpenes is under strong genetic control.

Strategies to increase terpene content are presented based on modern genetics and genomics approaches.

Research toward renewable and sustainable energy has identified specific terpenes capable of supplementing or replacing current petroleum-derived fuels. Despite being naturally produced and stored by many plants, there are few examples of commercial recovery of terpenes from plants because of low yields. Plant terpene biosynthesis is regulated at multiple levels, leading to wide variability in terpene content and chemistry. Advances in the plant molecular toolkit, including annotated genomes, high-throughput omics profiling, and genome editing, have begun to elucidate plant terpene metabolism, and such information is useful for bioengineering metabolic pathways for specific terpenes. We review here the status of terpenes as a specialty biofuel and discuss the potential of plants as a viable agronomic solution for future terpene-derived 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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    This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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