Harnessing plant biomass for biofuels and biomaterials

Modern societies have relied for more than a century on fossil carbon sources for the production of fuels or chemicals ranging from plastic polymers to drugs to food additives. The consequences of this utilization of fossil carbon-derived fuels – the release of carbon dioxide into the atmosphere – are no longer deniable. There is general agreement among leading scientists that the increased concentration of carbon dioxide in the atmosphere has contributed to the all-too-apparent global-warming trend. It is also clear that the increasingly high demand for fossil carbon will eventually deplete the existing stocks, with consequences not only in the area of energy but also in the wider chemical industry. Concerned citizens from scientists to policy makers have recognized the need for a reliable, renewable and affordable source of carbon in its chemically reduced form that can sustain future economic developments without having a negative impact on the environment. Discussion of solutions to overcome the current dependence on fossil carbon is conducted at all levels of society. The conversion of light energy into chemical energy by plant photosynthesis ranks prominently among the natural processes that can potentially meet the challenge. Plant biologists and biochemists are therefore at the forefront of developing schemes and ideas for an emerging bioeconomy that sustainably harnesses plant biomass. To contribute to the ongoing discussion in this area, we present in this special issue a series of reviews that describe the multiple biochemical processes that plants can or could use to convert their fixed carbon into fuels and other useful products. Rather than advocate a specific process or compound, these invited peer-reviewed articles by leading plant biologists and biochemists focus on the scientific facts behind the production of plant biofuels such as ethanol or biodiesel, as well as other important chemicals that are often unique to plants. It should be remembered that the basic precursors for the synthesis of drugs, plastics and other industrially important products of the petrochemical industry are also derived from fossil carbon. While only a small proportion of fossil carbon is used for the production of these specialized feedstock chemicals, such chemicals are nevertheless just as important as fuels to our economy, and when fossil carbon sources are depleted or become too expensive, new sources for these chemicals will have to be found. Plants already synthesize many of these feedstock chemicals, and have the potential to economically produce many more, in particular some compounds that are extremely difficult to manufacture, e.g. compounds with chirally pure precursors. Therefore, we have also included articles on biomaterials and plant-derived compounds that one day might replace petrochemical compounds. Traditionally, The Plant Journal focuses on fundamental questions in plant biology and does not publish articles with a heavy emphasis on plant biotechnology. There will be no change in this current editorial policy. However, discoveries in basic plant sciences ultimately provide the basis for solutions to applied problems. Thus, we have asked the authors to focus on the basic science behind the conversion of plant biomass into useful compounds. It is not the intention to provide a single best solution or to identify the most promising path to a sustainable bioeconomy. Rather, we have chosen to cover the most prominent areas currently discussed and have asked the authors to be critical with regard to the promise and potential of the proposed ideas. We also requested from the authors to make their reviews as accessible as possible so that they can be used by interested individuals, educators and members of the media as a scientific backdrop to inform discussion. To this end, we are making all the articles available free online from the day of publication. Starting with a review by Nikolau et al. (pp. 536–545), the concept of a biorefinery that produces biofuels and platform chemicals for the chemical industry from plant biomass is introduced. The most readily available biofuel, ethanol, is currently produced from grain starch or sugar cane sucrose. As Smith (pp. 546–558) suggests, the yield potential of easily converted carbohydrates such as starch and sucrose per unit of land is not even close to theoretical capacity. Crop plants with a high weight fraction of these carbohydrates in seeds and other tissues could substitute for lignocellulose as the feedstock for ethanol production until lignocellulose can be converted at an industrial scale and at reasonable costs. Lignocellulose, essentially plant cell walls, is by far the most abundant land-based plant biomaterial, and its conversion to transportation fuels holds great promise. However, plant cell walls are naturally designed to resist degradation by micro-organisms, making conversion of this biomass into biofuels a challenge. Currently, plant biologists are working on schemes to modify the carbohydrate components of plant cell walls, as discussed by Pauly and Keegstra (pp. 559–568), and the lignin matrix, as described by Li et al. (pp. 569– 581). The overall goal is to improve conversion of the wall polymers into feedstocks for microbial fermentation processes without compromising plant growth. To provide a context for the conversion of plant biomass into fuels and chemicals, DoranPeterson et al. (pp. 582–592) discuss the microbiological aspects of the process.