Genetic and biotechnological approaches for biofuel crop improvement

Abstract

Research and development efforts for biofuel production are targeted at converting plant biomass into renewable liquid fuels. Major obstacles for biofuel production include lack of biofuel crop domestication, low oil yields from crop plants as well as recalcitrance of lignocellulose to chemical and enzymatic breakdown. Researchers are expanding the genetic and genomic resources available for crop improvement, elucidating lipid metabolism to facilitate manipulation of fatty acid biosynthetic pathways and studying how plant cell walls are synthesized and assembled. This knowledge will be used to produce the next generation of biofuel crops by increasing fatty acid content and by optimizing the hydrolysis of plant cell walls to release fermentable sugars. Introduction Biofuels are commonly defined as fuels derived from renewable biological products and are often regarded as an attractive, 'green' alternative to fossil sources of energy due to their potential contribution to lowering carbon dioxide emissions [1]. Globally, plants produce an estimated 200 billion tones of biomass per year [2] in the form of sugars, polysaccharides, oils and other biopolymers, representing an unprecedented resource for biofuel production. However, despite its abundance and potential environmental benefits, the efficient and sustainable use of plant biomass for energy purposes remains a challenging endeavor, requiring major investments in science and technology [3,4].With the exception of sugar cane ethanol, biofuels are a nascent industry in many parts of the world. A few of the commercialized products include bioethanol derived from corn starch and biodiesel obtained from plants with a high content in fatty acids such as soybean, canola and sunflower ( Figure 1). However, the status of corn and soybean as major food crops, coupled to the fact that yields of starch and plant oil are too modest to cover the huge demand of transportation fuels has prompted the development of alternative biofuel production based on lignocellulosic biomass [4][5][6]. Lignocellulose, composed of the polysaccharides cellulose and hemicellulose, and lignin, a phenolic polymer, is the most abundant biomaterial on earth [2,7]. Most lignocellulosic feedstocks in consideration are perennial, non-food grasses such as switchgrass and Miscanthus, as well as woody plants such as poplar (Table 1).None of the current and potential crops has been domesticated or bred for improved polysaccharide or oil extraction for biofuel production. For this reason, biofuel research is focused towards understanding the plant biomass characteristics and traits that need to be modified to optimize crops for biofuel production. The wealth of genetic and genomic resources in model plants such as rice, Arabidopsis and Brachypodium are being used to answer fundamental scientific questions that cannot be addressed directly using potential biofuel crops. This review will discuss the recent advances in understanding lignocellulosic biomass recalcitrance and lipid metabolism for increasing oi...

Innovative Biological Solutions to Challenges in Sustainable Biofuels Production

“…Inclusion of monolignol substitutes, such as feruloylquinic acid, methyl caffeate, or caffeoylquinic acid with normal monolignols could considerably suppress lignin formation and substantially improve cell wall hydrolysis and fermentation (Grabber et al, 2010). Besides lignin, hemicellulose (including xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan) also contributes to plant cell wall recalcitrance (Vega-Sanchez & Ronald, 2010). It has been demonstrated that modification of hemicellulose could help overcome biomass recalcitrance.…”

“…A major bottleneck for conversion of lignocellulosic biomass to simple sugars (saccharification), to be subsequently converted by microorganisms into ethanol or other products, is the recalcitrance to enzymatic saccharification (Chen & Dixon, 2007;Lionetti et al, 2010). Recalcitrance is mainly due to the heterogeneity and molecular structure of lignocellulose where cellulose is arranged into a network of tight, inter-chain hydrogen bonds that form a crystalline core of microfibrils, embedded in a matrix of hemicellulosic polysaccharides that are covalently linked to lignin, a highly complex aromatic polymer (Vega-Sanchez & Ronald, 2010). Lignin contributes to biomass recalcitrance and consequently increases the costs associated with conversion (Simmons et al, 2010;Vega-Sanchez & Ronald, 2010).…”

“…For example, the maize (Zea mays) R2R3-MYB factor ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic Arabidopsis plants over-expressing ZmMYB31 show a significantly reduced lignin content with unaltered polymer composition, and consequently increased cell wall degradability of the transgenic plants (Fornale et al, 2010). An alternative approach to address the lignin issue is to replace monolignols with compounds containing easily cleavable chemical linkages, such as ester and amide bonds, avoiding the undesirable developmental and structural phenotypes associated with the down-regulation of lignin biosynthetic enzymes in transgenic plants (Vega-Sanchez & Ronald, 2010). Inclusion of monolignol substitutes, such as feruloylquinic acid, methyl caffeate, or caffeoylquinic acid with normal monolignols could considerably suppress lignin formation and substantially improve cell wall hydrolysis and fermentation (Grabber et al, 2010).…”

“…Recalcitrance is mainly due to the heterogeneity and molecular structure of lignocellulose where cellulose is arranged into a network of tight, inter-chain hydrogen bonds that form a crystalline core of microfibrils, embedded in a matrix of hemicellulosic polysaccharides that are covalently linked to lignin, a highly complex aromatic polymer (Vega-Sanchez & Ronald, 2010). Lignin contributes to biomass recalcitrance and consequently increases the costs associated with conversion (Simmons et al, 2010;Vega-Sanchez & Ronald, 2010). Lignins are complex aromatic biopolymers, consisting of (mainly) syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units (Simmons et al, 2010).…”

“…Two approaches can be used to address the issue of biodiesel quality. The first approach is to reduce the levels of both saturated and polyunsaturated fatty acids while increasing the amount of monounsaturated fatty acids, such as oleate (C18:1) or palmitoleate (C16:1) (Durrett et al, 2008;Pinzi et al, 2009;Vega-Sanchez & Ronald, 2010). For example, simultaneous down-regulation of two embryo-specific genes in soybean, Delta-12 fatty acid desaturase FAD2-1 gene and the FatB gene encoding a palmitoyl-thioesterase, increased oleic acid levels to greater than 85% compared with less than 18% in wild-type, and lowered saturated fatty acids levels to less than 6% (Buhr et al, 2002).…”