Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol

Abstract

Biofuels provide a potential route to avoiding the global political instability and environmental issues that arise from reliance on petroleum. Currently, most biofuel is in the form of ethanol generated from starch or sugar, but this can meet only a limited fraction of global fuel requirements. Conversion of cellulosic biomass, which is both abundant and renewable, is a promising alternative. However, the cellulases and pretreatment processes involved are very expensive. Genetically engineering plants to produce cellulases and hemicellulases, and to reduce the need for pretreatment processes through lignin modification, are promising paths to solving this problem, together with other strategies, such as increasing plant polysaccharide content and overall biomass.

Recombinant hyperthermophilic enzyme expression in plants: a novel approach for lignocellulose digestion

“…The most obvious is the relatively low level of in situ protein expression/accumulation observed in most studies (Table S1 in the supplementary material online), in which an order-of-magnitude increase in expression level is required for complete conversion of biomass [8,33]. In addition to the obvious requirement for careful codon optimization [8,46,59] and/or codon harmonization [69], the further development of combinations of transit peptides and promoters (strong promoters and chimeric promoters), with or without promoter engineering, may be effective strategies [52,70]. Carbohydrate-binding modules can also be used to target better the recombinant enzymes and ultimately improve biomass digestion efficiency [45].…”

“…enzymes expressed in host plants often represent <10% of the total soluble protein (TSP), but would have to be increased to 30% for effective hydrolysis of lignocellulosic tissues without the need for addition of exogenous enzymes [8]. Although in planta expression of thermophilic endoglucanases has been achieved in the range of 10-26% TSP without any detrimental effects on transgenic plants [37][38][39][40][41][42][43][44][45][46][47][48], yields of 0.1-5% TSP are more typical, and several studies have described negative phenotypes associated with in planta expression of recombinant mesophilic cellulolytic enzymes [32,49].…”

“…Subcellular targeting of heterologous proteins generally yields higher accumulation levels in transgenic plants than expression in the cytosol [32][33][34][35][36][37][38][39][40][41][42]47,51]. When expressed in subcellular compartments, heterologous proteins typically display correct folding and activity, appropriate glycosylation, reduced degradation, and increased stability, as compared to expression and accumulation in the cytosol [8,44].…”

“…The crystallinity and surface area accessibility of cellulose, protection of cellulose by lignin, the heterogeneous character of biomass particles, and cellulose sheathing by hemicellulose all contribute to the recalcitrance of lignocellulosic biomass to hydrolysis, and collectively represent a technical barrier to commercial deployment of cellulosic biofuel [7][8][9][10][11][12][13]. Typically, lignocellulosic biomass is preprocessed ( Figure 1A) by steam explosion, solvent treatment, or acid or alkaline treatment to reduce its recalcitrance.…”

Cultura de células em suspensão como ferramenta para estudos em parede celular secundária em gramí­neas C4

“…A celulose é o biopolimero mais abundante na terra (FestucciBuselli et al, 2007), representando cerca de 15-20% da massa seca em parede celular primária e até 40% da parede celular secundária (Sticklen, 2008). As microfibrilas de celulose são formadas pelo "empacotamento" e cristalinização de dezenas de cadeias…”