Genetics and bioenergy potential of forage maize: deconstructing the cell wall

Despite gaining prominence in scientific spheres and political agendas worldwide, the production of biofuels from plant biomass is yet to achieve an economic stronghold in the renewable-energy sector. Plant lignocellulose has evolved to resist chemical and enzymatic deconstruction, and its conversion into liquid fuels requires energetically stringent processes that currently render the industry economically and environmentally unviable. To address this challenge, experts have envisioned the development of advanced bioenergy crops which require lower energetic and chemical inputs for their effective fractionation. At its core, this approach requires an in-depth understanding of the composition, synthesis and breeding amenability of the plant cell wall; the principal constituent of total plant dry biomass and the most recalcitrant fraction of the crop at physiological maturity to deconstruction. To this end, the primary aim of this thesis was to dissect and elucidate the biochemical and genetic factors controlling cell wall characteristics relevant to the development of bioenergy grasses with improved processing quality for cellulosic based fuel production. A focus on maize was warranted as it currently represents the de facto model system for bioenergy crop research; offering an unrivalled platform to underpin the complex genetic architecture of cell wall biosynthesis, develop advanced bioenergy-crop breeding strategies and translate cell wall research into innovations and commercial products. This thesis exposed that the biomass-to-fuel conversion of crops is a highly complex trait dependent on both, the balance and synergy between multiple cell wall components, and the inherent effectiveness of the conversion technology. Concerning the production of cellulosic ethanol via the combined operations of dilute-acid pretreatment and enzymatic saccharification, our results revealed that the chemical mechanisms affecting biomass conversion efficiency depend on pretreatment severity. Whereas at harsh pretreatments biomass conversion efficiency was primarily influenced by the inherent efficacy of thermochemical cell wall deconstruction, at milder pretreatments, maximum fermentable glucose release was observed for maize genotypes exhibiting systematic cell wall changes leading to higher ruminal cell wall digestibility. These results confirmed that the selection and use of cellulosic feedstocks that best match the processing conditions used in the industry can aid in reaching industrial goals aimed at improving the commercial and environmental performance of cellulosic fuels. In turn, the exhaustive characterization of a forage maize doubled haploid (DH) population demonstrated the vast degree of genetic diversity in maize cell wall composition and bioconversion potential amenable to breeding. Principally, these findings suggest that natural diversity in the biochemical composition of the maize cell wall and its physical properties is primarily ascribed to variation in the balance, monomeric make-up, and extent of cross-linking of non-cellulosic cell wall polymers (i.e. lignin and hemicellulose). Indeed, correlation analyses confirmed that the extent of enzymatic depolymerization of maize biomass was strongly and negatively associated to the concentration of cell wall phenolics, but positively impacted by the degree of glucuronoarabinoxylan (GAX) glycosylation and extent of hemicellulose-to-hemicellulose cross-linking. Our results also showed that natural variation in cell wall content and composition is quantitatively inherited and putatively ascribed to the segregation of multiple genetic loci with minor additive effects. In our population, genotypic diversity for cell wall composition and quality was found to be controlled by 52 quantitative trait loci (QTLs). From eight QTLs regulating bioconversion properties, five were previously unidentified and warrant further investigation. Despite the apparent complexity of cell wall genetics, however, the high heritability and environmentally stability of cell wall compositional and degradability properties guarantee high selection efficacy during the development of superior DH/inbred material, and predispose that multi-environment testing will only be necessary at advanced stages of bioenergy-maize breeding programs. Moreover, because genetic variation for complex cell wall characteristics appears to be predominantly additive, preliminary selection at the inbred level will expectedly lead to successful hybrid selection; thereby minimizing the need for recurrent test-crossing procedures and evaluations. In this regard, maize cell wall bioconversion efficiency constitutes an excellent selection criterion for immediate application in modern maize breeding programs. Ultimately, the convergence of classical selection schemes with inexpensive genotyping, advanced biometric models, high-throughput cell wall phenotyping and doubled haploid (DH) production technologies can accelerate development and commercial release of maize cultivars for bioenergy applications. To play a determinant role in the development and realization of sustainable and cost-effective cellulosic fuel processing technologies, however, novel dual-purpose maize cultivars (i.e. delivering both, grain for feed or food and fiber materials for bioconversion) will have to surpass the performance in lignocellulose processing quality and biomass yields of the best elite germplasm. These prospects seem realistic as the parallel advance of grain yield and stover productivity and quality characteristics is a feasible undertaking. Conceptually, the advance of superior bioenergy cultivars (surpassing the performance of modern elite material) would allow us to make the currently available biomass-to-fuel conversion systems more cost-effective and sustainable, and may also have favorable consequences for the ideal size and geographical distribution of biofuel refineries.