Bioethanol from maize cell walls: Genes, molecular tools, and breeding prospects

In the last decade, cellulosic ethanol has caught the growing interest of governments and private investors worldwide as it brings the promise of responsible renewable-energy and an opportunity to depart from an oil-reliant economy. Alongside advances in bioprocessing technologies, the development of specialized bioenergy crops is seen as a pressing industrial necessity, and while C4 perennials (e.g., Miscanthus, switchgrass, and sugarcane) have been coined the most promising candidates for the production of lignocellulosic biomass, maize should not be overlooked. In this review, we have addressed the benefits of advancing maize as a second-generation bioenergy feedstock. We have also analyzed current knowledge on the maize cell wall and promising genetic strategies for its modification, given that lignocellulose recalcitrance represents the most crucial breeding target in bioenergy crop research programs. In addition to lignin, a focus on the underlying genetic basis of cellulose, hemicellulose, and ferulate cross-linking patterns, as well as their regulation, has been warranted. A comprehensive overview of the state-of-art of genomic and phenotyping strategies available for bioenergy crop research is also provided. Overall, maize represents an outstanding model organism for understanding complex cell wall characteristics and defining the path for breeders looking to improve this and other promising bioenergy grasses. With an extensive array of dedicated agronomic and genomic resources at hand, we believe that breeding maize with improved processing amenability is a likely prospect but would like to remind readers that advances in high-biomass yielding properties, improved agronomic hardiness, and enhanced processing efficiency will also be necessary.