is considered as a promising bioenergy crop. However, plant cell wall recalcitrance determines its cost-effective conversion into biofuels. Considering that the genetic modification of plant cell walls is proposed to reduce recalcitrance, the crucial factor in wall polymers that affects biomass digestibility needs to be identified [25
]. Due to the complicated structures and diverse functions of plant cell walls [25
], the effects of the three major wall polymers (cellulose, hemicelluloses, and lignin) on biomass digestion were initially compared. Hence, this study focused on the analysis of six pairs of Miscanthus
samples that had different cell wall compositions.
Studies on the effects of the three major wall polymers, particularly cellulose and hemicelluloses, on lignocellulose digestibility in plants are very limited. The effective hydrolysis of hemicelluloses due to the soluble and extractable properties of these polymers has been described [28
]. In the current work, total hemicelluloses level was demonstrated to be the positive and dominant factor that affected the high biomass saccharification efficiency due to its negative effect on lignocellulose crystallinity (CrI). Hemicelluloses are proposed to deposit into cell walls via crosslink to cellulose by hydrogen bonds [14
]. Thus, hemicelluloses can reduce cellulose crystallization and the negative effect of the hemicelluloses level may not depend on the cellulose as well as lignin levels (Table ).
Cellulose crystallinity (CrI) reportedly affects biomass digestibility negatively because the reduction of cellulose CrI may result in efficient cellulase enzyme penetration and high affinity to cellulose substrate [30
]. However, little is known about the impact of the three wall polymers on cellulose crystallinity. Apart from the abovementioned negative hemicelluloses effect, the cellulose and lignin levels were found to be positive factors (Table ). The positive effect of cellulose levels may be due to the relatively low hemicelluloses proportion or smaller non-crystalline cellulose region. With respect to the positive effect of lignin, lignin was assumed to interact with hemicelluloses rather than with cellulose, which may indirectly reduce hemicelluloses cross-linking to cellulose. This assumption also confirmed that increased cellulose and lignin levels can lead to increased cellulose CrI at higher rates as observed in the pair VI samples (Table ). Although the S/G ratio in lignin is recently reported to be a dual factor that affects biomass digestibility [20
], the mechanism remains unknown. In the present study, Miscanthus
samples with high S/G ratios were found to have relatively higher cellulose CrI values, which suggested that S monomer may have a different interlinking with wall polymers.
Acid and alkali chemicals such as H2
and NaOH are extensively used in biomass pretreatments. However, the two chemicals are found to have different mechanisms for biomass depolymerization [33
]. Alkali pretreatment can mostly cause the dissociation of entire wall polymers by breaking hydrogen and other covalent bonds, whereas acid pretreatment induces the partial release of monosaccharides, oligosaccharides, and lignin monomers by splitting strong chemical bonds under high temperature [34
]. Hence, smaller residues remained on the cellulose surface after 1% H2
pretreatment than after 1% NaOH pretreatment (Figure ), leading to a relatively lower biomass saccharification rate after 1% H2
pretreatment (Additional file 1
: Table S1). This result also indicated that increasing the cellulose or lignin levels of pairs II and III can result in much lower hexose yields after H2
pretreatments than after NaOH (Figure ). In other words, the result confirmed that increasing the hemicellulose level of pair I or decreasing the cellulose and lignin levels of pair VI (Table ) can lead to increased biomass digestibility at similar rates (1.3-times to 1.5-times for pair I and 1.8-times to 2.2-times for pair IV) between H2
and NaOH pretreatments (Additional file 1
: Table S1).
Plant cell wall mutants have been used to account for wall polymers and biomass digestibility. Generally, most mutants show growth-defective and biomass-reduced phenotypes [37
], these mutants can not be directly used as energy crops for biofuel purposes. In particular, multiple alterations in cell wall compositions and structures render some mutants not even worthy of consideration as experimental materials. Based on a rich natural germplasm resource, we selected six pairs of Miscanthus
samples with different cell wall compositions. These samples were able to demonstrate the effect of each wall polymer on biomass saccharification, and can thus be used as genetic materials for energy crop breeding. Thus, this study provides a fundamental strategy for the genetic engineering of plant cell walls toward bioenergy crop selection.