Biomass digestibility is predominantly affected by three factors of wall polymer features distinctive in wheat accessions and rice mutants

Abstract

BackgroundWheat and rice are important food crops with enormous biomass residues for biofuels. However, lignocellulosic recalcitrance becomes a crucial factor on biomass process. Plant cell walls greatly determine biomass recalcitrance, thus it is essential to identify their key factors on lignocellulose saccharification. Despite it has been reported about cell wall factors on biomass digestions, little is known in wheat and rice. In this study, we analyzed nine typical pairs of wheat and rice samples that exhibited distinct cell wall compositions, and identified three major factors of wall polymer features that affected biomass digestibility.ResultsBased on cell wall compositions, ten wheat accessions and three rice mutants were classified into three distinct groups each with three typical pairs. In terms of group I that displayed single wall polymer alternations in wheat, we found that three wall polymer levels (cellulose, hemicelluloses and lignin) each had a negative effect on biomass digestibility at similar rates under pretreatments of NaOH and H2SO4 with three concentrations. However, analysis of six pairs of wheat and rice samples in groups II and III that each exhibited a similar cell wall composition, indicated that three wall polymer levels were not the major factors on biomass saccharification. Furthermore, in-depth detection of the wall polymer features distinctive in rice mutants, demonstrated that biomass digestibility was remarkably affected either negatively by cellulose crystallinity (CrI) of raw biomass materials, or positively by both Ara substitution degree of non-KOH-extractable hemicelluloses (reverse Xyl/Ara) and p-coumaryl alcohol relative proportion of KOH-extractable lignin (H/G). Correlation analysis indicated that Ara substitution degree and H/G ratio negatively affected cellulose crystallinity for high biomass enzymatic digestion. It was also suggested to determine whether Ara and H monomer have an interlinking with cellulose chains in the future.ConclusionsUsing nine typical pairs of wheat and rice samples having distinct cell wall compositions and wide biomass saccharification, Ara substitution degree and monolignin H proportion have been revealed to be the dominant factors positively determining biomass digestibility upon various chemical pretreatments. The results demonstrated the potential of genetic modification of plant cell walls for high biomass saccharification in bioenergy crops.

Three Fiber Crops Show Distinctive Biomass Saccharification under Physical and Chemical Pretreatments by Altered Wall Polymer Features

“…Notably, a similar increase in hexoses yields was also observed in the sequential dilute alkali (NaOH) pretreatments (Fig. 1B), which was different from previous observations that alkali pretreatments can lead to much higher hexoses yields than those of acid pretreatments with raw biomass in many plant species (Xu et al 2012;Wu et al 2013;Li et al 2014a;Jia et al 2014). As steam explosion is somewhat like the acid pretreatment, which could predominately remove hemicelluloses (Jørgensen et al 2007;Alvira et al 2010), the present findings may imply that the sequential dilute acid and alkali pretreatments could lead to a similar enhancement on biomass saccharification in the three fiber crops.…”

“…Ramie showed a relatively high cellulose level (41.5% dry matter) and low lignin (22.5%), whereas jute had relatively high hemicelluloses content (26.0%; Table A.1), and kenaf had low cellulose content (34.8%; Table A.1). However, all three fiber crops exhibited much higher cellulose levels than that reported for most grass plants (Xu et al 2012;Wu et al 2013;Li et al 2013;Jia et al 2014). Hence, the three fiber crops could provide cellulose-rich biomass in the core stalks.…”

“…Total lignin content was measured by the two-step acid hydrolysis method as described by Wu et al (2013). All experiments were carried out in triplicate.…”

“…The biomass enzymatic saccharification (or digestibility) was measured by calculating the hexoses yield (% cellulose) of enzymatic hydrolysis of biomass after pretreatment (Wu et al 2013). To test steam explosion enhancement on biomass saccharification, two-step pretreatments (steam explosion followed with acid/alkali) were performed in the three fiber crops.…”

“…Cellulose crystallinity and degree of polymerization (DP) have been characterized as negative factors affecting biomass digestibility (Chang and Holtzapple 2000;Klemm et al 2005;Pan et al 2008;Xu et al 2012;Zhang et al 2013). Furthermore, the hemicellulose content is negatively correlated with the degree of cellulose crystallinity for high biomass saccharification under H2SO4 and NaOH pretreatments in many plant species examined (Xu et al 2012;Zhang et al 2013;Wu et al 2013;Jia et al 2014). In particular, arabinose substitution degree of xylans (reverse Xyl/Ara) has been discovered as a positive factor relative to biomass digestibility upon various chemical pretreatments (Wu et al 2013;Li et al 2013).…”