Tassel removal positively affects biomass production coupled with significantly increasing stem digestibility in switchgrass

FRAGILE CULM 18 encodes a UDP-glucuronic acid decarboxylase required for xylan biosynthesis and plant growth in rice

Although UDP-glucuronic acid decarboxylases (UXSs) have been well studied with regard to catalysing the conversion of UDP-glucuronic acid into UDP-xylose, their biological roles in grasses remain largely unknown. The rice (Oryza sativa) genome contains six UXSs, but none of them has been genetically characterized. Here, we reported on the characterization of a novel rice fragile culm mutant, fc18, which exhibited brittleness with altered cell wall and pleiotropic defects in growth. Map-based cloning and transgenic analyses revealed that the FC18 gene encodes a cytosol-localized OsUXS3 and is widely expressed with higher expression in xylan-rich tissues. Monosaccharide analysis showed that the xylose level was decreased in fc18, and cell wall fraction determinations confirmed that the xylan content in fc18 was lower, suggesting that UDP-xylose from FC18 participates in xylan biosynthesis. Moreover, the fc18 mutant displayed defective cellulose properties, which led to an enhancement in biomass saccharification. Furthermore, expression of genes involved in sugar metabolism and phytohormone signal transduction was largely altered in fc18. Consistent with this, the fc18 mutant exhibited significantly reduced free auxin (indole-3-acetic acid) content and lower expression levels of PIN family genes compared with wild type. Our work reveals the physiological roles of FC18/UXS3 in xylan biosynthesis, cellulose deposition, and plant growth in rice.

A grass-specific cellulose-xylan interaction dominates in sorghum secondary cell walls

Sorghum (Sorghum bicolor L. Moench) is a promising source of lignocellulosic biomass for the production of renewable fuels and chemicals, as well as for forage. Understanding secondary cell wall architecture is key to understanding recalcitrance i.e. identifying features which prevent the efficient conversion of complex biomass to simple carbon units. Here, we use multi-dimensional magic angle spinning solid-state NMR to characterize the sorghum secondary cell wall. We show that xylan is mainly in a three-fold screw conformation due to dense arabinosyl substitutions, with close proximity to cellulose. We also show that sorghum secondary cell walls present a high ratio of amorphous to crystalline cellulose as compared to dicots. We propose a model of sorghum cell wall architecture which is dominated by interactions between three-fold screw xylan and amorphous cellulose. This work will aid the design of low-recalcitrance biomass crops, a requirement for a sustainable bioeconomy.

Sorghum is a source of lignocellulosic biomass for the production of renewable fuels. Here the authors characterise the sorghum secondary cell wall using multi-dimensional magic angle spinning solid-state NMR and present a model dominated by interactions between three-fold screw xylan and amorphous cellulose.

Hemicellulose characterization of deuterated switchgrass

This work describes the structural characterization of hemicellulose isolated from hydroponically grown switchgrass in H₂O medium (protiated) or 50% D₂O medium (deuterated) through compositional analysis, GPC, FTIR, ¹³C and ¹H/¹³C HSQC NMR. 4-O-methyl glucuronoarabinoxylan (GAX), the major hemicellulose in switchgrass isolated from deuterated switchgrass, had structural properties similar to hemicellulose isolated from protiated switchgrass. Both had comparable arabinose to xylose ratio (0.25) and molecular weight (47-50 kDa). Structural similarities show that deuterated switchgrass hemicellulose can be used as a model carbohydrate polymer in neutron scattering, or pharmaceutical studies due to their immunomodulatory activity and gastroprotective effects.

Extended light exposure increases stem digestibility and biomass production of switchgrass

Switchgrass is a photoperiod-sensitive energy grass suitable for growing in the marginal lands of China. We explored the effects of extended photoperiods of low-irradiance light (7 μmol·m-2·s-1, no effective photosynthesis) on the growth, the biomass dry weight, the biomass allocation, and, especially, the stem digestibility and cell wall characteristics of switchgrass. Two extended photoperiods (i.e., 18 and 24 h) were applied over Alamo. Extended light exposure (18 and 24 h) resulted in delayed heading and higher dry weights of vegetative organs (by 32.87 and 35.94%, respectively) at the expense of reducing the amount of sexual organs (by 40.05 and 50.87%, respectively). Compared to the control group (i.e., natural photoperiod), the yield of hexoses (% dry matter) in the stems after a direct enzymatic hydrolysis (DEH) treatment significantly increased (by 44.02 and 46.10%) for those groups irradiated during 18 and 24 h, respectively. Moreover, the yield of hexoses obtained via enzymatic hydrolysis increased after both basic (1% NaOH) and acid (1% H2SO4) pretreatments for the groups irradiated during 18 and 24 h. Additionally, low-irradiance light extension (LILE) significantly increased the content of non-structural carbohydrates (NSCs) while notably reducing the lignin content and the syringyl to guaiacyl (S/G) ratio. These structural changes were in part responsible for the observed improved stem digestibility. Remarkably, LILE significantly decreased the cellulose crystallinity index (CrI) of switchgrass by significantly increasing both the arabinose substitution degree in xylan and the content of ammonium oxalate-extractable uronic acids, both favoring cellulose digestibility. Despite this LILE technology is not applied to the cultivation of switchgrass on a large scale yet, we believe that the present work is important in that it reveals important relationships between extended day length irradiations and biomass production and quality. Additionally, this study paves the way for improving biomass production and digestibility via genetic modification of day length sensitive transcription factors or key structural genes in switchgrass leaves.