Abstract
Nucleotide-activated sugars are essential substrates for plant cell-wall carbohydrate-polymer biosynthesis. The most prevalent grass cell wall (CW) sugars are glucose (Glc), xylose (Xyl), and arabinose (Ara). These sugars are biosynthetically related via the UDP–sugar interconversion pathway. We sought to target and generate UDP–sugar interconversion pathway transgenic Brachypodium distachyon lines resulting in CW carbohydrate composition changes with improved digestibility and normal plant stature. Both RNAi-mediated gene-suppression and constitutive gene-expression approaches were performed. CWs from 336 T0 transgenic plants with normal appearance were screened for complete carbohydrate composition. RNAi mutants of BdRGP1, a UDP-arabinopyranose mutase, resulted in large alterations in CW carbohydrate composition with significant decreases in CW Ara content but with minimal change in plant stature. Five independent RNAi-RGP1 T1 plant lines were used for in-depth analysis of plant CWs. Real-time PCR analysis indicated that gene expression levels for BdRGP1, BdRGP2, and BdRGP3 were reduced in RNAi-RGP1 plants to 15–20% of controls. CW Ara content was reduced by 23–51% of control levels. No alterations in CW Xyl and Glc content were observed. Corresponding decreases in CW ferulic acid (FA) and ferulic acid-dimers (FA-dimers) were observed. Additionally, CW p-coumarates (pCA) were decreased. We demonstrate the CW pCA decrease corresponds to Ara-coupled pCA. Xylanase-mediated digestibility of RNAi-RGP1 Brachypodium CWs resulted in a near twofold increase of released total carbohydrate. However, cellulolytic hydrolysis of CW material was inhibited in leaves of RNAi-RGP1 mutants. Our results indicate that targeted manipulation of UDP–sugar biosynthesis can result in biomass with substantially altered compositions and highlights the complex effect CW composition has on digestibility.
Highlights
The plant cell wall (CW) is a complex mixture of carbohydrates, aromaticcompounds, and protein that is critical for plant form and function whilst comprising the largest source of renewable biomass on earth (Pauly and Keegstra, 2008)
RNA-mediated interference (RNAi)-mutants exhibit increased xylan digestibility and decreased cellulose digestibility while not affecting plant stature. These data support the efficacy of a selection-scheme where mutant plants of near wildtype stature are screened for CW composition phenotypes. These data highlight the complexity of grass CW composition and ways to manipulate that complexity to improve digestibility
Putative candidate protein sequences were obtained and sequence alignments and phylogenetic analysis were performed for UDP-α-D-Glc dehydrogenase (UGD) (Supplementary Figure S1), UDP-α-D-glucuronic acid (GlcA) decarboxylase (UXS) (Supplementary Figures S2 and S3), UDP-α-D-Xyl epimerase (UXE) (Supplementary Figure S4), reversibly glycosylated protein (RGP)/UDP-β-L-Arap mutase (UAM) (Supplementary Figure S5), and apiose/UDPα-D-Xyl synthase (AXS) (Supplementary Figure S6)
Summary
The plant CW is a complex mixture of carbohydrates, aromaticcompounds, and protein that is critical for plant form and function whilst comprising the largest source of renewable biomass on earth (Pauly and Keegstra, 2008). This lignocellulosic biomass material is used for food, feed, fiber, and energy-inputs. Approaches to improve biomass quality, use-efficiency, and yields are needed. The feasibility and efficacy of the proposed approaches may be plant species-dependent
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
