Interactions of Arabinoxylan and (1,3)(1,4)-β-Glucan with Cellulose Networks Deirdre Mikkelsen, † Bernadine M. Flanagan, † Sarah M. Wilson, ‡ Antony Bacic, ‡ and Michael J. Gidley* ,† † The University of Queensland, ARC Centre of Excellence in Plant Cell Walls, Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, Brisbane, Queensland 4072, Australia ‡ The University of Melbourne, ARC Centre of Excellence in Plant Cell Walls, School of Botany and Bio21 Molecular Science and Biotechnology Institute, Melbourne, Victoria 3010, Australia *S Supporting Information ABSTRACT: To identify interactions of relevance to the structure and properties of the primary cell walls of cereals and grasses, we used arabinoxylan and (1,3)(1,4)-β-glucan, major polymers in cereal/grass primary cell walls, to construct composites with cellulose produced by Gluconacetobacter xylinus. Both polymers associated prolifically with cellulose without becoming rigid or altering the nature or extent of cellulose crystallinity. Mechanical properties were modestly affected compared with xyloglucan or pectin (characteristic components of nongrass primary cell walls) composites with cellulose. In situ depletion of arabinoxylan arabinose side chains within preformed cellulose composites resulted in phase separation, with only limited enhancement of xylan−cellulose interactions. These results suggest that arabinoxylan and (1 → 3)(1 → 4)-β-D-glucan are not functional homologues for either xyloglucan or pectin in the way they interact with cellulose networks. Association of cell-wall polymers with cellulose driven by entropic amelioration of high energy cellulose/water interfaces should be considered as a third type of interaction within cellulose-based cell walls, in addition to molecular binding (enthalpic driving force) exhibited by, for example, xyloglucans or mannans, and interpenetrating networks based on, for example, pectins. ■ INTRODUCTION Cell walls provide the structural framework of plants, playing a critical role in their growth and development. Furthermore, they are an integral part of the human diet and a major source of renewal biomass. Therefore, structural features of plant cell wall (PCW) polymers have been the subject of research for decades and are now largely defined. 1−6 Recently, research has intensified to identify genes responsible for the synthesis and assembly of individual PCW polymers. 5−8 Despite these significant advances, there is still limited understanding of how individual polymers come together to form the PCW matrix and what the functional consequences of different matrix compositions and architectures are on cell-wall material properties. The assembly of PCWs has been modeled using an in vitro construction approach with the cellulose-producing bacterium Gluconacetobacter xylinus (formerly Acetobacter xylinus). Ga. xylinus produces extracellular cellulose via transmembrane synthesis and an extrusion process; 9 similar to that which occurs in plants. 10,11 Incorporating certain PCW polymers in the fermentation growth medium results in stable cellulose composites being produced through spontaneous self-assembly processes. 12−14 The relatively homogeneous nature of these composites enables material testing studies to be performed under a range of imposed stresses, 15,16 both before and after the action of PCW proteins (expansins, xyloglucan endotransgly- cosylases, xyloglucanases) hypothesized to have a mechanical effect on native PCWs. 17,18 Previous work studied composites of cellulose with xyloglucans (XG) or pectins, the major polysaccharides of the primary cell walls (“Type I” walls) of most dicots and nongraminaceous monocots but minor polysaccharides in commelinoid monocots. However, the primary cell walls (“Type II” walls) of cereals/grasses (Poaceae) and related commelinoid monocots differ in their polysaccharide compo- sition, with cellulose being the only major conserved feature between the two PCW types. 2,19−21 Instead, heteroxylans decorated with various amounts of arabinose (AXs) or glucuronic acid (GAXs/GXs) are the major noncellulosic components, while xyloglucan and pectin levels are low. 1,2,20,21 In addition, (1 → 3)(1 → 4)-β-D-glucans (mixed linkage glucans or MLG) are deposited in certain tissues and at particular stages of development. Various studies have hypothesized that heteroxylans or MLGs are able to form Received: January 6, 2015 Revised: March 6, 2015 Published: March 10, 2015 Article pubs.acs.org/Biomac © 2015 American Chemical Society 1232 DOI: 10.1021/acs.biomac.5b00009 Biomacromolecules 2015, 16, 1232−1239