Xyloglucan in the primary cell wall: assessment by FESEM, selective enzyme digestions and nanogold affinity tags. (22nd December 2017)

• Record Type: Journal Article
• Authors: Zheng, Yunzhen; Wang, Xuan; Chen, Yuning; Wagner, Edward; Cosgrove, Daniel J.
• Is Part Of: Plant journal. Volume 93:Number 2(2018)
• Page Start: 211
• Page End: 226
• Publication Date: 2017-12-22
• Subjects: backscattered electron detection -- carbohydrate‐binding module -- CBM3 -- CBM76 -- cell wall structure -- cellulose microfibril organization -- field emission scanning electron microscopy -- immunogold labelling -- nanogold -- xyloglucan; Plant molecular biology -- Periodicals; Plant cells and tissues -- Periodicals; Botany -- Periodicals; 580
• DOI: 10.1111/tpj.13778
• Languages: English
• ISSNs: 0960-7412

Abstract

Summary: Xyloglucan has been hypothesized to bind extensively to cellulose microfibril surfaces and to tether microfibrils into a load‐bearing network, thereby playing a central role in wall mechanics and growth, but this view is challenged by newer results. Here we combined high‐resolution imaging by field emission scanning electron microscopy (FESEM) with nanogold affinity tags and selective endoglucanase treatments to assess the spatial location and conformation of xyloglucan in onion cell walls. FESEM imaging of xyloglucanase‐digested cell walls revealed an altered microfibril organization but did not yield clear evidence of xyloglucan conformations. Backscattered electron detection provided excellent detection of nanogold affinity tags in the context of wall fibrillar organization. Labelling with xyloglucan‐specific CBM76 conjugated with nanogold showed that xyloglucans were associated with fibril surfaces in both extended and coiled conformations, but tethered configurations were not observed. Labelling with nanogold‐conjugated CBM3, which binds the hydrophobic surface of crystalline cellulose, was infrequent until the wall was predigested with xyloglucanase, whereupon microfibril labelling was extensive. When tamarind xyloglucan was allowed to bind to xyloglucan‐depleted onion walls, CBM76 labelling gave positive evidence for xyloglucans in both extended and coiled conformations, yet xyloglucan chains were not directly visible by FESEM. These results indicate that an appreciable, but still small, surface of cellulose microfibrils in the onion wall is tightly bound with extended xyloglucan chains and that some of the xyloglucan has a coiled conformation. Significance Statement: The nm‐scale location and conformation of xyloglucan, as well as its interaction with cellulose, are important for the architecture, growth and mechanics of primary cell walls, yet are not well established by a consensus of experimental results. From high‐resolution FESEM imaging of onion epidermal walls, in combination with xyloglucanase treatments and nanogold affinity tags, we conclude that xyloglucan assumes both random‐coil conformations and an extended conformation that binds to the hydrophobic surfaces of cellulose microfibrils, altering cellulose aggregation.