Resonant soft X-ray scattering reveals cellulose microfibril spacing in plant primary cell walls

Dan Ye,Sarah N Kiemle,Xuan Wang,Enrique D Gomez,Cheng Wang,Sintu Rongpipi,Esther W Gomez,Daniel J Cosgrove

doi:10.1038/s41598-018-31024-1

Dan Ye, Sarah N Kiemle + Show 6 more

Open Access

https://doi.org/10.1038/s41598-018-31024-1

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Abstract

Cellulose microfibrils are crucial for many of the remarkable mechanical properties of primary cell walls. Nevertheless, many structural features of cellulose microfibril organization in cell walls are not yet fully described. Microscopy techniques provide direct visualization of cell wall organization, and quantification of some aspects of wall microstructure is possible through image processing. Complementary to microscopy techniques, scattering yields structural information in reciprocal space over large sample areas. Using the onion epidermal wall as a model system, we introduce resonant soft X-ray scattering (RSoXS) to directly quantify the average interfibril spacing. Tuning the X-ray energy to the calcium L-edge enhances the contrast between cellulose and pectin due to the localization of calcium ions to homogalacturonan in the pectin matrix. As a consequence, RSoXS profiles reveal an average center-to-center distance between cellulose microfibrils or microfibril bundles of about 20 nm.

Highlights

Primary cell walls in plants are constructed of stiff cellulose microfibrils that are 3 to 5 nm wide and several microns long,[1,2] and that are embedded in a soft, hydrated matrix of pectin and hemicellulose[3,4,5]
scanning electron microscopy (SEM) relies on differences in electron backscattering to visualize the topology, whereas transmission electron microscopy (TEM) relies on rotary shadowing of a replica; both EM techniques benefit from partial removal of matrix polymers to enhance microfibril visualization
The presence of calcium associated with carboxylic acids in homogalacturonan, the predominant pectic polysaccharide, provides an opportunity to generate contrast based on differences in elemental composition using analytical TEM techniques

Summary

Introduction

Primary cell walls in plants are constructed of stiff cellulose microfibrils that are 3 to 5 nm wide and several microns long,[1,2] and that are embedded in a soft, hydrated matrix of pectin and hemicellulose[3,4,5]. The physical connections of microfibrils with adjacent microfibrils and with the matrix are considered to be important determinants of primary cell wall mechanics and the ability for growth (i.e., extensibility)[14,15,16,17], yet some aspects of the complex spatial organization of cellulose microfibrils remain challenging to characterize. Previous work has demonstrated variation of calcium in plant cell walls using electron energy-loss spectroscopy (EELS),[26,27] the resolution was limited such that microfibrils are not apparent. In principle, software such as SOAX28 and Ridge Detection[29] can identify fibrils in acquired images and enable quantitative estimates of microfibril organization. Other factors can affect this ratio and it is not yet feasible to extract values for the spacing from SFG spectra

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