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Fall 2019

Windows on Walls

Looking into the growth of plant cell walls to enhance biomass conversion

Sally Jiao

Plant cell walls are a potentially vast source of renewable fuel. Biomass is projected to have the capacity to produce approximately 700 billion kilowatt-hours of electricity annually by 2030 according to an article by the Union of Concerned Scientists. Unfortunately, neither we nor our bodies have figured out how to efficiently convert plant material to useful energy on a large scale, resulting in 35 million tons of biomass wasted every year from households alone, according to the same article.

Despite the fact that we have been eating (or pretending to eat) vegetables since the first gatherer, we still lack a fundamental understanding of how all the components of a cell wall are arranged and how they work together. This means that, for us, converting biomass into fuels is like trying to reassemble Legos blindfolded. Scientists at the Center for Lignocellulose Structure and Function (CLSF), an Energy Frontier Research Center, are helping us remove that blindfold.

“Our mission as an EFRC is to develop a nano- to meso-scale understanding of cellulosic cell walls…forming the foundation for new technologies in sustainable energy and novel biomaterials,” said Yunzhen Zheng, a research technologist in CLSF.

The complementary experimental and computational techniques of their multidisciplinary team allow them to access scales from the microscopic to the macromolecular to construct a complete picture of the cell wall. Recently published projects from CLSF have used spectroscopy and microscopy to elucidate the factors that influence cell wall stability and growth, so that we can better understand the switches we can toggle to affect that stability and growth. A more in-depth knowledge of these factors will help develop processes to efficiently break down biomass by converting cell wall material into useful chemical products. “This knowledge will form the scientific foundation for designing rational pretreatment methods to deconstruct cell walls,” said Pyae Phyo, a graduate student at the CLSF.

Understanding how acidic conditions affect cell wall components

A model of cell wall components. Image adapted from Phyo et al. Cellulose (2019)

Researchers at CLSF use solid-state Nuclear Magnetic Resonance (ssNMR) to probe cell wall components. An atomic nucleus has a specific response to a given magnetic field depending on its chemical identity—whether it is hydrogen or carbon—and its chemical environment. Experiments can detect these responses, and therefore deduce the chemical environments of the nuclei in the molecule of interest, which provide information about the system’s composition, structure, and dynamics. Under the umbrella of NMR fall many specific techniques that CLSF researchers have used to understand how an acidic environment affects cell growth.

While previous studies on cell wall growth in acidic conditions focused on how these conditions activate enzyme proteins that loosen the cell wall, Phyo, the graduate student who led the study, focused on the pre-enzymatic effect of acidification on cell wall structure in order to understand its connection with cell wall growth. Phyo studied the cell walls of Arabidopsis thaliana, a distant relative of watercress, in acid using several ssNMR techniques, each of which gave her a piece of the puzzle. For instance, one ssNMR technique allowed Phyo to determine the types of molecules that are linked together to form the long chains in the cell walls, while another provided information about how those molecules are connected to each other.

“I can walk through the whole sugar ring and connect all the dots…which carbons are connected to which other carbons,” said Phyo. By comparing the spectra of plants under acidic conditions to those under neutral conditions, Phyo could directly assess the impact of acidification on plant cell wall structure in the absence of protein activity. Putting all the puzzle pieces together, Phyo found that exposing a cell wall to acid weakens the interactions between cellulose, the stiff strings that give the cell wall structural stability, and pectin, a substance that surrounds the cellulose fibrils. The weakened interaction prepares the cell wall for further modification by enzymes to allow for cell wall loosening and growth. This finding suggests that the strength of cellulose-pectin interactions plays a role in cell wall loosening, contributing to our overall understanding of and ability to manipulate cell wall structure and growth.

Elucidating the behavior of a specific component of cell walls

FESEM image of the cell wall. The strings are cellulose microfibrils and the bright spots are nanogold particles tethered to CBM76, which binds to xyloglucan. Image from Zheng et al. The Plant Journal (2018)

The cell wall, however, has many other components that CLSF is also working to understand, such as xyloglucan, a molecule hypothesized to link the cellulose strings or “microfibrils” together. Because of this suggested linking activity, previous works have suggested that it plays a major role in cell wall stability. Studying xyloglucan’s interactions with cellulose uniquely requires field emission scanning electron microscopy (FESEM) because FESEM is able to image the cellulose microfibrils at the nanoscale. To obtain images that clearly distinguish the precise locations of the xyloglucan and cellulose, researchers aided by Zheng attached nanoparticles made of gold to proteins called CBM76 and CBM3, which bind to xyloglucan and cellulose respectively, and then exposed onion cell walls to these proteins before imaging.

In the resulting FESEM images, bright spots from the gold nanoparticles show where the xyloglucan and cellulose are. Changes in these images before and after exposure to enzymes that break up xyloglucan informed Zheng about the structural impact of xyloglucan on cellulose microfibrils. From these images, Zheng determined that there were more visible gold nanoparticles attached to cellulose-binding CBM3 after exposure to the enzyme that breaks up xyloglucan, suggesting that the intact xyloglucan prevented CBM3 from binding. Thus, xyloglucan binds to cellulose.

However, even before treatment with the enzyme, xyloglucan-binding CBM76 did not show extensive binding in between cellulose microfibrils, suggesting that xyloglucan in fact does not tether together cellulose microfibrils. Knowing this can help researchers design processes that help break down the network of cellulose microfibrils that make up cell walls.

In addition to these findings related to xyloglucans, this work has also brought a new technique to the scientific community’s attention: “Now we know it is feasible to study cell wall structure using nanogold affinity tags assessed by FESEM, which is groundbreaking,” said Zheng.

Windows on walls

Yesterday, you probably ate a plant’s cell wall. Today, thanks to CLSF, we are beginning to understand its structure and mechanisms at a microscopic level. Tomorrow, we may finally begin to digest it to produce electricity.

More Information

Phyo P, Y Gu, and M Hong. 2019. “Impact of acidic pH on plant cell wall polysaccharide structure and dynamics: insights into the mechanism of acid growth in plants from solid-state NMR.” Cellulose 26:291. DOI: 10.1007/s10570-018-2094-7

Zheng Y, X Wang, Y Chen, E Wagner, and DJ Cosgrove. 2018. “Xyloglucan in the primary cell wall: assessment by FESEM, selective enzyme digestions and nanogold affinity tags.The Plant Journal 93:211. DOI: 10.1111/tpj.13778

Union of Concerned Scientists. 2012. “The Promise of Biomass: Clean Power and Fuel--If Handled Right.”

Acknowledgments

Phyo et al. This research was supported by the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0001090.

Zheng et al. This work was supported as part of the Center for LignoCellulose Structure and Formation, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0001090. We thank Prof. Harry Gilbert, Newcastle University, UK, for the CBM76 clone, Prof. Enrique Gomez (Penn State University, USA) for discussions of polymer physics, and Laura Ullrich, Liza Wilson, Sarah Kiemle, Greg Ning, and Trevor Clark (all at Penn State University) for technical support. We thank Dr. Kirk Schnorr (Novozymes) for XGase. The authors declare no conflict of interest.

About the author(s):

  • Sally Jiao is a graduate student in chemical engineering at the University of California, Santa Barbara advised by M. Scott Shell. She is part of the Center for Materials for Water and Energy Systems (M-WET) Energy Frontier Research Center. She works on controlling the properties and interactions of hydration water and solutes through surface patterning and functionalization.

More Information

Phyo P, Y Gu, and M Hong. 2019. “Impact of acidic pH on plant cell wall polysaccharide structure and dynamics: insights into the mechanism of acid growth in plants from solid-state NMR.” Cellulose 26:291. DOI: 10.1007/s10570-018-2094-7

Zheng Y, X Wang, Y Chen, E Wagner, and DJ Cosgrove. 2018. “Xyloglucan in the primary cell wall: assessment by FESEM, selective enzyme digestions and nanogold affinity tags.The Plant Journal 93:211. DOI: 10.1111/tpj.13778

Union of Concerned Scientists. 2012. “The Promise of Biomass: Clean Power and Fuel--If Handled Right.”

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.