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

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.