A paper describing the research appeared in the October issue of the journal The plant cell.

Cellulose, a structural component of plant cell walls, is an abundant and promising source of biofuels. However, common techniques for extracting cellulose from cell walls, which involve removing other large entangled molecules called polymers, require chemical solvents, enzymes and high-temperature reactions that add cost and complexity to the process. Improving understanding of how cell walls are built could illuminate new, more cost-effective ways to extract cellulose, according to the researchers.

“In recent years, researchers have explored a variety of ways to potentially improve the efficiency of the cellulose extraction process, for example by manipulating other cell wall polymers that can get in the way, such as xylan and lignin,” said Sarah Pfaff . postdoctoral researcher at Penn State Eberly College of Science, who led the research. “But the unique structures formed by the cells of the ‘xylem tracheal element’ often fail to develop properly in these mutant plants, causing the cells to collapse and ultimately reducing plant growth and the amount of extractable cellulose. In this study, we explore how these unique cell walls are assembled in healthy plant cells, and also how this process goes awry in mutants.”

Xylem tracheal elements (XTE) are a type of cell that allow water to move from a plant’s roots to leaves and have remarkably thick cell walls. Unlike other cells, Pfaff said, polymers such as cellulose, xylan and lignin are deposited in specific places in XTE’s cell walls, creating a band pattern. When these patterns are not formed properly in the mutant cells, the cells can collapse due to the pressure of water moving against gravity.

“The band patterns in the tracheal elements of the xylem act much like the wavy pattern in cardboard, adding stability to the cell wall,” Pfaff said. “Using traditional methods, it was difficult to see individual cells to understand how this banding pattern breaks down in mutant cells. So we developed a method that allows us to observe individual cells without any of the neighboring cells getting in the way.”

The new method takes advantage of protoplasts, individual cells that have had their cell walls removed, which the researchers provide nutrients and what Pfaff calls a “genetic trigger” to differentiate into a new type of cell. Although protoplasts have been used in a variety of previous plant studies, the new method allows researchers to observe the cells as they differentiate into the unique XTE cell type.

“We give the protoplasts a transcription factor—a kind of genetic trigger—so that they develop into a new type of cell based on that cue,” Pfaff said. “It’s a bit like stem cells in that we can reprogram their developmental fate and watch them turn into completely different cell types. In this study, we specifically induced protoplasts from both healthy and mutant plants to transform into xylem tracheal elements and observed how the bands. patterns formed in their cell walls’.

The researchers discovered that certain interactions between cellulose and xylan are necessary for the bands to form correctly and that a properly assembled polymer network of the cell wall acts as a scaffold to dictate the banding pattern. They also found that in different mutant cells, the banding pattern failed in different ways.

“Previous research focused on how the inside of the cell might affect the cell wall, which is synthesized outside the cell, but we found it works the other way,” Pfaff said. “The structure of the cell wall can also have an impact on what happens inside the cell, and they can feed into each other. This work provides important insights into how cell walls are made and how these types of mutants might be viable in the future.”

According to Pfaff, understanding how cell walls are built is of interest to forestry, materials science, as well as biofuel production. The research team plans to use their new method to explore how other types of cell walls are made.

“Instead of breeding mutant plants together to get several different genetic traits in one plant, which could take many months, you can now explore different combinations in individual cells,” Pfaff said. “You could also use different types of genetic triggers to study other cell types, which could have implications for plant biology.”

In addition to Pfaff, the Penn State research team includes Edward Wagner, senior research technician, and Daniel Cosgrove, Eberly Family Chair in Biology. Penn State’s Center for Lignocellulosic Structure and Formation, an Energy Frontier Research Center funded by the US Department of Energy, and the Human Frontier Science Program supported this research.