Stem cell-like approach in plants sheds light on specialized cell wall formation

An article 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 to extract cellulose from cell walls, which involve removing other entangled large molecules called polymers, require chemical solvents, enzymes and reactions at high temperatures, adding to the cost and complexity of the process. Improving the understanding of how cell walls are built could yield new, more cost-efficient ways to extract cellulose, according to the researchers.

“In recent years, researchers have explored various ways to potentially improve the efficiency of the cellulose extraction process, for example by manipulating other polymers in the cell wall that can get in the way, such as xylan and lignin,” says Sarah Pfaff. postdoctoral researcher at Penn State Eberly College of Science who led the study. “But the unique structures formed by ‘xylem tracheal element’ cells often do not 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 investigate how these unique cell walls are assembled in healthy plant cells and also how this process goes wrong in mutants.”

Xylem tracheary elements (XTEs) are a type of cell that allows water to flow from a plant’s roots to its leaves and have remarkably thick cell walls. Unlike in other cells, Pfaff says, polymers such as cellulose, xylan and lignin are deposited at specific locations in the cell walls of XTEs, creating a banding pattern. When these patterns are not formed properly in mutant cells, the cells can collapse from the pressure of moving water against gravity.

“The banding patterns in xylem tracheal elements are very similar to the wave pattern in cardboard, adding stability to the cell wall,” Pfaff said. “Using traditional methods, it has been 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 uses protoplasts, individual cells stripped of their cell walls, which provide the researchers with nutrients and what Pfaff calls a “genetic trigger” to differentiate into a new type of cell. Although protoplasts have been used in several previous plant studies, the new method allows the researchers to observe the cells as they differentiate into the unique XTE cell type.

“We provide protoplasts with a transcription factor — a kind of genetic trigger — so that they develop into a new cell type based on that signal,” 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 turn into xylem tracheal elements and observed how banding formed patterns 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 well-assembled cell wall network of polymers acts as a platform to dictate the band pattern. They also found that in different mutant cells the band pattern failed in different ways.

“Previous research has focused on how the inside of the cell might influence the cell wall, which is synthesized outside the cell, but we found that it also works in the other direction,” Pfaff said. “Cell wall structure can also influence what happens inside the cell, and they can interact with each other. This work provides important insights into how cell walls form and how these types of mutants could be viable in the future.”

According to Pfaff, understanding how cell walls are built is important in forestry, materials science and biofuel production. The research team plans to use their new method to investigate how other types of cell walls form.

“Instead of growing mutant plants together to get several different genetic traits in one plant, which can take many months, you can now investigate 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, Penn State’s research team includes Edward Wagner, senior research technician, and Daniel Cosgrove, Eberly Family Chair of Biology. The Center for Lignocellulosic Structure and Formation at Penn State, an Energy Frontier Research Center funded by the U.S. Department of Energy, and the Human Frontier Science Program supported this research.