Cellulose—the primary building block of the walls of most plant cells—is synthesized at the plasma membrane by cellulose synthase complexes (CSCs). The ability to deliver CSCs to discrete sites at the cell surface is critical for cellulose synthesis.

Now,researchers at Penn State have identified the major steps in the process as well as the tools used by plant cells to create cellulose, including proteins that transport critical components to the location where cellulose is made. The findings, published in an open-access paper in the journal Proceedings of the National Academy of Sciences, could have important implications for biofuels.

Cellulose is the single most abundant biopolymer on earth. It makes up about 95 percent of paper and 90 percent of cotton, and its derivatives are even in the emulsifiers in ice cream. In the past ten years or so, cellulose has also been considered as a major component of biofuels. Understanding how cellulose is synthesized may allow us to optimize its use as a renewable energy source.

We knew that cellulose is synthesized in the plasma membrane that surrounds plant cells within a heteromeric protein complex—a grouping of different kinds of proteins—called the cellulose synthase complex, and that the main component of this complex is a unique cargo protein called cellulose synthase. But we didn’t know if other proteins are involved in the complex, or how the proteins get to the plasma membrane. To start answering these questions, we used a combination of approaches, including cell imaging, functional genetics, and proteomics, to create a timeline of events and to identify the main proteins involved in preparing the cell for synthesis.

—Ying Gu, associate professor of biochemistry and molecular biology at Penn State, senior author

The researchers showed that a protein called cellulose synthase interactive 1 (CSI1) interacts with the cellulose synthase complex prior to synthesis and may help mark the site at the plasma membrane where synthesis occurs. They also demonstrated that CSI1 interacts with a separate complex called the exocyst complex, which is involved in transporting materials to the plasma membrane in a variety of species, and a protein called PATROL1. These components may contribute to how quickly the cellulose synthase complex travels to the cell’s outer membrane before synthesis.

We knew that the exocyst complex is evolutionarily conserved, with essentially unchanged structure in yeast and mammals, and here we confirmed its role in plants. But PATROL1 is a plant-specific protein that is not like anything we see in mammals or yeast. We are puzzled by what PATROL1 actually does and are excited to continue to investigate its function.

—Ying Gu

Because CSI1 interacts with many components that are integral to cellulose synthesis, the research team plans to use it as a tool to further elucidate this important process and its evolution.

We eventually hope to translate what we know about how plant cells build cellulose to more efficiently break it apart again for use in biofuels, ultimately increasing the efficiency of biomass-based energy production.

—Ying Gu

In addition to Gu, the research team also includes Xiaoyu Zhu, Shundai Li, and Xiaoran Xin at Penn State and Songqin Pan at the University of California, Riverside. This work was funded by the National Science Foundation. Additional support was provided by the Institutes of Energy and the Environment and the Huck Institutes of the Life Sciences at Penn State.

Resources

• Xiaoyu Zhu, Shundai Li, Songqin Pan, Xiaoran Xin, Ying Gu (2018) “CSI1, PATROL1, and exocyst complex cooperate in delivery of cellulose synthase complexes to the plasma membrane” Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1800182115