URI-led study finds two classes of enzymes ignore common rule of evolution

Made up of tiny threads known as cellulose microfibrils, plant cell walls are important for regulating a plant’s growth and protecting them from pests and pathogens. Previously only one route for producing these microfibrils was known: a class of enzymes called CESA. But research by University of Rhode Island Professor of Biological Sciences Alison Roberts and colleagues has revealed that plants also make cellulose microfibrils using a different class of enzymes called CSLD – an important discovery with potential implications for everything from textiles to renewable energy. 

In collaboration with colleagues at Rhode Island College, Dartmouth College, North Carolina State University, University of Cambridge and University of Warwick in the UK, and URI, Roberts recently published “An alternate route for cellulose microfibril biosynthesis in plants” in Science Advances. 

As the main component of paper, wood and many textiles, we all use cellulose every day, and expanding the knowledge base about how it is made has the potential to benefit these industries, Roberts explains. CSLD enzymes and the microfibrils they produce are also essential for pollen tube development and root hair growth in plants. Pollen tubes are necessary for reproduction, and root hairs absorb water and minerals from the soil–ultimately impacting a plant’s ability to deal with stress. 

“Understanding cellulose and cell walls in general is important for understanding how plants develop, and for being able to modify crops for environmental resilience, which will be important as our climate changes,” Roberts says. 

To investigate CSLD enzymes directly, researchers had to eliminate all CESA enzymes from a plant–a challenging task because most plants can’t survive without them. In the process of working with a moss affectionately known as “Physco,” a fortuitous discovery materialized: the plant did not need CESAs to survive. This enabled the team to study CSLDs on their own.

While it was already known that CSLD enzymes made some form of cellulose important for the growth of specialized cell types, it was widely assumed that the cellulose made by CSLDs would have a different structure from the cellulose microfibrils made by CESAs. The latter cluster together to form tiny spinneret-like structures, earning the name “rosettes” for their similar appearance to tiny flowers with six petals. The size and shape of rosettes determine the structure and properties of the cellulose microfibrils they make. After eliminating all CESAs from the Physco plant, another surprise emerged: they still had rosettes and made cellulose microfibrils that are the same size and shape as those made by CESAs. 

Discovering that CESA and CSLD enzymes are much more similar than previously thought is surprising because of how evolution typically works: when genes duplicate and become two genes that encode the same protein, one of them usually takes on another function or is weeded out by natural selection. In this case, two classes of enzymes that do very much the same thing have persisted for more than 500 million years. 

The researchers have uncovered a few hints about how the roles of CSLDs and CESAs may differ: “In a study we finished a year ago with collaborators from Dartmouth College, we found that CESA and CSLD enzymes differ in the way they move in the cell membrane and in how that movement is controlled,” Roberts adds. “So next we want to study how the different ways CESA and CSLD rosettes move enables development of different types of plant cells, including the pollen tubes and root hairs that use CSLD enzymes to make their cellulose.”

This project originally started with a URI Division of Research and Economic Development Bridge Grant and has since been supported by an NSF collaborative research grant with additional funding from the Center for Lignocellulose Structure and Formation, supported by the U.S. Department of Energy. 

Roberts notes that this research underscores the value of interdisciplinary collaboration and the evolving nature of scientific knowledge. “Collaborations are important for bringing in diverse experimental approaches but also for bringing in diverse ideas,” Roberts says – an important aspect of research that explores observations no one understands yet. 

“You have to imagine how something might work, and then you test that idea,” she says. “In this case, these two proteins are so much more alike than we thought, but how are they different? And people imagine that differently, and that results in designing different experiments–and a much higher likelihood that you’re going to find the answer.”