Research

The Roberts lab studies the evolution of the proteins and biochemical mechanisms responsible for synthesis of cellulose microfibrils, the major component of plant cell walls. In addition to its role in plant development and function, cellulose is a major carbon sink and has value as a component of wood and textile fibers, and potentially as a carbon-neutral energy source. However, the same properties that give cellulose its strength and resilience also hinder its conversion to biofuels. Our work is aimed at understanding how cellulose microfibril structure is regulated through the biosynthetic process, with the goal of informing efforts to modify the properties of cellulose produced by plant species used for lumber, paper, textiles, and biofuel feedstocks. Cellulose microfibril structure is controlled at the point of synthesis by membrane-embedded ‘rosettes’ complexes composed of cellulose synthase (CESA) enzymes. By investigating the interactions among the CESAs, we aim to understand the evolution and assembly of rosettes, and in turn the determinants of cellulose microfibril structure. Our work in this field is distinctive in its use of the model moss species Physcomitrium patens, which has unique advantages due to its reduced morphological complexity and ease of culture and genetic manipulation.

Ongoing projects:

The Center for Lignocellulose Structure and Formation (CLSF) is a multi-institutional Department of Energy-funded Energy Frontiers Research Center focused on developing a detailed understanding of lignocellulose, the complex composite of polysaccharides, proteins and phenolics that forms plant cell walls and is the most abundant biomass on earth. Our contributions to this effort include developing and employing an assay for testing engineered modifications of CESA enzymes and analysis of rosette complexes by freeze-fracture transmission electron microscopy. In 2019 our team of eight collaborators was awarded the Ten at Ten Scientific Ideas Team Award as part of the Energy Frontiers Research Centers program tenth anniversary celebration.

Reducing complexity in vivo enables investigation of Cellulose Synthase-like D complex formation, trafficking and function is an NSF funded project with collaborator Magdalena Bezanilla (Dartmouth College). The rosettes that make most plant cellulose contain 18 CESA enzymes and the 18 glucan chains they produce bundle together to form a strong, durable microfibril. A related enzyme, Cellulose Synthase-Like D (CSLD), also produces glucan chains but is only active during specific cellular growth processes. Although it is not known whether CSLDs associate to form complexes, they appear to produce cellulose that is structurally distinct from the cellulose produced by CESA proteins. Attempts to study the cellulose produced by CSLDs have been hampered by the abundance of cellulose produced by CESAs, which are essential for survival of most plants. We have engineered moss lines that lack CESAs and rely solely on CSLDs to make cellulose, enabling us to study CSLDs and the structure and properties of the cellulose they produce.

Arabinoglucan—phylogenetic distribution, biosynthesis and function of a novel mixed-linkage cell wall polysaccharide. During my 2014-2015 sabbatical, I identified a novel arabinoglucan (AGlc) cell wall polysaccharide and the enzyme that synthesizes it in moss. AGlc is structurally similar to mixed-linkage glucan (MLG), an important dietary fiber known to occur in the cell walls of cereals and other grasses, some seedless vascular plants, some fungi and a few species of green algae. The enzymes that synthesize AGlc and MLG are especially interesting because they catalyze the formation of both 1,4-β- and 1,3-β-linkages and have a complex evolutionary history. The methods we have developed, including a sensitive assay for AGlc in cell walls and a heterologous expression system for testing AGlc synthase activity, will enable us to survey a broad sampling of algae and non-seed plants for the presence of cell wall AGlc and to functionally characterize putative AGlc synthase proteins. Because the assays can also detect MLG and MLG synthase activity, this approach could clarify the early evolutionary history of MLG synthesis. Comparative structural analysis of AGlc and MLG synthases could contribute to efforts to understand the catalytic mechanism for synthesis of mixed-linkage polysaccharides by a single enzyme.