The cellulose microfibrils in the cell walls of charophycean green algae, bryophytes (for example mosses), and vascular plants are synthesized by rosette cellulose synthesis complexes (CSCs) composed of at least 18 cellulose synthase proteins (CESAs), which are responsible for polymerization of the constituent β-1,4-glucan chains. The CESAs of vascular plants appear to function in vivo as hetero-oligomers composed of three isoforms in a 1:1:1 stoichiometry. Based on genetic evidence, the isoforms that form a complex are not interchangeable, and each is thought to occupy distinct positions defined by its interfaces with other components of the complex. However, phylogenetic analysis suggests that the common ancestor of bryophytes and vascular plants had rosette CSCs and a single CESA gene. Furthermore, genetic evidence indicates that cellulose required for development of the leafy shoots of the moss Physcomitrella patens may be synthesized by homo-oligomeric rosette CSCs. This implies that modification of the interfaces between CESA proteins occurred subsequent to the evolution of the rosette CSC morphology. Evolutionary mechanisms that can explain functional differentiation of CESA proteins within rosette CSCs will be presented, along with their implications for the nature of CESA-CESA interactions. A collaborative effort to compare divergent CESA proteins, informed by structural modeling of selected CESA isoforms, has revealed structural features that potentially define isoform-specific function. To evaluate hypotheses arising through this approach, a complementation assay was developed in P. patens for testing the effects of engineered mutations on CESA function. The results of these assays are beginning to shed light on which structural features define the functional differences between CESA isoforms, and potentially their interfaces with other components of the CSCs.

Evolutionary perspectives on functional differentiation of CESA proteins

253rd National Meeting (2017)

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