Evolution of cellulose synthesis complexes in plants: Cellulose synthase-like D rosettes in the charophyte green alga <i>Coleochaete</i>

The cellulose microfibrils of land plants are synthesized by two families of enzymes, Cellulose Synthase (CESA) and CESA-Like-D (CSLD). Both form rosette Cellulose Synthesis Complexes (CSCs), which appear as hexagonal groups of six particles with freeze fracture transmission electron microscopy. Throughout the green plant lineage, CSC morphology is correlated with cellulose microfibril structure and properties. Charophyte green algae (CGA) have CESAs, CSLDs, and/or CESA/CSLD-like enzymes from which the other two families likely evolved, and most have rosette CSCs. The apparent exception is Coleochaete, previously reported to have unique octagonal CSCs. Using a specimen preparation method that promotes periclinal fractures, we show that Coleochaete has typical six-particle rosette CSCs consistent with expression of CSLD genes, thus resolving a long-standing anomaly. Coleochaetophyceae also express CESA/CSLD-like genes, but no CESAs were detected in 27 Coleochaete and Chaetosphaeridium transcriptomes. The phylogenetic distribution and exon-intron structure of CESAs, CSLDs, and their CESA/CSLD-like common ancestors are consistent with an evolutionary history that included losses and gains of genes and introns, with implications for the divergence and functional specialization of the CESA and CSLD families. Together, these findings help clarify the structural basis of cellulose microfibril synthesis in CGA and the evolution of cellulose synthases in plants.