Plants produce a wide range of polysaccharides in their cell walls, some of which are restricted to certain species. Roberts et al. investigated a moss gene encoding an enzyme that resembles a mixed-linkage (1,3;1,4)-β-glucan (MLG) synthase. When expressed in tobacco, this enzyme produces a novel unbranched, unsubstituted arabinoglucan (AGlc) polysaccharide that consists of both glucose (Glcp) and arabinose (Araf) residues. The similarity between this novel arabinoglucan synthase and MLG synthases, as well as phylogenetic analysis of related genes, suggests that “the capacities to synthesize AGlc and MLG have a common evolutionary history” that was lost in the angiosperm lineage, followed by a later, independent origin for MLG synthases in monocots. (Summary by Mary Williams) Plant Cell 10.1105/tpc.18.00082.