Abstract text: Bioengineering the production of complex carbohydrates with desired functions remains challenging to the complexity of most plant cell walls and genetic redundancy which limits our functional understanding of the underlying parts. The liverwort Marchantia polymorpha, a bryophyte, provides a unique system to understand how hemicelluloses are made and function during plant development. The Marchantia genome contains a single CELLULOSE SYNTHASE-LIKE A (CSLA) gene for mannan elongation, simplifying functional analysis compared to most land plants. For comparison, Arabidopsis thaliana encodes nine CSLA paralogs. Using Agrobacterium-mediated transformation and CRISPR/Cas9 editing, we generated several csla mutant alleles that are essentially devoid of mannan. Despite the loss of this abundant polymer, representing at least 10% of Marchantia biomass, the csla mutants completed a full life cycle with a minimal developmental penalty. However, they exhibited reduced thallus expansion under osmotic stress, accompanied by disordered epidermal cell organization. Proteomic analysis revealed upregulation of a host of uncharacterized players that may compensate for cell surface defects, such as dirigent-like and Late Embryogenesis Abundant (LEA) proteins. Therefore, mannans may be dispensable under ideal growth conditions but likely maintain structural integrity under stress.