The role of COBRA-LIKE 2 function, as part of the complex network of interacting pathways regulating Arabidopsis seed mucilage polysaccharide matrix organization

The production of hydrophilic mucilage along the course of seed coat epidermal cell differentiation is a common adaptation in angiosperms. Previous studies have identified COBRA-LIKE 2 (COBL2), a member of the COBRA-LIKE gene family, as a novel component required for crystalline cellulose deposition in seed coat epidermal cells. In recent years, Arabidopsis seed coat epidermal cells (SCEs), also called mucilage secretory cells, have emerged as a powerful model system for the study of plant cell wall components biosynthesis, secretion, assembly and de muro modification. Despite accumulating data, the molecular mechanism of COBL function remains largely unknown. In the current research, we utilized genetic interactions to study the role of COBL2 as part of the protein network required for seed mucilage production. Using correlative phenotyping of structural and biochemical characteristics, unique features of the cobl2 extruded mucilage are revealed, including: ‘unraveled’ ray morphology, loss of primary cell wall ‘pyramidal’ organization, reduced Ruthenium red staining intensity of the adherent mucilage layer, and increased levels of the monosaccharides arabinose and galactose. Examination of the cobl2cesa5 double mutant provides insight into the interface between COBL function and cellulose deposition. Additionally, genetic interactions between cobl2 and fei1fei2 as well as between each of these mutants to mucilage-modified 2 (mum2) suggest that COBL2 functions independently of the FEI-SOS pathway. Altogether, the presented data place COBL2 within the complex protein network required for cell wall deposition in the context of seed mucilage and introduce new methodology expending the seed mucilage phenotyping toolbox.