Identification of Key Enzymes for Pectin Synthesis in Seed Mucilage

Pectin is a vital component of the plant cell wall and provides the molecular glue that maintains cell-cell adhesion, among other functions. As the most complex wall polysaccharide, pectin is composed of several covalently linked domains, such as homogalacturonan (HG) and rhamnogalacturonan I (RG I). Pectin has widespread uses in the food industry and has emerging biomedical applications, but its synthesis remains poorly understood. For instance, the enzymes that catalyze RG I elongation remain unknown. Recently, a coexpression- and sequence-based MUCILAGE-RELATED (MUCI) reverse genetic screen uncovered hemicellulose biosynthetic enzymes in the Arabidopsis (Arabidopsis thaliana) seed coat. Here, we use an extension of this strategy to identify MUCI70 as the founding member of a glycosyltransferase family essential for the accumulation of seed mucilage, a gelatinous wall rich in unbranched RG I. Detailed biochemical and histological characterization of two muci70 mutants and two galacturonosyltransferase11 (gaut11) mutants identified MUCI70 and GAUT11 as required for two distinct RG I domains in seed mucilage. We demonstrate that, unlike MUCI70, GAUT11 catalyzes HG elongation in vitro and, thus, likely is required for the synthesis of an HG region important for RG I elongation. Analysis of a muci70 gaut11 double mutant confirmed that MUCI70 and GAUT11 are indispensable for the production and release of the bulk of mucilage RG I and for shaping the surface morphology of seeds. In addition, we uncover relationships between pectin and hemicelluloses and show that xylan is essential for the elongation of at least one RG I domain.