Loss of UBIQUITIN-PROTEIN LIGASE 3 (UPL3) impairs cellulose-xylan structural organization in Arabidopsis seed mucilage.

Arabidopsis thaliana seeds release large capsules of mucilaginous polysaccharides, whose adherence depends not only cellulose and hemicellulose biosynthesis but also their interactions. However, the key proteins regulating the connection within this polysaccharide network in the seed coat mucilage remain unclear and require further investigation. Through reverse genetics, we demonstrate that UBIQUITIN-PROTEIN LIGASE 3 (UPL3), a nucleus-localized, HECT E3 ligase protein, is required for the proper organization of cellulosic rays in seed coat mucilage. Disruption of UPL3 results in an enlarged seed mucilage halo due to elongated, diffusely stained cellulose structures, while not affecting the biosynthesis, structure, and chemical composition of pectic polymers. Furthermore, the distribution, rather than the biosynthesis, of xylans is significantly affected in upl3 seeds. Genetic analysis revealed that upl3 partially suppresses the mucilage defects of cellulose synthase 5 (cesa5) and completely rescues those of irregular xylem 7 (irx7), but not mucilage modified 5 (mum5) and irx14. Our findings suggest that UPL3 plays a regulatory role in xylan organization (and possibly in xylan synthesis in the irx7 background), and is essential for maintaining the proper co-organization of cellulose and xylans, which is likely important for their functional interplay in mucilage architecture.