Xyloglucan galactosylation is essential for proper cell wall assembly that facilitates stabilization of the actin cytoskeleton and the endomembrane system.

Xyloglucan, a major hemicellulose, interacts with cellulose and pectin to assemble primary cell walls in plants. Loss of the xyloglucan galactosyltransferase MURUS3 (MUR3) leads to the deficiency of galactosylated xyloglucan and perturbs plant growth. However, it is unclear whether defects in xyloglucan galactosylation influence the synthesis of other wall polysaccharides, cell wall integrity, cytoskeleton behavior and endomembrane homeostasis. We found that mur3-7 etiolated seedlings have reduced cellulose and that CESA genes are down-regulated, the density and mobility of cellulose synthase complexes (CSCs) are reduced, and cellulose microfibrils become discontinuous. Pectin, RGII, and boron contents are reduced in mur3-7 plants, and B-RGII cross-linking is abnormal. Wall porosity and thickness are significantly increased in mur3-7 seedlings. The endomembrane aggregation is also apparent in mur3-7 mutant. Furthermore, mutant seedlings and their actin filaments are more sensitive to LatA treatment. However, all defects in mur3-7 mutants can be substantially restored by exogenous boric acid application. Our study reveals the importance of MUR3-mediated xyloglucan galactosylation for wall structural assembly and cell wall homeostasis, which is required to stabilize the actin cytoskeleton and the endomembrane system.