Pectins are abundant in the cell walls of dicotyledonous plants, but how they interact with other wall polymers and influence wall integrity and cell growth has remained mysterious. Here, we verified that QUASIMODO2 (QUA2) is a pectin methyltransferase and determined that QUA2 is required for normal pectin biosynthesis. To gain further insight into how pectin affects wall assembly and integrity maintenance, we investigated cellulose biosynthesis, cellulose organization, cortical microtubules, and wall integrity signaling in two mutant alleles of Arabidopsis (Arabidopsis thaliana) QUA2, qua2 and tsd2. In both mutants, crystalline cellulose content is reduced, cellulose synthase particles move more slowly, and cellulose organization is aberrant. NMR analysis shows higher mobility of cellulose and matrix polysaccharides in the mutants. Microtubules in mutant hypocotyls have aberrant organization and depolymerize more readily upon treatment with oryzalin or external force. The expression of genes related to wall integrity, wall biosynthesis, and microtubule stability is dysregulated in both mutants. These data provide insights into how homogalacturonan is methylesterified upon its synthesis, the mechanisms by which pectin functionally interacts with cellulose, and how these interactions are translated into intracellular regulation to maintain the structural integrity of the cell wall during plant growth and development.

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.