It has been known that the transverse orientation of cortical microtubules (MTs) along the elongation axis is essential for normal cell morphogenesis, but whether cortical MTs are essential for normal cell wall synthesis is still not clear. In the present study, we have investigated whether cortical MTs affect cell wall synthesis by direct alteration of the cortical MT organization in Arabidopsis thaliana. Disruption of the cortical MT organization by expression of an excess amount of green fluorescent protein-tagged α-tubulin 6 (GFP-TUA6)in transgenic Arabidopsis plants was found to cause a marked reduction in cell wall thickness and a decrease in the cell wall sugars glucose and xylose. Concomitantly, the stem strength of the GFP-TUA6overexpressors was markedly reduced compared with the wild type. In addition, expression of excess GFPTUA6 results in an alteration in cell morphogenesis and a severe effect on plant growth and development.Together, these results suggest that the proper organization of cortical MTs is essential for the normal synthesis of plant cell walls.