FIGURE 1.

COBRA is localized at the plasma membrane, is sensitive to isoxaben, and when mutated disrupts cellulose synthesis. A, FlAsH-COBRA and GFP-CESA3 were visualized in the hypocotyl elongation zone of 5-day-old dark grown Arabidopsis seedlings. FlAsH-COBRA showed a punctate pattern of labeling in the plasma membrane focal plane. B, plasma membrane localization was verified by plasmolysis with 0.8 m NaCl, which causes shrinkage and condensation of the cytoplasm and vacuole, and detachment of the plasma membrane from the cell wall (arrowhead). C, treatment of seedlings with 100 nm isoxaben caused clearance of GFP-CESA3 particles from the plasma membrane resulting in intracellular accumulation. The same observation was made for FlAsH-COBRA treated with isoxaben. D, time-lapse confocal images of YFP-CESA6 in hypocotyl cells of 5-day-old etiolated Col-0 (left) and cob-6 (right) plants were compared. Each image is an average of 36 frames taken at 5-s intervals (n = 9). CESA complex velocities were measured using Imaris. Data passed normality and equal distribution tests after natural log transformation. The complex velocity in cob-6 (289 ± 162 nm/min) was found to be significantly different from that of wild type (225 ± 147 nm/min) (t test, p < 0.01). Aside from the differences in the velocity rate, in cob-6, cellulose synthesis occurs in a relatively linear direction. Scale bars are 5 μm in A, B, and D and 10 μm in C.