The herbicide dichlobenil disrupts cell plate formation: Immunogold characterization

We have utilized light and transmission electron microscopy and immunocytochemistry to examine onion roots treated with the herbicide dichlobenil (2,6-dichlorobenzonitrile; DCB), a purported disrupter of cellulose biosynthesis. The most salient effect of DCB is observed on cell plate formation, the process that gives rise to new cell walls. In the presence of DCB, cell plates develop normally up to the tubular network stage. They are the result of fusion of Golgi-derived vesicles and the accumulation of callose and the first strands of cellulose. The DCB-treated cell plates retain the reticulate and malleable nature of the tubular network/early fenestrated plate stage of cell plate formation, but fail to display signs of the stiffening and straightening associated with an accumulation of cellulose. Instead, the malleable cell plates in the DCB-treated cells retain a wavy architecture, accumulate pockets of electron opaque material, and produce plasmodesmata in abnormal orientations. Immunocytochemical investigations of the abnormal cell plates formed after DCB treatment show 20-fold increase in the level of callose labelling found in the control cell plates. Xyloglucans and rhamnogalacturonans can be detected in the partially-formed cell plates, with the labelling density of xyloglucan 4-5 times greater than in the control cell plates and that of the rhamnogalacturonans being similar to the controls. These data support the hypothesis that DCB inhibits cellulose biosynthesis as a primary mechanism of action, and that in the absence of cellulose synthesis the cell plates fail to mature and to give rise to new cross walls.