Ultrastructural effects of cellulose biosynthesis inhibitor herbicides on developing cotton fibers

Cotton fibers are often utilized as a model system to investigate cellulose biosynthesis and cell wall elongation. In this study we grew cotton fibers in vitro, with ovules dissected at day zero post anthesis as the explant s ource, in the presence of three herbicides that inhibit cellulose biosynthe sis. Cultures were sampled for electron microscopy and immunocytochemistry 1-2 days after beginning the treatments. After dichlobenil treatment, the f ibers were much shorter than the controls and assumed a variety of abnormal shapes, from shortened versions of the control fiber to nearly spherical. The inner layers of the fiber wall. often contained juxtaposed electron-tra nslucent and -transparent areas: this layer reacted strongly with antibodie s to callose. Cellulase-gold labeling in these newly developed fibers grown in the presence of dichlobenil was present at only about 3% of the control labeling. After treatment with either isoxaben or flupoxam, the fibers ass umed spherical shapes and frequently (more than 60% of fibers) exhibited a new cell plate within the fiber, indicating that cell division had occurred , a process that rarely occurred in the controls. Unlike the dichlobenil-tr eated fibers, fibers grown in the presence of isoxaben or flupoxam containe d an extensive accumulation of chiefly deesterified pectins, replacing the entire wall with an elaborated version of the pectin sheath found in contro l cotton fibers These data indicate that all three herbicides are effective disrupters of cellulose biosynthesis and cause radical changes in cell wal l structure and composition. Moreover, these data indicate that the composi tion of the walls may influence indirectly cell cycle kinetics, keeping the se fiber cells in a more meristematic mode.