RECOGNITION OF A PATHOGEN AND A NONPATHOGEN BY BARLEY COLEOPTILE CELLS .3. RESPONSES OF MICROTUBULES AND ACTIN-FILAMENTS IN BARLEY COLEOPTILE CELLS TO PENETRATION ATTEMPTS

New arrangements of microtubules and actin filaments in coleoptile cells of barley that had been inoculated with either a nonpathogen, Erysiphe pisi, or a pathogen, E. graminis, were observed by cytochemistry and confocal laser scanning microscopy. In uninoculated coleoptile cells, microtubules were oriented almost obliquely or transversely to the long axis of the cells and actin filaments almost obliquely or longitudinally. A thick actin bundle was located beneath approximately 70% of appressoria of E. pisi when the appressoria matured 3-4 h before they attempted penetration. This phenomenon occurred below approximately 30% of appressoria of E. graminis. Microtubules were gathered beneath the appressoria when and after the inoculated fungi induced cytoplasmic aggregation. This phenomenon also occurred more frequently below appressoria of E. pisi than those of E. graminis. Confocal laser scanning microscopy confirmed the localization of microtubules and actin filaments in a cortical region of the coleoptile cell beneath the appressorium. The time-course study revealed that the new arrangement of actin filaments was initiated 3-4 h prior to the fungal penetration attempt, whereas that of microtubules began at the time of initiation of cytoplasmic aggregation. The incidence of cells with newly arranged cytoskeletons was distinctly higher when E. pisi rather than E. graminis was used as inoculum. The possibilities that actin filaments might be involved in sensing the presence of the fungi and that both microtubules and actin filaments might be involved in localized resistance mechanisms are discussed.