INTERPHASE AND PREPROPHASE MICROTUBULE ORGANIZATION IN SOME POLARIZED CELL-TYPES OF THE LIVERWORT MARCHANTIA-PALEACEA BERT

The organization of interphase and preprophase-prophase microtubules was studied in photosynthetic filament mother cells (PFMCs), photosynthetic filament cells (PFCs), mucilage papilla mother cells (MPMCs) and mucilage papillae (MP) of Marchantia paleacea Bert. These cell types exhibit polarized growth resulting, (a) in the formation of cell outgrowths followed by asymmetrical division (PFMCs and MPMCs), or (b) in the development of a tubular cell shape (PFCs and MP). The above cell types display an interphase cortical microtubule ring perpendicular to the axis of growth. In PFMCs and MPMCs it resides at the base of the cell outgrowth, while in PFCs and MP it has a median location. This microtubule ring is involved in the deposition of transverse, circumferentially aligned, cellulose microfibrils, defines the site where cell protrusion formation occurs and affects cell morphogenesis. In differentiated MP microtubules are rearranged in longitudinal or oblique cortical arrays. PFCs and MP also contain a prominent system of endoplasmic microtubules which may be involved in cytoplasmic polarization. In PFMCs, PFCs and MPMCs the interphase microtubule ring seems to function as a preprophase microtubule band (PPB). In the asymmetrically dividing PFMCs and MPMCs the PPB is complete but in symmetrically dividing PFCs it may be interrupted. The cell plate meets the parent wall at sites adjacent to the PPB cortical zone. These observations reveal further peculiarities in cortical microtubule organization and particularly in the PPBs of M. paleacea. Possible factors underlying the formation of complete or interrupted PPBs are discussed.