MICROTUBULE ORIENTATION AND DYNAMICS IN ELONGATING CHARACEAN INTERNODAL CELLS FOLLOWING CYTOSOLIC ACIDIFICATION, INDUCTION OF PH BANDS, OR PREMATURE GROWTH ARREST

Cortical microtubules (MTs) at indifferent zones in immature Nitella i nternodes were investigated by injection of fluorescently tagged sheep brain tubulin into living cells and by immunofluorescence on fixed ma terial. Nearly identical MT patterns and numbers were detected with th e two techniques, indicating that sheep brain tubulin incorporated int o all cortical MTs. MTs were aligned transversely to the long axis of the cell and approximately one MT was present every micrometer of long itudinal cell distance. Treatment of internodes with propionic acid to acidify cytosolic pH caused depolymerization of MTs and an increase i n the unpolymerized tubulin pool. Transfer of young, vigorously elonga ting cells to media inducing premature growth cessation resulted in a slight decrease in microtubule numbers but did not significantly alter microtubule orientation patterns or microtubule lifespans. MTs remain ed transverse for days following growth cessation before finally assum ing a more random alignment characteristic of mature, non-growing inte rnodes. No differences in MT numbers, orientation, or dynamics were de tected between acid and alkaline bands in internodes incubated in a ba nd-inducing medium. Thus, properties of cortical MT arrays were not cl osely coupled to growth status or to regional differences in cellular physiology associated with pH banding.