Gibberellic-acid-induced reorientation of cortical microtubules in living plant cells

By microinjecting rhodamine-conjugated porcine tubulin into pea epidermis we recently showed how cortical microtubules reorientate from transverse to longitudinal in living cells (Yuan et nl,, 1994, Proc. Nat. Acad. Sci, USA, 91, 6050-6053). In the present paper we compare this reorientation with the contrary longitudinal to transverse realignment induced by adding gibberellic acid to preinjected cells on the microscope slide, Both kinds of reorientation are initiated by the appearance of 'discordant' microtubules which do not share the existing alignment but anticipate the new direction. These increase in number as the existing microtubules depolymerize, one alignment apparently replacing the other in a continuous process, By rotating stacks of confocal sections by computer methods we have previously shown that microtubules at the outer tangential cell wall do not necessarily have the same orientation as microtubules at the adjoining anticlinal walls of the same cell (Yuan et al., 1995. Plant J. 7, 17-23), This suggests that microtubule reorientations epidermal cells occur mainly (or, at least, first) at the outer wall, indicating that the array may not reorientate as a whole, Collectively, these data emphasize the discontinuous nature of the realignment process, the importance of new microtubule polymerization, and the special property of the outer epidermal surface as a sensitive domain.