GIBBERELLIC-ACID-INDUCED REORIENTATION OF CORTICAL MICROTUBULES IN LIVING PLANT-CELLS

By microinjecting rhodamine-conjugated porcine tubulin into pea epider mis we recently showed how cortical microtubules reorientate from tran sverse to longitudinal in living cells (Yuan et nl,, 1994, Proc. Nat. Acad. Sci, USA, 91, 6050-6053). In the present paper we compare this r eorientation with the contrary longitudinal to transverse realignment induced by adding gibberellic acid to preinjected cells on the microsc ope slide, Both kinds of reorientation are initiated by the appearance of 'discordant' microtubules which do not share the existing alignmen t but anticipate the new direction. These increase in number as the ex isting microtubules depolymerize, one alignment apparently replacing t he other in a continuous process, By rotating stacks of confocal secti ons by computer methods we have previously shown that microtubules at the outer tangential cell wall do not necessarily have the same orient ation as microtubules at the adjoining anticlinal walls of the same ce ll (Yuan et al., 1995. Plant J. 7, 17-23), This suggests that microtub ule 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 polymerizat ion, and the special property of the outer epidermal surface as a sens itive domain.