Orientation of microtubules against transverse cell walls in roots ofAzolla pinnata R. Br.

Abstract

Orientation of cortical microtubules against transverse cell walls was examined inAzolla root primordia at different stages of development. All transverse walls in the zone of formative divisions, where the rate of increase in girth of the root is maximal and contributed to by all cell layers, show circumferential orientation of their associated microtubules with respect to the shape of the root as a whole. In the zone of proliferative divisions, expansion of inner cortex cells remains predominantly radial and they retain the same microtubule orientation. However, in the endodermal cells which bound the stele and alter very little in radial dimension, circumferential orientation gives way to radial orientation. In none of these zones or cell types is the orientation of cortical microtubules against transverse walls “random”. Individual cells in the root can deploy their microtubules in specific, and different, orientations on different cell faces. The hypothesis that, inAzolla, nucleation of microtubules occurs along selected cell edges receives further support from the observations.