Plant and Soil 226: 117–128, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
117
Relationship between microtubule orientation and patterns of cell
expansion in developing cortical cells of Lemna minor roots
S. Inada and S. Sato
1
Department of Biology and Earth Sciences, Faculty of Science, Ehime University, Bunkyo-cho 2-5, Matsuyama
790-8577, Japan.
1
Corresponding author
∗
Key words: cell wall, cortical cells, cortical microtubules, immunofluorescence,Lemna minor
Abstract
In actively growing cortical cells in the elongation zone of Lemna minor L. roots, both longitudinal (radial and
tangential) and transverse walls expand in both length and width. The longitudinal walls of the three types of
cortical cells in the root (i.e. outer, middle and inner) showed the largest expansion in the longitudinal axis. In
contrast, the inner cortical cells exhibited the least expansion in width, whereas the middle cortical cells displayed
the largest expansion in width. Thus, the profiles of the expansion of longitudinal walls were characteristic for
the three types of cortical cells. In this study, both the orientation of cortical microtubule (MT) arrays and their
dynamic reorientation, and the density of cortical MTs, were documented and correlated to the patterns of cell wall
expansion. Significantly, transverse arrays of cortical MTs were most prominent in the radial walls of the inner
corticalcells, andleastso inthoseof themiddlecorticalcells. Toward the baseof roots,beyondtheelongationzone,
the orientationof corticalMTsshiftedcontinuouslyfromtransverse toobliqueand thento longitudinal. Inthis case,
the rate of shift in the orientation of cortical MTs along the root axis was appreciably faster in the middle cortical
cells than in the other two types of cortical cells. Interestingly, the continuous change in cortical MT orientation
was not confirmed in the transverse walls which showed much smaller two-dimensional expansion than the radial
walls. Additionally, the presence of fragmentedor shortened cortical MTs rapidly increased concomitantly with the
decrease of transversely oriented cortical MTs. This relationship was especially prominent in the transverse walls
of the inner cortical cells, which displayed the least expansion among the three types of cortical cells investigated.
In the root elongation zone, the density of cortical MTs in the inner cortical cells was about three times higher
than that in the other two cortical cell types. These results indicate that in the early stage of cell expansion, the
orientation of cortical MTs determines a preferential direction of cell expansion and both the shifting orientation
and density of cortical MTs affect the magnitude of expansion in width of the cell wall.
Introduction
Evidence from numerous studies has demonstrated
that cortical MTs are involved in the control of plant
cell morphogenesis. In rapidly elongating roots of
Raphanus sativus, MTs in cortical cells of the elong-
ation zone are oriented perpendicular to the root axis,
whereas in non-elongatingcellsthese cortical MTs are
predominantly oblique (Traas et al., 1984). Similar
observations have also been reported in other plants
such as pea (Hogetsu and Oshima, 1986), Nitella
(Wasteneys and Williamson, 1987) and maize (Bal-
∗
FAX No: 899279630. E-mail: ssato@sci.ehime-u.ac.jp
uska et al., 1992). Furthermore, cell growth is isodia-
metric when corticalMTs are eitherrandomlyoriented
or disrupted with drugs (Hogetsu and Shibaoka, 1978;
Melan, 1990; Shibaoka, 1991). Collectively, these
data suggest that the orientation of cortical MTs plays
an important role in deciding the direction of cell
expansion because cell expansion is typically perpen-
dicular to the orientation of cortical MTs.
In roots of Azolla pinnata, the interpolation of
cortical MTs diminished or ceased in cortical cells
which were expanding in all dimensions (Hardham
andGunning, 1979).In contrast, thedensityofcortical
MTs increased rapidly in elongating sieve elements
but diminished when these cells ceased to elongate