Dynamics of cortical microtubules in developing maize internodes

The transverse arrangement of cortical microtubules (MTs) in the intercalary meristem of an elongating grass internode has been considered a major determinant of its marked cell elongation. We examined how the transverse orientation of cortical MTs is established and maintained during grass internode development. The developmental sequence of the arrangement of cortical MTs in the maize internode, from its initiation to maturity, was studied using fluorescence microscopy. Undifferentiated cells in a shoot apex had randomly arranged cortical MTs. Internodal cells originated from a portion of these cells, with reorientation of cortical MTs from random to transverse. This transverse orientation remained unchanged throughout the proliferation of the internodal cells. Thereafter, the intercalary meristem was formed as the residuum of these cells. By contrast, the arrangement of cortical MTs in nodal cells remained random. These results demonstrate that the transverse orientation of cortical MTs in the intercalary meristem originates from the reorientation of cortical MTs during the initiation of the internodal cells. This reorientation of MTs might be a manifestation of the establishment of a cell fate in these cells.