Microtubule arrays in maize root cells. Interplay between the cytoskeleton, nuclear organization and post‐mitotic cellular growth patterns

summary Results from studies with metabolic inhibitors suggest that, in growing cells of the maize root apex, nuclear metabolism is involved in the organization of the cortical microtubular arrays (CMTs) located at the cell periphery. Nucleus‐derived factors intrinsic to each individual cell are inferred as being responsible for the accomplishment of the MT‐dependent transition from the approximately isodiametric mode of cellular growth, characteristic of the immediate post‐mitotic root region, to the strictly directional cell lengthening in the elongation zone. These hypothetical factors seem to be related to RNA but not DNA synthesis, as indicated by our inhibitor studies. For instance 5‐fluorodeoxyuridine, an inhibitor of DNA synthesis, had little effect on orientation of CMTs, when these were present, and allowed the developmental switch from more or less isodiametric mode of cell growth into highly polarized cell elongation to occur. In contrast, actinomycin D, a specific inhibitor of nuclear metabolism which rapidly halts rRNA synthesis, affected both the polarity and efficacy of postmitotic cell growth and impaired the transversal arrangement of CMTs as well as inducing the appearance of prominent ‘holes’ in their bundled arrays. These latter effects of actinomycin D cannot be simply explained by its indirect effect on translation, through impairment of functional ribosome assembly, as they were visible after only 15 min. Moreover, inhibition of protein synthesis by cycloheximide resulted in a different response. There was rapid and complete loss of the preferred transverse orientation of CMTs and the individual CMTs failed to organize themselves into bundles. Endoplasmic microtubules (EMTs), located more internally between plasma membrane and nucleus, seem to be important for determining nuclear size and chromatin architecture, since whenever EMTs were caused to disintegrate (cold and anti‐MT drug treatments), meristematic nuclei swelled and their chromatin dispersed. By contrast, when EMTs were stabilized and increased in number by taxol, which specifically favours MT polymerization, the nuclei became smaller and the chromatin more condensed. Nevertheless, the mechanisms of these complex interplays between the MT cytoskeleton and the nuclear organization remain unclear.