MICROTUBULE ARRAYS IN MAIZE ROOT-CELLS - INTERPLAY BETWEEN THE CYTOSKELETON, NUCLEAR-ORGANIZATION AND POSTMITOTIC CELLULAR GROWTH-PATTERNS

Results from studies with metabolic inhibitors suggest that, in growin g 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 M T-dependent transition from the approximately isodiametric mode of cel lular 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 synt hesis, as indicated by our inhibitor studies. For instance 5-fluorodeo xyuridine, an inhibitor of DNA synthesis, had little effect on orienta tion of CMTs, when these were present, and allowed the developmental s witch from more or less isodiametric mode of cell growth into highly p olarized cell elongation to occur. In contrast, actinomycin D, a speci fic inhibitor of nuclear metabolism which rapidly halts rRNA synthesis , affected both the polarity and efficacy of postmitotic cell growth a nd impaired the transversal arrangement of CMTs as well as inducing th e appearance of prominent 'holes' in their bundled arrays. These latte r effects of actinomycin D cannot be simply explained by its indirect effect on translation, through impairment of functional ribosome assem bly, as they were Visible after only 15 min. Moreover, inhibition of p rotein synthesis by cycloheximide resulted in a different response. Th ere was rapid and complete loss of the preferred transverse orientatio n of CMTs and the individual CMTs failed to organize themselves into b undles. Endoplasmic microtubules (EMTs), located more internally betwe en plasma membrane and nucleus, seem to be important for determining n uclear size and chromatin architecture, since whenever EMTs were cause d to disintegrate (cold and anti-MT drug treatments), meristematic nuc lei swelled and their chromatin dispersed. By contrast, when EMTs were stabilized and increased in number by taxol, which specifically favou rs MT polymerization, the nuclei became smaller and the chromatin more condensed. Nevertheless, the mechanisms of these complex interplays b etween the MT cytoskeleton and the nuclear organization remain unclear .