Growth is “anisotropic” when growth rates in different directions are not equal. Anisotropic elongation is controlled by cortical microtubules and cellulose microfibrils of the cell wall. Distorted anisotropic growth results when there are aberrations in either the cellulose network or microtubule cytoskeleton. in this ultrastructural study, the roots of wild type (control) and mutants of Arabidopsis thaliana (L.) Heynh, ecotype Columbia, were compared to determine the role of microtubule organisation, cellulose synthesis and cytokinesis on root expansion.

Three mutations, obtained by treating seeds of A. thaliana with ethane-methylsulfonate and backcrossing once to wild type after four selfed generations, were isolated and designated as rsw (radially swollen). These phenotypes are all temperature sensitive, growing and appearing as wild type at 19°C but expressing the mutant phenotype at or above 30°C. Segments of intact root tips from the three mutants and wild type were prepared and examined for TEM with a Jeol 1200EX. The overall appearance of rsw seedlings at 19°C is the same as wild type. At 30°C these phenotypes lead to extensive swelling of the root apex. Despite considerable alteration in the morphology of the root apex, the appearance of the root cap and root hairs seems essentially normal. When mutant roots expressing these phenotypes at 30°C are returned to 19°C, a wild type appearance of the root apex and elongation gradually returns.

The primary root of the wild type has a remarkably uniform cellular organisation with regularly arranged dense cytoplasmic cells (Fig. 1). At 19°C, rsw4, rsw6 and rsw7 were indistinguishable from wild type; however, at 30°C, the well-defined anatomy was distorted in all tissues by additional cell walls in unusual positions (Fig. 2).