Microtubules orchestrate cell division and morphogenesis, but how they disassemble and reappear at different subcellular locations is unknown. Microtubule organizing centres are thought to have an important role, but in higher plants microtubules assemble in ordered configurations even though microtubule organizing centres are inconspicuous or absent. Plant cells generate highly organized microtubule arrays that coordinate mitosis, cytokinesis and expansion. Inhibiting microtubule assembly prevents chromosome separation, blocks cell division and impairs growth polarity. Microtubules are essential for the formation of cell walls, through an array of plasma-membrane-associated cortical microtubules whose control mechanisms are unknown. Using a genetic strategy to identify microtubule organizing factors in Arabidopsis thaliana, we isolated temperature-sensitive mutant alleles of the MICROTUBULE ORGANIZATION 1 (MOR1) gene. Here we show that MOR1 is the plant version of an ancient family of microtubule-associated proteins. Point mutations that substitute single amino-acid residues in an amino-terminal HEAT repeat impart reversible temperature-dependent cortical microtubule disruption, showing that MOR1 is essential for cortical microtubule organization.