Description:
<jats:p>Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four<jats:italic>Arabidopsis</jats:italic> genes collectively called the <jats:italic>PILZ</jats:italic> group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four <jats:italic>PILZ</jats:italic> group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of α/β-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype,<jats:italic>KIESEL</jats:italic>, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase.</jats:p>