Abstract
The classic maize mutant divergent spindle-1 (dv1) causes failures in meiotic spindle assembly and a decrease in pollen viability. By analyzing two independent dv1 alleles we demonstrate that this phenotype is caused by mutations in a member of the kinesin-14A subfamily, a class of C-terminal, minus-end directed microtubule motors. Further analysis demonstrates that defects in early spindle assembly are rare, but that later stages of spindle organization promoting the formation of finely focused spindle poles are strongly dependent on Dv1. Anaphase is error-prone in dv1 lines but not severely so, and the majority of cells show normal chromosome segregation. Live-cell imaging of wild type and mutant plants carrying CFP-tagged β-tubulin confirm that meiosis in dv1 lines fails primarily at the pole-sharpening phase of spindle assembly. These data indicate that plant kinesin-14A proteins help to enforce bipolarity by focusing spindle poles and that this stage of spindle assembly is not required for transition through the spindle checkpoint but improves the accuracy of chromosome segregation.
Highlights
The plant cytoskeleton, comprised of actin-based microfilaments and tubulin-based microtubules, is involved in a number of critical cellular processes, including cell elongation, cell wall deposition, and cell division
The sequences of genes of this subclass were collected from different species, including the Arabidopsis genes AtKIN14a (Chen et al, 2002), and AtKIN14b (Ambrose et al, 2005), the Drosophila melanogaster gene NCD (Mcdonald et al, 1990) and the Xenopus laevis gene XCTK2 (Walczak et al, 1997)
The resulting tree identified two maize genes that cluster with the kinesin-14As of other species (Figure S1)
Summary
The plant cytoskeleton, comprised of actin-based microfilaments and tubulin-based microtubules, is involved in a number of critical cellular processes, including cell elongation, cell wall deposition, and cell division. Microtubules are hollow tube-shaped structures comprised of polymerized dimers of α- and β-tubulin and are polarized into dynamically growing and shrinking plus ends as well as relatively stable minus ends. This polar quality of microtubules is important in a number of their roles in plant cells, including meiosis, the process by which diploid somatic cells undergo reductional division to form haploid gametes. Organisms have evolved different structures known as microtubule organizing centers (MTOCs) to assist in this process These include the spindle pole body in budding yeast (reviewed in Kilmartin, 2014), and centrosomes in animals (reviewed in Conduit et al, 2015)
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