Augmin Plays a Critical Role in Organizing the Spindle and Phragmoplast Microtubule Arrays in Arabidopsis

Abstract

In higher plant cells, microtubules (MTs) are nucleated and organized in a centrosome-independent manner. It is unclear whether augmin-dependent mechanisms underlie spindle MT organization in plant cells as they do in animal cells. When AUGMIN subunit3 (AUG3), which encodes a homolog of animal dim g-tubulin 3/human augmin-like complex, subunit 3, was disrupted in Arabidopsis thaliana, gametogenesis frequently failed due to defects in cell division. Compared with the control microspores, which formed bipolar spindles at the cell periphery, the mutant cells often formed peripheral half spindles that only attached to condensed chromosomes or formed elongated spindles with unfocused interior poles. In addition, defective cells exhibited disorganized phragmoplast MT arrays, which caused aborted cytokinesis. The resulting pollen grains were either shrunken or contained two nuclei in an undivided cytoplasm. AUG3 was localized along MTs in the spindle and phragmoplast, and its signal was pronounced in anaphase spindle poles. An AUG3-green fluorescent protein fusion exhibited a dynamic distribution pattern, similar to that of the g-tubulin complex protein2. When AUG3 was enriched from seedlings by affinity chromatography, AUG1 was detected by immunoblotting, suggesting an augmin-like complex was present in vivo. We conclude that augmin plays a critical role in MT organization during plant cell division.

Branching Microtubule Nucleation in Xenopus Egg Extracts Mediated by Augmin and TPX2

“…This result, combined with the fact that augmin homologues have not been found in yeast, suggests that branching nucleation in yeast and metazoans occur by fundamentally different mechanisms. In plants, augmin has recently been shown to be important for microtubule production in the spindle and phragmoplasts of plants (Ho et al, 2011; Hotta et al, 2012; Nakaoka et al, 2012). Branching microtubule nucleation also has been observed in plants, although not yet identified as an activity of augmin (Chan et al, 2009; Murata et al, 2005).…”

Mechanisms of Mitotic Spindle Assembly

“…How these factors work together to induce this reaction and what the branch point looks like remain to be resolved. In parallel, augmin was identified in plants, where it plays an important role in the formation of the plant spindle, the MT-rich phragmoplast, and the plant cortex (148–151). Augmin and TPX2 provide the first clear connection between RanGTP as the upstream factor and a γ-tubulin-dependent MT nucleation pathway.…”

Chromosome Dynamics Visualized with an Anti-Centromeric Histone H3 Antibody in Allium

“…Abnormal PPB was found to induce abnormal cell divisions [37]. Similar cell dynamics was also visualized by GFP-fused proteins in Arabidopsis [39]. However, the information from these plant experiments is limited, because information about the position and movement of the centromeres was not provided.…”