Characterization of the Arabidopsis Augmin Complex Uncovers Its Critical Function in the Assembly of the Acentrosomal Spindle and Phragmoplast Microtubule Arrays

Abstract

Plant cells assemble the bipolar spindle and phragmoplast microtubule (MT) arrays in the absence of the centrosome structure. Our recent findings in Arabidopsis thaliana indicated that AUGMIN subunit3 (AUG3), a homolog of animal dim gtubulin 3, plays a critical role in g-tubulin-dependent MT nucleation and amplification during mitosis. Here, we report the isolation of the entire plant augmin complex that contains eight subunits. Among them, AUG1 to AUG6 share low sequence similarity with their animal counterparts, but AUG7 and AUG8 share homology only with proteins of plant origin. Genetic analyses indicate that the AUG1, AUG2, AUG4, and AUG5 genes are essential, as stable mutations in these genes could only be transmitted to heterozygous plants. The sterile aug7-1 homozygous mutant in which AUG7 expression is significantly reduced exhibited pleiotropic phenotypes of seriously retarded vegetative and reproductive growth. The aug7-1 mutation caused delocalization of g-tubulin in the mitotic spindle and phragmoplast. Consequently, spindles were abnormally elongated, and their poles failed to converge, as MTs were splayed to discrete positions rendering deformed arrays. In addition, the mutant phragmoplasts often had disorganized MT bundles with uneven edges. We conclude that assembly of MT arrays during plant mitosis depends on the augmin complex, which includes two plant-specific subunits.

The rise and fall of the phragmoplast microtubule array

“…g-Tubulin localization to phragmoplast MTs is largely dependent on augmin as it becomes dissociated from the phragmoplast in augmin mutant cells in A. thaliana [42 ]. In the mutant, the phragmoplast MT array often fails to expand toward the cell periphery and MTs ultimately became disorganized [42 ]. Similarly when genes encoding augmin subunits are silenced in the moss P. patens, phragmoplast expansion is inhibited because of compromised MT formation in the array [43 ].…”

“…The 8-subunit complex augmin contains a MAP and serves as a docking factor for the g-tubulin complex to localize to spindle MTs in animal and plant cells [40,41 ,42 ]. g-Tubulin localization to phragmoplast MTs is largely dependent on augmin as it becomes dissociated from the phragmoplast in augmin mutant cells in A. thaliana [42 ]. In the mutant, the phragmoplast MT array often fails to expand toward the cell periphery and MTs ultimately became disorganized [42 ].…”

Differential Methylation during Maize Leaf Growth Targets Developmentally Regulated Genes

“…For example, GRMZM2G311883 is a gene homologous to Arabidopsis MODIFIER OF SNC1-1 14 (MOS14), which is known to form a link between splicing and RNAdependent DNA methylation . Also, genes involved in cell cycle regulation, such as CYCD4, APC10, TIP120-encoding (Wang et al, 2011b), FORMIN8-like (Xue et al, 2011), and AUGMIN6-like (Hotta et al, 2012), are represented in the data set, as are developmental processes affecting the vasculature (Petricka et al, 2008), meristem maintenance , and cell shape and plant architecture (O'Hara et al, 2013). Several genes that are differentially methylated in the older zones play a known role in development.…”

Update: Plant Cortical Microtubule Arrays

“…Results from mutant studies suggest that GIP1a may increase the nucleation capacity of g-TuRCs in vivo (Nakamura et al, 2012). Many of the proteins that direct the localization or activity of g-TuRCs turn out to be essential genes (Ho et al, 2011;Hotta et al, 2012;Nakamura et al, 2012;Walia et al, 2014), indicating a critical role for the spatiotemporal regulation of g-TuRC-dependent microtubule nucleation in plant cells.…”

“…The AUGMIN complex, composed of six subunits common to all eukaryotes and two plant-specific subunits (Hotta et al, 2012), was initially found to recruit g-TuRC to microtubules within the mitotic spindle and the phragmoplast (Ho et al, 2011). Further work revealed that this complex recruits g-TuRC to interphase microtubules to promote nucleation activity (Liu et al, 2014).…”