γ-Tubulin Complexes and Fibrillar Arrays: Two Conserved High Molecular Forms with Many Cellular Functions

Higher plants represent a large group of eukaryotes where centrosomes are absent. The functions of γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs) in metazoans and fungi in microtubule nucleation are well established and the majority of components found in the complexes are present in plants. However, plant microtubules are also nucleated in a γ-tubulin-dependent but γ-TuRC-independent manner. There is growing evidence that γ-tubulin is a microtubule nucleator without being complexed in γ-TuRC. Fibrillar arrays of γ-tubulin were demonstrated in plant and animal cells and the ability of γ-tubulin to assemble into linear oligomers/polymers was confirmed in vitro for both native and recombinant γ-tubulin. The functions of γ-tubulin as a template for microtubule nucleation or in promoting spontaneous nucleation is outlined. Higher plants represent an excellent model for studies on the role of γ-tubulin in nucleation due to their acentrosomal nature and high abundancy and conservation of γ-tubulin including its intrinsic ability to assemble filaments. The defining scaffolding or sequestration functions of plant γ-tubulin in microtubule organization or in nuclear processes will help our understanding of its cellular roles in eukaryotes

γ-Tubulin Complexes and Fibrillar Arrays: Two Conserved High Molecular Forms with Many Cellular Functions

Higher plants represent a large group of eukaryotes where centrosomes are absent. The functions of γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs) in metazoans and fungi in microtubule nucleation are well established and the majority of components found in the complexes are present in plants. However, plant microtubules are also nucleated in a γ-tubulin-dependent but γ-TuRC-independent manner. There is growing evidence that γ-tubulin is a microtubule nucleator without being complexed in γ-TuRC. Fibrillar arrays of γ-tubulin were demonstrated in plant and animal cells and the ability of γ-tubulin to assemble into linear oligomers/polymers was confirmed in vitro for both native and recombinant γ-tubulin. The functions of γ-tubulin as a template for microtubule nucleation or in promoting spontaneous nucleation is outlined. Higher plants represent an excellent model for studies on the role of γ-tubulin in nucleation due to their acentrosomal nature and high abundancy and conservation of γ-tubulin including its intrinsic ability to assemble filaments. The defining scaffolding or sequestration functions of plant γ-tubulin in microtubule organization or in nuclear processes will help our understanding of its cellular roles in eukaryotes

Microtubular and Nuclear Functions of γ-Tubulin: Are They LINCed?

γ-Tubulin is a conserved member of the tubulin superfamily with a function in microtubule nucleation. Proteins of γ-tubulin complexes serve as nucleation templates as well as a majority of other proteins contributing to centrosomal and non-centrosomal nucleation, conserved across eukaryotes. There is a growing amount of evidence of γ-tubulin functions besides microtubule nucleation in transcription, DNA damage response, chromatin remodeling, and on its interactions with tumor suppressors. However, the molecular mechanisms are not well understood. Furthermore, interactions with lamin and SUN proteins of the LINC complex suggest the role of γ-tubulin in the coupling of nuclear organization with cytoskeletons. γ-Tubulin that belongs to the clade of eukaryotic tubulins shows characteristics of both prokaryotic and eukaryotic tubulins. Both human and plant γ-tubulins preserve the ability of prokaryotic tubulins to assemble filaments and higher-order fibrillar networks. γ-Tubulin filaments, with bundling and aggregating capacity, are suggested to perform complex scaffolding and sequestration functions. In this review, we discuss a plethora of γ-tubulin molecular interactions and cellular functions, as well as recent advances in understanding the molecular mechanisms behind them

Box-and-whisker plots showing holoploid genome sizes (2C-values) for eight groups representing different species and cytotypes of <i>Anthoxanthum</i>.

<p>(A) (ploidy categories are marked as “2x”–diploids, “4x”–tetraploids and “poly”–high polyploid). (B) Box-and-whisker plots showing monoploid genome sizes (1Cx-values) for six groups representing different species and cytotypes of <i>Anthoxanthum</i> (Cx-values in the high-polyploid <i>A</i>. <i>amarum</i> and the <i>A</i>. <i>aristatum/ovatum</i> complex could not be calculated due to uncertain ploidy levels).</p

Variation in holoploid genome sizes (sorted according to increasing 2C-values) in (A) the <i>A</i>. <i>aristatum/ovatum</i> complex (total variation 64.8%); (B) 4x <i>A</i>. <i>odoratum</i> (total variation 14.4%); (C) ‘Mediterranean diploid’ (total variation 9.9%); and (D) 2x <i>A</i>. <i>alpinum</i> (total variation 6.2%).

<p>Individuals with determined numbers of somatic chromosomes are indicated by solid circles. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133748#pone.0133748.s003" target="_blank">S1 Table</a> for population details.</p

Summary of recognized <i>Anthoxanthum</i> species, their ploidy levels, genome sizes (both 2C-values and 1Cx-values given in DNA picograms), intraspecific/intraploidy genome size variation and numbers of somatic chromosomes.

<p>* Different letters indicate groups of taxa that are significantly different at α = 0.05 in Tukey HSD test.</p><p>() Mean 1Cx-value for the <i>A</i>. <i>aristatum/ovatum</i> complex was calculated from 25 diploid individuals with known chromosomes number (2n = 10).</p><p>** Mean Cx-value in the highly polyploid <i>A</i>. <i>amarum</i> could not be reliably determined due to the lack of exact chromosome counts.</p><p>Summary of recognized <i>Anthoxanthum</i> species, their ploidy levels, genome sizes (both 2C-values and 1Cx-values given in DNA picograms), intraspecific/intraploidy genome size variation and numbers of somatic chromosomes.</p

Evolutionary and Taxonomic Implications of Variation in Nuclear Genome Size: Lesson from the Grass Genus <i>Anthoxanthum</i> (Poaceae)

<div><p>The genus <i>Anthoxanthum</i> (sweet vernal grass, Poaceae) represents a taxonomically intricate polyploid complex with large phenotypic variation and its evolutionary relationships still poorly resolved. In order to get insight into the geographic distribution of ploidy levels and assess the taxonomic value of genome size data, we determined C- and Cx-values in 628 plants representing all currently recognized European species collected from 197 populations in 29 European countries. The flow cytometric estimates were supplemented by conventional chromosome counts.</p><p>In addition to diploids, we found two low (rare 3x and common 4x) and one high (~16x–18x) polyploid levels. Mean holoploid genome sizes ranged from 5.52 pg in diploid <i>A</i>. <i>alpinum</i> to 44.75 pg in highly polyploid <i>A</i>. <i>amarum</i>, while the size of monoploid genomes ranged from 2.75 pg in tetraploid <i>A</i>. <i>alpinum</i> to 9.19 pg in diploid <i>A</i>. <i>gracile</i>. In contrast to Central and Northern Europe, which harboured only limited cytological variation, a much more complex pattern of genome sizes was revealed in the Mediterranean, particularly in Corsica. Eight taxonomic groups that partly corresponded to traditionally recognized species were delimited based on genome size values and phenotypic variation. Whereas our data supported the merger of <i>A</i>. <i>aristatum</i> and <i>A</i>. <i>ovatum</i>, eastern Mediterranean populations traditionally referred to as diploid <i>A</i>. <i>odoratum</i> were shown to be cytologically distinct, and may represent a new taxon. Autopolyploid origin was suggested for 4x <i>A</i>. <i>alpinum</i>. In contrast, 4x <i>A</i>. <i>odoratum</i> seems to be an allopolyploid, based on the amounts of nuclear DNA. Intraspecific variation in genome size was observed in all recognized species, the most striking example being the <i>A</i>. <i>aristatum/ovatum</i> complex.</p><p>Altogether, our study showed that genome size can be a useful taxonomic marker in <i>Anthoxathum</i> to not only guide taxonomic decisions but also help resolve evolutionary relationships in this challenging grass genus.</p></div

Distribution of species and cytotypes of <i>Anthoxanthum</i> in the area studied, based on analysis of 628 individuals from 197 populations sampled in 29 European countries.

<p>Distribution of species and cytotypes of <i>Anthoxanthum</i> in the area studied, based on analysis of 628 individuals from 197 populations sampled in 29 European countries.</p