The γ-tubulin complex protein GCP6 is crucial for spindle morphogenesis but not essential for microtubule reorganization in Arabidopsis

Plant microtubule nucleating apparatus and its potential signaling pathway

Plant cell cortical microtubules are located beneath the plasma membrane and direct the location of cellulose synthases during interphase, influencing cell morphology. Microtubule-associated proteins (MAPs) regulate these microtubules in response to growth and environmental stimuli. This review focuses on recent advances in understanding microtubule nucleation mechanisms in plants and the spatiotemporal regulation of cortical arrays via phytohormone signaling. Emphasis is placed on the conserved nature of the gamma-tubulin ring complex (γTuRC) and plant-specific components. The discussion includes the role of the Augmin complex and the distinct function of the Msd1-Wdr8 complex in plants. We also explore the effects of hormone signaling, particularly brassinosteroids, on the microtubule regulatory apparatus. The interplay between hormone signaling pathways and microtubule dynamics, including phosphorylation events and post-translational modifications, is also addressed. Finally, the impact of environmental signals and the role of protein post-translational modifications in regulating microtubule organization are suggested for future research.

An Arabidopsis Kinesin-14D motor is associated with midzone microtubules for spindle morphogenesis

The acentrosomal spindle apparatus has kinetochore fibers organized and converged toward opposite poles; however, mechanisms underlying the organization of these microtubule fibers into an orchestrated bipolar array were largely unknown. Kinesin-14D is one of the four classes of Kinesin-14 motors that are conserved from green algae to flowering plants. In Arabidopsis thaliana, three Kinesin-14D members displayed distinct cell cycle-dependent localization patterns on spindle microtubules in mitosis. Notably, Kinesin-14D1 was enriched on the midzone microtubules of prophase and mitotic spindles and later persisted in the spindle and phragmoplast midzones. The kinesin-14d1 mutant had kinetochore fibers disengaged from each other during mitosis and exhibited hypersensitivity to the microtubule-depolymerizing herbicide oryzalin. Oryzalin-treated kinesin-14d1 mutant cells had kinetochore fibers tangled together in collapsed spindle microtubule arrays. Kinesin-14D1, unlike other Kinesin-14 motors, showed slow microtubule plus end-directed motility, and its localization and function were dependent on its motor activity and the novel malectin-like domain. Our findings revealed a Kinesin-14D1-dependent mechanism that employs interpolar microtubules to regulate the organization of kinetochore fibers for acentrosomal spindle morphogenesis.

Genome-wide identification and evolution of the tubulin gene family in Camelina sativa

BACKGROUND

Tubulins play crucial roles in numerous fundamental processes of plant development. In flowering plants, tubulins are grouped into α-, β- and γ-subfamilies, while α- and β-tubulins possess a large isotype diversity and gene number variations among different species. This circumstance leads to insufficient recognition of orthologous isotypes and significantly complicates extrapolation of obtained experimental results, and brings difficulties for the identification of particular tubulin isotype function. The aim of this research is to identify and characterize tubulins of an emerging biofuel crop Camelina sativa.

RESULTS

We report comprehensive identification and characterization of tubulin gene family in C. sativa, including analyses of exon-intron organization, duplicated genes comparison, proper isotype designation, phylogenetic analysis, and expression patterns in different tissues. 17 α-, 34 β- and 6 γ-tubulin genes were identified and assigned to a particular isotype. Recognition of orthologous tubulin isotypes was cross-referred, involving data of phylogeny, synteny analyses and genes allocation on reconstructed genomic blocks of Ancestral Crucifer Karyotype. An investigation of expression patterns of tubulin homeologs revealed the predominant role of N6 (A) and N7 (B) subgenomes in tubulin expression at various developmental stages, contrarily to general the dominance of transcripts of H7 (C) subgenome.

CONCLUSIONS

For the first time a complete set of tubulin gene family members was identified and characterized for allohexaploid C. sativa species. The study demonstrates the comprehensive approach of precise inferring gene orthology. The applied technique allowed not only identifying C. sativa tubulin orthologs in model Arabidopsis species and tracking tubulin gene evolution, but also uncovered that A. thaliana is missing orthologs for several particular isotypes of α- and β-tubulins.

Smooth Elongation of Pavement Cells Induced by RIC1 Overexpression Leads to Marginal Protrusions of the Cotyledon in Arabidopsis thaliana

The interdigitated pavement cell shape is suggested to be mechanically rational at both the cellular and tissue levels, but the biological significance of the cell shape is not fully understood. In this study, we explored the potential importance of the jigsaw puzzle-like cell shape for cotyledon morphogenesis in Arabidopsis. We used a transgenic line overexpressing a Rho-like GTPase-interacting protein, ROP-INTERACTIVE CRIB MOTIF-CONTAINING PROTEIN 1 (RIC1), which causes simple elongation of pavement cells. Computer-assisted microscopic analyses, including virtual reality observation, revealed that RIC1 overexpression resulted in abnormal cotyledon shapes with marginal protrusions, suggesting that the abnormal organ shape might be explained by changes in the pavement cell shape. Microscopic, biochemical and mechanical observations indicated that the pavement cell deformation might be due to reduction in the cell wall cellulose content with alteration of cortical microtubule organization. To examine our hypothesis that simple elongation of pavement cells leads to an abnormal shape with marginal protrusion of the cotyledon, we developed a mathematical model that examines the impact of planar cell growth geometry on the morphogenesis of the organ that is an assemblage of the cells. Computer simulations supported experimental observations that elongated pavement cells resulted in an irregular cotyledon shape, suggesting that marginal protrusions were due to local growth variation possibly caused by stochastic bias in the direction of cell elongation cannot be explained only by polarity-based cell elongation, but that an organ-level regulatory mechanism is required.

Developmental Selection and the Perception of Mutation Bias

The notion that mutations are random relative to their fitness effects is central to the Neo-Darwinian view of evolution. However, a recent interpretation of the patterns of mutation accumulation in the genome of Arabidopsis thaliana has challenged this notion, arguing for the presence of a targeted DNA repair mechanism that causes a nonrandom association of mutation rates and fitness effects. Specifically, this mechanism was suggested to cause a reduction in the rates of mutations on essential genes, thus lowering the rates of deleterious mutations. Central to this argument were attempts to rule out selection at the population level. Here, we offer an alternative and parsimonious interpretation of the patterns of mutation accumulation previously attributed to mutation bias, showing how they can instead or additionally be caused by developmental selection, that is selection occurring at the cellular level during the development of a multicellular organism. Thus, the depletion of deleterious mutations in A. thaliana may indeed be the result of a selective process, rather than a bias in mutation. More broadly, our work highlights the importance of considering development in the interpretation of population-genetic analyses of multicellular organisms, and it emphasizes that efforts to identify mechanisms involved in mutational biases should explicitly account for developmental selection.

Brassinosteroid signals cooperate with katanin-mediated microtubule severing to control stamen filament elongation

Proper stamen filament elongation is essential for pollination and plant reproduction. Plant hormones are extensively involved in every stage of stamen development; however, the cellular mechanisms by which phytohormone signals couple with microtubule dynamics to control filament elongation remain unclear. Here, we screened a series of Arabidopsis thaliana mutants showing different microtubule defects and revealed that only those unable to sever microtubules, lue1 and ktn80.1234, displayed differential floral organ elongation with less elongated stamen filaments. Prompted by short stamen filaments and severe decrease in KTN1 and KTN80s expression in qui-2 lacking five BZR1-family transcription factors (BFTFs), we investigated the crosstalk between microtubule severing and brassinosteroid (BR) signaling. The BFTFs transcriptionally activate katanin-encoding genes, and the microtubule-severing frequency was severely reduced in qui-2. Taken together, our findings reveal how BRs can regulate cytoskeletal dynamics to coordinate the proper development of reproductive organs.

Two Kinesin-14A Motors Oligomerize to Drive Poleward Microtubule Convergence for Acentrosomal Spindle Morphogenesis in Arabidopsis thaliana

Plant cells form acentrosomal spindles with microtubules (MTs) converged toward two structurally undefined poles by employing MT minus end-directed Kinesin-14 motors. To date, it is unclear whether the convergent bipolar MT array assumes unified poles in plant spindles, and if so, how such a goal is achieved. Among six classes of Kinesin-14 motors in Arabidopsis thaliana, the Kinesin-14A motors ATK1 (KatA) and ATK5 share the essential function in spindle morphogenesis. To understand how the two functionally redundant Kinesin-14A motors contributed to the spindle assembly, we had ATK1-GFP and ATK5-GFP fusion proteins expressed in their corresponding null mutants and found that they were functionally comparable to their native forms. Although ATK1 was a nuclear protein and ATK5 cytoplasmic prior to nuclear envelop breakdown, at later mitotic stages, the two motors shared similar localization patterns of uniform association with both spindle and phragmoplast MTs. We found that ATK1 and ATK5 were rapidly concentrated toward unified polar foci when cells were under hyperosmotic conditions. Concomitantly, spindle poles became perfectly focused as if there were centrosome-like MT-organizing centers where ATK1 and ATK5 were highly enriched and at which kinetochore fibers pointed. The separation of ATK1/ATK5-highlighted MTs from those of kinetochore fibers suggested that the motors translocated interpolar MTs. Our protein purification and live-cell imaging results showed that ATK1 and ATK5 are associated with each other in vivo. The stress-induced spindle pole convergence was also accompanied by poleward accumulation of the MT nucleator γ-tubulin. These results led to the conclusion that the two Kinesin-14A motors formed oligomeric motor complexes that drove MT translocation toward the spindle pole to establish acentrosomal spindles with convergent poles.

Spindle Assembly and Mitosis in Plants

In contrast to well-studied fungal and animal cells, plant cells assemble bipolar spindles that exhibit a great deal of plasticity in the absence of structurally defined microtubule-organizing centers like the centrosome. While plants employ some evolutionarily conserved proteins to regulate spindle morphogenesis and remodeling, many essential spindle assembly factors found in vertebrates are either missing or not required for producing the plant bipolar microtubule array. Plants also produce proteins distantly related to their fungal and animal counterparts to regulate critical events such as the spindle assembly checkpoint. Plant spindle assembly initiates with microtubule nucleation on the nuclear envelope followed by bipolarization into the prophase spindle. After nuclear envelope breakdown, kinetochore fibers are assembled and unified into the spindle apparatus with convergent poles. Of note, compared to fungal and animal systems, relatively little is known about how plant cells remodel the spindle microtubule array during anaphase. Uncovering mitotic functions of novel proteins for spindle assembly in plants will illuminate both common and divergent mechanisms employed by different eukaryotic organisms to segregate genetic materials.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Imaging Chambers for Arabidopsis Seedlings for Mitotic Studies

Flowering plants evolved away from creating centrosomes or conventional microtubule organizing centers. Therein, plants have posed a long-standing challenge to many of the conventional ideas for mitotic spindle construction and the process of chromosome segregation. The Arabidopsis seedling has emerged as a leading model for plant cell biological studies of the cytoskeleton and vesicle trafficking. Here we describe methods for creating a reusable chamber for mitotic studies in both seedling root and shoot cells with instruction for best practices with conventional microscopic techniques.

Effects of the protein GCP4 on gametophyte development in Arabidopsis thaliana

γ-Tubulin complex protein 4 (GCP4, encoded by AT3G53760) participates in microtubule (MT) nucleation in Arabidopsis thaliana, affecting the MT nucleation angles in cortical MTs, and the formation of the spindle and phragmoplasts during mitosis. Here, we report that GCP4 plays a critical role in gametophyte development. The results indicate that the gcp4 mutant caused by T-DNA insertion may express an aberrant gene product interfering with normal GCP4 expression, ultimately leading to the formation of desiccated ovules and aborted seeds. An analysis of transmission efficiency (TE) indicated that female gametophytes were more impaired in development than male gametophytes, and so observation and analysis of gametophyte defects were conducted. Complementation lines obtained by the native promoter and GCP4-coded CDS gene sequence fused with GFP reduced the numbers of lethal phenotypes of the gcp4 mutant. The localization of GCP4 in the gametophyte was detected in cytoplasm around nuclei and in vicinity of plasma membrane of pollen grains, and also detected in full cytoplasm and around the nuclei of ovules in complementation line. Thus, it was established that GCP4 influences the functionality of gametophytes during gametophyte development.

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.

Genome-Wide Analysis of Tubulin Gene Family in Cassava and Expression of Family Member FtsZ2-1 during Various Stress

Filamentous temperature-sensitive protein Z (Tubulin/FtsZ) family is a group of conserved GTP-binding (guanine nucleotide-binding) proteins, which are closely related to plant tissue development and organ formation as the major component of the cytoskeleton. According to the published genome sequence information of cassava (Manihot esculenta Crantz), 23 tubulin genes (MeTubulins) were identified, which were divided into four main groups based on their type and phylogenetic characteristics. The same grouping generally has the same or similar motif composition and exon–intron structure. Collinear analysis showed that fragment repetition event is the main factor in amplification of cassava tubulin superfamily gene. The expression profiles of MeTubulin genes in various tissue were analyzed, and it was found that MeTubulins were mainly expressed in leaf, petiole, and stem, while FtsZ2-1 was highly expressed in storage root. The qRT-PCR results of the FtsZ2-1 gene under hormone and abiotic stresses showed that indole-3-acetic acid (IAA) and gibberellin A3 (GA3) stresses could significantly increase the expression of the FtsZ2-1 gene, thereby revealing the potential role of FtsZ2-1 in IAA and GA3 stress-induced responses.

Microtubule organization defects in Arabidopsis thaliana

Microtubules (MT) are critical cytoskeletal filaments that have several functions in cell morphogenesis, cell division, vesicle transport and cytoplasmic separation in the spatiotemporal regulation of eukaryotic cells. Formation of MT requires the co-interaction of MT nucleation and α-β-tubulins, as well as MT-associated proteins (MAP). Many key MAP contributing to MT nucleation and elongation are essential for MT nucleation and regulation of MT dynamics, and are conserved in the plant kingdom. Therefore, the deletion or decrease of γ-tubulin ring complex (γTuRC) components and related MAP, such as the augmin complex, NEDD1, MZT1, EB1, MAP65, etc., in Arabidopsis thaliana results in MT organizational defects in the spindle and phragmoplast MT, as well as in chromosome defects. In addition, similar defects in MT organization and chromosome structure have been observed in plants under abiotic stress conditions, such as under high UV-B radiation. The MT can sense the signal from UV-B radiation, resulting in abnormal MT arrangement. Further studies are required to determine whether the abnormal chromosomes induced by UV-B radiation can be attributed to the involvement of abnormal MT arrays in chromosome migration after DNA damage.