Establishment of a mitotic model system by transient expression of the D-type cyclin in differentiated leaf cells of tobacco (Nicotiana benthamiana)

Spatially resolved proteomics of the Arabidopsis stomatal lineage identifies polarity complexes for cell divisions and stomatal pores

Cell polarity is used to guide asymmetric divisions and create morphologically diverse cells. We find that two oppositely oriented cortical polarity domains present during the asymmetric divisions in the Arabidopsis stomatal lineage are reconfigured into polar domains marking ventral (pore-forming) and outward-facing domains of maturing stomatal guard cells. Proteins that define these opposing polarity domains were used as baits in miniTurboID-based proximity labeling. Among differentially enriched proteins, we find kinases, putative microtubule-interacting proteins, and polar SOSEKIs with their effector ANGUSTIFOLIA. Using AI-facilitated protein structure prediction models, we identify potential protein-protein interaction interfaces among them. Functional and localization analyses of the polarity protein OPL2 and its putative interaction partners suggest a positive interaction with mitotic microtubules and a role in cytokinesis. This combination of proteomics and structural modeling with live-cell imaging provides insights into how polarity is rewired in different cell types and cell-cycle stages.

Functional characterization of D-type cyclins involved in cell division in rice

BACKGROUND

D-type cyclins (CYCD) regulate the cell cycle G1/S transition and are thus closely involved in cell cycle progression. However, little is known about their functions in rice.

RESULTS

We identified 14 CYCD genes in the rice genome and confirmed the presence of characteristic cyclin domains in each. The expression of the OsCYCD genes in different tissues was investigated. Most OsCYCD genes were expressed at least in one of the analyzed tissues, with varying degrees of expression. Ten OsCYCD proteins could interact with both retinoblastoma-related protein (RBR) and A-type cyclin-dependent kinases (CDKA) forming holistic complexes, while OsCYCD3;1, OsCYCD6;1, and OsCYCD7;1 bound only one component, and OsCYCD4;2 bound to neither protein. Interestingly, all OsCYCD genes except OsCYCD7;1, were able to induce tobacco pavement cells to re-enter mitosis with different efficiencies. Transgenic rice plants overexpressing OsCYCD2;2, OsCYCD6;1, and OsCYCD7;1 (which induced cell division in tobacco with high-, low-, and zero-efficiency, respectively) were created. Higher levels of cell division were observed in both the stomatal lineage and epidermal cells of the OsCYCD2;2- and OsCYCD6;1-overexpressing plants, with lower levels seen in OsCYCD7;1-overexpressing plants.

CONCLUSIONS

The distinct expression patterns and varying effects on the cell cycle suggest different functions for the various OsCYCD proteins. Our findings will enhance understanding of the CYCD family in rice and provide a preliminary foundation for the future functional verification of these genes.

Microtubule-associated proteins MAP65-1 and SUN18/IQD26 coordinately regulate tomato fruit shape by affecting cell division

Microtubule-associated proteins MAP65-1 and SUN18 function additively in fruit shape regulation by modulating cell division patterns but not changing cell morphology.

Microtubule-associated protein SlMAP70 interacts with IQ67-domain protein SlIQD21a to regulate fruit shape in tomato

Tomato (Solanum lycopersicum) fruit shape is related to microtubule organization and the activity of microtubule-associated proteins (MAPs). However, insights into the mechanism of fruit shape formation from a cell biology perspective remain limited. Analysis of the tissue expression profiles of different microtubule regulators revealed that functionally distinct classes of MAPs, including members of the plant-specific MICROTUBULE-ASSOCIATED PROTEIN 70 (MAP70) and IQ67 DOMAIN (IQD, also named SUN in tomato) families, are differentially expressed during fruit development. SlMAP70-1-3 and SlIQD21a are highly expressed during fruit initiation, which relates to the dramatic microtubule pattern rearrangements throughout this developmental stage of tomato fruits. Transgenic tomato lines overexpressing SlMAP70-1 or SlIQD21a produced elongated fruits with reduced cell circularity and microtubule anisotropy, while their loss-of-function mutants showed the opposite phenotype, harboring flatter fruits. Fruits were further elongated in plants coexpressing both SlMAP70-1 and SlIQD21a. We demonstrated that SlMAP70s and SlIQD21a physically interact and that the elongated fruit phenotype is likely due to microtubule stabilization induced by the SlMAP70-SlIQD21a interaction. Together, our results identify SlMAP70 proteins and SlIQD21a as important regulators of fruit elongation and demonstrate that manipulating microtubule function during early fruit development provides an effective approach to alter fruit shape.

The canonical E2Fs together with RETINOBLASTOMA-RELATED are required to establish quiescence during plant development

Maintaining stable and transient quiescence in differentiated and stem cells, respectively, requires repression of the cell cycle. The plant RETINOBLASTOMA-RELATED (RBR) has been implicated in stem cell maintenance, presumably by forming repressor complexes with E2F transcription factors. Surprisingly we find that mutations in all three canonical E2Fs do not hinder the cell cycle, but similarly to RBR silencing, result in hyperplasia. Contrary to the growth arrest that occurs when exit from proliferation to differentiation is inhibited upon RBR silencing, the e2fabc mutant develops enlarged organs with supernumerary stem and differentiated cells as quiescence is compromised. While E2F, RBR and the M-phase regulatory MYB3Rs are part of the DREAM repressor complexes, and recruited to overlapping groups of targets, they regulate distinct sets of genes. Only the loss of E2Fs but not the MYB3Rs interferes with quiescence, which might be due to the ability of E2Fs to control both G1-S and some key G2-M targets. We conclude that collectively the three canonical E2Fs in complex with RBR have central roles in establishing cellular quiescence during organ development, leading to enhanced plant growth.

SEC14-like condensate phase transitions at plasma membranes regulate root growth in Arabidopsis

Protein function can be modulated by phase transitions in their material properties, which can range from liquid- to solid-like; yet, the mechanisms that drive these transitions and whether they are important for physiology are still unknown. In the model plant Arabidopsis, we show that developmental robustness is reinforced by phase transitions of the plasma membrane-bound lipid-binding protein SEC14-like. Using imaging, genetics, and in vitro reconstitution experiments, we show that SEC14-like undergoes liquid-like phase separation in the root stem cells. Outside the stem cell niche, SEC14-like associates with the caspase-like protease separase and conserved microtubule motors at unique polar plasma membrane interfaces. In these interfaces, SEC14-like undergoes processing by separase, which promotes its liquid-to-solid transition. This transition is important for root development, as lines expressing an uncleavable SEC14-like variant or mutants of separase and associated microtubule motors show similar developmental phenotypes. Furthermore, the processed and solidified but not the liquid form of SEC14-like interacts with and regulates the polarity of the auxin efflux carrier PINFORMED2. This work demonstrates that robust development can involve liquid-to-solid transitions mediated by proteolysis at unique plasma membrane interfaces.

A polarized nuclear position specifies the correct division plane during maize stomatal development

Asymmetric cell division generates different cell types and is a feature of development in multicellular organisms. Prior to asymmetric cell division, cell polarity is established. Maize (Zea mays) stomatal development serves as an excellent plant model system for asymmetric cell division, especially the asymmetric division of the subsidiary mother cell (SMC). In SMCs, the nucleus migrates to a polar location after the accumulation of polarly localized proteins but before the appearance of the preprophase band. We examined a mutant of an outer nuclear membrane protein that is part of the LINC (linker of nucleoskeleton and cytoskeleton) complex that localizes to the nuclear envelope in interphase cells. Previously, maize linc kash sine-like2 (mlks2) was observed to have abnormal stomata. We confirmed and identified the precise defects that lead to abnormal asymmetric divisions. Proteins that are polarly localized in SMCs prior to division polarized normally in mlks2. However, polar localization of the nucleus was sometimes impaired, even in cells that have otherwise normal polarity. This led to a misplaced preprophase band and atypical division planes. MLKS2 localized to mitotic structures; however, the structure of the preprophase band, spindle, and phragmoplast appeared normal in mlks2. Time-lapse imaging revealed that mlks2 has defects in premitotic nuclear migration toward the polarized site and unstable position at the division site after formation of the preprophase band. Overall, our results show that nuclear envelope proteins promote premitotic nuclear migration and stable nuclear position and that the position of the nucleus influences division plane establishment in asymmetrically dividing cells.

Confocal Microscopic Assays of Mitotically Active Proteins in an Agrobacterial Infiltration-Based, Cell Division-Enabled Leaf System of Tobacco (Nicotiana benthamiana)

The production of tissues and organs in plants is brought about by mitotic cell divisions, starting from the zygote. Successful mitosis and cytokinesis harness the functional input of proteins that are expressed in cell cycle-dependent manners to regulate cytoskeletal reorganization and intracellular motility. Fluorescence microscopic assays of mitotically active proteins have been dependent on time-consuming transformation experiments in a host plant or cultured cells. To facilitate the detection and observation of cell cycle-dependent localization and dynamics of plant proteins, we demonstrate, in this chapter, a transiently induced cell division system in Nicotiana benthamiana, named the cell division-enabled leaf system (CDELS). Plasmid constructs which express the D-type cyclin along with a fluorescent fusion protein(s) of interest are delivered to the leaves of N. benthamiana by agrobacterial infiltration. Ectopic expression of cyclin D induces leaf epidermal cells to re-enter mitosis and subsequently cytokinesis, allowing the dynamic localization of fluorescent fusion protein(s) to be observed throughout the course of mitotic cell division using live-cell fluorescence microscopy. This effective approach not only allows one to detect mitotic activities of novel proteins but also record their dynamics and relationship with others during mitosis and cytokinesis in a greatly shortened period of time.

A nuclear Pandora's box: functions of nuclear envelope proteins in cell division

The nucleus is characteristic of eukaryotic cells and nuclear envelope proteins are conserved across the kingdoms. Over the years, the function of these proteins was studied in the intact nuclear envelope. Knowledge regarding the localization and function of nuclear envelope proteins during mitosis, after the nuclear envelope breaks down, is limited. Until recently, the localization of nuclear envelope proteins during mitosis has been observed with the mitotic apparatus. In this context, research in plant cell biology is more advanced compared to non-plant model systems. Although current studies shed light on the localization of nuclear envelope proteins, further experiments are required to determine what, if any, functional role different nuclear envelope proteins play during mitosis. This review will highlight our current knowledge about the role of nuclear envelope proteins and point out the unanswered questions as future direction.

A base substitution in OsphyC disturbs its Interaction with OsphyB and affects flowering time and chlorophyll synthesis in rice

BACKGROUND

Phytochromes are important photoreceptors in plants, and play essential roles in photomorphogenesis. The functions of PhyA and PhyB in plants have been fully analyzed, while those of PhyC in plant are not well understood.

RESULTS

A rice mutant, late heading date 3 (lhd3), was characterized, and the gene LHD3 was identified with a map-based cloning strategy. LHD3 encodes phytochrome C in rice. Animo acid substitution in OsphyC disrupted its interaction with OsphyB or itself, restraining functional forms of homodimer or heterodimer formation. Compared with wild-type plants, the lhd3 mutant exhibited delayed flowering under both LD (long-day) and SD (short-day) conditions, and delayed flowering time was positively associated with the day length via the Ehd1 pathway. In addition, lhd3 showed a pale-green-leaf phenotype and a slower chlorophyll synthesis rate during the greening process. The transcription patterns of many key genes involved in photoperiod-mediated flowering and chlorophyll synthesis were altered in lhd3.

CONCLUSION

The dimerization of OsPhyC is important for its functions in the regulation of chlorophyll synthesis and heading. Our findings will facilitate efforts to further elucidate the function and mechanism of OsphyC and during light signal transduction in rice.

Rice microtubule-associated protein OsMAP65-3.1, but not OsMAP65-3.2, plays a critical role in phragmoplast microtubule organization in cytokinesis

In plants, MAP65 preferentially cross-links the anti-parallel microtubules (MTs) and plays an important role for cytokinesis. However, the functions of MAP65 isoforms in rice (Oryza sativa. L) are largely unknown. Here, we identified two MAP65-3 homologs in rice, OsMAP65-3.1 and OsMAP65-3.2. We found that both OsMAP65-3.1 and OsMAP65-3.2 were similar in dimerization and location to AtMAP65-3, and the expression of either rice genes driven by the AtMAP65-3 promoter suppressed the cytokinesis failure and growth defect of atmap65-3. However, OsMAP65-3.1 with native promoter also recovered the atmap65-3, but OsMAP65-3.2 with its own promoter had no effects. OsMAP65-3.1 but not OsMAP65-3.2 was actively expressed in tissues enriched with dividing cells. R1R2R3-Myb (MYB3R) transcription factors directly bound to the OsMAP65-3.1 promoter but not that of OsMAP65-3.2. Furthermore, osmap65-3.2 had no obvious phenotype, while either osmap65-3.1 or osmap65-3.1(+/-) was lethal. The eminent MTs around the daughter nuclei and cytokinesis defects were frequently observed in OsMAP65-3.1-defective plants. Taken together, our findings suggest that OsMAP65-3.1, rather than OsMAP65-3.2, plays essential roles in rice cytokinesis resulting from their differential expression which were passably directly regulated by OsMYB3Rs.

Tools for studying the cytoskeleton during plant cell division

The plant cytoskeleton regulates fundamental biological processes, including cell division. How to experimentally perturb the cytoskeleton is a key question if one wants to understand the role of both actin filaments (AFs) and microtubules (MTs) in a given biological process. While a myriad of mutants are available, knock-out in cytoskeleton regulators, when nonlethal, often produce little or no phenotypic perturbation because such regulators are often part of a large family, leading to functional redundancy. In this review, alternative techniques to modify the plant cytoskeleton during plant cell division are outlined. The different pharmacological and genetic approaches already developed in cell culture, transient assays, or in whole organisms are presented. Perspectives on the use of optogenetics to perturb the plant cytoskeleton are also discussed.

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.

The B-Type Cyclin CYCB1-1 Regulates Embryonic Development and Seed Size in Maize

Progress through the cell cycle is a critical process during plant embryo and seed development and its progression is regulated by cyclins. Despite extensive study of cyclins in other systems, their role in embryo and seed development of maize is unclear. In this study, we demonstrate that ZmCYCB1-1 overexpression significantly accelerated embryo growth and increased seed size. In situ hybridization and toluidine blue staining indicated that ZmCYCB1-1 was highly expressed in the plumule of embryos, and the cells of the plumule were smaller, denser, and more regularly arranged in ZmCYCB1-1 overexpression plants. Overexpression of ZmCYCB1-1 in maize also resulted in an increased ear length and enhanced kernel weight by increasing kernel width. Transcriptome analysis indicated that the overexpression of ZmCYCB1-1 affected several different metabolic pathways, including photosynthesis in embryos and leaves, and lipid metabolism in leaves. Conversely, knocking out ZmCYCB1-1 resulted in plants with slow growth. Our results suggest that ZmCYCB1-1 regulates embryo growth and seed size, making it an ideal target for efforts aimed at maize yield improvement.

Transient Overexpression of E2Fb to Induce Cell Divisions in Nicotiana benthamiana Pavement Cells

The availability of a fast and controlled mitotic model system that could simplify the generation of genetic material and reduce the experimental time from months to days would largely benefit research in plant cell division. In this protocol, we propose the use of pavement cells of Nicotiana benthamiana leaves to study cell division, which is artificially induced by Agrobacterium-mediated transient overexpression of the transcription factor E2Fb. The cell division-inducing overexpression of E2Fb can be combined with the expression of fluorescent protein-tagged proteins of interest or with dyes, which could be visualized throughout the cell cycle under the microscope. This simple and affordable method enables the study of cell cycle regulation and cell division in plants, from genome replication to cell wall formation, in a fast and controlled manner, and can be used for functional studies when coupled with chemical inhibitors or reverse genetic approaches.

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.

MdWRKY75e enhances resistance to Alternaria alternata in Malus domestica

The Alternaria alternata apple pathotype adversely affects apple (Malus domestica Borkh.) cultivation. However, the molecular mechanisms underlying enhanced resistance to this pathogen in apple remain poorly understood. We have previously reported that MdWRKY75 expression is upregulated by A. alternata infection in 'Sushuai' apples. In this study, we discovered that overexpression of MdWRKY75e increased the resistance of transgenic apple lines to A. alternata infection, whereas silencing this gene enhanced susceptibility to A. alternata infection. Furthermore, we found that MdWRKY75e directly binds to the MdLAC7 promoter to regulate the biosynthesis of laccase and increase the biosynthesis of lignin during A. alternata infection. Moreover, the thickening of the cell wall enhanced the mechanical defense capabilities of apple. In addition, we found that jasmonic acid remarkably induced MdWRKY75e expression, and its levels in transgenic apple lines were elevated. These results indicate that MdWRKY75e confers resistance to the A. alternata apple pathotype mainly via the jasmonic acid pathway and that pathogenesis-related genes and antioxidant-related enzyme activity are involved in the disease resistance of MdWRKY75e transgenic plants. In conclusion, our findings provide insights into the importance of MdWRKY75e for resistance to A. alternata infection in apples.

Distinct EH domains of the endocytic TPLATE complex confer lipid and protein binding

Clathrin-mediated endocytosis (CME) is the gatekeeper of the plasma membrane. In contrast to animals and yeasts, CME in plants depends on the TPLATE complex (TPC), an evolutionary ancient adaptor complex. However, the mechanistic contribution of the individual TPC subunits to plant CME remains elusive. In this study, we used a multidisciplinary approach to elucidate the structural and functional roles of the evolutionary conserved N-terminal Eps15 homology (EH) domains of the TPC subunit AtEH1/Pan1. By integrating high-resolution structural information obtained by X-ray crystallography and NMR spectroscopy with all-atom molecular dynamics simulations, we provide structural insight into the function of both EH domains. Both domains bind phosphatidic acid with a different strength, and only the second domain binds phosphatidylinositol 4,5-bisphosphate. Unbiased peptidome profiling by mass-spectrometry revealed that the first EH domain preferentially interacts with the double N-terminal NPF motif of a previously unidentified TPC interactor, the integral membrane protein Secretory Carrier Membrane Protein 5 (SCAMP5). Furthermore, we show that AtEH/Pan1 proteins control the internalization of SCAMP5 via this double NPF peptide interaction motif. Collectively, our structural and functional studies reveal distinct but complementary roles of the EH domains of AtEH/Pan1 in plant CME and connect the internalization of SCAMP5 to the TPLATE complex.

Whether Gametophytes are Reduced or Unreduced in Angiosperms Might Be Determined Metabolically

In angiosperms, meiotic failure coupled with the formation of genetically unreduced gametophytes in ovules (apomeiosis) constitute major components of gametophytic apomixis. These aberrant developmental events are generally thought to be caused by mutation. However, efforts to locate the responsible mutations have failed. Herein, we tested a fundamentally different hypothesis: apomeiosis is a polyphenism of meiosis, with meiosis and apomeiosis being maintained by different states of metabolic homeostasis. Microarray analyses of ovules and pistils were used to differentiate meiotic from apomeiotic processes in Boechera (Brassicaceae). Genes associated with translation, cell division, epigenetic silencing, flowering, and meiosis characterized sexual Boechera (meiotic). In contrast, genes associated with stress responses, abscisic acid signaling, reactive oxygen species production, and stress attenuation mechanisms characterized apomictic Boechera (apomeiotic). We next tested whether these metabolic differences regulate reproductive mode. Apomeiosis switched to meiosis when premeiotic ovules of apomicts were cultured on media that increased oxidative stress. These treatments included drought, starvation, and H₂O₂ applications. In contrast, meiosis switched to apomeiosis when premeiotic pistils of sexual plants were cultured on media that relieved oxidative stress. These treatments included antioxidants, glucose, abscisic acid, fluridone, and 5-azacytidine. High-frequency apomeiosis was initiated in all sexual species tested: Brassicaceae, Boechera stricta, Boechera exilis, and Arabidopsis thaliana; Fabaceae, Vigna unguiculata; Asteraceae, Antennaria dioica. Unreduced gametophytes formed from ameiotic female and male sporocytes, first division restitution dyads, and nucellar cells. These results are consistent with modes of reproduction and types of apomixis, in natural apomicts, being regulated metabolically.

A method for studying cell division in Nicotiana benthamiana pavement cells based on Agrobacterium infiltration

This article is a Commentary on Xu et al., 226: 1213–1221.