The Arabidopsis KNOLLE protein is a cytokinesis-specific syntaxin

Electron tomographic analysis of cytokinesis in the brown alga Silvetia babingtonii (Fucales, Phaeophyceae)

In brown algae, membrane resources for the new cell partition during cytokinesis are mainly flat cisternae (FCs) and Golgi-derived vesicles. We used electron tomography coupled with rapid freezing/freeze substitution of zygotes to clarify the structure of transient membrane compartments during cytokinesis in Silvetia zygotes. After mitosis, an amorphous membranous structure, considered to be an FC intermediate was observed near endoplasmic reticulum clusters, lying between two daughter nuclei. FCs were arrayed at the cytokinetic plane, and a tubular membranous network was formed around them. This network might be formed by the consecutive fusion of spherical vesicles that are linked to the edges of FCs to form a membranous network (MN). At the initial stage of the formation of a membranous sac (MS) from the MN, the MS had flat and swollen parts, with the latter showing membranous tunnels. Coated pits were detected with high frequency at the swollen parts of the MS. This observation indicated that membranous tunnels disappeared by recycling of excess membrane via endocytosis, and the swollen part became flat. The MN appeared at the edges of the growing MS. MN and the MN-MS complex were observed along the cytokinetic plane in several spaces. The MS expanded by the incorporation of MN or other MS in its neighborhood. With the maturation of the new cell partition membrane, the thickness of the MS became constant and the membrane cavity disappeared. The changes in the surface area and volume of the transient membrane compartment during cytokinesis were analyzed from the tomographic data.

The ASH1-RELATED3 SET-domain protein controls cell division competence of the meristem and the quiescent center of the Arabidopsis primary root

The stem cell niche of the Arabidopsis (Arabidopsis thaliana) primary root apical meristem is composed of the quiescent (or organizing) center surrounded by stem (initial) cells for the different tissues. Initial cells generate a population of transit-amplifying cells that undergo a limited number of cell divisions before elongating and differentiating. It is unclear whether these divisions occur stochastically or in an orderly manner. Using the thymidine analog 5-ethynyl-2'-deoxyuridine to monitor DNA replication of cells of Arabidopsis root meristems, we identified a pattern of two, four, and eight neighboring cells with synchronized replication along the cortical, epidermal, and endodermal cell files, suggested to be daughters, granddaughters, and great-granddaughters of the direct progeny of each stem cell. Markers of mitosis and cytokinesis were not present in the region closest to the transition zone where the cells start to elongate, suggesting that great-granddaughter cells switch synchronously from the mitotic cell cycle to endoreduplication. Mutations in the stem cell niche-expressed ASH1-RELATED3 (ASHR3) gene, encoding a SET-domain protein conferring histone H3 lysine-36 methylation, disrupted this pattern of coordinated DNA replication and cell division and increased the cell division rate in the quiescent center. E2Fa/E2Fb transcription factors controlling the G1-to-S-phase transition regulate ASHR3 expression and bind to the ASHR3 promoter, substantiating a role for ASHR3 in cell division control. The reduced length of the root apical meristem and primary root of the mutant ashr3-1 indicate that synchronization of replication and cell divisions is required for normal root growth and development.

Endosidin 7 Specifically Arrests Late Cytokinesis and Inhibits Callose Biosynthesis, Revealing Distinct Trafficking Events during Cell Plate Maturation

Although cytokinesis is vital for plant growth and development, our mechanistic understanding of the highly regulated membrane and cargo transport mechanisms in relation to polysaccharide deposition during this process is limited. Here, we present an in-depth characterization of the small molecule endosidin 7 (ES7) inhibiting callose synthase activity and arresting late cytokinesis both in vitro and in vivo in Arabidopsis (Arabidopsis thaliana). ES7 is a specific inhibitor for plant callose deposition during cytokinesis that does not affect endomembrane trafficking during interphase or cytoskeletal organization. The specificity of ES7 was demonstrated (1) by comparing its action with that of known inhibitors such as caffeine, flufenacet, and concanamycin A and (2) across kingdoms with a comparison in yeast. The interplay between cell plate-specific post-Golgi vesicle traffic and callose accumulation was analyzed using ES7, and it revealed unique and temporal contributions of secretory and endosomal vesicles in cell plate maturation. While RABA2A-labeled vesicles, which accumulate at the early stage of cell plate formation, were not affected by ES7, KNOLLE was differentially altered by the small molecule. In addition, the presence of clathrin-coated vesicles in cells containing elevated levels of callose and their reduction under ES7 treatment further support the role of endocytic membrane remodeling in the maturing cell plate while the plate is stabilized by callose. Taken together, these data show the essential role of callose during the late stages of cell plate maturation and establish the temporal relationship between vesicles and regulatory proteins at the cell plate assembly matrix during polysaccharide deposition.

Arabidopsis SABRE and CLASP interact to stabilize cell division plane orientation and planar polarity

The orientation of cell division and the coordination of cell polarity within the plane of the tissue layer (planar polarity) contribute to shape diverse multicellular organisms. The root of Arabidopsis thaliana displays regularly oriented cell divisions, cell elongation and planar polarity providing a plant model system to study these processes. Here we report that the SABRE protein, which shares similarity with proteins of unknown function throughout eukaryotes, has important roles in orienting cell division and planar polarity. SABRE localizes at the plasma membrane, endomembranes, mitotic spindle and cell plate. SABRE stabilizes the orientation of CLASP-labelled preprophase band microtubules predicting the cell division plane, and of cortical microtubules driving cell elongation. During planar polarity establishment, sabre is epistatic to clasp at directing polar membrane domains of Rho-of-plant GTPases. Our findings mechanistically link SABRE to CLASP-dependent microtubule organization, shedding new light on the function of SABRE-related proteins in eukaryotes.

Plant cytokinesis is orchestrated by the sequential action of the TRAPPII and exocyst tethering complexes

Plant cytokinesis is initiated in a transient membrane compartment, the cell plate, and completed by a process of maturation during which the cell plate becomes a cross wall. How the transition from juvenile to adult stages occurs is poorly understood. In this study, we monitor the Arabidopsis transport protein particle II (TRAPPII) and exocyst tethering complexes throughout cytokinesis. We show that their appearance is predominantly sequential, with brief overlap at the onset and end of cytokinesis. The TRAPPII complex is required for cell plate biogenesis, and the exocyst is required for cell plate maturation. The TRAPPII complex sorts plasma membrane proteins, including exocyst subunits, at the cell plate throughout cytokinesis. We show that the two tethering complexes physically interact and propose that their coordinated action may orchestrate not only plant but also animal cytokinesis.

Syntaxin 31 functions in Glycine max resistance to the plant parasitic nematode Heterodera glycines

A Glycine max syntaxin 31 homolog (Gm-SYP38) was identified as being expressed in nematode-induced feeding structures known as syncytia undergoing an incompatible interaction with the plant parasitic nematode Heterodera glycines. The observed Gm-SYP38 expression was consistent with prior gene expression analyses that identified the alpha soluble NSF attachment protein (Gm-α-SNAP) resistance gene because homologs of these genes physically interact and function together in other genetic systems. Syntaxin 31 is a protein that resides on the cis face of the Golgi apparatus and binds α-SNAP-like proteins, but has no known role in resistance. Experiments presented here show Gm-α-SNAP overexpression induces Gm-SYP38 transcription. Overexpression of Gm-SYP38 rescues G. max [Williams 82/PI 518671], genetically rhg1 (-/-), by suppressing H. glycines parasitism. In contrast, Gm-SYP38 RNAi in the rhg1 (+/+) genotype G. max [Peking/PI 548402] increases susceptibility. Gm-α-SNAP and Gm-SYP38 overexpression induce the transcriptional activity of the cytoplasmic receptor-like kinase BOTRYTIS INDUCED KINASE 1 (Gm-BIK1-6) which is a family of defense proteins known to anchor to membranes through a 5' MGXXXS/T(R) N-myristoylation sequence. Gm-BIK1-6 had been identified previously by RNA-seq experiments as expressed in syncytia undergoing an incompatible reaction. Gm-BIK1-6 overexpression rescues the resistant phenotype. In contrast, Gm-BIK1-6 RNAi increases parasitism. The analysis demonstrates a role for syntaxin 31-like genes in resistance that until now was not known.

Delivery of endocytosed proteins to the cell-division plane requires change of pathway from recycling to secretion

Membrane trafficking is essential to fundamental processes in eukaryotic life, including cell growth and division. In plant cytokinesis, post-Golgi trafficking mediates a massive flow of vesicles that form the partitioning membrane but its regulation remains poorly understood. Here, we identify functionally redundant Arabidopsis ARF guanine-nucleotide exchange factors (ARF-GEFs) BIG1–BIG4 as regulators of post-Golgi trafficking, mediating late secretion from the trans-Golgi network but not recycling of endocytosed proteins to the plasma membrane, although the TGN also functions as an early endosome in plants. In contrast, BIG1-4 are absolutely required for trafficking of both endocytosed and newly synthesized proteins to the cell–division plane during cytokinesis, counteracting recycling to the plasma membrane. This change from recycling to secretory trafficking pathway mediated by ARF-GEFs confers specificity of cargo delivery to the division plane and might thus ensure that the partitioning membrane is completed on time in the absence of a cytokinesis-interphase checkpoint.

DOI:http://dx.doi.org/10.7554/eLife.02131.001

Cells are surrounded by a plasma membrane, and when a cell divides to create two new cells, it must grow a new membrane to keep the two new cells apart. Animal cells and plant cells tackle this challenge in different ways: in animal cells the new membrane grows inwards from the surface of the cell, whereas the new membrane grows outwards from the centre of the cell in plant cells.

The materials needed to make the plasma membrane are delivered in packages called vesicles: most of these materials arrive from a structure within the cell called the trans-Golgi network, but some materials are recycled from the existing plasma membrane. In plants the formation of the new cell membrane is orchestrated by scaffold-like structure that forms in the plant cell called the ‘phragmoplast’. It is widely thought that this structure guides the vesicles bringing materials from the trans-Golgi network, but the details of this process are not fully understood.

Now, Richter et al. have discovered four proteins, called BIG1 to BIG4, that control the formation of the new cell membrane in the flowering plant Arabidopsis thaliana, a species that is routinely studied by plant biologists. These four proteins belong to a larger family of proteins that control the trafficking of vesicles within a cell. Richter et al show that a plant cell can lose up to three of these four proteins and still divide, as the plant can still grow and develop as normal. Thus, BIG1 to BIG4 appear to perform essentially the same role in the plant.

Richter et al. also show that, when a plant cell is not dividing, these proteins are involved in controlling the delivery of new materials to surface membrane, and not the recycling of material. However, when a cell is dividing, these proteins switch to regulate both processes, but direct all the material to a new destination—the newly forming membrane, instead of the established surface membrane. Richter et al. suggest that this switch is important to stop any recycling to the plasma membrane that might move material away from the new membrane. The next challenge will be to identify the molecular signals and mechanisms that enable the proteins BIG1 to BIG4 to re-route the recycling of membrane material during cell division.

DOI:http://dx.doi.org/10.7554/eLife.02131.002

Divergent roles for maize PAN1 and PAN2 receptor-like proteins in cytokinesis and cell morphogenesis

Pangloss1 (PAN1) and PAN2 are leucine-rich repeat receptor-like proteins that function cooperatively to polarize the divisions of subsidiary mother cells (SMCs) during stomatal development in maize (Zea mays). PANs colocalize in SMCs, and both PAN1 and PAN2 promote polarization of the actin cytoskeleton and nuclei in these cells. Here, we show that PAN1 and PAN2 have additional functions that are unequal or divergent. PAN1, but not PAN2, is localized to cell plates in all classes of dividing cells examined. pan1 mutants exhibited no defects in cell plate formation or in the recruitment or removal of a variety of cell plate components; thus, they did not demonstrate a function for PAN1 in cytokinesis. PAN2, in turn, plays a greater role than PAN1 in directing patterns of postmitotic cell expansion that determine the shapes of mature stomatal subsidiary cells and interstomatal cells. Localization studies indicate that PAN2 impacts subsidiary cell shape indirectly by stimulating localized cortical actin accumulation and polarized growth in interstomatal cells. Localization of PAN1, Rho of Plants2, and PIN1a suggests that PAN2-dependent cell shape changes do not involve any of these proteins, indicating that PAN2 function is linked to actin polymerization by a different mechanism in interstomatal cells compared with SMCs. Together, these results demonstrate that PAN1 and PAN2 are not dedicated to SMC polarization but instead play broader roles in plant development. We speculate that PANs may function in all contexts to regulate polarized membrane trafficking either directly or indirectly via their influence on actin polymerization.

Syntaxin of plant proteins SYP123 and SYP132 mediate root hair tip growth in Arabidopsis thaliana

Root hairs are fast-growing tubular protrusions on root epidermal cells that play important roles in water and nutrient uptake in plants. The tip-focused polarized growth of root hairs is accomplished by the secretion of newly synthesized materials to the tip via the polarized membrane trafficking mechanism. Here, we report the function of two different types of plasma membrane (PM) Qa-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), SYP123 and SYP132, in the growth of root hair in Arabidopsis. We found that SYP123, but not SYP132, localizes in the tip region of root hairs by recycling between the brefeldin A (BFA)-sensitive endosomes and the PM of the expanding tip in an F-actin-dependent manner. The vesicle-associated membrane proteins VAMP721/722/724 also exhibited tip-focused localization in root hairs and formed ternary SNARE complexes with both SYP123 and SYP132. These results demonstrate that SYP123 and SYP132 act in a coordinated fashion to mediate tip-focused membrane trafficking for root hair tip growth.

The use of multidrug approach to uncover new players of the endomembrane system trafficking machinery

Chemical Biology is a strong tool to perform experimental procedures to study the Endomembrane System (ES) in plant biology. In the last few years, several bioactive compounds and their effects upon protein trafficking as well as organelle distribution, identity, and size in plants and yeast have been characterized. Today, several of these chemical tools are widely used to perform mutant screens and establish the trafficking pathway of a given cellular component. This chapter is a guideline to perform multidrug approaches to study the endomembrane system in plant cells. This type of approach is a powerful and useful strategy to thoroughly determine the trafficking of a specific protein as well as to perform mutant screens based on phenotypes produced by drug treatments. On the other hand, a multidrug approach can address the characterization of a new bioactive molecule and find its cellular pathway target. Overall, this approach can unravel mechanisms and identify new players in endomembrane trafficking.

Sterol dynamics during endocytic trafficking in Arabidopsis

Sterols are lipids found in membranes of eukaryotic cells. Functions of sterols have been demonstrated for various cellular processes including endocytic trafficking in animal, fungal, and plant cells. The ability to visualize sterols at the subcellular level is crucial to understand sterol distribution and function during endocytic trafficking. In plant cells, the polyene antibiotic filipin is the most extensively used tool for the specific detection of fluorescently labeled 3-β-hydroxysterols in situ. Filipin can to some extent be used to track sterol internalization in live cells, but this application is limited, due to the inhibitory effects filipin exerts on sterol-dependent endocytosis. Nevertheless, filipin-sterol labeling can be performed on aldehyde-fixed cells which allows for sterol detection in endocytic compartments. This approach can combine studies correlating sterol distribution with experimental manipulations of endocytic trafficking pathways. Here, we describe step-by-step protocols and troubleshooting for procedures on live and fixed cells to visualize sterols during endocytic trafficking. We also provide a detailed discussion of advantages and limitations of both methods. Moreover, we illustrate the use of the endocytic recycling inhibitor brefeldin A and a genetically modified version of one of its target molecules for studying endocytic sterol trafficking.

Immunocytochemical fluorescent in situ visualization of proteins in Arabidopsis

The understanding of cellular and subcellular functions often relies on the ability to visualize proteins as close as possible to their endogenous locations. A number of immunocytochemical techniques have been developed to detect proteins in situ using specific antibodies raised against proteins of interest. Here, we describe in detail two protocols commonly, successfully employed in Arabidopsis research. The first allows for immunolocalization of proteins in whole-mount Arabidopsis roots without the need for physical sectioning. The second allows for immunolocalization of proteins on semi-thin microtome sections of wax-embedded swamples. This approach is particularly useful when sectioning of Arabidopsis roots or other thicker plant organs is required for immunolocalization. We provide step-by-step protocols with extensive troubleshooting for both the whole-mount and sectioning protocols. Furthermore, critical steps, advantages, and limitations of the two protocols described here are discussed.

Spatio-temporal analysis of cellulose synthesis during cell plate formation in Arabidopsis

During cytokinesis a new crosswall is rapidly laid down. This process involves the formation at the cell equator of a tubulo-vesicular membrane network (TVN). This TVN evolves into a tubular network (TN) and a planar fenestrated sheet, which extends at its periphery before fusing to the mother cell wall. The role of cell wall polymers in cell plate assembly is poorly understood. We used specific stains and GFP-labelled cellulose synthases (CESAs) to show that cellulose, as well as three distinct CESAs, accumulated in the cell plate already at the TVN stage. This early presence suggests that cellulose is extruded into the tubular membrane structures of the TVN. Co-localisation studies using GFP-CESAs suggest the delivery of cellulose synthase complexes (CSCs) to the cell plate via phragmoplast-associated vesicles. In the more mature TN part of the cell plate, we observed delivery of GFP-CESA from doughnut-shaped organelles, presumably Golgi bodies. During the conversion of the TN into a planar fenestrated sheet, the GFP-CESA density diminished, whereas GFP-CESA levels remained high in the TVN zone at the periphery of the expanding cell plate. We observed retrieval of GFP-CESA in clathrin-containing structures from the central zone of the cell plate and from the plasma membrane of the mother cell, which may contribute to the recycling of CESAs to the peripheral growth zone of the cell plate. These observations, together with mutant phenotypes of cellulose-deficient mutants and pharmacological experiments, suggest a key role for cellulose synthesis already at early stages of cell plate assembly.

Genome-wide annotation, expression profiling, and protein interaction studies of the core cell-cycle genes in Phalaenopsis aphrodite

Orchidaceae is one of the most abundant and diverse families in the plant kingdom and its unique developmental patterns have drawn the attention of many evolutionary biologists. Particular areas of interest have included the co-evolution of pollinators and distinct floral structures, and symbiotic relationships with mycorrhizal flora. However, comprehensive studies to decipher the molecular basis of growth and development in orchids remain scarce. Cell proliferation governed by cell-cycle regulation is fundamental to growth and development of the plant body. We took advantage of recently released transcriptome information to systematically isolate and annotate the core cell-cycle regulators in the moth orchid Phalaenopsis aphrodite. Our data verified that Phalaenopsis cyclin-dependent kinase A (CDKA) is an evolutionarily conserved CDK. Expression profiling studies suggested that core cell-cycle genes functioning during the G1/S, S, and G2/M stages were preferentially enriched in the meristematic tissues that have high proliferation activity. In addition, subcellular localization and pairwise interaction analyses of various combinations of CDKs and cyclins, and of E2 promoter-binding factors and dimerization partners confirmed interactions of the functional units. Furthermore, our data showed that expression of the core cell-cycle genes was coordinately regulated during pollination-induced reproductive development. The data obtained establish a fundamental framework for study of the cell-cycle machinery in Phalaenopsis orchids.

Regulation of cytokinesis by exocyst subunit SEC6 and KEULE in Arabidopsis thaliana

Proper vesicle tethering and membrane fusion at the cell plate are essential for cytokinesis. Both the vesicle tethering complex exocyst and membrane fusion regulator KEULE were shown to function in cell plate formation, but the exact mechanisms still remain to be explored. In this study, using yeast two-hybrid (Y-2-H) assay, we found that SEC6 interacted with KEULE, and that a small portion of C-terminal region of KEULE was required for the interaction. The direct SEC6-KEULE interaction was supported by further studies using in vitro pull-down assay, immunoprecipitation, and in vivo bimolecular fluorescence complementation (BIFC) microscopy. sec6 mutants were male gametophytic lethal as reported; however, pollen-rescued sec6 mutants (PRsec6) displayed cytokinesis defects in the embryonic cells and later in the leaf pavement cells and the guard cells. SEC6 and KEULE proteins were co-localized to the cell plate during cytokinesis in transgenic Arabidopsis. Furthermore, only SEC6 but not other exocyst subunits located in the cell plate interacted with KEULE in vitro. These results demonstrated that, like KEULE, SEC6 plays a physiological role in cytokinesis, and the SEC6-KEULE interaction may serve as a novel molecular linkage between arriving vesicles and membrane fusion machinery or directly regulate membrane fusion during cell plate formation in plants.

Expression of α-taxilin in the murine gastrointestinal tract: potential implication in cell proliferation

α-Taxilin, a binding partner of the syntaxin family, is a candidate tumor marker. To gain insight into the physiological role of α-taxilin in normal tissues, we examined α-taxilin expression by Western blot and performed immunochemical analysis in the murine gastrointestinal tract where cell renewal vigorously occurs. α-Taxilin was expressed in the majority of the gastrointestinal tract and was prominently expressed in epithelial cells positive for Ki-67, a marker of actively proliferating cells. In the small intestine, α-taxilin was expressed in transient-amplifying cells and crypt base columnar cells intercalated among Paneth cells. In the corpus and antrum of the stomach, α-taxilin was expressed in cells localized in the lower pit and at the gland, respectively, but not in parietal or zymogenic cells. During development of the small intestine, α-taxilin was expressed in Ki-67-positive regions. Inhibition of cell proliferation by suppression of the Notch cascade using a γ-secretase inhibitor led to a decrease in α-taxilin- and Ki-67-positive cells in the stomach. These results suggest that expression of α-taxilin is regulated in parallel with cell proliferation in the murine gastrointestinal tract.

Mediation of clathrin-dependent trafficking during cytokinesis and cell expansion by Arabidopsis stomatal cytokinesis defective proteins

Stomatal cytokinesis defective1 (SCD1) encodes a putative Rab guanine nucleotide exchange factor that functions in membrane trafficking and is required for cytokinesis and cell expansion in Arabidopsis thaliana. Here, we show that the loss of SCD2 function disrupts cytokinesis and cell expansion and impairs fertility, phenotypes similar to those observed for scd1 mutants. Genetic and biochemical analyses showed that SCD1 function is dependent upon SCD2 and that together these proteins are required for plasma membrane internalization. Further specifying the role of these proteins in membrane trafficking, SCD1 and SCD2 proteins were found to be associated with isolated clathrin-coated vesicles and to colocalize with clathrin light chain at putative sites of endocytosis at the plasma membrane. Together, these data suggest that SCD1 and SCD2 function in clathrin-mediated membrane transport, including plasma membrane endocytosis, required for cytokinesis and cell expansion.

The endoplasmic reticulum is the main membrane source for biogenesis of the lytic vacuole in Arabidopsis

Vacuoles are multifunctional organelles essential for the sessile lifestyle of plants. Despite their central functions in cell growth, storage, and detoxification, knowledge about mechanisms underlying their biogenesis and associated protein trafficking pathways remains limited. Here, we show that in meristematic cells of the Arabidopsis thaliana root, biogenesis of vacuoles as well as the trafficking of sterols and of two major tonoplast proteins, the vacuolar H(+)-pyrophosphatase and the vacuolar H(+)-adenosinetriphosphatase, occurs independently of endoplasmic reticulum (ER)-Golgi and post-Golgi trafficking. Instead, both pumps are found in provacuoles that structurally resemble autophagosomes but are not formed by the core autophagy machinery. Taken together, our results suggest that vacuole biogenesis and trafficking of tonoplast proteins and lipids can occur directly from the ER independent of Golgi function.

cell- and tissue-specific transcriptome analyses of Medicago truncatula root nodules

Legumes have the unique ability to host nitrogen-fixing Rhizobium bacteria as symbiosomes inside root nodule cells. To get insight into this key process, which forms the heart of the endosymbiosis, we isolated specific cells/tissues at different stages of symbiosome formation from nodules of the model legume Medicago truncatula using laser-capture microdissection. Next, we determined their associated expression profiles using Affymetrix Medicago GeneChips. Cells were collected from the nodule infection zone divided into a distal (where symbiosome formation and division occur) and proximal region (where symbiosomes are mainly differentiating), as well as infected cells from the fixation zone containing mature nitrogen fixing symbiosomes. As non-infected cells/tissue we included nodule meristem cells and uninfected cells from the fixation zone. Here, we present a comprehensive gene expression map of an indeterminate Medicago nodule and selected genes that show specific enriched expression in the different cells or tissues. Validation of the obtained expression profiles, by comparison to published gene expression profiles and experimental verification, indicates that the data can be used as digital “in situ”. This digital “in situ” offers a genome-wide insight into genes specifically associated with subsequent stages of symbiosome and nodule cell development, and can serve to guide future functional studies.

cell- and tissue-specific transcriptome analyses of Medicago truncatula root nodules

Legumes have the unique ability to host nitrogen-fixing Rhizobium bacteria as symbiosomes inside root nodule cells. To get insight into this key process, which forms the heart of the endosymbiosis, we isolated specific cells/tissues at different stages of symbiosome formation from nodules of the model legume Medicago truncatula using laser-capture microdissection. Next, we determined their associated expression profiles using Affymetrix Medicago GeneChips. Cells were collected from the nodule infection zone divided into a distal (where symbiosome formation and division occur) and proximal region (where symbiosomes are mainly differentiating), as well as infected cells from the fixation zone containing mature nitrogen fixing symbiosomes. As non-infected cells/tissue we included nodule meristem cells and uninfected cells from the fixation zone. Here, we present a comprehensive gene expression map of an indeterminate Medicago nodule and selected genes that show specific enriched expression in the different cells or tissues. Validation of the obtained expression profiles, by comparison to published gene expression profiles and experimental verification, indicates that the data can be used as digital "in situ". This digital "in situ" offers a genome-wide insight into genes specifically associated with subsequent stages of symbiosome and nodule cell development, and can serve to guide future functional studies.

SNARE complexes of different composition jointly mediate membrane fusion in Arabidopsis cytokinesis

Membrane fusion is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. Although membrane fusion is required for separating daughter cells in eukaryotic cytokinesis, the SNARE complexes involved are not known. In plants, membrane vesicles targeted to the cell division plane fuse with one another to form the partitioning membrane, progressing from the center to the periphery of the cell. In Arabidopsis, the cytokinesis-specific Qa-SNARE KNOLLE interacts with two other Q-SNAREs, SNAP33 and novel plant-specific SNARE 11 (NPSN11), whose roles in cytokinesis are not clear. Here we show by coimmunoprecipitation that KNOLLE forms two SNARE complexes that differ in composition. One complex is modeled on the trimeric plasma membrane type of SNARE complex and includes, in addition to KNOLLE, the promiscuous Qb,c-SNARE SNAP33 and the R-SNARE vesicle-associated membrane protein (VAMP) 721,722, also involved in innate immunity. In contrast, the other KNOLLE-containing complex is tetrameric and includes Qb-SNARE NPSN11, Qc-SNARE SYP71, and VAMP721,722. Elimination of only one or the other type of KNOLLE complex by mutation, including the double mutant npsn11 syp71, causes a mild or no cytokinesis defect. In contrast, the two double mutants snap33 npsn11 and snap33 syp71 eliminate both types of KNOLLE complexes and display knolle-like cytokinesis defects. Thus the two distinct types of KNOLLE complexes appear to jointly mediate membrane fusion in Arabidopsis cytokinesis.

Cytokinesis in plant male meiosis

In somatic cell division, cytokinesis is the final step of the cell cycle and physically divides the mother cytoplasm into two daughter cells. In the meiotic cell division, however, pollen mother cells (PMCs) undergo two successive nuclear divisions without an intervening S-phase and consequently generate four haploid daughter nuclei out of one parental cell. In line with this, the physical separation of meiotic nuclei does not follow the conventional cytokinesis pathway, but instead is mediated by alternative processes, including polar-based phragmoplast outgrowth and RMA-mediated cell wall positioning. In this review, we outline the different cytological mechanisms of cell plate formation operating in different types of PMCs and additionally focus on some important features associated with male meiotic cytokinesis, including cytoskeletal dynamics and callose deposition. We also provide an up-to-date overview of the main molecular actors involved in PMC wall formation and additionally highlight some recent advances on the effect of cold stress on meiotic cytokinesis in plants.

Cytoskeletal and membrane dynamics during higher plant cytokinesis

Following mitosis, cytoplasm, organelles and genetic material are partitioned into daughter cells through the process of cytokinesis. In somatic cells of higher plants, two cytoskeletal arrays, the preprophase band and the phragmoplast, facilitate the positioning and de novo assembly of the plant-specific cytokinetic organelle, the cell plate, which develops across the division plane and fuses with the parental plasma membrane to yield distinct new cells. The coordination of cytoskeletal and membrane dynamics required to initiate, assemble and shape the cell plate as it grows toward the mother cell cortex is dependent upon a large array of proteins, including molecular motors, membrane tethering, fusion and restructuring factors and biosynthetic, structural and regulatory elements. This review focuses on the temporal and molecular requirements of cytokinesis in somatic cells of higher plants gleaned from recent studies using cell biology, genetics, pharmacology and biochemistry.

GLUCAN SYNTHASE-LIKE8 and STEROL METHYLTRANSFERASE2 are required for ploidy consistency of the sexual reproduction system in Arabidopsis

In sexually reproducing plants, the meiocyte-producing archesporal cell lineage is maintained at the diploid state to consolidate the formation of haploid gametes. In search of molecular factors that regulate this ploidy consistency, we isolated an Arabidopsis thaliana mutant, called enlarged tetrad2 (et2), which produces tetraploid meiocytes through the stochastic occurrence of premeiotic endomitosis. Endomitotic polyploidization events were induced by alterations in cell wall formation, and similar cytokinetic defects were sporadically observed in other tissues, including cotyledons and leaves. ET2 encodes GLUCAN SYNTHASE-LIKE8 (GSL8), a callose synthase that mediates the deposition of callose at developing cell plates, root hairs, and plasmodesmata. Unlike other gsl8 mutants, in which defects in cell plate formation are seedling lethal, cytokinetic defects in et2 predominantly occur in flowers and have little effect on vegetative growth and development. Similarly, mutations in STEROL METHYLTRANSFERASE2 (SMT2), a major sterol biosynthesis enzyme, also lead to weak cytokinetic defects, primarily in the flowers. In addition, SMT2 allelic mutants also generate tetraploid meiocytes through the ectopic induction of premeiotic endomitosis. These observations demonstrate that appropriate callose and sterol biosynthesis are required for maintaining the ploidy level of the premeiotic germ lineage and that subtle defects in cytokinesis may lead to diploid gametes and polyploid offspring.

Functional analysis of small Rab GTPases in cytokinesis in Arabidopsis thaliana

Rab proteins are key regulators of membrane transport in eukaryotes. Recent evidence from different species supports the notion that some Rab proteins are crucial for cytokinesis, a pivotal procedure for successful cell division. As a family of monomeric small GTPases of the Ras superfamily, the function of Rab proteins is modulated by guanine nucleotide binding and hydrolysis. To investigate the function of Rab proteins, creating dominant negative or constitutively active mutant forms of a Rab protein is a widely used approach. To study cytokinesis in plant cells, using fluorescent dye to highlight the cell shape and the nuclei, and to monitor the formation of the newly formed cell plate in mitotic cells, is easy and useful. In this chapter, we describe detailed methods for (1) generating transgenic plants expressing dominant negative or constitutively active form of RAB-A1c; (2) fluorescent staining of cell shape, cell wall, and nuclei of mitotic root tip cells; (3) fluorescent staining of newly formed cell plate; and (4) detecting fluorescent signals using Confocal Laser Scanning Microscopy in the genetic model plant species Arabidopsis thaliana.

Imaging of phenotypes, gene expression, and protein localization during embryonic root formation in Arabidopsis

Plants grow elaborate architectures by repeatedly initiating new organs post-embryonically. The competence to do so depends on the activity of meristems, stem cell niches located at the tips of shoot and root. These meristems are first specified early during embryogenesis. Therefore, important insight into the activity of factors that are central to the establishment of stem cell niches in plants can be gained from studying early embryogenesis. However, embryos are not directly accessible to microscopic observation since they are embedded within the seed, which is itself enveloped by the fruit. Here we describe a suite of methods for the analysis of mutant phenotypes, fluorescent reporter gene expression and protein localization in Arabidopsis embryos, and show how these methods can be used to visualize key factors in embryonic root formation.

ROS-mediated lipid peroxidation and RES-activated signaling

Nonenzymatic lipid oxidation is usually viewed as deleterious. But if this is the case, then why does it occur so frequently in cells? Here we review the mechanisms of membrane peroxidation and examine the genesis of reactive electrophile species (RES). Recent evidence suggests that during stress, both lipid peroxidation and RES generation can benefit cells. New results from genetic approaches support a model in which entire membranes can act as supramolecular sinks for singlet oxygen, the predominant reactive oxygen species (ROS) in plastids. RES reprogram gene expression through a class II TGA transcription factor module as well as other, unknown signaling pathways. We propose a framework to explain how RES signaling promotes cell "REScue" by stimulating the expression of genes encoding detoxification functions, cell cycle regulators, and chaperones. The majority of the known biological activities of oxygenated lipids (oxylipins) in plants are mediated either by jasmonate perception or through RES signaling networks.

Arabidopsis SABRE and CLASP interact to stabilize cell division plane orientation and planar polarity

The orientation of cell division and the coordination of cell polarity within the plane of the tissue layer (planar polarity) contribute to shape diverse multicellular organisms. The root of Arabidopsis thaliana displays regularly oriented cell divisions, cell elongation and planar polarity providing a plant model system to study these processes. Here we report that the SABRE protein, which shares similarity with proteins of unknown function throughout eukaryotes, has important roles in orienting cell division and planar polarity. SABRE localizes at the plasma membrane, endomembranes, mitotic spindle and cell plate. SABRE stabilizes the orientation of CLASP-labelled preprophase band microtubules predicting the cell division plane, and of cortical microtubules driving cell elongation. During planar polarity establishment, sabre is epistatic to clasp at directing polar membrane domains of Rho-of-plant GTPases. Our findings mechanistically link SABRE to CLASP-dependent microtubule organization, shedding new light on the function of SABRE-related proteins in eukaryotes.

Sec1/Munc18 protein stabilizes fusion-competent syntaxin for membrane fusion in Arabidopsis cytokinesis

Intracellular membrane fusion requires complexes of syntaxins with other SNARE proteins and regulatory Sec1/Munc18 (SM) proteins. In membrane fusion mediating, e.g., neurotransmitter release or glucose-stimulated insulin secretion in mammals, SM proteins preferentially interact with the inactive closed, rather than the active open, conformation of syntaxin or with the assembled SNARE complex. Other membrane fusion processes such as vacuolar fusion in yeast involve like membranes carrying cis-SNARE complexes, and the role of SM protein is unknown. We investigated syntaxin-SM protein interaction in membrane fusion of Arabidopsis cytokinesis, which involves cytokinesis-specific syntaxin KNOLLE and SM protein KEULE. KEULE interacted with an open conformation of KNOLLE that complemented both knolle and keule mutants. This interaction occurred at the cell division plane and required the KNOLLE linker sequence between helix Hc and SNARE domain. Our results suggest that in cytokinesis, SM protein stabilizes the fusion-competent open form of syntaxin, thereby promoting trans-SNARE complex formation.

An A/ENTH domain-containing protein functions as an adaptor for clathrin-coated vesicles on the growing cell plate in Arabidopsis root cells

Cytokinesis is the process of partitioning the cytoplasm of a dividing cell, thereby completing mitosis. Cytokinesis in the plant cell is achieved by the formation of a new cell wall between daughter nuclei using components carried in Golgi-derived vesicles that accumulate at the midplane of the phragmoplast and fuse to form the cell plate. Proteins that play major roles in the development of the cell plate in plant cells are not well defined. Here, we report that an AP180 amino-terminal homology/epsin amino-terminal homology domain-containing protein from Arabidopsis (Arabidopsis thaliana) is involved in clathrin-coated vesicle formation from the cell plate. Arabidopsis Epsin-like Clathrin Adaptor1 (AtECA1; At2g01600) and its homologous proteins AtECA2 and AtECA4 localize to the growing cell plate in cells undergoing cytokinesis and also to the plasma membrane and endosomes in nondividing cells. AtECA1 (At2g01600) does not localize to nascent cell plates but localizes at higher levels to expanding cell plates even after the cell plate fuses with the parental plasma membrane. The temporal and spatial localization patterns of AtECA1 overlap most closely with those of the clathrin light chain. In vitro protein interaction assays revealed that AtECA1 binds to the clathrin H chain via its carboxyl-terminal domain. These results suggest that these AP180 amino-terminal homology/epsin amino-terminal homology domain-containing proteins, AtECA1, AtECA2, and AtECA4, may function as adaptors of clathrin-coated vesicles budding from the cell plate.

Secretory pathway research: the more experimental systems the better

Transient gene expression, in plant protoplasts or specific plant tissues, is a key technique in plant molecular cell biology, aimed at exploring gene products and their modifications to examine functional subdomains, their interactions with other biomolecules, and their subcellular localization. Here, we highlight some of the major advantages and potential pitfalls of the most commonly used transient gene expression models and illustrate how ectopic expression and the use of dominant mutants can provide insights into protein function.

Expression of the bacterial heavy metal transporter MerC fused with a plant SNARE, SYP121, in Arabidopsis thaliana increases cadmium accumulation and tolerance

The bacterial merC gene from the Tn21-encoded mer operon is a potential molecular tool for improving the efficiency of metal phytoremediation. Arabidopsis SNARE molecules, including SYP111, SYP121, and AtVAM3 (SYP22), were attached to the C-terminus of MerC to target the protein to various organelles. The subcellular localization of transiently expressed GFP-fused MerC-SYP111, MerC-SYP121, and MerC-AtVAM3 was examined in Arabidopsis suspension-cultured cells. We found that GFP-MerC-SYP111 and GFP-MerC-SYP121 localized to the plasma membrane, whereas GFP-AtVAM3 localized to the vacuolar membranes. These results demonstrate that SYP111/SYP121 and AtVAM3 target foreign molecules to the plasma membrane and vacuolar membrane, respectively. To enhance the efficiency and potential of plants to sequester and accumulate cadmium from contaminated sites, transgenic Arabidopsis plants expressing MerC, MerC-SYP111, MerC-SYP121, or MerC-AtVAM3 were generated. The transgenic plants that expressed MerC, MerC-SYP121, or MerC-AtVAM3 appeared to be normal, whereas the transgenic that expressed MerC-SYP111 exhibited severe growth defects. The transgenic plants expressing merC-SYP121 were more resistant to cadmium than the wild type and accumulated significantly more cadmium. Thus, the expression of MerC-SYP121 in the plant plasma membrane may provide an ecologically compatible approach for the phytoremediation of cadmium pollution.

Cryo-section immunolabelling of difficult to preserve specimens: advantages of cryofixation, freeze-substitution and rehydration

BACKGROUND INFORMATION

Electron microscopic immunolabelling of ultrathin thawed cryo-sections, according to the method of Tokuyasu, is widely used as a very sensitive high-resolution localization technique. Its main advantages are that antigens remain in a hydrated environment prior to immunolabelling, and that antigen accessibility is improved compared with resin section labelling. However, the quality of structural appearance and antigenicity depends highly on the limitations of the initial conventional chemical fixation step, such as slow diffusion and selective reaction/cross-linking of fixative molecules.

RESULTS AND CONCLUSIONS

Cryofixation, instead of conventional chemical fixation, followed by freeze-substitution/chemical fixation, rehydration and further processing for Tokuyasu cryo-sectioning leads to an improved preservation of both ultrastructure and antigenicity. This is especially true for tissues which are difficult to preserve by conventional chemical fixation at ambient temperatures, such as plant material, Drosophila embryos or nematode tissue. In particular labile and highly dynamic structures (for example, microtubules and Golgi apparatus) are remarkably better preserved. These improvements are also valid for light microscopic applications.

Conserved and plant-unique mechanisms regulating plant post-Golgi traffic

Membrane traffic plays crucial roles in diverse aspects of cellular and organelle functions in eukaryotic cells. Molecular machineries regulating each step of membrane traffic including the formation, tethering, and fusion of membrane carriers are largely conserved among various organisms, which suggests that the framework of membrane traffic is commonly shared among eukaryotic lineages. However, in addition to the common components, each organism has also acquired lineage-specific regulatory molecules that may be associated with the lineage-specific diversification of membrane trafficking events. In plants, comparative genomic analyses also indicate that some key machineries of membrane traffic are significantly and specifically diversified. In this review, we summarize recent progress regarding plant-unique regulatory mechanisms for membrane traffic, with a special focus on vesicle formation and fusion components in the post-Golgi trafficking pathway.

News and Views into the SNARE Complexity in Arabidopsis

Secretory organelles are engaged in a continuous flux of membranes, which is believed to occur mostly via transport vesicles. Being critical in maintaining several cellular functions, transport vesicles are membrane-enclosed sacs that temporarily store and then deliver membrane lipids, protein, and polysaccharides. SNAREs have a crucial role in vesicle traffic by driving membrane fusion and conferring fidelity through the formation of specific SNARE complexes. Additionally, specific roles of SNAREs in growth and development implicate that they are versatile components for the life of a plant. Here, we summarize the recent progress on the understanding of the role of SNAREs and highlight some of the questions that are still unsolved.

The trafficking protein SYP121 of Arabidopsis connects programmed stomatal closure and K⁺ channel activity with vegetative growth

The vesicle-trafficking protein SYP121 (SYR1/PEN1) was originally identified in association with ion channel control at the plasma membrane of stomatal guard cells, although stomata of the Arabidopsis syp121 loss-of-function mutant close normally in ABA and high Ca²⁺. We have now uncovered a set of stomatal phenotypes in the syp121 mutant that reduce CO₂ assimilation, slow vegetative growth and increase water use efficiency in the whole plant, conditional upon high light intensities and low relative humidity. Stomatal opening and the rise in stomatal transpiration of the mutant was delayed in the light and following Ca²⁺-evoked closure, consistent with a constitutive form of so-called programmed stomatal closure. Delayed reopening was observed in the syp121, but not in the syp122 mutant lacking the homologous gene product; the delay was rescued by complementation with wild-type SYP121 and was phenocopied in wild-type plants in the presence of the vesicle-trafficking inhibitor Brefeldin A. K⁺ channel current that normally mediates K⁺ uptake for stomatal opening was suppressed in the syp121 mutant and, following closure, its recovery was slowed compared to guard cells of wild-type plants. Evoked stomatal closure was accompanied by internalisation of GFP-tagged KAT1 K⁺ channels in both wild-type and syp121 mutant guard cells, but their subsequently recycling was slowed in the mutant. Our findings indicate that SYP121 facilitates stomatal reopening and they suggest that K⁺ channel traffic and recycling to the plasma membrane underpins the stress memory phenomenon of programmed closure in stomata. Additionally, they underline the significance of vesicle traffic for whole-plant water use and biomass production, tying SYP121 function to guard cell membrane transport and stomatal control.

Polarized cell growth in Arabidopsis requires endosomal recycling mediated by GBF1-related ARF exchange factors

Polarized tip growth is a fundamental cellular process in many eukaryotic organisms, mediating growth of neuronal axons and dendrites or fungal hyphae. In plants, pollen and root hairs are cellular model systems for analysing tip growth. Cell growth depends on membrane traffic. The regulation of this membrane traffic is largely unknown for tip-growing cells, in contrast to cells exhibiting intercalary growth. Here we show that in Arabidopsis, GBF1-related exchange factors for the ARF GTPases (ARF GEFs) GNOM and GNL2 play essential roles in polar tip growth of root hairs and pollen, respectively. When expressed from the same promoter, GNL2 (in contrast to the early-secretory ARF GEF GNL1) is able to replace GNOM in polar recycling of the auxin efflux regulator PIN1 from endosomes to the basal plasma membrane in non-tip growing cells. Thus, polar recycling facilitates polar tip growth, and GNL2 seems to have evolved to meet the specific requirement of fast-growing pollen in higher plants.

Functional anatomy of the Arabidopsis cytokinesis-specific syntaxin KNOLLE

In plant cytokinesis, Golgi/trans-Golgi network-derived vesicles are targeted to the plane of cell division where they fuse with one another to form the partitioning membrane (cell plate). This membrane fusion requires a specialised syntaxin (Qa-SNARE), named KNOLLE in Arabidopsis. KNOLLE is only made during the M-phase of the cell cycle, targeted to the plane of cell division and degraded in the vacuole at the end of cytokinesis. To identify the parts of KNOLLE required for proper targeting and function in membrane fusion, we generated chimeric syntaxins comprising complementary fragments from KNOLLE and MVB-localized PEP12 (SYP21). Surprisingly, targeting of the chimeric protein was not specified by the C-terminal membrane anchor. Rather the N-terminal region including helix Ha and the adjacent linker to helix Hb appeared to played a critical role. However, deletion of this N-terminal fragment from KNOLLE (KN(Δ1-82) ) had the same effect as its presence in the chimeric protein (KN(1-82) -PEP12(64-279) ), suggesting that targeting to the plane of cell division occurs by default, i.e. when no sorting signal would target the syntaxin to a specific endomembrane compartment. Once the full-length syntaxin accumulated at the plane of division, phenotypic rescue of the knolle mutant only required the SNARE domain plus the adjacent linker connecting helix Hc to the SNARE domain from KNOLLE. Our results suggest that targeting of syntaxin to the plane of cell division occurs without active sorting, whereas syntaxin-mediated membrane fusion requires sequence-specific features.

A cellular expression map of the Arabidopsis AUXIN RESPONSE FACTOR gene family

The plant hormone auxin triggers a wide range of developmental and growth responses throughout a plant's life. Most well-known auxin responses involve changes in gene expression that are mediated by a short pathway involving an auxin-receptor/ubiquitin-ligase, DNA-binding auxin response factor (ARF) transcription factors and their interacting auxin/indole-3-acetic acid (Aux/IAA) transcriptional inhibitors. Auxin promotes the degradation of Aux/IAA proteins through the auxin receptor and hence releases the inhibition of ARF transcription factors. Although this generic mechanism is now well understood, it is still unclear how developmental specificity is generated and how individual gene family members of response components contribute to local auxin responses. We have established a collection of transcriptional reporters for the ARF gene family and used these to generate a map of expression during embryogenesis and in the primary root meristem. Our results demonstrate that transcriptional regulation of ARF genes generates a complex pattern of overlapping activities. Genetic analysis shows that functions of co-expressed ARFs converge on the same biological processes, but can act either antagonistically or synergistically. Importantly, the existence of an 'ARF pre-pattern' could explain how cell-type-specific auxin responses are generated. Furthermore, this resource can now be used to probe the functions of ARF in other auxin-dependent processes.

Arabidopsis R-SNARE proteins VAMP721 and VAMP722 are required for cell plate formation

Background

Cell plate formation during plant cytokinesis is facilitated by SNARE complex-mediated vesicle fusion at the cell-division plane. However, our knowledge regarding R-SNARE components of membrane fusion machinery for cell plate formation remains quite limited.

Methodology/Principal Findings

We report the in vivo function of Arabidopsis VAMP721 and VAMP722, two closely sequence-related R-SNAREs, in cell plate formation. Double homozygous vamp721vamp722 mutant seedlings showed lethal dwarf phenotypes and were characterized by rudimentary roots, cotyledons and hypocotyls. Furthermore, cell wall stubs and incomplete cytokinesis were frequently observed in vamp721vamp722 seedlings. Confocal images revealed that green fluorescent protein-tagged VAMP721 and VAMP722 were preferentially localized to the expanding cell plates in dividing cells. Drug treatments and co-localization analyses demonstrated that punctuate organelles labeled with VAMP721 and VAMP722 represented early endosomes overlapped with VHA-a1-labeled TGN, which were distinct from Golgi stacks and prevacuolar compartments. In addition, protein traffic to the plasma membrane, but not to the vacuole, was severely disrupted in vamp721vamp722 seedlings by subcellular localization of marker proteins.

Conclusion/Significance

These observations suggest that VAMP721 and VAMP722 are involved in secretory trafficking to the plasma membrane via TGN/early endosomal compartment, which contributes substantially to cell plate formation during plant cytokinesis.

A specific role for Arabidopsis TRAPPII in post-Golgi trafficking that is crucial for cytokinesis and cell polarity

Cytokinesis and cell polarity are supported by membrane trafficking from the trans-Golgi network (TGN), but the molecular mechanisms that promote membrane trafficking from the TGN are poorly defined in plant cells. Here we show that TRAPPII in Arabidopsis regulates the post-Golgi trafficking that is crucial for assembly of the cell plate and cell polarity. Disruptions of AtTRS120 or AtTRS130, two genes encoding two key subunits of TRAPPII, result in defective cytokinesis and cell polarity in embryogenesis and seedling development. In attrs120 and attrs130, the organization and trafficking in the endoplasmic reticulum (ER)-Golgi interface are normal. However, post-Golgi trafficking to the cell plate and to the cell wall, but not to the vacuole, is impaired. Furthermore, TRAPPII is required for the selective transport of PIN2, but not PIN1, to the plasma membrane. We revealed that AtTRS130 is co-localized with RAB-A1c. Expression of constitutively active RAB-A1c partially rescues attrs130. RAB-A1c, which resides at the TGN, is delocalized to the cytosol in attrs130. We propose that TRAPPII in Arabidopsis acts upstream of Rab-A GTPases in post-Golgi membrane trafficking in plant cells.

Mutations in MYB3R1 and MYB3R4 cause pleiotropic developmental defects and preferential down-regulation of multiple G2/M-specific genes in Arabidopsis

R1R2R3-Myb proteins represent an evolutionarily conserved class of Myb family proteins important for cell cycle regulation and differentiation in eukaryotic cells. In plants, this class of Myb proteins are believed to regulate the transcription of G2/M phase-specific genes by binding to common cis-elements, called mitosis-specific activator (MSA) elements. In Arabidopsis (Arabidopsis thaliana), MYB3R1 and MYB3R4 act as transcriptional activators and positively regulate cytokinesis by activating the transcription of KNOLLE, which encodes a cytokinesis-specific syntaxin. Here, we show that the double mutation myb3r1 myb3r4 causes pleiotropic developmental defects, some of which are due to deficiency of KNOLLE whereas other are not, suggesting that multiple target genes are involved. Consistently, microarray analysis of the double mutant revealed altered expression of many genes, among which G2/M-specific genes showed significant overrepresentation of the MSA motif and a strong tendency to be down-regulated by the double mutation. Our results demonstrate, on a genome-wide level, the importance of the MYB3R-MSA pathway for regulating G2/M-specific transcription. In addition, MYB3R1 and MYB3R4 may have diverse roles during plant development by regulating G2/M-specific genes with various functions as well as genes possibly unrelated to the cell cycle.

Very-long-chain fatty acids are required for cell plate formation during cytokinesis in Arabidopsis thaliana

Acyl chain length is thought to be crucial for biophysical properties of the membrane, in particular during cell division, when active vesicular fusion is necessary. In higher plants, the process of cytokinesis is unique, because the separation of the two daughter cells is carried out by de novo vesicular fusion to generate a laterally expanding cell plate. In Arabidopsis thaliana, very-long-chain fatty acid (VLCFA) depletion caused by a mutation in the microsomal elongase gene PASTICCINO2 (PAS2) or by application of the selective elongase inhibitor flufenacet altered cytokinesis. Cell plate expansion was delayed and the formation of the endomembrane tubular network altered. These defects were associated with specific aggregation of the cell plate markers YFP-Rab-A2a and KNOLLE during cytokinesis. Changes in levels of VLCFA also resulted in modification of endocytosis and sensitivity to brefeldin A. Finally, the cytokinesis impairment in pas2 cells was associated with reduced levels of very long fatty acyl chains in phospholipids. Together, our findings demonstrate that VLCFA-containing lipids are essential for endomembrane dynamics during cytokinesis.

Interaction of antiparallel microtubules in the phragmoplast is mediated by the microtubule-associated protein MAP65-3 in Arabidopsis

In plant cells, microtubules (MTs) in the cytokinetic apparatus phragmoplast exhibit an antiparallel array and transport Golgi-derived vesicles toward MT plus ends located at or near the division site. By transmission electron microscopy, we observed that certain antiparallel phragmoplast MTs overlapped and were bridged by electron-dense materials in Arabidopsis thaliana. Robust MT polymerization, reported by fluorescently tagged End Binding1c (EB1c), took place in the phragmoplast midline. The engagement of antiparallel MTs in the central spindle and phragmoplast was largely abolished in mutant cells lacking the MT-associated protein, MAP65-3. We found that endogenous MAP65-3 was selectively detected on the middle segments of the central spindle MTs at late anaphase. When MTs exhibited a bipolar appearance with their plus ends placed in the middle, MAP65-3 exclusively decorated the phragmoplast midline. A bacterially expressed MAP65-3 protein was able to establish the interdigitation of MTs in vitro. MAP65-3 interacted with antiparallel microtubules before motor Kinesin-12 did during the establishment of the phragmoplast MT array. Thus, MAP65-3 selectively cross-linked interdigitating MTs (IMTs) to allow antiparallel MTs to be closely engaged in the phragmoplast. Although the presence of IMTs was not essential for vesicle trafficking, they were required for the phragmoplast-specific motors Kinesin-12 and Phragmoplast-Associated Kinesin-Related Protein2 to interact with MT plus ends. In conclusion, we suggest that the phragmoplast contains IMTs and highly dynamic noninterdigitating MTs, which work in concert to bring about cytokinesis in plant cells.