ZYP1-mediated recruitment of PCH2 to the synaptonemal complex remodels the chromosome axis leading to crossover restriction.

Chromosome axis-associated HORMA domain proteins (HORMADs), e.g. ASY1 in Arabidopsis, are crucial for meiotic recombination. ASY1, as other HORMADs, is assembled on the axis at early meiosis and depleted when homologous chromosomes synapse. Puzzlingly, both processes are catalyzed by AAA+ ATPase PCH2 together with its cofactor COMET. Here, we show that the ASY1 remodeling complex is temporally and spatially differently assembled. While PCH2 and COMET appear to directly interact in the cytoplasm in early meiosis, PCH2 is recruited by the transverse filament protein ZYP1 and brought to the ASY1-bound COMET assuring the timely removal of ASY1 during chromosome synapsis. Since we found that the PCH2 homolog TRIP13 also binds to the ZYP1 homolog SYCP1 in mouse, we postulate that this mechanism is conserved among eukaryotes. Deleting the PCH2 binding site of ZYP1 led to a failure of ASY1 removal. Interestingly, the placement of one obligatory crossover per homologous chromosome pair, compromised by ZYP1 depletion, is largely restored in this separation-of-function zyp1 allele suggesting that crossover assurance is promoted by synapsis. In contrast, this zyp1 allele, similar to the zyp1 null mutant, showed elevated type I crossover numbers indicating that PCH2-mediated eviction of ASY1 from the axis restricts crossover formation.

Non-cell-autonomous regulation of petal initiation in Arabidopsis thaliana.

In many flowering plants, petals initiate in alternate positions from first whorl sepals, suggesting possible signaling between sepal boundaries and petal initiation sites. PETAL LOSS (PTL) and RABBIT EARS (RBE) regulate petal initiation in Arabidopsis thaliana and their transcripts are expressed in sepal boundary and petal initiation sites, respectively, suggesting that PTL acts in a non-cell-autonomous manner. Here, we determined that cells expressing PTL and RBE fusion proteins did not overlap but were adjacent, confirming the non-cell-autonomous function of PTL. Genetic ablation of intersepal cells by expressing the diphtheria toxin-A chain gene driven by the PTL promoter resulted in flowers lacking petals, suggesting these cells are required for petal initiation. Transcriptome analysis combined with a PTL induction system revealed 42 genes that were upregulated under PTL activation, including UNUSUAL FLORAL ORGANS (UFO), which likely plays an important role in petal initiation. These findings suggest a molecular mechanism in which PTL indirectly regulates petal initiation and UFO mediates positional signaling between the sepal boundary and petal initiation sites.

Caught in the Act: Live-Cell Imaging of Plant Meiosis.

Live-cell imaging is a powerful method to obtain insights into cellular processes, particularly with respect to their dynamics. This is especially true for meiosis, where chromosomes and other cellular components such as the cytoskeleton follow an elaborate choreography over a relatively short period of time. Making these dynamics visible expands understanding of the regulation of meiosis and its underlying molecular forces. However, the analysis of meiosis by live-cell imaging is challenging; specifically in plants, a temporally resolved understanding of chromosome segregation and recombination events is lacking. Recent advances in live-cell imaging now allow the analysis of meiotic events in plants in real time. These new microscopy methods rely on the generation of reporter lines for meiotic regulators and on the establishment of ex vivo culture and imaging conditions, which stabilize the specimen and keep it alive for several hours or even days. In this review, we combine an overview of the technical aspects of live-cell imaging in plants with a summary of outstanding questions that can now be addressed to promote live-cell imaging in Arabidopsis and other plant species and stimulate ideas on the topics that can be addressed in the context of plant meiotic recombination.

The inhibitory effect of canine interferon gamma on the growth of canine tumors.

Recombinant canine interferon-γ (rc-IFNγ; InterdogⓇ) was exclusively approved as a therapeutic for canine atopic dermatitis. However, it has been used off-label for the treatment of canine cancer. We examined the inhibitory effect of rc-IFNγ on the growth of canine tumor cell lines and analyzed its mechanism of action. Three (CTB-p, CTB-m, and CNM-m) out of seven mammary gland tumor cell lines and two (VIMC and CoMS) out of four mast cell tumor cell lines showed remarkable growth inhibition after treatment with rc-IFNγ. However, one (CLBL-1) out of nine lymphoma cell lines showed a significant amount of cell death. Using CTB-p and CTB-m cell lines, we showed that STAT1 was essential for inducing the growth inhibitory effect of rc-IFNγ. Although rc-IFNγ induced G1 growth arrest in CTB-p cell line, treatment with rc-IFNγ did not alter the expression of cell cycle regulatory proteins. In this study, we observed direct cytotoxicity or cytostatic effects of rc-IFNγ in canine tumor cell lines. However, the detailed mechanisms responsible for these effects need to be elucidated in the future.

CDKD-dependent activation of CDKA;1 controls microtubule dynamics and cytokinesis during meiosis.

Precise control of cytoskeleton dynamics and its tight coordination with chromosomal events are key to cell division. This is exemplified by formation of the spindle and execution of cytokinesis after nuclear division. Here, we reveal that the central cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CDKA;1), the Arabidopsis homologue of Cdk1 and Cdk2, partially in conjunction with CYCLIN B3;1 (CYCB3;1), is a key regulator of the microtubule cytoskeleton in meiosis. For full CDKA;1 activity, the function of three redundantly acting CDK-activating kinases (CAKs), CDKD;1, CDKD;2, and CDKD;3, is necessary. Progressive loss of these genes in combination with a weak loss-of-function mutant in CDKA;1 allowed a fine-grained dissection of the requirement of cell-cycle kinase activity for meiosis. Notably, a moderate reduction of CDKA;1 activity converts the simultaneous cytokinesis in Arabidopsis, i.e., one cytokinesis separating all four meiotic products concurrently into two successive cytokineses with cell wall formation after the first and second meiotic division, as found in many monocotyledonous species.

Functional Analysis of the Plant Chromosomal Passenger Complex.

The Aurora B kinase, encoded by the AURORA 3 (AUR3) gene in Arabidopsis (Arabidopsis thaliana), is a key regulator of cell division in all eukaryotes. Aurora B has at least two central functions during cell division; it is essential for the correct, i.e. balanced, segregation of chromosomes in mitosis and meiosis by controlling kinetochore function, and it acts at the division plane, where it is necessary to complete cytokinesis. To accomplish these two spatially distinct functions, Aurora B in animals is guided to its sites of action by Borealin, inner centromere protein (INCENP), and Survivin, which, together with Aurora B, form the chromosome passenger complex (CPC). However, besides Aurora homologs, only a candidate gene with restricted homology to INCENP has been described in Arabidopsis, raising the question of whether a full complement of the CPC exists in plants and how Aurora homologs are targeted subcellularly. Here, we have identified and functionally characterized a Borealin homolog, BOREALIN RELATED (BORR), in Arabidopsis. Together with detailed localization studies including the putative Arabidopsis INCENP homolog, these results support the existence of a CPC in plants.

The Arabidopsis Cdk1/Cdk2 homolog CDKA;1 controls chromosome axis assembly during plant meiosis.

Meiosis is key to sexual reproduction and genetic diversity. Here, we show that the Arabidopsis cyclin-dependent kinase Cdk1/Cdk2 homolog CDKA;1 is an important regulator of meiosis needed for several aspects of meiosis such as chromosome synapsis. We identify the chromosome axis protein ASYNAPTIC 1 (ASY1), the Arabidopsis homolog of Hop1 (homolog pairing 1), essential for synaptonemal complex formation, as a target of CDKA;1. The phosphorylation of ASY1 is required for its recruitment to the chromosome axis via ASYNAPTIC 3 (ASY3), the Arabidopsis reductional division 1 (Red1) homolog, counteracting the disassembly activity of the AAA+ ATPase PACHYTENE CHECKPOINT 2 (PCH2). Furthermore, we have identified the closure motif in ASY1, typical for HORMA domain proteins, and provide evidence that the phosphorylation of ASY1 regulates the putative self-polymerization of ASY1 along the chromosome axis. Hence, the phosphorylation of ASY1 by CDKA;1 appears to be a two-pronged mechanism to initiate chromosome axis formation in meiosis.

A Practical Guide to Live-Cell Imaging of Meiosis in Arabidopsis.

Plants are powerful model systems to study meiosis. Our knowledge about the cytology of plant meiosis is mainly based on the analysis of fixed material. Although highly informative, this approach is limited in understanding the dynamics of meiosis. Here, we present a step-by-step instruction for a newly developed method to follow meiosis in male meiocytes of Arabidopsis in real time by confocal laser scanning microscopy. We envision that this method can be easily translated to other plant species and especially crops (e.g., Brassica, maize, and potato).

SWITCH 1/DYAD is a WINGS APART-LIKE antagonist that maintains sister chromatid cohesion in meiosis.

Mitosis and meiosis both rely on cohesin, which embraces the sister chromatids and plays a crucial role for the faithful distribution of chromosomes to daughter cells. Prior to the cleavage by Separase at anaphase onset, cohesin is largely removed from chromosomes by the non-proteolytic action of WINGS APART-LIKE (WAPL), a mechanism referred to as the prophase pathway. To prevent the premature loss of sister chromatid cohesion, WAPL is inhibited in early mitosis by Sororin. However, Sororin homologs have only been found to function as WAPL inhibitors during mitosis in vertebrates and Drosophila. Here we show that SWITCH 1/DYAD defines a WAPL antagonist that acts in meiosis of Arabidopsis. Crucially, SWI1 becomes dispensable for sister chromatid cohesion in the absence of WAPL. Despite the lack of any sequence similarities, we found that SWI1 is regulated and functions in a similar manner as Sororin hence likely representing a case of convergent molecular evolution across the eukaryotic kingdom.

Dynamic F-actin movement is essential for fertilization in Arabidopsis thaliana.

In animals, microtubules and centrosomes direct the migration of gamete pronuclei for fertilization. By contrast, flowering plants have lost essential components of the centrosome, raising the question of how flowering plants control gamete nuclei migration during fertilization. Here, we use Arabidopsis thaliana to document a novel mechanism that regulates F-actin dynamics in the female gametes and is essential for fertilization. Live imaging shows that F-actin structures assist the male nucleus during its migration towards the female nucleus. We identify a female gamete-specific Rho-GTPase that regulates F-actin dynamics and further show that actin-myosin interactions are also involved in male gamete nucleus migration. Genetic analyses and imaging indicate that microtubules are dispensable for migration and fusion of male and female gamete nuclei. The innovation of a novel actin-based mechanism of fertilization during plant evolution might account for the complete loss of the centrosome in flowering plants.

Live imaging of calcium spikes during double fertilization in Arabidopsis.

Ca(2+) waves and oscillation are key signalling elements during the fertilization process of animals, and are involved, for example, in egg activation. In the unique double fertilization process in flowering plants, both the egg cell and the neighbouring central cell fuse with a sperm cell each. Here we succeeded in imaging cytosolic Ca(2+) in these two cells, and in the two synergid cells that accompany the gametes during semi-in vivo double fertilization. Following pollen tube discharge and plasmogamy, the egg and central cells displayed transient Ca(2+) spikes, but not oscillations. Only the events in the egg cell correlated with the plasmogamy. In contrast, the synergid cells displayed Ca(2+) oscillations on pollen tube arrival. The two synergid cells showed distinct Ca(2+) dynamics depending on their respective roles in tube reception. These Ca(2+) dynamics in the female gametophyte seem to represent highly specific signatures that coordinate successful double fertilization in the flowering plants.

An EAR-Dependent Regulatory Module Promotes Male Germ Cell Division and Sperm Fertility in Arabidopsis.

The production of the sperm cells in angiosperms requires coordination of cell division and cell differentiation. In Arabidopsis thaliana, the germline-specific MYB protein DUO1 integrates these processes, but the regulatory hierarchy in which DUO1 functions is unknown. Here, we identify an essential role for two germline-specific DUO1 target genes, DAZ1 and DAZ2, which encode EAR motif-containing C2H2-type zinc finger proteins. We show that DAZ1/DAZ2 are required for germ cell division and for the proper accumulation of mitotic cyclins. Importantly, DAZ1/DAZ2 are sufficient to promote G2- to M-phase transition and germ cell division in the absence of DUO1. DAZ1/DAZ2 are also required for DUO1-dependent cell differentiation and are essential for gamete fusion at fertilization. We demonstrate that the two EAR motifs in DAZ1/DAZ2 mediate their function in the male germline and are required for transcriptional repression and for physical interaction with the corepressor TOPLESS. Our findings uncover an essential module in a regulatory hierarchy that drives mitotic transition in male germ cells and implicates gene repression pathways in sperm cell formation and fertility.

Ca2+-activated reactive oxygen species production by Arabidopsis RbohH and RbohJ is essential for proper pollen tube tip growth.

In flowering plants, pollen germinates on the stigma and pollen tubes grow through the style to fertilize the ovules. Enzymatic production of reactive oxygen species (ROS) has been suggested to be involved in pollen tube tip growth. Here, we characterized the function and regulation of the NADPH oxidases RbohH and RbohJ (Respiratory burst oxidase homolog H and J) in pollen tubes in Arabidopsis thaliana. In the rbohH and rbohJ single mutants, pollen tube tip growth was comparable to that of the wild type; however, tip growth was severely impaired in the double mutant. In vivo imaging showed that ROS accumulation in the pollen tube was impaired in the double mutant. Both RbohH and RbohJ, which contain Ca(2+) binding EF-hand motifs, possessed Ca(2+)-induced ROS-producing activity and localized at the plasma membrane of the pollen tube tip. Point mutations in the EF-hand motifs impaired Ca(2+)-induced ROS production and complementation of the double mutant phenotype. We also showed that a protein phosphatase inhibitor enhanced the Ca(2+)-induced ROS-producing activity of RbohH and RbohJ, suggesting their synergistic activation by protein phosphorylation and Ca(2+). Our results suggest that ROS production by RbohH and RbohJ is essential for proper pollen tube tip growth, and furthermore, that Ca(2+)-induced ROS positive feedback regulation is conserved in the polarized cell growth to shape the long tubular cell.

Spatial distribution of the RABBIT EARS protein and effects of its ectopic expression in Arabidopsis thaliana flowers.

In many flowering plants, flowers consist of two peripheral organs, sepals and petals, occurring in outer two whorls, and two inner reproductive organs, stamens and carpels. These organs are arranged in a concentric pattern in a floral meristem, and the organ identity is established by the combined action of floral homeotic genes expressed along the whorls. Floral organ primordia arise at fixed positions in the floral meristem within each whorl. The RABBIT EARS (RBE) gene is transcribed in the petal precursor cells and primordia, and regulates petal initiation and early growth in Arabidopsis thaliana. We investigated the spatial and temporal expression pattern of a RBE protein fused to the green fluorescent protein (GFP). Expression of the GFP:RBE fusion gene under the RBE cis-regulatory genomic fragment rescues the rbe petal defects, indicating that the fusion protein is functional. The GFP signal is located to the cells where RBE is transcribed, suggesting that RBE function is cell-autonomous. Ectopic expression of GFP:RBE under the APETALA1 promoter causes the homeotic conversion of floral organs, resulting in sterile flowers. In these plants, the class B homeotic genes APETALA3 and PISTILLATA are down-regulated, suggesting that the restriction of the RBE expression to the petal precursor cells is crucial for flower development.

The simplest integrated multicellular organism unveiled.

Volvocine green algae represent the "evolutionary time machine" model lineage for studying multicellularity, because they encompass the whole range of evolutionary transition of multicellularity from unicellular Chlamydomonas to >500-celled Volvox. Multicellular volvocalean species including Gonium pectorale and Volvox carteri generally have several common morphological features to survive as integrated multicellular organisms such as "rotational asymmetry of cells" so that the cells become components of the individual and "cytoplasmic bridges between protoplasts in developing embryos" to maintain the species-specific form of the multicellular individual before secretion of new extracellular matrix (ECM). However, these morphological features have not been studied in the four-celled colonial volvocine species Tetrabaena socialis that is positioned in the most basal lineage within the colonial or multicellular volvocine greens. Here we established synchronous cultures of T. socialis and carried out immunofluorescence microscopic and ultrastructural observations to elucidate these two morphological attributes. Based on immunofluorescence microscopy, four cells of the mature T. socialis colony were identical in morphology but had rotational asymmetry in arrangement of microtubular rootlets and separation of basal bodies like G. pectorale and V. carteri. Ultrastructural observations clearly confirmed the presence of cytoplasmic bridges between protoplasts in developing embryos of T. socialis even after the formation of new flagella in each daughter protoplast within the parental ECM. Therefore, these two morphological attributes might have evolved in the common four-celled ancestor of the colonial volvocine algae and contributed to the further increase in cell number and complexity of the multicellular individuals of this model lineage. T. socialis is one of the simplest integrated multicellular organisms in which four identical cells constitute the individual.

Septins promote dendrite and axon development by negatively regulating microtubule stability via HDAC6-mediated deacetylation.

Neurite growth requires two guanine nucleotide-binding protein polymers of tubulins and septins. However, whether and how those cytoskeletal systems are coordinated was unknown. Here we show that the acute knockdown or knockout of the pivotal septin subunit SEPT7 from cerebrocortical neurons impairs their interhemispheric and cerebrospinal axon projections and dendritogenesis in perinatal mice, when the microtubules are severely hyperacetylated. The resulting hyperstabilization and growth retardation of microtubules are demonstrated in vitro. The phenotypic similarity between SEPT7 depletion and the pharmacological inhibition of α-tubulin deacetylase HDAC6 reveals that HDAC6 requires SEPT7 not for its enzymatic activity, but to associate with acetylated α-tubulin. These and other findings indicate that septins provide a physical scaffold for HDAC6 to achieve efficient microtubule deacetylation, thereby negatively regulating microtubule stability to an optimal level for neuritogenesis. Our findings shed light on the mechanisms underlying the HDAC6-mediated coupling of the two ubiquitous cytoskeletal systems during neural development.

Independent control by each female gamete prevents the attraction of multiple pollen tubes.

In flowering plants, double fertilization is normally accomplished by the first pollen tube, with the fertilized ovule subsequently inhibiting the attraction of a second pollen tube. However, the mechanism of second-pollen-tube avoidance remains unknown. We discovered that failure to fertilize either the egg cell or the central cell compromised second-pollen-tube avoidance in Arabidopsis thaliana. A similar disturbance was caused by disrupting the fertilization-independent seed (FIS) class polycomb-repressive complex 2 (FIS-PRC2), a central cell- and endosperm-specific chromatin-modifying complex for gene silencing. Therefore, the two female gametes have evolved their own signaling pathways. Intriguingly, second-pollen-tube attraction induced by half-successful fertilization allowed the ovules to complete double fertilization, producing a genetically distinct embryo and endosperm. We thus propose that each female gamete independently determines second-pollen-tube avoidance to maximize reproductive fitness in flowering plants.

Live-cell analysis of plant reproduction: live-cell imaging, optical manipulation, and advanced microscopy technologies.

Sexual reproduction ensures propagation of species and enhances genetic diversity within populations. In flowering plants, sexual reproduction requires complicated and multi-step cell-to-cell communications among male and female cells. However, the confined nature of plant reproduction processes, which occur in the female reproductive organs and several cell layers of the pistil, limits our ability to observe these events in vivo. In this review, we discuss recent live-cell imaging in in vitro systems and the optical manipulation techniques that are used to capture the dynamic mechanisms representing molecular and cellular communications in sexual plant reproduction.

Fertilization recovery after defective sperm cell release in Arabidopsis.

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (∼80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.

Double fertilization on the move.

Double fertilization is a flowering plant mechanism whereby two immotile sperm cells fertilize two different female gametes. One of the two sperm cells fertilizes the egg cell to produce the embryo and the other fertilizes the central cell to produce the endosperm. Despite the biological and agricultural significance of double fertilization, the mechanism remains largely unknown owing to difficulties associated with the embedded structure of female gametes in the maternal tissue. However, molecular genetic approaches combined with novel live-cell imaging techniques have begun to clarify the actual behavior of the sperm cells, which is different from that described by previous hypotheses. In this review article, we discuss the mechanism of double fertilization based on the dynamics of the two sperm cells in Arabidopsis.