Control of oriented cell division in the Arabidopsis embryo

Comparative Embryogenesis in Angiosperms: Activation and Patterning of Embryonic Cell Lineages

Following fertilization in flowering plants (angiosperms), egg and sperm cells unite to form the zygote, which generates an entire new organism through a process called embryogenesis. In this review, we provide a comparative perspective on early zygotic embryogenesis in flowering plants by using the Poaceae maize and rice as monocot grass and crop models as well as Arabidopsis as a eudicot model of the Brassicaceae family. Beginning with the activation of the egg cell, we summarize and discuss the process of maternal-to-zygotic transition in plants, also taking recent work on parthenogenesis and haploid induction into consideration. Aspects like imprinting, which is mainly associated with endosperm development and somatic embryogenesis, are not considered. Controversial findings about the timing of zygotic genome activation as well as maternal versus paternal contribution to zygote and early embryo development are highlighted. The establishment of zygotic polarity, asymmetric division, and apical and basal cell lineages represents another chapter in which we also examine and compare the role of major signaling pathways, cell fate genes, and hormones in early embryogenesis. Except for the model Arabidopsis, little is known about embryopatterning and the establishment of the basic body plan in angiosperms. Using available in situ hybridization, RNA-sequencing, and marker data, we try to compare how and when stem cell niches are established. Finally, evolutionary aspects of plant embryo development are discussed.

A ClearSee-Based Clearing Protocol for 3D Visualization of Arabidopsis thaliana Embryos

Tissue clearing methods combined with confocal microscopy have been widely used for studying developmental biology. In plants, ClearSee is a reliable clearing method that is applicable to a wide range of tissues and is suitable for gene expression analysis using fluorescent reporters, but its application to the Arabidopsis thaliana embryo, a model system to study morphogenesis and pattern formation, has not been described in the original literature. Here, we describe a ClearSee-based clearing protocol which is suitable for obtaining 3D images of Arabidopsis thaliana embryos. The method consists of embryo dissection, fixation, washing, clearing, and cell wall staining and enables high-quality 3D imaging of embryo morphology and expression of fluorescent reporters with the cellular resolution. Our protocol provides a reliable method that is applicable to the analysis of morphogenesis and gene expression patterns in Arabidopsis thaliana embryos.

Emerging Functions for Cell Wall Polysaccharides Accumulated during Eudicot Seed Development

The formation of seeds is a reproductive strategy in higher plants that enables the dispersal of offspring through time and space. Eudicot seeds comprise three main components, the embryo, the endosperm and the seed coat, where the coordinated development of each is important for the correct formation of the mature seed. In addition, the seed coat protects the quiescent progeny and can provide transport mechanisms. A key underlying process in the production of seed tissues is the formation of an extracellular matrix termed the cell wall, which is well known for its essential function in cytokinesis, directional growth and morphogenesis. The cell wall is composed of a macromolecular network of polymers where the major component is polysaccharides. The attributes of polysaccharides differ with their composition and charge, which enables dynamic remodeling of the mechanical and physical properties of the matrix by adjusting their production, modification or turnover. Accordingly, the importance of specific polysaccharides or modifications is increasingly being associated with specialized functions within seed tissues, often through the spatio-temporal accumulation or remodeling of particular polymers. Here, we review the evolution and accumulation of polysaccharides during eudicot seed development, what is known of their impact on wall architecture and the diverse roles associated with these in different seed tissues.

A toolkit for studying cellular reorganization during early embryogenesis in Arabidopsis thaliana

Considerable progress has been made in understanding the influence of physical and genetic factors on the patterns of cell division in various model systems. However, how each of these factors directs changes in subcellular structures has remained unclear. Generic machineries for the execution of cell expansion and division have been characterized, but how these are influenced by genetic regulators and physical cell properties remains an open question. To a large degree, the complexity of growing post-embryonic tissues and a lack of precise predictability have prevented the extraction of rigid correlations between subcellular structures and future orientation of cell division. The Arabidopsis embryo offers an exquisitely predictable and simple model for studying such correlations, but so far the tools and methodology for studying subcellular structures in the early embryo have been lacking. Here, we describe a set of markers to visualize a range of subcellular structures in the early Arabidopsis embryo. We have designed a series of fluorescent cellular reporters optimized for embryos, and demonstrate the effectiveness of using these 'ACE' reporters with simple three-dimensional imaging procedures that preserve delicate cellular structures. We describe the ontogeny of subcellular structures in the early embryo and find that central/peripheral cell polarity is established much earlier than suspected. In addition, we show that the actin and microtubule cytoskeleton has distinct topologies in the embryo. These tools and methods will allow detailed analysis of the events of cellular reorganization that underlie morphogenesis in the Arabidopsis embryo.

Identification and Functional Characterization of a Microtubule-Associated Protein, GhCLASP2, From Upland Cotton (Gossypium hirsutum L.)

Cytoplasmic linker-associated proteins (CLASPs) are microtubule-associated proteins (MAPs) involved in regulation of dynamics of microtubules (MTs) that play an important role in plant growth and development. In this study, we identified cotton CLASP genes and investigated the function of GhCLASP2. GhCLASP2 was mainly expressed in stem and developing fibers, especially in fibers of the secondary cell wall deposition stage. Ectopic expression of GhCLASP2 in Arabidopsis increased the branching number of leaf trichomes and rescued the defective phenotypes of clasp-1. In cotton, overexpression of GhCLASP2 increased fiber strength, probably related to enhanced expression levels of tubulin, cellulose synthase, and expansin genes. Suppression of GhCLASP2 caused shorter internodes and semi-dwarfism, abnormal flower stigma, aborted anthers without pollen grains, and sterility. These changed phenotypes were similar to those observed in the Arabidopsis clasp-1 mutant. GhCLASP2 was co-localized with MTs according to transient experiment. These results suggest that GhCLASP2 functions similarly as AtCLASP, acting as a MAP and controlling cotton growth and development by regulating MTs.

Maternal control of embryogenesis by MPK6 and its upstream MKK4/MKK5 in Arabidopsis

In flowering plants, developing embryos reside in maternal sporophytes. It is known that maternal generation influences the development of next-generation embryos; however, little is known about the signaling components in the process. Previously, we demonstrated that Arabidopsis mitogen-activated protein kinase 6 (MPK6) and MPK3 play critical roles in plant reproduction. In addition, we noticed that a large fraction of seeds from mpk6 single-mutant plants showed a wrinkled seed coat or a burst-out embryo phenotype. Here, we report that these seed phenotypes can be traced back to defective embryogenesis. The defective embryos have shorter suspensors and reduced growth along the longitudinal axis. Furthermore, the cotyledons fail to bend over to progress to the bent-cotyledon stage. As a result of the uneven circumference along the axis, the seed coat wrinkles to develop raisin-like morphology after dehydration. In more severe cases, the embryo can be pushed out from the micropylar end, resulting in the burst-out embryo seed phenotype. Genetic analyses demonstrated that the defective embryogenesis of the mpk6 mutant is a maternal effect. Heterozygous or homozygous mpk6 embryos have defects only in mpk6 homozygous maternal plants, but not in wild-type or heterozygous maternal plants. The loss of function of MKK4/MKK5 also results in the same phenotypes, suggesting that MKK4/MKK5 might act upstream of MPK6 in this pathway. The maternal-mediated embryo defects are associated with changes in auxin activity maxima and PIN localization. In summary, this research demonstrates that the Arabidopsis MKK4/MKK5-MPK6 cascade is an important player in the maternal control of embryogenesis.

Regulation and evolution of the interaction of the seed B3 transcription factors with NF-Y subunits

The LAFL genes (LEC2, ABI3, FUS3, LEC1) encode transcription factors that regulate different aspects of seed development, from early to late embryogenesis and accumulation of storage compounds. These transcription factors form a complex network, with members able to interact with various other players to control the switch between embryo development and seed maturation and, at a later stage in the plant life cycle, between the mature seed and germination. In this review, we first summarize our current understanding of the role of each member in the network in the light of recent advances regarding their regulation and structure/function relationships. In a second part, we discuss new insights concerning the evolution of the LAFL genes to address the more specific question of the conservation of LEAFY COTYLEDONS 2 in both dicots and monocots and the putative origin of the network. Last we examine the current major limitations to current knowledge and future prospects to improve our understanding of this regulatory network.

Proper division plane orientation and mitotic progression together allow normal growth of maize

How growth, microtubule dynamics, and cell-cycle progression are coordinated is one of the unsolved mysteries of cell biology. A maize mutant, tangled1, with known defects in growth and proper division plane orientation, and a recently characterized cell-cycle delay identified by time-lapse imaging, was used to clarify the relationship between growth, cell cycle, and proper division plane orientation. The tangled1 mutant was fully rescued by introduction of cortical division site localized TANGLED1-YFP. A CYCLIN1B destruction box was fused to TANGLED1-YFP to generate a line that mostly rescued the division plane defect but still showed cell-cycle delays when expressed in the tangled1 mutant. Although an intermediate growth phenotype between wild-type and the tangled1 mutant was expected, these partially rescued plants grew as well as wild-type siblings, indicating that mitotic progression delays alone do not alter overall growth. These data indicate that division plane orientation, together with proper cell-cycle progression, is critical for plant growth.

Asymmetric zygote division: A mystery starting point of embryogenesis

In angiosperm, asymmetric zygote division is critical for embryogenesis. The molecular mechanism underlying this process has gained a great attention recently. Some players involve in the control of both accurate position and correct orientation of zygote division plane have been found, which provide useful clues for the extensive investigations. It is getting clear that both internal and external factors are involved in this complex regulatory mechanism and the asymmetric zygote division seems with great impact in cell fate determination and embryo pattern formation.

Genetic and hormonal control of vascular tissue proliferation

The plant vascular system develops from a handful of provascular initial cells in the early embryo into a whole range of different cell types in the mature plant. In order to account for such proliferation and to generate this kind of diversity, vascular tissue development relies on a large number of highly oriented cell divisions. Different hormonal and genetic pathways have been implicated in this process and several of these have been recently interconnected. Nevertheless, how such networks control the actual division plane orientation and how they interact with the generic cell cycle machinery to coordinate these divisions remains a major unanswered question.

Characterization of Actin Filament Dynamics during Mitosis in Wheat Protoplasts under UV-B Radiation

Enhanced ultraviolet-B (UV-B) radiation is caused by the thinning ozone and affects photosynthesis and crop yield. Recently, UV-B radiation has been considered as an environmental signal that regulates plant growth. Elucidating the downstream effectors in UV-B-triggered pathways is of particular interest. Previous studies have shown that actin filaments (AFs) play many roles during cell physiological processes. However, the underlying response of AFs to UV-B radiation remains unclear. In this study, wheat protoplasts were isolated from 7-d-old leaves. The dynamics of AFs during mitosis were observed under different treatments. The protoplasts were treated with UV-B radiation, cytochalasin B (CB) and jasplakinolide (JAS). Ph-FITC labelling results revealed typical actin filament structures in the control group; AFs were rearranged under UV-B radiation. AFs polymerized into bundles during interphase, the preprophase band (PPB) structure was destroyed during prophase, and the AFs gathered into plaques during metaphase in response to UV-B radiation. During anaphase and telophase, the distribution of AFs was dispersed. Pharmacologic experiments revealed that CB induced apoptosis and JAS induced nuclear division without cytokinesis in wheat protoplasts. These results indicated that AFs respond to UV-B radiation during mitosis, supplying evidence of UV-B signal transduction in plants.