Establishment of cell polarity during early plant development

Cell polarity in plants: when two do the same, it is not the same

In unicellular and multicellular organisms, cell polarity is essential for a wide range of biological processes. An important feature of cell polarity is the asymmetric distribution of proteins in or at the plasma membrane. In plants such polar localized proteins play various specific roles ranging from organizing cell morphogenesis, asymmetric cell division, pathogen defense, nutrient transport and establishment of hormone gradients for developmental patterning. Moreover, flexible respecification of cell polarities enables plants to adjust their physiology and development to environmental changes. Having evolved multicellularity independently and lacking major cell polarity mechanisms of animal cells, plants came up with alternative solutions to generate and respecify cell polarity as well as to regulate polar domains at the plasma membrane.

The secret to life is being different: asymmetric divisions in plant development

Asymmetric cell divisions (ACDs) are used to create organismal form and cellular diversity during plant development. In several embryonic and postembryonic contexts, genes that specify cell fates and networks that provide positional information have been identified. The cellular mechanisms that translate this information into a physically ACD, however, are still obscure. In this review we examine the cell polarization events that precede asymmetric divisions in plants. Using principles derived from studies of other organisms and from postmitotic polarity generation in plants, we endeavor to provide a framework of what is known, what is on the horizon and what is critically needed to develop a rigorous mechanistic understanding of ACDs in plants.

Cell polarity, auxin transport, and cytoskeleton-mediated division planes: who comes first?

In plants, cell polarity is an issue more recurring than in other systems, because plants, due to their adaptive and flexible development, often change cell polarity postembryonically according to intrinsic cues and demands of the environment. Recent findings on the directional movement of the plant signalling molecule auxin provide a unique connection between individual cell polarity and the establishment of polarity at the tissue, organ, and whole-plant levels. Decisions about the subcellular polar targeting of PIN auxin transport components determine the direction of auxin flow between cells and consequently mediate multiple developmental events. In addition, mutations or chemical interference with PIN-based auxin transport result in abnormal cell divisions. Thus, the complicated links between cell polarity establishment, auxin transport, cytoskeleton, and oriented cell divisions now begin to emerge. Here we review the available literature on the issues of cell polarity in both plants and animals to extend our understanding on the generation, maintenance, and transmission of cell polarity in plants.

In vitro somatic embryogenesis from cell suspension cultures of cowpea [Vigna unguiculata (L.) Walp]

We report, an efficient protocol for plantlet regeneration from the cell suspension cultures of cowpea through somatic embryogenesis. Primary leaf-derived, embryogenic calli initiated in MMS [MS salts (Murashige and Skoog 1962) with B5 (Gamborg et al. 1968) vitamins] medium containing 2,4-Dichlorophenoxyacetic acid (2,4-D), casein hydrolysate (CH), and L: -Glutamic acid-5-amide (Gln). Fast-growing embryogenic cell suspensions were established in 0.5 mg l(-1) 2,4-D, which resulted in the highest recovery of early stages of somatic embryos in liquid MMS medium. Embryo development was asynchronous and strongly influenced by the 2,4-D concentration. Mature monocotyledonary-stage somatic embryos were induced in liquid B5 medium containing 0.1 mg l(-1) 2,4-D, 20 mg l(-1) L: -Proline (Pro), 5 muM Abscisic acid (ABA), and 2% mannitol. B5 medium was found superior for the maturation of somatic embryos compared to MS and MMS media. The importance of duration (5 d) for effective maturation of somatic embryos is demonstrated. A reduction in the 2,4-D level in suspensions increased the somatic embryo induction and maturation with decreased abnormalities. Sucrose was found to be the best carbon source for callus induction while mannitol for embryo maturation and maltose for embryo germination. Extension of hypocotyls and complete development of plantlet was achieved in half-strength B5 medium supplemented with 3% maltose, 2500 mg l(-1) potassium nitrate, and 0.05 mg l(-1) thidiazuron (TDZ) with 32% regeneration frequency. Field-established plants were morphologically normal and fertile. This regeneration protocol assures a high frequency of embryo induction, maturation, and plantlet conversion.

Comparative analysis of early embryonic sunflower cDNA libraries

To gain information concerning cell functions and activities during sunflower embryogenesis, an expressed sequence tag (EST) approach was used to analyse gene expression in the early stages of sunflower embryos development. Confocal microscopy observations of whole-mounted embryos allowed us to identify precisely the major steps of the zygotic embryonic development. A time-course analysis was then employed to collect the embryonic material. Three cDNA libraries were constructed from microdissected embryos, and three other cDNA libraries were created using a classical day after pollination schedule. A total of 7106 ESTs were produced and assembled. The total number of putative different genes represents about 43.1 (3064 tentative contigs and singlets) of the analysed sequences. The unigenes that showed similarity to proteins with known or predicted functions (50.3) were classified into 15 different functional categories. The functional profiles were found to be quite similar for all studied embryo stages but statistical analysis revealed that successive and coordinate sets of genes are expressed at each embryonic stage. The analysis allowed us to identify abundant and differentially expressed genes at the early stages of embryos development as well as some putatively interesting genes, showing strong similarities with genes playing key roles in plant and animal embryogenesis. The data presented in this study not only provide a first global overview of the genes expression profile during sunflower embryogenesis but also represent an original and valuable tool for developmental genomics studies on exalbuminous dicots.

Sequence evaluation of four specific cDNA libraries for developmental genomics of sunflower

Four different cDNA libraries were constructed from sunflower protoplasts growing under embryogenic and non-embryogenic conditions: one standard library from each condition and two subtractive libraries in opposite sense. A total of 22,876 cDNA clones were obtained and 4800 ESTs were sequenced, giving rise to 2479 high quality ESTs representing an unigene set of 1502 sequences. This set was compared with ESTs represented in public databases using the programs BLASTN and BLASTX, and its members were classified according to putative function using the catalog in the Kyoto Encyclopedia of Genes and Genomes (KEGG). Some 33% of sequences failed to align with existing plant ESTs and therefore represent putative novel genes. The libraries show a low level of redundancy and, on average, 50% of the present ESTs have not been previously reported for sunflower. Several potentially interesting genes were identified, based on their homology with genes involved in animal zygotic division or plant embryogenesis. We also identified two ESTs that show significantly different levels of expression under embryogenic and non-embryogenic conditions. The libraries described here represent an original and valuable resource for the discovery of yet unknown genes putatively involved in dicot embryogenesis and improving our knowledge of the mechanisms involved in polarity acquisition by plant embryos.

Cell axiality and polarity in plants--adding pieces to the puzzle

Plant cell polarity is important for cellular function and multicellular development. Classical physiological and cell biological analyses identified cues that orient cell polarity and suggested molecules that translate a cue into intracellular asymmetry. A range of proteins that either mark or are involved in the establishment of a (polar) axis are now available, as are many relevant mutants. These tools are likely to facilitate a dissection of the molecular mechanisms behind cell and organ polarity in the near future.

Motile plant cell body: a "bug" within a "cage"

Analysis of the cytoskeleton in morphogenetically active plant cells allows us to propose a unified concept for the structural organization of eukaryotic cells. Their cytoarchitecture is determined by two principal structural complexes: nucleus-microtubule-based cell bodies ("bugs") and plasma-membrane-F-actin-based cell periphery complexes ("cages"). There are dynamic interactions between each of these entities in response to extracellular and intracellular signals. In the case of the cell body, these signals determine its polarization, rotation and migration. Interactions between cell body and cell periphery complexes determine cell growth polarity and morphogenesis throughout the eukaryotic kingdom.

Latrunculin B-induced plant dwarfism: Plant cell elongation is F-actin-dependent

Marine macrolides latrunculins are highly specific toxins which effectively depolymerize actin filaments (generally F-actin) in all eukaryotic cells. We show that latrunculin B is effective on diverse cell types in higher plants and describe the use of this drug in probing F-actin-dependent growth and in plant development-related processes. In contrast to other eukaryotic organisms, cell divisions occurs in plant cells devoid of all actin filaments. However, the alignment of the division planes is often distorted. In addition to cell division, postembryonic development and morphogenesis also continue in the absence of F-actin. These experimental data suggest that F-actin is of little importance in the morphogenesis of higher plants, and that plants can develop more or less normally without F-actin. In contrast, F-actin turns out to be essential for cell elongation. When latrunculin B was added during germination, morphologically normal Arabidopsis and rye seedlings developed but, as a result of the absence of cell elongation, these were stunted, resembling either genetic dwarfs or environmental bonsai plants. In conclusion, F-actin is essential for the plant cell elongation, while this F-actin-dependent cell elongation is not an essential feature of plant-specific developmental programs.

Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: random orientation of septa in spherical cells

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the cell wall inherited from the mother cell. The septum that divides the cylindrical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells also extend at their ends and form their septa at right angles to the longitudinal axis, their septal (division) planes lie parallel to those of the mother cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morphologically predetermined cell ends to establish poles for growth. We studied four mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a random distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicating that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcofluor staining and by the analysis of electron microscopic images. Freeze-substitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determination of a division plane that can separate the postmitotic nuclei. However, it seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesize that the shape of the cell indirectly affects the positioning of the septum by directing the extension of the spindle.

ASYMMETRIC CELL DIVISION IN PLANTS

Asymmetric cell divisions generate cells with different fates. In plants, where cells do not move relative to another cell, the specification and orientation of these divisions is an important mechanism to generate the overall cellular pattern during development. This review summarizes our knowledge of selected cases of asymmetric cell division in plants, in the context of recent insights into mechanisms underlying this process in bacteria, algae, yeast, and animals.

Digging out roots: pattern formation, cell division, and morphogenesis in plants

The analysis of plant development by genetic, molecular, and surgical approaches has accumulated a large body of data, and yet it remains a challenge to uncover the basic mechanisms that are operating. Early steps of development, when the zygote and its daughter cells organize the embryonic plant, are poorly understood despite considerable efforts toward the identification of relevant genes. Reported cases of genetic redundancy suggest that the difficulty in uncovering patterning genes may reflect overlapping gene activities. Our current knowledge on plant embryo development still leaves open whether mechanisms for axis formation and subsequent pattern formation are fundamentally different in animals and plants. Axis formation may follow the general principle of establishing a peripheral asymmetric cue and mobilizing the cytoskeleton toward this cue--in the case of plants possibly located in the cell wall--but the molecules involved may be entirely different. Embryonic pattern formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet. Pattern formation continues postembryonically in the meristem, and the flexibility of this process may be explained by a feed-forward system of patterning cues originating from more mature cells. Control of cell division and differentiation, which is important in the meristems--regions of continuous development--has been studied intensively and appears to involve short-range signaling and transmembrane receptor kinase activation. Finally, although high importance of control of cell division rates and planes for plant morphogenesis have been often inferred, recent genetic studies as well as comparative morphological data point to a less decisive role of cell division and to global controls of as yet unknown nature.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.