Control of cell proliferation during plant development

Knowledge of the control of cell division in eukaryotes has increased tremendously in recent years. The isolation and characterization of the major players from a number of systems and the study of their interactions have led to a comprehensive understanding of how the different components of the cell cycle apparatus are brought together and assembled in a fine-tuned machinery. Many parts of this machine are highly conserved in organisms as evolutionary distant as yeast and animals. Some key regulators of cell division have also been identified in higher plants and have been shown to be functional homologues of the yeast or animal proteins. Although still in its early days, investigations into the regulation of these molecules have provided some clues on how cell division is coupled to plant development.     

Organelle fission in eukaryotes

The cellular machineries that power chloroplast and mitochondrial division in eukaryotes carry out the topologically challenging job of constricting and severing these double-membraned organelles. Consistent with their endosymbiotic origins, mitochondria in protists and chloroplasts in photosynthetic eukaryotes have evolved organelle-targeted forms of FtsZ, the prokaryotic ancestor of tubulin, as key components of their fission complexes. In fungi, animals and plants, mitochondria no longer utilize FtsZ for division, but several mitochondrial division proteins that localize to the outer membrane and intermembrane space, including two related to the filament-forming dynamins, have been identified in yeast and animals. Although the reactions that mediate organelle division are not yet understood, recent progress in uncovering the constituents of the organelle division machineries promises rapid advancement in our understanding of the biochemical mechanisms underlying the distinct but related processes of chloroplast and mitochondrial division in eukaryotes.     

Wie Hormone die Zellteilung der Pflanzen kontrollieren. Auxin und Cytokinin

Control of plant cell division by two groups of phytohormones, auxins and cytokinins, has been known for half a century. Only recently, the biochemical mechanisms driving and controlling cell division in plants became clearer and similarities as well as differences between the cell cycles of plants, fungi and animals promoted its understanding. Most important elements are protein kinases which require small and specific regulatory proteins, the so called cyclins, for activation. The phytohormones, in particular the cytokinins apparently control synthesis and degradation of these cyclins, as oscillations of their concentrations accompany or even promote the transitions from one cycle phase to the next. The biochemical model presented here presents some details of phytohormone action in plant cell division.     

Microtubule Associated Proteins in Plants and the Processes They Manage

Microtubule associated proteins （MAPs） are proteins that physically bind to microtubules in eukaryotes. MAPs play important roles in regulating the polymerization and organization of microtubules and in using the ensuing microtubule arrays to carry out a variety of cellular functions. In plants, MAPs manage the construction, repositioning, and dismantling of four distinct microtubule arrays throughout the cell cycle. Three of these arrays, the cortical array, the preprophase band, and the phragmoplast, are prominent to plants and are responsible for facilitating cell wall deposition and modification, transducing signals, demarcating the plane of cell division, and forming the new cell plate during cytokinesis. This review highlights important aspects of how MAPs in plants establish and maintain microtubule arrays as well as regulate cell growth, cell division, and cellular responses to the environment.     

The generation and modification of cell polarity

The generation of polarity in the cells of eukaryotes in most cases requires an external asymmetrical input. This signal may originate from a random event such as the point of penetration of an egg by a sperm or from asymmetries present in the maternal environment as, for example, in megasporogenesis in flowering plants, or from an asymmetry in the physical environment usually involving either light or gravity. The latter affords the best opportunities for experimental manipulation and single cell systems in algae, mosses and ferns allow both cell biological and genetical investigation of the mechanisms establishing polarity. These simple systems have already shown that the generation of polarity involves two distinguishable processes, axis alignment and axis orientation, but a detailed understanding of the mechanisms by which the environmental inputs are transduced to set up a polar axis is still lacking. Non-motile organisms must also be able to modify cell polarity in response to changed environmental inputs, and here too, lower plant systems afford the best prospect for understanding the mechanisms responsible.     

Deep phylogeny,ancestral groups and the four ages of life

Organismal phylogeny depends on cell division, stasis, mutational divergence, cell mergers (by sex or symbiogenesis), lateral gene transfer and death. The tree of life is a useful metaphor for organismal genealogical history provided we recognize that branches sometimes fuse. Hennigian cladistics emphasizes only lineage splitting, ignoring most other major phylogenetic processes. Though methodologically useful it has been conceptually confusing and harmed taxonomy, especially in mistakenly opposing ancestral (paraphyletic) taxa. The history of life involved about 10 really major innovations in cell structure. In membrane topology, there were five successive kinds of cell: (i) negibacteria, with two bounding membranes, (ii) unibacteria, with one bounding and no internal membranes, (iii) eukaryotes with endomembranes and mitochondria, (iv) plants with chloroplasts and (v) finally, chromists with plastids inside the rough endoplasmic reticulum. Membrane chemistry divides negibacteria into the more advanced Glycobacteria (e.g. Cyanobacteria and Proteobacteria) with outer membrane lipolysaccharide and primitive Eobacteria without lipopolysaccharide (deserving intenser study). It also divides unibacteria into posibacteria, ancestors of eukaryotes, and archaebacteria—the sisters (not ancestors) of eukaryotes and the youngest bacterial phylum. Anaerobic eobacteria, oxygenic cyanobacteria, desiccation-resistant posibacteria and finally neomura (eukaryotes plus archaebacteria) successively transformed Earth. Accidents and organizational constraints are as important as adaptiveness in body plan evolution.     

Cell cycle control and plant morphogenesis: is there an essential link?

Plant morphogenesis has some interesting features that may have consequences for the regulation of cell division. In particular, the immobility of plant cells implies the necessity for highly accurate controls, in contrast with the flexibility of many developmental processes in animals. An important question in plant development concerns the status of the relationship between plant morphogenesis and cell division. In this review, we discuss the current knowledge of the molecular mechanisms controlling the plant cell cycle and how this could be differentially regulated during plant morphogenesis. The plant genes involved are homologous to those of other higher eukaryotes, suggesting a similar cell cycle machinery. A variety of mechanisms control these genes, reflecting the complexity of internal and environmental signals to which plants should respond. This intricate network requires an upstream control mechanism to function as a failsafe system and to govern cell division and growth to produce the correct plant shape. BioEssays 21:29–37, 1999. © 1999 John Wiley & Sons, Inc.     

Arabidopsis epigenetics: when RNA meets chromatin

Recent work in plants and other eukaryotes has uncovered a major role for RNA interference in silent chromatin formation. The heritability of the silent state through multiple cell division cycles and, in some instances, through meiosis is assured by epigenetic marks. In plants, transposable elements and transgenes provide striking examples of the stable inheritance of repressed states, and are characterized by dense DNA methylation and heterochromatin histone modifications. Arabidopsis is a useful higher eukaryotes model with which to explore the crossroads between silent chromatin and RNA interference both during development and in the genome-wide control of repeat elements.     

Triggering the cell cycle in plants

In essence, the mitotic cell cycle in eukaryotes involves the duplication and separation of chromosomes, coupled to the process of dividing one cell into two. Cytokinesis is therefore the culmination of a series of events that were triggered during G1 phase, and brings the daughter cells back to the starting position in G1 for another possible round of division. In all eukaryotes, progression through the cell cycle is controlled by cyclin-dependent kinases that bind to positive regulators called cyclins. This review explores some of the pathways that trigger the plant cell cycle, with emphasis on the G1 phase. Examples include signalling pathways involving glutathione and cellular redox potential, the possible existence of a G1 DNA-damage checkpoint, and the plant hormones auxin and cytokinin. Progress in understanding the link between cell proliferation, cell differentiation and the cell-cycle machinery in a developmental context is discussed.     

Microtubule dynamics: new surprises from an old MAP

Dis1/XMAP215 family microtubule-binding proteins are essential for cell division in animals, plants and fungi, suggesting a conserved cell-division mechanism used by all eukaryotes. Two new studies, however, reveal that different family members can have very different effects on microtubule dynamics.     

The coupling of cell growth to the cell cycle

The development of a complex multicellular organism requires a coordination of growth and cell division under the control of patterning mechanisms. Studies in yeast have pioneered our understanding of the relationship between growth and cell division. In recent years, many of the pathways that regulate growth in multicellular eukaryotes have been identified. This work has revealed interesting and unexpected relationships between mechanisms that regulate growth and the cell cycle machinery.     

植物丝裂原活化蛋白激酶级联信号转导通路研究进展

丝裂原活化蛋白激酶(MAPK)是酵母、动物和植物等真核生物中普遍存在和高度保守的一类信号转导通路,由MAPKKK、MAPKK和MAPK等3部分组成,在应对生物非生物胁迫、激素、细胞分裂调控及植物生长发育等过程中发挥重要作用。该文对近年来国内外有关MAPK级联通路的组成、在植株体内的生物学功能以及MAPK通路的失活进行了概述,旨在为今后MAPK通路介导的信号转导机制的研究提供参考依据。     

Feast or famine: autophagy control and engineering in eukaryotic cell culture

Autophagy is a degradative process playing a role in both cell death and cell survival. Its presence is well conserved both in lower and higher eukaryotes. Recent studies have shown that activation or inhibition of autophagy may be possible in biotechnologically important species including mammalian cells and filamentous fungi using both environmental manipulation and genetic engineering. As our understanding of the autophagic biochemical pathways increases and monitoring methods become more user-friendly, the potential exists to obtain an optimum level of autophagy. This may allow for maximum cell survival and production of proteins and other metabolites in these industrially important eukaryotes.     

New insights into cyclins, CDKs, and cell cycle control

Since their initial discovery in yeast, cyclin-dependent kinases have proven to be universal regulators of the cell cycle in all eukaryotes. In unicellular eukaryotes, cell cycle progression is principally governed by one catalytic subunit (cyclin-dependent kinase) that pairs with cell cycle-specific regulatory subunits known as cyclins. Progression through a specific phase of the cell cycle is under the control of a specific class of cyclin. Cell cycle control in multicellular eukaryotes has an additional layer of complexity, as multiple CDKs and cyclins are required. In this review, we will discuss recent advances in the area of cyclins and CDKs, with emphasis on the role of the mammalian proteins in cell cycle control at the cellular and at the organismal level. Many recent surprises have come to light recently as a result of genetic manipulation of cells and mice, and these findings suggest that our understanding of the intricacies of the cell cycle is still rudimentary at best.     

Regulating the reapers: activating metacaspases for programmed cell death

Research during the past two decades has revealed that specialized cysteine proteases act as conserved initiators or executioners for programmed cell death (PCD) in eukaryotes. Caspases were first identified as common regulators of PCD in metazoans, whereas the role of metacaspases (MCs) as regulators of cellular suicide in plants has only been shown genetically in the past several years. Together with recent biochemical and molecular characterizations of some of the representative MCs from different model systems, multiple mechanisms that can mediate the post-translational regulation of these proteases are beginning to emerge. Further elucidation of these regulatory pathways and definition of the downstream degradomes targeted by MCs should lead to a better understanding of cell death control in plants, protozoans, and fungi.     

FtsZ and the division of prokaryotic cells and organelles

Binary fission of many prokaryotes as well as some eukaryotic organelles depends on the FtsZ protein, which self-assembles into a membrane-associated ring structure early in the division process. FtsZ is homologous to tubulin, the building block of the microtubule cytoskeleton in eukaryotes. Recent advances in genomics and cell-imaging techniques have paved the way for the remarkable progress in our understanding of fission in bacteria and organelles.     

Mitogen-activated protein kinase cascades in plants: a new nomenclature

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes, including yeasts, animals and plants. These protein phosphorylation cascades link extracellular stimuli to a wide range of cellular responses. In plants, MAPK cascades are involved in responses to various biotic and abiotic stresses, hormones, cell division and developmental processes. Completion of the Arabidopsis genome-sequencing project has revealed the existence of 20 MAPKs, 10 MAPK kinases and 60 MAPK kinase kinases. Here, we propose a simplified nomenclature for Arabidopsis MAPKs and MAPK kinases that might also serve as a basis for standard annotation of these gene families in all plants.