Plant cell divisions: variations from the shortest symmetric path.

In plants, the spatial arrangement of cells within tissues and organs is a direct consequence of the positioning of the new cell walls during cell division. Since the nineteenth century, scientists have proposed rules to explain the orientation of plant cell divisions. Most of these rules predict the new wall will follow the shortest path passing through the cell centroid halving the cell into two equal volumes. However, in some developmental contexts, divisions deviate significantly from this rule. In these situations, mechanical stress, hormonal signalling, or cell polarity have been described to influence the division path. Here we discuss the mechanism and subcellular structure required to define the cell division placement then we provide an overview of the situations where division deviates from the shortest symmetric path

Possible origins of macroscopic left-right asymmetry in organisms

I consider the microscopic mechanisms by which a particular left-right (L/R) asymmetry is generated at the organism level from the microscopic handedness of cytoskeletal molecules. In light of a fundamental symmetry principle, the typical pattern-formation mechanisms of diffusion plus regulation cannot implement the "right-hand rule"; at the microscopic level, the cell's cytoskeleton of chiral filaments seems always to be involved, usually in collective states driven by polymerization forces or molecular motors. It seems particularly easy for handedness to emerge in a shear or rotation in the background of an effectively two-dimensional system, such as the cell membrane or a layer of cells, as this requires no pre-existing axis apart from the layer normal. I detail a scenario involving actin/myosin layers in snails and in C. elegans, and also one about the microtubule layer in plant cells. I also survey the other examples that I am aware of, such as the emergence of handedness such as the emergence of handedness in neurons, in eukaryote cell motility, and in non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue. Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec

Mechanobiology of cell division in plant growth.

Cell division in plants is particularly important as cells cannot rearrange. It therefore determines the arrangement of cells (topology) and their size and shape (geometry). Cell division reduces mechanical stress locally by producing smaller cells and alters mechanical properties by reinforcing the mechanical wall network, both of which can alter overall tissue morphology. Division orientation is often regarded as following geometric rules, however recent work has suggested that divisions align with the direction of maximal tensile stress. Mechanical stress has already been shown to feed into many processes of development including those that alter mechanical properties. Such an alignment may enable cell division to selectively reinforce the cell wall network in the direction of maximal tensile stress. Therefore there exists potential feedback between cell division, mechanical stress and growth. Improving our understanding of this topic will help to shed light on the debated role of cell division in organ scale growth

Cellular reorganization in auxin-dependent pattern formation during early embryogenesis in Arabidopsis thaliana

A fundamental question in developmental biology is how the complex cellular pattern in multicellular organisms arises from a single cell. In land plants, the biosynthesis, transport, and signaling of phytohormone auxin is essential for pattern formation in embryogenesis. In Chapter 1, a brief introduction on plant embryogenesis, the roles of auxin signaling in pattern formation in early embryo, cellular basis on oriented cell division, and auxin-regulated oriented cell division during early embryogenesis are described as the foundation of this thesis aimed to answer the domain and cellular structures regulated by auxin that lead precise pattern formation during early embryo development of Arabidopsis thaliana. In Chapter 2, two novel fluorescent protein-based reporters for auxin perception and response, respectively, were developed to overcome technical bottlenecks for dissecting auxin signaling in embryos. The novel reporters offer higher sensitivity and responsiveness compared to existing tools. Our reporters revealed the gradients and maxima of auxin perception and response that had been hypothesized, but not yet detected. In addition, these new tools now offer a wider scope of application beyond the embryo and are generic tools for the auxin biology research community. In Chapter 3, the auxin reporters described in the previous chapter were improved to overcome their limitations, and the first comprehensive auxin reporter that was able to simultaneously visualize both auxin perception and response was characterized. With this new auxin reporter, the differential auxin signaling capacity between different cell types and differentiation states was demonstrated. In addition, the reporter for auxin response described in the previous chapter was applied in mutant embryos with a local auxin response defect, revealing its broad impact on auxin output. In Chapter 4, a toolkit of fluorescent protein-based markers labeling specific cellular structures was established. The structures included the plasma membrane, cytoskeletons, organelles, and the nucleus, structures excepted to participate in the oriented cell divisions that shape the early embryo. Expression of the protein markers was optimized for early Arabidopsis embryos, and topologies of subcellular structures were mapped during cellular reorganization in early embryogenesis. In addition, a specialized imaging technique was developed to allow high-resolution 3-dimensional imaging within the special embryo geometry. Combining the embryo-specific cellular structure marker set and the optimized imaging approach, 3-dimentional imaging of cellular structures in early embryos was achieved, and the dynamic organizations of organelles and cytoskeletons along with the unexpected discovery of early establishment of central/peripheral polarity in early embryos are described in this chapter. In Chapter 5, part of the toolkit established in the previous chapter was applied to embryos with inducible suppression on auxin response. Previously, it was shown that suppression of auxin response leads to divisions that follow only the cellular geometry, while auxin response allows cells to divide asymmetrically by deviating from this mode. It was unclear if and how the cytoskeleton mediates this auxin output, which was tested by visualizing the effect of auxin response on cytoskeleton organization. Distinct effects on both actin and microtubule properties were identified, and this provides an indication for further investigation into the biochemical and biomechanical mechanisms of pattern formation. In Chapter 6, the discoveries described in this thesis are placed in a broader context and discussed along with the latest technological and scientific advances related to the topic to offer future perspective in understanding the mechanisms underlying pattern formation.</p

Příspěvek rodičovských genomů ke genové expresi během pohlavního rozmnožování rostlin

Pohlavní rozmnožování je důležitým procesem vzniku semen u krytosemenných rostlin. Embryo se vyvíjí v semeni spolu s triploidním pletivem - endospermem. U živočichů je časný vývoj embrya řízen mateřskou RNA a proteiny uloženými ve vajíčku před oplozením. Mateřský a otcovský genom nemají zaměnitelné role ani po aktivaci genomu zygoty. Některé geny ovlivňující růst a vývoj embrya jsou přepisovány jenom z maternálně nebo paternálně zděděné alely. To je příkladem fenoménu genového imprintingu. U rostlin je imprinting dobře znám z endospermu. Rodičovský příspěvek ke genové expresi v embryu byl studován méně intenzivně. Tato práce si klade za cíl kriticky zhodnotit aktuálně dostupné informace o rodičovském příspěvku ve vývoji embrya u různých druhů rostlin.Sexual reproduction in flowering plants is fundamental to seed formation. After fertilization, the embryo is enclosed and develops in a seed together with a triploid tissue - the endosperm. In animals, early embryogenesis is well-known to be controlled by maternal RNA and proteins deposited in the ovule before fertilization. Even after the activation of zygotic genome, paternal and maternal genome do not play interchangeable roles. A few genes affecting embryo growth and development are transcribed either from maternally or paternally inherited alleles only. Such genes are example of the well-known phenomenon of gene imprinting. In plants, imprinting is well documented in the endosperm. The role of parent-of-origin contributions to gene expression has been studied less extensively in embryo. The aim of this work is to critically assess current knowledge of parent-of-origin contributions to embryo development in different plant species.Department of Experimental Plant BiologyKatedra experimentální biologie rostlinFaculty of SciencePřírodovědecká fakult

A Task for Molecular Motors: Maintaining the Cortical Division Zone in Arabidopsis thaliana

Plant cells are confined and immobilized by rigid cell walls, which determine their shape and location within tissues. Therefore plant cells demand specific spatial control over cell division and have evolved unique cytoskeletal structures, which aid in coping with these spatial demands. Prior to mitosis a prominent but transient band of microtubules and actin filaments, called the preprophase band (PPB) marks the site where the future cell plate will fuse with the parental plasma membrane. Thus, the PPB is a faithful predictor of the division plane. PPBs spatial information is preserved after its disassembly by the cortical division zone (CDZ), which provides guidance for localized cell plate fusion. Cell plate formation by vesicle fusion initiates in the center of the cell and is aided by the plant specific cytokinetic apparatus, the phragmoplast. Yet only little is known about the molecular identity of the CDZ and how it attracts the phragmoplast and cell plate. A pair of kinesin-12 class motor proteins, PHRAGMOPLAST ORIENTING KINESIN 1 (POK1) and POK2, is essential for the spatial control of cytokinesis. Here we report that a functional full length POK1 fusion protein (YFP-POK1) is dynamically recruited to the PPB and permanently resides at the CDZ until cell plate fusion takes place. In vivo analysis of phragmoplast expansion in pok1 pok2 double mutants revealed a PPB – phragmoplast misalignment caused by phragmoplast tilting, which results in cell wall mis-positioning. Moreover, maintenance of the CDZ identity marker TANGLED is dependent on POK function, suggesting that POK motor proteins might act as a scaffold to retain CDZ identity markers at the plasma membrane, thus maintaining the molecular memory of the division plane. The second project intends to elucidate the effects of a specific posttranslational tubulin modification (PTM) on microtubule organization and CDZ establishment in Arabidopsis thaliana. We demonstrate, that low concentrations of NO2Tyr, which is incorporated into the C-terminus of α-tubulin, are not detrimental for plant health. However the NO2Tyr treatment affects the organization of the cortical microtubule array, resulting in non-polar cell expansion and induces oblique cell wall integration. The results indicate that PTM of α-tubulin might be important for microtubule organization and for division plane establishment during plant development

What determines cell size?

AbstractFirst paragraph (this article has no abstract) For well over 100 years, cell biologists have been wondering what determines the size of cells. In modern times, we know all of the molecules that control the cell cycle and cell division, but we still do not understand how cell size is determined. To check whether modern cell biology has made any inroads on this age-old question, BMC Biology asked several heavyweights in the field to tell us how they think cell size is controlled, drawing on a range of different cell types. The essays in this collection address two related questions - why does cell size matter, and how do cells control it

What is quantitative plant biology?

Quantitative plant biology is an interdisciplinary field that builds on a long history of biomathematics and biophysics. Today, thanks to high spatiotemporal resolution tools and computational modelling, it sets a new standard in plant science. Acquired data, whether molecular, geometric or mechanical, are quantified, statistically assessed and integrated at multiple scales and across fields. They feed testable predictions that, in turn, guide further experimental tests. Quantitative features such as variability, noise, robustness, delays or feedback loops are included to account for the inner dynamics of plants and their interactions with the environment. Here, we present the main features of this ongoing revolution, through new questions around signalling networks, tissue topology, shape plasticity, biomechanics, bioenergetics, ecology and engineering. In the end, quantitative plant biology allows us to question and better understand our interactions with plants. In turn, this field opens the door to transdisciplinary projects with the society, notably through citizen science.Peer reviewe