1,721,126 research outputs found
A model system to study the effects of elevated CO2 on the developmental physiology of roots: the use of Arabidopsis thaliana
No full textThree developmental changes were observed in the roots of Arabidopsis thaliana (Columbia) when shoots were exposed to elevated CO2. (i) The allometric coefficient, k, was enhanced significantly (P<0.001), (ii) primary root length and root extension rate were enhanced (P<0.001). Accelerated cortical cell expansion contributed to this effect and was associated with increased cell wall extensibility, measured as % plasticity. (iii) Lateral root formation and extension were also increased in elevated CO2 (P<0.05). These results illustrate that root growth and structure was altered following exposure to elevated CO2. The changes observed suggest that Arabidopsis provides a useful model which should, in future, be amenable to study using appropriate mutants allowing the genetic basis of the responses to be identified
Control of cell growth in root hairs
No full textDuring development plant cells acquire different shapes and often these shapes are correlated with their function. To understand how these cell shapes and forms develop, it is necessary to understand the underlying regulatory mechanisms of cell growth. The growth of root hair cells is a good example of cells undergoing localized cell expansion to acquire their final form. In Arabidopsis two bHLH transcription factors RHD6 and its target RSL4 have been identified as key regulators of the initiation and elongation phases of root hair (H) cell development. However in order to obtain a better understanding of this RHD6-RSL4 transcriptional pathway controlling root hair growth, the characterization of the immediate targets of RSL4 is necessary. Here I describe the use of a glucocorticoid inducible expression system coupled with the microarray analysis to identify the direct targets of RSL4. Thirty four putative direct target genes of RSL4 were identified. I assessed the requirement of nine of these genes by characterising the phenotypes of plants homozygous for the loss of function alleles. Plants homozygous for loss of function mutations in AT1G22620/SAC1, AT1G35670/CPK11/CDPK2, AT1G30870/PRX7 and AT5G03540/EXO70A1 (44% of the genes analysed) exhibited defects in root hair growth. Taken together my results not only identified potential RSL4 target genes but also demonstrated that RSL4 controls root hair growth by regulating genes that encode proteins involved in cell signalling, vesicle trafficking and cell wall modification
Evolution and morphology of lycophyte root systems
Full text linkThe evolution of plant roots transformed the Earthâs surface, engineering new ecosystems
and environments, and providing the nutrient and water uptake, as well as anchorage
necessary to support the first trees. The lycophytes (clubmosses) were the first group of
plants to evolve roots and the study of their morphology and evolution has been a major goal
for evolutionary scientists working on both extant and extinct lycopsids. The aim of the
research described in this thesis is to increase our understanding of both the morphology and
evolution of lycopsid roots. This is achieved by presenting three papers on the theme of root
morphology and evolution. First, I report the discovery of root hairs on extinct stigmarian
rootlets, highlighting the conserved morphology of all Isoetalean rootlets. Second, in my
discovery of the oldest fossilized root meristem, I illustrate how the interpretation of
exceptionally well preserved fossils can change the way we think about the evolution of
development of living plant roots. Third, I identify that the rootlets of Isoetes echinospora
and roots of Selaginella moellendorffii have similar gene expression profiles. The new
results reported in this thesis taken together with a review of the literature of extant and
extinct lycopsid rooting structures, enabled me to identify two contrasting evolutionary
patterns: conservatism of lycopsid roots, and huge disparity in the structures to which roots
are attached. The highly conserved nature of lycopsid roots, supported by the new data
presented in this thesis, is consistent with the hypothesis that all lycopsid roots are
homologous (described as the lycopsid root hypothesis). In recognising the homology of
lycopsid roots, and the two contrasting patterns of rooting structure evolution, the research
presented in this thesis makes a significant contribution to our understanding of the
morphology and evolution of lycopsid roots.</p
The genetic control of microtubule-mediated tip-growth stability in the liverwort Marchantia polymorpha
Full text linkPolar growth is an important mechanism for plant cell morphogenesis. Tip-growth represents an extreme mode of polar growth where cell expansion is stably restricted to a narrow domain of the cell periphery resulting in the formation of a tubular cell projection. The microtubule cytoskeleton controls the stable positioning of the growth region in tip-growing cells of flowering plants and mosses. I show that this holds true in the earliest diverging clade of land plants, the liverworts. In Marchantia polymorpha, pharmacological destabilization of the microtubule cytoskeleton leads to the formation of wavy or bifurcating rhizoids, a tip-growing cell type analogous to root hairs of flowering plants and to caulonema cells of mosses. Characterization of the organisation of the microtubule cytoskeleton in growing rhizoids of Marchantia polymorpha revealed longitudinally oriented microtubules that grow towards and converge into the apical dome. Because microtubule-associated proteins (MAPs) control the organisation of the microtubule cytoskeleton I generated the first de novo genome assembly of Marchantia polymorpha and compared the MAP repertoire of the liverwort model with that of existing model organisms of the green lineage. A mutant screen in Marchantia polymorpha identified the function of MpWAVE DAMPENED LIKE (MpWDL) and MpNIMA-RELATED KINASE (MpNEK) in microtubule-mediated tip-growth stability. MpWDL localizes preferentially to microtubules in the shank of growing rhizoids, where it promotes the longitudinal orientation of the microtubule cytoskeleton. These results are discussed in the context of the evolution of microtubule-mediated tip-growth stability and the tentative hypothesis that the underlying mechanism differs between flowering plants and bryophytes is proposed.</p
The genetic control of rhizoid development in the liverwort Marchantia polymorpha
Full text linkThe first land plants faced a harsh terrestrial environment when they emerged from the water over 470 million years ago, and one of the key adaptations that allowed them radiate across the land was the development of a rooting system. To investigate the genetic mechanism that controlled the differentiation of rooting cells in ancient land plants, I carried out a mutant screen to identify genes that regulate rhizoid development in the liverwort Marchantia polymorpha, a member of the earliest-diverging lineage of land plants.
I used insertional mutagenesis to generate a population of 105,000 lines from which I selected 61 mutants with defects in rhizoid development, and identified 10 genes that are part of the network of genes that influence the differentiation and growth of rhizoids. Eight of these are late-acting genes that are required for the elongation of rhizoids by tip growth, while two are transcription factors that direct early events in the adoption of rhizoid fate. I identified the bHLH transcription factor MpROOT HAIR DEFECTIVE 6-LIKE1 (MpRSL1) as a key regulator of rhizoid differentiation, as gain-of-function mutations in MpRSL1 cause rhizoids to develop in ectopic locations. The homologues of MpRSL1 in the angiosperm Arabidopsis and the moss Physcomitrella control the differentiation of their root hairs and rhizoids, respectively, which suggests that a gene regulatory network that included RSL genes controlled the development of filamentous rooting cells in the last common ancestor of all land plants. I also identified MpWIP, which encodes a member of a plant-specific family of zinc finger proteins, as a putative regulator of the development of both rhizoids and the cells of the air pore complex, a second specialized epidermal cell type. WIP genes have not been implicated in the control of rooting cell development in other species, so this role in Marchantia may be either inherited from the earliest land plants or a derived character.
This work demonstrates the suitability of M. polymorpha as a subject for large-scale mutageneses and screens for gene discovery. The genes I have found to be involved in rhizoid development indicate that the last common ancestor of all land plants already possessed a gene regulatory network that controlled the development of rooting cells, and that at least some of its components, such as RSL genes, have been conserved in its descendents since the divergence of the liverworts and other land plants.
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Evolution and function of RHO cell polarity signalling in plants
Full text linkSince their origin from a unicellular ancestor 630–890 million years ago, the streptophytes, which comprise streptophyte algae and land plants, have evolved mechanisms to develop morphologically complex body plans. Much remains unknown about how such mechanisms evolved and how they regulate plant morphogenesis. To start, I hypothesise that cell polarity signalling, which can spatially regulate fundamental cellular processes like growth and division, contributed to morphological evolution in the streptophytes. In this context, the aim of this thesis is to investigate the evolution and function of RHO Of Plant (ROP) signalling, the plant specific form of the ancient eukaryotic RHO cell polarity signalling. First, I report phylogenetic and sequence analyses resulting in the discovery that ROP signalling genes became established early in the streptophyte lineage, during the period when multicellular filamentous body plans are believed to have evolved from unicellular/colonial forms. Second, by functionally characterising the single ROP gene in the liverwort Marchantia polymorpha, I demonstrate that ROP protein regulates the morphogenesis of complex plant tissues by controlling cell growth, division, and possibly adhesion. Finally, through a series of cross-species complementation experiments with M. polymorpha rop mutants, I show that ROP function has remained largely conserved since the time land plants last shared a common ancestor with the filamentous streptophyte algae, Klebsormidium nitens. Collectively, the findings from this thesis reveal that an ancient signalling mechanism which became established early in the streptophyte lineage is required for the morphogenesis of complex tissues in extant land plants. They also indicate a potential role of ROP signalling function in the morphological evolution of early streptophytes
The role of the cytoskeleton in the polarisation of Marchantia polymorpha spores
Full text linkAll multicellular land plants start life as a single cell. By polarising – developing asymmetry – and dividing asymmetrically, this single cell can form two distinct cell lineages and establish the first plant body axis. How these cells establish polarity from a non-polar state is little known. A novel single-celled polarity system is the spore of the liverwort Marchantia polymorpha. This haploid cell polarises, by unknown mechanisms, and divides asymmetrically to form a proliferating stem cell and differentiated rhizoid cell. My project aimed to define the dynamics of the cytoskeleton - microtubules and actin filaments - in the establishment of spore polarity. By live timelapse imaging of spores expressing fluorescent reporters, I show that the nucleus migrates from the cell centre to the basal cortex to orient the first asymmetric division. Movement of the nucleus is led by a microtubule organising centre (MTOC) nucleating a dense astral array. An actin filament network also forms in the space between the migrating nucleus and the basal cortex. These data support the hypothesis that the cytoskeleton is required for nuclear migration during spore polarisation. Cytoskeleton organisation and dynamics requires tight regulation by proteins such as the microtubule severing enzyme, katanin. By mutating katanin, I discovered that katanin severing is required for MTOC formation and microtubule organisation in M. polymorpha. Collectively these findings propose that microtubules, MTOCs, actin filaments and possibly katanin are required for the establishment of spore polarity and to orient the first asymmetric division plane, thereby defining the first apical-basal body axis of M. polymorpha
Evolution of bHLH transcription factors that control epidermal cell development in plants
Full text linkThe colonization of the arid continental surface by plants was one of the milestones in Earth's history. Morphological innovations, such as the origin of complex 3D tissues, allowed the successful colonization and radiation of plants on land. The epidermis is the outermost plant tissue that constitutes the interface between the plant and the environment. Thus, the evolution of epidermal cells was crucial for the adaptation of plants on the terrestrial arid environment. I undertook a combined approach that aims to understand the evolutionary trends that drove land plant colonization and the genetic mechanisms that underlie the development of the epidermis. This approach includes: 1) analyses of plant transcription factors (TFs) families distribution and diversification, with a particular focus on the basic Helix-Loop-Helix (bHLH) TF family, and 2) functional characterization of a putatively conserved bHLH TF subfamily involved in epidermal cell development in land plants. Here, I showed that there was a stepwise increase in the number of transcription factor (TF) families and bHLH subfamilies that predated the colonization of the terrestrial surface by plants. The subsequent increase in TF number on land was through duplication within pre-existing TF families and subfamilies. Moreover, a similar trend occurred in metazoan bHLH TF, suggesting that the majority of innovation in plant and metazoan TF families occurred in the Precambrian before the Phanerozoic radiation of land plants and metazoans. Furthermore, I demonstrated that the function of IIIf bHLH TFs in controlling the development of the epidermal cell layer is conserved between liverworts and angiosperms. This suggests that IIIf bHLH TFs are ancient and conserved regulators of epidermal cell development since the early colonization of the land by plants. Moreover, these bHLH TFs were recruited during the evolution of land plants to control the development of seemingly unrelated morphological characters in specific lineages of extant land plants. The recruitment of ancient developmental regulators to control distinct and unrelated developmental processes in land plants might underlie the huge morphological and taxonomic radiation of plants on land.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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