Coleochaete Orbicularis Research Articles

Rethinking the Origin of Auxin Biosynthesis in Plants.

Rethinking the Origin of Auxin Biosynthesis in Plants.

Direction of illumination controls gametophyte orientation in seedless plants and related algae

The environmental influences that determine dorsiventral or axial gametophyte orientation are unknown for most modern seedless plants. To fill this gap, an experimental laboratory system was employed to evaluate the relative effects of light direction and gravity on body orientation of the dorsiventral green alga Coleochaete orbicularis, and gametophytes of liverworts Blasia pusilla and Marchantia polymorpha, early-diverging moss Sphagnum compactum, and fern Ceratopteris richardii, the latter functioning as experimental control. Replicate clonal cultures were experimentally illuminated only from above, only from below, or from multiple directions, with the same near-saturation PAR level for periods brief enough to minimize nutrient limitation effects, and orientation of new growth was evaluated. For all species tested, direction of illumination exerted stronger control over gametophyte body orientation than gravity. When illuminated only from below: 1) axial Sphagnum gametophores that had initially grown into an overlying air space inverted growth by 180°, burrowing into the substrate; 2) new growth of dorsiventral Blasia, Marchantia, and Ceratopteris gametophytes–whose ventral rhizoids initially penetrated agar substrate and dorsal surfaces initially faced overlying airspace–twisted 180° so that ventral surfaces bearing rhizoids faced overlying air space and rhizoids extended into the air; and 3) Coleochaete lost typical dorsiventral organization and diagnostic dorsal hairs. Direction of illumination also exerted stronger control over orientation of liverwort new growth than surface contact did. These results indicate that early land plants likely inherited light-directed gametophyte body orientation from ancestral streptophyte algae and suggest a mechanism for reorientation of gametophyte-dominant land plants after spatial disturbance.

Evidence for land plant cell wall biosynthetic mechanisms in charophyte green algae.

The charophyte green algae (CGA) are thought to be the closest living relatives to the land plants, and ancestral CGA were unique in giving rise to the land plant lineage. The cell wall has been suggested to be a defining structure that enabled the green algal ancestor to colonize land. These cell walls provide support and protection, are a source of signalling molecules, and provide developmental cues for cell differentiation and elongation. The cell wall of land plants is a highly complex fibre composite, characterized by cellulose cross-linked by non-cellulosic polysaccharides, such as xyloglucan, embedded in a matrix of pectic polysaccharides. How the land plant cell wall evolved is currently unknown: early-divergent chlorophyte and prasinophyte algae genomes contain a low number of glycosyl transferases (GTs), while land plants contain hundreds. The number of GTs in CGA is currently unknown, as no genomes are available, so this study sought to give insight into the evolution of the biosynthetic machinery of CGA through an analysis of available transcriptomes. Available CGA transcriptomes were mined for cell wall biosynthesis GTs and compared with GTs characterized in land plants. In addition, gene cloning was employed in two cases to answer important evolutionary questions. Genetic evidence was obtained indicating that many of the most important core cell wall polysaccharides have their evolutionary origins in the CGA, including cellulose, mannan, xyloglucan, xylan and pectin, as well as arabino-galactan protein. Moreover, two putative cellulose synthase-like D family genes (CSLDs) from the CGA species Coleochaete orbicularis and a fragment of a putative CSLA/K-like sequence from a CGA Spirogyra species were cloned, providing the first evidence that all the cellulose synthase/-like genes present in early-divergent land plants were already present in CGA. The results provide new insights into the evolution of cell walls and support the notion that the CGA were pre-adapted to life on land by virtue of the their cell wall biosynthetic capacity. These findings are highly significant for understanding plant cell wall evolution as they imply that some features of land plant cell walls evolved prior to the transition to land, rather than having evolved as a result of selection pressures inherent in this transition.

Mixed evolutionary traits of Tolypella (section Rothia, Charales) compared with Chara and Nitella shown by ultrastructure of vegetative internodal cells

The ultrastructure of vegetative internodal cells in a representative of the genus Tolypella (section Rothia) is described for the first time for Tolypella intricata. The most striking difference compared with the cells of Chara and Nitella is that the four-layered lateral cell wall shows the regular presence of unique refractive lenticular thickenings, which we designate as “tolysomes”, considering their unique ultrastructure elucidated here for the first time. Tolysomes greatly amplify the plasma membrane surface; thereby defining particular spaces of the cytoplasm. There are similar structures in the cell walls of the non-charalean charophyte Coleochaete orbicularis and also in bryophytes, angiosperms and gymnosperms. As the function of tolysomes is hitherto elusive, a bona fide analysis of evolutionary significance is not yet possible. C-shaped amitotic nuclei are similar to those described for young internodal Chara cells. Resting amitotic nuclei exhibit normal nuclear envelope, dispersed granular chromatin and numerous nucleoli with variable size. Nevertheless, they show neither the single, central nucleolus characteristic of dividing Chara nuclei nor the bundles of tubular elements running parallel to the long axis of the nucleus, which are also normal in Chara and Nitella amitotic nuclei. The presence of C-shaped resting nuclei of Tolypella and Chara and their absence in Nitella and the presence of spindle-shaped nuclei in Nitella, but not in Tolypella, suggest that Tolypella (section Rothia) divergence took place earlier than did Nitella in accordance with, but also in contradiction to molecular data. Mature echinoid bodies and chloroplasts in Tolypella intricata are entirely similar to those previously described for Chara and Nitella.

Unraveling the Evolution of Auxin Signaling

Auxin signaling is central to plant growth and development, yet hardly anything is known about its evolutionary origin. While the presence of key players in auxin signaling has been analyzed in various land plant species, similar analyses in the green algal lineages are lacking. Here, we survey the key players in auxin biology in the available genomes of Chlorophyta species. We found that the genetic potential for auxin biosynthesis and AUXIN1 (AUX1)/LIKE AUX1- and P-GLYCOPROTEIN/ATP-BINDING CASSETTE subfamily B-dependent transport is already present in several single-celled and colony-forming Chlorophyta species. In addition, our analysis of expressed sequence tag libraries from Coleochaete orbicularis and Spirogyra pratensis, green algae of the Streptophyta clade that are evolutionarily closer to the land plants than those of the Chlorophyta clade, revealed the presence of partial AUXIN RESPONSE FACTORs and/or AUXIN/INDOLE-3-ACETIC ACID proteins (the key factors in auxin signaling) and PIN-FORMED-like proteins (the best-characterized auxin-efflux carriers). While the identification of these possible AUXIN RESPONSE FACTOR- and AUXIN/INDOLE-3-ACETIC ACID precursors and putative PIN-FORMED orthologs calls for a deeper investigation of their evolution after sequencing more intermediate genomes, it emphasizes that the canonical auxin response machinery and auxin transport mechanisms were, at least in part, already present before plants "moved" to land habitats.

Uncovering the evolutionary origin of plant molecular processes: comparison of Coleochaete (Coleochaetales) and Spirogyra (Zygnematales) transcriptomes

BackgroundThe large and diverse land plant lineage is nested within a clade of fresh water green algae, the charophytes. Collection of genome-scale data for land plants and other organisms over the past decade has invigorated the field of evolutionary biology. One of the core questions in the field asks: how did a colonization event by a green algae over 450 mya lead to one of the most successful lineages on the tree of life? This question can best be answered using the comparative method, the first step of which is to gather genome-scale data across closely related lineages to land plants. Before sequencing an entire genome it is useful to first gather transcriptome data: it is less expensive, it targets the protein coding regions of the genome, and provides support for gene models for future genome sequencing. We built Expressed Sequence Tag (EST) libraries for two charophyte species, Coleochaete orbicularis (Coleochaetales) and Spirogyra pratensis (Zygnematales). We used both Sanger sequencing and next generation 454 sequencing to cover as much of the transcriptome as possible.ResultsOur sequencing effort for Spirogyra pratensis yielded 9,984 5' Sanger reads plus 598,460 GS FLX Standard 454 sequences; Coleochaete orbicularis yielded 4,992 5' Sanger reads plus 673,811 GS FLX Titanium 454 sequences. After clustering S. pratensis yielded 12,000 unique transcripts, or unigenes, and C. orbicularis yielded 19,000. Both transcriptomes were very plant-like, i.e. most of the transcripts were more similar to streptophytes (land plants + charophyte green algae) than to other green algae in the sister group chlorophytes. BLAST results of several land plant genes hypothesized to be important in early land plant evolution resulted in high quality hits in both transcriptomes revealing putative orthologs ripe for follow-up studies.ConclusionsTwo main conclusions were drawn from this study. One illustrates the utility of next generation sequencing for transcriptome studies: larger scale data collection at a lower cost enabled us to cover a considerable portion of the transcriptome for both species. And, two, that the charophyte green algal transcriptoms are remarkably plant-like, which gives them the unique capacity to be major players for future evolutionary genomic studies addressing origin of land plant questions.

Cytokinesis in Coleochaete orbicularis (Charophyceae): an ancestral mechanism inherited by plants

Recently, highly vacuolate cells of Arabidopsis were shown to exhibit "polarized" cytokinesis, in which the phragmoplast and cell plate contact the mother cell wall and then progress from one side of the cell to the other, rather than forming uniformly outward from the cell center (Cutler and Ehrhardt, 2002, Proceedings of the National Academy of Sciences, USA 99: 2812-2817). It was not known if such a mechanism was unique to flowering plants or whether it occurred more broadly in the plant clade. To determine if a polar mechanism of cell division might have been characteristic of the first plants, differential interference contrast optics were used to examine living cells of the charophycean green alga Coleochaete orbicularis, a close relative of plants, with cytokinesis involving a phragmoplast. By recording images in different focal planes over time, such "polarized" cytokinesis was found in cells dividing either parallel or perpendicular to the edge of this radially symmetrical organism. Previously reported differences between these two types of division in Coleochaete were clarified. Polarized cytokinesis appears to be an ancestral mechanism of plant cell division inherited from the highly vacuolate cells of the charophycean algal ancestors of plants.

Sequence Diversity of the Gene tufA in Coleochaete: An Algal Model for Horizontal Gene Transfer

AbstractStudying the organellar gene transfer of tufA in Coleochaete serves as a model system for understanding the effect of functional horizontal gene transfer on sequence conservation and divergence. Previous work indicates that tufA, which codes for protein synthesis elongation factor TU, is encoded only in the chloroplast genome in most green algae, red algae, and glaucocystophytes, but only in the nuclear genome of embryophytes. In the charophytes, the lineage of green algae that gave rise to embryophytes, tufA is present variously within the chloroplast and/or nuclear genome. This implies that tufA was probably transferred to the nucleus from the chloroplast after the charophytes diverged from other green algae but before the diversification of the group. The chloroplast copy has undergone three separate fates within the charophytes: maintained in Charales, divergent in Coleochaetales, and lost in Zygnematales. Previous research indicates the presence of multiple nuclear copies and a highly divergent chloroplast encoded tufA in Coleochaete orbicularis. This research aims to isolate the chloroplast‐encoded tufA from a representative panel of Coleochaete in order to characterize patterns of conservation in the divergent chloroplast tufA. Putative chloroplast copies of tufA have been isolated through PCR and subsequent molecular cloning. By comparing the DNA sequences from copies of the chloroplast‐encoded tufA from a representative panel of Coleochaete, we hope to gain insight into patterns of sequence conservation and from this infer the current function of the divergent chloroplast tufA.

Estimation of directional division frequencies in vascular cambium and in marginal meristematic cells of plants.

Using simple arithmetical formulae, it is shown that, when the meristematic initial cells of a growing plant organ are arranged in a ring, the cellular dimensions predict the relative frequencies of anticlinal and periclinal divisions which these cells undergo. The pattern of cell file branching which appears during the course of development, and which is predicted by this mathematical model, is validated using data pertaining to the numbers and dimensions of initial cells within the secondary vascular cambium of hybrid aspen trees. Data pertaining to a second, simpler set of initial cells which comprises the outer cellular ring of the thallus of the alga Coleochaete orbicularis, and from which all the radial cell files of the circular disc-like thallus are descended, have also been used for model validation. Combining the mathematical approach to division frequencies with data of actual cell sizes permits inferences about the course of the increase of the number of cell files (generated by the anticlinal divisions) and the number of cells within each file (generated by the periclinal divisions) during the earlier stages of secondary tissue or thallus development, and also about how they will develop at future stages. The question whether or not cell division patterns conform to the geometry of the system in which the cells are embedded is also discussed.

Structural Similarities between Surface Layers of Selected Charophycean Algae and Bryophytes and the Cuticles of Vascular Plants

Land plants are a monophyletic group that arose from a common ancestor that would be classified with the charophycean green algae. Thus early divergent plants and charophycean algae can be used as simple model systems for studying the evolution of more complex features of higher plants. We used TEM and SEM to examine surface layers of the charophycean green algae Nitella gracilis, Chara zeylanica, and Coleochaete orbicularis and of gametophytes of the putatively early divergent bryophytes Monoclea gottschei (liverwort), Notothylas orbicularis (hornwort), and Sphagnum fimbriatum (moss) in an attempt to learn how the cuticle of vascular plants may have originated In all of the taxa we studied, TEM data indicate that there is an osmiophilic layer on the outer cell wall that bears some structural resemblance to early developmental stages of vascular plant cuticles. SEM shows further that the surface layer of Monoclea is nodular, that Notothylas may have either a nodular or a sheetlike surface, and that Coleochaete and Sphagnum bear sheetlike surface layers with regular ridges running parallel to the edges of their thalli. The numerous ridges at the edge of the thallus in Coleochaete resemble those at the base of branches in charalean algae. These algae have a distinctive sheetlike or ridged surface layer, composed of strands, which appears to protect young, unexpanded shoots and is then pulled apart as growth takes place This may be a specialization resulting from the charalean lifestyle. Based on our results, it appears that some structures that are at least positionally analogous to the plant cuticle arose before the charophycean algae and bryophytes diverged from a common ancestor. While structural correlation is not proof of homology, it indicates that further examination at the biochemical level is warranted.

Occurrence and phylogenetic significance of glucose utilization by charophycean algae: glucose enhancement of growth in Coleochaete orbicularis

Two critical innovations had a profound influence upon the evolutionary history of plants: the nutritionally dependent embryo and apoplastic phloem loading processes. Both depend upon the ability of the plant cell membrane to transport sugars. The evolutionary origin of sugar transport by plants is, therefore, of special phylogenetic importance. Recent evidence suggests that hexoses such as glucose are the main form of sugar transported apoplastically across the placental junction between gametophyte and sporophyte of the moss Polytrichum (Renault et al., 1992, Plant Physiology 100: 1815–1822). There is also considerable evidence that hexose transport may be involved in apoplastic phloem loading in Arabidopsis and other flowering plants. Results of numerous molecular, biochemical, immunofluorescence, and ultrastructural studies indicate that bryophytes and charophycean algae are related to the ancestors of vascular plants. This report demonstrates that the charophyte Coleochaete orbicularis exhibits enhanced growth in the presence of glucose under conditions in which inorganic carbon sources are limiting. Computer image analysis was used to demonstrate that nonaerated cultures of C. orbicularis grown for 7 weeks in an inorganic medium supplemented with 1% glucose produce as much as 13.6 times the biomass of clonal cultures grown for the same length of time in the same medium without glucose. Furthermore, addition of 1% glucose to nonaerated cultures yielded chlorophyll a concentration 20 times higher after 4 weeks growth than cultures grown under the same conditions without added glucose. This and other evidence derived from comparative growth measurements strongly suggest that Coleochaete takes up (or loads) glucose. Experimental results also suggest that hexose uptake by charophytes (such as Coleochaete) that typically grow in low alkalinity waters might provide a supplementary source of organic carbon when dissolved carbon dioxide levels are low. This capability could have served as an evolutionary precursor to hexose import by plant embryos, as well as leptom and phloem loading in bryophytes and vascular plants.

Occurrence and Phylogenetic Significance of Glucose Utilization by Charophycean Algae: Glucose Enhancement of Growth in Coleochaete orbicularis

Occurrence and Phylogenetic Significance of Glucose Utilization by Charophycean Algae: Glucose Enhancement of Growth in Coleochaete orbicularis

Morphogenetic Plastid Migration and Microtubule Arrays in Mitosis and Cytokinesis in the Green Alga Coleochaete orbicularis

This study provides data on cell division in Coleochaete orbicularis, an important taxon in evolutionary theories deriving land plants from green algae. Vegetative growth in discoid species of Coleochaete results from marginal cell division in two planes—radial and circumferential. Like many algae and certain of the simple land plants, Coleochaete is monoplastidic. Prior to mitosis, the single plastid migrates to a position where it will divide and be distributed into the daughter cells. Unlike monoplastidic cell division in hornworts, mosses, and lycopsids; microtubule nucleation is not intimately associated with the plastids. Instead, microtubule organization is associated with centriolar centrosomes throughout the cell cycle, as is common in algae. The cytokinetic apparatus lacks preprophase bands of microtubules, but includes typical phragmoplasts consisting of brushlike arrays of microtubules on either side of a dark zone. However, the origin and role of phragmoplasts is unusual. Phragmoplasts appear to develop among microtubules that emanate from the polar centrosomes rather than from nuclear envelopes and/or plastids. The function of phragmoplasts in Coleochaete is unclear, as the process of cytokinesis is not strictly centrifugal. Some infurrowing occurs in radial division, and cytokinesis appears to be entirely centripetal by infurrowing in circumferential division. The cortical arrays of microtubules differ from those typical of land plants in that they develop as a network in association with centrosomes after mitosis.

Phylogenetic relationships of four charophycean green algae inferred from complete nuclear‐encoded small subunit RRNA gene sequences

Complete nuclear‐encoded (18S) small subunit rRNA gene sequences were determined for four charophycean green algae, Chlorokybus atmophyticus, Coleochaete orbicularis, Klebsormidium flaccidum, and Nitella sp. Chlorokybus atmophyticus and Coleochaete orbicularis have been previously suggested to represent the most basal and most derived taxa within the charophytes, respectively. However, parsimony analysis of our 18S rDNA sequences along with a selection of other complete green algal and land plant 18S rDNA sequences yields a gene tree topology in which Chlorokybus is the most basal taxon, followed by the branching of Coleochaete and Klebsormidium. Two “sister” clades then diverge, one including Nitella and the land plants, and the second, members of the Chlorophyceae and Pleurastrophyceae. Despite producing slightly diiferent gene tree topologies than those inferred from parsimony, distance analyses of the 18S rDNA sequences also do not indicate a strong affinity between the land plants and Coleochaete. Rather, Klebsormidium and Coleochaete are virtually equidistant from the land plant taxa. Other data are needed in order to assess the unexpected findings reported here, particularly the position of Coleochaete.

Phylogenetic Relationships of Four Charophycean Green Algae Inferred from Complete Nuclear-Encoded Small Subunit rRNA Gene Sequences

Complete nuclear-encoded (18S) small subunit rRNA gene sequences were determined for four charophycean green algae, Chlorokybus atmophyticus, Coleochaete orbicularis, Klebsormidium flaccidum, and Nitella sp. Chlorokybus atmophyticus and Coleochaete orbicularis have been previously suggested to represent the most basal and most derived taxa within the charophytes, respectively. However, parsimony analysis of our 18S rDNA sequences along with a selection of other complete green algal and land plant 18S rDNA sequences yields a gene tree topology in which Chlorokybus is the most basal taxon, followed by the branching of Coleochaete and Klebsormidium. Two “sister” clades then diverge, one including Nitella and the land plants, and the second, members of the Chlorophyceae and Pleurastrophyceae. Despite producing slightly diiferent gene tree topologies than those inferred from parsimony, distance analyses of the 18S rDNA sequences also do not indicate a strong affinity between the land plants and Coleochaete. Rather, Klebsormidium and Coleochaete are virtually equidistant from the land plant taxa. Other data are needed in order to assess the unexpected findings reported here, particularly the position of Coleochaete.

Different fates of the chloroplast tufA gene following its transfer to the nucleus in green algae.

Previous work suggested that the tufA gene, encoding protein synthesis elongation factor Tu, was transferred from the chloroplast to the nucleus within the green algal lineage giving rise to land plants. In this report we investigate the timing and mode of transfer by examining chloroplast and nuclear DNA from the three major classes of green algae, with emphasis on the class Charophyceae, the proposed sister group to land plants. Filter hybridizations reveal a chloroplast tufA gene in all Ulvophyceae and Chlorophyceae and in some but not all Charophyceae. One charophycean alga, Coleochaete orbicularis, is shown to contain an intact but highly divergent chloroplast tufA gene, whose product is predicted to be non-functional in protein synthesis. We propose that a copy of the tufA gene was functionally transferred from the chloroplast to the nucleus early in the evolution of the Charophyceae, with chloroplast copies of varying function being retained in some but not all of the subsequently diverging lineages. This proposal is supported by the demonstration of multiple tufA-like sequences in Coleochaete nuclear DNA and in nuclear DNA from all other Charophyceae examined.

Lignin-Like Compounds and Sporopollen in Coleochaete , an Algal Model for Land Plant Ancestry

Unusual cell wall structure and resistance to microbial degradation led to an investigation of resistant biopolymers in Coleochaete (Chlorophyta, class Charophyceae), a green alga on the evolutionary lineage that led to land plants. In Coleochaete that are undergoing sexual reproduction, vegetative cell walls contain material similar to lignin, a substance generally thought absent from green algae, and the zygote wall includes sporopollenin. Knowledge of chemically resistant compounds in Coleochaete may facilitate interpretation of the fossil record. Placental transfer cells in Coleochaete orbicularis and in the hornwort Anthoceros survive acetolysis and contain lignin-like compounds, implying a close relation between these taxa.

The Occurrence and Phylogenetic Significance of Putative Placental Transfer Cells in the Green Alga Coleochaete

Following fertilization, zygotes of the green alga Coleochaete orbicularis, which are retained on the haploid thallus, first enlarge, then become covered with a layer of vegetative cells. Light microscopy and high-voltage electron microscopy revealed the presence of localized wall ingrowths in vegetative cells adjacent to zygotes. These covering cells resemble the gametophytic placental transfer cells of embryophytes in their morphology, location, and time of development. If Coleochaete cells with wall protuberances function as do placental transfer cells of embryophytes, their presence is evidence that photosynthates may be transported between haploid thallus cells and zygotes. Thus, a nutritional relationship between different phases of the life cycle, similar to that which occurs in embryophytes, may also have evolved in green algae. This first report of putative placental transfer cells in a green alga supports Bower's (1908) ideas concerning the origin of land plant sporophytes and alternation of generations. The presence or absence of cells with wall ingrowths in several species of Coleochaete was correlated with estimates of zygote-plant area ratios.

THE OCCURRENCE AND PHYLOGENETIC SIGNIFICANCE OF PUTATIVE PLACENTAL TRANSFER CELLS IN THE GREEN ALGA COLEOCHAETE.

Following fertilization, zygotes of the green alga Coleochaete orbicularis, which are retained on the haploid thallus, first enlarge, then become covered with a layer of vegetative cells. Light microscopy and high-voltage electron microscopy revealed the presence of localized wall ingrowths in vegetative cells adjacent to zygotes. These covering cells resemble the gametophytic placental transfer cells of embryophytes in their morphology, location, and time of development. If Coleochaete cells with wall protuberances function as do placental transfer cells of embryophytes, their presence is evidence that photosynthates may be transported between haploid thallus cells and zygotes. Thus, a nutritional relationship between different phases of the life cycle, similar to that which occurs in embryophytes, may also have evolved in green algae. This first report of putative placental transfer cells in a green alga supports Bower's (1908) ideas concerning the origin of land plant sporophytes and alternation of generations. The presence or absence of cells with wall ingrowths in several species of Coleochaete was correlated with estimates of zygote-plant area ratios.

The microdistribution of epiphytes on the leaves of natural and artificial macrophytes

The microdistribution of two diatoms (Cocconeis placentula, Eunotia spp.) and two epiphytic green algae (Coleochaete orbicularis and Coleochaete nitellarum) on leaves of Potamogeton richardsonii and on those of plastic aquarium plants was compared using a new “peel-off” technique. Differential colonization of the upper and underside of the leaves was much more evident on the living plants, which were characterized by a distinct CaCO3 incrustation on their upper-side. The diatom preference for the edge of leaves appear to have a physical basis because it was equally pronounced on both natural and plastic leaves.