Occurrence and phylogenetic significance of cytokinesis-related callose in green algae, bryophytes, ferns and seed plants

 In order to investigate the occurrence of callose in dividing cells, we cultivated a selection of 30 organisms (the prokaryotic cyanobacterium Anabaena and eukaryotic green algae, bryophytes, ferns and seed plants) under defined conditions in the laboratory. Samples from these photoautotrophs, which are members of the evolutionary 'green lineage' leading from freshwater algae to land plants, were analysed by fluorescence microscopy. The β-1,3-glucan callose was identified by its staining properties with aniline blue and sirofluor. With the exception of the prokaryotic cyanobacterium, all of the eukaryotic organisms studied were capable of producing wound-induced callose. No callose was detected during cytokinesis of dividing cells of unicellular green algae (and Anabaena). However, in all of the multicellular green algae and land plants (embryophytes) investigated, callose was identified in newly made septae by an intense yellow fluorescence. The formation of wound callose was never detected in cells with callose in the newly formed septae. Additional experiments verified that no fixation-induced artefacts occurred. Our results show that callose is a regular component of developing septae in juvenile cells during cytokinesis in multicellular green algae and embryophytes. The implications of our results with respect to the evolutionary relationships between extant charophytes and land plants are discussed. Received: 15 September 2000 / Revision received: 23 October 2000 / Accepted: 23 October 2000     

SUMMARY OF GREEN PLANT PHYLOGENY AND CLASSIFICATION

Abstract— A cladogram of green plants involving all major extant groups of green algae, bryophytes, pteridophytes, and seed plants is presented. It is partly based on contributions by B. Mishler and S. Churchill, H. Wagner, and P. Crane. The relationships of green plants to other green organisms ( Prochloron , euglenophytes) are discussed. The characters and subclades of the cladogram are briefly discussed, with an attempt to indicate weak points. The possibility of including some major extinct groups is considered. A cladistic classification consistent with the cladogram is presented. Grades are abandoned as taxa and major clades like the division Chlorophyta (green algae excluding micro-monadophytes and charophytes sensu Mattox and Stewart), the division Streptophyta (charophytes + embryophytes), the subdivision Embryophytina (land plants or embryophytes), the superclass Tracheidatae (tracheophytes), and the class Spermatopsida (seed plants) are recognized.     

Chloroplast gene arrangement variation within a closely related group of green algae (Trebouxiophyceae, Chlorophyta)

The 22 published chloroplast genomes of green algae, representing sparse taxonomic sampling of diverse lineages that span over one billion years of evolution, each possess a unique gene arrangement. In contrast, many of the >190 published embryophyte (land plant) chloroplast genomes have relatively conserved architectures. To determine the phylogenetic depth at which chloroplast gene rearrangements occur in green algae, a 1.5-4 kb segment of the chloroplast genome was compared across nine species in three closely related genera of Trebouxiophyceae (Chlorophyta). In total, four distinct gene arrangements were obtained for the three genera Elliptochloris, Hemichloris, and Coccomyxa. In Elliptochloris, three distinct chloroplast gene arrangements were detected, one of which is shared with members of its sister genus Hemichloris. Both species of Coccomyxa examined share the fourth arrangement of this genome region, one characterized by very long spacers. Next, the order of genes found in this segment of the chloroplast genome was compared across green algae and land plants. As taxonomic ranks are not equivalent among different groups of organisms, the maximum molecular divergence among taxa sharing a common gene arrangement in this genome segment was compared. Well-supported clades possessing a single gene order had similar phylogenetic depth in green algae and embryophytes. When the dominant gene order of this chloroplast segment in embryophytes was assumed to be ancestral for land plants, the maximum molecular divergence was found to be over two times greater in embryophytes than in trebouxiophyte green algae. This study greatly expands information about chloroplast genome variation in green algae, is the first to demonstrate such variation among congeneric green algae, and further illustrates the fluidity of green algal chloroplast genome architecture in comparison to that of many embryophytes.     

Green algae and the evolution of land plants: inferences from nuclear-encoded rRNA gene sequences.

Phylogenetic analysis of 381 informative sites in partial sequences of nuclear-encoded large and small subunit ribosomal RNAs from 38 chlorophyll a- and b-containing plants (Chlorobionta sensu Bremer) including tracheophytes, bryophytes, charophytes and chlorophytes, supports the hypotheses of: (1) monophyly of the green plants (excluding Euglenophyta); (2) monophyly of the embryophytes; (3) non-monophyly of the bryophytes; (4) monophyly of the tracheophytes; and (5) a single origin of embryophytes from charophycean green algae. The Charales and Klebsormidium appear to be the green algae most closely related to the land plants. The unexpected basal divergence of Coleochaete and the apparent non-monophyly of the Zygnematales are not robustly supported and, thus, are interpreted to be sources of new questions, rather than new phylogenetic hypotheses.     

The charophycean green algae provide insights into the early origins of plant cell walls

Numerous evolutionary innovations were required to enable freshwater green algae to colonize terrestrial habitats and thereby initiate the evolution of land plants (embryophytes). These adaptations probably included changes in cell-wall composition and architecture that were to become essential for embryophyte development and radiation. However, it is not known to what extent the polymers that are characteristic of embryophyte cell walls, including pectins, hemicelluloses, glycoproteins and lignin, evolved in response to the demands of the terrestrial environment or whether they pre-existed in their algal ancestors. Here we show that members of the advanced charophycean green algae (CGA), including the Charales, Coleochaetales and Zygnematales, but not basal CGA (Klebsormidiales and Chlorokybales), have cell walls that are comparable in several respects to the primary walls of embryophytes. Moreover, we provide both chemical and immunocytochemical evidence that selected Coleochaete species have cell walls that contain small amounts of lignin or lignin-like polymers derived from radical coupling of hydroxycinnamyl alcohols. Thus, the ability to synthesize many of the components that characterize extant embryophyte walls evolved during divergence within CGA. Our study provides new insight into the evolutionary window during which the structurally complex walls of embryophytes originated, and the significance of the advanced CGA during these events.     

Recent gene duplications dominate evolutionary dynamics of adaptor protein complex subunits in embryophytes

Adaptor protein complexes and the related complexes COPI and TSET function in packaging vesicles for transport among endomembrane compartments in eukaryotic cells. Differences in the complement of these complexes in lineages such as yeast and mammals as well as apicomplexan and kinetoplastid parasites via loss or duplication of subunits appears to reflect specialization in their respective trafficking systems. The model plant Arabidopsis thaliana possesses multiple paralogues for adaptor protein complex subunits, raising questions as to the timing and extent of these duplications in embryophytes (land plants). However, adaptor protein complex evolution in embryophytes is unexplored. Therefore, we analyzed genomes of diverse embryophytes and closely related green algae using extensive homology searches and phylogenetic analysis of 35 complex subunit proteins. The results reveal numerous paralogues, the vast majority of which, approximately 97%, arose from recent duplication events. This suggests that specialization of these protein complexes may occur frequently but independently in embryophytes.     

Green algae to land plants: An evolutionary transition

Studies focused upon the evolutionary transition from ancestral green algae to the earliest land plants are important from a range of ecological, molecular and evolutionary perspectives. A substantial suite of ultrastructural, biochemical and molecular data supports the concept that land plants (embryophytes) are monophyletically derived from an ancestral charophycean alga. However, the details of phylogenetic branching patterns linking extant charophytes and seedless embryophytes are currently unclear. Moreover, the fossil record has so far been mute regarding the algae-land plant transition. Nevertheless, an accurate reflection of major evolutionary events in the history of the earliest land plants can be obtained by comparative paleontological-neontological studies, and comparative molecular, cellular and developmental investigations of extant charophytes and bryophytes. This review focuses upon research progress toward understanding three clade-specific adaptations that were important in the successful colonization of land by plants: the histogenetic apical meristem, the matrotrophic embryo, and decay-resistant cell wall polymers.     

A cladistic approach to the phylogeny of the “Bryophytes”

The importance of a cladistic approach in reconstructing the phylogeny of bryophytes is discussed and illustrated by an analysis of the major groups of bryophytes with respect to the tracheophytes and the green algae. The cladistic analysis, using 51 characters taken from the literature, gives the following tentative results: (1) the embryophytes as a whole are monophyletic; (2) the bryophytes (sensu lato) are paraphyletic; (3) the mosses share a more recent common ancestor with the tracheophytes than do the liverworts or hornworts; (4) the hornworts appear to share a more recent common ancestor with the moss-tracheophyte lineage than with the liverworts; however, the existence of several homoplasies makes this placement more problematical; (5) the origin of alternation of generations in the embryophytes, based on out-group comparison with their oogamous, haplontic, algal sister groups, was by progressive elaboration of the primitively epiphytic sporophyte generation; and (6) the presence of vascular tissue (xylem and phloem) can best be interpreted as a synapomorphy of the moss-tracheophyte clade, and tracheids (xylem with ornamented walls) as a synapomorphy of the tracheophytes; therefore, the prevailing designation of “vascular plants” for the tracheophytes alone is inaccurate.     

The Transition From Algae to Embryophytes: Chloroplast Phylogenomic Evidence (II)

The transition of plant life from aquatic algae to land plants was one of the major events in the history of life. However, in hypothesizing the evolutionary path of the transition, limited shared phenotypic characters in aquatic algae and land plants (embryophytes) have been a major hinderance. Chloroplast genomes contain characters useful in tracing evolutionary histories. Embryophyte chloroplast genomes are distinguished from algal cpDNAs by the presence of over 20 group II introns and three ribosomal protein operons (rpl23, clpP and 3?rps12 operons). These phylogenomic features indicate a phylogenetic relationship of charophytes and embryophytes. In addition to these operons and introns, the evolution of rRNA and psbB operon evolution of streptophyte lineages will be incorporated with major biological phenotypic features to produce a phylogenetic tree. Basal embryophytes, the antithetic hypothesis, monophyly of embryophytes, and paraphyly of charophytes will be discussed. Strepotophytes are classified into three major groups (basal streptophytes, mid‐divergent streptophytes and late divergent charophytes‐embryophytes).     

The origin of alternation of generations in land plants: a focus on matrotrophy and hexose transport

A life history involving alternation of two developmentally associated, multicellular generations (sporophyte and gametophyte) is an autapomorphy of embryophytes (bryophytesphytes + vascular plants). Microfossil data indicate that Mid Late Ordovician land plants possessed such a life cycle, and that the origin of alternation of generations preceded this date. Molecular phylogenetic data unambiguously relate charophycean green algae to the ancestry of monophyletic embryophytes, and identify bryophytes as early-divergent land plants. Comparison of reproduction in charophyceans and bryophytes suggests that the following stages occurred during evolutionary origin of embryophytic alternation of generations: (i) origin of oogamy; (ii) retention of eggs and zygotes on the parental thallus; (iii) origin of matrotrophy (regulated transfer of nutritional and morphogenetic solutes from parental cells to the next generation); (iv) origin of a multicellular sporophyte generation; and (v) origin of non-flagellate, walled spores. Oogamy, egg/zygote retention and matrotrophy characterize at least some modern charophvceans, and are postulated to represent pre-adaptative features inherited by embryophytes from ancestral charophyceans. Matrotrophy is hypothesized to have preceded origin of the multicellular sporophytes of' plants, and to represent a critical innovation. Molecular approaches to the study of the origins of matrotrophy include assessment of hexose transporter genes and protein family members and their expression patterns. The occurrence in modern charophyceans and bryophytes of chemically resistant tissues that exhibit distinctive morphology correlated with matrotrophy suggests that Early-Mid Ordovician or older microfossils relevant to the origin of land plant alternation of generations may be found.     

Streptophyte algae and the origin of embryophytes

Background

Land plants (embryophytes) evolved from streptophyte green algae, a small group of freshwater algae ranging from scaly, unicellular flagellates (Mesostigma) to complex, filamentous thalli with branching, cell differentiation and apical growth (Charales). Streptophyte algae and embryophytes form the division Streptophyta, whereas the remaining green algae are classified as Chlorophyta. The Charales (stoneworts) are often considered to be sister to land plants, suggesting progressive evolution towards cellular complexity within streptophyte green algae. Many cellular (e.g. phragmoplast, plasmodesmata, hexameric cellulose synthase, structure of flagellated cells, oogamous sexual reproduction with zygote retention) and physiological characters (e.g. type of photorespiration, phytochrome system) originated within streptophyte algae.

Recent Progress

Phylogenetic studies have demonstrated that Mesostigma (flagellate) and Chlorokybus (sarcinoid) form the earliest divergence within streptophytes, as sister to all other Streptophyta including embryophytes. The question whether Charales, Coleochaetales or Zygnematales are the sister to embryophytes is still (or, again) hotly debated. Projects to study genome evolution within streptophytes including protein families and polyadenylation signals have been initiated. In agreement with morphological and physiological features, many molecular traits believed to be specific for embryophytes have been shown to predate the Chlorophyta/Streptophyta split, or to have originated within streptophyte algae. Molecular phylogenies and the fossil record allow a detailed reconstruction of the early evolutionary events that led to the origin of true land plants, and shaped the current diversity and ecology of streptophyte green algae and their embryophyte descendants.

Conclusions

The Streptophyta/Chlorophyta divergence correlates with a remarkably conservative preference for freshwater/marine habitats, and the early freshwater adaptation of streptophyte algae was a major advantage for the earliest land plants, even before the origin of the embryo and the sporophyte generation. The complete genomes of a few key streptophyte algae taxa will be required for a better understanding of the colonization of terrestrial habitats by streptophytes.Key words: Chlorophyta, Streptophyta, Embryophyta, Charales, Coleochaetales, Zygnematales, viridiplant phylogeny, land plants, genome evolution, freshwater adaptation, sporophyte origin, diversification, extinction     

Origin of land plants: Do conjugating green algae hold the key?

Background  

The terrestrial habitat was colonized by the ancestors of modern land plants about 500 to 470 million years ago. Today it is widely accepted that land plants (embryophytes) evolved from streptophyte algae, also referred to as charophycean algae. The streptophyte algae are a paraphyletic group of green algae, ranging from unicellular flagellates to morphologically complex forms such as the stoneworts (Charales). For a better understanding of the evolution of land plants, it is of prime importance to identify the streptophyte algae that are the sister-group to the embryophytes. The Charales, the Coleochaetales or more recently the Zygnematales have been considered to be the sister group of the embryophytes However, despite many years of phylogenetic studies, this question has not been resolved and remains controversial.     

Influence of UV-B radiation on photosynthetic light-response curves, absorption spectra and motility of four phytoplankton species

Several unicellular algae were exposed to artificial UV-B (280–320 nm) radiation after adaptation to high (43 W m⁻²) and low (19 W m⁻²) visible light. UV-B radiation had different effects on rates of photosynthesis, motility and absorption spectra for these species. Photosynthesis of Euglena gracilis and the diatom Phaeodactylum tricomution was more sensitive to UV-B inhibition than that of the dinoflagellates Heterocapsa triquetra and Prorocentrum minimum . Not only UV-B radiation but also high visible light had a photoinhibitory effect on photosynthesis in all four organisms. The effect on photosynthesis was observed both on the quantum yield and on the light saturation rate of photosynthesis. The dinoflagellates, in contrast to E. gracilis and P. tricorntum , absorbed strongly in the UV region (334 nm) and their absorption peaks increased after growth under high visible light or with or without UV-B radiation for one week. The swimming speed of H. triquetra decreased more after low visible light and UV-B radiation compared to high visible light and UV-B radiation. The negative effects of UV-B radiation on P. minimum and E. gracilis were most pronounced after high visible light.     

Amino Acid Compositional Shifts During Streptophyte Transitions to Terrestrial Habitats

Across the streptophyte lineage, which includes charophycean algae and embryophytic plants, there have been at least four independent transitions to the terrestrial habitat. One of these involved the evolution of embryophytes (bryophytes and tracheophytes) from a charophycean ancestor, while others involved the earliest branching lineages, containing the monotypic genera Mesostigma and Chlorokybus, and within the Klebsormidiales and Zygnematales lineages. To overcome heat, water stress, and increased exposure to ultraviolet radiation, which must have accompanied these transitions, adaptive mechanisms would have been required. During periods of dehydration and/or desiccation, proteomes struggle to maintain adequate cytoplasmic solute concentrations. The increased usage of charged amino acids (DEHKR) may be one way of maintaining protein hydration, while increased use of aromatic residues (FHWY) protects proteins and nucleic acids by absorbing damaging UV, with both groups of residues thought to be important for the stabilization of protein structures. To test these hypotheses we examined amino acid sequences of orthologous proteins representing both mitochondrion- and plastid-encoded proteomes across streptophytic lineages. We compared relative differences within categories of amino acid residues and found consistent patterns of amino acid compositional fluxuation in extra-membranous regions that correspond with episodes of terrestrialization: positive change in usage frequency for residues with charged side-chains, and aromatic residues of the light-capturing chloroplast proteomes. We also found a general decrease in the usage frequency of hydrophobic, aliphatic, and small residues. These results suggest that amino acid compositional shifts in extra-membrane regions of plastid and mitochondrial proteins may represent biochemical adaptations that allowed green plants to colonize the land.     

Responses of aquatic algae and cyanobacteria to solar UV-B

Continuous depletion of the stratospheric ozone layer has resulted in an increase in solar ultraviolet-B (UV-B; 280–315 nm) radiation reaching the Earth's surface. The consequences for aquatic phototrophic organisms of this small change in the solar spectrum are currently uncertain. UV radiation has been shown to adversely affect a number of photochemical and photobiological processes in a wide variety of aquatic organisms, such as cyanobacteria, phytoplankton and macroalgae. However, a number of photosynthetic organisms counteract the damaging effects of UV-B by synthesizing UV protective compounds such as mycosporine-like amino acids (MAAs) and the cyanobacterial sheath pigment, scytonemin. The aim of this contribution is to discuss the responses of algae and cyanobacteria to solar UV-B radiation and the role of photoprotective compounds in mitigating UV-B damage.     

Green algal phylogeny

Studies on the fine structure of green algal cells in the 1970s fundamentally revised theories on the evolution of green algae (Division Chlorophyta) and their relation to higher and drier green plants (i.e. embryophytes or land plants). Recent molecular phylogenetic work has largely confirmed some rather unorthodox proposals about which of the green algae represent the closest living relatives of higher plants. Resolution of the most ancient divergences on the green algal-land plant lineage remains elusive because of the rapidity of these evolutionary radiations and because branch topology varies with the taxa and molecular sequences sampled (as well as method of analysis). Molecular analyses within green algal groups have reinforced the value of ultrastructural characters and challenged the use of vegetative form as on overriding feature in classification.     

The Size of Cells and Organisms in Relation to the Evolution of Embryophytes

Abstract: The earliest O₂-_-evolvers were marine cyanobacteria (3.5 billion years ago) with marine eukaryotic phototrophs from 2.0 billion years ago. These organisms were, and are, poikilo-hydric, i.e., cannot remain hydrated when exposed to a desiccating atmosphere (as can occur for intertidal benthic algae and cy anobacteria at low tide). The smallest marine primarily poikilo-hydric O₂-_-evolvers are close to the lower size limit imposed by non-scaleable components such as minimum genome size and constant membrane thickness, with cyanobacterial unicells 0.65 μn in diameter and eukaryotic unicells 0.95 μm in diameter. The largest (multicellular) marine primarily aquatic poikilohydric O₂-_-evolvers are brown algae at least 60 m long and over 100 kg fresh mass; there are no obvious constraints on the max imum size of such organisms. In freshwaters the size range for primarily poikilohydric O₂-_-evolving organisms is smaller, due to the absence of very large organisms. An even smaller size range characterizes terrestrial algae and cyanobacteria which have occurred for about 1 billion years. Desiccation-tolerant cyanobacterium and algae (intertidal, freshwater, terrestrial) are at the lower end of the size ranges. Embryophytic terrestrial O₂-_-evolvers arose some 450 million years ago and were than all poikilohydric and (probably) desiccation-tolerant. Embryophytic defining structural features re quire organisms of at least 100 μm equivalent spherical diameter for both gametophyte and sporophyte phases. Primarily poi kilohydric embryophytes are not more than 1 m tall as a result of a mechanistically mysterious size limit for desiccation-tolerant organisms. Homoiohydric embryophytes evolved some 420 mil lion years ago in the sporophyte phase (later to become the dominant terrestrial vegetation) and possibly in the gameto phyte phase (although no such homoiohydric gametophytes are known today). The homoiohydric features of gas spaces, stomata, cuticle, endohydric water conducting system and water and nutrient uptake structures require an organism at least 5 mm high; this has implications for the minimum size of mega-spores and seeds. The tallest homoiohydric plants are (or were within historic times) 130 m high, with height constrained by re source costs of the synthesis and maintenance of the mechanical and water conduction systems, andbr of xylem water trans port. Secondarily poikilohydric embryophytes in aquatic, or very damp terrestrial, habitats are derived from homoiohydric plants; they retain most homoiohydric features but are not functionally homoiohydric. The smaller secondarily poikilohydric plants are less than one tenth of the size of the smallest functionally homoiohydric plants.     

Stable nuclear transformation of the Closterium peracerosum-strigosum-littorale complex

Although charophycean algae form a relevant monophyly with embryophytes and hence occupy a fundamental place in the development of Streptophyta, no tools for genetic transformation in these organisms have been developed. Here we present the first stable nuclear transformation system for the unicellular Zygnematales, the Closterium peracerosum-strigosum-littorale complex (C. psl complex), which is one of the most useful organisms for experimental research on charophycean algae. When a vector, pSA106, containing the dominant selectable marker ble (phleomycin-resistant) gene and a reporter cgfp (Chlamydomonas-adapted green fluorescent protein) gene was introduced into cells via particle bombardment, a total of 19 phleomycin-resistant cells were obtained in the presence of a low concentration of phleomycin. Six isogenic strains isolated using conditioned medium showed consecutive cgfp expression and long-term stability for phleomycin resistance. DNA analyses verified single or tandem/redundant integration of ~10 copies of pSA106 into the C. psl complex genome. We also constructed an overexpression vector, pSA1102, and then integrated a CpPI gene encoding minus-specific sex pheromone into pSA1102. Ectopic overexpression of CpPI and the pheromonal function were confirmed when the vector pSA1102_CpPI was introduced into mt(+) cells. The present efficient transformation system for the C. psl complex should provide not only a basis for molecular investigation of Closterium but also an insight into important processes in early development and evolution of Streptophyta.     

一株北极小球藻Chlorella sp.的分离鉴定及其对增强的UV-B辐射的生理生化响应

为了了解微藻对UV-B辐射增强效应的响应,以一种分离纯化于北极冰川融水的淡水微藻（Chlorella sp.）为实验材料,在不同强度UV-B辐射下对其生长、生化组分和细胞超微结构等进行了研究。研究结果显示：3种不同强度的UV-B（22μW/cm2,45μW/cm2,70μW/cm2）辐射均可导致藻的比生长速率及色素含量下降,且辐射强度越强,两者的下降越明显;而MDA含量和SOD活性会随辐照强度的增强而提高。表明辐射强度增强,UV-B对藻的伤害程度加大,而该小球藻SOD活性随UV-B强度增强而提高,表明其对上升的UV-B辐射有一定的适应能力。