Martha E. Cook

Comparative ultrastructure of plasmodesmata of Chara and selected bryophytes: toward an elucidation of the evolutionary origin of plant plasmodesmata

We have used transmission electron microscopy to examine plasmodesmata of the charophycean green alga Chara zeylanica, and of the putatively early divergent bryophytes Monoclea gottschei (liverwort), Notothylas orbicularis (hornwort), and Sphagnum fimbriatum (moss), in an attempt to learn when seed plant plasmodesmata may have originated. The three bryophytes examined have desmotubules. In addition, Monoclea was found to have branched plasmodesmata, and plasmodesmata of Sphagnum displayed densely staining regions around the neck region, as well as ring-like wall specializations. In Chara, longitudinal sections revealed endoplasmic reticulum (ER) that sometimes appeared to be associated with plasmodesmata, but this was rare, despite abundant ER at the cell periphery. Across all three fixation methods, cross-sectional views showed an internal central structure, which in some cases appeared to be connected to the plasma membrane via spoke-like structures. Plasmodesmata were present even in the incompletely formed reticulum of forming cell plates, from which we conclude that primary plasmodesmata are formed at cytokinesis in Chara zeylanica. Based on these results it appears that plasmodesmata of Chara may be less specialized than those of seed plants, and that complex plasmodesmata probably evolved in the ancestor of land plants before extant lineages of bryophytes diverged.

Aeroterrestrial Coleochaete (Streptophyta, Coleochaetales) models early plant adaptation to land

PREMISE OF THE STUDY The streptophyte water-to-land transition was a pivotal, but poorly understood event in Earth history. While some early-diverging modern streptophyte algae are aeroterrestrial (living in subaerial habitats), aeroterrestrial survival had not been tested for Coleochaete, widely regarded as obligately aquatic and one of the extant green algal genera most closely related to embryophytes. This relationship motivated a comparison of aeroterrestrial Coleochaete to lower Paleozoic microfossils whose relationships have been uncertain. METHODS We tested the ability of two species of the experimentally tractable, complex streptophyte algal genus Coleochaete Bréb. to (1) grow and reproduce when cultivated under conditions that mimic humid subaerial habitats, (2) survive desiccation for some period of time, and (3) produce degradation-resistant remains comparable to enigmatic Cambrian microfossils. KEY RESULTS When grown on mineral agar media or on quartz sand, both species displayed bodies structurally distinct from those expressed in aquatic habitats. Aeroterrestrial Coleochaete occurred as hairless, multistratose, hemispherical bodies having unistratose lobes or irregular clusters of cells with thick, layered, and chemically resistant walls that resemble certain enigmatic lower Paleozoic microfossils. Whether grown under humid conditions or air-dried for a week, then exposed to liquid water, aeroterrestrial Coleochaete produced typical asexual zoospores and germlings. Cells that had been air-dried for periods up to several months maintained their integrity and green pigmentation. CONCLUSIONS Features of modern aeroterrestrial Coleochaete suggest that ancient complex streptophyte algae could grow and reproduce in moist subaerial habitats, persist through periods of desiccation, and leave behind distinctive microfossil remains.

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.

Tracheid Structure in a Primitive Extant Plant Provides an Evolutionary Link to Earliest Fossil Tracheids

Most attempts to understand the early evolution of tracheids have centered on fossil Silurian and Devonian vascular plants, and these efforts have led to a wealth of new information on early water‐conducting cells. All of these early tracheids appear to possess secondary cell wall thickenings composed of two distinct layers: a layer adjacent to the primary cell wall that is prone to degradation (presumably during the process of fossilization) and a degradation‐resistant (possibly lignified) layer next to the cell lumen. Developmental studies of secondary wall formation in tracheary elements of extant vascular plants have been confined to highly derived seed plants, and it is evident that the basic structure of these secondary cell wall thickenings does not correspond well to those of tracheids of the Late Silurian and Early Devonian. Significantly, secondary cell wall thickenings of tracheary elements of seed plants are not known to display the coupled degradation‐prone and degradation‐resistant layers characteristic of tracheids in early tracheophytes. We report a previously unknown pattern of cell wall formation in the tracheids of a living plant. We show that in Huperzia, one of the most primitive extant vascular plants, secondary cell wall deposition in tracheids includes a first‐formed layer of wall material that is degradation‐prone (“template layer”) and a later‐formed degradation‐resistant layer (“resistant layer”). These layers match precisely the pattern of wall thickenings in the tracheids of early fossil vascular plants and provide an evolutionary link between tracheids of living vascular plants and those of their earliest fossil ancestors. Moreover, our developmental data provide the essential information for an explicit model of the early evolution of tracheid secondary wall thickenings. Finally, congruence of tracheid structure in extant Huperzia and Late Silurian and Early Devonian vascular plants supports the hypothesis of a single origin of tracheids in land plants.

MICROBIOMES OF STREPTOPHYTE ALGAE AND BRYOPHYTES SUGGEST THAT A FUNCTIONAL SUITE OF MICROBIOTA FOSTERED PLANT COLONIZATION OF LAND

Premise of research. The origin of land plants catalyzed key changes in Earth’s atmosphere and biota. Microbial associations likely nurtured earliest plants and influenced their biogeochemical roles. Because angiosperm and animal microbiomes—bacteria, archaea, microbial eukaryotes, and genes that promote host survival—are known to display lineage effects, we hypothesized that microbiomes of early-diverging modern bryophytes and phylogenetically closely related green algae might likewise reveal commonalities reflecting ancestral traits. Methodology. New metagenomic sequence data were obtained for the late-diverging streptophyte algae Chaetosphaeridium globosum and Coleochaete pulvinata and the liverwort Conocephalum conicum, representing early-diverging land plants. New 16S rDNA amplicon sequences were acquired for the charalean Nitella tenuissima. Sequence data were used to infer bacterial genera and fungi for comparisons among streptophyte microbiota and with our published microbiome data for the outgroup chlorophyte Cladophora. To enhance evolutionary signal, taxa were sampled in the same time frame and from geographically close locales. Streptophyte metagenomic data were also probed for protein markers of significant physiological and biogeochemical functions: NifH indicating nitrogen fixation, particulate MMo indicating methane oxidation, and vitamin B12 (cobalamin) indicating biosynthetic pathway enzymes. Pivotal results. Microbiota of studied streptophytes consistently included diverse N-fixing cyanobacteria and/or Rhizobiales, as well as methanotrophs and early-diverging fungi, and were more similar to each other than to Cladophora microbiota. Streptophyte metagenomic data indicated diverse nifH (nitrogen fixation) and pMMo (methane oxidation) marker sequences and vitamin B12 pathway genes. Glomalean fungi occurred with Conocephalum, consistent with field studies of modern liverworts and microfossil evidence for co-occurrence of glomaleans and early land plants. Conclusions. A suite of N fixers, methanotrophs, cobalamin producers, and early-diverging fungi was consistently associated with modern streptophyte algae and bryophytes studied, suggesting features of early land plants that have played significant, previously unrecognized roles in global nitrogen and carbon cycling for hundreds of millions of years.

Cytokinesis and nodal anatomy in the charophycean green algaChara zeylanica

SummaryCell plate formation inChara zeylanica was compared with recent models of cytokinesis in higher plants in order to gain insight into the evolutionary origin of plant cytokinetic processes. Transmission electron microscopy (TEM) reveals that while cytokinesis inC. zeylanica bears many features in common with that in higher plants, there are significant differences. Unlike that in higher plants, cytokinesis inC. zeylanica begins with a congregation of smooth membrane tubules that are closely associated with endoplasmic reticulum (ER) and Golgi membranes. Mitochondria and other organelles excluded by the phragmoplast in higher plants are present as well. Unlike in higher plants, phragmoplast microtubules persist throughout cytokinesis inC. zeylanica, and the cell plate generally forms across the whole cell at once, though development is patchy, due to small regions developing at different rates; the ends of the plate form last. By identifying aspects of cytokinesis that are different inC. zeylanica and plants, our study indicates which cytokinetic features are more likely to be derived, and which are more likely to be ancestral. In addition, we demonstrated that all nodal cells ofC. zeylanica are interconnected via plasmodesmata, lending support to the idea that, whileChara spp. are generally considered to be filamentous organisms, nodal regions may be thought of as meristemlike tissues.

STRUCTURE AND ASEXUAL REPRODUCTION OF THE ENIGMATIC CHAROPHYCEAN GREEN ALGA ENTRANSIA FIMBRIATA (KLEBSORMIDIALES, CHAROPHYCEAE)1

As the closest relatives of embryophytes, the charophycean green algae (sensu Mattox and Stewart) may reveal the evolutionary history of characters in this lineage. Recent molecular phylogenetic analysis indicates that the little‐known species Entransia fimbriata Hughes is a member of the charophycean order Klebsormidiales. In this study LM and EM were used to identify and describe additional structural characters of Entransia so that comparisons could be made with Klebsormidium and with other charophycean algae outside the order Klebsormidiales. Features that Entransia shares with various members of the genus Klebsormidium include cylindrical cells in unbranched filaments that may spiral, parietal chloroplasts that cover only part of the circumference of the cell, H‐shaped cross walls, and vegetative reproduction by both fragmentation and formation of zoospores or aplanospores. Among the characteristics that distinguish Entransia from Klebsormidium are a highly lobed chloroplast with multiple pyrenoids; a single large vacuole; short cells that die and collapse, apparently facilitating filament fragmentation; and germinating filaments with condensed adhesive at the base and a tapering spine at the tip. Although Entransia has sometimes been tentatively considered to be a member of the Zygnemataceae, the presence of a flagellate life history stage distinguishes Entransia from this group. The pyrenoids of Entransia are typical of those of charophycean algae in having traversing membranes and surrounding starch. Presence of multiple such pyrenoids in each chloroplast of Entransia supports the hypothesis that the common ancestor of charophycean algae and embryophytes had a single chloroplast with multiple pyrenoids.

Early land plant adaptations to terrestrial stress: A focus on phenolics

Publisher Summary This chapter maps the stress-related physiological traits onto a robust phylogeny for modern charophycean algae and bryophytes. Trait mapping suggests that early phenolics could have been preadaptive to the development of stable plant–microbe relationships. As in modern plants, phenolic compounds may have controlled microbial behavior, allowing microbes to live in close proximity to algae and early land plants without becoming pathogenic. The chapter also compares the aspects of phenolic chemistry among charophyceans, bryophytes, and pteridophytes and estimates the extent to which nonvascular plants could have contributed to carbon sequestration prior to the origin of vascular plants. Thioacidolysis was used as an assay for lignin-specific β-O-4 phenolic linkages in representative green algae and early-divergent land plants. Selected green algae and bryophytes were surveyed for the presence of resistant biomass and the percentages of resistant cell wall biomass were quantitatively determined. The amount of resistant organic carbon that might have been generated by early non-vascular land plants was also estimated. Adaptive utility for high levels of wall phenolics might include (1) resistance to attack by pathogenic bacteria, protists and fungi, (2) increased stability of cell walls, contributing to the ability to achieve increased height, (3) UV-damage resistance, and (4) desiccation resistance.

Evolution of Plasmodesmata

In the past20 years, improved methods of preservation, microscopy, and data analysis have lead to increased understanding of the ultrastructure of the plasmodesmata of flowering plants. Features such as proteinaceous particles, spokes, and cell wall specializations have been described. In addition, we have begun to understand the molecular components of plasmodesmata. Dynamic activity of plasmodesmata is thought to be associated with the molecules myosin, actin, and possibly centrin. Because of these advances (reviewed elsewhere in this Volume), we can now more fully appreciate the complexity of higher plant intercellular connections. Moreover, this wealth of new information provides numerous characters that can potentially be used to trace the evolution of complex plasmodesmata. In this chapter, we will review briefly the types of intercellular connections found in diverse multicellular organisms, including the green algae most closely related to plants, and the seedless plants. We will then consider whether comparisons between the intercellular connections of these diverse taxa and those of higher plants may lead to an understanding of the evolution of complex plant plasmodesmata.

Early Terrestrialization: Transition from Algal to Bryophyte Grade

Terrestrialization of planet Earth likely began more than a billion years ago with the colonization of land by bacteria, followed by eukaryotic algae much like those occupying modern soils and shallow freshwaters and the earliest embryophytes, close relatives of modern bryophytes. Colonization of land by algae and the first plants was prerequisite to the development of organic-rich soils that later supported more complex plant communities dominated by vascular plants, and the rise of land animals. Consequently, understanding terrestrialization sheds light on Earth’s early biological carbon cycling processes, which aids our understanding of global biogeochemistry in particular, and planetary science in general.