Genome-wide analysis of the GDSL esterase/lipase family genes in Physcomitrium patens and the involvement of GELP31 in spore germination

In plants, the ability to produce hydrophobic substances that would provide protection from dehydration was required for the transition to land. This genome-wide investigation outlines the evolution of GDSL-type esterase/lipase (GELP) proteins in the moss Physcomitrium patens and suggests possible functions of some genes. GELP proteins play roles in the formation of hydrophobic polymers such as cutin and suberin that protect against dehydration and pathogen attack. GELP proteins are also implicated in processes such as pollen development and seed metabolism and germination. The P. patens GELP gene family comprises 48 genes and 14 pseudogenes. Phylogenetic analysis of all P. patens GELP sequences along with vascular plant GELP proteins with reported functions revealed that the P. patens genes clustered within previously identified A, B and C clades. A duplication model predicting the expansion of the GELP gene family within the P. patens lineage was constructed. Expression analysis combined with phylogenetic analysis suggested candidate genes for functions such as defence against pathogens, cutin metabolism, spore development and spore germination. The presence of relatively fewer GELP genes in P. patens may reduce the occurrence of functional redundancy that complicates the characterization of vascular plant GELP genes. Knockout lines of GELP31, which is highly expressed in sporophytes, were constructed. Gelp31 spores contained amorphous oil bodies and germinated late, suggesting (a) role(s) of GELP31 in lipid metabolism in spore development or germination. Future knockout studies of other candidate GELP genes will further elucidate the relationship between expansion of the family and the ability to withstand the harsh land environment.

An optimized transformation protocol for Anthoceros agrestis and three more hornwort species

Land plants comprise two large monophyletic lineages, the vascular plants and the bryophytes, which diverged from their most recent common ancestor approximately 480 million years ago. Of the three lineages of bryophytes, only the mosses and the liverworts are systematically investigated, while the hornworts are understudied. Despite their importance for understanding fundamental questions of land plant evolution, they only recently became amenable to experimental investigation, with Anthoceros agrestis being developed as a hornwort model system. Availability of a high-quality genome assembly and a recently developed genetic transformation technique makes A. agrestis an attractive model species for hornworts. Here we describe an updated and optimized transformation protocol for A. agrestis, which can be successfully used to genetically modify one more strain of A. agrestis and three more hornwort species, Anthoceros punctatus, Leiosporoceros dussii, and Phaeoceros carolinianus. The new transformation method is less laborious, faster, and results in the generation of greatly increased numbers of transformants compared with the previous method. We have also developed a new selection marker for transformation. Finally, we report the development of a set of different cellular localization signal peptides for hornworts providing new tools to better understand the hornwort cell biology.

Sporogenesis in Physcomitrium patens: Intergenerational collaboration and the development of the spore wall and aperture

Although the evolution of spores was critical to the diversification of plants on land, sporogenesis is incompletely characterized for model plants such as Physcomitrium patens. In this study, the complete process of P. patens sporogenesis is detailed from capsule expansion to mature spore formation, with emphasis on the construction of the complex spore wall and proximal aperture. Both diploid (sporophytic) and haploid (spores) cells contribute to the development and maturation of spores. During capsule expansion, the diploid cells of the capsule, including spore mother cells (SMCs), inner capsule wall layer (spore sac), and columella, contribute a locular fibrillar matrix that contains the machinery and nutrients for spore ontogeny. Nascent spores are enclosed in a second matrix that is surrounded by a thin SMC wall and suspended in the locular material. As they expand and separate, a band of exine is produced external to a thin foundation layer of tripartite lamellae. Dense globules assemble evenly throughout the locule, and these are incorporated progressively onto the spore surface to form the perine external to the exine. On the distal spore surface, the intine forms internally, while the spiny perine ornamentation is assembled. The exine is at least partially extrasporal in origin, while the perine is derived exclusively from outside the spore. Across the proximal surface of the polar spores, an aperture begins formation at the onset of spore development and consists of an expanded intine, an annulus, and a central pad with radiating fibers. This complex aperture is elastic and enables the proximal spore surface to cycle between being compressed (concave) and expanded (rounded). In addition to providing a site for water intake and germination, the elastic aperture is likely involved in desiccation tolerance. Based on the current phylogenies, the ancestral plant spore contained an aperture, exine, intine, and perine. The reductive evolution of liverwort and hornwort spores entailed the loss of perine in both groups and the aperture in liverworts. This research serves as the foundation for comparisons with other plant groups and for future studies of the developmental genetics and evolution of spores across plants.

Cell wall polymers in the Phaeoceros placenta reflect developmental and functional differences across generations

The placenta of hornworts is unique among bryophytes in the restriction of transfer cells that are characterized by elaborate wall labyrinths to the gametophyte generation. During development, cells around the periphery of the sporophyte foot elongate, forming smooth-walled haustorial cells that interdigitate with gametophyte cells. Using immunogold labeling with 22 antibodies to diverse cell wall polymers, we examined compositional differences in the developmentally and morphologically distinct cell walls of gametophyte transfer cells and sporophyte haustorial cells in the placenta of Phaeoceros. As detected by Calcofluor White fluorescence, cellulose forms the cell wall scaffolding in cells on both sides of the placenta. Homogalacturonan (HG) and rhamnogalacturonan I (RG-I) pectins are abundant in both cell types, and haustrorial cells are further enriched in methyl-esterified HGs. The abundance of pectins in placental cell walls is consistent with the postulated roles of these polymers in cell wall porosity and in maintaining an acidic apoplastic pH favorable to solute transport. Xyloglucan hemicellulose, but not mannans or glucuronoxylans, are present in cell walls at the interface between the two generations with a lower density in gametophytic wall ingrowths. Arabinogalactan proteins (AGPs) are diverse along the plasmalemma of placental cells and are absent in surrounding cells in both generations. AGPs in placental cell walls may play a role in calcium binding and release associated with signal transduction as has been speculated for these glycoproteins in other plants. Callose is restricted to thin areas in cell walls of gametophyte transfer cells. In contrast to studies of transfer cells in other systems, no reaction to the JIM12 antibody against extensin was observed in Phaeoceros.

HAG1 and SWI3A/B control of male germ line development in P. patens suggests conservation of epigenetic reproductive control across land plants

KEY MESSAGE

Bryophytes as models to study the male germ line: loss-of-function mutants of epigenetic regulators HAG1 and SWI3a/b demonstrate conserved function in sexual reproduction. With the water-to-land transition, land plants evolved a peculiar haplodiplontic life cycle in which both the haploid gametophyte and the diploid sporophyte are multicellular. The switch between these phases was coined alternation of generations. Several key regulators that control the bauplan of either generation are already known. Analyses of such regulators in flowering plants are difficult due to the highly reduced gametophytic generation, and the fact that loss of function of such genes often is embryo lethal in homozygous plants. Here we set out to determine gene function and conservation via studies in bryophytes. Bryophytes are sister to vascular plants and hence allow evolutionary inferences. Moreover, embryo lethal mutants can be grown and vegetatively propagated due to the dominance of the bryophyte gametophytic generation. We determined candidates by selecting single copy orthologs that are involved in transcriptional control, and of which flowering plant mutants show defects during sexual reproduction, with a focus on the under-studied male germ line. We selected two orthologs, SWI3a/b and HAG1, and analyzed loss-of-function mutants in the moss P. patens. In both mutants, due to lack of fertile spermatozoids, fertilization and hence the switch to the diploid generation do not occur. Pphag1 additionally shows arrested male and impaired female gametangia development. We analyzed HAG1 in the dioecious liverwort M. polymorpha and found that in Mphag1 the development of gametangiophores is impaired. Taken together, we find that involvement of both regulators in sexual reproduction is conserved since the earliest divergence of land plants.

The hornworts: morphology, evolution and development

Extant land plants consist of two deeply divergent groups, tracheophytes and bryophytes, which shared a common ancestor some 500 million years ago. While information about vascular plants and the two of the three lineages of bryophytes, the mosses and liverworts, is steadily accumulating, the biology of hornworts remains poorly explored. Yet, as the sister group to liverworts and mosses, hornworts are critical in understanding the evolution of key land plant traits. Until recently, there was no hornwort model species amenable to systematic experimental investigation, which hampered detailed insight into the molecular biology and genetics of this unique group of land plants. The emerging hornwort model species, Anthoceros agrestis, is instrumental in our efforts to better understand not only hornwort biology but also fundamental questions of land plant evolution. To this end, here we provide an overview of hornwort biology and current research on the model plant A. agrestis to highlight its potential in answering key questions of land plant biology and evolution.

Differential localization of cell wall polymers across generations in the placenta of Marchantia polymorpha

To further knowledge on cell wall composition in early land plants, we localized cell wall constituents in placental cells of the liverwort Marchantia polymorpha L. using monoclonal antibodies (MAbs) in the transmission electron microscope and histochemical staining. The placenta of M. polymorpha is similar to the majority of bryophytes in that both generations contain transfer cells with extensive wall ingrowths. Although the four major cell wall polymers, i.e., cellulose, pectins, hemicelluloses, and arabinogalactan proteins, are present, there are variations in the richness and specificity across generations. An abundance of homogalacturonan pectins in all placental cell walls is consistent with maintaining cell wall permeability and an acidic apoplastic pH necessary for solute transport. Although similar in ultrastructure, transfer cell walls on the sporophyte side in M. polymorpha are enriched with xyloglucans and diverse AGPs not detected on the gametophyte side of the placenta. Gametophyte wall ingrowths are more uniform in polymer composition. Lastly, extensins and callose are not components of transfer cell walls of M. polymorpha, which deviates from studies on transfer cells in other plants. The difference in polymer localizations in transfer cell walls between generations is consistent with directional movement from gametophyte to sporophyte in this liverwort.

Reactive oxygen species are required for spore wall formation in Physcomitrella patens

A robust spore wall was a key requirement of terrestrialization by early plants. Sporopollenin in spore and pollen grain walls is thought to be polymerized and cross-linked to other macromolecular components partly through oxidative processes involving H2O2. Therefore, we investigated effects of scavengers of reactive oxygen species (ROS) on formation of spore walls in the moss, Physcomitrella patens. Exposure of sporophytes, containing spores in the process of forming walls, to ascorbate, dimethylthiourea or 4-hydroxy-TEMPO prevented normal wall development in a dose, chemical and stage-dependent manner. Mature spores, exposed while developing to a ROS scavenger, burst when mounted in water on a flat slide under a coverslip (a phenomenon we named "augmented osmolysis" since they did not burst in phosphate-buffered saline or in water on a depression slide). Additionally, walls of exposed spores were more susceptible to alkaline hydrolysis than those of control spores and some were characterized by discontinuities in the exine, anomalies in perine spine structure, abnormal intine and aperture and occasionally wall shedding. Our data support involvement of oxidative cross-linking in spore wall development, including sporopollenin polymerization or deposition, as well as a role for ROS in intine/aperture development.

Characterisation of evolutionarily conserved key players affecting eukaryotic flagellar motility and fertility using a moss model

Defects in flagella/cilia are often associated with infertility and disease. Motile male gametes (sperm cells) are an ancestral eukaryotic trait that has been lost in several lineages like flowering plants. Here, we made use of a phenotypic male fertility difference between two moss (Physcomitrella patens) ecotypes to explore spermatozoid function. We compare genetic and epigenetic variation as well as expression profiles between the Gransden and Reute ecotype to identify a set of candidate genes associated with moss male infertility. We generated a loss-of-function mutant of a coiled-coil domain containing 39 (ccdc39) gene that is part of the flagellar hydin network. Defects in mammal and algal homologues of this gene coincide with a loss of fertility, demonstrating the evolutionary conservation of flagellar function related to male fertility across kingdoms. The Ppccdc39 mutant resembles the Gransden phenotype in terms of male fertility. Potentially, several somatic (epi-)mutations occurred during prolonged vegetative propagation of Gransden, causing regulatory differences of for example the homeodomain transcription factor BELL1. Probably these somatic changes are causative for the observed male fertility defect. We propose that moss spermatozoids might be employed as an easily accessible system to study male infertility of humans and animals in terms of flagellar structure and movement.

With Over 60 Independent Losses, Stomata Are Expendable in Mosses

Because stomata in bryophytes are uniquely located on sporangia, the physiological and evolutionary constraints placed on bryophyte stomata are fundamentally different from those on leaves of tracheophytes. Although losses of stomata have been documented in mosses, the extent to which this evolutionary process occurred remains relatively unexplored. We initiated this study by plotting the known occurrences of stomata loss and numbers per capsule on the most recent moss phylogeny. From this, we identified 40 families and 74 genera that lack stomata, of which at least 63 are independent losses. No trends in stomata losses or numbers are evident in any direction across moss diversity. Extant taxa in early divergent moss lineages either lack stomata or produce pseudostomata that do not form pores. The earliest land plant macrofossils from 400 ma exhibit similar sporangial morphologies and stomatal distribution to extant mosses, suggesting that the earliest mosses may have possessed and lost stomata as is common in the group. To understand why stomata are expendable in mosses, we conducted comparative anatomical studies on a range of mosses with and without stomata. We compared the anatomy of stomate and astomate taxa and the development of intercellular spaces, including substomatal cavities, across mosses. Two types of intercellular spaces that develop differently are seen in peristomate mosses, those associated with stomata and those that surround the spore sac. Capsule architecture in astomate mosses ranges from solid in the taxa in early divergent lineages to containing an internal space that is directly connected to the conducing tissue and is involved in capsule expansion and the nourishment, hydration and development of spores. This anatomy reveals there are different architectural arrangements of tissues within moss capsules that are equally effective in accomplishing the essential processes of sporogenesis and spore dispersal. Stomata are not foundational to these processes.

Contrasting pectin polymers in guard cell walls of Arabidopsis and the hornwort Phaeoceros reflect physiological differences

BACKGROUND AND AIMS

In seed plants, stomata regulate CO2 acquisition and water relations via transpiration, while minimizing water loss. Walls of guard cells are strong yet flexible because they open and close the pore by changing shape over the substomatal cavity. Pectins are necessary for wall flexibility and proper stomata functioning. This study investigates the differences in pectin composition in guard cells of two taxa that represent key lineages of plants with stomata: Arabidopsis, an angiosperm with diurnal stomatal activity, and Phaeoceros, a bryophyte that lacks active stomatal movement.

METHODS

Using immunolocalization techniques in transmission electron microscopy, this study describes and compares the localization of pectin molecule epitopes essential to stomata function in guard cell walls of Arabidopsis and Phaeoceros.

KEY RESULTS

In Arabidopsis, unesterified homogalacturonans very strongly localize throughout guard cell walls and are interspersed with arabinan pectins, while methyl-esterified homogalacturonans are restricted to the exterior of the wall, the ledges and the junction with adjacent epidermal cells. In contrast, arabinans are absent in Phaeoceros, and both unesterified and methyl-esterified homogalacturonans localize throughout guard cell walls.

CONCLUSIONS

Arabinans and unesterified homogalacturonans are required for wall flexibility, which is consistent with active regulation of pore opening in Arabidopsis stomata. In contrast, the lack of arabinans and high levels of methyl-esterified homogalacturonans in guard cell walls of Phaeoceros are congruent with the inability of hornwort stomata to open and close with environmental change. Comparisons across groups demonstrate that variations in guard cell wall composition reflect different physiological activity of stomata in land plants.

Hornwort stomata do not respond actively to exogenous and environmental cues

Backgrounds and Aims

Because stomata in bryophytes occur on sporangia, they are subject to different developmental and evolutionary constraints from those on leaves of tracheophytes. No conclusive experimental evidence exists on the responses of hornwort stomata to exogenous stimulation.

Methods

Responses of hornwort stomata to abscisic acid (ABA), desiccation, darkness and plasmolysis were compared with those in tracheophyte leaves. Potassium ion concentrations in the guard cells and adjacent cells were analysed by X-ray microanalysis, and the ontogeny of the sporophytic intercellular spaces was compared with those of tracheophytes by cryo-scanning electron microscopy.

Key Results

The apertures in hornwort stomata open early in development and thereafter remain open. In hornworts, the experimental treatments, based on measurements of >9000 stomata, produced only a slight reduction in aperture dimensions after desiccation and plasmolysis, and no changes following ABA treatments and darkness. In tracheophytes, all these treatments resulted in complete stomatal closure. Potassium concentrations are similar in hornwort guard cells and epidermal cells under all treatments at all times. The small changes in hornwort stomatal dimensions in response to desiccation and plasmolysis are probably mechanical and/or stress responses of all the epidermal and spongy chlorophyllose cells, affecting the guard cells. In contrast to their nascent gas-filled counterparts across tracheophytes, sporophytic intercellular spaces in hornworts are initially liquid filled.

Conclusions

Our experiments demonstrate a lack of physiological regulation of opening and closing of stomata in hornworts compared with tracheophytes, and support accumulating developmental and structural evidence that stomata in hornworts are primarily involved in sporophyte desiccation and spore discharge rather than the regulation of photosynthesis-related gaseous exchange. Our results run counter to the notion of the early acquisition of active control of stomatal movements in bryophytes as proposed from previous experiments on mosses.

The Ceratopteris (fern) developing motile gamete walls contain diverse polysaccharides, but not pectin

Unlike most plant cell walls, the five consecutive walls laid down during spermatogenesis in the model fern Ceratopteris contain sparse cellulose, lack pectin and are enriched with callose and hemicelluloses. Seed-free plants like bryophytes and pteridophytes produce swimming male gametes for sexual reproduction. During spermatogenesis, unique walls are formed that are essential to the appropriate development and maturation of the motile gametes. Other than the detection of callose and general wall polysaccharides in scattered groups, little is known about the sequence of wall formation and the composition of these walls during sperm cell differentiation in plants that produce swimming sperm. Using histochemistry and immunogold localizations, we examined the distribution of callose, cellulose, mannan and xylan-containing hemicelluloses, and homogalacturonan (HG) pectins in the special walls deposited during spermatogenesis in Ceratopteris. Five walls are produced in sequence and each has a unique fate. The first wall (W1) contains callose and sparse xylan-containing hemicelluloses. Wall two (W2) is thin and composed of cellulose crosslinked by xylan-containing hemicelluloses. The third wall (W3) is thick and composed entirely of callose, and the fourth wall (W4) is built of cellulose heavily crosslinked by galactoxyloglucan hemicelluloses. Wall five (W5) is an arabinogalactan protein (AGP)-rich matrix in which the gamete changes shape and multiple flagella elongate. We detected no esterified or unesterified HG pectins in any of the walls laid down during spermatogenesis. To consider evolutionary modifications in cell walls associated with motile gametes, comparisons are presented with male gametophyte and spermatogenous cell walls across plant groups.

Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves

As one of the earliest plant groups to evolve stomata, hornworts are key to understanding the origin and function of stomata. Hornwort stomata are large and scattered on sporangia that grow from their bases and release spores at their tips. We present data from development and immunocytochemistry that identify a role for hornwort stomata that is correlated with sporangial and spore maturation. We measured guard cells across the genera with stomata to assess developmental changes in size and to analyze any correlation with genome size. Stomata form at the base of the sporophyte in the green region, where they develop differential wall thickenings, form a pore, and die. Guard cells collapse inwardly, increase in surface area, and remain perched over a substomatal cavity and network of intercellular spaces that is initially fluid filled. Following pore formation, the sporophyte dries from the outside inwardly and continues to do so after guard cells die and collapse. Spore tetrads develop in spore mother cell walls within a mucilaginous matrix, both of which progressively dry before sporophyte dehiscence. A lack of correlation between guard cell size and DNA content, lack of arabinans in cell walls, and perpetually open pores are consistent with the inactivity of hornwort stomata. Stomata are expendable in hornworts, as they have been lost twice in derived taxa. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. Our findings identify an architecture and fate of stomata in hornworts that is ancient and common to plants without sporophytic leaves.

Patterning of stomata in the moss Funaria: a simple way to space guard cells

BACKGROUND AND AIMS

Studies on stomatal development and the molecular mechanisms controlling patterning have provided new insights into cell signalling, cell fate determination and the evolution of these processes in plants. To fill a major gap in knowledge of stomatal patterning, this study describes the pattern of cell divisions that give rise to stomata and the underlying anatomical changes that occur during sporophyte development in the moss Funaria.

METHODS

Developing sporophytes at different stages were examined using light, fluorescence and electron microscopy; immunogold labelling was used to investigate the presence of pectin in the newly formed cavities.

KEY RESULTS

Substomatal cavities are liquid-filled when formed and drying of spaces is synchronous with pore opening and capsule expansion. Stomata in mosses do not develop from a self-generating meristemoid as in Arabidopsis, but instead they originate from a protodermal cell that differentiates directly into a guard mother cell. Epidermal cells develop from protodermal or other epidermal cells, i.e. there are no stomatal lineage ground cells.

CONCLUSIONS

Development of stomata in moss occurs by differentiation of guard mother cells arranged in files and spaced away from each other, and epidermal cells that continue to divide after stomata are formed. This research provides evidence for a less elaborated but effective mechanism for stomata spacing in plants, and we hypothesize that this operates by using some of the same core molecular signalling mechanism as angiosperms.

Arabinogalactan proteins and arabinan pectins abound in the specialized matrices surrounding female gametes of the fern Ceratopteris richardii

Both male and female gametes of archegoniates are highly specialized cells surrounded by an extraprotoplasmic matrix rich in AGPs, which are speculated to facilitate development and gamete fusion through Ca 2+) oscillations. An additional layer, the egg envelope, forms around the egg periphery, except at the fertilization pore, and contains arabinose-rich polymers that presumably impart flexibility for the rapidly growing zygote and embryo. The abundant AGPs and arabinan pectins associated with the eggs of C. richardii not only are integral to development, fertilization, and early embryogenesis, but also may be involved in desiccation tolerance important to the survival of the reproductive gametophyte. A defining feature of gametogenesis in archegoniates is the deposition of a special matrix outside of the plasmalemma of both egg and sperm cells that displaces the primary cell wall away from the protoplasm. It is within this matrix that gamete differentiation occurs. In leptosporangiate ferns, maturation of the egg cell involves the deposition of a second specialized wall, the so-called egg envelope that surrounds the cell except at the fertilization pore, a narrow site where gamete fusion takes place. We provide the first conclusive evidence of the macromolecular constituents in the unique structures surrounding fern egg cells before and after fertilization. To test the hypotheses that the egg extracellular matrix contains arabinogalactan proteins (AGPs) as does the sperm cell matrix, and that cell wall polysaccharides, especially pectins, are components of the egg envelope, we examined the expression patterns of AGPs and cell wall constituents during oogenesis in Ceratopteris richardii. Utilizing histochemical stains for callose, cellulose and AGPs coupled with immunogold localizations employing a suite of monoclonal antibodies to cell wall components (JIM13, JIM8, LM2, LM5, LM6, LM19, LM20 and anticallose), we demonstrate that AGPs, but not pectins, are abundant in the matrix around egg cells and degrading neck canal and ventral canal cells during archegonial development. A striking finding is that both AGPs and (1,5)-α-L-arabinan pectin epitopes are principle components of the egg envelope before and after fertilization, suggesting that they are important in both egg maturation and gamete fusion.

Callose is integral to the development of permanent tetrads in the liverwort Sphaerocarpos

A striking feature of the liverwort Sphaerocarpos is that pairs of male and female spores remain united in permanent tetrads. To identify the nature of this phenomenon and to test the hypothesis that callose is involved, we examined spore wall development in Sphaerocarpos miche lii, with emphasis on the appearance, location and fate of callose vis-à-vis construction of the sculptoderm. All stages of sporogenesis were examined using differential interference contrast optics, and aniline blue fluorescence to locate callose. For precise localization, specimens were immunogold labeled with anti-callose antibody and observed in the transmission electron microscope. Callose plays a role in Sphaerocarpos spore wall development not described in any other plant, including other liverworts. A massive callose matrix forms outside of the sculptured sporocyte plasmalemma that predicts spore wall ornamentation. Consequently, layers of exine form across adjacent spores uniting them. Spore wall development occurs entirely within the callose and involves the production of six layers of prolamellae that give rise to single or stacked tripartite lamellae (TPL). Between spores, an anastomosing network of exine layers forms in lieu of intersporal septum development. As sporopollenin assembles on TPL, callose progressively disappears from the inside outward leaving layers of sporopollenin impregnated exine, the sculptoderm, overlying a thick fibrillar intine. This developmental mechanism provides a direct pathway from callose deposition to sculptured exine that does not involve the intermediary primexine found in pollen wall development. The resulting tetrad, encased in a single wall, provides a simple model for development of permanent dyads and tetrads in the earliest fossil plants.

Multiflagellated sperm cells of Ceratopteris richardii are bathed in arabinogalactan proteins throughout development

•

PREMISE OF THE STUDY

Sperm cell differentiation in ferns involves the origin of an elaborate locomotory apparatus, including 70+ flagella, and the structural modification of every cellular component. Because arabinogalactan proteins (AGPs) are implicated in molecular signaling and in regulation of plant development, we speculated that these glycoproteins would be present during spermiogenesis in ferns.•

METHODS

Using β-glucosyl Yariv reagents that specifically bind to and inhibit AGPs and immunogold localizations with monoclonal antibodies JIM13, JIM8, and LM6, we examined the specific expression patterns of AGPs and inhibited their function during sperm cell development in the model fern Ceratopteris richardii.•

KEY RESULTS

Developing sperm cells stained intensely with Yariv phenylglycosides, demonstrating the presence of AGPs. JIM13-AGP epitopes were widespread throughout development in the expanding extraprotoplasmic matrix (EPM) in which flagella elongate, cytoplasm is eliminated, and spherical spermatids become coiled. JIM8 and LM6 epitopes localized to the plasmalemma on growing flagella and on the rapidly changing sperm cell body. Spermatids treated with β-glucosyl lacked an EPM and formed fewer, randomly arranged flagella.•

CONCLUSIONS

We demonstrated that AGPs are abundant in the EPM and along the plasmalemma and that the three AGP epitopes have specific expression patterns during development. Coupled with inhibition studies, these results identify AGPs as critical to the formation of an extraprotoplasmic matrix and the consequent origin and development of flagella in an orderly and precise fashion around the cell. We speculate that AGPs may play additional roles as signaling molecules involved in cell shaping, cytoskeletal development, vesicle trafficking, and cytoplasmic elimination.

Bioinformatic analysis of ciliary transition zone proteins reveals insights into the evolution of ciliopathy networks

Background

Cilia are critical for diverse functions, from motility to signal transduction, and ciliary dysfunction causes inherited diseases termed ciliopathies. Several ciliopathy proteins influence developmental signalling and aberrant signalling explains many ciliopathy phenotypes. Ciliary compartmentalisation is essential for function, and the transition zone (TZ), found at the proximal end of the cilium, has recently emerged as a key player in regulating this process. Ciliary compartmentalisation is linked to two protein complexes, the MKS and NPHP complexes, at the TZ that consist largely of ciliopathy proteins, leading to the hypothesis that ciliopathy proteins affect signalling by regulating ciliary content. However, there is no consensus on complex composition, formation, or the contribution of each component.

Results

Using bioinformatics, we examined the evolutionary patterns of TZ complex proteins across the extant eukaryotic supergroups, in both ciliated and non-ciliated organisms. We show that TZ complex proteins are restricted to the proteomes of ciliated organisms and identify a core conserved group (TMEM67, CC2D2A, B9D1, B9D2, AHI1 and a single TCTN, plus perhaps MKS1) which are present in >50% of all ciliate/flagellate organisms analysed in each supergroup. The smaller NPHP complex apparently evolved later than the larger MKS complex; this result may explain why RPGRIP1L, which forms the linker between the two complexes, is not one of the core conserved proteins. We also uncovered a striking correlation between lack of TZ proteins in non-seed land plants and loss of TZ-specific ciliary Y-links that link microtubule doublets to the membrane, consistent with the interpretation that these proteins are structural components of Y-links, or regulators of their formation.

Conclusions

This bioinformatic analysis represents the first systematic analysis of the cohort of TZ complex proteins across eukaryotic evolution. Given the near-ubiquity of only 6 proteins across ciliated eukaryotes, we propose that the MKS complex represents a dynamic complex built around these 6 proteins and implicated in Y-link formation and ciliary permeability.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-531) contains supplementary material, which is available to authorized users.

Moss stomata in highly elaborated Oedipodium (Oedipodiaceae) and highly reduced Ephemerum (Pottiaceae) sporophytes are remarkably similar

PREMISE OF THE STUDY

Mosses are central in understanding the origin, diversification, and early function of stomata in land plants. Oedipodium, the first extant moss with true stomata, has an elaborated capsule with numerous long-pored stomata; in contrast, the reduced and short-lived Ephemerum has few round-pored stomata. Here we present a comparative study of sporophyte anatomy and ultrastructure of stomata in two divergent mosses and its implications for stomata diversity and function.

METHODS

Mature sporophytes of two moss species were studied using light, fluorescence, and scanning and transmission electron microscopy. Immunolocalization of pectin was conducted on Oedipodium using the LM19 antibody.

KEY RESULTS

OEDIPODIUM capsules have extensive spongy tissue along the apophysis, whereas those of Ephemerum have minimal substomatal cavities. Stomatal ultrastructure and wall thickenings are highly similar. Sporophytes are covered by a cuticle that is thicker on guard cells and extends along walls surrounding the pore. Epicuticular waxes and pectin clog pores in old capsules.

CONCLUSIONS

Ultrastructure of stomata in these mosses is similar to each other and less variable than that of tracheophytes. Anatomical features such as the presence of a cuticle, water-conducting cells, and spongy tissues with large areas for gas exchange are more pronounced in Oedipodium sporophytes and support the role of stomata in gas exchange and water transport during development and maturation. These features are modified in the reduced sporophytes of Ephemerum. Capsule anatomy coupled with the exclusive existence of stomata on capsules supports the concept that stomata in moss may also facilitate drying and dispersal of spores.

Microanatomy of the placenta of Lycopodium obscurum: novel design in an underground embryo

BACKGROUND AND AIMS

Long-lived underground populations of mycoheterotrophic gametophytes and attached sporophytes at various developmental stages occur in lycophytes. Young underground sporophytes obtain carbon solely from the gametophyte and establish nutritional independence only after reaching the soil surface, which may take several years. This prolonged period of matrotrophy exceeds that of bryophytes. The foot is massive and provides the lifeline for sporophyte establishment, yet the fine structure of the placental region is unexplored in lycophytes with underground gametophytes.

METHODS

Gametophytes with attached embryos/young sporophytes of Lycopodium obscurum were collected in nature, processed and examined by light and transmission electron microscopy.

KEY RESULTS

Three ultrastructurally distinct regions were identified within a single foot of a sporophyte emerging from the soil. Young foot regions actively divide, and have direct contact with and show little differentiation from gametophyte cells. In unlobed foot areas, cells in both generations exhibit polarity in content and indicate unidirectional transport of carbon reserves into the foot toward the developing shoot and root. The foot has inconspicuous wall ingrowths. Highly lobed foot regions contain peripheral transfer cells with prominent wall ingrowths that absorb nutrients from degenerating gametophyte cells.

CONCLUSIONS

Variability within a single placenta is consistent with an invasive and long-lived foot. The late appearance of wall ingrowths in transfer cells reflects this dynamic ever-growing embryo. Placental features in lycophytes are related to the unique reorientation of all embryonic regions during development. Small placentas with wall ingrowths in both generations characterize ephemeral embryos in green gametophytes, while short-lived and repositioning embryos of heterosporous taxa are devoid of transfer cells. Transfer cell evolution across embryophytes is riddled with homoplasy and reflects diverse patterns of embryology. Scrutiny of placental evolution must include consideration of nutritional status and life history strategies of the gametophyte and young sporophyte.

The origin of the sporophyte shoot in land plants: a bryological perspective

BACKGROUND

Land plants (embryophytes) are monophyletic and encompass four major clades: liverworts, mosses, hornworts and polysporangiophytes. The liverworts are resolved as the earliest divergent lineage and the mosses as sister to a crown clade formed by the hornworts and polysporangiophytes (lycophytes, monilophytes and seed plants). Alternative topologies resolving the hornworts as sister to mosses plus polysporangiophytes are less well supported. Sporophyte development in liverworts depends only on embryonic formative cell divisions. A transient basal meristem contributes part of the sporophyte in mosses. The sporophyte body in hornworts and polysporangiophytes develops predominantly by post-embryonic meristematic activity.

SCOPE

This paper explores the origin of the sporophyte shoot in terms of changes in embryo organization. Pressure towards amplification of the sporangium-associated photosynthetic apparatus was a major driver of sporophyte evolution. Starting from a putative ancestral condition in which a transient basal meristem produced a sporangium-supporting seta, we postulate that in the hornwort-polysporangiophyte lineage the basal meristem acquired indeterminate meristematic activity and ectopically expressed the sporangium morphogenetic programme. The resulting sporophyte body plan remained substantially unaltered in hornworts, whereas in polysporangiophytes the persistent meristem shifted from a mid-embryo to a superficial position and was converted into an ancestral shoot apical meristem with the evolution of sequential vegetative and reproductive growth.

CONCLUSIONS

The sporophyte shoot is interpreted as a sterilized sporangial axis interpolated between the embryo and the fertile sporangium. With reference to the putatively ancestral condition found in mosses, the sporophyte body plans in hornworts and polysporangiophytes are viewed as the product of opposite heterochronic events, i.e. an anticipation and a delay, respectively, in the development of the sporangium. In either case the result was a pedomorphic sporophyte permanently retaining juvenile characters.

Major transitions in the evolution of early land plants: a bryological perspective

Background Molecular phylogeny has resolved the liverworts as the earliest-divergent clade of land plants and mosses as the sister group to hornworts plus tracheophytes, with alternative topologies resolving the hornworts as sister to mosses plus tracheophytes less well supported. The tracheophytes plus fossil plants putatively lacking lignified vascular tissue form the polysporangiophyte clade. Scope This paper reviews phylogenetic, developmental, anatomical, genetic and paleontological data with the aim of reconstructing the succession of events that shaped major land plant lineages. Conclusions Fundamental land plant characters primarily evolved in the bryophyte grade, and hence the key to a better understanding of the early evolution of land plants is in bryophytes. The last common ancestor of land plants was probably a leafless axial gametophyte bearing simple unisporangiate sporophytes. Water-conducting tissue, if present, was restricted to the gametophyte and presumably consisted of perforate cells similar to those in the early-divergent bryophytes Haplomitrium and Takakia. Stomata were a sporophyte innovation with the possible ancestral functions of producing a transpiration-driven flow of water and solutes from the parental gametophyte and facilitating spore separation before release. Stomata in mosses, hornworts and polysporangiophytes are viewed as homologous, and hence these three lineages are collectively referred to as the 'stomatophytes'. An indeterminate sporophyte body (the sporophyte shoot) developing from an apical meristem was the key innovation in polysporangiophytes. Poikilohydry is the ancestral condition in land plants; homoiohydry evolved in the sporophyte of polysporangiophytes. Fungal symbiotic associations ancestral to modern arbuscular mycorrhizas evolved in the gametophytic generation before the separation of major present-living lineages. Hydroids are imperforate water-conducting cells specific to advanced mosses. Xylem vascular cells in polysporangiophytes arose either from perforate cells or de novo. Food-conducting cells were a very early innovation in land plant evolution. The inferences presented here await testing by molecular genetics.

Complete plastome sequences of Equisetum arvense and Isoetes flaccida: implications for phylogeny and plastid genome evolution of early land plant lineages

Background

Despite considerable progress in our understanding of land plant phylogeny, several nodes in the green tree of life remain poorly resolved. Furthermore, the bulk of currently available data come from only a subset of major land plant clades. Here we examine early land plant evolution using complete plastome sequences including two previously unexamined and phylogenetically critical lineages. To better understand the evolution of land plants and their plastomes, we examined aligned nucleotide sequences, indels, gene and nucleotide composition, inversions, and gene order at the boundaries of the inverted repeats.

Results

We present the plastome sequences of Equisetum arvense, a horsetail, and of Isoetes flaccida, a heterosporous lycophyte. Phylogenetic analysis of aligned nucleotides from 49 plastome genes from 43 taxa supported monophyly for the following clades: embryophytes (land plants), lycophytes, monilophytes (leptosporangiate ferns + Angiopteris evecta + Psilotum nudum + Equisetum arvense), and seed plants. Resolution among the four monilophyte lineages remained moderate, although nucleotide analyses suggested that P. nudum and E. arvense form a clade sister to A. evecta + leptosporangiate ferns. Results from phylogenetic analyses of nucleotides were consistent with the distribution of plastome gene rearrangements and with analysis of sequence gaps resulting from insertions and deletions (indels). We found one new indel and an inversion of a block of genes that unites the monilophytes.

Conclusions

Monophyly of monilophytes has been disputed on the basis of morphological and fossil evidence. In the context of a broad sampling of land plant data we find several new pieces of evidence for monilophyte monophyly. Results from this study demonstrate resolution among the four monilophytes lineages, albeit with moderate support; we posit a clade consisting of Equisetaceae and Psilotaceae that is sister to the "true ferns," including Marattiaceae.

Novel localization of callose in the spores of Physcomitrella patens and phylogenomics of the callose synthase gene family

BACKGROUND AND AIMS

Callose involvement in spore development is a plesiomorphic feature of land plants. Correlated light, fluorescence and immuno-electron microscopy was conducted on the developing spores of Physcomitrella patens to probe for callose. Using a bioinformatic approach, the callose synthase (PpCalS) genes were annotated and PpCalS and AtCalS gene families compared, testing the hypothesis that an exine development orthologue is present in P. patens based on deduced polypeptide similarity with AtCalS5, a known exine development gene.

METHODS

Spores were stained with aniline blue fluorescent dye. Capsules were prepared for immuno-light and immuno-electron microscopy by gold labelling callose epitopes with monoclonal antibody. BLAST searches were conducted using the AtCalS5 sequence as a query against the P. patens genome. Phylogenomic analysis of the CalS gene family was conducted using PAUP (v.4.1b10).

KEY RESULTS

Callose is briefly present in the aperture of developing P. patens spores. The PpCalS gene family consists of 12 copies that fall into three distinct clades with AtCalS genes. PpCalS5 is an orthologue to AtCalS5 with highly conserved domains and 64 % similarity of their deduced polypeptides.

CONCLUSIONS

This is the first study to identify the presence of callose in moss spores. AtCalS5 was previously shown to be involved in pollen exine development, thus making PpCalS5 a suspect gene involved in moss spore exine development.

Exploding a myth: the capsule dehiscence mechanism and the function of pseudostomata in Sphagnum

The nineteenth century air-gun explanation for explosive spore discharge in Sphagnum has never been tested experimentally. Similarly, the function of the numerous stomata ubiquitous in the capsule walls has never been investigated. Both intact and pricked Sphagnum capsules, that were allowed to dry out, all dehisced over an 8-12 h period during which time the stomatal guard cells gradually collapsed and their potassium content, measured by X-ray microanalysis in a cryoscanning electron microscope, gradually increased. By contrast, guard cell potassium fell in water-stressed Arabidopsis. The pricking experiments demonstrate that the air-gun notion for explosive spore discharge in Sphagnum is inaccurate; differential shrinkage of the capsule walls causes popping off the rigid operculum. The absence of evidence for a potassium-regulating mechanism in the stomatal guard cells and their gradual collapse before spore discharge indicates that their sole role is facilitation of sporophyte desiccation that ultimately leads to capsule dehiscence. Our novel functional data on Sphagnum, when considered in relation to bryophyte phylogeny, suggest the possibility that stomata first appeared in land plants as structures that facilitated sporophyte drying out before spore discharge and only subsequently acquired their role in the regulation of gaseous exchange.

Embryology of Ceratopteris richardii (Pteridaceae, tribe Ceratopterideae), with emphasis on placental development

This comprehensive study of early embryology in Ceratopteris richardii combines light microscopy with the first ultrastructural evaluation of any pteridophyte embryo. Emphasis is placed on ontogeny of the foot and placental transfer cells. The embryology of C. richardii shares many similarities with that of other polypodiacious ferns while exhibiting distinctive division patterns. Formative embryonic stages have been reconstructed into three-dimensional models for ease of interpretation. The zygote divides perpendicular to the gametophyte plane and anterioposterior axis. This division establishes a prone embryological habit that maximizes rapid independent establishment of a leaf-root axis in a cordate gametophyte. After the formation of a globular eight-celled stage, initials of the first leaf, and root and shoot apical meristems are defined early by discrete formative divisions. Concomitantly, the foot expands and differentiates to transport nutrients from the gametophyte for the developing embryonic organs. Transfer cell wall ingrowth deposition begins in the gametophyte placental cells before the adjacent sporophyte cells just after the eight-celled stage. These observations provide an anatomical framework for future comparative developmental genetic studies of embryogenesis in free-sporing plants.

Structural and immunocytochemical characterization of the Ginkgo biloba L. sperm motility apparatus

Ginkgo biloba and the cycads are the only extant seed plants with motile sperm cells. However, there has been no immunocytochemical characterization of these gametes to determine if they share characteristics with the flagellated sperm found in bryophytes and pteridophytes or might give clues as to the relationships to nonflagellated sperm in all other seed plants. To determine characteristics of proteins associated with the motility apparatus in these motile sperm, we probed thin sections of developing spermatogenous cells of Ginkgo biloba with antibodies to acetylated and tyrosinated tubulin and monoclonal antibodies that recognize mammalian centrosomes and centrin. The blepharoplast that occurs as a precursor to the motility apparatus consists of an amorphous core, pitted with cavities containing microtubules and a surface studded with probasal bodies. The probasal bodies and microtubules within the blepharoplast cavities are labeled with antibodies specific to acetylated tubulin. Positive but weak reactions of the blepharoplast core occur with the centrosomereactive antibodies MPM-2 and C-9. Reactions to centrin antibodies are negative at this developmental stage. From this pre-motility apparatus structure, an assemblage of about 1,000 flagella and associated structures arises as the precursor to the motility apparatus for the sperm. The flagellar apparatus consists of a three-layered multilayered structure that subtends a layer of spline microtubules, a zone of amorphous material similar to that in the blepharoplast, and the flagellar band. Centrin antibodies react strongly with the multilayered structure, the transition zone of the flagella, and fibrillar material near the flagellar base at the surface of the amorphous material. Both the spline microtubules and all of the tubules in the flagella react strongly with the antibodies to acetylated tubulin. These localizations are consistent with the localizations of these components in pteridophyte and bryophyte spermatogenous cells, although the blepharoplast material surrounding and connecting flagellar bases does not occur in the seedless (nonseed) land plants. These data indicate that despite the large size of ginkgo gametes and the taxonomic separation between pteridophytes and Ginkgo biloba, similar proteins in gametes of both groups perform similar functions and are therefore homologous among these plants. Moreover, the presence of acetylated tubulin in bands of microtubules may be a characteristic shared with more derived non-flagellated sperm of other conifers and angiosperms.

Characterization of thesleepy spermmutant in the fernCeratopteris richardii: A new model for the study of axonemal function

Structural and motility characteristics of the zzz1 "sleepy sperm" mutant of Ceratopteris richardii Brongn. are described using scanning electron, transmission electron, light, and fluorescence microscopy. Although the zzz1 phenotype segregates as the product of a single gene mutation, the expression of the mutation varies within a single haploid gametophyte. The majority of mutant sperm cells are slow to initiate motility and typically swim in a slow, spiraling pattern. However, motility phenotypes range from immotile to wild-type (normal). This variable phenotypic expression is associated with a wide range of defects in the microtubule systems, especially the flagellar axonemes and the spline, a structure that provides a structural backbone for the cell. Defects in the spline microtubule array are associated with atypical cell shape and organellar positioning. Axonemal aberrations include an absence of the central pair complex and clumped flagella. We hypothesize that the gene product encoded by the zzz1 locus is not required for the establishment of the cytoskeletal elements necessary for sperm motility but rather is needed for stability and (or) repair (recycling) of these structures. This interpretation is consistent with the variable expression of zzz1 sperm, which appears to be age dependent.Key words: axoneme, microtubule, motility mutant, sperm cell, ultrastructure.

Architecture of the sperm cell of Psilotum

In this correlated SEM (scanning electron microscope) and TEM (transmission electron microscope) investigation, we describe architectural details of the multiflagellated sperm cell of Psilotum nudum. Comparisons with other pteridophytes are made to (1) assess the placement of Psilotum among pteridophyte taxa and (2) evaluate structural modifications of sperm cells during land plant evolution. The released spermatozoid of Psilotum coils 2.0 revolutions and is outlined by a parallel band of up to 190 microtubules. The elongated nucleus is highly compacted and parallels the cellular coils with numerous mitochondria and starch-laden plastids distributed along its length. Along the anterior coil is an elaborate locomotory apparatus that includes ∼36 flagella that are inserted into the cell by basal bodies. Subtending the basal bodies is the multilayered structure, which consists of a long narrow lamellar strip and an overlying band of microtubules. An elongated anterior mitochondrion underlies the multilayered structure. Additional amyloplasts and mitochondria are aggregated along the anterior coil in association with the locomotory apparatus, while a fibrous band encircles the leading edge of the cell. Salient features of this cell, including details of the locomotory apparatus, structure and position of organelles, and arrangement of the spline, are shared by spermatozoids of Equisetum and ferns (including eusporangiate and leptosporangiate taxa). Thus, this study provides morphological support for the hypothesis that Psilotum nudum is a member of an assemblage that includes ferns and Equisetum. However, the less streamlined architecture of Psilotum gametes and the lack of architectural features shared with any specific taxon examined to date suggest that Psilotum is an early divergent fern, with relatively remote affinities to Ophioglossaceae and Equisetaceae.

Vegetative and reproductive innovations of early land plants: implications for a unified phylogeny

As the oldest extant lineages of land plants, bryophytes provide a living laboratory in which to evaluate morphological adaptations associated with early land existence. In this paper we examine reproductive and structural innovations in the gametophyte and sporophyte generations of hornworts, liverworts, mosses and basal pteridophytes. Reproductive features relating to spermatogenesis and the architecture of motile male gametes are overviewed and evaluated from an evolutionary perspective. Phylogenetic analyses of a data set derived from spermatogenesis and one derived from comprehensive morphogenetic data are compared with a molecular analysis of nuclear and mitochondrial small subunit rDNA sequences. Although relatively small because of a reliance on water for sexual reproduction, gametophytes of bryophytes are the most elaborate of those produced by any land plant. Phenotypic variability in gametophytic habit ranges from leafy to thalloid forms with the greatest diversity exhibited by hepatics. Appendages, including leaves, slime papillae and hairs, predominate in liverworts and mosses, while hornwort gametophytes are strictly thalloid with no organized external structures. Internalization of reproductive and vegetative structures within mucilage-filled spaces is an adaptive strategy exhibited by hornworts. The formative stages of gametangial development are similar in the three bryophyte groups, with the exception that in mosses apical growth is intercalated into early organogenesis, a feature echoed in moss sporophyte ontogeny. A monosporangiate, unbranched sporophyte typifies bryophytes, but developmental and structural innovations suggest the three bryophyte groups diverged prior to elaboration of this generation. Sporophyte morphogenesis in hornworts involves non-synchronized sporogenesis and the continued elongation of the single sporangium, features unique among archegoniates. In hepatics, elongation of the sporophyte seta and archegoniophore is rapid and requires instantaneous wall expandability and hydrostatic support. Unicellular, spiralled elaters and capsule dehiscence through the formation of four regular valves are autapomorphies of liverworts. Sporophytic sophistications in the moss clade include conducting tissue, stomata, an assimilative layer and an elaborate peristome for extended spore dispersal. Characters such as stomata and conducting cells that are shared among sporophvtes of mosses, hornworts and pteridophytes are interpreted as parallelisms and not homologies. Our phylogenetic analysis of three different data sets is the most comprehensive to date and points to a single phylogenetic solution for the evolution of basal embryophytes. Hornworts are supported as the earliest divergent embryophyte clade with a moss/liverwort clade sister to tracheophytes. Among pteridophytes, lycophytes are monophyletic and an assemblage containing ferns, Equisetum and psilophytes is sister to seed plants. Congruence between morphological and molecular hypotheses indicates that these data sets are tracking the same phylogenetic signal and reinforces our phylogenetic conclusions. It appears that total evidence approaches are valuable in resolving ancient radiations such as those characterizing the evolution of early embryophytes. More information on land plant phylogeny can be found at: http: //www.science.siu.edu/ landplants/index.html.

Conducting tissues and phyletic relationships of bryophytes

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Conducting tissues are unknown in the anthocerotes. Water-conducting cells (WCCs) with walls perforated by plasmodesma-derived pores occur in the Calobryales and Pallaviciniaceae (Metzgeriales among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchyma cells of the leafy stem and seta in other mosses including Sphagnumn. A similar organization, suggested to reflect specialization in long-distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putative food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.

Microtubule organizing centers and the origin of centrioles during spermatogenesis in the pteridophyte Phylloglossum

Spermatogenesis in the lycophyte Phylloglossum is characterized by profound ultrastructural changes that involve complex microtubule arrays and discrete microtubule organizing centers (MTOCs). The first visible MTOC is an electron-opaque acentriolar centrosome that organizes the mitotic spindles in late spermatogeneous cells. In the spermatid mother cell, centrioles arise de novo within the pericentriolar matrix of the MTOC. Approximately 20 centrioles, which ultimately function as basal bodies, originate in each of two branched "blepharoplasts." Constituent centrioles of each organelle radiate from a central region where they are interconnected by cartwheel cylinders, each with nine-fold symmetry. The development and structure of this novel centriolar-generating organelle suggests a direct evolutionary link with the bicentriole of other lycophytes, and are consistent with the concept that multiflagellated spermatozoids in Phylloglossum evolved independently of those in other pteridophytes. During spermiogenesis, two additional structurally defined MTOCs organize the ton and locomotory apparatus, which comprises 20 staggered flagella over a multilayered structure. An MTOC that overlies the multilayered structure and consists of a cloud of electron-opaque material is involved in repositioning basal bodies and generating flagella. The spline, a band of up to 200 microtubules, provides the architectural framework for development and maintenance of cell shape and is organized by the lamellar strip, a highly structured MTOC. Microtubule arrays during spermatogenesis in Phylloglossum are diverse and include mitotic, cytokinetic, cytoskeletal, and locomotory assemblages. MTOCs responsible for the nucleation and organization of these arrays are among the most elaborate and morphologically distinct of any described in land plants.

A light and electron microscope study of rhizoid-ascomycete associations and flagelliform axes in British hepatics with observations on the effects of the fungi on host morphology

Ascomycetous fungi, identified by the presence of simple septa and Woronin bodies, form a range of highly specialized associations with the unicellular rhizoids of 46 (16.2%) of the 284 species of British liverworts. The majority of these are in the two jungermannialian suborders Lepidoziineae and Cephaloziineae. Thirty three species (11.6%) also possess flagelliform axes. These extend to depths of 20-30 cm in peaty substrata and in most species bear abundant fungus-infected rhizoids. When grown in sand or water culture, isolated from their natural substrata (sandstone rocks, peat, Sphagnum, rotten wood), the hepatics are free from rhizoid infections. On returning the plants to their native substrata, the rhizoid-fungus associations are re-established. Each rhizoid or fascicle of rhizoids is infected independently and a continuous strand of fungus-containing cells is absent. Internally the flagelliform axes contain elongate parenchyma cells with abundant plasmodesmata in their transverse end walls. The apices of the axes are invested by mucilage and subapical amyloplasts appear to act as graviperceptive particles. The morphology of the flagelliform axes and the ultrastructure of the rhizoid-fungus association varies greatly between taxa. The Lepidoziaceae and Calypogeiaceae produce fascicles of rhizoids from the bases of rudimentary leaves whereas the flagelliform axes in the Cephaloziaceae are leafless. In Cladopodiella the fungus forms a pseudoparenchymatous sheath around the rips of swollen rhizoids. Development of swollen rhizoids precedes fungal infection in Lepidozia, Kurzia and Telaranea but not in other genera. Hyphae penetrating the basal walls of the rhizoids in Cephalosia, Nowellia, Calypogeia and Odantoschisma are ensheathed by host Wall material. These diverse patterns of fungal infection suggest independent, and probably relatively recent, evolution in the various genera. The flagelliform axes in hepatics appear to be major organs of assimilation and perennation structurally and functionally analogous in the former capacity to vascular plant mycorrhizas and particularly the hairs roots in Ericaceae.

Desiccation causes the proliferation of multicellular hairs, but not mucilage papillae, in Cryptothallus mirabilis (Hepatophyta): a correlated light and electron microscope study

When Cryptothallus dries out over periods of 4–20 days, the dorsal surfaces of the thalli become covered with multicellular hairs. The distribution of mucilage papillae and the endophytic fungus are not affected by desiccation. The hairs are thin walled and highly vacuolated whereas the mucilage papillae, like their secretory counterparts in Marchantia and mosses, are thick walled with dense cytoplasm containing stacks of endoplasmic reticulum and numerous Golgi bodies. Cytochemistry shows that the secretion is rich in carbohydrates and is derived from Golgi vesicles. After an active secretory phase, senescence of the mucilage papillae is associated with acid phosphatase activity. Key words: Aneura, Cryptothallus, desiccation, liverwort, mucilage papilla, multicellular hair, ultrastructure.

The sporophyte-gametophyte junction in the hornwort, Dendroceros tubercularis Hatt (Anthocerotophyta)

The placenta of the anthocerote, Dendroceros tubercularis Hatt., consists of long and branched haustorial cells, that arise from the foot and gametophyte transfer cells. Both cell types contain electron-dense vacuolar deposits that were digested by pronase and therefore are assumed to be protein. These deposits were negative to the PATAg test for carbohydrates. Protein bodies were also found in the parenchyma cells of the foot and younger meristematic cells at the base of the capsule. Vacuolar deposits of osmiophilic material in the gametophyte cells external to the placenta were stained non-specifically with PATAg method and were not affected by pronase. The haustorial cells have pleomorphic plastids lacking starch and a thylakoid system, whereas the transfer cells have well developed chloroplasts. No pronase-sensitive material was detected in the apo plastic space separating gametophyte and sporophyte cells. These results suggest that protein is synthesized in the haustorial cells, perhaps from precursors provided by transfer cells, and is then transferred, via plasmodesmata, to the parenchyma cells of the foot and eventually to the cells of the growing capsule.

The ultrastructure of the placenta in Sphagnum

The placenta of two Sphagnum species was examined by electron microscopy. In contrast to all mosses so far investigated, neither sporophyte nor gametophyte placental cells of Sphagnum develop wall ingrowths. The sporophyte cells are highly vacuolate and the gametophyte cells close to them degenerate to produce a system of spaces filled with mucilage. Whether this type of placenta represents a primitive or derived condition in mosses is discussed.