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

The sexual lability hypothesis for the origin of the land plant generation cycle

The evolution of the land plant alternation of generations has been an open question for the past 150 years. Two hypotheses have dominated the discussion: the antithetic hypothesis, which posits that the diploid sporophyte generation arose de novo and gradually increased in complexity, and the homologous hypothesis, which holds that land plant ancestors had independently living sporophytes and haploid gametophytes of similar complexity. Changes in ploidy levels were unknown to early researchers. The antithetic hypothesis is contradicted by generation cycles in Lower Devonian Rhynie chert plants, whose sporophytes and gametophytes have similar morphologies and by some Silurian sporophytes whose complexity exceeds that of Rhynie chert sporophytes. The oldest unambiguous bryophyte gametophytes (thalli) are from the upper Middle Devonian, with an unconnected sporophyte nearby. Based on the 2024 discovery that conjugate algae are paraphyletic to land plants, we present a new hypothesis for the evolution of the land plant generation cycle, focusing on labile ploidy levels and types of reproduction found in conjugate algae. Our 'sexual lability' hypothesis assumes a period of unstable generation cycles (as regards ploidy), likely with predominant clonal growth, as is common in conjugate algae, resulting in sporophytes and gametophytes of similar morphology. When sexual reproduction became stabilized, the timing of gamete fusion, meiosis, and resistant wall formation, which are heterochronic in some conjugate algae, became standardized, with wall formation permanently delayed. In our scenario, independently living adult sporophytes are the land plant ancestral condition, and life-long sporophyte retention on the gametophyte is a bryophyte apomorphy.

MpMLO1 controls sperm discharge in liverwort

In bryophytes, sexual reproduction necessitates the release of motile sperm cells from a gametophyte into the environment. Since 1856, this process, particularly in liverworts, has been known to depend on water. However, the molecular mechanism underlying this phenomenon has remained elusive. Here we identify the plasma membrane protein MpMLO1 in Marchantia polymorpha, a model liverwort, as critical for sperm discharge from antheridia. The MpMLO1-expressing tip cells among the sperm-wrapping jacket cells undergo programmed cell death upon antheridium maturation to facilitate sperm discharge after the application of water and even hypertonic solutions. The absence of MpMLO1 leads to reduced cytoplasmic Ca2+ levels in tip cells, preventing cell death and consequently sperm discharge. Our findings reveal that MpMLO1-mediated programmed cell death in antheridial tip cells, regulated by cytosolic Ca2+ dynamics, is essential for sperm release, elucidating a key mechanism in bryophyte sexual reproduction and providing insights into terrestrial plant evolution.

Proteome plasticity during Physcomitrium patens spore germination - from the desiccated phase to heterotrophic growth and reconstitution of photoautotrophy

The establishment of moss spores is considered a milestone in plant evolution. They harbor protein networks underpinning desiccation tolerance and accumulation of storage compounds that can be found already in algae and that are also utilized in seeds and pollen. Furthermore, germinating spores must produce proteins that drive the transition through heterotrophic growth to the autotrophic plant. To get insight into the plasticity of this proteome, we investigated it at five timepoints of moss (Physcomitrium patens) spore germination and in protonemata and gametophores. The comparison to previously published Arabidopsis proteome data of seedling establishment showed that not only the proteomes of spores and seeds are functionally related, but also the proteomes of germinating spores and young seedlings. We observed similarities with regard to desiccation tolerance, lipid droplet proteome composition, control of dormancy, and β-oxidation and the glyoxylate cycle. However, there were also striking differences. For example, spores lacked any obvious storage proteins. Furthermore, we did not detect homologs to the main triacylglycerol lipase in Arabidopsis seeds, SUGAR DEPENDENT1. Instead, we discovered a triacylglycerol lipase of the oil body lipase family and a lipoxygenase as being the overall most abundant proteins in spores. This finding indicates an alternative pathway for triacylglycerol degradation via oxylipin intermediates in the moss. The comparison of spores to Nicotiana tabacum pollen indicated similarities for example in regards to resistance to desiccation and hypoxia, but the overall developmental pattern did not align as in the case of seedling establishment and spore germination.

Callose in leptoid cell walls of the moss Polytrichum and the evolution of callose synthase across bryophytes

Introduction

Leptoids, the food-conducting cells of polytrichaceous mosses, share key structural features with sieve elements in tracheophytes, including an elongated shape with oblique end walls containing modified plasmodesmata or pores. In tracheophytes, callose is instrumental in developing the pores in sieve elements that enable efficient photoassimilate transport. Aside from a few studies using aniline blue fluorescence that yielded confusing results, little is known about callose in moss leptoids.

Methods

Callose location and abundance during the development of leptoid cell walls was investigated in the moss Polytrichum commune using aniline blue fluorescence and quantitative immunogold labeling (label density) in the transmission electron microscope. To evaluate changes during abiotic stress, callose abundance in leptoids of hydrated plants was compared to plants dried for 14 days under field conditions. A bioinformatic study to assess the evolution of callose within and across bryophytes was conducted using callose synthase (CalS) genes from 46 bryophytes (24 mosses, 15 liverworts, and 7 hornworts) and one representative each of five tracheophyte groups.

Results

Callose abundance increases around plasmodesmata from meristematic cells to end walls in mature leptoids. Controlled drying resulted in a significant increase in label density around plasmodesmata and pores over counts in hydrated plants. Phylogenetic analysis of the CalS protein family recovered main clades (A, B, and C). Different from tracheophytes, where the greatest diversity of homologs is found in clade A, the majority of gene duplication in bryophytes is in clade B.

Discussion

This work identifies callose as a crucial cell wall polymer around plasmodesmata from their inception to functioning in leptoids, and during water stress similar to sieve elements of tracheophytes. Among bryophytes, mosses exhibit the greatest number of multiple duplication events, while only two duplications are revealed in hornwort and none in liverworts. The absence in bryophytes of the CalS 7 gene that is essential for sieve pore development in angiosperms, reveals that a different gene is responsible for synthesizing the callose associated with leptoids in mosses.

Relationships among sporophytic and gametophytic traits of 27 subtropical montane moss species

PREMISE

Moss sporophytes differ strongly in size and biomass partitioning, potentially reflecting reproductive and dispersal strategies. Understanding how sporophyte traits are coordinated is essential for understanding moss functioning and evolution. This study aimed to answer: (1) how the size and proportions of the sporophyte differ between moss species with and without a prominent central strand in the seta, (2) how anatomical and morphological traits of the seta are related, and (3) how sporophytic biomass relates to gametophytic biomass and nutrient concentrations.

METHODS

We studied the relationships between seta anatomical and morphological traits, the biomass of seta, capsule, and gametophyte, and carbon, nitrogen, and phosphorus concentrations of 27 subtropical montane moss species.

RESULTS

(1) Moss species with a prominent central strand in the seta had larger setae and heavier capsules than those without a prominent strand. (2) With increasing seta length, setae became thicker and more rounded for both groups, while in species with a prominent central strand, the ratio of transport-cell area to epidermal area decreased. (3) In both groups, mosses with greater gametophytic biomass tended to have heavier sporophytes, but nitrogen and phosphorus concentrations in the gametophyte were unrelated to sporophytic traits.

CONCLUSIONS

Our study highlights that the central strand in the seta may have an important functional role and affect the allometry of moss sporophytes. The coordinated variations in sporophyte morphological and anatomical traits follow basic biomechanical principles of cylinder-like structures, and these traits relate only weakly to the gametophytic nutrient concentrations. Research on moss sporophyte functional traits and their relationships to gametophytes is still in its infancy but could provide important insights into their adaptative strategies.

Non-seed plants are emerging gene sources for agriculture and insect control proteins

The non-seed plants (e.g., charophyte algae, bryophytes, and ferns) have multiple human uses, but their contributions to agriculture and research have lagged behind seed plants. While sharing broadly conserved biology with seed plants and the major crops, non-seed plants sometimes possess alternative molecular and physiological adaptations. These adaptations may guide crop improvements. One such area is the presence of multiple classes of insecticidal proteins found in non-seed plant genomes which are either absent or widely diverged in seed plants. There are documented uses of non-seed plants, and ferns for example have been used in human diets. Among the occasional identifiable toxins or antinutritive components present in non-seed plants, none include these insecticidal proteins. Apart from these discrete risk factors which can be addressed in the safety assessment, there should be no general safety concern about sourcing genes from non-seed plant species.

The TOPLESS corepressor regulates developmental switches in the bryophyte Physcomitrium patens that were critical for plant terrestrialisation

The plant-specific TOPLESS (TPL) family of transcriptional corepressors is integral to multiple angiosperm developmental processes. Despite this, we know little about TPL function in other plants. To address this gap, we investigated the roles TPL plays in the bryophyte Physcomitrium patens, which diverged from angiosperms approximately 0.5 billion years ago. Although complete loss of PpTPL function is lethal, transgenic lines with reduced PpTPL activity revealed that PpTPLs are essential for two fundamental developmental switches in this plant: the transitions from basal photosynthetic filaments (chloronemata) to specialised foraging filaments (caulonemata) and from two-dimensional (2D) to three-dimensional (3D) growth. Using a transcriptomics approach, we integrated PpTPL into the regulatory network governing 3D growth and we propose that PpTPLs represent another important class of regulators that are essential for the 2D-to-3D developmental switch. Transcriptomics also revealed a previously unknown role for PpTPL in the regulation of flavonoids. Intriguingly, 3D growth and the formation of caulonemata were crucial innovations that facilitated the colonisation of land by plants, a major transformative event in the history of life on Earth. We conclude that TPL, which existed before the land plants, was co-opted into new developmental pathways, enabling phytoterrestrialisation and the evolution of land plants.

Terrestrial surface stabilisation by modern analogues of the earliest land plants: A multi-dimensional imaging study

The evolution of the first plant-based terrestrial ecosystems in the early Palaeozoic had a profound effect on the development of soils, the architecture of sedimentary systems, and shifts in global biogeochemical cycles. In part, this was due to the evolution of complex below-ground (root-like) anchorage systems in plants, which expanded and promoted plant-mineral interactions, weathering, and resulting surface sediment stabilisation. However, little is understood about how these micro-scale processes occurred, because of a lack of in situ plant fossils in sedimentary rocks/palaeosols that exhibit these interactions. Some modern plants (e.g., liverworts, mosses, lycophytes) share key features with the earliest land plants; these include uni- or multicellular rhizoid-like anchorage systems or simple roots, and the ability to develop below-ground networks through prostrate axes, and intimate associations with fungi, making them suitable analogues. Here, we investigated cryptogamic ground covers in Iceland and New Zealand to better understand these interactions, and how they initiate the sediment stabilisation process. We employed multi-dimensional and multi-scale imaging, including scanning electron microscopy (SEM) and X-ray Computed Tomography (μCT) of non-vascular liverworts (Haplomitriopsida and complex thalloids) and mosses, with additional imaging of vascular lycopods. We find that plants interact with their substrate in multiple ways, including: (1) through the development of extensive surface coverings as mats; (2) entrapment of sediment grains within and between networks of rhizoids; (3) grain entwining and adherence by rhizoids, through mucilage secretions, biofilm-like envelopment of thalli on surface grains; and (4) through grain entrapment within upright 'leafy' structures. Significantly, μCT imaging allows us to ascertain that rhizoids are the main method for entrapment and stabilisation of soil grains in the thalloid liverworts. This information provides us with details of how the earliest land plants may have significantly influenced early Palaeozoic sedimentary system architectures, promoted in situ weathering and proto-soil development, and how these interactions diversified over time with the evolution of new plant organ systems. Further, this study highlights the importance of cryptogamic organisms in the early stages of sediment stabilisation and soil formation today.

What can hornworts teach us?

The hornworts are a small group of land plants, consisting of only 11 families and approximately 220 species. Despite their small size as a group, their phylogenetic position and unique biology are of great importance. Hornworts, together with mosses and liverworts, form the monophyletic group of bryophytes that is sister to all other land plants (Tracheophytes). It is only recently that hornworts became amenable to experimental investigation with the establishment of Anthoceros agrestis as a model system. In this perspective, we summarize the recent advances in the development of A. agrestis as an experimental system and compare it with other plant model systems. We also discuss how A. agrestis can help to further research in comparative developmental studies across land plants and to solve key questions of plant biology associated with the colonization of the terrestrial environment. Finally, we explore the significance of A. agrestis in crop improvement and synthetic biology applications in general.

Stomatal regulators are co-opted for seta development in the astomatous liverwort Marchantia polymorpha

The evolution of special types of cells requires the acquisition of new gene regulatory networks controlled by transcription factors (TFs). In stomatous plants, a TF module formed by subfamilies Ia and IIIb basic helix-loop-helix TFs (Ia-IIIb bHLH) regulates stomatal formation; however, how this module evolved during land plant diversification remains unclear. Here we show that, in the astomatous liverwort Marchantia polymorpha, a Ia-IIIb bHLH module regulates the development of a unique sporophyte tissue, the seta, which is found in mosses and liverworts. The sole Ia bHLH gene, MpSETA, and a IIIb bHLH gene, MpICE2, regulate the cell division and/or differentiation of seta lineage cells. MpSETA can partially replace the stomatal function of Ia bHLH TFs in Arabidopsis thaliana, suggesting that a common regulatory mechanism underlies setal and stomatal formation. Our findings reveal the co-option of a Ia-IIIb bHLH TF module for regulating cell fate determination and/or cell division of distinct types of cells during land plant evolution.

How plants conquered land: evolution of terrestrial adaptation

The transition of plants from water to land is considered one of the most significant events in the evolution of life on Earth. The colonization of land by plants, accompanied by their morphological, physiological and developmental changes, resulted in plant biodiversity. Besides significantly influencing oxygen levels in the air and on land, plants manufacture organic matter from CO₂ and water with the help of sunlight, paving the way for the diversification of nonplant lineages ranging from microscopic organisms to animals. Land plants regulate the climate by adjusting total biomass and energy flow. At the genetic level, these innovations are achieved through the rearrangement of pre-existing genetic information. Advances in genome sequencing technology are revamping our understanding of plant evolution. This study highlights the morphological and genomic innovations that allow plants to integrate life on Earth.

The phylogeny of Syntrichia: An ecologically diverse clade of mosses with an origin in South America

PREMISE

To address the biodiversity crisis, we need to understand the evolution of all organisms and how they fill geographic and ecological space. Syntrichia is one of the most diverse and dominant genera of mosses, ranging from alpine habitats to desert biocrusts, yet its evolutionary history remains unclear.

METHODS

We present a comprehensive phylogenetic analysis of Syntrichia, based on both molecular and morphological data, with most of the named species and closest outgroups represented. In addition, we provide ancestral-state reconstructions of water-related traits and a global biogeographic analysis.

RESULTS

We found 10 major well-resolved subclades of Syntrichia that possess geographical or morphological coherence, in some cases representing previously accepted genera. We infer that the extant species diversity of Syntrichia likely originated in South America in the early Eocene (56.5-43.8 million years ago [Mya]), subsequently expanded its distribution to the neotropics, and finally dispersed to the northern hemisphere. There, the clade experienced a recent diversification (15-12 Mya) into a broad set of ecological niches (e.g., the S. caninervis and S. ruralis complexes). The transition from terricolous to either saxicolous or epiphytic habitats occurred more than once and was associated with changes in water-related traits.

CONCLUSIONS

Our study provides a framework for understanding the evolutionary history of Syntrichia through the combination of morphological and molecular characters, revealing that migration events that shaped the current distribution of the clade have implications for morphological character evolution in relation to niche diversity.

The origin of a land flora

The origin of a land flora fundamentally shifted the course of evolution of life on earth, facilitating terrestrialization of other eukaryotic lineages and altering the planet's geology, from changing atmospheric and hydrological cycles to transforming continental erosion processes. Despite algal lineages inhabiting the terrestrial environment for a considerable preceding period, they failed to evolve complex multicellularity necessary to conquer the land. About 470 million years ago, one lineage of charophycean alga evolved complex multicellularity via developmental innovations in both haploid and diploid generations and became land plants (embryophytes), which rapidly diversified to dominate most terrestrial habitats. Genome sequences have provided unprecedented insights into the genetic and genomic bases for embryophyte origins, with some embryophyte-specific genes being associated with the evolution of key developmental or physiological attributes, such as meristems, rhizoids and the ability to form mycorrhizal associations. However, based on the fossil record, the evolution of the defining feature of embryophytes, the embryo, and consequently the sporangium that provided a reproductive advantage, may have been most critical in their rise to dominance. The long timeframe and singularity of a land flora were perhaps due to the stepwise assembly of a large constellation of genetic innovations required to conquer the terrestrial environment.

A Changing Light Environment Induces Significant Lateral CO2 Diffusion within Maize Leaves

A leaf structure with high porosity is beneficial for lateral CO₂ diffusion inside the leaves. However, the leaf structure of maize is compact, and it has long been considered that lateral CO₂ diffusion is restricted. Moreover, lateral CO₂ diffusion is closely related to CO₂ pressure differences (ΔCO₂). Therefore, we speculated that enlarging the ΔCO₂ between the adjacent regions inside maize leaves may result in lateral diffusion when the diffusion resistance is kept constant. Thus, the leaf structure and gas exchange of maize (C₄), cotton (C₃), and other species were explored. The results showed that maize and sorghum leaves had a lower mesophyll porosity than cotton and cucumber leaves. Similar to cotton, the local photosynthetic induction resulted in an increase in the ΔCO₂ between the local illuminated and the adjacent unilluminated regions, which significantly reduced the respiration rate of the adjacent unilluminated region. Further analysis showed that when the adjacent region in the maize leaves was maintained under a steady high light, the photosynthesis induction in the local regions not only gradually reduced the ΔCO₂ between them but also progressively increased the steady photosynthetic rate in the adjacent region. Under field conditions, the ΔCO₂, respiration, and photosynthetic rate of the adjacent region were also markedly changed by fluctuating light in local regions in the maize leaves. Consequently, we proposed that enlarging the ΔCO₂ between the adjacent regions inside the maize leaves results in the lateral CO₂ diffusion and supports photosynthesis in adjacent regions to a certain extent under fluctuating light.

Deep origin and gradual evolution of transporting tissues: Perspectives from across the land plants

The evolution of transporting tissues was an important innovation in terrestrial plants that allowed them to adapt to almost all nonaquatic environments. These tissues consist of water-conducting cells and food-conducting cells and bridge plant-soil and plant-air interfaces over long distances. The largest group of land plants, representing about 95% of all known plant species, is associated with morphologically complex transporting tissue in plants with a range of additional traits. Therefore, this entire clade was named tracheophytes, or vascular plants. However, some nonvascular plants possess conductive tissues that closely resemble vascular tissue in their organization, structure, and function. Recent molecular studies also point to a highly conserved toolbox of molecular regulators for transporting tissues. Here, we reflect on the distinguishing features of conductive and vascular tissues and their evolutionary history. Rather than sudden emergence of complex, vascular tissues, plant transporting tissues likely evolved gradually, building on pre-existing developmental mechanisms and genetic components. Improved knowledge of the intimate structure and developmental regulation of transporting tissues across the entire taxonomic breadth of extant plant lineages, combined with more comprehensive documentation of the fossil record of transporting tissues, is required for a full understanding of the evolutionary trajectory of transporting tissues.

Plant Cysteine Oxidase Oxygen-Sensing Function Is Conserved in Early Land Plants and Algae

All aerobic organisms require O₂ for survival. When their O₂ is limited (hypoxia), a response is required to reduce demand and/or improve supply. A hypoxic response mechanism has been identified in flowering plants: the stability of certain proteins with N-terminal cysteine residues is regulated in an O₂-dependent manner by the Cys/Arg branch of the N-degron pathway. These include the Group VII ethylene response factors (ERF-VIIs), which can initiate adaptive responses to hypoxia. Oxidation of their N-terminal cysteine residues is catalyzed by plant cysteine oxidases (PCOs), destabilizing these proteins in normoxia; PCO inactivity in hypoxia results in their stabilization. Biochemically, the PCOs are sensitive to O₂ availability and can therefore act as plant O₂ sensors. It is not known whether oxygen-sensing mechanisms exist in other phyla from the plant kingdom. Known PCO targets are only conserved in flowering plants, however PCO-like sequences appear to be conserved in all plant species. We sought to determine whether PCO-like enzymes from the liverwort, Marchantia polymorpha (MpPCO), and the freshwater algae, Klebsormidium nitens (KnPCO), have a similar function as PCO enzymes from Arabidopsis thaliana. We report that MpPCO and KnPCO show O₂-sensitive N-terminal cysteine dioxygenase activity toward known AtPCO ERF-VII substrates as well as a putative endogenous substrate, MpERF-like, which was identified by homology to the Arabidopsis ERF-VIIs transcription factors. This work confirms functional and O₂-dependent PCOs from Bryophyta and Charophyta, indicating the potential for PCO-mediated O₂-sensing pathways in these organisms and suggesting PCO O₂-sensing function could be important throughout the plant kingdom.

The cell wall of hornworts and liverworts: innovations in early land plant evolution?

An important step for plant diversification was the transition from freshwater to terrestrial habitats. The bryophytes and all vascular plants share a common ancestor that was probably the first to adapt to life on land. A polysaccharide-rich cell wall was necessary to cope with newly faced environmental conditions. Therefore, some pre-requisites for terrestrial life have to be shared in the lineages of modern bryophytes and vascular plants. This review focuses on hornwort and liverwort cell walls and aims to provide an overview on shared and divergent polysaccharide features between these two groups of bryophytes and vascular plants. Analytical, immunocytochemical, and bioinformatic data were analysed. The major classes of polysaccharides-cellulose, hemicelluloses, and pectins-seem to be present but have diversified structurally during evolution. Some polysaccharide groups show structural characteristics which separate hornworts from the other bryophytes or are too poorly studied in detail to be able to draw absolute conclusions. Hydroxyproline-rich glycoprotein backbones are found in hornworts and liverworts, and show differences in, for example, the occurrence of glycosylphosphatidylinositol (GPI)-anchored arabinogalactan-proteins, while glycosylation is practically unstudied. Overall, the data are an appeal to researchers in the field to gain more knowledge on cell wall structures in order to understand the changes with regard to bryophyte evolution.

Diversity, phylogeny, and adaptation of bryophytes: insights from genomic and transcriptomic data

Bryophytes including mosses, liverworts, and hornworts are among the earliest land plants, and occupy a crucial phylogenetic position to aid in the understanding of plant terrestrialization. Despite their small size and simple structure, bryophytes are the second largest group of extant land plants. They live ubiquitously in various habitats and are highly diversified, with adaptive strategies to modern ecosystems on Earth. More and more genomes and transcriptomes have been assembled to address fundamental questions in plant biology. Here, we review recent advances in bryophytes associated with diversity, phylogeny, and ecological adaptation. Phylogenomic studies have provided increasing supports for the monophyly of bryophytes, with hornworts sister to the Setaphyta clade including liverworts and mosses. Further comparative genomic analyses revealed that multiple whole-genome duplications might have contributed to the species richness and morphological diversity in mosses. We highlight that the biological changes through gene gain or neofunctionalization that primarily evolved in bryophytes have facilitated the adaptation to early land environments; among the strategies to adapt to modern ecosystems in bryophytes, desiccation tolerance is the most remarkable. More genomic information for bryophytes would shed light on key mechanisms for the ecological success of these 'dwarfs' in the plant kingdom.

Phylotranscriptomics Illuminates the Placement of Whole Genome Duplications and Gene Retention in Ferns

Ferns are the second largest clade of vascular plants with over 10,000 species, yet the generation of genomic resources for the group has lagged behind other major clades of plants. Transcriptomic data have proven to be a powerful tool to assess phylogenetic relationships, using thousands of markers that are largely conserved across the genome, and without the need to sequence entire genomes. We assembled the largest nuclear phylogenetic dataset for ferns to date, including 2884 single-copy nuclear loci from 247 transcriptomes (242 ferns, five outgroups), and investigated phylogenetic relationships across the fern tree, the placement of whole genome duplications (WGDs), and gene retention patterns following WGDs. We generated a well-supported phylogeny of ferns and identified several regions of the fern phylogeny that demonstrate high levels of gene tree-species tree conflict, which largely correspond to areas of the phylogeny that have been difficult to resolve. Using a combination of approaches, we identified 27 WGDs across the phylogeny, including 18 large-scale events (involving more than one sampled taxon) and nine small-scale events (involving only one sampled taxon). Most inferred WGDs occur within single lineages (e.g., orders, families) rather than on the backbone of the phylogeny, although two inferred events are shared by leptosporangiate ferns (excluding Osmundales) and Polypodiales (excluding Lindsaeineae and Saccolomatineae), clades which correspond to the majority of fern diversity. We further examined how retained duplicates following WGDs compared across independent events and found that functions of retained genes were largely convergent, with processes involved in binding, responses to stimuli, and certain organelles over-represented in paralogs while processes involved in transport, organelles derived from endosymbiotic events, and signaling were under-represented. To date, our study is the most comprehensive investigation of the nuclear fern phylogeny, though several avenues for future research remain unexplored.

Dependence on clade II bHLH transcription factors for nursing of haploid products by tapetal-like cells is conserved between moss sporangia and angiosperm anthers

Clade II basic helix-loop-helix transcription factors (bHLH TFs) are essential for pollen production and tapetal nursing functions in angiosperm anthers. As pollen has been suggested to be related to bryophyte spores by descent, we characterized two Physcomitrium (Physcomitrella) patens clade II bHLH TFs (PpbHLH092 and PpbHLH098), to test if regulation of sporogenous cells and the nursing cells surrounding them is conserved between angiosperm anthers and bryophyte sporangia. We made CRISPR-Cas9 reporter and loss-of-function lines to address the function of PpbHLH092/098. We sectioned and analyzed WT and mutant sporophytes for a comprehensive stage-by-stage comparison of sporangium development. Spore precursors in the P. patens sporangium are surrounded by nursing cells showing striking similarities to tapetal cells in angiosperms. Moss clade II bHLH TFs are essential for the differentiation of these tapetal-like cells and for the production of functional spores. Clade II bHLH TFs provide a conserved role in controlling the sporophytic somatic cells surrounding and nursing the sporogenous cells in both moss sporangia and angiosperm anthers. This supports the hypothesis that such nursing functions in mosses and angiosperms, lineages separated by c. 450 million years, are related by descent.

A, Christian C, Demiri K, Evans D, Evans N, Flavin C, Gillis J, Gogol V, Heublein E, Huang E, Hutchinson J, Jackson C, Jackson O, Johnson L, Kirihara M, Kivarkis H, Kowalczyk A, Labontu A, Levi B, Lyu I, Martin-Eberhardt S, Mata G, Martinec J, McDonald B, Mira M, Nguyen M, Nguyen P, Nolimal S, Reese V, Ritchie W, Rodriguez J, Rodriguez Y, Shuler J, Silvestre J, Simpson G, Somarriba G, Ssozi R, Suwa T, Syring C, Thirthamattur N, Thompson K, Vaughn C, Viramontes M, Wong CS, Wszolek L. People-Powered Research and Experiential Learning: Unravelling Hidden Biodiversity

Globally, thousands of institutions house nearly three billion scientific collections offering unparallelled resources that contribute to both science and society. For herbaria alone - facilities housing dried plant collections - there are over 3,000 herbaria worldwide with an estimated 350 million specimens that have been collected over the past four centuries. Digitisation has greatly enhanced the use of herbarium data in scientific research, impacting diverse research areas, including biodiversity informatics, global climate change, analyses using next-generation sequencing technologies and many others. Despite the entrance of herbaria into a new era with enhanced scientific, educational and societal relevance, museum specimens remain underused. Natural history museums can enhance learning and engagement in science, particularly for school-age and undergraduate students. Here, we outline a novel approach of a natural history museum using touchscreen technology that formed part of an interactive kiosk in a temporary museum exhibit on biological specimens. We provide some preliminary analysis investigating the efficacy of the tool, based on the Zooniverse platform, in an exhibit environment to engage patrons in the collection of biological data. We conclude there is great potential in using crowd‐sourced science, coupled with online technology to unlock data and information from digital images of natural history specimens themselves. Sixty percent of the records generated by community scientists (citizen scientists) were of high enough quality to be utilised by researchers. All age groups produced valid, high quality data that could be used by researchers, including children (10 and under), teens and adults. Significantly, the paper outlines the implementation of experiential learning through an undergraduate mathematics course that focuses on projects with actual data to gain a deep, practical knowledge of the subject, including observations, the collection of data, analysis and problem solving. We here promote an intergenerational model including children, high school students, undergraduate students, early career scientists and senior scientists, combining experiential learning, museum patrons, researchers and data derived from natural history collections. Natural history museums with their dual remit of education and collections-based research can play a significant role in the field of community engagement and people-powered research. There also remains much to investigate on the use of interactive displays to help learners interpret and appreciate authentic research. We conclude with a brief insight into the next phase of our ongoing people-powered research activities developed and designed by high school students using the Zooniverse platform.

Developmental regulation of leaf venation patterns: monocot versus eudicots and the role of auxin

Organisation and patterning of the vascular network in land plants varies in different taxonomic, developmental and environmental contexts. In leaves, the degree of vascular strand connectivity influences both light and CO₂ harvesting capabilities as well as hydraulic capacity. As such, developmental mechanisms that regulate leaf venation patterning have a direct impact on physiological performance. Development of the leaf venation network requires the specification of procambial cells within the ground meristem of the primordium and subsequent proliferation and differentiation of the procambial lineage to form vascular strands. An understanding of how diverse venation patterns are manifest therefore requires mechanistic insight into how procambium is dynamically specified in a growing leaf. A role for auxin in this process was identified many years ago, but questions remain. In this review we first provide an overview of the diverse venation patterns that exist in land plants, providing an evolutionary perspective. We then focus on the developmental regulation of leaf venation patterns in angiosperms, comparing patterning in eudicots and monocots, and the role of auxin in each case. Although common themes emerge, we conclude that the developmental mechanisms elucidated in eudicots are unlikely to fully explain how parallel venation patterns in monocot leaves are elaborated.

Auxin/Cytokinin Antagonistic Control of the Shoot/Root Growth Ratio and Its Relevance for Adaptation to Drought and Nutrient Deficiency Stresses

The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective.

Convergent tip-to-base widening of water-conducting conduits in the tallest bryophytes

PREMISE

Tip-to-base conduit widening is considered a key mechanism that enables vascular plants to grow tall by decreasing the hydraulic resistance imposed by increasing height. Widening of hydraulic anatomy (larger conducting elements toward the base of the vascular system) minimizes gradients in leaf-specific hydraulic conductance with plant height, allowing uniform photosynthesis across the crown of trees. Tip-to-base conduit widening has also been associated with changes in conduit number. However, in bryophytes, despite having representatives with internal water-conducting tissue, conduit widening has been scarcely investigated.

METHODS

Here, we examined the changes in hydroid diameter and number with distance from plant tip in Dawsonia superba and D. polytrichoides, two representatives of the genus containing the tallest extant bryophytes.

RESULTS

The position of these moss species on the global scale of conduit size and plant size was consistent with a general scaling among plants with internal water transport. Within plants, patterns of conduit widening and number with distance from plant tip in endohydric mosses were similar to those observed in vascular plants.

CONCLUSIONS

This study demonstrated that land plants growing upward in the atmosphere show analogous conduit widening of hydraulic structures, suggesting that efficient internal water transport is a convergent adaptation for photosynthesis on land.

Piecing together the eophytes - a new group of ancient plants containing cryptospores

The earliest evidence for land plants comes from dispersed cryptospores from the Ordovician, which dominated assemblages for 60 million years. Direct evidence of their parent plants comes from minute fossils in Welsh Borderland Upper Silurian to Lower Devonian rocks. We recognize a group that had forking, striated axes with rare stomata terminating in valvate sporangia containing permanent cryptospores, but their anatomy was unknown especially regarding conducting tissues. Charcoalified fossils extracted from the rock using HF were selected from macerates and observed using scanning electron microscopy. Promising examples were split for further examination and compared with electron micrographs of the anatomy of extant bryophytes. Fertile fossil axes possess central elongate cells with thick walls bearing globules, occasional strands and plasmodesmata-sized pores. The anatomy of these cells best matches desiccation-tolerant food-conducting cells (leptoids) of bryophytes. Together with thick-walled epidermal cells and extremely small size, these features suggest that these plants were poikilohydric. Our new data on conducting cells confirms a combination of characters that distinguish the permanent cryptospore-producers from bryophytes and tracheophytes. We therefore propose the erection of a new group, here named the Eophytidae (eophytes).

Pole position: How plant cells polarize along the axes

Having a sense of direction is a fundamental cellular trait that can determine cell shape, division orientation, or function, and ultimately the formation of a functional, multicellular body. Cells acquire and integrate directional information by establishing discrete subcellular domains along an axis with distinct molecular profiles, a process known as cell polarization. Insight into the principles and mechanisms underlying cell polarity has been propelled by decades of extensive research mostly in yeast and animal models. Our understanding of cell polarity establishment in plants, which lack most of the regulatory molecules identified in other eukaryotes, is more limited, but significant progress has been made in recent years. In this review, we explore how plant cells coordinately establish stable polarity axes aligned with the organ axes, highlighting similarities in the molecular logic used to polarize both plant and animal cells. We propose a classification system for plant cell polarity events and nomenclature guidelines. Finally, we provide a deep phylogenetic analysis of polar proteins and discuss the evolution of polarity machineries in plants.

Seasonal Dynamics of Photochemical Performance of PS II of Terrestrial Mosses from Different Elevations

Mosses are critical components of tropical forest ecosystems and have multiple essential ecological functions. The drying and rehydrating and often hot environments in tropical regions present some of the greatest challenges for their photosynthetic activities. There is limited knowledge available on the physiological responses to the changing environments such as temperature and water pattern changes for terrestrial mosses. We examined the seasonal dynamics of photochemical performance of PS II through the measuring of chlorophyll fluorescence of 12 terrestrial mosses in situ from five different elevations by Photosynthesis Yield Analyzer MINI-PAM-II, along with the seasonal changes of climatic factors (air temperature, dew point, relative humidity and rainfall), which were collected by local weather stations and self-deployed mini weather stations. The results showed a great seasonality during observing periods, which, mainly the changes of rainfall and relative humidity pattern, presented significant impacts on the photochemical performance of PS II of terrestrial mosses. All these tested moss species developed a suitable regulated and non-regulated strategy to avoid the detrimental effect of abiotic stresses. We found that only Hypnum plumaeforme, Pterobryopsis crassicaulis and Pogonatum inflexum were well adapted to the changes of habitat temperature and water patterns, even though they still experienced a lower CO₂ assimilation efficiency in the drier months. The other nine species were susceptible to seasonality, especially during the months of lower rainfall and relative humidity when moss species were under physiologically reduced PS II efficiency. Anomobryum julaceum, Pogonatum neesii, Sematophyllum subhumile, Pseudotaxiphyllum pohliaecarpum and Leucobryum boninense, and especially Brachythecium buchananii, were sensitive to the changes of water patterns, which enable them as ideal ecological indicators of photosynthetic acclimation to stressed environments as a result of climate change.

Fundamental mechanisms of the stem cell regulation in land plants: lesson from shoot apical cells in bryophytes

This review compares the molecular mechanisms of stem cell control in the shoot apical meristems of mosses and angiosperms and reveals the conserved features and evolution of plant stem cells. The establishment and maintenance of pluripotent stem cells in the shoot apical meristem (SAM) are key developmental processes in land plants including the most basal, bryophytes. Bryophytes, such as Physcomitrium (Physcomitrella) patens and Marchantia polymorpha, are emerging as attractive model species to study the conserved features and evolutionary processes in the mechanisms controlling stem cells. Recent studies using these model bryophyte species have started to uncover the similarities and differences in stem cell regulation between bryophytes and angiosperms. In this review, we summarize findings on stem cell function and its regulation focusing on different aspects including hormonal, genetic, and epigenetic control. Stem cell regulation through auxin, cytokinin, CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) signaling and chromatin modification by Polycomb Repressive Complex 2 (PRC2) and PRC1 is well conserved. Several transcription factors crucial for SAM regulation in angiosperms are not involved in the regulation of the SAM in mosses, but similarities also exist. These findings provide insights into the evolutionary trajectory of the SAM and the fundamental mechanisms involved in stem cell regulation that are conserved across land plants.

Distinct Evolutionary Profiles and Functions of microRNA156 and microRNA529 in Land Plants

MicroRNA156 (miR156) and miR529 have high sequence similarity and recognize overlapping sites in the same target genes, SQUAMOSA promoter binding protein-like (SPL or SBP box) genes, making it difficult to accurately distinguish their roles in regulatory networks that affect numerous biological functions. Here, we collected data about miR156 and miR529 family members from representative land plants and performed sequence comparisons, phylogenetic analysis, small RNA sequencing, and parallel analysis of RNA ends (PARE) analysis to dissect their evolutionary and functional differences. Although miR156 and miR529 are highly similar, there are differences in their mismatch-sensitive regions, which are essential for target recognition. In land plants, miR156 precursors are conserved mainly within the hairpin region, whereas miR529 precursors are conserved outside the hairpin region, including both the 5’ and 3’ arms. Phylogenetic analysis showed that MIR156 and MIR529 evolved independently, through divergent evolutionary patterns. The two genes also exhibit different expression patterns, with MIR529 preferentially expressed in reproductive tissues and MIR156 in other tissues. PARE analysis revealed that miR156 and miR529 possess specific targets in addition to common targets in maize, pointing to functional differences between them. Based on our findings, we developed a method for the rapid identification of miR529 and miR156 family members and uncovered the evolutionary divergence of these families, providing insights into their different regulatory roles in plant growth and development.

The evolutionary emergence of land plants

There can be no doubt that early land plant evolution transformed the planet but, until recently, how and when this was achieved was unclear. Coincidence in the first appearance of land plant fossils and formative shifts in atmospheric oxygen and CO2 are an artefact of the paucity of earlier terrestrial rocks. Disentangling the timing of land plant bodyplan assembly and its impact on global biogeochemical cycles has been precluded by uncertainty concerning the relationships of bryophytes to one another and to the tracheophytes, as well as the timescale over which these events unfolded. New genome and transcriptome sequencing projects, combined with the application of sophisticated phylogenomic modelling methods, have yielded increasing support for the Setaphyta clade of liverworts and mosses, within monophyletic bryophytes. We consider the evolution of anatomy, genes, genomes and of development within this phylogenetic context, concluding that many vascular plant (tracheophytes) novelties were already present in a comparatively complex last common ancestor of living land plants (embryophytes). Molecular clock analyses indicate that embryophytes emerged in a mid-Cambrian to early Ordovician interval, compatible with hypotheses on their role as geoengineers, precipitating early Palaeozoic glaciations.

Terrestrial Green Algae Show Higher Tolerance to Dehydration than Do Their Aquatic Sister-Species

Diverse algae possess the ability to recover from extreme desiccation without forming specialized resting structures. Green algal genera such as Tetradesmus (Sphaeropleales, Chlorophyceae) contain temperate terrestrial, desert, and aquatic species, providing an opportunity to compare physiological traits associated with the transition to land in closely related taxa. We subjected six species from distinct habitats to three dehydration treatments varying in relative humidity (RH 5%, 65%, 80%) followed by short- and long-term rehydration. We tested the capacity of the algae to recover from dehydration using the effective quantum yield of photosystem II as a proxy for physiological activity. The degree of recovery was dependent both on the habitat of origin and the dehydration scenario, with terrestrial, but not aquatic, species recovering from dehydration. Distinct strains of each species responded similarly to dehydration and rehydration, with the exception of one aquatic strain that recovered from the mildest dehydration treatment. Cell ultrastructure was uniformly maintained in both aquatic and desert species during dehydration and rehydration, but staining with an amphiphilic styryl dye indicated damage to the plasma membrane from osmotically induced water loss in the aquatic species. These analyses demonstrate that terrestrial Tetradesmus possess a vegetative desiccation tolerance phenotype, making these species ideal for comparative omics studies.

Ecophysiological differentiation between life stages in filmy ferns (Hymenophyllaceae)

Desiccation tolerance was a key trait that allowed plants to colonize land. However, little is known about the transition from desiccation tolerant non-vascular plants to desiccation sensitive vascular ones. Filmy ferns (Hymenophyllaceae) represent a useful system to investigate how water-stress strategies differ between non-vascular and vascular stages within a single organism because they have vascularized sporophytes and nonvascular gametophytes that are each capable of varying degrees of desiccation tolerance. To explore this, we surveyed sporophytes and gametophytes of 19 species (22 taxa including varieties) of filmy ferns on Moorea (French Polynesia) and used chlorophyll fluorescence to measure desiccation tolerance and light responses. We conducted phylogenetically informed analyses to identify differences in physiology between life stages and growth habits. Gametophytes had similar or less desiccation tolerance (ability to recover from 2 days desiccation at - 86 MPa) and lower photosynthetic optima (maximum electron transport rate of photosystem II and light level at 95% of that rate) than sporophytes. Epiphytes were more tolerant of desiccation than terrestrial species in both life stages. Despite their lack of greater physiological tolerances, gametophytes of several species occurred over a wider elevational range than conspecific sporophytes. Our results demonstrate that filmy fern gametophytes and sporophytes differ in their physiology and niche requirements, and point to the importance of microhabitat in shaping the evolution of water-use strategies in vascular plants.

An archipelago within an archipelago: A checklist of liverworts and hornworts of Kepulauan Sunda Kecil (Lesser Sunda Islands), Indonesia and Timor-Leste (East Timor)

The first ever liverwort and hornwort checklist is provided for the Kepulauan Sunda Kecil (Lesser Sunda Islands) of Indonesia and Timor-Leste (East Timor). We report 129 accepted taxa, 12 doubtful taxa and three rejected taxa previously reported for the Lesser Sunda Islands. The list is based on over 130 literature references, including monographs, regional studies, and molecular investigations. It is clear that bryophytes from this region have been overlooked historically, and under collected, compared to seed plants, birds, and other organisms, forming a remarkable gap in the flora of Indonesia. Publications dealing with liverworts of Lesser Sunda Islands are few and scattered. We predict that further fieldwork, in addition to collections unveiled from regional herbaria, will uncover a number of new records that remain to be reported, especially considering that regionally widespread species have been recorded elsewhere.

Biological Nanofactories: Using Living Forms for Metal Nanoparticle Synthesis

Metal nanoparticles are nanosized entities with dimensions of 1-100 nm that are increasingly in demand due to applications in diverse fields like electronics, sensing, environmental remediation, oil recovery and drug delivery. Metal nanoparticles possess large surface energy and properties different from bulk materials due to their small size, large surface area with free dangling bonds and higher reactivity. High cost and pernicious effects associated with the chemical and physical methods of nanoparticle synthesis are gradually paving the way for biological methods due to their eco-friendly nature. Considering the vast potentiality of microbes and plants as sources, biological synthesis can serve as a green technique for the synthesis of nanoparticles as an alternative to conventional methods. A number of reviews are available on green synthesis of nanoparticles but few have focused on covering the entire biological agents in this process. Therefore present paper describes the use of various living organisms like bacteria, fungi, algae, bryophytes and tracheophytes in the biological synthesis of metal nanoparticles, the mechanisms involved and the advantages associated therein.

The sequenced genomes of non-flowering land plants reveal the innovative evolutionary history of peptide signaling

An understanding of land plant evolution is a prerequisite for in-depth knowledge of plant biology. Here we extract and explore information hidden in the increasing number of sequenced plant genomes, from bryophytes to angiosperms, to elucidate a specific biological question - how peptide signaling evolved. To conquer land and cope with changing environmental conditions, plants have gone through transformations that must have required innovations in cell-to-cell communication. We discuss peptides mediating endogenous and exogenous changes by interaction with receptors activating intracellular molecular signaling. Signaling peptides were discovered in angiosperms and operate in tissues and organs such as flowers, seeds, vasculature, and 3D meristems that are not universally conserved across land plants. Nevertheless, orthologs of angiosperm peptides and receptors have been identified in non-angiosperms. These discoveries provoke questions regarding co-evolution of ligands and their receptors, and whether de novo interactions in peptide signaling pathways may have contributed to generate novel traits in land plants. The answers to such questions will have profound implications for the understanding of the evolution of cell-to-cell communication and the wealth of diversified terrestrial plants. Under this perspective we have generated, analyzed, and reviewed phylogenetic, genomic, structural, and functional data to elucidate the evolution of peptide signaling.

On the evolution of plant thermomorphogenesis

Plants have a remarkable capacity to acclimate to their environment. Acclimation is enabled to a large degree by phenotypic plasticity, the extent of which confers a selective advantage, especially in natural habitats. Certain key events in evolution triggered adaptive bursts necessary to cope with drastic environmental changes. One such event was the colonization of land 400-500 mya. Compared to most aquatic habitats, fluctuations in abiotic parameters became more pronounced, generating significant selection pressure. To endure these harsh conditions, plants needed to adapt their physiology and morphology and to increase the range of phenotypic plasticity. In addition to drought stress and high light, high temperatures and fluctuation thereof were among the biggest challenges faced by terrestrial plants. Thermomorphogenesis research has emerged as a new sub-discipline of the plant sciences and aims to understand how plants acclimate to elevated ambient temperatures through changes in architecture. While we have begun to understand how angiosperms sense and respond to elevated ambient temperature, very little is known about thermomorphogenesis in plant lineages with less complex body plans. It is unclear when thermomorphogenesis initially evolved and how this depended on morphological complexity. In this review, we take an evolutionary-physiological perspective and generate hypotheses about the emergence of thermomorphogenesis.

Large-Scale Phylogenomic Analyses Reveal the Monophyly of Bryophytes and Neoproterozoic Origin of Land Plants

The relationships among the four major embryophyte lineages (mosses, liverworts, hornworts, vascular plants) and the timing of the origin of land plants are enigmatic problems in plant evolution. Here, we resolve the monophyly of bryophytes by improving taxon sampling of hornworts and eliminating the effect of synonymous substitutions. We then estimate the divergence time of crown embryophytes based on three fossil calibration strategies, and reveal that maximum calibration constraints have a major effect on estimating the time of origin of land plants. Moreover, comparison of priors and posteriors provides a guide for evaluating the optimal calibration strategy. By considering the reliability of fossil calibrations and the influences of molecular data, we estimate that land plants originated in the Precambrian (980–682 Ma), much older than widely recognized. Our study highlights the important contribution of molecular data when faced with contentious fossil evidence, and that fossil calibrations used in estimating the timescale of plant evolution require critical scrutiny.

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.

Adaptive innovation of green plants by horizontal gene transfer

Horizontal gene transfer (HGT) refers to the movement of genetic material between distinct species by means other than sexual reproduction. HGT has contributed tremendously to the genome plasticity and adaptive evolution of prokaryotes and certain unicellular eukaryotes. The evolution of green plants from chlorophyte algae to angiosperms and from water to land represents a process of adaptation to diverse environments, which has been facilitated by acquisition of genetic material from other organisms. In this article, we review the occurrence of HGT in major lineages of green plants, including chlorophyte and charophyte green algae, bryophytes, lycophytes, ferns, and seed plants. In addition, we discuss the significance of horizontally acquired genes in the adaptive innovations of green plants and their potential applications to crop breeding and improvement.

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.

What drives photosynthesis during desiccation? Mosses and other outliers from the photosynthesis-elasticity trade-off

In vascular plants, more rigid leaves have been linked to lower photosynthetic capacity, associated with low CO2 diffusion across the mesophyll, indirectly resulting in a trade-off between photosynthetic capacity (An) and bulk modulus of elasticity (ε). However, we evaluated mosses, liverworts, and Chara sp., plus some lycophytes and ferns, and found that they behaved as clear outliers of the An-ε relationship. Despite this finding, when vascular and non-vascular plants were plotted together, ε still linearly determined the cessation of net photosynthesis during desiccation both in species with stomata (either actively or hydro-passively regulated) and in species lacking stomata, and regardless of their leaf structure. The latter result challenges our current view of photosynthetic responses to desiccation and/or water stress. Structural features and hydric strategy are discussed as possible explanations for the deviation of these species from the An-ε trade-off, as well as for the general linear dependency between ε and the full cessation of An during desiccation.

Plant glucose transporter structure and function

The carbohydrate D-glucose is the main source of energy in living organisms. In contrast to animals, as well as most fungi, bacteria, and archaea, plants are capable to synthesize a surplus of sugars characterizing them as autothrophic organisms. Thus, plants are de facto the source of all food on earth, either directly or indirectly via feed to livestock. Glucose is stored as polymeric glucan, in animals as glycogen and in plants as starch. Despite serving a general source for metabolic energy and energy storage, glucose is the main building block for cellulose synthesis and represents the metabolic starting point of carboxylate- and amino acid synthesis. Finally yet importantly, glucose functions as signalling molecule conveying the plant metabolic status for adjustment of growth, development, and survival. Therefore, cell-to-cell and long-distance transport of photoassimilates/sugars throughout the plant body require the fine-tuned activity of sugar transporters facilitating the transport across membranes. The functional plant counterparts of the animal sodium/glucose transporters (SGLTs) are represented by the proton-coupled sugar transport proteins (STPs) of the plant monosaccharide transporter(-like) family (MST). In the framework of this special issue on “Glucose Transporters in Health and Disease,” this review gives an overview of the function and structure of plant STPs in comparison to the respective knowledge obtained with the animal Na⁺-coupled glucose transporters (SGLTs).

Eukaryotic and Prokaryotic Phytochelatin Synthases Differ Less in Functional Terms Than Previously Thought: A Comparative Analysis of Marchantia polymorpha and Geitlerinema sp. PCC 7407

This paper reports functional studies on the enzyme phytochelatin synthase in the liverwort Marchantia polymorpha and the cyanobacterium Geitlerinema sp. strain PCC 7407. In vitro activity assays in control samples (cadmium-untreated) showed that phytochelatin synthase was constitutively expressed in both organisms. In the presence of 100 µM cadmium, in both the liverwort and the cyanobacterium, the enzyme was promptly activated in vitro, and produced phytochelatins up to the oligomer PC₄. Likewise, in vivo exposure to 10–36 µM cadmium for 6-120 h induced in both organisms phytochelatin synthesis up to PC₄. Furthermore, the glutathione (GSH) levels in M. polymorpha were constitutively low (compared with the average content in higher plants), but increased considerably under cadmium stress. Conversely, the GSH levels in Geitlerinema sp. PCC 7407 were constitutively high, but were halved under metal treatments. At odds with former papers, our results demonstrate that, as in M. polymorpha and other plants, the cyanobacterial phytochelatin synthase exposed to cadmium possesses manifest transpeptidasic activity, being able to synthesize phytochelatins with a degree of oligomerization higher than PC₂. Therefore, prokaryotic and eukaryotic phytochelatin synthases differ less in functional terms than previously thought.

Stomata and Sporophytes of the Model Moss Physcomitrium patens

Mosses are an ancient land plant lineage and are therefore important in studying the evolution of plant developmental processes. Here, we describe stomatal development in the model moss species Physcomitrium patens (previously known as Physcomitrella patens) over the duration of sporophyte development. We dissect the molecular mechanisms guiding cell division and fate and highlight how stomatal function might vary under different environmental conditions. In contrast to the asymmetric entry divisions described in Arabidopsis thaliana, moss protodermal cells can enter the stomatal lineage directly by expanding into an oval shaped guard mother cell (GMC). We observed that when two early stage P. patens GMCs form adjacently, a spacing division can occur, leading to separation of the GMCs by an intervening epidermal spacer cell. We investigated whether orthologs of Arabidopsis stomatal development regulators are required for this spacing division. Our results indicated that bHLH transcription factors PpSMF1 and PpSCRM1 are required for GMC formation. Moreover, the ligand and receptor components PpEPF1 and PpTMM are also required for orientating cell divisions and preventing single or clustered early GMCs from developing adjacent to one another. The identification of GMC spacing divisions in P. patens raises the possibility that the ability to space stomatal lineage cells could have evolved before mosses diverged from the ancestral lineage. This would have enabled plants to integrate stomatal development with sporophyte growth and could underpin the adoption of multiple bHLH transcription factors and EPF ligands to more precisely control stomatal patterning in later diverging plant lineages. We also observed that when P. patens sporophyte capsules mature in wet conditions, stomata are typically plugged whereas under drier conditions this is not the case; instead, mucilage drying leads to hollow sub-stomatal cavities. This appears to aid capsule drying and provides further evidence for early land plant stomata contributing to capsule rupture and spore release.

Functional expression and characterization of cinnamic acid 4-hydroxylase from the hornwort Anthoceros agrestis in Physcomitrella patens

Cinnamic acid 4-hydroxylase from the hornwort Anthoceros agrestis (AaC4H) was functionally expressed in the moss Physcomitrella patens and characterized at biochemical and molecular levels. Cinnamic acid 4-hydroxylase (C4H), a cytochrome P450-dependent hydroxylase, catalyzes the formation of 4-coumaric acid (=4-hydroxycinnamic acid) from trans-cinnamic acid. In the hornwort Anthoceros agrestis (Aa), this enzyme is supposed to be involved in the biosynthesis of rosmarinic acid (a caffeic acid ester of 3-(3,4-dihydroxyphenyl)lactic acid) and other related compounds. The coding sequence of AaC4H (CYP73A260) was expressed in the moss Physcomitrella patens (Pp_AaC4H). Protein extracts from the transformed moss showed considerably increased C4H activity driven by NADPH:cytochrome P450 reductase of the moss. Since Physcomitrella has own putative cinnamic acid 4-hydroxylases, enzyme characterization was carried out in parallel with the untransformed Physcomitrella wild type (Pp_WT). Apparent K_m-values for cinnamic acid and NADPH were determined to be at 17.3 µM and 88.0 µM for Pp_AaC4H and 25.1 µM and 92.3 µM for Pp_WT, respectively. Expression levels of AaC4H as well as two Physcomitrella patens C4H isoforms were analyzed by quantitative real-time PCR. While PpC4H_1 displayed constantly low levels of expression during the whole 21-day culture period, AaC4H and PpC4H_2 increased their expression during the first 6-8 days of the culture period and then decreased again. This work describes the biochemical in vitro characterization of a cytochrome P450-dependent enzyme, namely C4H, heterologously expressed in the haploid model plant Physcomitrella patens.

Functional expression and characterization of cinnamic acid 4-hydroxylase from the hornwort Anthoceros agrestis in Physcomitrella patens

Cinnamic acid 4-hydroxylase from the hornwort Anthoceros agrestis (AaC4H) was functionally expressed in the moss Physcomitrella patens and characterized at biochemical and molecular levels. Cinnamic acid 4-hydroxylase (C4H), a cytochrome P450-dependent hydroxylase, catalyzes the formation of 4-coumaric acid (=4-hydroxycinnamic acid) from trans-cinnamic acid. In the hornwort Anthoceros agrestis (Aa), this enzyme is supposed to be involved in the biosynthesis of rosmarinic acid (a caffeic acid ester of 3-(3,4-dihydroxyphenyl)lactic acid) and other related compounds. The coding sequence of AaC4H (CYP73A260) was expressed in the moss Physcomitrella patens (Pp_AaC4H). Protein extracts from the transformed moss showed considerably increased C4H activity driven by NADPH:cytochrome P450 reductase of the moss. Since Physcomitrella has own putative cinnamic acid 4-hydroxylases, enzyme characterization was carried out in parallel with the untransformed Physcomitrella wild type (Pp_WT). Apparent K_m-values for cinnamic acid and NADPH were determined to be at 17.3 µM and 88.0 µM for Pp_AaC4H and 25.1 µM and 92.3 µM for Pp_WT, respectively. Expression levels of AaC4H as well as two Physcomitrella patens C4H isoforms were analyzed by quantitative real-time PCR. While PpC4H_1 displayed constantly low levels of expression during the whole 21-day culture period, AaC4H and PpC4H_2 increased their expression during the first 6-8 days of the culture period and then decreased again. This work describes the biochemical in vitro characterization of a cytochrome P450-dependent enzyme, namely C4H, heterologously expressed in the haploid model plant Physcomitrella patens.

A mycorrhizae-like gene regulates stem cell and gametophore development in mosses

Plant colonization of land has been intimately associated with mycorrhizae or mycorrhizae-like fungi. Despite the pivotal role of fungi in plant adaptation, it remains unclear whether and how gene acquisition following fungal interaction might have affected the development of land plants. Here we report a macro2 domain gene in bryophytes that is likely derived from Mucoromycota, a group that includes some mycorrhizae-like fungi found in the earliest land plants. Experimental and transcriptomic evidence suggests that this macro2 domain gene in the moss Physcomitrella patens, PpMACRO2, is important in epigenetic modification, stem cell function, cell reprogramming and other processes. Gene knockout and over-expression of PpMACRO2 significantly change the number and size of gametophores. These findings provide insights into the role of fungal association and the ancestral gene repertoire in the early evolution of land plants.

Callose in sporogenesis: Novel composition of the inner spore wall in hornworts

Sporogenesis is a developmental process that defines embryophytes and involves callose, especially in the production of the highly protective and recalcitrant spore/pollen wall. Until now, hornworts, leptosporangiate ferns and homosporous lycophytes are the only major plant groups in which the involvement of callose in spore development is equivocal. Through aniline blue fluorescence and immunogold labeling in the transmission electron microscope, we provide indisputable evidence for the presence of callose in the spore wall of five hornwort genera, but not in the derived Dendroceros, an epiphyte that produces multicellular spores. We present evidence that callose appears in the developing spore wall and is retained throughout development as a wall constituent of the intine or inner spore wall, a novel location for this polysaccharide in embryophytes. In endosporic and multicellular spores/pollen of Dendroceros, the liverwort Pellia, and Arabidopsis, callose appears in the newly formed cell walls only following the first mitotic division. Further probing for other wall polymers in hornworts reveals the presence of cellulose (Calcofluor fluorescence) in the spore intine, aperture and around the equatorial girdle. Further immunogold labeling with monoclonal antibodies identifies pectin and hemicellulose in hornwort intines. The persistence of callose, a typically transient cell wall constituent, with cellulose, pectins and hemicellulose in the intine, supports specialized functions of callose in spores of hornworts that include reduced water loss when spores are dry and mechanical flexibility to withstand desiccation.

The Evolution of Flavonoid Biosynthesis: A Bryophyte Perspective

The flavonoid pathway is one of the best characterized specialized metabolite pathways of plants. In angiosperms, the flavonoids have varied roles in assisting with tolerance to abiotic stress and are also key for signaling to pollinators and seed dispersal agents. The pathway is thought to be specific to land plants and to have arisen during the period of land colonization around 550-470 million years ago. In this review we consider current knowledge of the flavonoid pathway in the bryophytes, consisting of the liverworts, hornworts, and mosses. The pathway is less characterized for bryophytes than angiosperms, and the first genetic and molecular studies on bryophytes are finding both commonalities and significant differences in flavonoid biosynthesis and pathway regulation between angiosperms and bryophytes. This includes biosynthetic pathway branches specific to each plant group and the apparent complete absence of flavonoids from the hornworts.

Organellomic data sets confirm a cryptic consensus on (unrooted) land-plant relationships and provide new insights into bryophyte molecular evolution

PREMISE

Phylogenetic trees of bryophytes provide important evolutionary context for land plants. However, published inferences of overall embryophyte relationships vary considerably. We performed phylogenomic analyses of bryophytes and relatives using both mitochondrial and plastid gene sets, and investigated bryophyte plastome evolution.

METHODS

We employed diverse likelihood-based analyses to infer large-scale bryophyte phylogeny for mitochondrial and plastid data sets. We tested for changes in purifying selection in plastid genes of a mycoheterotrophic liverwort (Aneura mirabilis) and a putatively mycoheterotrophic moss (Buxbaumia), and compared 15 bryophyte plastomes for major structural rearrangements.

RESULTS

Overall land-plant relationships conflict across analyses, generally weakly. However, an underlying (unrooted) four-taxon tree is consistent across most analyses and published studies. Despite gene coverage patchiness, relationships within mosses, liverworts, and hornworts are largely congruent with previous studies, with plastid results generally better supported. Exclusion of RNA edit sites restores cases of unexpected non-monophyly to monophyly for Takakia and two hornwort genera. Relaxed purifying selection affects multiple plastid genes in mycoheterotrophic Aneura but not Buxbaumia. Plastid genome structure is nearly invariant across bryophytes, but the tufA locus, presumed lost in embryophytes, is unexpectedly retained in several mosses.

CONCLUSIONS

A common unrooted tree underlies embryophyte phylogeny, [(liverworts, mosses), (hornworts, vascular plants)]; rooting inconsistency across studies likely reflects substantial distance to algal outgroups. Analyses combining genomic and transcriptomic data may be misled locally for heavily RNA-edited taxa. The Buxbaumia plastome lacks hallmarks of relaxed selection found in mycoheterotrophic Aneura. Autotrophic bryophyte plastomes, including Buxbaumia, hardly vary in overall structure.