Advanced vascular function discovered in a widespread moss

Lignin developmental patterns and Casparian strip as apoplastic barriers: A review

Lignin and Casparian strips are two essential components of plant cells that play critical roles in plant development regulate nutrients and water across the plants cell. Recent studies have extensively investigated lignin diversity and Casparian strip formation, providing valuable insights into plant physiology. This review presents the established lignin biosynthesis pathway, as well as the developmental patterns of lignin and Casparian strip and transcriptional network associated with Casparian strip formation. It describes the biochemical and genetic mechanisms that regulate lignin biosynthesis and deposition in different plants cell types and tissues. Additionally, the review highlights recent studies that have uncovered novel lignin biosynthesis genes and enzymatic pathways, expanding our understanding of lignin diversity. This review also discusses the developmental patterns of Casparian strip in roots and their role in regulating nutrient and water transport, focusing on recent genetic and molecular studies that have identified regulators of Casparian strip formation. Previous research has shown that lignin biosynthesis genes also play a role in Casparian strip formation, suggesting that these processes are interconnected. In conclusion, this comprehensive overview provides insights into the developmental patterns of lignin diversity and Casparian strip as apoplastic barriers. It also identifies future research directions, including the functional characterization of novel lignin biosynthesis genes and the identification of additional regulators of Casparian strip formation. Overall, this review enhances our understanding of the complex and interconnected processes that drive plant growth, pathogen defense, regulation and development.

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.

The dynamics of external water conduction in the dryland moss Syntrichia

Syntrichia relies on external water conduction for photosynthesis, survival, and reproduction, a condition referred to as ectohydry. Capillarity spaces are abundant in Syntrichia, but the link between function and morphology is complex. The aim of this study was to provide a better understanding of species-specific morphological traits underlying the functions of water conduction and storage. We used an environmental scanning electron microscope and confocal microscopy for observing anatomical characters in the leaves of Syntrichia species. We also measured hydration/dehydration curves to understand the rate of conduction and dehydration by experimental approaches. Syntrichia is an ectohydric moss that can externally transport and store water from the base of the stem using capillary action. We propose a new framework to study ectohydric capabilities, which incorporates three morphological scales and the timing of going from completely dehydrated to fully hydrated. Characters of interest in this model include cell anatomy (papillae development, hyaline basal cells and laminar cells), architecture of the stem (concavity and orientation) and whole clump characteristics (density of stems). We report significant variations in the speed of conduction, water holding capacity and hydration associated with each species studied (11 in total). All Syntrichia species are capable of external water conduction and storage, but the relevant traits differ among species. These results help to understand potential evolutionary and ecological trade-offs among speed of water conduction, water holding capacity, ontogeny, and differing habitat requirements. An integrative view of ectohydry in Syntrichia contributes to understanding the water relationships of mosses.

Phloem development

The evolution of the plant vascular system is a key process in Earth history because it enabled plants to conquer land and transform the terrestrial surface. Among the vascular tissues, the phloem is particularly intriguing because of its complex functionality. In angiosperms, its principal components are the sieve elements, which transport phloem sap, and their neighboring companion cells. Together, they form a functional unit that sustains sap loading, transport, and unloading. The developmental trajectory of sieve elements is unique among plant cell types because it entails selective organelle degradation including enucleation. Meticulous analyses of primary, so-called protophloem in the Arabidopsis thaliana root meristem have revealed key steps in protophloem sieve element formation at single-cell resolution. A transcription factor cascade connects specification with differentiation and also orchestrates phloem pole patterning via noncell-autonomous action of sieve element-derived effectors. Reminiscent of vascular tissue patterning in secondary growth, these involve receptor kinase pathways, whose antagonists guide the progression of sieve element differentiation. Receptor kinase pathways may also safeguard phloem formation by maintaining the developmental plasticity of neighboring cell files. Our current understanding of protophloem development in the A. thaliana root has reached sufficient detail to instruct molecular-level investigation of phloem formation in other organs.

Cell corner middle lamella in hydroids of dendroid moss Hypnodendron menziesii gametophyte is prominently thickened: a proposed role in the mechanical support function

Significantly thickened corner middle lamella of the hydroid cell wall in the stipe of dendroid moss Hypnodendron menziesii has a mechanical support function. The hydroid cell walls of the erect stipe of Hypnodendron menziesii were investigated using light microscopy (LM), transmission electron microscopy (TEM), and TEM-immunogold labeling in support of the proposed biomechanical function for the highly thickened cell corner middle lamellae. The statistical analyses of dimensions of hydroid cell and wall parameters revealed a strong positive correlation between the area of hydroid cell and (i) the hydroid cell walls adhering to thick corner middle lamella, (ii) the area of the thick cell wall at hydroid corners, and (iii) the maximum thickness of cell wall at hydroid corners. The total area of the thick cell wall at the hydroid corners concomitantly increased with the area of the hydroid cell wall adhering to the middle lamella, and with the increased number of hydroids surrounding a reference hydroid. The results suggest that markedly thickened middle lamellae of the hydroid cell wall in Hypnodendron likely function by preventing hydroid cells from collapsing under the tensile forces generated from the transpirational pull on the water column. The specific localization of (1→4)- β-D-galactan and (1,5)-α-L-arabinan in the interface region of the hydroid cell wall and the thick middle lamella is consistent with these cell wall components being involved in the mechanical strengthening of the interface through firm adhesion as well as elasticity, ensuring the structural stability of this cell wall region, which may be prone to delamination/fracturing from the various internal and external pressures imposed. The copious presence of homogalacturonan in the thick middle lamella may further enhance the strength and flexibility of hydroid cell walls.

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.

The limiting effect of genome size on xylem vessel diameter is shifted by environmental pressures in seed plants

Current and previous studies have extensively studied the physiological and ecological consequences of genome size (GS) on plants because of the limiting effect of GS on cell size. However, it is still obscure whether such limiting effect could be shifted by environmental pressures, or not. Here, we compiled a global dataset comprised of GS, xylem vessel diameter (V _dia), xylem hydraulic conductivity (K _S), P ₅₀ (xylem water potential at the loss of 50% maximum K _S), and climate factors of 251 phylogeny and habitat divergent species from 59 families. The results showed that GS could limit the V _dia of the species from the same family sampled in the similar climate conditions. But the expected positive relationship between GS and V _dia became uncertain and even negative across different environmental conditions. V _dia was strongly positively coordinated with mean annual temperature (MAT), mean annual precipitation (MAP), and potential evapotranspiration (PET). Furthermore, V _dia as the anatomic foundation of plant hydraulic performance was strongly positively coordinated with K _S and negatively coordinated with -P ₅₀. The strong environmental selection on K _S and P ₅₀ explained the concerted regulation of V _dia by environmental factors. The findings revealed the combined regulation of GS and environmental pressures on xylem cell size and thus affected plant eco-physiological performance. The shifted cell size limiting effect of GS by environmental factors manifests plants great plasticity under changed environmental conditions.

Reference bioimaging to assess the phenotypic trait diversity of bryophytes within the family Scapaniaceae

Macro- and microscopic images of organisms are pivotal in biodiversity research. Despite that bioimages have manifold applications such as assessing the diversity of form and function, FAIR bioimaging data in the context of biodiversity are still very scarce, especially for difficult taxonomic groups such as bryophytes. Here, we present a high-quality reference dataset containing macroscopic and bright-field microscopic images documenting various phenotypic characters of the species belonging to the liverwort family of Scapaniaceae occurring in Europe. To encourage data reuse in biodiversity and adjacent research areas, we annotated the imaging data with machine-actionable metadata using community-accepted semantics. Furthermore, raw imaging data are retained and any contextual image processing like multi-focus image fusion and stitching were documented to foster good scientific practices through source tracking and provenance. The information contained in the raw images are also of particular interest for machine learning and image segmentation used in bioinformatics and computational ecology. We expect that this richly annotated reference dataset will encourage future studies to follow our principles.

Measurement(s)	phenotype
Technology Type(s)	bright-field microscopy
Factor Type(s)	taxonomic identification of different species
Sample Characteristic - Organism	Scapaniaceae

Chemotaxonomic and evolutionary perspectives of Bryophyta based on multivariate analysis of fatty acid fingerprints of Eastern Himalayan mosses

Bryophyta comprises one of the earliest lineages of land plants that had implemented remarkable innovations to their lipid metabolic systems for successful adaptation to terrestrial habitat. This study presents a comprehensive investigation of fatty acid profiles of mosses from Eastern Himalayas with an aim to trace their chemotaxonomic and evolutionary implications. Fatty acid compositions of 40 random mosses belonging to major families of Bryophyta were explored by gas chromatographic analysis. A diverse array of saturated, monounsaturated and polyunsaturated fatty acids including rare acetylenic fatty acids were detected. Hexadecanoic acid (C16:0), 9,12 (Z,Z)-octadecadienoic acid (C18:2n6) and 9,12,15 (Z,Z,Z)-octadecatrienoic acid (C18:3n3) were the predominant fatty acids in all the mosses. However, quantitative variation of C20 polyunsaturated fatty acids (PUFAs), specifically 5,8,11,14 (Z,Z,Z,Z)-eicosatetraenoic acid (C20:4n6), among the investigated mosses was the most prominent outcome. The diplolepidous members of Bryidae, especially the mosses of Hypnales, Bryales and Bartramiales contained higher amount of C20 PUFAs compared with the haplolepidous orders. Principal component analyses based on individual fatty acids and other related parameters validated C20:4n6 content and the ratio of C20:4n6/C18:2n6 as the apparent chemotaxonomic discriminants. The prevalent notion of considering 9,12,15-octadecatrien-6-ynoic acid (C18:4a) as the chemomarker of Dicranaceae has also been challenged, since the compound was detected not only in different families of Dicranales, but also in a Pottiales member, Leptodontium viticulosoides. Therefore, an ensemble of fatty acids instead of a single one can be considered as the chemical signature for taxonomic interpretation which may also be vital from an evolutionary standpoint.

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.

Development and diversity of lignin patterns

Different patterns of lignified cell walls are associated with diverse functions in a variety of plant tissues. These functions rely on the stiffness and hydrophobicity that lignin polymers impart to the cell wall. The precise pattern of subcellular lignin deposition is critical for the structure-function relationship in each lignified cell type. Here, we describe the role of xylem vessels as water pipes, Casparian strips as apoplastic barriers, and the role of asymmetrically lignified endocarp b cells in exploding seed pods. We highlight similarities and differences in the genetic mechanisms underpinning local lignin deposition in these diverse cell types. By bringing together examples from different developmental contexts and different plant species, we propose that comparative approaches can benefit our understanding of lignin patterning mechanisms.

Coordination of hydraulic thresholds across roots, stems, and leaves of two co-occurring mangrove species

Mangroves are frequently inundated with saline water and have evolved different anatomical and physiological mechanisms to filter and, in some species, excrete excess salt from the water they take up. Because salts impose osmotic stress, interspecific differences in salt tolerance and salt management strategy may influence physiological responses to drought throughout the entire plant hydraulic pathway, from roots to leaves. Here, we characterized embolism vulnerability simultaneously in leaves, stems, and roots of seedlings of two mangrove species (Avicennia marina and Bruguiera gymnorrhiza) along with turgor-loss points in roots and leaves and xylem anatomical traits. In both species, the water potentials causing 50% of total embolism were less negative in roots and leaves than they were in stems, but the water potentials causing incipient embolism (5%) were similar in roots, stems, and leaves. Stomatal closure in leaves and turgor loss in both leaves and roots occurred at water potentials only slightly less negative than the water potentials causing 5% of total embolism. Xylem anatomical traits were unrelated to vulnerability to embolism. Vulnerability segmentation may be important in limiting embolism spread into stems from more vulnerable roots and leaves. Interspecific differences in salt tolerance affected hydraulic traits from roots to leaves: the salt-secretor A. marina lost turgor at more negative water potentials and had more embolism-resistant xylem than the salt-excluder B. gymnorrhiza. Characterizing physiological thresholds of roots may help to explain recent mangrove mortality after drought and extended saltwater inundation.

Seeing is believing: what visualising bubbles in the xylem has revealed about plant hydraulic function

Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K ) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to directly visualising embolism in conduits. One method allowing a direct quantification of embolised xylem area is the optical vulnerability (OV) technique. This method has been used across different organs and has a high spatial and temporal resolution. Here, we review studies using the OV technique, discuss the main advantages and disadvantages of this method, and summarise key advances arising from its use. Vulnerability curves generated by the OV method are regularly comparable to other methods, including the centrifuge and X-ray microtomography. A major advantage of the OV technique over other methods is that it can be simultaneously used to determine in situ embolism formation in leaves, stems and roots, in species spanning the phylogeny of land plants. The OV method has been used to experimentally investigate the spreading of embolism through xylem networks, associate embolism with downstream tissue death, and observe embolism formation in the field.

When time is not of the essence: constraints to the carbon balance of bryophytes

The data available so far indicate that the photosynthetic and relative growth rates of bryophytes are 10% of those reported for tracheophytes. By examining the existing literature and reanalysing data published in over 100 studies, this review examines the ecophysiological, biochemical, and structural reasons behind this phenomenon. The limiting Rubisco content and surface for gas exchange are the internal factors that can explain the low photosynthetic and growth rates of bryophytes. The role of the thicker cell walls of bryophytes in limiting CO2 diffusion is unclear, due to the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite bryophytes having low photosynthetic rates, their positive carbon balance is tightly related to their capacity to deal with extreme conditions. Contributing factors include their capacity to deal with large daily temperature oscillations, and their capacity to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on their remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rate.

Unraveling host-microbe interactions and ecosystem functions in moss-bacteria symbioses

Mosses are non-vascular plants usually found in moist and shaded areas, with great ecological importance in several ecosystems. This is especially true in northern latitudes, where mosses are responsible for up to 100% of primary production in some ecosystems. Mosses establish symbiotic associations with unique bacteria that play key roles in the carbon and nitrogen cycles. For instance, in boreal environments, more than 35% of the nitrogen fixed by diazotrophic symbionts in peatlands is transferred to mosses, directly affecting carbon fixation by the hosts, while moss-associated methanotrophic bacteria contribute 10-30% of moss carbon. Further, half of ecosystem N input may derive from moss-cyanobacteria associations in pristine ecosystems. Moss-bacteria interactions have consequences on a global scale since northern environments sequester 20% of all the carbon generated by forests in the world and stock at least 32% of global terrestrial carbon. Different moss hosts influence bacteria in distinct ways, which suggests that threats to mosses also threaten unique microbial communities with important ecological and biogeochemical consequences. Since their origin ~500 Ma, mosses have interacted with bacteria, making these associations ideal models for understanding the evolution of plant-microbe associations and their contribution to biogeochemical cycles.

Water-related innovations in land plants evolved by different patterns of gene cooption and novelty

The origin of land plants and their descendants was marked by the evolution of key adaptations to life in terrestrial environments such as roots, vascular tissue and stomata. Though these innovations are well characterized, the evolution of the genetic toolkit underlying their development and function is poorly understood. We analysed molecular data from 532 species to investigate the evolutionary origin and diversification of genes involved in the development and regulation of these adaptations. We show that novel genes in the first land plants led to the single origin of stomata, but the stomatal closure of seed plants resulted from later gene expansions. By contrast, the major mechanism leading to the origin of vascular tissue was cooption of genes that emerged in the first land plants, enabling continuous water transport throughout the ancestral vascular plant. In turn, new key genes in the ancestors of plants with true leaves and seed plants led to the emergence of roots and lateral roots. The analysis highlights the different modes of evolution that enabled plants to conquer land, suggesting that gene expansion and cooption are the most common mechanisms of biological innovation in plant evolutionary history.

Xylem embolism spread is largely prevented by interconduit pit membranes until the majority of conduits are gas-filled

Xylem embolism resistance varies across species influencing drought tolerance, yet little is known about the determinants of the embolism resistance of an individual conduit. Here we conducted an experiment using the optical vulnerability method to test whether individual conduits have a specific water potential threshold for embolism formation and whether pre-existing embolism in neighbouring conduits alters this threshold. Observations were made on a diverse sample of angiosperm and conifer species through a cycle of dehydration, rehydration and subsequent dehydration to death. Upon rehydration after the formation of embolism, no refilling was observed. When little pre-existing embolism was present, xylem conduits had a conserved, individual embolism-resistance threshold that varied across the population of conduits. The consequence of a variable conduit-specific embolism threshold is that a small degree of pre-existing embolism in the xylem results in apparently more resistant xylem in subsequent dehydrations, particularly in angiosperms with vessels. While our results suggest that pit membranes separating xylem conduits are critical for maintaining a conserved individual conduit threshold for embolism when little pre-existing embolism is present, as the percentage of embolized conduits increases, gas movement, local pressure differences and connectivity between conduits increasingly contribute to embolism spread.

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).

Stable isotope signals provide seasonal climatic markers for moss functional groups

Living moss biomass and archival peat deposits represent key indicators of present and past climatic conditions, but prediction of future climatic impacts requires appropriate marker species to be characterized under a range of contemporary conditions. Stable isotope signals in high latitude moss deposits offer potential climatic proxies. Seasonal changes in δ13C and δ18O of organic material (cellulose) in representative functional groups, and associated photosynthetic activity (as chlorophyll fluorescence) have been compared across East Anglia, UK, as a function of tissue water content. Representative species from contrasting acid bog, heathland, and fen woodland habitats were selected for monthly sampling of recent growth tissues between spring 2017 and autumn 2018, with isotopic signals in purified cellulose compared with tissue water, precipitation, and nearby groundwater signals. Sphagnum and Polytrichum groups, which tend to dominate peat formation, provided contrasting and complementary indicators of seasonal variations in carbon assimilation. Cellulose δ18O signals from Sphagnum spp. demonstrate seasonal variations in source precipitation inputs; carbon isotope signals in Polytrichum spp. indicate evaporative demand and photosynthetic limitation.

Cell wall thickness and composition are related to photosynthesis in Antarctic mosses

Cell wall thickness (T_cw ) has been proposed as an important anatomical trait that could determine photosynthesis through land plants' phylogeny, bryophytes being the plant group presenting the thickest walls and the lowest photosynthetic rates. Also, it has recently been suggested that cell wall composition may have the potential to influence both thickness and mesophyll conductance (g_m ), representing a novel trait that could ultimately affect photosynthesis. However, only a few studies in spermatophytes have demonstrated this issue. In order to explore the role of cell wall composition in determining both T_cw and g_m in mosses, we tested six species grown under field conditions in Antarctica. We performed gas exchange and chlorophyll fluorescence measurements, an anatomical characterization, and a quantitative analysis of cell wall main composition (i.e., cellulose, hemicelluloses and pectins) in these six species. We found the photosynthetic rates to vary between the species, and they also presented differences in anatomical characteristics and in cell wall composition. Whilst g_m correlated negatively with T_cw and pectins content, a positive relationship between T_cw and pectins emerged, suggesting that pectins could contribute to determine cell wall porosity. Although our results do not allow us to provide conclusive statements, we suggest for the first time that cell wall composition-with pectins playing a key role-could strongly influence T_cw and g_m in Antarctic mosses, ultimately defining photosynthesis.

Different Metabolic Roles for Alternative Oxidase in Leaves of Palustrine and Terrestrial Species

The alternative oxidase pathway (AOP) is associated with excess energy dissipation in leaves of terrestrial plants. To address whether this association is less important in palustrine plants, we compared the role of AOP in balancing energy and carbon metabolism in palustrine and terrestrial environments by identifying metabolic relationships between primary carbon metabolites and AOP in each habitat. We measured oxygen isotope discrimination during respiration, gas exchange, and metabolite profiles in aerial leaves of ten fern and angiosperm species belonging to five families organized as pairs of palustrine and terrestrial species. We performed a partial least square model combined with variable importance for projection to reveal relationships between the electron partitioning to the AOP (τa) and metabolite levels. Terrestrial plants showed higher values of net photosynthesis (AN) and τa, together with stronger metabolic relationships between τa and sugars, important for water conservation. Palustrine plants showed relationships between τa and metabolites related to the shikimate pathway and the GABA shunt, to be important for heterophylly. Excess energy dissipation via AOX is less crucial in palustrine environments than on land. The basis of this difference resides in the contrasting photosynthetic performance observed in each environment, thus reinforcing the importance of AOP for photosynthesis.

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.

Hydraulic architecture with high root-resistance fraction contributes to efficient carbon gain of plants in temperate habitats

PREMISE

The hydraulic architecture in the leaves, stems and roots of plants constrains water transport and carbon gain through stomatal limitation to CO₂ absorption. Because roots are the main bottleneck in water transport for a range of plant species, we assessed the ecophysiological mechanism and importance of a high fraction of root hydraulic resistance in woody and herbaceous species.

METHODS

Biomass partitioning and hydraulic conductance of leaves, stems, and roots of Japanese knotweed (Fallopia japonica, a perennial herb) and Japanese zelkova (Zelkova serrata, a deciduous tall tree) were measured. Theoretical analyses were used to examine whether the measured hydraulic architecture and biomass partitioning maximized the plant photosynthetic rate (the product of leaf area and photosynthetic rate per leaf area).

RESULTS

Root hydraulic resistance accounted for 83% and 68% of the total plant resistance for Japanese knotweed and Japanese zelkova, respectively. Comparisons of hydraulic and biomass partitioning revealed that high root-resistance fractions were attributable to low biomass partitioning to root organs rather than high mass-specific root conductance. The measured partitioning of hydraulic resistance closely corresponded to the predicted optimal partitioning, maximizing the plant photosynthetic rate for the two species. The high fraction of root resistance was predicted to be optimal with variations in air humidity and soil water potential.

CONCLUSIONS

These results suggest that the hydraulic architecture of plants growing in mesic and fertile habitats not only results in high root resistance due to small biomass partitioning to root organs, but contributes to efficient carbon gain.

Comparative transcriptomic analysis reveals conserved programmes underpinning organogenesis and reproduction in land plants

The appearance of plant organs mediated the explosive radiation of land plants, which shaped the biosphere and allowed the establishment of terrestrial animal life. The evolution of organs and immobile gametes required the coordinated acquisition of novel gene functions, the co-option of existing genes and the development of novel regulatory programmes. However, no large-scale analyses of genomic and transcriptomic data have been performed for land plants. To remedy this, we generated gene expression atlases for various organs and gametes of ten plant species comprising bryophytes, vascular plants, gymnosperms and flowering plants. A comparative analysis of the atlases identified hundreds of organ- and gamete-specific orthogroups and revealed that most of the specific transcriptomes are significantly conserved. Interestingly, our results suggest that co-option of existing genes is the main mechanism for evolving new organs. In contrast to female gametes, male gametes showed a high number and conservation of specific genes, which indicates that male reproduction is highly specialized. The expression atlas capturing pollen development revealed numerous transcription factors and kinases essential for pollen biogenesis and function.

Comparative transcriptomic analysis reveals conserved programmes underpinning organogenesis and reproduction in land plants

The appearance of plant organs mediated the explosive radiation of land plants, which shaped the biosphere and allowed the establishment of terrestrial animal life. The evolution of organs and immobile gametes required the coordinated acquisition of novel gene functions, the co-option of existing genes and the development of novel regulatory programmes. However, no large-scale analyses of genomic and transcriptomic data have been performed for land plants. To remedy this, we generated gene expression atlases for various organs and gametes of ten plant species comprising bryophytes, vascular plants, gymnosperms and flowering plants. A comparative analysis of the atlases identified hundreds of organ- and gamete-specific orthogroups and revealed that most of the specific transcriptomes are significantly conserved. Interestingly, our results suggest that co-option of existing genes is the main mechanism for evolving new organs. In contrast to female gametes, male gametes showed a high number and conservation of specific genes, which indicates that male reproduction is highly specialized. The expression atlas capturing pollen development revealed numerous transcription factors and kinases essential for pollen biogenesis and function.

The relationship of C and N stable isotopes to high-latitude moss-associated N2 fixation

Moss-associated N₂ fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N₂ fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N₂ fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N₂ fixation with ¹⁵N₂ gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that δ¹⁵N values would increase toward 0‰ with higher N₂ fixation to reflect the increasing contribution of fixed N₂ in moss biomass. Second, we hypothesized that δ¹³C and N₂ fixation would be positively related, as enriched δ¹³C signatures reflect abiotic conditions favorable to N₂ fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N₂ fixation rates and δ¹⁵N in a structural equation model. We found a significant positive relationship between δ¹³C and N₂ fixation only in Hypnales, where the probability of N₂ fixation activity reached 95% when δ¹³C values exceeded - 30.4‰. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N₂ fixation rates.

Moss stomata do not respond to light and CO2 concentration but facilitate carbon uptake by sporophytes: a gas exchange, stomatal aperture, and 13 C-labelling study

Stomata exert control on fluxes of CO₂ and water (H₂ O) in the majority of vascular plants and thus are pivotal for planetary fluxes of carbon and H₂ O. However, in mosses, the significance and possible function of the sporophytic stomata are not well understood, hindering understanding of the ancestral function and evolution of these key structures of land plants. Infrared gas analysis and ¹³ CO₂ labelling, with supporting data from gravimetry and optical and scanning electron microscopy, were used to measure CO₂ assimilation and water exchange on young, green, ± fully expanded capsules of 11 moss species with a range of stomatal numbers, distributions, and aperture sizes. Moss sporophytes are effectively homoiohydric. In line with their open fixed apertures, moss stomata, contrary to those in tracheophytes, do not respond to light and CO₂ concentration. Whereas the sporophyte cuticle is highly impermeable to gases, stomata are the predominant sites of ¹³ CO₂ entry and H₂ O loss in moss sporophytes, and CO₂ assimilation is closely linked to total stomatal surface areas. Higher photosynthetic autonomy of moss sporophytes, consequent on the presence of numerous stomata, may have been the key to our understanding of evolution of large, gametophyte-independent sporophytes at the onset of plant terrestrialization.

Cell wall thickness and composition are involved in photosynthetic limitation

The key role of cell walls in setting mesophyll conductance to CO2 (gm) and, consequently, photosynthesis is reviewed. First, the theoretical properties of cell walls that can affect gm are presented. Then, we focus on cell wall thickness (Tcw) reviewing empirical evidence showing that Tcw varies strongly among species and phylogenetic groups in a way that correlates with gm and photosynthesis; that is, the thicker the mesophyll cell walls, the lower the gm and photosynthesis. Potential interplays of gm, Tcw, dehydration tolerance, and hydraulic properties of leaves are also discussed. Dynamic variations of Tcw in response to the environment and their implications in the regulation of photosynthesis are discussed, and recent evidence suggesting an influence of cell wall composition on gm is presented. We then propose a hypothetical mechanism for the influence of cell walls on photosynthesis, combining the effects of thickness and composition, particularly pectins. Finally, we discuss the prospects for using biotechnology for enhancing photosynthesis by altering cell wall-related genes.

Evolutionary relationships between drought-related traits and climate shape large hydraulic safety margins in western North American oaks

Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks (Quercus spp.). Optical visualization showed that 50% of embolism occurs at water potentials below -2.7 MPa in all 19 species, and -6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere's most ecologically and economically important plants.

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.

The evolving role of abscisic acid in cell function and plant development over geological time

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the function of key genes in the ABA signalling pathway of angiosperms has revealed that this signal transduction pathway is ancient, yet considerable change in the physiological roles of this hormone have occurred over geological time. With recent advances in our capacity to characterise gene function in non-angiosperms we are on the cusp of revealing the origins of this critical hormonal signalling pathway in plants, and understanding how a simple hormone may have shaped land plant diversity, ecology and adaptation over the past 500 million years.

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.