Cryogenian Origins of Multicellularity in Archaeplastida

Earth was impacted by global glaciations during the Cryogenian (720 to 635 million years ago; Ma), events invoked to explain both the origins of multicellularity in Archaeplastida and radiation of the first land plants. However, the temporal relationship between these environmental and biological events is poorly established, due to a paucity of molecular and fossil data, precluding resolution of the phylogeny and timescale of archaeplastid evolution. We infer a time-calibrated phylogeny of early archaeplastid evolution based on a revised molecular dataset and reappraisal of the fossil record. Phylogenetic topology testing resolves deep archaeplastid relationships, identifying two clades of Viridiplantae and placing Bryopsidales as sister to the Chlorophyceae. Our molecular clock analysis infers an origin of Archaeplastida in the late-Paleoproterozoic to early-Mesoproterozoic (1712 to 1387 Ma). Ancestral state reconstruction of cytomorphological traits on this time-calibrated tree reveals many of the independent origins of multicellularity span the Cryogenian, consistent with the Cryogenian multicellularity hypothesis. Multicellular rhodophytes emerged 902 to 655 Ma while crown-Anydrophyta (Zygnematophyceae and Embryophyta) originated 796 to 671 Ma, broadly compatible with the Cryogenian plant terrestrialization hypothesis. Our analyses resolve the timetree of Archaeplastida with age estimates for ancestral multicellular archaeplastids coinciding with the Cryogenian, compatible with hypotheses that propose a role of Snowball Earth in plant evolution.

Multicellularity and the Need for Communication-A Systematic Overview on (Algal) Plasmodesmata and Other Types of Symplasmic Cell Connections

In the evolution of eukaryotes, the transition from unicellular to simple multicellular organisms has happened multiple times. For the development of complex multicellularity, characterized by sophisticated body plans and division of labor between specialized cells, symplasmic intercellular communication is supposed to be indispensable. We review the diversity of symplasmic connectivity among the eukaryotes and distinguish between distinct types of non-plasmodesmatal connections, plasmodesmata-like structures, and 'canonical' plasmodesmata on the basis of developmental, structural, and functional criteria. Focusing on the occurrence of plasmodesmata (-like) structures in extant taxa of fungi, brown algae (Phaeophyceae), green algae (Chlorophyta), and streptophyte algae, we present a detailed critical update on the available literature which is adapted to the present classification of these taxa and may serve as a tool for future work. From the data, we conclude that, actually, development of complex multicellularity correlates with symplasmic connectivity in many algal taxa, but there might be alternative routes. Furthermore, we deduce a four-step process towards the evolution of canonical plasmodesmata and demonstrate similarity of plasmodesmata in streptophyte algae and land plants with respect to the occurrence of an ER component. Finally, we discuss the urgent need for functional investigations and molecular work on cell connections in algal organisms.

The origin and early evolution of plants

Plant (archaeplastid) evolution has transformed the biosphere, but we are only now beginning to learn how this took place through comparative genomics, phylogenetics, and the fossil record. This has illuminated the phylogeny of Archaeplastida, Viridiplantae, and Streptophyta, and has resolved the evolution of key characters, genes, and genomes - revealing that many key innovations evolved long before the clades with which they have been casually associated. Molecular clock analyses estimate that Streptophyta and Viridiplantae emerged in the late Mesoproterozoic to late Neoproterozoic, whereas Archaeplastida emerged in the late-mid Palaeoproterozoic. Together, these insights inform on the coevolution of plants and the Earth system that transformed ecology and global biogeochemical cycles, increased weathering, and precipitated snowball Earth events, during which they would have been key to oxygen production and net primary productivity (NPP).

Phylogenetic distribution and expression pattern analyses identified a divergent basal body assembly protein involved in land plant spermatogenesis

Land plant spermatozoids commonly possess characteristic structures such as the spline, which consists of a microtubule array, the multilayered structure (MLS) in which the uppermost layer is a continuum of the spline, and multiple flagella. However, the molecular mechanisms underpinning spermatogenesis remain to be elucidated. We successfully identified candidate genes involved in spermatogenesis, deeply divergent BLD10s, by computational analyses combining multiple methods and omics data. We then examined the functions of BLD10s in the liverwort Marchantia polymorpha and the moss Physcomitrium patens. MpBLD10 and PpBLD10 are required for normal basal body (BB) and flagella formation. Mpbld10 mutants exhibited defects in remodeling of the cytoplasm and nucleus during spermatozoid formation, and thus MpBLD10 should be involved in chromatin reorganization and elimination of the cytoplasm during spermiogenesis. We identified orthologs of MpBLD10 and PpBLD10 in diverse Streptophyta and found that MpBLD10 and PpBLD10 are orthologous to BLD10/CEP135 family proteins, which function in BB assembly. However, BLD10s evolved especially quickly in land plants and MpBLD10 might have acquired additional functions in spermatozoid formation through rapid molecular evolution.

Cryogenian Origin and Subsequent Diversification of the Plant Cell-Wall Enzyme XTH Family

All land plants encode large multigene families of xyloglucan endotransglucosylase/hydrolases (XTHs), plant-specific enzymes that cleave and reconnect plant cell-wall polysaccharides. Despite the ubiquity of these enzymes, considerable uncertainty remains regarding the evolutionary history of the XTH family. Phylogenomic and comparative analyses in this study traced the non-plant origins of the XTH family to Alphaproteobacteria ExoKs, bacterial enzymes involved in loosening biofilms, rather than Firmicutes licheninases, plant biomass digesting enzymes, as previously supposed. The relevant horizontal gene transfer (HGT) event was mapped to the divergence of non-swimming charophycean algae in the Cryogenian geological period. This HGT event was the likely origin of charophycean EG16-2s, which are putative intermediates between ExoKs and XTHs. Another HGT event in the Cryogenian may have led from EG16-2s or ExoKs to fungal Congo Red Hypersensitive proteins (CRHs) to fungal CRHs, enzymes that cleave and reconnect chitin and glucans in fungal cell walls. This successive transfer of enzyme-encoding genes may have supported the adaptation of plants and fungi to the ancient icy environment by facilitating their sessile lifestyles. Furthermore, several protein evolutionary steps, including coevolution of substrate-interacting residues and putative intra-family gene fusion, occurred in the land plant lineage and drove diversification of the XTH family. At least some of those events correlated with the evolutionary gain of broader substrate specificities, which may have underpinned the expansion of the XTH family by enhancing duplicated gene survival. Together, this study highlights the Precambrian evolution of life and the mode of multigene family expansion in the evolutionary history of the XTH family.

Cryogenian Glacial Habitats as a Plant Terrestrialisation Cradle - The Origin of the Anydrophytes and Zygnematophyceae Split

For tens of millions of years (Ma), the terrestrial habitats of Snowball Earth during the Cryogenian period (between 720 and 635 Ma before present-Neoproterozoic Era) were possibly dominated by global snow and ice cover up to the equatorial sublimative desert. The most recent time-calibrated phylogenies calibrated not only on plants but on a comprehensive set of eukaryotes indicate that within the Streptophyta, multicellular charophytes (Phragmoplastophyta) evolved in the Mesoproterozoic to the early Neoproterozoic. At the same time, Cryogenian is the time of the likely origin of the common ancestor of Zygnematophyceae and Embryophyta and later, also of the Zygnematophyceae-Embryophyta split. This common ancestor is proposed to be called Anydrophyta; here, we use anydrophytes. Based on the combination of published phylogenomic studies and estimated diversification time comparisons, we deem it highly likely that anydrophytes evolved in response to Cryogenian cooling. Also, later in the Cryogenian, secondary simplification of multicellular anydrophytes and loss of flagella resulted in Zygnematophyceae diversification as an adaptation to the extended cold glacial environment. We propose that the Marinoan geochemically documented expansion of first terrestrial flora has been represented not only by Chlorophyta but also by Streptophyta, including the anydrophytes, and later by Zygnematophyceae, thriving on glacial surfaces until today. It is possible that multicellular early Embryophyta survived in less abundant (possibly relatively warmer) refugia, relying more on mineral substrates, allowing the retention of flagella-based sexuality. The loss of flagella and sexual reproduction by conjugation evolved in Zygnematophyceae and zygomycetous fungi during the Cryogenian in a remarkably convergent way. Thus, we support the concept that the important basal cellular adaptations to terrestrial environments were exapted in streptophyte algae for terrestrialization and propose that this was stimulated by the adaptation to glacial habitats dominating the Cryogenian Snowball Earth. Including the glacial lifestyle when considering the rise of land plants increases the parsimony of connecting different ecological, phylogenetic, and physiological puzzles of the journey from aquatic algae to terrestrial floras.

Step by step evolution of Indeterminate Domain (IDD) transcriptional regulators: from algae to angiosperms

INTRODUCTION

The Indeterminate Domain (IDD) proteins are a plant-specific subclass of C2H2 Zinc Finger transcription factors. Some of these transcription factors play roles in diverse aspects of plant metabolism and development, but the function of most of IDD genes is unknown and the molecular evolution of the subfamily has not been explored in detail.

METHODS

In this study, we mined available genome sequences of green plants (Viridiplantae) to reconstruct the phylogeny and then described the motifs/expression patterns of IDD genes.

KEY RESULTS

We identified the complete set of IDD genes of 16 Streptophyta genomes. We found that IDD and its sister clade STOP arose by a duplication at the base of Streptophyta. Once on land, the IDD genes duplicated extensively, giving rise to at least ten lineages. Some of these lineages were lost in extant non-vascular plants and gymnosperms, but all of them were retained in angiosperms, duplicating profoundly in dicots and monocots and acquiring, at the same time, surprising heterogeneity in their C-terminal regions and expression patterns.

CONCLUSIONS

IDDs were present in the last common ancestor of Streptophyta. On land, IDDs duplicated extensively, leading to ten lineages. Later, IDDs were recruited by angiosperms where they diversified greatly in number, C-terminal and expression patterns. Interestingly, such diversification occurred during the evolution of novel traits of the plant body. This study provides a solid framework of the orthology relationships of green land plant IDD transcription factors, thus increasing the accuracy of orthologue identification in model and non-model species and facilitating the identification of agronomically important genes related to plant metabolism and development.

Desiccation tolerance in streptophyte algae and the algae to land plant transition: evolution of LEA and MIP protein families within the Viridiplantae

The present review summarizes the effects of desiccation in streptophyte green algae, as numerous experimental studies have been performed over the past decade particularly in the early branching streptophyte Klebsormidium sp. and the late branching Zygnema circumcarinatum. The latter genus gives its name to the Zygenmatophyceae, the sister group to land plants. For both organisms, transcriptomic investigations of desiccation stress are available, and illustrate a high variability in the stress response depending on the conditions and the strains used. However, overall, the responses of both organisms to desiccation stress are very similar to that of land plants. We highlight the evolution of two highly regulated protein families, the late embryogenesis abundant (LEA) proteins and the major intrinsic protein (MIP) family. Chlorophytes and streptophytes encode LEA4 and LEA5, while LEA2 have so far only been found in streptophyte algae, indicating an evolutionary origin in this group. Within the MIP family, a high transcriptomic regulation of a tonoplast intrinsic protein (TIP) has been found for the first time outside the embryophytes in Z. circumcarinatum. The MIP family became more complex on the way to terrestrialization but simplified afterwards. These observations suggest a key role for water transport proteins in desiccation tolerance of streptophytes.

Mesostigma viride Genome and Transcriptome Provide Insights into the Origin and Evolution of Streptophyta

The Streptophyta include unicellular and multicellular charophyte green algae and land plants. Colonization of the terrestrial habitat by land plants is a major evolutionary event that has transformed the planet. So far, lack of genome information on unicellular charophyte algae hinders the understanding of the origin and the evolution from unicellular to multicellular life in Streptophyta. This work reports the high-quality reference genome and transcriptome of Mesostigma viride, a single-celled charophyte alga with a position at the base of Streptophyta. There are abundant segmental duplications and transposable elements in M. viride, which contribute to a relatively large genome with high gene content compared to other algae and early diverging land plants. This work identifies the origin of genetic tools that multicellular Streptophyta have inherited and key genetic innovations required for the evolution of land plants from unicellular aquatic ancestors. The findings shed light on the age-old questions of the evolution of multicellularity and the origin of land plants.

Complete mitogenome of the streptophyte green alga Coleochaete scutata (Coleochaetophyceae)

We have sequenced the mitogenome of Coleochaete scutata strain SAG 110.80M. This mitogenome is the largest among the streptophyte green algae examined to date. At 242,024 bp, it is 4.3-fold larger than the mitogenome of Chaetosphaeridium globosum, the only other mitogenome available for the Coleochaetophyceae. This size difference is mainly explained by differences in the abundance of introns and in the length of intergenic regions containing vestiges of coding sequences thought to be of foreign origin. With 31 group I and 26 group II introns, the C. scutata mitogenome is the most intron-rich organelle genome known among streptophyte algae.

Light and Dehydration but Not Temperature Drive Photosynthetic Adaptations of Basal Streptophytes (Hormidiella, Streptosarcina and Streptofilum) Living in Terrestrial Habitats

Streptophyte algae are the ancestors of land plants, and several classes contain taxa that are adapted to an aero-terrestrial lifestyle. In this study, four basal terrestrial streptophytes from the class Klebsormidiophyceae, including Hormidiella parvula; two species of the newly described genus Streptosarcina (S. costaricana and S. arenaria); and the newly described Streptofilum capillatum were investigated for their responses to radiation, desiccation and temperature stress conditions. All the strains showed low-light adaptation (Ik < 70 μmol photons m-2 s-1) but differed in photoprotective capacities (such as non-photochemical quenching). Acclimation to enhanced photon fluence rates (160 μmol photons m-2 s-1) increased photosynthetic performance in H. parvula and S. costaricana but not in S. arenaria, showing that low-light adaptation is a constitutive trait for S. arenaria. This lower-light adaptation of S. arenaria was coupled with a higher desiccation tolerance, providing further evidence that dehydration is a selective force shaping species occurrence in low light. For protection against ultraviolet radiation, all species synthesised and accumulated different amounts of mycosporine-like amino acids (MAAs). Biochemically, MAAs synthesised by Hormidiella and Streptosarcina were similar to MAAs from closely related Klebsormidium spp. but differed in retention time and spectral characteristics in S. capillatum. Unlike the different radiation and dehydration tolerances, Hormidiella, Streptosarcina and Streptofilum displayed preferences for similar thermal conditions. These species showed a temperature dependence of photosynthesis similar to respiration, contrasting with Klebsormidium spp. and highlighting an interspecific diversity in thermal requirements, which could regulate species distributions under temperature changes.

Usual alga from unusual habitats: Biodiversity of Klebsormidium (Klebsormidiophyceae, Streptophyta) from the phylogenetic superclade G isolated from biological soil crusts

Seven new species and two varieties of Klebsormidium were described using an integrative approach on the base of 28 strains from the poorly studied phylogenetic superclade G. These strains originated from the unusual and exotic habitats (semi-deserts, semi-arid shrublands, Mediterranean shrub and deciduous vegetation, temperate Araucaria forests, peat bogs, dumps after coal mining, maritime sand dunes etc.) of four continents (Africa, South and North America, and Europe). Molecular phylogenies based on ITS-1,2, rbcL gene and concatenated dataset of ITS-1,2-rbcL, secondary structure of ITS-2, morphology, ecology and biogeography, micrographs and drawings of the investigated strains were assessed. Additionally, phylogeny and morphology of 18 Klebsormidium strains from other lineages isolated from the same localities (different vegetation types of Chile and maritime sand dunes of Germany) were investigated for the comparison with representatives of clade G. Clade G Klebsormidium is characterized by distant phylogenetic position from the other Klebsormidium lineages and prominent morphology: four-lobed chloroplasts and mostly short swollen cells in young culture, compact small pyrenoids, curved or disintegrated filaments, unusual elongation of cells in old culture, formation of specific cluster- and knot-like colonies on agar surface, especially prominent in strains isolated from desert regions, from which the group probably originated. Comparison of Klebsormidium diversity from different biogeographic regions showed that the representatives of clade G are common algae in regions of the southern hemisphere (South Africa and Chile) and rare representatives in terrestrial ecosystems of the northern hemisphere. Further investigation of mostly unstudied territories of the southern hemisphere could bring many surprises and discoveries, leading to a change of the present concept that Klebsormidium is cosmopolitan in distribution.

Comprehensive Genome-Wide Classification Reveals That Many Plant-Specific Transcription Factors Evolved in Streptophyte Algae

Plant genomes encode many lineage-specific, unique transcription factors. Expansion of such gene families has been previously found to coincide with the evolution of morphological complexity, although comparative analyses have been hampered by severe sampling bias. Here, we make use of the recently increased availability of plant genomes. We have updated and expanded previous rule sets for domain-based classification of transcription associated proteins (TAPs), comprising transcription factors and transcriptional regulators. The genome-wide annotation of these protein families has been analyzed and made available via the novel TAPscan web interface. We find that many TAP families previously thought to be specific for land plants actually evolved in streptophyte (charophyte) algae; 26 out of 36 TAP family gains are inferred to have occurred in the common ancestor of the Streptophyta (uniting the land plants—Embryophyta—with their closest algal relatives). In contrast, expansions of TAP families were found to occur throughout streptophyte evolution. 17 out of 76 expansion events were found to be common to all land plants and thus probably evolved concomitant with the water-to-land-transition.

New Taxa of Streptophyte Algae (Streptophyta) from Terrestrial Habitats Revealed Using an Integrative Approach

Two new genera (Streptosarcina and Streptofilum) and three new species (Streptosarcina arenaria, S. costaricana and Streptofilum capillatum) of streptophyte algae were detected in cultures isolated from terrestrial habitats of Europe and Central America and described using an integrative approach. Additionally, a strain isolated from soil in North America was identified as Hormidiella parvula and proposed as an epitype of this species. The molecular phylogeny based on 18S rRNA and rbcL genes, secondary structure of ITS-2, as well as the morphology of vegetative and reproductive stages, cell ultrastructure, ecology and distribution of the investigated strains were assessed. The new genus Streptosarcina forms a sister lineage to the genus Hormidiella (Klebsormidiophyceae). Streptosarcina is characterized by packet-like (sarcinoid) and filamentous thalli with true branching and a cell organization typical for Klebsormidiophyceae. Streptofilum forms a separate lineage within Streptophyta. This genus represents an easily disintegrating filamentous alga which exhibits a cell coverage of unique structure: layers of submicroscopic scales of piliform shape covering the plasmalemma and exfoliate inside the mucilage envelope surrounding cells. The implications of the discovery of the new taxa for understanding evolutionary tendencies in the Streptophyta, a group of great evolutionary interest, are discussed.

Why we need more non-seed plant models

Contents 355 I. 355 II. 356 III. 356 IV. 357 V. 358 VI. 359 359 References 359 SUMMARY: Out of a hundred sequenced and published land plant genomes, four are not of flowering plants. This severely skewed taxonomic sampling hinders our comprehension of land plant evolution at large. Moreover, most genetically accessible model species are flowering plants as well. If we are to gain a deeper understanding of how plants evolved and still evolve, and which of their developmental patterns are ancestral or derived, we need to study a more diverse set of plants. Here, I thus argue that we need to sequence genomes of so far neglected lineages, and that we need to develop more non-seed plant model species.

Conserved Roles of CrRLK1L Receptor-Like Kinases in Cell Expansion and Reproduction from Algae to Angiosperms

Receptor-like kinases (RLKs) are regulators of plant development through allowing cells to sense their extracellular environment. They facilitate detection of local endogenous signals, in addition to external biotic and abiotic stimuli. The Catharanthus roseus RLK1-like (CrRLK1L) protein kinase subfamily, which contains FERONIA, plays a central role in regulating fertilization and in cell expansion mechanisms such as cell elongation and tip growth, as well as having indirect links to plant-pathogen interactions. Several components of CrRLK1L signaling pathways have been identified, including an extracellular ligand, coreceptors, and downstream signaling elements. The presence and abundance of the CrRLK1L proteins in the plant kingdom suggest an origin within the Streptophyta lineage, with a notable increase in prevalence in the seeded land plants. Given the function of the sole CrRLK1L protein in a charophycean alga, the possibility of a conserved role in detection and/or regulation of cell wall integrity throughout the Strephtophytes is discussed. Orthologs of signaling pathway components are also present in extant representatives of non-vascular land plants and early vascular land plants including the liverwort Marchantia polymorpha, the moss Physcomitrella patens and the lycophyte Selaginella moellendorffii. Deciphering the roles in development of the CrRLK1L protein kinases in early diverging land plants will provide insights into their ancestral function, furthering our understanding of this diversified subfamily of receptors in higher plants.

A new microscopic method to analyse desiccation-induced volume changes in aeroterrestrial green algae

Aeroterrestrial green algae are exposed to desiccation in their natural habitat, but their actual volume changes have not been investigated. Here, we measure the relative volume reduction (RVRED ) in Klebsormidium crenulatum and Zygnema sp. under different preset relative air humidities (RH). A new chamber allows monitoring RH during light microscopic observation of the desiccation process. The RHs were set in the range of ∼4 % to ∼95% in 10 steps. RVRED caused by the desiccation process was determined after full acclimation to the respective RHs. In K. crenulatum, RVRED (mean ± SE) was 46.4 ± 1.9%, in Zygnema sp. RVRED was only 34.3 ± 2.4% at the highest RH (∼95%) tested. This indicates a more pronounced water loss at higher RHs in K. crenulatum versus Zygnema sp. By contrast, at the lowest RH (∼4%) tested, RVRED ranged from 75.9 ± 2.7% in K. crenulatum to 83.9 ± 2.2% in Zygnema sp. The final volume reduction is therefore more drastic in Zygnema sp. These data contribute to our understanding of the desiccation process in streptophytic green algae, which are considered the closest ancestors of land plants.

Comparative Chloroplast Genome Analyses of Streptophyte Green Algae Uncover Major Structural Alterations in the Klebsormidiophyceae, Coleochaetophyceae and Zygnematophyceae

The Streptophyta comprises all land plants and six main lineages of freshwater green algae: Mesostigmatophyceae, Chlorokybophyceae, Klebsormidiophyceae, Charophyceae, Coleochaetophyceae and Zygnematophyceae. Previous comparisons of the chloroplast genome from nine streptophyte algae (including four zygnematophyceans) revealed that, although land plant chloroplast DNAs (cpDNAs) inherited most of their highly conserved structural features from green algal ancestors, considerable cpDNA changes took place during the evolution of the Zygnematophyceae, the sister group of land plants. To gain deeper insights into the evolutionary dynamics of the chloroplast genome in streptophyte algae, we sequenced the cpDNAs of nine additional taxa: two klebsormidiophyceans (Entransia fimbriata and Klebsormidium sp. SAG 51.86), one coleocheatophycean (Coleochaete scutata) and six zygnematophyceans (Cylindrocystis brebissonii, Netrium digitus, Roya obtusa, Spirogyra maxima, Cosmarium botrytis and Closterium baillyanum). Our comparative analyses of these genomes with their streptophyte algal counterparts indicate that the large inverted repeat (IR) encoding the rDNA operon experienced loss or expansion/contraction in all three sampled classes and that genes were extensively shuffled in both the Klebsormidiophyceae and Zygnematophyceae. The klebsormidiophycean genomes boast greatly expanded IRs, with the Entransia 60,590-bp IR being the largest known among green algae. The 206,025-bp Entransia cpDNA, which is one of the largest genome among streptophytes, encodes 118 standard genes, i.e., four additional genes compared to its Klebsormidium flaccidum homolog. We inferred that seven of the 21 group II introns usually found in land plants were already present in the common ancestor of the Klebsormidiophyceae and its sister lineages. At 107,236 bp and with 117 standard genes, the Coleochaete IR-less genome is both the smallest and most compact among the streptophyte algal cpDNAs analyzed thus far; it lacks eight genes relative to its Chaetosphaeridium globosum homolog, four of which represent unique events in the evolutionary scenario of gene losses we reconstructed for streptophyte algae. The 10 compared zygnematophycean cpDNAs display tremendous variations at all levels, except gene content. During zygnematophycean evolution, the IR disappeared a minimum of five times, the rDNA operon was broken at four distinct sites, group II introns were lost on at least 43 occasions, and putative foreign genes, mainly of phage/viral origin, were gained.

Sugar composition of the pectic polysaccharides of charophytes, the closest algal relatives of land-plants: presence of 3-O-methyl-D-galactose residues

BACKGROUND AND AIMS

During evolution, plants have acquired and/or lost diverse sugar residues as cell-wall constituents. Of particular interest are primordial cell-wall features that existed, and in some cases abruptly changed, during the momentous step whereby land-plants arose from charophytic algal ancestors.

METHODS

Polysaccharides were extracted from four charophyte orders [Chlorokybales (Chlorokybus atmophyticus), Klebsormidiales (Klebsormidium fluitans, K. subtile), Charales (Chara vulgaris, Nitella flexilis), Coleochaetales (Coleochaete scutata)] and an early-diverging land-plant (Anthoceros agrestis). 'Pectins' and 'hemicelluloses', operationally defined as extractable in oxalate (100 °C) and 6 m NaOH (37 °C), respectively, were acid- or Driselase-hydrolysed, and the monosaccharides analysed chromatographically. One unusual monosaccharide, 'U', was characterized by (1)H/(13)C-nuclear magnetic resonance spectroscopy and also enzymically.

KEY RESULTS

'U' was identified as 3-O-methyl-D-galactose (3-MeGal). All pectins, except in Klebsormidium, contained acid- and Driselase-releasable galacturonate, suggesting homogalacturonan. All pectins, without exception, released rhamnose and galactose on acid hydrolysis; however, only in 'higher' charophytes (Charales, Coleochaetales) and Anthoceros were these sugars also efficiently released by Driselase, suggesting rhamnogalacturonan-I. Pectins of 'higher' charophytes, especially Chara, contained little arabinose, instead possessing 3-MeGal. Anthoceros hemicelluloses were rich in glucose, xylose, galactose and arabinose (suggesting xyloglucan and arabinoxylan), none of which was consistently present in charophyte hemicelluloses.

CONCLUSIONS

Homogalacturonan is an ancient streptophyte feature, albeit secondarily lost in Klebsormidium. When conquering the land, the first embryophytes already possessed rhamnogalacturonan-I. In contrast, charophyte and land-plant hemicelluloses differ substantially, indicating major changes during terrestrialization. The presence of 3-MeGal in charophytes and lycophytes but not in the 'intervening' bryophytes confirms that cell-wall chemistry changed drastically between major phylogenetic grades.

Phylotranscriptomic analysis of the origin and early diversification of land plants

Reconstructing the origin and evolution of land plants and their algal relatives is a fundamental problem in plant phylogenetics, and is essential for understanding how critical adaptations arose, including the embryo, vascular tissue, seeds, and flowers. Despite advances in molecular systematics, some hypotheses of relationships remain weakly resolved. Inferring deep phylogenies with bouts of rapid diversification can be problematic; however, genome-scale data should significantly increase the number of informative characters for analyses. Recent phylogenomic reconstructions focused on the major divergences of plants have resulted in promising but inconsistent results. One limitation is sparse taxon sampling, likely resulting from the difficulty and cost of data generation. To address this limitation, transcriptome data for 92 streptophyte taxa were generated and analyzed along with 11 published plant genome sequences. Phylogenetic reconstructions were conducted using up to 852 nuclear genes and 1,701,170 aligned sites. Sixty-nine analyses were performed to test the robustness of phylogenetic inferences to permutations of the data matrix or to phylogenetic method, including supermatrix, supertree, and coalescent-based approaches, maximum-likelihood and Bayesian methods, partitioned and unpartitioned analyses, and amino acid versus DNA alignments. Among other results, we find robust support for a sister-group relationship between land plants and one group of streptophyte green algae, the Zygnematophyceae. Strong and robust support for a clade comprising liverworts and mosses is inconsistent with a widely accepted view of early land plant evolution, and suggests that phylogenetic hypotheses used to understand the evolution of fundamental plant traits should be reevaluated.

Morphology and ultrastructure of Interfilum and Klebsormidium (Klebsormidiales, Streptophyta) with special reference to cell division and thallus formation

Representatives of the closely related genera, Interfilum and Klebsormidium, are characterized by unicells, dyads or packets in Interfilum and contrasting uniseriate filaments in Klebsormidium. According to the literature, these distinct thallus forms originate by different types of cell division, sporulation (cytogony) versus vegetative cell division (cytotomy), but investigations of their morphology and ultrastructure show a high degree of similarity. Cell walls of both genera are characterized by triangular spaces between cell walls of neighbouring cells and the parental wall or central space among the walls of a cell packet, exfoliations and projections of the parental wall and cap-like and H-like fragments of the cell wall. In both genera, each cell has its individual cell wall and it also has part of the common parental wall or its remnants. Therefore, vegetative cells of Interfilum and Klebsormidium probably divide by the same type of cell division (sporulation-like). Various strains representing different species of the two genera are characterized by differences in cell wall ultrastructure, particularly the level of preservation, rupture or gelatinization of the parental wall surrounding the daughter cells. The differing morphologies of representatives of various lineages result from features of the parental wall during cell separation and detachment. Cell division in three planes (usual in Interfilum and a rare event in Klebsormidium) takes place in spherical or short cylindrical cells, with the chloroplast positioned perpendicularly or obliquely to the filament (dyad) axis. The morphological differences are mainly a consequence of differing fates of the parental wall after cell division and detachment. The development of different morphologies within the two genera mostly depends on characters such as the shape of cells, texture of cell walls, mechanical interactions between cells and the influence of environmental conditions.

Tracing the evolution of streptophyte algae and their mitochondrial genome

Six monophyletic groups of charophycean green algae are recognized within the Streptophyta. Although incongruent with earlier studies based on genes from three cellular compartments, chloroplast and nuclear phylogenomic analyses have resolved identical relationships among these groups, placing the Zygnematales or the Zygnematales + Coleochaetales as sister to land plants. The present investigation aimed at determining whether this consensus view is supported by the mitochondrial genome and at gaining insight into mitochondrial DNA (mtDNA) evolution within and across streptophyte algal lineages and during the transition toward the first land plants. We present here the newly sequenced mtDNAs of representatives of the Klebsormidiales (Entransia fimbriata and Klebsormidium spec.) and Zygnematales (Closterium baillyanum and Roya obtusa) and compare them with their homologs in other charophycean lineages as well as in selected embryophyte and chlorophyte lineages. Our results indicate that important changes occurred at the levels of genome size, gene order, and intron content within the Zygnematales. Although the representatives of the Klebsormidiales display more similarity in genome size and intron content, gene order seems more fluid and gene losses more frequent than in other charophycean lineages. In contrast, the two members of the Charales display an extremely conservative pattern of mtDNA evolution. Collectively, our analyses of gene order and gene content and the phylogenies we inferred from 40 mtDNA-encoded proteins failed to resolve the relationships among the Zygnematales, Coleochaetales, and Charales; however, they are consistent with previous phylogenomic studies in favoring that the morphologically complex Charales are not sister to land plants.

DESICCATION STRESS CAUSES STRUCTURAL AND ULTRASTRUCTURAL ALTERATIONS IN THE AEROTERRESTRIAL GREEN ALGA KLEBSORMIDIUM CRENULATUM (KLEBSORMIDIOPHYCEAE, STREPTOPHYTA) ISOLATED FROM AN ALPINE SOIL CRUST1

Klebsormidium crenulatum (Kütz.) Lokhorst (Klebsormidiophyceae, Streptophyta) isolated from an alpine soil in Tyrol, Austria, was experimentally exposed to desiccation under various relative air humidities (RH 5, 75, and >95%, ambient air 55%-60%). The effects on the structure and ultrastructure of K. crenulatum after 1, 4, or 7 d of desiccation at 5, 75, and >95% RH were investigated. The cross walls were deformed to an undulated shape, and the cell diameter was reduced to ∼60% of the control. Regardless of the RH applied, in all cases the cytoplasm appeared denser compared to that of liquid-culture-grown cells. Electron-dense particles with diameters of 0.4 μm-0.8 μm were observed in the cytoplasm, likely representing lipid droplets. The chloroplasts of desiccated samples contained a large number of plastoglobules. The number and appearance of mitochondria were not visibly altered, as also verified by 3,3' dihexyloxacarbocyanine iodine (DIOC₆ ) staining. The amphiphilic styryl dye FM 1-43 resulted in staining of the plasma membrane in cells from liquid culture. In 7 d desiccated samples, a marked fluorescence is seen in ∼40%-50% of the cells, which were dead. Actin microfilaments (MFs) were drastically disrupted after desiccation; only dotlike actin batches remained. These results demonstrate that flexibility of the cell walls and maintenance of the key organelles play a key role in the tolerance of desiccation stress in K. crenulatum.

DESICCATION STRESS CAUSES STRUCTURAL AND ULTRASTRUCTURAL ALTERATIONS IN THE AEROTERRESTRIAL GREEN ALGA KLEBSORMIDIUM CRENULATUM (KLEBSORMIDIOPHYCEAE, STREPTOPHYTA) ISOLATED FROM AN ALPINE SOIL CRUST1

Klebsormidium crenulatum (Kütz.) Lokhorst (Klebsormidiophyceae, Streptophyta) isolated from an alpine soil in Tyrol, Austria, was experimentally exposed to desiccation under various relative air humidities (RH 5, 75, and >95%, ambient air 55%-60%). The effects on the structure and ultrastructure of K. crenulatum after 1, 4, or 7 d of desiccation at 5, 75, and >95% RH were investigated. The cross walls were deformed to an undulated shape, and the cell diameter was reduced to ∼60% of the control. Regardless of the RH applied, in all cases the cytoplasm appeared denser compared to that of liquid-culture-grown cells. Electron-dense particles with diameters of 0.4 μm-0.8 μm were observed in the cytoplasm, likely representing lipid droplets. The chloroplasts of desiccated samples contained a large number of plastoglobules. The number and appearance of mitochondria were not visibly altered, as also verified by 3,3' dihexyloxacarbocyanine iodine (DIOC₆ ) staining. The amphiphilic styryl dye FM 1-43 resulted in staining of the plasma membrane in cells from liquid culture. In 7 d desiccated samples, a marked fluorescence is seen in ∼40%-50% of the cells, which were dead. Actin microfilaments (MFs) were drastically disrupted after desiccation; only dotlike actin batches remained. These results demonstrate that flexibility of the cell walls and maintenance of the key organelles play a key role in the tolerance of desiccation stress in K. crenulatum.

NEW STREPTOPHYTE GREEN ALGAE FROM TERRESTRIAL HABITATS AND AN ASSESSMENT OF THE GENUS INTERFILUM (KLEBSORMIDIOPHYCEAE, STREPTOPHYTA)(1)

Sarcinoid aeroterrestrial green algae were isolated from three arid locations in Ukraine and the Czech Republic. Although gross morphology suggested an affinity with Desmococcus (for taxonomic authorities, see Table S1 in the supplementary material), the cellular morphological characteristics were reminiscent of those of Geminella terricola. However, the presence of a complex of ultrastructural features indicated that these isolates were members of the streptophyte lineage in the green plants. 18S rDNA sequence phylogenies provided evidence of a close relationship with Klebsormidium in the Streptophyta, while the position of Desmococcus was within the Trebouxiophyceae. In the internal transcribed spacer (ITS) rDNA phylogeny, the sarcinoid isolates were closely related with strains of G. terricola and Interfilum paradoxum. Strains of that clade were morphologically united by a specific type of cell division that involves the association of persistent, cap-shaped remains of the mother cell wall with daughter cells. Consequently, these strains were assigned to a redefined genus Interfilum, and a new species, I. massjukiae, was described to accommodate one of the sarcinoid isolates. As the position of the genus Geminella was in the Trebouxiophyceae, the streptophyte G. terricola was transferred to Interfilum, as I. terricola comb. nov., but the ITS rDNA analyses proved inconclusive to resolve its affinities with other species of Interfilum due to intragenomic polymorphisms. The species of Interfilum had a closer relationship with K. flaccidum than with other species of Klebsormidium. The latter genus may not be monophyletic in its present circumscription.