Gene targeting in Physcomitrella patens

Development of PEG-mediated genetic transformation and gene editing system of Bryum argenteum as an abiotic stress tolerance model plant

KEY MESSAGE

To establish a sterile culture system and protoplast regeneration system for Bryum argenteum, and to establish and apply CRISPR/Cas9 system in Bryum argenteum. Bryum argenteum is a fascinating, cosmopolitan, and versatile moss species that thrives in various disturbed environments. Because of its comprehensive tolerance to the desiccation, high UV and extreme temperatures, it is emerging as a model moss for studying the molecular mechanisms underlying plant responses to abiotic stresses. However, the lack of basic tools such as gene transformation and targeted genome modification has hindered the understanding of the molecular mechanisms underlying the survival of B. argenteum in different environments. Here, we reported the protonema of B. argenteum can survive up to 95.4% water loss. In addition, the genome size of B. argenteum is approximately 313 Mb by kmer analysis, which is smaller than the previously reported 700 Mb. We also developed a simple method for protonema induction and an efficient protoplast isolation and regeneration protocol for B. argenteum. Furthermore, we established a PEG-mediated protoplast transient transfection and stable transformation system for B. argenteum. Two homologues of ABI3(ABA-INSENSITIVE 3) gene were successfully cloned from B. argenteum. To further investigate the function of the ABI3 gene in B. argenteum, we used the CRISPR/Cas9 genetic editing system to target the BaABI3A and BaABI3B gene in B. argenteum protoplasts. This resulted in mutagenesis at the target in about 2-5% of the regenerated plants. The isolated abi3a and abi3b mutants exhibited increased sensitivity to desiccation, suggesting that BaABI3A and BaABI3B play redundant roles in desiccation stress. Overall, our results provide a rapid and simple approach for molecular genetics in B. argenteum. This study contributes to a better understanding of the molecular mechanisms of plant adaptation to extreme environmental.

CRISPR/Cas-mediated inplanta gene targeting: current advances and challenges

We can use gene targeting (GT) to make modifications at a specific region in a plant's genome and create high-precision tools for plant biotechnology and breeding. However, its low efficiency is a major barrier to its use in plants. The discovery of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-Cas-based site-specific nucleases capable of inducing double-strand breaks in desired loci resulted in the development of novel approaches for plant GT. Several studies have recently demonstrated improvements in GT efficiency through cell-type-specific expression of Cas nucleases, the use of self-amplified GT-vector DNA, or manipulation of RNA silencing and DNA repair pathways. In this review, we summarize recent advances in CRISPR/Cas-mediated GT in plants and discuss potential efficiency improvements. Increasing the efficiency of GT technology will help us pave the way for increased crop yields and food safety in environmentally friendly agriculture.

Targeted Gene Knockouts by Protoplast Transformation in the Moss Physcomitrella patens

Targeted gene knockout is particularly useful for analyzing gene functions in plant growth, signaling, and development. By transforming knockout cassettes consisting of homologous sequences of the target gene into protoplasts, the classical gene targeting method aims to obtain targeted gene replacement, allowing for the characterization of gene functions in vivo. The moss Physcomitrella patens is a known model organism for a high frequency of homologous recombination and thus harbors a remarkable rate of gene targeting. Other moss features, including easy to culture, dominant haploidy phase, and sequenced genome, make gene targeting prevalent in Physcomitrella patens. However, even gene targeting was powerful to generate knockouts, researchers using this method still experienced technical challenges. For example, obtaining a good number of targeted knockouts after protoplast transformation and regeneration disturbed the users. Off-target mutations such as illegitimate random integration mediated by nonhomologous end joining and targeted insertion wherein one junction on-target but the other end off-target is commonly present in the knockouts. Protoplast fusion during transformation and regeneration was also a problem. This review will discuss the advantages and technical challenges of gene targeting. Recently, CRISPR-Cas9 is a revolutionary technology and becoming a hot topic in plant gene editing. In the second part of this review, CRISPR-Cas9 technology will be focused on and compared to gene targeting regarding the practical use in Physcomitrella patens. This review presents an updated perspective of the gene targeting and CRISPR-Cas9 techniques to plant biologists who may consider studying gene functions in the model organism Physcomitrella patens.

A blueprint for gene function analysis through Base Editing in the model plant Physcomitrium (Physcomitrella) patens

CRISPR-Cas9 has proven to be highly valuable for genome editing in plants, including the model plant Physcomitrium patens. However, the fact that most of the editing events produced using the native Cas9 nuclease correspond to small insertions and deletions is a limitation. CRISPR-Cas9 base editors enable targeted mutation of single nucleotides in eukaryotic genomes and therefore overcome this limitation. Here, we report two programmable base-editing systems to induce precise cytosine or adenine conversions in P. patens. Using cytosine or adenine base editors, site-specific single-base mutations can be achieved with an efficiency up to 55%, without off-target mutations. Using the APT gene as a reporter of editing, we could show that both base editors can be used in simplex or multiplex, allowing for the production of protein variants with multiple amino-acid changes. Finally, we set up a co-editing selection system, named selecting modification of APRT to report gene targeting (SMART), allowing up to 90% efficiency site-specific base editing in P. patens. These two base editors will facilitate gene functional analysis in P. patens, allowing for site-specific editing of a given base through single sgRNA base editing or for in planta evolution of a given gene through the production of randomly mutagenised variants using multiple sgRNA base editing.

Dependence of Biocatalysis on D/H Ratio: Possible Fundamental Differences for High-Level Biological Taxons

The kinetics of biological reactions depends on the deuterium/protium (D/H) ratio in water. In this work, we describe the kinetic model of biocatalytic reactions in living organisms depending on the D/H ratio. We show that a change in the lifetime or other characteristics of the vital activity of some organisms in response to a decrease or increase in the content of deuterium in the environment can be a sign of a difference in taxons. For animals-this is a curve with saturation according to the Gauss's principle, for plants-it is the Poisson dependence, for bacteria a weakly saturated curve with a slight reaction to the deuterium/protium ratio toward increasing deuterium. The biological activity of the aquatic environment with reduced, elevated, and natural concentrations of deuterium is considered. The results of the study are presented in different vital indicators of some taxons: the bacteria kingdom-the colony forming units (CFU) index (Escherichia coli); animals-the activation energy of the death of ciliates (Spirostomum ambiguum), embryogenesis of fish (Brachydanio rerio); plants-germination and accumulation of trace elements Callisia fragrans L., sprouting of gametophores and peptidomics of moss Physcomitrella patens. It was found that many organisms change their metabolism and activity, responding to both high and low concentrations of deuterium in water.

Mosses: Versatile plants for biotechnological applications

Mosses have long been recognized as powerful experimental tools for the elucidation of complex processes in plant biology. Recent increases in the availability of sequenced genomes and mutant collections, the establishment of novel technologies for targeted mutagenesis, and the development of viable protocols for large-scale production in bioreactors are now transforming mosses into one of the most versatile tools for biotechnological applications. In the present review, we highlight the astonishing biotechnological potential of mosses and how these plants are being exploited for industrial, pharmaceutical, and environmental applications. We focus on the biological features that support their use as model organisms for basic and applied research, and how these are being leveraged to explore the biotechnological potential in an increasing number of species. Finally, we also provide an overview of the available moss cultivation protocols from an industrial perspective, offering insights into batch operations that are not yet well established or do not even exist in the literature. Our goal is to bolster the use of mosses as factories for the biosynthesis of molecules of interest and to show how these species can be harnessed for the generation of novel and commercially useful bioproducts.

Heterologous expression of HpBHY and CrBKT increases heat tolerance in Physcomitrella patens

Heat stress can restrict plant growth, development, and crop yield. As essential plant antioxidants, carotenoids play significant roles in plant stress resistance. β-carotene hydroxylase (BHY) and β-carotene ketolase (BKT), which catalyze the conversions of β-carotene to zeaxanthin and β-carotene to canthaxanthin, respectively, are key enzymes in the carotenoid biosynthetic pathway, but little is known about their potential functions in stress resistance. Here, we investigated the roles of β-carotene hydroxylase and β-carotene ketolase during heat stress in Physcomitrella patens through expressing a β-carotene ketolase gene from Chlamydomonas reinhardtii (CrBKT) and a β-carotene hydroxylase gene from Haematococcus pluvialis (HpBHY) in the moss P. patens. In transgenic moss expressing these genes, carotenoids content increased (especially lutein content), and heat stress tolerance increased, with reduced leafy tissue necrosis. To investigate the mechanism of this heat stress resistance, we measured various physiological indicators and found a lower malondialdehyde level, higher peroxidase and superoxide dismutase activities, and higher endogenous abscisic acid and salicylate content in the transgenic plants in response to high-temperature stress. These results demonstrate that CrBKT and HpBHY increase plant heat stress resistance through the antioxidant and damage repair metabolism, which is related to abscisic acid and salicylate signaling.

Involvement of Lhcb6 and Lhcb5 in Photosynthesis Regulation in Physcomitrella patens Response to Abiotic Stress

There are a number of highly conserved photosystem II light-harvesting antenna proteins in moss whose functions are unclear. Here, we investigated the involvement of chlorophyll-binding proteins, Lhcb6 and Lhcb5, in light-harvesting and photosynthesis regulation in Physcomitrella patens. Lhcb6 or Lhcb5 knock-out resulted in a disordered thylakoid arrangement, a decrease in the number of grana membranes, and an increase in the number of starch granule. The absence of Lhcb6 or Lhcb5 did not noticeably alter the electron transport rates. However, the non-photochemical quenching activity in the lhcb5 mutant was dramatically reduced when compared to wild-type or lhcb6 plants under abiotic stress. Lhcb5 plants were more sensitive to photo-inhibition, while lhcb6 plants showed little difference compared to the wild-type plants under high-light stress. Moreover, both mutants showed a growth malformation phenotype with reduced chlorophyll content in the gametophyte. These results suggested that Lhcb6 or Lhcb5 played a unique role in plant development, thylakoid organization, and photoprotection of PSII in Physcomitrella, especially when exposed to high light or osmotic environments.

Targeting mutations to the plastidial psbA gene of Chlamydomonas reinhardtii without direct positive selection

Targeting mutations to specific genomic loci is invaluable for assessing in vivo the effect of these changes on the biological role of the gene in study. Here, we attempted to introduce a mutation that was previously implicated in an increased heat stability of the mesophilic cyanobacterium Synechocystis sp. PCC6803 via homologous recombination to the psbA gene of Chlamydomonas reinhardtii. For that, we established a strategy for targeted mutagenesis that was derived from the efficient genome-wide homologous-recombination-based methodology that was used to target individual genes of Saccharomyces cerevisiae. While the isolated mutants did not show any benefit under elevated temperature conditions, the new strategy proved to be efficient for C. reinhardtii even in the absence of direct positive selection.

PpSARK Regulates Moss Senescence and Salt Tolerance through ABA Related Pathway

Senescence-associated receptor-like kinase (SARK) family members in Arabidopsis, soybean, and rice are known to be positive regulators of leaf senescence. In the meantime, SARKs are extensively involved in stress response. However, their function and underlying molecular mechanism in stress responses in moss are not well known. Here, we investigated functional roles of SARK isolated from Physcomitrella patens (PpSARK) in salt stress response and senescence. PpSARK transcripts significantly accumulated under NaCl and abscisic acid (ABA) treatments, with higher expression in the moss gametophyte stage. Insertional gain-of-function mutants of PpSARK (PpSARKg) were more tolerant to salt stress and ABA than wild type (WT), whereas senescence of mutants was delayed during the protonema stage. Expression of stress-responsive genes in the ABA related pathway, such as PpABI3, PpABI5, PpPP2C, and PpLEA were significantly higher in PpSARKg and WT under salt stress conditions, suggesting that PpSARK might positively regulate salt tolerance via an ABA-related pathway. Endogenous ABA contents also increased 3-fold under salt stress conditions. These results indicate that PpSARK functions as a positive regulator in salt stress responses, while possibly functioning as a negative regulator in senescence in moss.

Optimization of Production Conditions for Protoplasts and Polyethylene Glycol-Mediated Transformation of Gaeumannomyces tritici

Take-all, caused by Gaeumannomyces tritici, is one of the most important wheat root diseases worldwide, as it results in serious yield losses. In this study, G. tritici was transformed to express the hygromycin B phosphotransferase using a combined protoplast and polyethylene glycol (PEG)-mediated transformation technique. Based on a series of single-factor experimental results, three major factors-temperature, enzyme lysis time, and concentration of the lysing enzyme-were selected as the independent variables, which were optimized using the response surface methodology. A higher protoplast yield of 9.83 × 10⁷ protoplasts/mL was observed, and the protoplast vitality was also high, reaching 96.27% after optimization. Protoplasts were isolated under the optimal conditions, with the highest transformation frequency (46⁻54 transformants/μg DNA). Polymerase chain reaction and Southern blotting detection indicated that the genes of hygromycin phosphotransferase were successfully inserted into the genome of G. tritici. An optimised PEG-mediated protoplast transformation system for G. tritici was established. The techniques and procedures described will lay the foundation for establishing a good mutation library of G. tritici and could be used to transform other fungi.

An intragenic mutagenesis strategy in Physcomitrella patens to preserve intron splicing

Gene targeting is a powerful reverse genetics technique for site-specific genome modification. Intrinsic homologous recombination in the moss Physcomitrella patens permits highly effective gene targeting, a characteristic that makes this organism a valuable model for functional genetics. Functional characterization of domains located within a multi-domain protein depends on the ability to generate mutants harboring genetic modifications at internal gene positions while maintaining the reading-frames of the flanking exons. In this study, we designed and evaluated different gene targeting constructs for targeted gene manipulation of sequences corresponding to internal domains of the DEFECTIVE KERNEL1 protein in Physcomitrella patens. Our results show that gene targeting-associated mutagenesis of introns can have adverse effects on splicing, corrupting the normal reading frame of the transcript. We show that successful genetic modification of internal sequences of multi-exon genes depends on gene-targeting strategies which insert the selection marker cassette into the 5' end of the intron and preserve the nucleotide sequence of the targeted intron.

CRISPR-Cas9-mediated efficient directed mutagenesis and RAD51-dependent and RAD51-independent gene targeting in the moss Physcomitrella patens

The ability to address the CRISPR-Cas9 nuclease complex to any target DNA using customizable single-guide RNAs has now permitted genome engineering in many species. Here, we report its first successful use in a nonvascular plant, the moss Physcomitrella patens. Single-guide RNAs (sgRNAs) were designed to target an endogenous reporter gene, PpAPT, whose inactivation confers resistance to 2-fluoroadenine. Transformation of moss protoplasts with these sgRNAs and the Cas9 coding sequence from Streptococcus pyogenes triggered mutagenesis at the PpAPT target in about 2% of the regenerated plants. Mainly, deletions were observed, most of them resulting from alternative end-joining (alt-EJ)-driven repair. We further demonstrate that, in the presence of a donor DNA sharing sequence homology with the PpAPT gene, most transgene integration events occur by homology-driven repair (HDR) at the target locus but also that Cas9-induced double-strand breaks are repaired with almost equal frequencies by mutagenic illegitimate recombination. Finally, we establish that a significant fraction of HDR-mediated gene targeting events (30%) is still possible in the absence of PpRAD51 protein, indicating that CRISPR-induced HDR is only partially mediated by the classical homologous recombination pathway.

Moss-Produced, Glycosylation-Optimized Human Factor H for Therapeutic Application in Complement Disorders

Genetic defects in complement regulatory proteins can lead to severe renal diseases, including atypical hemolytic uremic syndrome and C3 glomerulopathies, and age-related macular degeneration. The majority of the mutations found in patients with these diseases affect the glycoprotein complement factor H, the main regulator of the alternative pathway of complement activation. Therapeutic options are limited, and novel treatments, specifically those targeting alternative pathway activation, are highly desirable. Substitution with biologically active factor H could potentially treat a variety of diseases that involve increased alternative pathway activation, but no therapeutic factor H is commercially available. We recently reported the expression of full-length recombinant factor H in moss (Physcomitrella patens). Here, we present the production of an improved moss-derived recombinant human factor H devoid of potentially immunogenic plant-specific sugar residues on protein N-glycans, yielding approximately 1 mg purified moss-derived human factor H per liter of initial P. patens culture after a multistep purification process. This glycosylation-optimized factor H showed full in vitro complement regulatory activity similar to that of plasma-derived factor H and efficiently blocked LPS-induced alternative pathway activation and hemolysis induced by sera from patients with atypical hemolytic uremic syndrome. Furthermore, injection of moss-derived factor H reduced C3 deposition and increased serum C3 levels in a murine model of C3 glomerulopathy. Thus, we consider moss-produced recombinant human factor H a promising pharmaceutical product for therapeutic intervention in patients suffering from complement dysregulation.

Simple and Efficient Targeting of Multiple Genes Through CRISPR-Cas9 in Physcomitrella patens

Powerful genome editing technologies are needed for efficient gene function analysis. The CRISPR-Cas9 system has been adapted as an efficient gene-knock-out technology in a variety of species. However, in a number of situations, knocking out or modifying a single gene is not sufficient; this is particularly true for genes belonging to a common family, or for genes showing redundant functions. Like many plants, the model organism Physcomitrella patens has experienced multiple events of polyploidization during evolution that has resulted in a number of families of duplicated genes. Here, we report a robust CRISPR-Cas9 system, based on the codelivery of a CAS9 expressing cassette, multiple sgRNA vectors, and a cassette for transient transformation selection, for gene knock-out in multiple gene families. We demonstrate that CRISPR-Cas9-mediated targeting of five different genes allows the selection of a quintuple mutant, and all possible subcombinations of mutants, in one experiment, with no mutations detected in potential off-target sequences. Furthermore, we confirmed the observation that the presence of repeats in the vicinity of the cutting region favors deletion due to the alternative end joining pathway, for which induced frameshift mutations can be potentially predicted. Because the number of multiple gene families in Physcomitrella is substantial, this tool opens new perspectives to study the role of expanded gene families in the colonization of land by plants.

Synthetic strategies for plant signalling studies: molecular toolbox and orthogonal platforms

Plants deploy a wide array of signalling networks integrating environmental cues with growth, defence and developmental responses. The high level of complexity, redundancy and connection between several pathways hampers a comprehensive understanding of involved functional and regulatory mechanisms. The implementation of synthetic biology approaches is revolutionizing experimental biology in prokaryotes, yeasts and animal systems and can likewise contribute to a new era in plant biology. This review gives an overview on synthetic biology approaches for the development and implementation of synthetic molecular tools and techniques to interrogate, understand and control signalling events in plants, ranging from strategies for the targeted manipulation of plant genomes up to the spatiotemporally resolved control of gene expression using optogenetic approaches. We also describe strategies based on the partial reconstruction of signalling pathways in orthogonal platforms, like yeast, animal and in vitro systems. This allows a targeted analysis of individual signalling hubs devoid of interconnectivity with endogenous interacting components. Implementation of the interdisciplinary synthetic biology tools and strategies is not exempt of challenges and hardships but simultaneously most rewarding in terms of the advances in basic and applied research. As witnessed in other areas, these original theoretical-experimental avenues will lead to a breakthrough in the ability to study and comprehend plant signalling networks.

In vivo assembly of DNA-fragments in the moss, Physcomitrella patens

Direct assembly of multiple linear DNA fragments via homologous recombination, a phenomenon known as in vivo assembly or transformation associated recombination, is used in biotechnology to assemble DNA constructs ranging in size from a few kilobases to full synthetic microbial genomes. It has also enabled the complete replacement of eukaryotic chromosomes with heterologous DNA. The moss Physcomitrella patens, a non-vascular and spore producing land plant (Bryophyte), has a well-established capacity for homologous recombination. Here, we demonstrate the in vivo assembly of multiple DNA fragments in P. patens with three examples of effective genome editing: we (i) efficiently deleted a genomic locus for diterpenoid metabolism yielding a biosynthetic knockout, (ii) introduced a salt inducible promoter, and (iii) re-routed endogenous metabolism into the formation of amorphadiene, a precursor of high-value therapeutics. These proof-of-principle experiments pave the way for more complex and increasingly flexible approaches for large-scale metabolic engineering in plant biotechnology.

A Transcriptome Atlas of Physcomitrella patens Provides Insights into the Evolution and Development of Land Plants

Identifying the genetic mechanisms that underpin the evolution of new organ and tissue systems is an aim of evolutionary developmental biology. Comparative functional genetic studies between angiosperms and bryophytes can define those genetic changes that were responsible for developmental innovations. Here, we report the generation of a transcriptome atlas covering most phases in the life cycle of the model bryophyte Physcomitrella patens, including detailed sporophyte developmental progression. We identified a comprehensive set of sporophyte-specific transcription factors, and found that many of these genes have homologs in angiosperms that function in developmental processes such as flowering and shoot branching. Deletion of the PpTCP5 transcription factor results in development of supernumerary sporangia attached to a single seta, suggesting that it negatively regulates branching in the moss sporophyte. Given that TCP genes repress branching in angiosperms, we suggest that this activity is ancient. Finally, comparison of P. patens and Arabidopsis thaliana transcriptomes led us to the identification of a conserved core of transcription factors expressed in tip-growing cells. We identified modifications in the expression patterns of these genes that could account for developmental differences between P. patens tip-growing cells and A. thaliana pollen tubes and root hairs.

Activation of Shikimate, Phenylpropanoid, Oxylipins, and Auxin Pathways in Pectobacterium carotovorum Elicitors-Treated Moss

Plants have developed complex defense mechanisms to cope with microbial pathogens. Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are perceived by pattern recognition receptors (PRRs), leading to the activation of defense. While substantial progress has been made in understanding the activation of plant defense by PAMPs and DAMPs recognition in tracheophytes, far less information exists on related processes in early divergent plants like mosses. The aim of this study was to identify genes that were induced in P. patens in response to elicitors of Pectobacterium carotovorum subsp. carotovorum, using a cDNA suppression subtractive hybridization (SSH) method. A total of 239 unigenes were identified, including genes involved in defense responses related to the shikimate, phenylpropanoid, and oxylipin pathways. The expression levels of selected genes related to these pathways were analyzed using quantitative RT-PCR, confirming their rapid induction by P.c. carotovorum derived elicitors. In addition, P. patens induced cell wall reinforcement after elicitor treatment by incorporation of phenolic compounds, callose deposition, and elevated expression of Dirigent-like encoding genes. Small molecule defense markers and phytohormones such as cinnamic acid, 12-oxo-phytodienoic acid, and auxin levels all increased in elicitor-treated moss tissues. In contrast, salicylic acid levels decreased while abscisic acid levels remained unchanged. P. patens reporter lines harboring an auxin-inducible promoter fused to β-glucuronidase revealed GUS activity in protonemal and gametophores tissues treated with elicitors of P.c. carotovorum, consistent with a localized activation of auxin signaling. These results indicate that P. patens activates the shikimate, phenylpropanoid, oxylipins, and auxin pathways upon treatment with P.c. carotovorum derived elicitors.

Gene Targeting for Precision Glyco-Engineering: Production of Biopharmaceuticals Devoid of Plant-Typical Glycosylation in Moss Bioreactors

One of the main challenges for the production of biopharmaceuticals in plant-based systems is the modulation of plant-specific glycosylation patterns towards a humanized form. Posttranslational modifications in plants are similar to those in humans, but several differences affect product quality and efficacy and can also cause immune responses in patients. In the moss Physcomitrella patens highly efficient gene targeting via homologous recombination enables glyco-engineering to obtain suitable platform lines for the production of recombinant proteins and biopharmaceuticals. Here we describe the methods which are effective for creating gene targeting constructs and transgenic moss lines as well as confirming successful homologous integration of the constructs and modification of target gene expression.

On the way to commercializing plant cell culture platform for biopharmaceuticals: present status and prospect

Plant cell culture is emerging as an alternative bioproduction system for recombinant pharmaceuticals. Growing plant cells in vitro under controlled environmental conditions allows for precise control over cell growth and protein production, batch-to-batch product consistency and a production process aligned with current good manufacturing practices. With the recent US FDA approval and commercialization of the world's first plant cell-based recombinant pharmaceutical for human use, β-glucocerebrosidase for treatment of Gaucher's disease, a new era has come in which plant cell culture shows high potential to displace some established platform technologies in niche markets. This review updates the progress in plant cell culture processing technology, highlights recent commercial successes and discusses the challenges that must be overcome to make this platform commercially viable.

Characterization of DNA repair deficient strains of Chlamydomonas reinhardtii generated by insertional mutagenesis

While the mechanisms governing DNA damage response and repair are fundamentally conserved, cross-kingdom comparisons indicate that they differ in many aspects due to differences in life-styles and developmental strategies. In photosynthetic organisms these differences have not been fully explored because gene-discovery approaches are mainly based on homology searches with known DDR/DNA repair proteins. Here we performed a forward genetic screen in the green algae Chlamydomonas reinhardtii to identify genes deficient in DDR/DNA repair. We isolated five insertional mutants that were sensitive to various genotoxic insults and two of them exhibited altered efficiency of transgene integration. To identify genomic regions disrupted in these mutants, we established a novel adaptor-ligation strategy for the efficient recovery of the insertion flanking sites. Four mutants harbored deletions that involved known DNA repair factors, DNA Pol zeta, DNA Pol theta, SAE2/COM1, and two neighbouring genes encoding ERCC1 and RAD17. Deletion in the last mutant spanned two Chlamydomonas-specific genes with unknown function, demonstrating the utility of this approach for discovering novel factors involved in genome maintenance.

Blue-light irradiation up-regulates the ent-kaurene synthase gene and affects the avoidance response of protonemal growth in Physcomitrella patens

We report a novel physiological response to blue light in the moss Physcomitrella patens . Blue light regulates ent -kaurene biosynthesis and avoidance response to protonemal growth. Gibberellins (GAs) are a group of diterpene-type plant hormones biosynthesized from ent-kaurenoic acid via ent-kaurene. While the moss Physcomitrella patens has part of the GA biosynthetic pathway, from geranylgeranyl diphosphate to ent-kaurenoic acid, no GA is found in this species. Caulonemal differentiation in a P. patens mutant with a disrupted bifunctional ent-copalyl diphosphate synthase/ent-kaurene synthase (PpCPS/KS) gene is suppressed under red light, and is recovered by application of ent-kaurene and ent-kaurenoic acid. This indicates that derivatives of ent-kaurenoic acid, not GAs, might act as endogenous developmental regulators. Here, we found unique responses in the protonemal growth of P. patens under unilateral blue light, and these regulators were involved in the responses. When protonemata of the wild type were incubated under blue light, the chloronemal filaments grew in the opposite direction to the light source. Although this avoidance was not observed in the ent-kaurene deficient mutant, chloronemal growth toward a blue-light source in the mutant was suppressed by application of ent-kaurenoic acid, and the growth was rescued to that in the wild type. Expression analysis of the PpCPS/KS gene showed that the mRNA level under blue light was rapidly increased and was five times higher than under red light. These results suggest that regulators derived from ent-kaurenoic acid are strongly involved not only in the growth regulation of caulonemal differentiation under red light, but also in the light avoidance response of chloronemal growth under blue light. In particular, growth under blue light is regulated via the PpCPS/KS gene.

Conserved function of Rho-related Rop/RAC GTPase signaling in regulation of cell polarity in Physcomitrella patens

Cell polarity is fundamentally important to growth and development in higher plants, from pollen tubes to root hairs. Basal land plants (mosses and ferns) also have cell polarity, developing protonemal apical cells that show polar tip growth. Flowering plants have a distinct group of Rho GTPases that regulate polarity in polarized cell growth. Rop/RAC signaling module components have been identified in non-flowering plants, but their roles remain unclear. To understand the importance and evolution of Rop/RAC signaling in polarity regulation in land plants, we examined the functions of PpRop and PpRopGEF in protonemal apical cells of the moss Physcomitrella patens. Inducible overexpression of PpRop2 or PpRopGEF3 caused depolarized growth of tip-growing apical cells. PpRop2 overexpression also caused aberrant cross wall formation. Fluorescent protein-tagged PpRop2 localized to the plasma membrane, including the cross wall membrane, and fluorescent-tagged PpRopGEF3 showed polarized localization to the tip region in apical cells. Thus, our results suggest common functions of PpRop and PpRopGEF in the tip-growing apical cells and the importance of a conserved Rop/RAC signaling module in the control of cell polarity in land plants.

Plastid transformation in Physcomitrella patens

The moss Physcomitrella patens performs efficient homologous recombination in both the nucleus and plastid enabling the study of individual gene function by generating precise inactivation or modification of genes. Polyethylene glycol (PEG)-mediated transformation of protoplasts is routinely used to study the nuclear gene function of P. patens. PEG-mediated protoplast transformation is also applied for plastid transformation of this moss. The efficiency of plastid transformation is quite reliable, and one or two homoplasmic transplastomic lines are obtained in a plastid transformation experiment (5 × 10(5) protoplasts) by selection for spectinomycin resistance.

High contents of very long-chain polyunsaturated fatty acids in different moss species

Polyunsaturated fatty acids (PUFAs) are important cellular compounds with manifold biological functions. Many PUFAs are essential for the human diet and beneficial for human health. In this study, we report on the high amounts of very long-chain (vl) PUFAs (≥C₂₀) such as arachidonic acid (AA) in seven moss species. These species were established in axenic in vitro culture, as a prerequisite for comparative metabolic studies under highly standardized laboratory conditions. In the model organism Physcomitrella patens, tissue-specific differences in the fatty acid compositions between the filamentous protonema and the leafy gametophores were observed. These metabolic differences correspond with differential gene expression of fatty acid desaturase (FADS)-encoding genes in both developmental stages, as determined via microarray analyses. Depending on the developmental stage and the species, AA amounts for 6-31 %, respectively, of the total fatty acids. Subcellular localization of the corresponding FADS revealed the endoplasmic reticulum as the cellular compartment for AA synthesis. Our results show that vlPUFAs are highly abundant metabolites in mosses. Standardized cultivation techniques using photobioreactors along with the availability of the P. patens genome sequence and the high rate of homologous recombination are the basis for targeted metabolic engineering in moss. The potential of producing vlPUFAs of interest from mosses will be highlighted as a promising area in plant biotechnology.

The potential of Physcomitrella patens as a platform for the production of plant-based vaccines

The moss Physcomitrella patens has a number of advantages for the production of biopharmaceuticals, including: i) availability of standardized conditions for cultivation in bioreactors; ii) not being part of the food chain; iii) high biosafety; iv) availability of highly efficient transformation methods; v) a haploid, fully sequenced genome providing genetic stability and uniform expression; vi) efficient gene targeting at the nuclear level allows for the generation of mutants with specific post-translational modifications (e.g., glycosylation patterns); and vii) oral formulations are a viable approach as no toxic effects are attributed to ingestion of this moss. In the light of this panorama, this opinion paper analyzes the possibilities of using P. patens for the production of oral vaccines and presents some specific cases where its use may represent significant progress in the field of plant-based vaccine development. The advantages represented by putative adjuvant effects of endogenous secondary metabolites and producing specific glycosylation patterns are highlighted.

Genetic modification stimulated by the induction of a site-specific break distant from the locus of correction in haploid and diploid yeast Saccharomyces cerevisiae

Generation of a site-specific break at a genomic locus to stimulate homologous recombination (HR) is used in many organisms to efficiently target genes for various types of genetic modification. Additionally, a site-specific chromosomal break can be used to trigger HR at genomic regions distant from the break, thereby largely expanding the region available for introducing desired mutations. In contrast to the former approach, the latter presents an alternative way in which genes can be efficiently modified also when it is not possible or desirable to introduce a break in the vicinity of the targeting locus. This type of in vivo site-directed mutagenesis distant from a break can be accomplished in the yeast model organism Saccharomyces cerevisiae because the generation of a double-strand break (DSB) in yeast chromosomal DNA activates HR at long regions upstream and downstream from the break site. Here we provide a protocol for efficiently altering a yeast chromosomal locus following the induction of a DSB several kilobase pairs distant from the site of gene correction. The techniques described can be used in both diploid and haploid yeast strains, and we provide examples of the gene correction assays.

Usefulness of Physcomitrella patens for studying plant organogenesis

In this chapter, we review the main organogenesis features and associated regulation processes of the moss Physcomitrella patens (P. patens), the model plant for the Bryophytes. We highlight how the study of this descendant of the earliest plant species that colonized earth, brings useful keys to understand the mechanisms that determine and control both vascular and non vascular plants organogenesis. Despite its simple morphogenesis pattern, P. patens still requires the fine tuning of organogenesis regulators, including hormone signalling, common to the whole plant kingdom, and which study is facilitated by a high number of molecular tools, among which the powerful possibility of gene targeting/replacement. The recent discovery of moss cells reprogramming capacity completes the picture of an excellent model for studying plant organogenesis.

Insights into the regenerative property of plant cells and their receptivity to transgenesis: wheat as a research case study

From a holistic perspective, the discovery of cellular plasticity, a very interesting property of totipotency, underlies many topical issues in biology with important medical applications, while transgenesis is a core research tool in biology. Partially known, some basic mechanisms involved in the regenerative property of cells and in their receptivity to transgenesis are common to plant and animal cells and highlight the principle of the unity of life. Transgenesis provides an important investigative instrument in plant physiology and is regarded as a valuable tool for crop improvement. The economic, social, cultural and scientific importance of cereals has led to a rich stream of research into their genetics, biology and evolution. Sustained efforts to achieve the results obtained in the fields of genetic engineering and applied biotechnology reflect this deep interest. Difficulties encountered in creating genetically modified cereals, especially wheat, highlighted the central notions of tissue culture regeneration and transformation competencies. From the perspective of combining or encountering these competencies in the same cell lineage, this reputedly recalcitrant species provides a stimulating biological system in which to explore the physiological and genetic complexity of both competencies. The former involves two phases, dedifferentiation and redifferentiation. Cells undergo development switches regulated by extrinsic and intrinsic factors. The re-entry into the cell division cycle progressively culminates in the development of organized structures. This is achieved by global chromatin reorganization associated with the reprogramming of the gene expression pattern. The latter is linked with surveillance mechanisms and DNA repair, aimed at maintaining genome integrity before cells move into mitosis, and with those mechanisms aimed at genome expression control and regulation. In order to clarify the biological basis of these two physiological properties and their interconnectedness, we look at both competencies at the core of defense/adaptive mechanisms and survival, between undifferentiated cell proliferation and organization, constituting a transition phase between two different dynamic regimes, a typical feature of critical dynamic systems. Opting for a candidate-gene strategy, several gene families could be proposed as relevant targets for investigating this hypothesis at the molecular level.

MicroRNAs in the moss Physcomitrella patens

Having diverged from the lineage that lead to flowering plants shortly after plants have established on land, mosses, which share fundamental processes with flowering plants but underwent little morphological changes by comparison with the fossil records, can be considered as an evolutionary informative place. Hence, they are especially useful for the study of developmental evolution and adaption to life on land. The transition to land exposed early plants to harsh physical conditions that resulted in key physiological and developmental changes. MicroRNAs (miRNAs) are an important class of small RNAs (sRNAs) that act as master regulators of development and stress in flowering plants. In recent years several groups have been engaged in the cloning of sRNAs from the model moss Physcomitrella patens. These studies have revealed a wealth of miRNAs, including novel and conserved ones, creating a unique opportunity to broaden our understanding of miRNA functions in land plants and their contribution to the latter's evolution. Here we review the current knowledge of moss miRNAs and suggest approaches for their functional analysis in P. patens.

MRE11 and RAD50, but not NBS1, are essential for gene targeting in the moss Physcomitrella patens

The moss Physcomitrella patens is unique among plant models for the high frequency with which targeted transgene insertion occurs via homologous recombination. Transgene integration is believed to utilize existing machinery for the detection and repair of DNA double-strand breaks (DSBs). We undertook targeted knockout of the Physcomitrella genes encoding components of the principal sensor of DNA DSBs, the MRN complex. Loss of function of PpMRE11 or PpRAD50 strongly and specifically inhibited gene targeting, whilst rates of untargeted transgene integration were relatively unaffected. In contrast, disruption of the PpNBS1 gene retained the wild-type capacity to integrate transforming DNA efficiently at homologous loci. Analysis of the kinetics of DNA-DSB repair in wild-type and mutant plants by single-nucleus agarose gel electrophoresis revealed that bleomycin-induced fragmentation of genomic DNA was repaired at approximately equal rates in each genotype, although both the Ppmre11 and Pprad50 mutants exhibited severely restricted growth and development and enhanced sensitivity to UV-B and bleomycin-induced DNA damage, compared with wild-type and Ppnbs1 plants. This implies that while extensive DNA repair can occur in the absence of a functional MRN complex; this is unsupervised in nature and results in the accumulation of deleterious mutations incompatible with normal growth and development.

Efficient polyethylene glycol (PEG) mediated transformation of the moss Physcomitrella patens

A simple and efficient method to transform Physcomitrella pantens protoplasts is described. This method is adapted from protocols for Physocmitrella protonemal protoplast and Arabidopsis mesophyll protoplast transformation. Due to its capacity to undergo efficient mitotic homologous recombination, Physcomitrella patens has emerged as an important model system in recent years. This capacity allows high frequencies of gene targeting, which is not seen in other model plants such as Arabidopsis. To take full advantage of this system, we need an effective and easy method to deliver DNA into moss cells. The most common ways to transform this moss are particle bombardment and PEG-mediated DNA uptake. Although particle bombardment can produce a high transformation efficiency, gene guns are not readily available to many laboratories and the protocol is difficult to standardize. On the other hand, PEG mediated transformation does not require specialized equipments, and can be performed in any laboratory with a sterile hood. Here, we show a simple and highly efficient method for transformation of moss protoplasts. This method can generate more than 120 transient transformants per microgram of DNA, which is an improvement from the most efficient protocol previously reported. Because of its simplicity, efficiency, and reproducibility, this method can be applied to projects requiring large number of transformants as well as for routine transformation.

Knocking out the wall: protocols for gene targeting in Physcomitrella patens

The moss Physcomitrella patens has become established as a model for investigating plant gene function due to the feasibility of gene targeting. The chemical composition of the P. patens cell wall is similar to that of vascular plants and phylogenetic analyses of glycosyltransferase sequences from the P. patens genome have identified genes that putatively encode cell wall biosynthetic enzymes, providing a basis for investigating the evolution of cell wall polysaccharides and the enzymes that synthesize them. The protocols described in this chapter provide methods for targeted gene knockout in P. patens, from constructing vectors and maintaining cultures to transforming protoplasts and analyzing the genotypes and phenotypes of the resulting transformed lines.

Evolutionary crossroads in developmental biology: Physcomitrella patens

The moss Physcomitrella patens has recently emerged as a powerful genetically tractable model plant system. As a member of the bryophytes, P. patens provides a unique opportunity to study the evolution of a myriad of plant traits, such as polarized cell growth, gametophyte-to-sporophyte transitions, and sperm-to-pollen transition. The availability of a complete genome sequence, together with the ability to perform gene targeting efficiently in P. patens has spurred a flurry of elegant reverse genetic studies in this plant model that address a variety of key questions in plant developmental biology.

Protection of Telomeres 1 is required for telomere integrity in the moss Physcomitrella patens

In vertebrates, the single-stranded telomeric DNA binding protein Protection of Telomeres 1 (POT1) shields chromosome ends and prevents them from eliciting a DNA damage response. By contrast, Arabidopsis thaliana encodes two divergent full-length POT1 paralogs that do not exhibit telomeric DNA binding in vitro and have evolved to mediate telomerase regulation instead of chromosome end protection. To further investigate the role of POT1 in plants, we established the moss Physcomitrella patens as a new model for telomere biology and a counterpoint to Arabidopsis. The sequence and architecture of the telomere tract is similar in P. patens and Arabidopsis, but P. patens harbors only a single-copy POT1 gene. Unlike At POT1 proteins, Pp POT1 efficiently bound single-stranded telomeric DNA in vitro. Deletion of the P. patens POT1 gene resulted in the rapid onset of severe developmental defects and sterility. Although telomerase activity levels were unperturbed, telomeres were substantially shortened, harbored extended G-overhangs, and engaged in end-to-end fusions. We conclude that the telomere capping function of POT1 is conserved in early diverging land plants but is subsequently lost in Arabidopsis.

High efficient gene targeting on the AGAMOUS gene in an ArabidopsisAtLIG4 mutant

Gene targeting induced by homologous integration of a foreign DNA segment into a chromosomal target sequence enables precise disruption or replacement of genes of interest and provides an effective means to analyze gene function, and also becomes an useful technique for breeding. But, integration of introduced DNA fragments is predominantly non-homologous in most species. However, we presented high-efficient homologous integration in disruptants of non-homologous end joining (NHEJ), that is, the Ku70-, Ku80- or Lig4-homologs deficient strain, in a model fungus Neurospora crassa. When the effect of NHEJ-defective plants for gene targeting was therefore examined in a model plant Arabidopsis (Arabidopsis thaliana), the efficiencies of gene targeting in the Atlig4/Atlig4 plant were 2/7 (28.6%) against calli obtained a selection-marker gene, 2/16 (12.5%) against selected calli, and about 2/540 (0.004%) against total cell particles at the starting point for transformation. The results of this paper show that the NHEJ-deficient system might cause a decrease in the efficiency of transformation but gives true targeted transformants with high efficiency in plant cell.

RAD51 loss of function abolishes gene targeting and de-represses illegitimate integration in the moss Physcomitrella patens

Gene targeting (GT) is a major tool for basic and applied research during which the transforming DNA, which shares sequence homology with a chromosomal target, integrates at the corresponding locus by homologous recombination (HR). In eukaryotes, GT recruits enzymes from the HR-mediated double strand break repair pathway. Different mechanisms of HR have been described which depend on the Rad52 epistasis group of genes, but which specific mechanism is used by the cell for GT remains unclear. In Saccharomyces cerevisiae, the RAD52 protein is essential for GT, and the RAD51 protein plays a minor role. In filamentous fungi and animal cells, however, GT depends on RAD51 and is weakly affected by suppression of RAD52. Genetic evidence also indicates that the non-homologous end-joining pathway of DSB repair has a negative impact on GT efficiencies, but how the balance between these two pathways is controlled is poorly understood. Here, we have examined the role of RAD51 in the only plant that exhibits high GT frequencies, the model bryophyte Physcomitrella patens. Our results show that the two RAD51 proteins have partially redundant functions in the maintenance of genome integrity and resistance to ionizing radiation. Furthermore, we demonstrate that loss of function of the two RAD51 proteins completely abolishes GT and strongly increases illegitimate integration rates in this moss. These findings demonstrate for the first time in plant the critical role of RAD51 in controlling the balance between targeted and random integration events observed upon transgenesis, and confirm that P. patens is a particularly interesting tool for studying GT in higher eukaryotes.

Genetic technologies for the identification of plant genes controlling environmental stress responses

Abiotic conditions such as light, temperature, water availability and soil parameters determine plant growth and development. The adaptation of plants to extreme environments or to sudden changes in their growth conditions is controlled by a well balanced, genetically determined signalling system, which is still far from being understood. The identification and characterisation of plant genes which control responses to environmental stresses is an essential step to elucidate the complex regulatory network, which determines stress tolerance. Here, we review the genetic approaches, which have been used with success to identify plant genes which control responses to different abiotic stress factors. We describe strategies and concepts for forward and reverse genetic screens, conventional and insertion mutagenesis, TILLING, gene tagging, promoter trapping, activation mutagenesis and cDNA library transfer. The utility of the various genetic approaches in plant stress research we review is illustrated by several published examples.

Suppression of repeat-mediated gross mitochondrial genome rearrangements by RecA in the moss Physcomitrella patens

RecA and its ubiquitous homologs are crucial components in homologous recombination. Besides their eukaryotic nuclear counterparts, plants characteristically possess several bacterial-type RecA proteins localized to chloroplasts and/or mitochondria, but their roles are poorly understood. Here, we analyzed the role of the only mitochondrial RecA in the moss Physcomitrella patens. Disruption of the P. patens mitochondrial recA gene RECA1 caused serious defects in plant growth and development and abnormal mitochondrial morphology. Analyses of mitochondrial DNA in disruptants revealed that frequent DNA rearrangements occurred at multiple loci. Structural analysis suggests that the rearrangements, which in some cases were associated with partial deletions and amplifications of mitochondrial DNA, were due to aberrant recombination between short (<100 bp) direct and inverted repeats in which the sequences were not always identical. Such repeats are abundant in the mitochondrial genome, and interestingly many are located in group II introns. These results suggest that RECA1 does not promote but rather suppresses recombination among short repeats scattered throughout the mitochondrial genome, thereby maintaining mitochondrial genome stability. We propose that RecA-mediated homologous recombination plays a crucial role in suppression of short repeat-mediated genome rearrangements in plant mitochondria.

Microtubules regulate dynamic organization of vacuoles in Physcomitrella patens

Eukaryotic cells have developed several essential membrane components. In flowering plants, appropriate structures and distributions of the major membrane components are predominantly regulated by actin microfilaments. In this study, we have focused on the regulatory mechanism of vacuolar structures in the moss, Physcomitrella patens. The high ability of P. patens to undergo homologous recombination enabled us stably to express green fluorescent protein (GFP) or red fluorescent protein (RFP) fusion proteins, and the simple body structure of P. patens enabled us to perform detailed visualization of the intracellular vacuolar and cytoskeletal structures. Three-dimensional analysis and high-speed time-lapse observations revealed surprisingly complex structures and dynamics of the vacuole, with inner sheets and tubular protrusions, and frequent rearrangements by separation and fusion of the membranes. Depolymerization of microtubules dramatically affected these structures and movements. Dual observation of microtubules and vacuolar membranes revealed that microtubules induced tubular protrusions and cytoplasmic strands of the vacuoles, indicative of interactions between microtubules and vacuolar membranes. These results demonstrate a novel function of microtubules in maintaining the distribution of the vacuole and suggest a functional divergence of cytoskeletal functions in land plant evolution.

Characterization of phosphatidylinositol phosphate kinases from the moss Physcomitrella patens: PpPIPK1 and PpPIPK2

Phosphoinositides (PIs) play a major role in eukaryotic cells, despite being a minor component of most membranes. This is the first report on PI metabolism in a bryophyte, the moss Physcomitrella patens. Moss PI composition is similar to that of other land plants growing under normal conditions. In contrast to the large number of PIPK genes present in flowering plants, the P. patens genome encodes only two type I/II PIPK genes, PpPIPK1 and PpPIPK2, which are very similar at both the nucleotide and protein product levels. However, the expression of the two genes is differentially regulated, and in vitro biochemical characterization shows that the resulting enzymes have different substrate specificities. PpPIPK1 uses PtdIns4P and PtdIns3P with similar preference and also metabolizes PtdIns(3,4)P(2) to produce PtdIns(3,4,5)P(3), a PI not yet detected in intact plant cells. PpPIPK2 prefers PtdIns as substrate and is much less active towards PtdIns4P and PtdIns3P. Thus, PpPIPK2 shows properties reminiscent of both PtdInsP-kinase and PtdIns-kinases. Moreover, a substitution of glutamic acid by alanine in the activation loop drastically reduced PpPIPK1 activity and altered the substrate specificity to PtdIns5P being the preferred substrate compared with PtdIns4P and PtdIns3P. These findings demonstrate that the substrate specificity of plant PIPKs is determined in a plant-specific manner, which provides new insights into the regulatory modes of PIPK activity in plants.

Quickly-released peroxidase of moss in defense against fungal invaders

Mosses (Bryophyta) are nonvascular plants that constitute a large part of the photosynthesizing biomass and carbon storage on Earth. Little is known about how this important portion of flora maintains its health status. This study assessed whether the moss, Physcomitrella patens, responds to treatment with chitosan, a fungal cell wall-derived compound inducing defense against fungal pathogens in vascular plants. Application of chitosan to liquid culture of P. patens caused a rapid increase in peroxidase activity in the medium. For identification of the peroxidase(s), matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF)/MS, other methods and the whole-genome sequence of P. patens were utilized. Peroxidase gene knock-out mutants were made and inoculated with fungi. The peroxidase activity resulted from a single secreted class III peroxidase (Prx34) which belonged to a P. patens specific phylogenetic cluster in analysis of the 45 putative class III peroxidases of P. patens and those of Arabidopsis and rice. Saprophytic and pathogenic fungi isolated from another moss killed the Prx34 knockout mutants but did not damage wild-type P. patens. The data point out the first specific host factor that is pivotal for pathogen defense in a nonvascular plant. Furthermore, results provide conclusive evidence that class III peroxidases in plants are needed in defense against hostile invasion by fungi.

Physcomitrella patens: a non-vascular plant for recombinant protein production

The moss Physcomitrella patens is a long-standing model for studying plant development, growth and cell differentiation in particular. Interest in this non-vascular plant arose following the discovery that homologous recombination is an efficient process. P. patens is, therefore, a tool of choice not only to study gene function but also for recombinant protein production. This system has many attributes that are advantageous for molecular farming: protein production in cell suspension, the possibility of generating targeted knockout mutants for glycoengineering and quantitative optimization for protein production. In terms of technical advances, P. patens is one of the most up-to-date plant expression systems and is a promising alternative to animal cell factories for the production of therapeutic proteins with either simple or highly complex structures.

Cellular differentiation in moss protonemata: a morphological and experimental study

BACKGROUND AND AIMS

Previous studies of protonemal morphogenesis in mosses have focused on the cytoskeletal basis of tip growth and the production of asexual propagules. This study provides the first comprehensive description of the differentiation of caulonemata and rhizoids, which share the same cytology, and the roles of the cytoskeleton in organelle shaping and spatial arrangement.

METHODS

Light and electron microscope observations were carried out on in vitro cultured and wild protonemata from over 200 moss species. Oryzalin and cytochalasin D were used to investigate the role of the cytoskeleton in the cytological organization of fully differentiated protonemal cells; time-lapse photography was employed to monitor organelle positions.

KEY RESULTS

The onset of differentiation in initially highly vacuolate subapical cells is marked by the appearance of tubular endoplasmic reticulum (ER) profiles with crystalline inclusions, closely followed by an increase in rough endoplasmic reticulum (RER). The tonoplast disintegrates and the original vacuole is replaced by a population of vesicles and small vacuoles originating de novo from RER. The cytoplasm then becomes distributed throughout the cell lumen, an event closely followed by the appearance of endoplasmic microtubules (MTs) in association with sheets of ER, stacks of vesicles that subsequently disperse, elongate mitochondria and chloroplasts and long tubular extensions at both poles of the nucleus. The production of large vesicles by previously inactive dictysomes coincides with the deposition of additional cell wall layers. At maturity, the numbers of endoplasmic microtubules decline, dictyosomes become inactive and the ER is predominantly smooth. Fully developed cells remain largely unaffected by cytochalasin; oryzalin elicits profound cytological changes. Both inhibitors elicit the formation of giant plastids. The plastids and other organelles in fully developed cells are largely stationary.

CONCLUSIONS

Differentiation of caulonemata and rhizoids involves a remarkable series of cytological changes, some of which closely recall major events in sieve element ontogeny in tracheophytes. The cytology of fully differentiated cells is remarkably similar to that of moss food-conducting cells and, in both, is dependent on an intact microtubule cytoskeleton. The disappearance of the major vacuolar apparatus is probably related to the function of caulonema and rhizoids in solute transport. Failure of fully differentiated caulonema and rhizoid cells to regenerate is attributed to a combination of endo-reduplication and irreversible tonoplast fragmentation. The formation of giant plastids, most likely by fusion, following both oryzalin and cytochalasin treatments, suggests key roles for both microtubules and microfilaments in the spatial arrangement and replication of plastids.

BRICK1 is required for apical cell growth in filaments of the moss Physcomitrella patens but not for gametophore morphology

When BRK1, a member of the Wave/SCAR complex, is deleted in Physcomitrella patens (Deltabrk1), we report a striking reduction of filament growth resulting in smaller and fewer cells with misplaced cross walls compared with the normal protonemal cells. Using an inducible green fluorescent protein-talin to detect actin in living tissue, a characteristic broad accumulation of actin is observed at the tip of wild-type apical cells, whereas in Deltabrk1, smaller, more distinct foci of actin are present. Insertion of brk1-yfp into Deltabrk1 rescues the mutant phenotype and results in BRK1 being localized only in the tip of apical cells, the exclusive site of cell extension and division in the filament. Like BRK1, ARPC4 and At RABA4d are normally localized at the tip of apical cells and their localization is correlated with rapid tip growth in filaments. However, neither marker accumulates in apical cells of Deltabrk1 filaments. Although the Deltabrk1 phenotypes in protonema are severe, the leafy shoots or gametophores are normally shaped but stunted. These and other results suggest that BRK1 functions directly or indirectly in the selective accumulation/stabilization of actin and other proteins required for polar cell growth of filaments but not for the basic structure of the gametophore.