The bryophytes Physcomitrium patens and Marchantia polymorpha as model systems for studying evolutionary cell and developmental biology in plants

Effects of fluorescent tags and activity status on the membrane localization of ROP GTPases

Plant-specific Rho-type GTPases (ROPs) are master regulators of cell polarity and development. Over the past 30 years, their localization and dynamics have been largely examined with fluorescent proteins fused at the amino terminus without investigating their impact on protein function. The moss Physcomitrium patens genome encodes four rop genes. In this study, we introduce a fluorescent tag at the endogenous amino terminus of ROP4 in wild-type and rop1,2,3 triple mutant via homologous recombination and demonstrate that the fluorescent tag severely impairs ROP4 function and inhibits its localization on the plasma membrane. This phenotype is exacerbated in mutants lacking ROP-related GTPase-activating proteins. By comparing the localization of nonfunctional and functional ROP4 fusion reporters, we provide insight into the mechanism that governs the membrane association of ROPs.

MpPUB9, a U-box E3 ubiquitin ligase, acts as a positive regulator by promoting the turnover of MpEXO70.1 under high salinity in Marchantia polymorpha

Marchantia polymorpha, occupying a basal position in the monophyletic assemblage of land plants, displays a notable expansion of plant U-box (PUB) proteins compared with those in animals. We elucidated the roles of MpPUB9 in regulating salt stress tolerance in M. polymorpha. MpPUB9 expression was rapidly induced by high salinity and dehydration. MpPUB9 possessed an intact U-box domain in the N-terminus. MpPUB9-Citrine localized to punctate structures and was peripherally associated with microsomal membranes. Phenotypic analyses demonstrate that the hyponastic and epinastic thallus growth phenotypes, which were induced by the overexpression and suppression of MpPUB9, may provoke salt stress-resistant and -susceptible phenotypes, respectively. MpPUB9 was also found to directly interact with the exocyst protein MpEXO70.1, leading to its ubiquitination. Under high-salinity conditions, though the stability of MpPUB9 was dramatically increased, MpEXO70.1 showed slightly faster turnover rates. Transcriptome analyses showed that salt treatment and the overexpression of MpPUB9 co-upregulated the genes related to the modulation of H2O2 and cell wall organization. Overall, our results suggest that MpPUB9 plays a crucial role in the positive regulation of salt stress tolerance, resulting from its interaction with MpEXO70.1 and modulating turnover of the protein under high-salt conditions via the coordination of UPS with autophagy.

How a liverwort reacts to wounding: tracing the evolution of defence responses in the nonvascular plant Marchantia polymorpha

This article is a Commentary on Sanmartín et al. (2024), 244: 870–882.

Plant synthetic genomics: Big lessons from the little yeast

Yeast has been extensively studied and engineered due to its genetic amenability. Projects like Sc2.0 and Sc3.0 have demonstrated the feasibility of constructing synthetic yeast genomes, yielding promising results in both research and industrial applications. In contrast, plant synthetic genomics has faced challenges due to the complexity of plant genomes. However, recent advancements of the project SynMoss, utilizing the model moss plant Physcomitrium patens, offer opportunities for plant synthetic genomics. The shared characteristics between P. patens and yeast, such as high homologous recombination rates and dominant haploid life cycle, enable researchers to manipulate P. patens genomes similarly, opening promising avenues for research and application in plant synthetic biology. In conclusion, harnessing insights from yeast synthetic genomics and applying them to plants, with P. patens as a breakthrough, shows great potential for revolutionizing plant synthetic genomics.

Manganese deficiency alters photosynthetic electron transport in Marchantia polymorpha

Manganese (Mn) is considered as an essential element for plant growth. Mn starvation has been shown to affect photosystem II, the site of the Mn4CaO5 cluster responsible for water oxidation. Less is known on the effect of Mn starvation on photosystem I. Here we studied the effects of Mn deficiency in vivo on redox changes of P700 and plastocyanin (Pc) in the liverwort Marchantia polymorpha using the KLAS-NIR spectrophotometer. Far-red illumination is used to excite preferentially photosystem I, thus facilitating cyclic electron transport. Under Mn starvation, we observed slower oxidation of P700 and a decrease in the Pc signal relative to P700. The lower Pc content under Mn deficiency was confirmed by western blots. Re-reduction kinetics of P700+ and Pc+ were faster in Mn deficient thalli than in the control. The above findings show that the kinetics studied under Mn deficiency not only depend on the number of available reductants but also on how quickly electrons are transferred from stromal donors via the intersystem chain to Pc+ and P700+. We suggest that under Mn deficiency a structural reorganization of the thylakoid membrane takes place favoring the formation of supercomplexes between ferredoxin, cytochrome b6f complex, Pc and photosystem I, and thus an enhanced cyclic electron transport.

An efficient sulfadiazine selection scheme for stable transformation in the model liverwort Marchantia polymorpha

Plant macroevolutionary studies leverage the phylogenetic position of non-flowering model systems like the liverwort Marchantia polymorpha to investigate the origin and evolution of key plant processes. To date, most molecular genetic studies in Marchantia rely on hygromycin and/or chlorsulfuron herbicide resistance markers for the selection of stable transformants. Here, we used a sulfonamide-resistant dihydropteroate synthase (DHPS) gene to enable sulfadiazine-based transformation selection in M. polymorpha. We demonstrate the reliability of sulfadiazine selection on its own and in combination with existing hygromycin and chlorsulfuron selection schemes through transgene stacking experiments. The utility of this system is further demonstrated through confocal microscopy of a triple transgenic line carrying fluorescent proteins labelling the plasma membrane, cortical microtubules, and the nucleus. Collectively, our findings and resources broaden the capacity to genetically manipulate the increasingly popular model liverwort M. polymorpha.

The BNB-GLID module regulates germline fate determination in Marchantia polymorpha

Germline fate determination is a critical event in sexual reproduction. Unlike animals, plants specify the germline by reprogramming somatic cells at the late stages of their development. However, the genetic basis of germline fate determination and how it evolved during the land plant evolution are still poorly understood. Here, we report that the plant homeodomain finger protein GERMLINE IDENTITY DETERMINANT (GLID) is a key regulator of the germline specification in liverwort, Marchantia polymorpha. Loss of the MpGLID function causes failure of germline initiation, leading to the absence of sperm and egg cells. Remarkably, the overexpression of MpGLID in M. polymorpha induces the ectopic formation of cells with male germline cell features exclusively in male thalli. We further show that MpBONOBO (BNB), with an evolutionarily conserved function, can induce the formation of male germ cell-like cells through the activation of MpGLID by directly binding to its promoter. The Arabidopsis (Arabidopsis thaliana) MpGLID ortholog, MALE STERILITY1 (AtMS1), fails to replace the germline specification function of MpGLID in M. polymorpha, demonstrating that a derived function of MpGLID orthologs has been restricted to tapetum development in flowering plants. Collectively, our findings suggest the presence of the BNB-GLID module in complex ancestral land plants that has been retained in bryophytes, but rewired in flowering plants for male germline fate determination.

Cytokinin and ALOG proteins regulate pluripotent stem cell identity in the moss Physcomitrium patens

The shoot apical meristem (SAM) contains pluripotent stem cells that produce all the aerial parts of the plant. Stem cells undergo asymmetric cell divisions to self-renew and to produce differentiating cells. Our research focused on unraveling the mechanisms governing the specification of these two distinct cell fates following the stem cell division. For this purpose, we used the model organism Physcomitrium patens, which features a singular pluripotent stem cell known as the gametophore apical cell. We show that the activity of cytokinins, critical stem cell regulators, is restricted to the gametophore apical cell due to the specific localization of PpLOG, the enzyme responsible for cytokinin activation. In turn, PpTAW, which promotes differentiating cell identity of the merophyte, is excluded from the gametophore apical cell by the action of cytokinins. We propose a cytokinin-based model for the establishment of asymmetry in the pluripotent stem cell division.

Syntrichia ruralis: emerging model moss genome reveals a conserved and previously unknown regulator of desiccation in flowering plants

Water scarcity, resulting from climate change, poses a significant threat to ecosystems. Syntrichia ruralis, a dryland desiccation-tolerant moss, provides valuable insights into survival of water-limited conditions. We sequenced the genome of S. ruralis, conducted transcriptomic analyses, and performed comparative genomic and transcriptomic analyses with existing genomes and transcriptomes, including with the close relative S. caninervis. We took a genetic approach to characterize the role of an S. ruralis transcription factor, identified in transcriptomic analyses, in Arabidopsis thaliana. The genome was assembled into 12 chromosomes encompassing 21 169 protein-coding genes. Comparative analysis revealed copy number and transcript abundance differences in known desiccation-associated gene families, and highlighted genome-level variation among species that may reflect adaptation to different habitats. A significant number of abscisic acid (ABA)-responsive genes were found to be negatively regulated by a MYB transcription factor (MYB55) that was upstream of the S. ruralis ortholog of ABA-insensitive 3 (ABI3). We determined that this conserved MYB transcription factor, uncharacterized in Arabidopsis, acts as a negative regulator of an ABA-dependent stress response in Arabidopsis. The new genomic resources from this emerging model moss offer novel insights into how plants regulate their responses to water deprivation.

Crosstalk between Rho of Plants GTPase signalling and plant hormones

Rho of Plants (ROPs) constitute a plant-specific subset of small guanine nucleotide-binding proteins within the Cdc42/Rho/Rac family. These versatile proteins regulate diverse cellular processes, including cell growth, cell division, cell morphogenesis, organ development, and stress responses. In recent years, the dynamic cellular and subcellular behaviours orchestrated by ROPs have unveiled a notable connection to hormone-mediated organ development and physiological responses, thereby expanding our knowledge of the functions and regulatory mechanisms of this signalling pathway. This review delineates advancements in understanding the interplay between plant hormones and the ROP signalling cascade, focusing primarily on the connections with auxin and abscisic acid pathways, alongside preliminary discoveries in cytokinin, brassinosteroid, and salicylic acid responses. It endeavours to shed light on the intricate, coordinated mechanisms bridging cell- and tissue-level signals that underlie plant cell behaviour, organ development, and physiological processes, and highlights future research prospects and challenges in this rapidly developing field.

Plasmodesmata dynamics in bryophyte model organisms: secondary formation and developmental modifications of structure and function

MAIN CONCLUSION

Developing bryophytes differentially modify their plasmodesmata structure and function. Secondary plasmodesmata formation via twinning appears to be an ancestral trait. Plasmodesmata networks in hornwort sporophyte meristems resemble those of angiosperms. All land-plant taxa use plasmodesmata (PD) cell connections for symplasmic communication. In angiosperm development, PD networks undergo an extensive remodeling by structural and functional PD modifications, and by postcytokinetic formation of additional secondary PD (secPD). Since comparable information on PD dynamics is scarce for the embryophyte sister groups, we investigated maturating tissues of Anthoceros agrestis (hornwort), Physcomitrium patens (moss), and Marchantia polymorpha (liverwort). As in angiosperms, quantitative electron microscopy revealed secPD formation via twinning in gametophytes of all model bryophytes, which gives rise to laterally adjacent PD pairs or to complex branched PD. This finding suggests that PD twinning is an ancient evolutionary mechanism to adjust PD numbers during wall expansion. Moreover, all bryophyte gametophytes modify their existing PD via taxon-specific strategies resembling those of angiosperms. Development of type II-like PD morphotypes with enlarged diameters or formation of pit pairs might be required to maintain PD transport rates during wall thickening. Similar to angiosperm leaves, fluorescence redistribution after photobleaching revealed a considerable reduction of the PD permeability in maturating P. patens phyllids. In contrast to previous reports on monoplex meristems of bryophyte gametophytes with single initials, we observed targeted secPD formation in the multi-initial basal meristems of A. agrestis sporophytes. Their PD networks share typical features of multi-initial angiosperm meristems, which may hint at a putative homologous origin. We also discuss that monoplex and multi-initial meristems may require distinct types of PD networks, with or without secPD formation, to control maintenance of initial identity and positional signaling.

A molecular atlas of plastid and mitochondrial proteins reveals organellar remodeling during plant evolutionary transitions from algae to angiosperms

Algae and plants carry 2 organelles of endosymbiotic origin that have been co-evolving in their host cells for more than a billion years. The biology of plastids and mitochondria can differ significantly across major lineages and organelle changes likely accompanied the adaptation to new ecological niches such as the terrestrial habitat. Based on organelle proteome data and the genomes of 168 phototrophic (Archaeplastida) versus a broad range of 518 non-phototrophic eukaryotes, we screened for changes in plastid and mitochondrial biology across 1 billion years of evolution. Taking into account 331,571 protein families (or orthogroups), we identify 31,625 protein families that are unique to primary plastid-bearing eukaryotes. The 1,906 and 825 protein families are predicted to operate in plastids and mitochondria, respectively. Tracing the evolutionary history of these protein families through evolutionary time uncovers the significant remodeling the organelles experienced from algae to land plants. The analyses of gained orthogroups identifies molecular changes of organelle biology that connect to the diversification of major lineages and facilitated major transitions from chlorophytes en route to the global greening and origin of angiosperms.

Unveiling the molecular mechanisms of arsenic tolerance and resilience in the primitive bryophyte Marchantia polymorpha L

This study addresses the pressing issue of high arsenic (As) contaminations, which poses a severe threat to various life forms in our ecosystem. Despite this prevailing concern, all organisms have developed some techniques to mitigate the toxic effects of As. Certain plants, such as bryophytes, the earliest land plants, exhibit remarkable tolerance to wide range of harsh environmental conditions, due to their inherent competence. In this study, bryophytes collected from West Bengal, India, across varying contamination levels were investigated for their As tolerance capabilities. Assessment of As accumulation potential and antioxidant defense efficiency, including SOD, CAT, APX, GPX etc. revealed Marchantia polymorpha as the most tolerant species. It exhibited highest As accumulation, antioxidative proficiency, and minimal damage. Transcriptomic analysis of M. polymorpha exposed to 40 μM As(III) for 24 and 48 h identified several early responsive differentially expressing genes (DEGs) associated with As tolerance. These includes GSTs, GRXs, Hsp20s, SULTR1;2, ABCC2 etc., indicating a mechanism involving vacuolar sequestration. Interestingly, one As(III) efflux-transporter ACR3, an extrusion pump, known to combat As toxicity was found to be differentially expressed compared to control. The SEM-EDX analysis, further elucidated the operation of As extrusion mechanism, which contributes added As resilience in M. polymorpha. Yeast complementation assay using Δacr3 yeast cells, showed increased tolerance towards As(III), compared to the mutant cells, indicating As tolerant phenotype. Overall, these findings significantly enhance our understanding of As tolerance mechanisms in bryophytes. This can pave the way for the development of genetically engineered plants with heightened As tolerance and the creation of improved plant varieties.

Re-examining meristems through the lens of evo-devo

The concept of the meristem was introduced in 1858 to characterize multicellular, formative, and proliferative tissues that give rise to the entire plant body, based on observations of vascular plants. Although its original definition did not encompass bryophytes, this concept has been used and continuously refined over the past 165 years to describe the diverse apices of all land plants. Here, we re-examine this matter in light of recent evo-devo research and show that, despite displaying high anatomical diversity, land plant meristems are unified by shared genetic control. We also propose a modular view of meristem function and highlight multiple evolutionary mechanisms that are likely to have contributed to the assembly and diversification of the varied meristems during the course of plant evolution.

Rubisco small subunit (RbCS) is co-opted by potyvirids as the scaffold protein in assembling a complex for viral intercellular movement

Plant viruses must move through plasmodesmata (PD) to complete their life cycles. For viruses in the Potyviridae family (potyvirids), three viral factors (P3N-PIPO, CI, and CP) and few host proteins are known to participate in this event. Nevertheless, not all the proteins engaging in the cell-to-cell movement of potyvirids have been discovered. Here, we found that HCPro2 encoded by areca palm necrotic ring spot virus (ANRSV) assists viral intercellular movement, which could be functionally complemented by its counterpart HCPro from a potyvirus. Affinity purification and mass spectrometry identified several viral factors (including CI and CP) and host proteins that are physically associated with HCPro2. We demonstrated that HCPro2 interacts with both CI and CP in planta in forming PD-localized complexes during viral infection. Further, we screened HCPro2-associating host proteins, and identified a common host protein in Nicotiana benthamiana-Rubisco small subunit (NbRbCS) that mediates the interactions of HCPro2 with CI or CP, and CI with CP. Knockdown of NbRbCS impairs these interactions, and significantly attenuates the intercellular and systemic movement of ANRSV and three other potyvirids (turnip mosaic virus, pepper veinal mottle virus, and telosma mosaic virus). This study indicates that a nucleus-encoded chloroplast-targeted protein is hijacked by potyvirids as the scaffold protein to assemble a complex to facilitate viral movement across cells.

Telomere Length Variation in Model Bryophytes

The ends of linear chromosomes of most eukaryotes consist of protein-bound DNA arrays called telomeres, which play essential roles in protecting genome integrity. Despite general evolutionary conservation in function, telomeric DNA is known to drastically vary in length and sequence between different eukaryotic lineages. Bryophytes are a group of early diverging land plants that include mosses, liverworts, and hornworts. This group of ancient land plants recently emerged as a new model for important discoveries in genomics and evolutionary biology, as well as for understanding plant adaptations to a terrestrial lifestyle. We measured telomere length in different ecotypes of model bryophyte species, including Physcomitrium patens, Marchantia polymorpha, Ceratodon purpureus, and in Sphagnum isolates. Our data indicate that all analyzed moss and liverwort genotypes have relatively short telomeres. Furthermore, all analyzed ecotypes and isolates of model mosses and liverworts display evidence of substantial natural variation in telomere length. Interestingly, telomere length also differs between male and female strains of the dioecious liverwort M. polymorpha and dioecious moss C. purpureus. Given that bryophytes are extraordinarily well adapted to different ecological niches from polar to tropical environments, our data will contribute to understanding the impact of natural telomere length variation on evolutionary adaptations in this ancient land plant lineage.

Two subtypes of GTPase-activating proteins coordinate tip growth and cell size regulation in Physcomitrium patens

The establishment of cell polarity is a prerequisite for many developmental processes. However, how it is achieved during tip growth in plants remains elusive. Here, we show that the RHO OF PLANTs (ROPs), ROP GUANINE NUCLEOTIDE EXCHANGE FACTORs (RopGEFs), and ROP GTPASE-ACTIVATING PROTEINs (RopGAPs) assemble into membrane domains in tip-growing cells of the moss Physcomitrium patens. The confinement of membrane domains requires redundant global inactivation of ROPs by PpRopGAPs and the PLECKSTRIN HOMOLOGY (PH) domain-containing RenGAP PpREN. Unexpectedly, PpRopGAPs and PpREN exert opposing effects on domain size and cell width upon overexpression. Biochemical and functional analyses indicate that PpRopGAPs are recruited to the membrane by active ROPs to restrict domain size through clustering, whereas PpREN rapidly inactivates ROPs and inhibits PpRopGAP-induced clustering. We propose that the activity- and clustering-based domain organization by RopGAPs and RenGAPs is a general mechanism for coordinating polarized cell growth and cell size regulation in plants.

Physcomitrium patens SUN2 Mediates MTOC Association with the Nuclear Envelope and Facilitates Chromosome Alignment during Spindle Assembly

Plant cells lack centrosomes and instead utilize acentrosomal microtubule organizing centers (MTOCs) to rapidly increase the number of microtubules at the onset of spindle assembly. Although several proteins required for MTOC formation have been identified, how the MTOC is positioned at the right place is not known. Here, we show that the inner nuclear membrane protein SUN2 is required for MTOC association with the nuclear envelope (NE) during mitotic prophase in the moss Physcomitrium patens. In actively dividing protonemal cells, microtubules accumulate around the NE during prophase. In particular, regional MTOC is formed at the apical surface of the nucleus. However, microtubule accumulation around the NE was impaired and apical MTOCs were mislocalized in sun2 knockout cells. Upon NE breakdown, the mitotic spindle was assembled with mislocalized MTOCs. However, completion of chromosome alignment in the spindle was delayed; in severe cases, the chromosome was transiently detached from the spindle body. SUN2 tended to localize to the apical surface of the nucleus during prophase in a microtubule-dependent manner. Based on these results, we propose that SUN2 facilitates the attachment of microtubules to chromosomes during spindle assembly by localizing microtubules to the NE. MTOC mispositioning was also observed during the first division of the gametophore tissue. Thus, this study suggests that microtubule-nucleus linking, a well-known function of SUN in animals and yeast, is conserved in plants.

Beyond a PPR-RNA recognition code: Many aspects matter for the multi-targeting properties of RNA editing factor PPR56

The mitochondrial C-to-U RNA editing factor PPR56 of the moss Physcomitrium patens is an RNA-binding pentatricopeptide repeat protein equipped with a terminal DYW-type cytidine deaminase domain. Transferred into Escherichia coli, PPR56 works faithfully on its two native RNA editing targets, nad3eU230SL and nad4eU272SL, and also converts cytidines into uridines at over 100 off-targets in the bacterial transcriptome. Accordingly, PPR56 is attractive for detailed mechanistic studies in the heterologous bacterial setup, allowing for scoring differential RNA editing activities of many target and protein variants in reasonable time. Here, we report (i) on the effects of numerous individual and combined PPR56 protein and target modifications, (ii) on the spectrum of off-target C-to-U editing in the bacterial background transcriptome for PPR56 and two variants engineered for target re-direction and (iii) on combinations of targets in tandem or separately at the 5'- and 3'-ends of large mRNAs. The latter experimentation finds enhancement of RNA editing at weak targets in many cases, including cox3eU290SF as a new candidate mitogenome target. We conclude that C-to-U RNA editing can be much enhanced by transcript features also outside the region ultimately targeted by PPRs of a plant editing factor, possibly facilitated by its enrichment or scanning along transcripts.

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

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

Comparative phyloproteomics identifies conserved plasmodesmal proteins

Plasmodesmata are cytosolic bridges, lined by the plasma membrane and traversed by endoplasmic reticulum; plasmodesmata connect cells and tissues, and are critical for many aspects of plant biology. While plasmodesmata are notoriously difficult to extract, tissue fractionation and proteomic analyses can yield valuable knowledge of their composition. Here we have generated two novel proteomes to expand tissue and taxonomic representation of plasmodesmata: one from mature Arabidopsis leaves and one from the moss Physcomitrium patens, and leveraged these and existing data to perform a comparative analysis to identify evolutionarily conserved protein families that are associated with plasmodesmata. Thus, we identified β-1,3-glucanases, C2 lipid-binding proteins, and tetraspanins as core plasmodesmal components that probably serve as essential structural or functional components. Our approach has not only identified elements of a conserved plasmodesmal proteome, but also demonstrated the added power offered by comparative analysis for recalcitrant samples. Conserved plasmodesmal proteins establish a basis upon which ancient plasmodesmal function can be further investigated to determine the essential roles these structures play in multicellular organism physiology in the green lineages.

A non-targeted metabolomics analysis identifies wound-induced oxylipins in Physcomitrium patens

Plant oxylipins are a class of lipid-derived signaling molecules being involved in the regulation of various biotic and abiotic stress responses. A major class of oxylipins are the circular derivatives to which 12-oxo-phytodienoic acid (OPDA) and its metabolite jasmonic acid (JA) belong. While OPDA and its shorter chain homologue dinor-OPDA (dnOPDA) seem to be ubiquitously found in land plants ranging from bryophytes to angiosperms, the occurrence of JA and its derivatives is still under discussion. The bryophyte Physcomitrium patens has received increased scientific interest as a non-vascular plant model organism over the last decade. Therefore, we followed the metabolism upon wounding by metabolite fingerprinting with the aim to identify jasmonates as well as novel oxylipins in P. patens. A non-targeted metabolomics approach was used to reconstruct the metabolic pathways for the synthesis of oxylipins, derived from roughanic, linoleic, α-linolenic, and arachidonic acid in wild type, the oxylipin-deficient mutants of Ppaos1 and Ppaos2, the mutants of Ppdes being deficient in all fatty acids harboring a Δ6-double bond and the C20-fatty acid-deficient mutants of Ppelo. Beside of OPDA, iso-OPDA, dnOPDA, and iso-dnOPDA, three additional C18-compounds and a metabolite being isobaric to JA were identified to accumulate after wounding. These findings can now serve as foundation for future research in determining, which compound(s) will serve as native ligand(s) for the oxylipin-receptor COI1 in P. patens.

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

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

An overview of bioinformatics, genomics, and transcriptomics resources for bryophytes

Bryophytes are useful models for the study of plant evolution, development, plant-fungal symbiosis, stress responses, and gametogenesis. Additionally, their dominant haploid gametophytic phase makes them great models for functional genomics research, allowing straightforward genome editing and gene knockout via CRISPR or homologous recombination. Until 2016, however, the only bryophyte genome sequence published was that of Physcomitrium patens. Throughout recent years, several other bryophyte genomes and transcriptome datasets became available, enabling better comparative genomics in evolutionary studies. The increase in the number of bryophyte genome and transcriptome resources available has yielded a plethora of annotations, databases, and bioinformatics tools to access the new data, which covers the large diversity of this clade and whose biology comprises features such as association with arbuscular mycorrhiza fungi, sex chromosomes, low gene redundancy, or loss of RNA editing genes for organellar transcripts. Here we provide a guide to resources available for bryophytes with regards to genome and transcriptome databases and bioinformatics tools.

A scoping review of bryophyte microbiota: diverse microbial communities in small plant packages

Plant health depends not only on the condition of the plant itself but also on its diverse community of microbes, or microbiota. Just like the better-studied angiosperms, bryophytes (mosses, liverworts, and hornworts) harbor diverse communities of bacteria, archaea, fungi, and other microbial eukaryotes. Bryophytes are increasingly recognized as important model systems for understanding plant evolution, development, physiology, and symbiotic interactions. Much of the work on bryophyte microbiota in the past focused on specific symbiont types for each bryophyte group, but more recent studies are taking a broader view acknowledging the coexistence of diverse microbial communities in bryophytes. Therefore, this review integrates studies of bryophyte microbes from both perspectives to provide a holistic view of the existing research for each bryophyte group and on key themes. The systematic search also reveals the taxonomic and geographic biases in this field, including a severe under-representation of the tropics, very few studies on viruses or eukaryotic microbes beyond fungi, and a focus on mycorrhizal fungi studies in liverworts. Such gaps may have led to errors in conclusions about evolutionary patterns in symbiosis. This analysis points to a wealth of future research directions that promise to reveal how the distinct life cycles and physiology of bryophytes interact with their microbiota.

Division site determination during asymmetric cell division in plants

During development, both animals and plants exploit asymmetric cell division (ACD) to increase tissue complexity, a process that usually generates cells dissimilar in size, morphology, and fate. Plants lack the key regulators that control ACD in animals. Instead, plants have evolved two unique cytoskeletal structures to tackle this problem: the preprophase band (PPB) and phragmoplast. The assembly of the PPB and phragmoplast and their contributions to division plane orientation have been extensively studied. However, how the division plane is positioned off the cell center during asymmetric division is poorly understood. Over the past 20 years, emerging evidence points to a critical role for polarly localized membrane proteins in this process. Although many of these proteins are species- or cell type specific, and the molecular mechanism underlying division asymmetry is not fully understood, common features such as morphological changes in cells, cytoskeletal dynamics, and nuclear positioning have been observed. In this review, we provide updates on polarity establishment and nuclear positioning during ACD in plants. Together with previous findings about symmetrically dividing cells and the emerging roles of developmental cues, we aim to offer evolutionary insight into a common framework for asymmetric division-site determination and highlight directions for future work.

Bacterial-type ferroxidase tunes iron-dependent phosphate sensing during Arabidopsis root development

Access to inorganic phosphate (Pi), a principal intermediate of energy and nucleotide metabolism, profoundly affects cellular activities and plant performance. In most soils, antagonistic Pi-metal interactions restrict Pi bioavailability, which guides local root development to maximize Pi interception. Growing root tips scout the essential but immobile mineral nutrient; however, the mechanisms monitoring external Pi status are unknown. Here, we show that Arabidopsis LOW PHOSPHATE ROOT 1 (LPR1), one key determinant of Fe-dependent Pi sensing in root meristems, encodes a novel ferroxidase of high substrate specificity and affinity (apparent KM ∼ 2 μM Fe2+). LPR1 typifies an ancient, Fe-oxidizing multicopper protein family that evolved early upon bacterial land colonization. The ancestor of streptophyte algae and embryophytes (land plants) acquired LPR1-type ferroxidase from soil bacteria via horizontal gene transfer, a hypothesis supported by phylogenomics, homology modeling, and biochemistry. Our molecular and kinetic data on LPR1 regulation indicate that Pi-dependent Fe substrate availability determines LPR1 activity and function. Guided by the metabolic lifestyle of extant sister bacterial genera, we propose that Arabidopsis LPR1 monitors subtle concentration differentials of external Fe availability as a Pi-dependent cue to adjust root meristem maintenance via Fe redox signaling and cell wall modification. We further hypothesize that the acquisition of bacterial LPR1-type ferroxidase by embryophyte progenitors facilitated the evolution of local Pi sensing and acquisition during plant terrestrialization.

Abscisic acid switches cell division modes of asymmetric cell division and symmetric cell division in stem cells of protonemal filaments in the moss Physcomitrium patens

Multicellular organisms regulate cell numbers and cell fate by using asymmetric cell division (ACD) and symmetric cell division (SCD) during their development and to adapt to unfavorable environmental conditions. A stem cell self-renews and generates differentiated cells. In plants, various types of cells are produced by ACD or SCD; however, the molecular mechanisms of ACD or SCD and the cell division mode switch are largely unknown. The moss Physcomitrium (Physcomitrella) patens is a suitable model to study plant stem cells due to its simple anatomy. Here, we report the cell division mode switch induced by abscisic acid (ABA) in P. patens. ABA is synthesized in response to abiotic stresses and induces round-shape cells, called brood cells, from cylindrical protonemal cells. Although two daughter cells with distinct sizes were produced by ACD in a protonemal stem cell on ABA-free media, the sizes of two daughter cells became similar with ABA treatment. Actin microfilaments were spatially localized on the apices of apical stem cells in protonemata on ABA-free media, but the polar accumulation was lost under the condition of ABA treatment. Moreover, ABA treatment conferred an identical cell fate to the daughter cells in terms of cell division activity. Collectively, the results indicate ABA may suppress the ACD characteristics but evoke SCD in cells. We also noticed that ABA-induced brood cells not only self-renewed but regenerated protonemal cells when ABA was removed from the media, suggesting that brood cells are novel stem cells that are induced by environmental signals in P. patens.