The RopGEF KARAPPO Is Essential for the Initiation of Vegetative Reproduction in Marchantia polymorpha

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.

The phosphorylated pathway of serine biosynthesis affects sperm, embryo, and sporophyte development, and metabolism in Marchantia polymorpha

Serine metabolism is involved in various biological processes. Here we investigate primary functions of the phosphorylated pathway of serine biosynthesis in a non-vascular plant Marchantia polymorpha by analyzing knockout mutants of MpPGDH encoding 3-phosphoglycerate dehydrogenase in this pathway. Growth phenotypes indicate that serine from the phosphorylated pathway in the dark is crucial for thallus growth. Sperm development requires serine from the phosphorylated pathway, while egg formation does not. Functional MpPGDH in the maternal genome is necessary for embryo and sporophyte development. Under high CO2 where the glycolate pathway of serine biosynthesis is inhibited, suppressed thallus growth of the mutants is not fully recovered by exogenously-supplemented serine, suggesting the importance of serine homeostasis involving the phosphorylated and glycolate pathways. Metabolomic phenotypes indicate that the phosphorylated pathway mainly influences the tricarboxylic acid cycle, the amino acid and nucleotide metabolism, and lipid metabolism. These results indicate the importance of the phosphorylated pathway of serine biosynthesis in the dark, in the development of sperm, embryo, and sporophyte, and metabolism in M. polymorpha.

RHO of plant signaling was established early in streptophyte evolution

The algal ancestors of land plants underwent a transition from a unicellular to a multicellular body plan.1 This transition likely took place early in streptophyte evolution, sometime after the divergence of the Chlorokybophyceae/Mesostigmatophyceae lineage, but before the divergence of the Klebsormidiophyceae lineage.2 How this transition was brought about is unknown; however, it was likely facilitated by the evolution of novel mechanisms to spatially regulate morphogenesis. In land plants, RHO of plant (ROP) signaling plays a conserved role in regulating polarized cell growth and cell division orientation to orchestrate morphogenesis.3,4,5,6,7,8 ROP constitutes a plant-specific subfamily of the RHO GTPases, which are more widely conserved throughout eukaryotes.9,10 Although the RHO family originated in early eukaryotes,11,12 how and when the ROP subfamily originated had remained elusive. Here, we demonstrate that ROP signaling was established early in the streptophyte lineage, sometime after the divergence of the Chlorokybophyceae/Mesostigmatophyceae lineage, but before the divergence of the Klebsormidiophyceae lineage. This period corresponds to when the unicellular-to-multicellular transition likely took place in the streptophytes. In addition to being critical for the complex morphogenesis of extant land plants, we speculate that ROP signaling contributed to morphological evolution in early streptophytes.

The maturation and aging trajectory of Marchantia polymorpha at single-cell resolution

Bryophytes represent a sister to the rest of land plants. Despite their evolutionary importance and relatively simple body plan, a comprehensive understanding of the cell types and transcriptional states that underpin the temporal development of bryophytes has not been achieved. Using time-resolved single-cell RNA sequencing, we define the cellular taxonomy of Marchantia polymorpha across asexual reproduction phases. We identify two maturation and aging trajectories of the main plant body of M. polymorpha at single-cell resolution: the gradual maturation of tissues and organs along the tip-to-base axis of the midvein and the progressive decline of meristem activities in the tip along the chronological axis. Specifically, we observe that the latter aging axis is temporally correlated with the formation of clonal propagules, suggesting an ancient strategy to optimize allocation of resources to producing offspring. Our work thus provides insights into the cellular heterogeneity that underpins the temporal development and aging of bryophytes.

RHO GTPase of plants regulates polarized cell growth and cell division orientation during morphogenesis

Cell polarity-broadly defined as the asymmetric distribution of cellular activities and subcellular components within a cell-determines the geometry of cell growth and division during development. RHO GTPase proteins regulate the establishment of cell polarity and are conserved among eukaryotes. RHO of plant (ROP) proteins are a subgroup of RHO GTPases that are required for cellular morphogenesis in plants. However, how ROP proteins modulate the geometry of cell growth and division during the morphogenesis of plant tissues and organs is not well understood. To investigate how ROP proteins function during tissue development and organogenesis, we characterized the function of the single-copy ROP gene of the liverwort Marchantia polymorpha (MpROP). M. polymorpha develops morphologically complex three-dimensional tissues and organs exemplified by air chambers and gemmae, respectively. Mprop loss-of-function mutants form defective air chambers and gemmae, indicating ROP function is required for tissue development and organogenesis. During air chamber and gemma development in wild type, the MpROP protein is enriched to sites of polarized growth at the cell surface and accumulates at the expanding cell plate of dividing cells. Consistent with these observations, polarized cell growth is lost and cell divisions are misoriented in Mprop mutants. We propose that ROP regulates both polarized cell growth and cell division orientation in a coordinated manner to orchestrate tissue development and organogenesis in land plants.

Comprehensive genomic analysis of the Rho GTPases regulators in seven Rosaceae species revealed that PbrGDI1 controls pollen tube growth in Pyrus via mediating cellulose deposition

The primary regulators of Rho GTPases are GTPase-activating protein (GAP), guanine nucleotide exchange factor (GEF), and GDP dissociation inhibitor (GDI), which function as signaling switches in several physiological processes involved in plant growth and development. This study compared how the Rho GTPase regulators functioned in seven Rosaceae species. Seven Rosaceae species, divided into three subgroups, had a total of 177 regulators of Rho GTPases. According to duplication analysis, the expansion of GEF, GAP, and GDI families was facilitated by whole genome duplication or a dispersed duplication event. The balance of cellulose deposition to control the growth of the pear pollen tube, as demonstrated by the expression profile and antisense oligonucleotide approach. Moreover, protein-protein interactions indicated that PbrGDI1 and PbrROP1 could directly interact, suggesting that PbrGDI1 regulated the growth of the pear pollen tube through PbrROP1 signaling downstream. These results lay the foundations for future functional characterization of the GAP, GEF, and GDI gene families in Pyrus bretschneideri.

Control of vegetative reproduction in Marchantiapolymorpha by the KAI2-ligand signaling pathway

In vegetative reproduction of Marchantia polymorpha (M. polymorpha), propagules, called gemmae, are formed in gemma cups. Despite its significance for survival, control of gemma and gemma cup formation by environmental cues is not well understood. We show here that the number of gemmae formed in a gemma cup is a genetic trait. Gemma formation starts from the central region of the floor of the gemma cup, proceeds to the periphery, and terminates when the appropriate number of gemmae is initiated. The MpKARRIKIN INSENSITIVE2 (MpKAI2)-dependent signaling pathway promotes gemma cup formation and gemma initiation. The number of gemmae in a cup is controlled by modulating the ON/OFF switch of the KAI2-dependent signaling. Termination of the signaling results in the accumulation of MpSMXL, a suppressor protein. In the Mpsmxl mutants, gemma initiation continues, leading to the formation of a highly increased number of gemmae in a cup. Consistent with its function, the MpKAI2-dependent signaling pathway is active in gemma cups where gemmae initiate, as well as in the notch region of the mature gemma and midrib of the ventral side of the thallus. In this work, we also show that GEMMA CUP-ASSOCIATED MYB1 works downstream of this signaling pathway to promote gemma cup formation and gemma initiation. We also found that the availability of potassium affects gemma cup formation independently from the KAI2-dependent signaling pathway in M. polymorpha. We propose that the KAI2-dependent signaling pathway functions to optimize vegetative reproduction by adapting to the environment in M. polymorpha.

ROP GTPase-dependent polarity establishment during tip growth in plants

Polar cell growth in plants requires a cell peripheral region that undergoes membrane extension and cell wall remodeling. Since the 1990s, RHO-RELATED GTPASES FROM PLANTS (ROPs) have been identified as master regulators that determine the site of cell growth. ROPs function to regulate actin and microtubule cytoskeletons, calcium gradients, and exocytosis, thus directing the delivery of materials for membrane and cell wall extension. In recent years, our understanding of the regulatory mechanisms underlying polar localization and the activation of ROPs has greatly advanced. Evidence points to the crucial roles of membrane lipids, receptor-like kinases, and cell wall components. In this review, we provide updates on the mechanisms underlying polarity control in tip-growing cells, with a focus on ROP effectors and membrane-associated signals. By integrating knowledge from pollen tubes, root hairs, and findings in bryophyte protonema cells and rhizoids, we hope to offer important insights into a common conceptual framework on polarity establishment governed by intercellular and extracellular signals.

Barley guanine nucleotide exchange factor HvGEF14 is an activator of the susceptibility factor HvRACB and supports host cell entry by Blumeria graminis f. sp. hordei

In barley (Hordeum vulgare), signalling rat sarcoma homolog (RHO) of plants guanosine triphosphate hydrolases (ROP GTPases) support the penetration success of Blumeria graminis f. sp. hordei but little is known about ROP activation. Guanine nucleotide exchange factors (GEFs) facilitate the exchange of ROP-bound GDP for GTP and thereby turn ROPs into a signalling-activated ROP-GTP state. Plants possess a unique class of GEFs harbouring a plant-specific ROP nucleotide exchanger domain (PRONE). Here, we performed phylogenetic analyses and annotated barley PRONE-GEFs. The leaf epidermal-expressed PRONE-GEF HvGEF14 undergoes a transcriptional down-regulation on inoculation with B. graminis f. sp. hordei and directly interacts with the ROP GTPase and susceptibility factor HvRACB in yeast and in planta. Overexpression of activated HvRACB or of HvGEF14 led to the recruitment of ROP downstream interactor HvRIC171 to the cell periphery. HvGEF14 further supported direct interaction of HvRACB with a HvRACB-GTP-binding CRIB (Cdc42/Rac Interactive Binding motif) domain-containing HvRIC171 truncation. Finally, the overexpression of HvGEF14 caused enhanced susceptibility to fungal entry, while HvGEF14 RNAi provoked a trend to more penetration resistance. HvGEF14 might therefore play a role in the activation of HvRACB in barley epidermal cells during fungal penetration.

The renaissance and enlightenment of Marchantia as a model system

The liverwort Marchantia polymorpha has been utilized as a model for biological studies since the 18th century. In the past few decades, there has been a Renaissance in its utilization in genomic and genetic approaches to investigating physiological, developmental, and evolutionary aspects of land plant biology. The reasons for its adoption are similar to those of other genetic models, e.g. simple cultivation, ready access via its worldwide distribution, ease of crossing, facile genetics, and more recently, efficient transformation, genome editing, and genomic resources. The haploid gametophyte dominant life cycle of M. polymorpha is conducive to forward genetic approaches. The lack of ancient whole-genome duplications within liverworts facilitates reverse genetic approaches, and possibly related to this genomic stability, liverworts possess sex chromosomes that evolved in the ancestral liverwort. As a representative of one of the three bryophyte lineages, its phylogenetic position allows comparative approaches to provide insights into ancestral land plants. Given the karyotype and genome stability within liverworts, the resources developed for M. polymorpha have facilitated the development of related species as models for biological processes lacking in M. polymorpha.

The liverwort Marchantia polymorpha, a model for all ages

The liverwort Marchantia polymorpha has been known to man for millennia due to its inclusion Greek herbals. Perhaps due to its familiarity and association with growth in, often, man-made disturbed habitats, it was readily used to address fundamental biological questions of the day, including elucidation of land plant life cycles in the late 18th century, the formulation of cell theory early in the 19th century and the discovery of the alternation of generations in land plants in the mid-19th century. Subsequently, Marchantia was used as model in botany classes. With the arrival of the molecular era, its organellar genomes, the chloroplast and mitochondrial, were some of the first to be sequenced from any plant. In the past two decades, molecular genetic tools have been applied such that genes may be manipulated seemingly at will. Here, are past, present, and some views to the future of Marchantia as a model.

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

Bryophytes are nonvascular spore-forming plants. Unlike in flowering plants, the gametophyte (haploid) generation of bryophytes dominates the sporophyte (diploid) generation. A comparison of bryophytes with flowering plants allows us to answer some fundamental questions raised in evolutionary cell and developmental biology. The moss Physcomitrium patens was the first bryophyte with a sequenced genome. Many cell and developmental studies have been conducted in this species using gene targeting by homologous recombination. The liverwort Marchantia polymorpha has recently emerged as an excellent model system with low genomic redundancy in most of its regulatory pathways. With the development of molecular genetic tools such as efficient genome editing, both P. patens and M. polymorpha have provided many valuable insights. Here, we review these advances with a special focus on polarity formation at the cell and tissue levels. We examine current knowledge regarding the cellular mechanisms of polarized cell elongation and cell division, including symmetric and asymmetric cell division. We also examine the role of polar auxin transport in mosses and liverworts. Finally, we discuss the future of evolutionary cell and developmental biological studies in plants.

Development and Molecular Genetics of Marchantia polymorpha

Bryophytes occupy a basal position in the monophyletic evolution of land plants and have a life cycle in which the gametophyte generation dominates over the sporophyte generation, offering a significant advantage in conducting genetics. Owing to its low genetic redundancy and the availability of an array of versatile molecular tools, including efficient genome editing, the liverwort Marchantia polymorpha has become a model organism of choice that provides clues to the mechanisms underlying eco-evo-devo biology in plants. Recent analyses of developmental mutants have revealed that key genes in developmental processes are functionally well conserved in plants, despite their morphological differences, and that lineage-specific evolution occurred by neo/subfunctionalization of common ancestral genes. We suggest that M. polymorpha is an excellent platform to uncover the conserved and diversified mechanisms underlying land plant development.

Physcomitrium patens: A Single Model to Study Oriented Cell Divisions in 1D to 3D Patterning

Development in multicellular organisms relies on cell proliferation and specialization. In plants, both these processes critically depend on the spatial organization of cells within a tissue. Owing to an absence of significant cellular migration, the relative position of plant cells is virtually made permanent at the moment of division. Therefore, in numerous plant developmental contexts, the (divergent) developmental trajectories of daughter cells are dependent on division plane positioning in the parental cell. Prior to and throughout division, specific cellular processes inform, establish and execute division plane control. For studying these facets of division plane control, the moss Physcomitrium (Physcomitrella) patens has emerged as a suitable model system. Developmental progression in this organism starts out simple and transitions towards a body plan with a three-dimensional structure. The transition is accompanied by a series of divisions where cell fate transitions and division plane positioning go hand in hand. These divisions are experimentally highly tractable and accessible. In this review, we will highlight recently uncovered mechanisms, including polarity protein complexes and cytoskeletal structures, and transcriptional regulators, that are required for 1D to 3D body plan formation.

Gemma cup and gemma development in Marchantia polymorpha

The basal land plant Marchantia polymorpha efficiently propagates in favourable environments through clonal progeny called gemmae. Gemmae develop in cup-shaped receptacles known as gemma cups, which are formed on the gametophyte body. Anatomical studies have described the developmental processes involved over a century ago; however, little is known about the underlying molecular mechanisms. Recent studies have started to unravel the mechanism underlying genetic and hormonal regulation of gemma cup and gemma development, showing that it shares some regulatory mechanisms with several sporophytic organs in angiosperms. Further study of these specialized organs will contribute to our understanding of the core regulatory modules underlying organ development in land plants and how these became so diversified morphologically over the course of evolution.

ARMADILLO REPEAT ONLY proteins confine Rho GTPase signalling to polar growth sites

Polar growth requires the precise tuning of Rho GTPase signalling at distinct plasma membrane domains. The activity of Rho of plant (ROP) GTPases is regulated by the opposing action of guanine nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs). Whereas plant-specific ROPGEFs have been shown to be embedded in higher-level regulatory mechanisms involving membrane-bound receptor-like kinases, the regulation of GAPs has remained enigmatic. Here, we show that three Arabidopsis ARMADILLO REPEAT ONLY (ARO) proteins are essential for the stabilization of growth sites in root hair cells and trichomes. AROs interact with ROP1 enhancer GAPs (RENGAPs) and bind to the plasma membrane via a conserved polybasic region at the ARO amino terminus. The ectopic spreading of ROP2 in aro2/3/4 mutant root hair cells and the preferential interaction of AROs with active ROPs and anionic phospholipids suggests that AROs recruit RENGAPs into complexes with ROPs to confine ROP signalling to distinct membrane regions.