Physcomitrella patens: a model to investigate the role of RAC/ROP GTPase signalling in tip growth

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.

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.

RHO OF PLANTS signalling and the activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS: specificity in cellular signal transduction in plants

Every cell constantly receives signals from its neighbours or the environment. In plants, most signals are perceived by RECEPTOR-LIKE KINASEs (RLKs) and then transmitted into the cell. The molecular switches RHO OF PLANTS (ROP) are critical proteins for polar signal transduction and regulate multiple cell polarity processes downstream of RLKs. Many ROP-regulating proteins and scaffold proteins of the ROP complex are known. However, the spatiotemporal ROP signalling complex composition is not yet understood. Moreover, how specificity is achieved in different ROP signalling pathways within one cell still needs to be determined. This review gives an overview of recent advances in ROP signalling and how specificity by downstream scaffold proteins can be achieved. The composition of the ROP signalling complexes is discussed, focusing on the possibility of the simultaneous presence of ROP activators and inactivators within the same complex to balance ROP activity. Furthermore, this review highlights the function of plant-specific ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS polarizing ROP signalling and defining the specificity of the initiated ROP signalling pathway.

Regulation of developmental gatekeeping and cell fate transition by the calpain protease DEK1 in Physcomitrium patens

Calpains are cysteine proteases that control cell fate transitions whose loss of function causes severe, pleiotropic phenotypes in eukaryotes. Although mainly considered as modulatory proteases, human calpain targets are directed to the N-end rule degradation pathway. Several such targets are transcription factors, hinting at a gene-regulatory role. Here, we analyze the gene-regulatory networks of the moss Physcomitrium patens and characterize the regulons that are misregulated in mutants of the calpain DEFECTIVE KERNEL1 (DEK1). Predicted cleavage patterns of the regulatory hierarchies in five DEK1-controlled subnetworks are consistent with a pleiotropic and regulatory role during cell fate transitions targeting multiple functions. Network structure suggests DEK1-gated sequential transitions between cell fates in 2D-to-3D development. Our method combines comprehensive phenotyping, transcriptomics and data science to dissect phenotypic traits, and our model explains the protease function as a switch gatekeeping cell fate transitions potentially also beyond plant development.

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 PpRIC, an ancestral CRIB-domain ROP effector, inhibits auxin-induced differentiation of apical initial cells

RHO guanosine triphosphatases are important eukaryotic regulators of cell differentiation and behavior. Plant ROP (RHO of plant) family members activate specific, incompletely characterized downstream signaling. The structurally simple land plant Physcomitrium patens is missing homologs of key animal and flowering plant RHO effectors but contains a single CRIB (CDC42/RAC interactive binding)-domain-containing RIC (ROP-interacting CRIB-containing) protein (PpRIC). Protonemal P. patens filaments elongate based on regular division and PpROP-dependent tip growth of apical initial cells, which upon stimulation by the hormone auxin differentiate caulonemal characteristics. PpRIC interacts with active PpROP1, co-localizes with this protein at the plasma membrane at the tip of apical initial cells, and accumulates in the nucleus. Remarkably, PpRIC is not required for tip growth but is targeted to the nucleus to block caulonema differentiation downstream of auxin-controlled gene expression. These observations establish functions of PpRIC in mediating crosstalk between ROP and auxin signaling, which contributes to the maintenance of apical initial cell identity.

Signaling network controlling ROP-mediated tip growth in Arabidopsis and beyond

Cell polarity operates across a broad range of spatial and temporal scales and is essential for specific biological functions of polarized cells. Tip growth is a special type of polarization in which a single and unique polarization site is established and maintained, as for the growth of root hairs and pollen tubes in plants. Extensive studies in past decades have demonstrated that the spatiotemporal localization and activity of Rho of Plants (ROPs), the only class of Rho GTPases in plants, are critical for tip growth. ROPs are switched on or off by different factors to initiate dynamic intracellular activities, leading to tip growth. Recent studies have also uncovered several feedback modules for ROP signaling. In this review, we summarize recent progress on ROP signaling in tip growth, focusing on molecular mechanisms that underlie the dynamic distribution and activity of ROPs in Arabidopsis. We also highlight feedback modules that control ROP-mediated tip growth and provide a perspective for building a complex ROP signaling network. Finally, we provide an evolutionary perspective for ROP-mediated tip growth in Physcomitrella patens and during plant-rhizobia interaction.

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 cellular function of ROP GTPase prenylation is important for multicellularity in the moss Physcomitrium patens

A complete picture of how signaling pathways lead to multicellularity is largely unknown. Previously, we generated mutations in a protein prenylation enzyme, GGB, and showed that it is essential for maintaining multicellularity in the moss Physcomitrium patens. Here, we show that ROP GTPases act as downstream factors that are prenylated by GGB and themselves play an important role in the multicellularity of P. patens. We also show that the loss of multicellularity caused by the suppression of GGB or ROP GTPases is due to uncoordinated cell expansion, defects in cell wall integrity and the disturbance of the directional control of cell plate orientation. Expressing prenylatable ROP in the ggb mutant not only rescues multicellularity in protonemata but also results in development of gametophores. Although the prenylation of ROP is important for multicellularity, a higher threshold of active ROP is required for gametophore development. Thus, our results suggest that ROP activation via prenylation by GGB is a key process at both cell and tissue levels, facilitating the developmental transition from one dimension to two dimensions and to three dimensions in P. patens.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

Here we review, from a quantitative point of view, the cell biology of protonemal tip growth in the model moss Physcomitrium patens. We focus on the role of the cytoskeleton, vesicle trafficking, and cell wall mechanics, including reviewing some of the existing mathematical models of tip growth. We provide a primer for existing cell biological tools that can be applied to the future study of tip growth in moss. Polarized cell growth is a ubiquitous process throughout the plant kingdom in which the cell elongates in a self-similar manner. This process is important for nutrient uptake by root hairs, fertilization by pollen, and gametophyte development by the protonemata of bryophytes and ferns. In this review, we will focus on the tip growth of moss cells, emphasizing the role of cytoskeletal organization, cytoplasmic zonation, vesicle trafficking, cell wall composition, and dynamics. We compare some of the existing knowledge on tip growth in protonemata against what is known in pollen tubes and root hairs, which are better-studied tip growing cells. To fully understand how plant cells grow requires that we deepen our knowledge in a variety of forms of plant cell growth. We focus this review on the model plant Physcomitrium patens, which uses tip growth as the dominant form of growth at its protonemal stage. Because mosses and vascular plants shared a common ancestor more than 450 million years ago, we anticipate that both similarities and differences between tip growing plant cells will provide mechanistic information of tip growth as well as of plant cell growth in general. Towards this mechanistic understanding, we will also review some of the existing mathematical models of plant tip growth and their applicability to investigate protonemal morphogenesis. We attempt to integrate the conclusions and data across cell biology and physical modeling to our current state of knowledge of polarized cell growth in P. patens and highlight future directions in the field.

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.

A Fully Functional ROP Fluorescent Fusion Protein Reveals Roles for This GTPase in Subcellular and Tissue-Level Patterning

Rho of Plants (ROPs) are GTPases that regulate polarity and patterned wall deposition in plants. As these small, globular proteins have many interactors, it has been difficult to ensure that methods to visualize ROP in live cells do not affect ROP function. Here, motivated by work in fission yeast (Schizosaccharomyces pombe), we generated a fluorescent moss (Physcomitrium [Physcomitrella] patens) ROP4 fusion protein by inserting mNeonGreen after Gly-134. Plants harboring tagged ROP4 and no other ROP genes were phenotypically normal. Plants lacking all four ROP genes comprised an unpatterned clump of spherical cells that were unable to form gametophores, demonstrating that ROP is essentially for spatial patterning at the cellular and tissue levels. The functional ROP fusion protein formed a steep gradient at the apical plasma membranes of growing tip cells. ROP also predicted the site of branch formation in the apical cell at the onset of mitosis, which occurs one to two cell cycles before a branch cell emerges. While fluorescence recovery after photobleaching studies demonstrated that ROP dynamics do not depend on the cytoskeleton, acute depolymerization of the cytoskeleton removed ROP from the membrane only in recently divided cells, pointing to a feedback mechanism between the cell cycle, cytoskeleton, and ROP.

Rho of Plants GTPases and Cytoskeletal Elements Control Nuclear Positioning and Asymmetric Cell Division during Physcomitrella patens Branching

Branching morphogenesis is a widely used mechanism for development [1, 2]. In plants, it is initiated by the emergence of a new growth axis, which is of particular importance for plants to explore space and access resources [1]. Branches can emerge either from a single cell or from a group of cells [3-5]. In both cases, the mother cells that initiate branching must undergo dynamic morphological changes and/or adopt oriented asymmetric cell divisions (ACDs) to establish the new growth direction. However, the underlying mechanisms are not fully understood. Here, using the bryophyte moss Physcomitrella patens as a model, we show that side-branch formation in P. patens protonemata requires coordinated polarized cell expansion, directional nuclear migration, and orientated ACD. By combining pharmacological experiments, long-term time-lapse imaging, and genetic analyses, we demonstrate that Rho of plants (ROP) GTPases and actin are essential for cell polarization and local cell expansion (bulging). The growing bulge acts as a prerequisite signal to guide long-distance microtubule (MT)-dependent nuclear migration, which determines the asymmetric positioning of the division plane. MTs play an essential role in nuclear migration but are less involved in bulge formation. Hence, cell polarity and cytoskeletal elements act cooperatively to modulate cell morphology and nuclear positioning during branch initiation. We propose that polarity-triggered nuclear positioning and ACD comprise a fundamental mechanism for increasing multicellularity and tissue complexity during plant morphogenesis.

Design Principles of Branching Morphogenesis in Filamentous Organisms

The radiation of life on Earth was accompanied by the diversification of multicellular body plans in the eukaryotic kingdoms Animalia, Plantae, Fungi and Chromista. Branching forms are ubiquitous in nature and evolved repeatedly in the above lineages. The developmental and genetic basis of branch formation is well studied in the three-dimensional shoot and root systems of land plants, and in animal organs such as the lung, kidney, mammary gland, vasculature, etc. Notably, recent thought-provoking studies combining experimental analysis and computational modeling of branching patterns in whole animal organs have identified global patterning rules and proposed unifying principles of branching morphogenesis. Filamentous branching forms represent one of the simplest expressions of the multicellular body plan and constitute a key step in the evolution of morphological complexity. Similarities between simple and complex branching forms distantly related in evolution are compelling, raising the question whether shared mechanisms underlie their development. Here, we focus on filamentous branching organisms that represent major study models from three distinct eukaryotic kingdoms, including the moss Physcomitrella patens (Plantae), the brown alga Ectocarpus sp. (Chromista), and the ascomycetes Neurospora crassa and Aspergillus nidulans (Fungi), and bring to light developmental regulatory mechanisms and design principles common to these lineages. Throughout the review we explore how the regulatory mechanisms of branching morphogenesis identified in other models, and in particular animal organs, may inform our thinking on filamentous systems and thereby advance our understanding of the diverse strategies deployed across the eukaryotic tree of life to evolve similar forms.

Analysis of the Localization of Fluorescent PpROP1 and PpROP-GEF4 Fusion Proteins in Moss Protonemata Based on Genomic "Knock-In" and Estradiol-Titratable Expression

Tip growth of pollen tubes, root hairs, and apical cells of moss protonemata is controlled by ROP (Rho of plants) GTPases, which were shown to accumulate at the apical plasma membrane of these cells. However, most ROP localization patterns reported in the literature are based on fluorescent protein tagging and need to be interpreted with caution, as ROP fusion proteins were generally overexpressed at undefined levels, in many cases without assessing effects on tip growth. ROP-GEFs, important regulators of ROP activity, were also described to accumulate at the apical plasma membrane during tip growth. However, to date only the localization of fluorescent ROP-GEF fusion proteins strongly overexpressed using highly active promoters have been investigated. Here, the intracellular distributions of fluorescent PpROP1 and PpROP-GEF4 fusion proteins expressed at essentially endogenous levels in apical cells of Physcomitrella patens "knock-in" protonemata were analyzed. Whereas PpROP-GEF4 was found to associate with a small apical plasma membrane domain, PpROP1 expression was below the detection limit. Estradiol-titratable expression of a fluorescent PpROP1 fusion protein at the lowest detectable level, at which plant development was only marginally affected, was therefore employed to show that PpROP1 also accumulates at the apical plasma membrane, although within a substantially larger domain. Interestingly, RNA-Seq data indicated that the majority of all genes active in protonemata are expressed at lower levels than PpROP1, suggesting that estradiol-titratable expression may represent an important alternative to "knock-in" based analysis of the intracellular distribution of fluorescent fusion proteins in protonemal cells.

Phosphatidic acid binds to and regulates guanine nucleotide exchange factor 8 (GEF8) activity in Arabidopsis

Phosphatidic acid (PA) forms part of plant lipid metabolism and is a signalling molecule used in response to various external stresses. Guanine nucleotide exchange factors (GEFs) activate small GTPase ROPs, serving as molecular switches in a wide range of signalling pathways. However, the interaction between PA and GEFs in plants has not yet been reported. Here we show that PA bound specifically to GEF8 by using fat-Western blot and isothermal titration calorimetry assays. A C-terminal truncation of GEF8 exhibited strong PA binding, and mutation of lysines 13 and 18 in GEF8 PRONE domain caused a total loss of binding to PA. Two ROPs, ROP7 and ROP10, were identified as preferred substrates of GEF8 by pull-down and bimolecular fluorescence complementation assays. GEF8 activity towards ROP7, but not ROP10, was stimulated by PA both in vitro and in cells. Moreover, the PA- or ABA-induced activation of GEF8 was completely lost in the mutant GEF8, which did not bind to PA. Together, these findings identify a direct interconnection between PA-mediated GEFs activity and small GTPase signalling in plants and provide evidence for a synergistic activation of GEF8 by direct PA-binding to its PRONE domain.

Wheat ROP proteins modulate defense response through lignin metabolism

ROP is a subfamily of small GTP-binding proteins that uniquely exist in plants. It acts as versatile molecular switches that regulate various developmental processes. Some ROP proteins are also reported to affect defense responses, although their exact mechanism is not fully understood. Herein, ROP members in wheat were mined; the functions of three wheat ROP proteins were studied. RT-PCR results showed that the expression of TaRac1 was rapidly and strongly induced after leaf rust infection. TaRac1 interacted with TaCCR in yeast-hybridization assay. The overexpression of TaRac1 in tobacco promoted CCR and CAD gene expression, increased the total lignin content and sinapyl lignin proportion, and then enhanced resistance to tobacco black shank and bacterial wilt diseases. In contrast, TaRac3 and TaRac4 did not show to interact with TaCCR. Furthermore, the overexpression of TaRac3 and TaRac4 did not increase lignin gene expression and lignin accumulation either. Unlike TaRac1, the overexpression of TaRac3 increased susceptibility to both black shank and bacterial wilt pathogens, while overexpression of TaRac4 showed no effect on disease resistance but promoted the root growth in tobacco seedling. These data collectively suggest that TaRac1 in Group II is mainly involved in regulating lignin metabolism which, in turn, responsible for the observed roles in pathogen resistance. TaRac3 and TaRac4 have the minor roles in defense response but may act on regulation in plant developmental processes. These results shed light on the complexity and diverse function of ROP in plant defense pathway.

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

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

(Why) Does Evolution Favour Embryogenesis?

Complex multicellular organisms typically possess life cycles in which zygotes (formed by gamete fusion) and meiosis occur. Canonical animal embryogenesis describes development from zygote to birth. It involves polarisation of the egg/zygote, asymmetric cell divisions, establishment of axes, symmetry breaking, formation of organs, and parental nutrition (at least in early stages). Similar developmental patterns have independently evolved in other eukaryotic lineages, including land plants and brown algae. The question arises whether embryo-like structures and associated developmental processes recurrently emerge because they are local optima of the evolutionary landscape. To understand which evolutionary principles govern complex multicellularity, we need to analyse why and how similar processes evolve convergently - von Baer's and Haeckel's phylotypic stage revisited in other phyla.

The Ectopic Expression of CaRop1 Modulates the Response of Tobacco Plants to Ralstonia solanacearum and Aphids

In plants, Rho-related GTPases (Rops) are versatile molecular switches that regulate various biological processes, although their exact roles are not fully understood. Herein, we provide evidence that the ectopic expression of a Rop derived from Capsicum annuum, designated CaRop1, in tobacco plants modulates the response of these plants to Ralstonia solanacearum or aphid attack. The deduced amino acid sequence of CaRop1 harbors a conserved Rho domain and is highly homologous to Rops of other plant species. Transient expression of a CaRop1-GFP fusion protein in Nicotiana benthamiana leaf epidermal cells revealed localization of the GFP signal to the plasma membrane, cytoplasm, and nucleus. Overexpression (OE) of the wild-type CaRop1 or its dominant-negative mutant (DN-CaRop1) conferred substantial resistance to R. solanacearum infection and aphid attack, and this effect was accompanied by enhanced transcriptional expression of the hypersensitive-reaction marker gene HSR201; the jasmonic acid (JA)-responsive PR1b and LOX1; the insect resistance-associated NtPI-I, NtPI-II, and NtTPI; the ethylene (ET) production-associated NtACS1; and NPK1, a mitogen-activated protein kinase kinase kinase (MAPKKK) that interferes with N-, Bs2-, and Rx-mediated disease resistance. In contrast, OE of the constitutively active mutant of CaRop1(CA-CaRop1) enhanced susceptibility of the transgenic tobacco plants to R. solanacearum infection and aphid attack and downregulated or sustained the expression of HSR201, PR1b, NPK1, NtACS1, NtPI-I, NtPI-II, and NtTPI. These results collectively suggest that CaRop1 acts as a signaling switch in the crosstalk between Solanaceaes's response to R. solanacearum infection and aphid attack possibly via JA/ET-mediated signaling machinery.

A family of ROP proteins that suppresses actin dynamics, and is essential for polarized growth and cell adhesion

In plants, the ROP family of small GTPases has been implicated in the polarized growth of tip-growing cells, such as root hairs and pollen tubes; however, most of the data derive from overexpressing ROP genes or constitutively active and dominant-negative isoforms, whereas confirmation by using loss-of-function studies has generally been lacking. Here, in the model moss Physcomitrella patens, we study ROP signaling during tip growth by using a loss-of-function approach based on RNA interference (RNAi) to silence the entire moss ROP family. We find that plants with reduced expression of ROP genes, in addition to failing to initiate tip growth, have perturbed cell wall staining, reduced cell adhesion and have increased actin-filament dynamics. Although plants subjected to RNAi against the ROP family also have reduced microtubule dynamics, this reduction is not specific to loss of ROP genes, as it occurs when actin function is compromised chemically or genetically. Our data suggest that ROP proteins polarize the actin cytoskeleton by suppressing actin-filament dynamics, leading to an increase in actin filaments at the site of polarized secretion.

Protein lipid modifications and the regulation of ROP GTPase function

In eukaryotes, the RHO superfamily of small G-proteins is implicated in the regulation of cell polarity and growth. Rho of Plants (ROPs)/RACs are plant-specific Rho family proteins that have been shown to regulate cell polarity, auxin transport and responses, ABA signalling, and response to pathogens. A hallmark of ROP/RAC function is their localization in specific plasma membrane domains. This short review focuses on the mechanisms responsible for membrane interactions of ROPs/RACs and how they affect ROP/RAC function.

RHO GTPase in plants: Conservation and invention of regulators and effectors

Plants possess a single subfamily of Rho GTPases, ROP, which does usual things as do Rho-family GTPases in animal and fungal systems, namely participating in the spatial control of cellular processes by signaling to the cytoskeleton and vesicular trafficking. As one would expect, ROPs are modulated by conserved regulators such as DHR2-type GEFs, RhoGAPs and Rho GDIs. What is surprising is that plants have invented new regulators such as PRONE-type GEFs (known as RopGEFs) and effectors such as RICs and ICRs/RIPs in the regulation of the cytoskeleton and vesicular trafficking. This review will discuss recent work on characterizing ROP regulators and effectors as well as addressing why and how a mixture of conserved and novel Rho signaling mechanisms is utilized to modulate fundamental cellular processes such as cytoskeletal dynamics/reorganization and vesicular trafficking.

Systemic acquired resistance in moss: further evidence for conserved defense mechanisms in plants

Vascular plants possess multiple mechanisms for defending themselves against pathogens. One well-characterized defense mechanism is systemic acquired resistance (SAR). In SAR, a plant detects the presence of a pathogen and transmits a signal throughout the plant, inducing changes in the expression of various pathogenesis-related (PR) genes. Once SAR is established, the plant is capable of mounting rapid responses to subsequent pathogen attacks. SAR has been characterized in numerous angiosperm and gymnosperm species; however, despite several pieces of evidence suggesting SAR may also exist in non-vascular plants^6–8, its presence in non-vascular plants has not been conclusively demonstrated, in part due to the lack of an appropriate culture system. Here, we describe and use a novel culture system to demonstrate that the moss species Amblystegium serpens does initiate a SAR-like reaction upon inoculation with Pythium irregulare, a common soil-borne oomycete. Infection of A. serpens gametophores by P. irregulare is characterized by localized cytoplasmic shrinkage within 34 h and chlorosis and necrosis within 7 d of inoculation. Within 24 h of a primary inoculation (induction), moss gametophores grown in culture became highly resistant to infection following subsequent inoculation (challenge) by the same pathogen. This increased resistance was a response to the pathogen itself and not to physical wounding. Treatment with β-1,3 glucan, a structural component of oomycete cell walls, was equally effective at triggering SAR. Our results demonstrate, for the first time, that this important defense mechanism exists in a non-vascular plant, and, together with previous studies, suggest that SAR arose prior to the divergence of vascular and non-vascular plants. In addition, this novel moss – pathogen culture system will be valuable for future characterization of the mechanism of SAR in moss, which is necessary for a better understanding of the evolutionary history of SAR in plants.

Prenylation is required for polar cell elongation, cell adhesion, and differentiation in Physcomitrella patens

Protein prenylation is required for a variety of growth and developmental processes in flowering plants. Here we report the consequences of loss of function of all known prenylation subunits in the moss Physcomitrella patens. As in Arabidopsis, protein farnesyltransferase and protein geranylgeranyltransferase type I are not required for viability. However, protein geranylgeranyltransferase type I activity is required for cell adhesion, polar cell elongation, and cell differentiation. Loss of protein geranylgeranyltransferase activity results in colonies of round, single-celled organisms that resemble unicellular algae. The loss of protein farnesylation is not as severe but also results in polar cell elongation and differentiation defects. The complete loss of Rab geranylgeranyltransferase activity appears to be lethal in P. patens. Labeling with antibodies to cell wall components support the lack of polarity establishment and the undifferentiated state of geranylgeranyltransferase type I mutant plants. Our results show that prenylated proteins play key roles in P. patens development and differentiation processes.

Auxin biology revealed by small molecules

The plant hormone auxin regulates virtually every aspect of plant growth and development and unraveling its molecular and cellular modes of action is fundamental for plant biology research. Chemical genomics is the use of small molecules to modify protein functions. This approach currently rises as a powerful technology for basic research. Small compounds with auxin-like activities or affecting auxin-mediated biological processes have been widely used in auxin research. They can serve as a tool complementary to genetic and genomic methods, facilitating the identification of an array of components modulating auxin metabolism, transport and signaling. The employment of high-throughput screening technologies combined with informatics-based chemical design and organic chemical synthesis has since yielded many novel small molecules with more instantaneous, precise and specific functionalities. By applying those small molecules, novel molecular targets can be isolated to further understand and dissect auxin-related pathways and networks that otherwise are too complex to be elucidated only by gene-based methods. Here, we will review examples of recently characterized molecules used in auxin research, highlight the strategies of unraveling the mechanisms of these small molecules and discuss future perspectives of small molecule applications in auxin biology.

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.

Phosphoproteomic analysis of the non-seed vascular plant model Selaginella moellendorffii

BACKGROUND

Selaginella (Selaginella moellendorffii) is a lycophyte which diverged from other vascular plants approximately 410 million years ago. As the first reported non-seed vascular plant genome, Selaginella genome data allow comparative analysis of genetic changes that may be associated with land plant evolution. Proteomics investigations on this lycophyte model have not been extensively reported. Phosphorylation represents the most common post-translational modifications and it is a ubiquitous regulatory mechanism controlling the functional expression of proteins inside living organisms.

RESULTS

In this study, polyethylene glycol fractionation and immobilized metal ion affinity chromatography were employed to isolate phosphopeptides from wild-growing Selaginella. Using liquid chromatography-tandem mass spectrometry analysis, 1593 unique phosphopeptides spanning 1104 non-redundant phosphosites with confirmed localization on 716 phosphoproteins were identified. Analysis of the Selaginella dataset revealed features that are consistent with other plant phosphoproteomes, such as the relative proportions of phosphorylated Ser, Thr, and Tyr residues, the highest occurrence of phosphosites in the C-terminal regions of proteins, and the localization of phosphorylation events outside protein domains. In addition, a total of 97 highly conserved phosphosites in evolutionary conserved proteins were identified, indicating the conservation of phosphorylation-dependent regulatory mechanisms in phylogenetically distinct plant species. On the other hand, close examination of proteins involved in photosynthesis revealed phosphorylation events which may be unique to Selaginella evolution. Furthermore, phosphorylation motif analyses identified Pro-directed, acidic, and basic signatures which are recognized by typical protein kinases in plants. A group of Selaginella-specific phosphoproteins were found to be enriched in the Pro-directed motif class.

CONCLUSIONS

Our work provides the first large-scale atlas of phosphoproteins in Selaginella which occupies a unique position in the evolution of terrestrial plants. Future research into the functional roles of Selaginella-specific phosphorylation events in photosynthesis and other processes may offer insight into the molecular mechanisms leading to the distinct evolution of lycophytes.

Turning moss into algae: prenylation targets in Physcomitrella patens

Prenylation is a series of lipid posttranslational modifications that are involved in several key aspects of plant development. We recently knocked out every prenylation subunit in Physcomitrella patens. Like in Arabidopsis, knockout of protein farnesyltransferase and protein geranylgeranyltransferase in P. patens does not result in lethality; however, effects on development are extensive. In particular, the knockout of protein geranylgeranyltransferase results in small unicellular plants that resemble algae. Here we perform an analysis of predicted geranylgeranyltransferase target proteins in P. patens, and draw attention to those most likely to play a role in the knockout phenotype.

The Arabidopsis small GTPase AtRAC7/ROP9 is a modulator of auxin and abscisic acid signalling

Rac-like GTPases or Rho-related GTPases from plants (RAC/ROPs) are important components of hormone signalling pathways in plants. Based on phylogeny, several groups can be distinguished, and the underlying premise is that members of different groups perform distinct functions in the plant. AtRAC7/ROP9 is phylogenetically unique among 11 Arabidopsis RAC/ROPs, and here it was shown that it functions as a modulator of auxin and abscisic acid (ABA) signalling, a dual role not previously assigned to these small GTPases. Plants with reduced levels of AtRAC7/ROP9 had increased sensitivity to auxin and were less sensitive to ABA. On the other hand, overexpressing AtRAC7/ROP9 activated ABA-induced gene expression but repressed auxin-induced gene expression. In addition, both hormones regulated the activity of the AtRAC7/ROP9 promoter, suggesting a feedback mechanism to modulate the signalling output from the AtRAC7/ROP9-controlled molecular switch. High levels of AtRAC7/ROP9 were detected specifically in embryos and lateral roots, underscoring the important role of this protein during embryo development and lateral root formation. These results place AtRAC7/ROP9 as an important signal transducer in recently described pathways that integrate auxin and ABA signalling in the plant.

Polar Expansion Dynamics in the Plant Kingdom: A Diverse and Multifunctional Journey on the Path to Pollen Tubes

Polar expansion is a widespread phenomenon in plants spanning all taxonomic groups from the Charophycean Green Algae to pollen tubes in Angiosperms and Gymnosperms. Current data strongly suggests that many common features are shared amongst cells displaying polar growth mechanics including changes to the structural features of localized regions of the cell wall, mobilization of targeted secretion mechanisms, employment of the actin cytoskeleton for directing secretion and in many cases, endocytosis and coordinated interaction of multiple signal transduction mechanisms prompted by external biotic and abiotic cues. The products of polar expansion perform diverse functions including delivery of male gametes to the egg, absorption, anchorage, adhesion and photo-absorption efficacy. A comparative analysis of polar expansion dynamics is provided with special emphasis on those found in early divergent plants.

Phytochrome cytoplasmic signaling

Extensive studies in both lower and higher plants indicate that plant phytochrome photoreceptors signal not only by regulating transcription in the nucleus but also by acting within the cytoplasm, the latter signaling routes acting within minutes or even seconds and also providing directional information. Directional signals seem to arise from phytochromes attached anisotropically to the plasma membrane. Neochromes-phytochrome-phototropin hybrid photoreceptors probably attached to the plasma membrane-provide this signal in various ferns and perhaps certain algae but are absent from other groups. In mosses and probably higher plants too, a subpopulation of canonical phytochromes interact with phototropins at the plasma membrane and thereby steer directional responses. Phytochromes also seem able to regulate translation in the cytoplasm. This review discusses putative phytochrome functions in these contexts.

Apical myosin XI anticipates F-actin during polarized growth of Physcomitrella patens cells

Tip growth is essential for land colonization by bryophytes, plant sexual reproduction and water and nutrient uptake. Because this specialized form of polarized cell growth requires both a dynamic actin cytoskeleton and active secretion, it has been proposed that the F-actin-associated motor myosin XI is essential for this process. Nevertheless, a spatial and temporal relationship between myosin XI and F-actin during tip growth is not known in any plant cell. Here, we use the highly polarized cells of the moss Physcomitrella patens to show that myosin XI and F-actin localize, in vivo, at the same apical domain and that both signals fluctuate. Surprisingly, phase analysis shows that increase in myosin XI anticipates that of F-actin; in contrast, myosin XI levels at the tip fluctuate in identical phase with a vesicle marker. Pharmacological analysis using a low concentration of the actin polymerization inhibitor latrunculin B showed that the F-actin at the tip can be significantly diminished while myosin XI remains elevated in this region, suggesting that a mechanism exists to cluster myosin XI-associated structures at the cell's apex. In addition, this approach uncovered a mechanism for actin polymerization-dependent motility in the moss cytoplasm, where myosin XI-associated structures seem to anticipate and organize the actin polymerization machinery. From our results, we inferred a model where the interaction between myosin XI-associated vesicular structures and F-actin polymerization-driven motility function at the cell's apex to maintain polarized cell growth. We hypothesize this is a general mechanism for the participation of myosin XI and F-actin in tip growing cells.

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.

Modification of plant Rac/Rop GTPase signalling using bacterial toxin transgenes

Bacterial protein toxins which modify Rho GTPase are useful for the analysis of Rho signalling in animal cells, but these toxins cannot be taken up by plant cells. We demonstrate in vitro deamidation of Arabidopsis Rop4 by Escherichia coli Cytotoxic Necrotizing Factor 1 (CNF1) and glucosylation by Clostridium difficile toxin B. Expression of the catalytic domain of CNF1 caused modification and activation of co-expressed Arabidopsis Rop4 GTPase in tobacco leaves, resulting in hypersensitive-like cell death. By contrast, the catalytic domain of toxin B modified and inactivated co-expressed constitutively active Rop4, blocking the hypersensitive response caused by over-expression of active Rops. In transgenic Arabidopsis, both CNF1 and toxin B inhibited Rop-dependent polar morphogenesis of leaf epidermal cells. Toxin B expression also inhibited Rop-dependent morphogenesis of root hairs and trichome branching, and resulted in root meristem enlargement and dwarf growth. Our results show that CNF1 and toxin B transgenes are effective tools in Rop GTPase signalling studies.

Physcomitrella patens: a model for tip cell growth and differentiation

The moss Physcomitrella patens has emerged as an excellent model system owing to its amenability to reverse genetics. The moss gametophyte has three filamentous tissues that grow by tip growth: chloronemata, caulonemata, and rhizoids. Because establishment of the moss plant relies on this form of growth, it is particularly suited for dissecting the molecular basis of tip growth. Recent studies demonstrate that a core set of actin cytoskeletal proteins is essential for tip growth. Additional actin cytoskeletal components are required for modulating growth to produce caulonemata and rhizoids. Differentiation into these cell types has previously been linked to auxin, light and nutrients. Recent studies have identified that core auxin signaling components as well as transcription factors that respond to auxin or nutrient levels are required for tip-growing cell differentiation. Future studies may establish a connection between the actin cytoskeleton and auxin or nutrient-induced cell differentiation.

New insights into Rho signaling from plant ROP/Rac GTPases

In animal and plant cells, a wide range of key cellular processes that require the establishment of cell polarity are governed by Rho-GTPases. In contrast to animals and yeast, however, plants possess a single Rho-GTPase subfamily called Rho-like GTPases from plants (ROPs). This raises the question of how plants achieve the high level of regulation required for polar cellular processes. It is becoming evident that plants have evolved specific regulators, including ROP-Guanine Exchange Factors (GEFs) and the Rop-interactive CRIB motif-containing protein (RIC) effectors. Recent research shows that the spatiotemporal dynamics of ROPs, the cytoskeleton, endocytosis, and exocytosis are intertwined. This review focuses on the proposed self-organizing nature of ROPs in plants and how ROP-mediated cellular mechanisms compare with those responsible for cell polarity in animals and yeast.

The unique plant RhoGAPs are dimeric and contain a CRIB motif required for affinity and specificity towards cognate small G proteins

Plant Rho proteins (ROPs) are inactivated by specific GTPase activating proteins, called RopGAPs. Many of these comprise the exclusive combination of a classic, catalytic Arg-containing RhoGAP domain, and a Cdc42/ Rac interactive binding (CRIB) motif which in animal and fungi has been identified in effectors for Cdc42 and Rac1, but never in any GAP protein. Both elements are required for an efficient RopGAP activity. Here, we analyzed the effect of the CRIB motif on the complex formation and the binding reaction with plant and human Rho proteins by using kinetic and equilibrium methods. We show that RopGAP2 from Arabidopsis thaliana dimerizes via its GAP domain and forms a 2:2 complex with ROP. The CRIB effector motif mediates high affinity and specificity in binding. The catalytic Arg in the context of the CRIB motif is inhibitory for binding. The unusually slow association and dissociation reactions suggest a major conformational change whereby the CRIB motif functions as a lid for binding and/or release of ROP. We propose a two-site interaction model where ROP binds to the CRIB motif as described for the human CRIB effectors and to the catalytic GAP domain as described for animal RhoGAPs.

A barley ROP GTPase ACTIVATING PROTEIN associates with microtubules and regulates entry of the barley powdery mildew fungus into leaf epidermal cells

Little is known about the function of host factors involved in disease susceptibility. The barley (Hordeum vulgare) ROP (RHO of plants) G-protein RACB is required for full susceptibility of the leaf epidermis to invasion by the biotrophic fungus Blumeria graminis f. sp hordei. Stable transgenic knockdown of RACB reduced the ability of barley to accommodate haustoria of B. graminis in intact epidermal leaf cells and to form hairs on the root epidermis, suggesting that RACB is a common element of root hair outgrowth and ingrowth of haustoria in leaf epidermal cells. We further identified a barley MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN (MAGAP1) interacting with RACB in yeast and in planta. Fluorescent MAGAP1 decorated cortical microtubules and was recruited by activated RACB to the cell periphery. Under fungal attack, MAGAP1-labeled microtubules built a polarized network at sites of successful defense. By contrast, microtubules loosened where the fungus succeeded in penetration. Genetic evidence suggests a function of MAGAP1 in limiting susceptibility to penetration by B. graminis. Additionally, MAGAP1 influenced the polar organization of cortical microtubules. These results add to our understanding of how intact plant cells accommodate fungal infection structures and suggest that RACB and MAGAP1 might be antagonistic players in cytoskeleton organization for fungal entry.

New insights into the functional roles of CrRLKs in the control of plant cell growth and development

Receptor-like kinases (RLKs) are a family of transmembrane proteins with a variable ligand-binding extracellular domain and a cytoplasmic kinase domain. In Arabidopsis, there are ~600 RLKs believed to have diverse functions during plant growth, development and interactions with the environment. Based on the variable extracellular domain, RLKs can be classified into different subfamilies. The CrRLK subfamily contains 17 members in Arabidopsis and characterization of some of its members suggests a role for these proteins in the regulation of growth and reproduction. The present review focuses on the roles of CrRLKs in the regulation of polarized growth with emphasis on the newly identified signal transduction pathways activated downstream of CrRLKs. A picture is emerging where CrRLKs are part of a conserved signal transduction cascade important for growth maintenance in different cell types.

Receptor-like kinases as surface regulators for RAC/ROP-mediated pollen tube growth and interaction with the pistil

BACKGROUND

RAC/ROPs are RHO-type GTPases and are known to play diverse signalling roles in plants. Cytoplasmic RAC/ROPs are recruited to the cell membrane and activated in response to extracellular signals perceived and mediated by cell surface-located signalling assemblies, transducing the signals to regulate cellular processes. More than any other cell types in plants, pollen tubes depend on continuous interactions with an extracellular environment produced by their surrounding tissues as they grow within the female organ pistil to deliver sperm to the female gametophyte for fertilization.

SCOPE

We review studies on pollen tube growth that provide compelling evidence indicating that RAC/ROPs are crucial for regulating the cellular processes that underlie the polarized cell growth process. Efforts to identify cell surface regulators that mediate extracellular signals also point to RAC/ROPs being the molecular switches targeted by growth-regulating female factors for modulation to mediate pollination and fertilization. We discuss a large volume of work spanning more than two decades on a family of pollen-specific receptor kinases and some recent studies on members of the FERONIA family of receptor-like kinases (RLKs).

SIGNIFICANCE

The research described shows the crucial roles that two RLK families play in transducing signals from growth regulatory factors to the RAC/ROP switch at the pollen tube apex to mediate and target pollen tube growth to the female gametophyte and signal its disintegration to achieve fertilization once inside the female chamber.