G protein activation without a GEF in the plant kingdom

Atomistic Mechanism Underlying the Regulation of the GPA1 G Protein Signaling Pathway Mediated by the Gibberellin A1 Phytohormone Binding to the GCR1 Plant G-Protein-Coupled Receptor

We propose an atomistic mechanism suggesting that fundamental plant processes, including seed germination, root elongation, and flower and fruit production, may be regulated by phytohormones such as Gibberellin A1 (GA1) binding to the GCR1 plant G-protein-coupled receptor. This parallels the central roles of GPCRs in animals for vision, taste, smell, pain, depression, and nerve signaling, among others. Validating GCR1 as a genuine GPCR in plants, particularly its interaction with GPA1, G-protein, would mark a groundbreaking advancement in understanding plant processes, both biologically and agronomically. However, experimental confirmation of this interaction and evidence supporting the idea that binding of GA1 to GCR1 would regulate GPA1 activation are lacking. Indeed, the design of experiments to explore these hypotheses is impeded by the absence of structural information relating to interactions of GPA1 with the GA1-GCR1 complex. To address this gap, we employ molecular dynamics and metadynamics simulations to demonstrate that binding GPA1 to the GA1-GCR1 complex induces conformational changes that open up the Ras and Helical domains of GPA1 to release GDP for exchange with GTP, thereby enabling signaling. Our results suggest numerous mutations involving GA1 binding at the GCR1 site and the coupling of GCR1 to the GPA1 G-protein that could be used to validate (or not validate) our predicted mechanism. Such validation would serve as a foundation for devising strategies to design novel agonists and inverse agonists to provide precise control of crucial plant processes.

Rice grain size: current regulatory mechanisms and future perspectives

Rice is a staple food for over half of the world's population. To feed the growing population, molecular breeders aim to increase grain yield. Grain size is an important factor for crop productivity, and it has been extensively studied. However, molecular breeders face a major challenge in further improving crop productivity in terms of grain yield and quality. Grain size is a complex trait controlled by multiple genes. Over the past few decades, genetic studies have identified various gene families involved in grain size development. The list of molecular mechanisms, and key regulators involved in grain size development is constantly expanding, making it difficult to understand the main regulators that play crucial roles in grain development. In this review, we focus on the major regulators of grain size, including G-protein signaling, the mitogen-activated protein kinase (MAPK) pathway, transcriptional regulation, the ubiquitin-proteasome degradation (UPD) pathway, and phytohormone signaling. These molecular mechanisms directly or indirectly regulate grain size. We provided a comprehensive understanding of the genes involved in these mechanisms and cross discussions about how these mechanisms are interlinked. This review serves as a valuable resource for understanding the molecular mechanisms that govern grain development and can aid in the development of molecular breeding strategies.

Plant PAQR-like sensors activate heterotrimeric G proteins to confer resistance against multiple pathogens

Human adiponectin receptors (AdipoRs) and membrane progestin receptors (mPRs, members of the progestin and adipoQ receptor [PAQR] family) are seven-transmembrane receptors involved in the regulation of metabolism and cancer development, which share structural similarities with G protein-coupled receptors. Plant PAQR-like sensors (PLSs) are homologous to human PAQRs but their molecular functions remain unclear. In this study, we found that PLSs associate with cell surface receptor-like kinases through KIN7 and positively regulate plant immune responses, stomatal defense, and disease resistance. Moreover, PLSs activate heterotrimeric G proteins (Gαβγ) to transduce immune signals and regulate the exchange of GDP for GTP on GPA1. Further analyses revealed that the immune function of PLSs is conserved in rice and soybean and contributes to resistance against multiple diseases. Notably, heterologous expression of human AdipoRs in Arabidopsis replicates the immune functions of PLSs. Collectively, our findings demonstrate that PLSs are key modulators of plant immunity via the G-protein pathway and highlight the potential application of human genes in enhancing plant disease resistance.

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins, composed of Gα, Gβ, and Gγ subunits, are a class of signal transduction complexes with broad roles in human health and agriculturally relevant plant physiological and developmental traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. We sought to develop improved methods for heterologous expression and rapid purification of Gα subunits, initially targeting GPA1, the sole canonical Gα subunit of the model plant species, Arabidopsis thaliana. Compared to conventional methods, our expression methodology and rapid StrepII-tag mediated purification facilitates substantially higher yield, and isolation of protein with increased GTP binding and hydrolysis activities. Human GNAI1 purified using our approach displayed the expected binding and hydrolysis activities, indicating our protocol is applicable to mammalian Gα subunits, potentially including those for which purification of enzymatically active protein has been historically problematic. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate that the inherent instability of GPA1 is a function of the interaction between the Ras and helical domains. Additionally, we found that GPA1-GNAO1 domain swaps partially uncouple the instability from the rapid nucleotide binding kinetics displayed by GPA1. In summary, our work provides insights into methods to optimally study heterotrimeric G proteins, and reveals roles of the helical domain in Gα kinetics and stability.

Proteome-wide analysis reveals G protein-coupled receptor-like proteins in rice (Oryza sativa)

G protein-coupled receptors (GPCRs) constitute the largest family of transmembrane proteins in metazoans that mediate the regulation of various physiological responses to discrete ligands through heterotrimeric G protein subunits. The existence of GPCRs in plant is contentious, but their comparable crucial role in various signaling pathways necessitates the identification of novel remote GPCR-like proteins that essentially interact with the plant G protein α subunit and facilitate the transduction of various stimuli. In this study, we identified three putative GPCR-like proteins (OsGPCRLPs) (LOC_Os06g09930.1, LOC_Os04g36630.1, and LOC_Os01g54784.1) in the rice proteome using a stringent bioinformatics workflow. The identified OsGPCRLPs exhibited a canonical GPCR 'type I' 7TM topology, patterns, and biologically significant sites for membrane anchorage and desensitization. Cluster-based interactome mapping revealed that the identified proteins interact with the G protein α subunit which is a characteristic feature of GPCRs. Computational results showing the interaction of identified GPCR-like proteins with G protein α subunit and its further validation by the membrane yeast-two-hybrid assay strongly suggest the presence of GPCR-like 7TM proteins in the rice proteome. The absence of a regulator of G protein signaling (RGS) box in the C- terminal domain, and the presence of signature motifs of canonical GPCR in the identified OsGPCRLPs strongly suggest that the rice proteome contains GPCR-like proteins that might be involved in signal transduction.

Alkaline tolerance in plants: The AT1 gene and beyond

Salt stress poses a serious challenge to crop production and a significant threat to global food security and ecosystem sustainability. Soil salinization commonly occurs in conjunction with alkalization, which causes combined saline-alkaline stress. Alkaline soil predominantly comprises NaHCO3 and Na2CO3 and is characterized by a high pH. The combined saline-alkaline stress is more harmful to crop production than neutral salt stress owing to the effects of both elevated salinity and high pH stress. Through genome association analysis of sorghum, a recent study has identified Alkaline tolerance 1 (AT1) as a contributor to alkaline sensitivity in crops. AT1, which is the first gene to be identified as being specifically associated with alkaline tolerance, encodes a G protein γ-subunit (Gγ). Editing of AT1 enhances the yields of sorghum, rice, maize, and millet grown in alkaline soils, indicating that AT1 has potential for generating alkaline-resistant crops. In this review, we summarize the role of AT1 in alkaline tolerance in plants and present a phylogenetic analysis along with a motif comparison of Gγ subunits of monocot and dicot plants across various species.

Trade-Off Regulation in Plant Growth and Stress Responses Through the Role of Heterotrimeric G Protein Signaling

Unlike animals, plants are sessile organisms that cannot migrate to more favorable conditions and must constantly adapt to a variety of biotic and abiotic stresses. Therefore, plants exhibit developmental plasticity to cope, which is probably based on the underlying trade-off mechanism that allocates energy expenditure between growth and stress responses to achieve appropriate growth and development under different environmental conditions. Plant heterotrimeric G protein signaling plays a crucial role in the trade-off involved in the regulation of normal growth and stress adaptation. This review examines the composition and signaling processes of heterotrimeric G proteins in plants, detailing how they balance growth and adaptive responses in plant immunity and thermomorphogenesis through recent advances in the field. Understanding the trade-offs associated with plant G protein signaling will have significant implications for agricultural innovation, particularly in the development of crops with improved resilience and minimal growth penalties under environmental stress.

GPCR-like Protein ZmCOLD1 Regulate Plant Height in an ABA Manner

G protein-coupled receptors (GPCRs) are sensors for the G protein complex to sense changes in environmental factors and molecular switches for G protein complex signal transduction. In this study, the homologous gene of GPCR-like proteins was identified from maize and named as ZmCOLD1. Subcellular analysis showed that the ZmCOLD1 protein is localized to the cell membrane and endoplasmic reticulum. A CRISPR/Cas9 knock-out line of ZmCOLD1 was further created and its plant height was significantly lower than the wild-type maize at both the seedling and adult stages. Histological analysis showed that the increased cell number but significantly smaller cell size may result in dwarfing of zmcold1, indicating that the ZmCOLD1 gene could regulate plant height development by affecting the cell division process. Additionally, ZmCOLD1 was verified to interact with the maize Gα subunit, ZmCT2, though the central hydrophilic loop domain by in vivo and in vitro methods. Abscisic acid (ABA) sensitivity analysis by seed germination assays exhibited that zmcold1 were hypersensitive to ABA, indicating its important roles in ABA signaling. Finally, transcriptome analysis was performed to investigate the transcriptional change in zmcold1 mutant. Overall, ZmCOLD1 functions as a GPCR-like protein and an important regulator to plant height.

E3 ligase DECREASED GRAIN SIZE 1 promotes degradation of a G-protein subunit and positively regulates grain size in rice

Grain size is one of the most important traits determining crop yield. However, the mechanism controlling grain size remains unclear. Here, we confirmed the E3 ligase activity of DECREASED GRAIN SIZE 1 (DGS1) in positive regulation of grain size in rice (Oryza sativa) suggested in a previous study. Rice G-protein subunit gamma 2 (RGG2), which negatively regulates grain size, was identified as an interacting protein of DGS1. Biochemical analysis suggested that DGS1 specifically interacts with canonical Gγ subunits (rice G-protein subunit gamma 1 [RGG1] and rice G-protein subunit gamma 2 [RGG2]) rather than non-canonical Gγ subunits (DENSE AND ERECT PANICLE 1 [DEP1], rice G-protein gamma subunit type C 2 [GCC2], GRAIN SIZE 3 [GS3]). We also identified the necessary domains for interaction between DGS1 and RGG2. As an E3 ligase, DGS1 ubiquitinated and degraded RGG2 via a proteasome pathway in several experiments. DGS1 also ubiquitinated RGG2 by its K140, K145, and S147 residues. Thus, this work identified a substrate of the E3 ligase DGS1 and elucidated the post-transcriptional regulatory mechanism of the G-protein signaling pathway in the control of grain size.

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins are a class of signal transduction complexes with broad roles in human health and agriculturally important plant traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. Using the Arabidopsis thaliana Gα subunit, GPA1, we developed a rapid StrepII-tag mediated purification method that facilitates isolation of protein with increased enzymatic activities as compared to conventional methods, and is demonstrably also applicable to mammalian Gα subunits. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate the instability of recombinant GPA1 is a function of the interaction between the Ras and helical domains, and can be partially uncoupled from the rapid nucleotide binding kinetics displayed by GPA1.

Pivotal role of heterotrimeric G protein in the crosstalk between sugar signaling and abiotic stress response in plants

Heterotrimeric G-proteins are key modulators of multiple signaling and developmental pathways in plants, in which they act as molecular switches to engage in transmitting various stimuli signals from outside into the cells. Substantial studies have identified G proteins as essential components of the organismal response to abiotic stress, leading to adaptation and survival in plants. Meanwhile, sugars are also well acknowledged key players in stress perception, signaling, and gene expression regulation. Connections between the two significant signaling pathways in stress response are of interest to a general audience in plant biology. In this article, advances unraveling a pivotal role of G proteins in the process of sugar signals outside the cells being translated into the operation of autophagy in cells during stress are reviewed. In addition, we have presented recent findings on G proteins regulating the response to drought, salt, alkali, cold, heat and other abiotic stresses. Perspectives on G-protein research are also provided in the end. Since G protein signaling regulates many agronomic traits, elucidation of detailed mechanism of the related pathways would provide useful insights for the breeding of abiotic stress resistant and high-yield crops.

Structure-function analysis of plant G-protein regulatory mechanisms identifies key Gα-RGS protein interactions

Heterotrimeric GTP-binding protein alpha subunit (Gα) and its cognate regulator of G-protein signaling (RGS) protein transduce signals in eukaryotes spanning protists, amoeba, animals, fungi, and plants. The core catalytic mechanisms of the GTPase activity of Gα and the interaction interface with RGS for the acceleration of GTP hydrolysis seem to be conserved across these groups; however, the RGS gene is under low selective pressure in plants, resulting in its frequent loss. Our current understanding of the structural basis of Gα:RGS regulation in plants has been shaped by Arabidopsis Gα, (AtGPA1), which has a cognate RGS protein. To gain a comprehensive understanding of this regulation beyond Arabidopsis, we obtained the x-ray crystal structures of Oryza sativa Gα, which has no RGS, and Selaginella moellendorffi (a lycophyte) Gα that has low sequence similarity with AtGPA1 but has an RGS. We show that the three-dimensional structure, protein-protein interaction with RGS, and the dynamic features of these Gα are similar to AtGPA1 and metazoan Gα. Molecular dynamic simulation of the Gα-RGS interaction identifies the contacts established by specific residues of the switch regions of GTP-bound Gα, crucial for this interaction, but finds no significant difference due to specific amino acid substitutions. Together, our data provide valuable insights into the regulatory mechanisms of plant G-proteins but do not support the hypothesis of adaptive co-evolution of Gα:RGS proteins in plants.

Moving beyond the arabidopsis-centric view of G-protein signaling in plants

Heterotrimeric G-protein-mediated signaling is a key mechanism to transduce a multitude of endogenous and environmental signals in diverse organisms. The scope and expectations of plant G-protein research were set by pioneering work in metazoans. Given the similarity of the core constituents, G-protein-signaling mechanisms were presumed to be universally conserved. However, because of the enormous diversity of survival strategies and endless forms among eukaryotes, the signal, its interpretation, and responses vary even among different plant groups. Earlier G-protein research in arabidopsis (Arabidopsis thaliana) has emphasized its divergence from Metazoa. Here, we compare recent evidence from diverse plant lineages with the available arabidopsis G-protein model and discuss the conserved and novel protein components, signaling mechanisms, and response regulation.

Lipid transport protein ORP2A promotes glucose signaling by facilitating RGS1 degradation

Heterotrimeric GTP-binding proteins (G proteins) are a group of regulators essential for signal transmission into cells. Regulator of G protein signaling 1 (AtRGS1) possesses intrinsic GTPase-accelerating protein (GAP) activity and could suppress G protein and glucose signal transduction in Arabidopsis (Arabidopsis thaliana). However, how AtRGS1 activity is regulated is poorly understood. Here, we identified a knockout mutant of oxysterol binding protein-related protein 2A, orp2a-1, which exhibits similar phenotypes to the arabidopsis g-protein beta 1-2 (agb1-2) mutant. Transgenic lines overexpressing ORP2A displayed short hypocotyls, a hypersensitive response to sugar, and lower intracellular AtRGS1 levels than the control. Consistently, ORP2A interacted with AtRGS1 in vitro and in vivo. Tissue-specific expression of 2 ORP2A alternative splicing isoforms implied functions in controlling organ size and shape. Bioinformatic data and phenotypes of orp2a-1, agb1-2, and the orp2a-1 agb1-2 double mutant revealed the genetic interactions between ORP2A and Gβ in the regulation of G protein signaling and sugar response. Both alternative protein isoforms of ORP2A localized in the endoplasmic reticulum (ER), plasma membrane (PM), and ER-PM contact sites and interacted with vesicle-associated membrane protein-associated protein 27-1 (VAP27-1) in vivo and in vitro through their two phenylalanines in an acidic track-like motif. ORP2A also displayed differential phosphatidyl phosphoinositide binding activity mediated by the pleckstrin homology domain in vitro. Taken together, the Arabidopsis membrane protein ORP2A interacts with AtRGS1 and VAP27-1 to positively regulate G protein and sugar signaling by facilitating AtRGS1 degradation.

Looking outside the box: a comparative cross-kingdom view on the cell biology of the three major lineages of eukaryotic multicellular life

Many cell biological facts that can be found in dedicated scientific textbooks are based on findings originally made in humans and/or other mammals, including respective tissue culture systems. They are often presented as if they were universally valid, neglecting that many aspects differ-in part considerably-between the three major kingdoms of multicellular eukaryotic life, comprising animals, plants and fungi. Here, we provide a comparative cross-kingdom view on the basic cell biology across these lineages, highlighting in particular essential differences in cellular structures and processes between phyla. We focus on key dissimilarities in cellular organization, e.g. regarding cell size and shape, the composition of the extracellular matrix, the types of cell-cell junctions, the presence of specific membrane-bound organelles and the organization of the cytoskeleton. We further highlight essential disparities in important cellular processes such as signal transduction, intracellular transport, cell cycle regulation, apoptosis and cytokinesis. Our comprehensive cross-kingdom comparison emphasizes overlaps but also marked differences between the major lineages of the three kingdoms and, thus, adds to a more holistic view of multicellular eukaryotic cell biology.

Predicted Three-Dimensional Structure of the GCR1 Putative GPCR in Arabidopsis thaliana and Its Binding to Abscisic Acid and Gibberellin A1

GCR1 has been proposed as a plant analogue to animal G-protein-coupled receptors that can promote or regulate several physiological processes by binding different phytohormones. For instance, abscisic acid (ABA) and gibberellin A1 (GA1) have been shown to promote or regulate germination and flowering, root elongation, dormancy, and biotic and abiotic stresses, among others. They may act through binding to GCR1, which would put GCR1 at the heart of key signaling processes of agronomic importance. Unfortunately, this GPCR function has yet to be fully validated due to the lack of an X-ray or cryo-EM 3D atomistic structure for GCR1. Here, we used the primary sequence data from Arabidopsis thaliana and the GEnSeMBLE complete sampling method to examine 13 trillion possible packings of the 7 transmembrane helical domains corresponding to GCR1 to downselect an ensemble of 25 configurations likely to be accessible to the binding of ABA or GA1. We then predicted the best binding sites and energies for both phytohormones to the best GCR1 configurations. To provide the basis for the experimental validation of our predicted ligand-GCR1 structures, we identify several mutations that should improve or weaken the interactions. Such validations could help establish the physiological role of GCR1 in plants.

Amino acid motifs for the identification of novel protein interactants

Biological systems consist of multiple components of different physical and chemical properties that require complex and dynamic regulatory loops to function efficiently. The discovery of ever more novel interacting sites in complex proteins suggests that we are only beginning to understand how cellular and biological functions are integrated and tuned at the molecular and systems levels. Here we review recently discovered interacting sites which have been identified through rationally designed amino acid motifs diagnostic for specific molecular functions, including enzymatic activities and ligand-binding properties. We specifically discuss the nature of the latter using as examples, novel hormone recognition and gas sensing sites that occur in moonlighting protein complexes. Drawing evidence from the current literature, we discuss the potential implications at the cellular, tissue, and/or organismal levels of such non-catalytic interacting sites and provide several promising avenues for the expansion of amino acid motif searches to discover hitherto unknown protein interactants and interaction networks. We believe this knowledge will unearth unexpected functions in both new and well-characterized proteins, thus filling existing conceptual gaps or opening new avenues for applications either as drug targets or tools in pharmacology, cell biology and bio-catalysis. Beyond this, motif searches may also support the design of novel, effective and sustainable approaches to crop improvements and the development of new therapeutics.

Novel Insights into Understanding the Molecular Dialogues between Bipolaroxin and the Gα and Gβ Subunits of the Wheat Heterotrimeric G-Protein during Host–Pathogen Interaction

Spot blotch disease of wheat, caused by the fungus Bipolaris sorokiniana (Sacc.) Shoem., produces several toxins which interact with the plants and thereby increase the blightening of the wheat leaves, and Bipolaroxin is one of them. There is an urgent need to decipher the molecular interaction between wheat and the toxin Bipolaroxin for in-depth understanding of host–pathogen interactions. In the present study, we have developed the three-dimensional structure of G-protein alpha subunit from Triticum aestivum. Molecular docking studies were performed using the active site of the modeled G-protein alpha and cryo-EM structure of beta subunit from T. aestivum and ‘Bipolaroxin’. The study of protein–ligand interactions revealed that six H-bonds are mainly formed by Glu29, Ser30, Lys32, and Ala177 of G-alpha with Bipolaroxin. In the beta subunit, the residues of the core beta strand domain participate in the ligand interaction where Lys256, Phe306, and Leu352 formed seven H-bonds with the ligand Bipolaroxin. All-atoms molecular dynamics (MD) simulation studies were conducted for G-alpha and -beta subunit and Bipolaroxin complexes to explore the stability, conformational flexibility, and dynamic behavior of the complex system. In planta studies clearly indicated that application of Bipolaroxin significantly impacted the physio-biochemical pathways in wheat and led to the blightening of leaves in susceptible cultivars as compared to resistant ones. Further, it interacted with the Gα and Gβ subunits of G-protein, phenylpropanoid, and MAPK pathways, which is clearly supported by the qPCR results. This study gives deeper insights into understanding the molecular dialogues between Bipolaroxin and the Gα and Gβ subunits of the wheat heterotrimeric G-protein during host–pathogen interaction.

G-protein couples MAPK cascade through maize heterotrimeric Gβ subunit

G protein couples MAPK cascade through maize heterotrimeric Gβ subunit MGB1. Heterotrimeric G protein Gβ interacts with Gγ subunit to generate Gβγ dimer in modulation of various biological processes. The modulatory events at transcriptome scale of plant Gβ subunit remain largely unknown. To reveal the regulatory basis of Gβ subunit at transcriptome level, we first identified a canonical maize Gβ subunit MGB1 that physically interacted with Type C Gγ protein MGG4. For transcriptome analysis, two independent CRISPR/Cas9-edited MGB1 lines were generated, which all exhibited growth arrest, suggestive of MGB1 essential for maize seedling establishment. Transcriptomic outcomes showed that MGB1 knockout resulted in elevated transcriptional abundance of plant immune response marker PR and immune receptor RPM1. Integrated GO, KEGG, and GSEA analyses pinpointed the enrichment of differentially expressed genes in defense response pathway. Functional association network construction revealed MAPK cascade components and PR protein as hub regulators of MGB1-mediated immune signaling. MGB1 and scaffold protein ZmRACK1 together with MAPK cascade components coordinately modulated maize immune responses. We built a modulatory hierarchy of Gβ subunit at transcriptome and interacting scales, which is informative for our understanding of the regulatory basis of G protein signaling.

Stomatal closure induced by hydrogen-rich water is dependent on GPA1 in Arabidopsis thaliana

Hydrogen (H₂) is a new signaling molecule that regulates stomatal closure via stimulating the generation of reactive oxygen species (ROS) and nitric oxide (NO) in Arabidopsis thaliana. GPA1 is the sole heterotrimeric G protein canonical α subunit found in Arabidopsis genome and functions in stomatal closure. Here, we estimated a possible role of Arabidopsis GPA1 in hydrogen-rich water (HRW)-induced stomatal closure. Our data indicated that HRW induced significant stomatal closure as well as the generation of ROS and NO in the Col-0 guard cells. However, the production of ROS and NO and stomatal closure induced by HRW were absent in the gpa1-4 mutant lacking the expression of AtGPA1. By contrast, overexpression of AtGPA1 in gpa1-4 (AtGPA1-HA/gpa1-4) restored stomatal closure and the generation of NO and ROS in the presence of HRW. Taken together, our results suggest that GPA1 is necessary for HRW-induced stomatal closure in Arabidopsis.

Distribution and the evolutionary history of G-protein components in plant and algal lineages

Heterotrimeric G-protein complexes comprising Gα-, Gβ-, and Gγ-subunits and the regulator of G-protein signaling (RGS) are conserved across most eukaryotic lineages. Signaling pathways mediated by these proteins influence overall growth, development, and physiology. In plants, this protein complex has been characterized primarily from angiosperms with the exception of spreading-leaved earth moss (Physcomitrium patens) and Chara braunii (charophytic algae). Even within angiosperms, specific G-protein components are missing in certain species, whereas unique plant-specific variants-the extra-large Gα (XLGα) and the cysteine-rich Gγ proteins-also exist. The distribution and evolutionary history of G-proteins and their function in nonangiosperm lineages remain mostly unknown. We explored this using the wealth of available sequence data spanning algae to angiosperms representing extant species that diverged approximately 1,500 million years ago, using BLAST, synteny analysis, and custom-built Hidden Markov Model profile searches. We show that a minimal set of components forming the XLGαβγ trimer exists in the entire land plant lineage, but their presence is sporadic in algae. Additionally, individual components have distinct evolutionary histories. The XLGα exhibits many lineage-specific gene duplications, whereas Gα and RGS show several instances of gene loss. Similarly, Gβ remained constant in both number and structure, but Gγ diverged before the emergence of land plants and underwent changes in protein domains, which led to three distinct subtypes. These results highlight the evolutionary oddities and summarize the phyletic patterns of this conserved signaling pathway in plants. They also provide a framework to formulate pertinent questions on plant G-protein signaling within an evolutionary context.

Self-activating G protein α subunits engage seven-transmembrane Regulator of G protein Signaling (RGS) proteins and a Rho guanine nucleotide exchange factor effector in the amoeba Naegleria fowleri

The free-living amoeba Naegleria fowleri is a causative agent of primary amoebic meningoencephalitis and is highly resistant to current therapies, resulting in mortality rates >97%. As many therapeutics target G protein-centered signal transduction pathways, further understanding the functional significance of G protein signaling within N. fowleri should aid future drug discovery against this pathogen. Here, we report that the N. fowleri genome encodes numerous transcribed G protein signaling components, including G protein-coupled receptors (GPCRs), heterotrimeric G protein subunits, Regulator of G protein Signaling (RGS) proteins, and candidate Gα effector proteins. We found N. fowleri Gα subunits have diverse nucleotide cycling kinetics; Nf Gα5 and Gα7 exhibit more rapid nucleotide exchange than GTP hydrolysis (i.e. "self-activating" behavior). A crystal structure of Nf Gα7 highlights the stability of its nucleotide-free state, consistent with its rapid nucleotide exchange. Variations in the phosphate binding loop (P-loop) also contribute to nucleotide cycling differences among Gα subunits. Similar to plant G protein signaling pathways, N. fowleri Gα subunits selectively engage members of a large seven-transmembrane RGS protein family, resulting in acceleration of GTP hydrolysis. We show Nf Gα2 and Gα3 directly interact with a candidate Gα effector protein, RGS-RhoGEF, similar to mammalian Gα_12/13 signaling pathways. We demonstrate Nf Gα2 and Gα3 each engage RGS-RhoGEF through a canonical Gα/RGS domain interface, suggesting a shared evolutionary origin with G protein signaling in the enteric pathogen Entamoeba histolytica. These findings further illuminate the evolution of G protein signaling and identify potential targets of pharmacological manipulation in Naegleria fowleri.

Developmental Changes in Ciliary Composition during Gametogenesis in Chlamydomonas

Chlamydomonas reinhardtii transitions from mitotically dividing vegetative cells to sexually competent gametes of two distinct mating types following nutrient deprivation. Gametes of opposite mating type interact via their cilia, initiating an intraciliary signaling cascade and ultimately fuse forming diploid zygotes. The process of gametogenesis is genetically encode, and a previous study revealed numerous significant changes in mRNA abundance during this life-cycle transition. Here we describe a proteomic analysis of cilia derived from vegetative and gametic cells of both mating types in an effort to assess the global changes that occur within the organelle during this process. We identify numerous membrane- and/or matrix-associated proteins in gametic cilia that were not detected in cilia from vegetative cells. This includes the pro-protein from which the GATI-amide gametic chemotactic modulator derives, as well as receptors, a dynamin-related protein, ammonium transporters, two proteins potentially involved in the intraciliary signaling cascade-driven increase in cAMP, and multiple proteins with a variety of interaction domains. These changes in ciliary composition likely directly affect the functional properties of this organelle as the cell transitions between life-cycle stages.

SymRK-dependent phosphorylation of Gα protein and its role in signaling during soybean (Glycine max) nodulation

Heterotrimeric G proteins, comprised of Gα, Gβ and Gγ subunits, influence signaling in most eukaryotes. In metazoans, G proteins are activated by G protein-coupled receptor (GPCR)-mediated GDP to GTP exchange on Gα; however, the role(s) of GPCRs in regulating plant G-protein signaling remains equivocal. Mounting evidence suggests the involvement of receptor-like kinases (RLKs) in regulating plant G-protein signaling, but their mechanistic details remain scarce. We have previously shown that during Glycine max (soybean) nodulation, the nod factor receptor 1 (NFR1) interacts with G-protein components and indirectly affects signaling. We explored the direct regulation of G-protein signaling by RLKs using protein-protein interactions, receptor-mediated in vitro phosphorylations and the effects of such phosphorylations on soybean nodule formation. Results presented in this study demonstrate a direct, phosphorylation-based regulation of Gα by symbiosis receptor kinase (SymRK). SymRKs interact with and phosphorylate Gα at multiple residues in vitro, including two in its active site, which abolishes GTP binding. Additionally, phospho-mimetic Gα fails to interact with Gβγ, potentially allowing for constitutive signaling by the freed Gβγ. These results uncover an unusual mechanism of G-protein cycle regulation in plants where the receptor-mediated phosphorylation of Gα not only affects its activity but also influences the availability of its signaling partners, thereby exerting a two-pronged check on signaling.

Characterization of the heterotrimeric G protein gene families in Triticum aestivum and related species

This study characterizes the heterotrimeric G protein gene families in Triticum aestivum, their tissue-specific expression patterns during development and in response to biotic and abiotic stress conditions. There are three Gα genes, three Gβ and 12 Gγ genes, totaling 18 genes encoding heterotrimeric G proteins in the hexaploid wheat genome. Each haploid genome of the hexaploid T. aestivum has a single gene encoding the α subunit of the heterotrimeric G protein complex, GA1, a single Gβ and four Gγ genes. Each gene has three homeologous copies in the A, B and D genomes. The physical interaction between the Gβ (Gpb) and two Gγ subunits, Gpg1 and Gpg2, was shown through bimolecular fluorescence complementation (BiFC). The gene expression in response to biotic and abiotic stresses showed both up-regulation and down-regulation of members of the gene families. Gγ2-B and Gγ2-D are significantly upregulated during heat stress, GA1-D is upregulated by cold stress and Gγ1-A and Gγ1-D were upregulated by Fusarium graminearum inoculation in a F. graminearum resistant cultivar. This suggests that these members may play roles in biotic and abiotic signaling pathways and the roles of these genes within these pathways need further investigation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03156-9.

Towards resolution of a paradox in plant G-protein signaling

G-proteins are molecular on-off switches that are involved in transmitting a variety of extracellular signals to their intracellular targets. In animal and yeast systems, the switch property is encoded through nucleotides: a GDP-bound state is the "off-state" and the GTP-bound state is the "on-state". The G-protein cycle consists of the switch turning on through nucleotide exchange facilitated by a G-protein coupled receptor and the switch turning off through hydrolysis of GTP back to GDP, facilitated by a protein designated REGULATOR OF G SIGNALING 1 (RGS). In plants, G-protein signaling dramatically differs from that in animals and yeast. Despite stringent conservation of the nucleotide binding and catalytic structures over the 1.6 billion years that separate the evolution of plants and animals, genetic and biochemical data indicate that nucleotide exchange is less critical for this switch to operate in plants. Also, the loss of the single RGS protein in Arabidopsis (Arabidopsis thaliana) confers unexpectedly weaker phenotypes consistent with a diminished role for the G cycle, at least under static conditions. However, under dynamic conditions, genetic ablation of RGS in Arabidopsis results in a strong phenotype. We explore explanations to this conundrum by formulating a mathematical model that takes into account the accruing evidence for the indispensable role of phosphorylation in G-protein signaling in plants and that the G-protein cycle is needed to process dynamic signal inputs. We speculate that the plant G-protein cycle and its attendant components evolved to process dynamic signals through signaling modulation rather than through on-off, switch-like regulation of signaling. This so-called change detection may impart greater fitness for plants due to their sessility in a dynamic light, temperature, and pest environment.

Melatonin functions in priming of stomatal immunity in Panax notoginseng and Arabidopsis thaliana

Melatonin (MT) plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that MT functions in stomatal immunity in Panax notoginseng and Arabidopsis thaliana. Biochemical analyses showed that MT-induced stomatal closure plays a prominent role in preventing invasion of bacteria Pseudomonas syringe pv. tomato (Pst) DC3000 via activation of mitogen-activated protein kinase (MAPK) and NADPH oxidase-mediated reactive oxygen species production in P. notoginseng. The first putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving MT signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and MT-induced MAPK activation in a heterotrimeric GTP-binding protein Gα subunit GPA1-independent manner. GPA1 functions in the same genetic pathways of FLS2/BAK1 (Flagellin Sensing 2/Brassinosteroid Insensitive 1-associated kinase 1)- as well as PMTR1-mediated flg22 and MT signaling in stomatal closure. The stomata in pmtr1 are insensitive to MT and flg22, but the application of MT induces stomatal closure and reduces the bacterial growth in fls2 and bak1 plants, indicating that PMTR1 might be a downstream signaling component in FLS2- and BAK1-mediated stomatal immunity. In summary, our results (i) demonstrate that phytomelatonin functions in the priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.

Heterotrimeric G Proteins in Plants: Canonical and Atypical Gα Subunits

Heterotrimeric GTP-binding proteins (G proteins), consisting of Gα, Gβ and Gγ subunits, transduce signals from a diverse range of extracellular stimuli, resulting in the regulation of numerous cellular and physiological functions in Eukaryotes. According to the classic G protein paradigm established in animal models, the bound guanine nucleotide on a Gα subunit, either guanosine diphosphate (GDP) or guanosine triphosphate (GTP) determines the inactive or active mode, respectively. In plants, there are two types of Gα subunits: canonical Gα subunits structurally similar to their animal counterparts and unconventional extra-large Gα subunits (XLGs) containing a C-terminal domain homologous to the canonical Gα along with an extended N-terminal domain. Both Gα and XLG subunits interact with Gβγ dimers and regulator of G protein signalling (RGS) protein. Plant G proteins are implicated directly or indirectly in developmental processes, stress responses, and innate immunity. It is established that despite the substantial overall similarity between plant and animal Gα subunits, they convey signalling differently including the mechanism by which they are activated. This review emphasizes the unique characteristics of plant Gα subunits and speculates on their unique signalling mechanisms.

Damage-Associated Molecular Patterns (DAMPs) in Plant Innate Immunity: Applying the Danger Model and Evolutionary Perspectives

Danger signals trigger immune responses upon perception by a complex surveillance system. Such signals can originate from the infectious nonself or the damaged self, the latter termed damage-associated molecular patterns (DAMPs). Here, we apply Matzinger's danger model to plant innate immunity to discuss the adaptive advantages of DAMPs and their integration into preexisting signaling pathways. Constitutive DAMPs (cDAMPs), e.g., extracellular ATP, histones, and self-DNA, fulfill primary, conserved functions and adopt a signaling role only when cellular damage causes their fragmentation or localization to aberrant compartments. By contrast, immunomodulatory peptides (also known as phytocytokines) exclusively function as signals and, upon damage, are activated as inducible DAMPs (iDAMPs). Dynamic coevolutionary processes between the signals and their emerging receptors and shared co-receptors have likely linked danger recognition to preexisting, conserved downstream pathways.

Molecular switch architecture determines response properties of signaling pathways

Many intracellular signaling pathways are composed of molecular switches, proteins that transition between two states-on and off Typically, signaling is initiated when an external stimulus activates its cognate receptor that, in turn, causes downstream switches to transition from off to on using one of the following mechanisms: activation, in which the transition rate from the off state to the on state increases; derepression, in which the transition rate from the on state to the off state decreases; and concerted, in which activation and derepression operate simultaneously. We use mathematical modeling to compare these signaling mechanisms in terms of their dose-response curves, response times, and abilities to process upstream fluctuations. Our analysis elucidates several operating principles for molecular switches. First, activation increases the sensitivity of the pathway, whereas derepression decreases sensitivity. Second, activation generates response times that decrease with signal strength, whereas derepression causes response times to increase with signal strength. These opposing features allow the concerted mechanism to not only show dose-response alignment, but also to decouple the response time from stimulus strength. However, these potentially beneficial properties come at the expense of increased susceptibility to upstream fluctuations. We demonstrate that these operating principles also hold when the models are extended to include additional features, such as receptor removal, kinetic proofreading, and cascades of switches. In total, we show how the architecture of molecular switches govern their response properties. We also discuss the biological implications of our findings.

Characterization of ZmCOLD1, novel GPCR-Type G Protein genes involved in cold stress from Zea mays L. and the evolution analysis with those from other species

Maize is one of the most vital staple crops worldwide. G proteins modulate plentiful signaling pathways, and G protein-coupled receptor-type G proteins (GPCRs) are highly conserved membrane proteins in plants. However, researches on maize G proteins and GPCRs are scarce. In this study, we identified three novel GPCR-Type G Protein (GTG) genes from chromosome 10 (Chr 10) in maize, designated as ZmCOLD1-10A, ZmCOLD1-10B and ZmCOLD1-10C. Their amino acid sequences had high similarity to TaCOLD1 from wheat and OsCOLD1 from rice. They contained the basic characteristics of GTG/COLD1 proteins, including GPCR-like topology, the conserved hydrophilic loop (HL) domain, DUF3735 (domain of unknown function 3735) domain, GTPase-activating domain, and ATP/GTP-binding domain. Subcellular localization analyses of ZmCOLD1 proteins suggested that ZmCOLD1 proteins localized on plasma membrane (PM) and endoplasmic reticulum (ER). Furthermore, amino acid sequence alignment verified the conservation of the key 187th amino acid T in maize and other wild maize-relative species. Evolutionary relationship among plants GTG/COLD1 proteins family displayed strong group-specificity. Expression analysis indicated that ZmCOLD1-10A was cold-induced and inhibited by light. Together, these results suggested that ZmCOLD1 genes had potential value to improve cold tolerance and to contribute crops growth and molecular breeding.

Signal transduction by plant heterotrimeric G-protein

Heterotrimeric G-proteins are complexes that regulate important signalling pathways essential for growth and development in both plants and animals. Although plant cells are composed of the core components (Gα, Gβ and Gγ subunits) found in animal G-proteins, the complexities of the architecture, function and signalling mechanisms of those in animals are dissimilar to those identified in some plants. Current studies on plant G-proteins have improved knowledge of the essential physiological and agronomic properties, which when harnessed, could potentially impact global food security. Extensive studies on the molecular mechanisms underlying these properties in diverse plant species will be imperative in improving our current understanding of G-protein signalling pathways involved in plant growth and development. The advancement of G-protein signalling networks in distinct plant species could significantly aid in better crop development. This review summarizes current progress, novel discoveries and future prospects for this area in potential crop improvement.

Heterotrimeric Gα subunit regulates plant architecture, organ size and seed weight in the oilseed Brassica juncea

Two BjuGα proteins exhibit conserved GTP-binding and GTP-hydrolysis activities, and function in maintaining overall plant architecture and controlling multiple yield-related traits in the oilseed Brassica juncea. Heterotrimeric G-protein (Gα, Gβ and Gγ) are key signal transducers, well characterized in model plants Arabidopsis and rice. However, our knowledge about the roles played by G-proteins in regulating various growth and developmental traits in polyploid crops, having a complex G-protein signalling network, is quite sparse. In the present study, two Gα encoding genes (BjuA.Gα1 and BjuB.Gα1) were isolated from the allotetraploid Brassica juncea, a globally cultivated oilseed crop of the Brassicaceae family. BjuGα1 genes share a close evolutionary relationship, and the encoded proteins exhibit highly conserved G-protein activities while showing expression differentiation, wherein BjuA.Gα1 was the highly abundant transcript during plant growth and developmental stages. RNAi based suppression of BjuGα1 displayed compromised effects on most of the tested vegetative and reproductive parameters, particularly plant height (32-58%), flower and siliques dimensions, and seed weight (11-13%). Further, over-expression of a constitutively active Gα, lacking the GTPase activity, produced plants with increased height, organ size and seed weight (7-25%), without altering seed quality traits like fatty acid composition, glucosinolates, oil and protein contents. Our study demonstrates that BjuGα1 proteins control overall plant architecture and multiple yield-related traits in the oilseed B. juncea, suggesting that BjuGα1 could be a promising target for crop improvement.

Pea Gβ subunit of G proteins has a role in nitric oxide-induced stomatal closure in response to heat and drought stress

Heterotrimeric G proteins consisting of Gα, Gβ and Gγ subunits act as downstream effectors to regulate multiple functions including abiotic stress tolerance. However, the mechanism of Gβ-mediated heat and drought tolerance is yet to be established. To explore the role of Pisum sativum Gβ subunit (PsGβ) in heat and drought stress, transgenic tobacco plants overexpressing (OEs) PsGβ were raised. Transgenic plants showing ectopic expression of PsGβ performed better under heat and drought stress in comparison with vector control plants. The seed germination, relative water content (RWC) and nitric oxide (NO) induction in the guard cells of transgenic plants were significantly higher in contrast to control plants. PsGβ promoter was isolated and several stress-responsive elements were identified. The change in Gβ expression in response to heat, methyl jasmonate (MeJA), abscisic acid (ABA), drought and salt confirms the presence of heat, low temperature and drought-responsive elements in the PsGβ promoter. Also, heat and drought stress caused the release of NO-induced stomatal closure in the leaves of transgenic tobacco plants OEs PsGβ. The better performance of transgenic plant OEs PsGβ is also attributed to the improved photosynthetic parameters as compared with control plants. These findings suggest a role of PsGβ in the signalling pathway leading to NO-induced stomatal closure during heat and drought stress.

Heterotrimeric G-proteins mediated hormonal responses in plants

Phytohormones not only orchestrate intrinsic developmental programs from germination to senescence but also regulate environmental inputs through complex signalling pathways. Despite building an own signalling network, hormones mutually contribute several signalling systems, which are also essential for plant growth and development, defense, and responses to abiotic stresses. One of such important signalling cascades is G-proteins, which act as critical regulators of a wide range of fundamental cellular processes by transducing receptor signals to the intracellular environment. G proteins are composed of α, β, and γ subunits, and the molecular switching between active and inactive conformation of Gα controls the signalling cycle. The active GTP bound Gα and freed Gβγ have both independent and tightly coordinated roles in the regulation of effector molecules, thereby modulating multiple responses, including hormonal responses. Therefore, an interplay of hormones with G-proteins fine-tunes multiple biological processes of plants; however, their molecular mechanisms are largely unknown. Functional characterization of hormone biosynthesis, perception, and signalling components, as well as identification of few effector molecules of G-proteins and their interaction networks, reduces the complexity of the hormonal signalling networks related to G-proteins. In this review, we highlight a valuable insight into the mechanisms of how the G-protein signalling cascades connect with hormonal responses to regulate increased developmental flexibility as well as remarkable plasticity of plants.

Plant receptor-like kinase signaling through heterotrimeric G-proteins

Heterotrimeric G-proteins regulate multiple aspects of plant growth, development, and response to biotic and abiotic stresses. While the core components of heterotrimeric G-proteins and their basic biochemistry are similar in plants and metazoans, key differences exist in their regulatory mechanisms. In particular, the activation mechanisms of plant G-proteins appear diverse and may include both canonical and novel modes. Classical G-protein-coupled receptor-like proteins exist in plants and interact with Gα proteins, but their ability to activate Gα by facilitating GDP to GTP exchange has not been demonstrated. Conversely, there is genetic and functional evidence that plant G-proteins interact with the highly prevalent receptor-like kinases (RLKs) and are phosphorylated by them. This suggests the exciting scenario that in plants the G-proteins integrate RLK-dependent signal perception at the plasma membrane with downstream effectors. Because RLKs are active kinases, it is also likely that the activity of plant G-proteins is regulated via phosphorylation/dephosphorylation rather than GTP-GDP exchange as in metazoans. This review discusses our current knowledge of the possible RLK-dependent regulatory mechanisms of plant G-protein signaling in the context of several biological systems and outlines the diversity that might exist in such regulation.

G protein α subunit suppresses sporangium formation through a serine/threonine protein kinase in Phytophthora sojae

Eukaryotic heterotrimeric guanine nucleotide-binding proteins consist of α, β, and γ subunits, which act as molecular switches to regulate a number of fundamental cellular processes. In the oomycete pathogen Phytophthora sojae, the sole G protein α subunit (Gα; encoded by PsGPA1) has been found to be involved in zoospore mobility and virulence, but how it functions remains unclear. In this study, we show that the Gα subunit PsGPA1 directly interacts with PsYPK1, a serine/threonine protein kinase that consists of an N-terminal region with unknown function and a C-terminal region with a conserved catalytic kinase domain. We generated knockout and knockout-complemented strains of PsYPK1 and found that deletion of PsYPK1 resulted in a pronounced reduction in the production of sporangia and oospores, in mycelial growth on nutrient poor medium, and in virulence. PsYPK1 exhibits a cytoplasmic-nuclear localization pattern that is essential for sporangium formation and virulence of P. sojae. Interestingly, PsGPA1 overexpression was found to prevent nuclear localization of PsYPK1 by exclusively binding to the N-terminal region of PsYPK1, therefore accounting for its negative role in sporangium formation. Our data demonstrate that PsGPA1 negatively regulates sporangium formation by repressing the nuclear localization of its downstream kinase PsYPK1.

The role of heterotrimeric G proteins in the control of symbiosis development in legume plants

Heterotrimeric G proteins are involved in the regulation of signaling pathways in eukaryotes. Previously, the data about possible participation of heterotrimeric G proteins in the regulation of nodulation in legumes were obtained, however, specific proteins, their composition and role in this process remain poorly understood. In this work searching of the genes encoding the alpha, beta, and gamma subunits of heterotrimeric G proteins based on an analysis of the Pisum sativum L. genome was performed, as well as the dynamics of the gene expression encoding the particular subunits of G proteins in the process of symbiosis was studied. In addition, a significant effect of beta 1-subunit gene suppression by RNA interference on the nodulation process was revealed.

Nucleotide exchange-dependent and nucleotide exchange-independent functions of plant heterotrimeric GTP-binding proteins

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile, guanine nucleotide-dependent, molecular on-off switches. In animals and fungi, the exchange of GDP for GTP on Gα controls G protein activation and is crucial for normal cellular responses to diverse extracellular signals. The model plant Arabidopsis thaliana has a single canonical Gα subunit, AtGPA1. We found that, in planta, the constitutively active, GTP-bound AtGPA1(Q222L) mutant and the nucleotide-free AtGPA1(S52C) mutant interacted with Gβγ1 and Gβγ2 dimers with similar affinities, suggesting that G protein heterotrimer formation occurred independently of nucleotide exchange. In contrast, AtGPA1(Q222L) had a greater affinity than that of AtGPA1(S52C) for Gβγ3, suggesting that the GTP-bound conformation of AtGPA1(Q222L) is distinct and tightly associated with Gβγ3. Functional analysis of transgenic lines expressing either AtGPA1(S52C) or AtGPA1(Q222L) in the gpa1-null mutant background revealed various mutant phenotypes that were complemented by either AtGPA1(S52C) or AtGPA1(Q222L). We conclude that, in addition to the canonical GDP-GTP exchange-dependent mechanism, plant G proteins can function independently of nucleotide exchange.

The heterotrimeric G protein β subunit RGB1 is required for seedling formation in rice

BACKGROUND

The heterotrimeric G protein β subunit RGB1 plays an important role in plant growth and development. However, the molecular mechanisms underlying the regulation of rice growth by RGB1 remain elusive.

RESULTS

Here, the rgb1 mutants rgb1-1 (+ 1 bp), rgb1-2 (- 1 bp), and rgb1-3 (- 11 bp) were isolated using the CRISPR/Cas9 system, and they were arrested at 1 day after germination and ultimately exhibited seedling lethality. The dynamic anatomical characteristics of the embryos of the rgb1 seedlings and WT during early postgermination and according to TUNEL assays showed that the suppressed growth of the rgb1 mutants was caused by cell death. In addition to the limited shoot and root development, the development of the embryo shoot-root axis was suppressed in the rgb1 mutants. RGB1 was expressed mainly in the root epidermal and vascular tissues of the embryo. Moreover, transcript profiling analysis revealed that the expression of a large number of auxin-, cytokinin-, and brassinosteroid-inducible genes was upregulated or downregulated in the rgb1 mutant compared to the wild type during seedling development.

CONCLUSIONS

Overall, the rgb1 mutants provide an ideal material for exploring the molecular mechanism underlying rice seedling formation during early postgermination development by G proteins.

SIGNIFICANCE STATEMENT

The heterotrimeric G protein β subunit RGB1 acts as a crucial factor in promoting early postgermination seedling development in rice.

Multifaceted plant G protein: interaction network, agronomic potential, and beyond

Heterotrimeric G protein and interacting effectors are relevant for agronomic significance. We can manipulate G protein and effectors, individually or in combination, to develop plant ideotypes by intelligent design breeding. Heterotrimeric guanine nucleotide-binding protein (G protein) is involved in a wide range of biological events, many of which with agronomic significance. In this review, we summarize recent advances of plant G protein research. We first retrieve maize G protein core subunits Gα, Gβ, and Gγ based on information of Arabidopsis and rice G proteins using integrated BLAST and domain confirmation. Then, we briefly introduce the distribution and function of G protein. We also describe the interaction between G protein and CLAVATA receptor, brassinosteroid signaling kinase complex, and MADS-domain transcription factor. Finally, we discuss the application of G protein knowledge in intelligent plant breeding with focus on the improvement of agronomically important traits.

Control of grain size by G protein signaling in rice

Heterotrimeric G proteins are involved in multiple cellular processes in eukaryotes by sensing and transducing various signals. G protein signaling in plants is quite different from that in animals, and the mechanisms of plant G protein signaling are still largely unknown. Several recent studies have provided new insights into the mechanisms of G protein signaling in rice grain size and yield control. In this review, we summarize recent advances on the function of G proteins in rice grain size control and discuss the potential genetic and molecular mechanisms of plant G protein signaling.

Heterotrimeric G-Protein Signaling in Plants: Conserved and Novel Mechanisms

Heterotrimeric GTP-binding proteins are key regulators of a multitude of signaling pathways in all eukaryotes. Although the core G-protein components and their basic biochemistries are broadly conserved throughout evolution, the regulatory mechanisms of G proteins seem to have been rewired in plants to meet specific needs. These proteins are currently the focus of intense research in plants due to their involvement in many agronomically important traits, such as seed yield, organ size regulation, biotic and abiotic stress responses, symbiosis, and nitrogen use efficiency. The availability of massive sequence information from a variety of plant species, extensive biochemical data generated over decades, and impressive genetic resources for plant G proteins have made it possible to examine their role, unique properties, and novel regulation. This review focuses on some recent advances in our understanding of the mechanistic details of this critical signaling pathway to enable the precise manipulation and generation of plants to meet future needs.

The Nucleotide-Dependent Interactome of Rice Heterotrimeric G-Protein α -Subunit

The rice heterotrimeric G-protein complex, a guanine-nucleotide-dependent on-off switch, mediates vital cellular processes and responses to biotic and abiotic stress. Exchange of bound GDP (resting state) for GTP (active state) is spontaneous in plants including rice and thus there is no need for promoting guanine nucleotide exchange in vivo as a mechanism for regulating the active state of signaling as it is well known for animal G signaling. As such, a master regulator controlling the G-protein activation state is unknown in plants. Therefore, an ab initio approach is taken to discover candidate regulators. The rice Gα subunit (RGA1) is used as bait to screen for nucleotide-dependent protein partners. A total of 264 proteins are identified by tandem mass spectrometry of which 32 were specific to the GDP-bound inactive state and 22 specific to the transition state. Approximately, 10% are validated as previously identified G-protein interactors.

GCR1 and GPA1 coupling regulates nitrate, cell wall, immunity and light responses in Arabidopsis

G-protein signaling components have been attributed many biological roles in plants, but the extent of involvement of G-protein coupled receptor 1 (GCR1) with the Gα (GPA1) remained unknown. To address this, we have performed transcriptomic analyses on Arabidopsis gpa1-5gcr1-5 double mutant and identified 656 differentially expressed genes (DEGs). MapMan and Gene Ontology analyses revealed global transcriptional changes associated with external stimulus, cell wall organization/biogenesis and secondary metabolite process among others. Comparative transcriptomic analyses using the single and double mutants of gcr1-5 and gpa1-5 identified 194, 139 and 391 exclusive DEGs respectively, whereas 64 DEGs were common to all three mutants. Further, pair wise comparison of DEGs of double mutant with single mutants of gcr1-5 or gpa1-5 showed about one-third and over half common DEGs, respectively. Further analysis of the DEGs exclusive to the double mutant using protein-protein interaction networks revealed molecular complexes associated with nitrate and light signaling and plant-pathogen interactions among others. Physiological and molecular validation of nitrate-response revealed the sensitivity of germination to low N in the double mutant and differential expression of nitrate transporter (and nitrate reductase in all three mutants). Taken together, GCR1 and GPA1 work in partnership as well as independently to regulate different pathways.

Heterotrimeric G protein signaling in plant immunity

In animals, heterotrimeric guanine nucleotide-binding proteins (G proteins) transduce signals perceived by numerous G protein-coupled receptors (GPCRs). However, no canonical GPCRs with guanine nucleotide exchange factor (GEF) activity are present in plant genomes. Accumulated evidence indicates that, instead of GPCRs, the receptor-like kinases (RLKs) function upstream of G proteins in plants. Regulator of G protein signaling 1 (RGS1) functions to convert the GTP-bound Gα to the GDP-bound form through its GTPase-accelerating protein (GAP) activity. Because of the intrinsic differences in the biochemical properties between Arabidopsis and animal Gα, the actions of animal and Arabidopsis RGS1 result in contrasting outcomes in G signaling activation/deactivation. Animal RGSs accelerate the deactivation of the activated G signaling, whereas Arabidopsis RGS1 prevents the activation of G signaling in the resting state. Phosphorylation of Arabidopsis RGS1 triggered by ligand-RLK recognition results in the endocytosis or degradation of RGS1, leading to the separation of RGS1 from Gα and thus the derepression of G signaling. Here, we summarize the involvement of the G proteins in plant immunity, with a special focus on the molecular mechanism of G signaling activation/deactivation regulated by RLKs and RGS1. We also provide a brief perspective on the outstanding questions that need to be addressed to fully understand G signaling in plant immunity.

Evaluation of differential qPE9-1/DEP1 protein domains in rice grain length and weight variation

BACKGROUND

qPE9-1/DEP1, encoding a G protein γ subunit, has multiple effects on plant architecture, grain size, and yield in rice. The qPE9-1 protein contains an N-terminal G gamma-like (GGL) domain, a putative transmembrane domain, and a C-terminal cysteine-rich domain. However, the roles of each domain remain unclear.

RESULTS

In the present study, we focused on the genetic effects of different domains of qPE9-1 in the regulation of grain length and weight. We generated a series of transgenic plants expressing different truncated qPE9-1 proteins through constitutive expression and clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 strategies. Phenotypic analysis indicated that the complete or long-tailed qPE9-1 contributed to the elongation of grains, while the GGL domain alone and short-tailed qPE9-1 led to short grains. The long C-terminus of qPE9-1 including two or three C-terminal von Willebrand factor type C domains effectively repressed the negative effects of the GGL domain on grain length and weight. qPE9-1-overexpressing lines in a Wuxianggeng 9 (carrying a qpe9-1 allele) background showed increased grain yield per plant, but lodging occurred in some years.

CONCLUSIONS

Manipulation of the C-terminal length of qPE9-1 through genetic engineering can be used to generate varieties with various grain lengths and weights according to different requirements in rice breeding. The genetic effects of qPE9-1/qpe9-1 are multidimensional, and breeders should take into account other factors including genetic backgrounds and planting conditions in the use of qPE9-1/qpe9-1.

Trithorax-group protein ATX5 mediates the glucose response via impacting the HY1-ABI4 signaling module

Trithorax-group Protein ARABIDOPSIS TRITHORAX5 modulates the glucose response. Glucose is an evolutionarily conserved modulator from unicellular microorganisms to multicellular animals and plants. Extensive studies have shown that the Trithorax-group proteins (TrxGs) play essential roles in different biological processes by affecting histone modifications and chromatin structures. However, whether TrxGs function in the glucose response and how they achieve the control of target genes in response to glucose signaling in plants remain unknown. Here, we show that the Trithorax-group Protein ARABIDOPSIS TRITHORAX5 (ATX5) affects the glucose response and signaling. atx5 loss-of-function mutants display glucose-oversensitive phenotypes compared to the wild-type (WT). Genome-wide RNA-sequencing analyses have revealed that ATX5 impacts the expression of a subset of glucose signaling responsive genes. Intriguingly, we have established that ATX5 directly controls the expression of HY1 by trimethylating H3 lysine 4 of the Arabidopsis Heme Oxygenase1 (HY1) locus. Glucose signaling causes the suppression of ATX5 activity and subsequently reduces the H3K4me3 levels at the HY1 locus, thereby leading to the increased expression of ABSCISIC ACID-INSENSITIVE4 (ABI4). This result suggests that an important ATX5-HY1-ABI4 regulatory module governs the glucose response. This idea is further supported by genetic evidence showing that an atx5 hy1-100 abi4 triple mutant showed a similar glucose-insensitive phenotype as compared to that of the abi4 single mutant. Our findings show that a novel ATX5-HY1-ABI4 module controls the glucose response in Arabidopsis thaliana.

Dual phosphorylation of Ric-8A enhances its ability to mediate G protein α subunit folding and to stimulate guanine nucleotide exchange

Resistance to inhibitors of cholinesterase-8A (Ric-8A) and Ric-8B are essential biosynthetic chaperones for heterotrimeric G protein α subunits. We provide evidence for the direct regulation of Ric-8A cellular activity by dual phosphorylation. Using proteomics, Western blotting, and mutational analyses, we determined that Ric-8A was constitutively phosphorylated at five serines and threonines by the protein kinase CK2. Phosphorylation of Ser⁴³⁵ and Thr⁴⁴⁰ in rat Ric-8A (corresponding to Ser⁴³⁶ and Thr⁴⁴¹ in human Ric-8A) was required for high-affinity binding to Gα subunits, efficient stimulation of Gα subunit guanine nucleotide exchange, and mediation of Gα subunit folding. The CK2 consensus sites that contain Ser⁴³⁵ and Thr⁴⁴⁰ are conserved in Ric-8 homologs from worms to mammals. We found that the homologous residues in mouse Ric-8B, Ser⁴⁶⁸ and Ser⁴⁷³, were also phosphorylated. Mutation of the genomic copy of ric-8 in Caenorhabditis elegans to encode alanine in the homologous sites resulted in characteristic ric-8 reduction-of-function phenotypes that are associated with defective G_q and G_s signaling, including reduced locomotion and defective egg laying. The C. elegans ric-8 phosphorylation site mutant phenotypes were partially rescued by chemical stimulation of G_q signaling. These results indicate that dual phosphorylation represents a critical form of conserved Ric-8 regulation and demonstrate that Ric-8 proteins are needed for effective Gα signaling. The position of the CK2-phosphorylated sites within a structural model of Ric-8A reveals that these sites contribute to a key acidic and negatively charged surface that may be important for its interactions with Gα subunits.

Emerging themes in heterotrimeric G-protein signaling in plants

Heterotrimeric G-proteins are key signaling components involved during the regulation of a multitude of growth and developmental pathways in all eukaryotes. Although the core proteins (Gα, Gβ, Gγ subunits) and their basic biochemistries are conserved between plants and non-plant systems, seemingly different inherent properties of specific components, altered wirings of G-protein network architectures, and the presence of novel receptors and effector proteins make plant G-protein signaling mechanisms somewhat distinct from the well-established animal paradigm. G-protein research in plants is getting a lot of attention recently due to the emerging roles of these proteins in controlling many agronomically important traits. New findings on both canonical and novel G-protein components and their conserved and unique signaling mechanisms are expected to improve our understanding of this important module in affecting critical plant growth and development pathways and eventually their utilization to produce plants for the future needs. In this review, we briefly summarize what is currently known in plant G-protein research, describe new findings and how they are changing our perceptions of the field, and discuss important issues that still need to be addressed.