Heterotrimeric G protein-coupled signaling in plants

Cracking the plant VOC sensing code and its practical applications

Volatile organic compounds (VOCs) are essential airborne mediators of interactions between plants. These plant-plant interactions require sophisticated VOC-sensing mechanisms that enable plants to regulate their defenses against pests. However, these interactions are not limited to specific plants or even conspecifics, and can function in very flexible interactions between plants. Sensing and responding to VOCs in plants is finely controlled by their uptake and transport systems as well as by cellular signaling via, for example, chromatin remodeling system-based transcriptional regulation for defense gene activation. Based on the accumulated knowledge about the interactions between plants and their major VOCs, companion plants and biostimulants are being developed for practical applications in agricultural and horticultural pest control, providing a sustainable alternative to harmful chemicals.

The Arabidopsis heterotrimeric G protein α subunit binds to and inhibits the inward rectifying potassium channel KAT1

In animal cells, Gα subunit of the heterotrimeric G proteins can bind to both the N-terminal and C-terminal domains of G-protein-activated inwardly rectifying K+ channels (GIRKs) to inhibit their activities. In Arabidopsis guard cells, the Gα subunit GPA1 mediates multiple stimuli-regulated stomatal movements via inhibiting guard cell inward-rectifying K+ (K+in) current, but it remains unclear whether GPA1 directly interacts with and inhibits the activities of K+in channels. Here, we found that GPA1 interacted with the transmembrane domain rather than the intracellular domain of the Shaker family K+in channel KAT1. Two-Electrode Voltage-Clamp experiments in Xenopus oocytes demonstrated that GPA1 significantly inhibited KAT1 channel activity. However, GPA1 could not inhibit the assembly of KAT1 as well as KAT2 as homo- and hetero-tetramers and alter the subcellular localization and protein stability of these channels. In conclusion, these findings reveal a novel regulatory mechanism for Gα inhibition of the Shaker family K+in channel KAT1 via binding to its channel transmembrane domains but without affecting its subcellular localization, protein stability and the formation of functional homo- and hetero-tetramers. This suggests that in both animal and plant cells, Gα can regulate K+in channels through physical interaction, albeit with differing mechanisms of interaction and regulation.

Biggest of tinies: natural variation in seed size and mineral distribution in the ancient crop tef [Eragrostis tef (Zucc.) Trotter]

Tef [Eragrostis tef (Zucc.) Trotter] is the major staple crop for millions of people in Ethiopia and Eritrea and is believed to have been domesticated several thousand years ago. Tef has the smallest grains of all the cereals, which directly impacts its productivity and presents numerous challenges to its cultivation. In this study, we assessed the natural variation in seed size of 189 tef and 11 accessions of its wild progenitor Indian lovegrass (Eragrostis pilosa (L.) P. Beauv.) and explored the mineral distribution of representative accessions. Our findings revealed significant natural variation in seed size and mineral concentration among both the tef and E. pilosa accessions. We observed significant variation in seed length, seed width, and seed area among the accessions of both Eragrostis spp. we analyzed. Using representative accessions of both species, we also found significant variation in 1000-grain weight. The observed variation in seed size attributes prompted us to use comparative genomics to identify seed size regulating genes based on the well-studied and closely related monocot cereal rice [Oryza sativa (L.)]. Using this approach, we identified putative orthologous genes in the tef genome that belong to a number of key pathways known to regulate seed size in rice. Phylogenetic analysis of putative tef orthologs of ubiquitin-proteasome, G-protein, MAPK, and brassinosteroid (BR)-family genes indicate significant similarity to seed size regulating genes in rice and other cereals. Because tef is known to be more nutrient-dense than other more common cereals such as rice, wheat, and maize, we also studied the mineral concentration of selected accessions using ICP-OES and explored their distribution within the seeds using synchrotron-based X-ray fluorescence (SXRF) microscopy. The findings showed significant variation in seed mineral concentration and mineral distribution among the selected accessions of both Eragrostis spp. This study highlights the natural variation in seed size attributes, mineral concentration, and distribution, while establishing the basis for understanding the genetic mechanisms regulating these traits. We hope our findings will lead to a better understanding of the evolution of tef at the genetic level and for the development of elite tef cultivars to improve seed size, yield, and quality of the grains.

Gβγ dimers mediate low K+ stress-inhibited root growth via modulating auxin redistribution in Arabidopsis

In the investigation of heterotrimeric G protein-mediated signal transduction in planta, their roles in the transmittance of low K+ stimuli remain to be elucidated. Here, we found that the primary root growth of wild-type Arabidopsis was gradually inhibited with the decrease of external K+ concentrations, while the primary root of the mutants for G protein β subunit AGB1 and γ subunits AGG1, AGG2 and AGG3 could still grow under low K+ conditions (LK). Exogenous NAA application attenuated primary root elongation in agb1 and agg1/2/3 but promoted the growth in wild-type seedlings under LK stress. Using ProDR5:GFP, ProPIN1:PIN1-GFP and ProPIN2:PIN2-GFP reporter lines, a diminishment in auxin concentration at the radicle apex and a reduction in PIN1and PIN2 efflux carrier abundance were observed in wild-type roots under LK, a phenomenon not recorded in the agb1 and agg1/2/3. Further proteolytic and transcriptional assessments revealed an enhanced degradation of PIN1 and a suppressed expression of PIN2 in the wild-type background under LK, contrasting with the stability observed in the agb1 and agg1/2/3 mutants. Our results indicate that the G protein β and γ subunits play pivotal roles in suppressing of Arabidopsis root growth under LK by modulating auxin redistribution via alterations in PIN1 degradation and PIN2 biosynthesis.

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.

Arabidopsis AGB1 participates in salinity response through bZIP17-mediated unfolded protein response

BACKGROUND

Plant heterotrimeric G proteins respond to various environmental stresses, including high salinity. It is known that Gβ subunit AGB1 functions in maintaining local and systemic Na+/K+ homeostasis to accommodate ionic toxicity under salt stress. However, whether AGB1 contributes to regulating gene expression for seedling's survival under high salinity remains unclear.

RESULTS

We showed that AGB1-Venus localized to nuclei when facing excessive salt, and the induction of a set of bZIP17-dependent salt stress-responsive genes was reduced in the agb1 mutant. We confirmed both genetic and physical interactions of AGB1 and bZIP17 in plant salinity response by comparing salt responses in the single and double mutants of agb1 and bzip17 and by BiFC assay, respectively. In addition, we show that AGB1 depletion decreases nuclei-localization of transgenic mRFP-bZIP17 under salt stress, as shown in s1p s2p double mutant in the Agrobacteria-mediated transient mRFP-bZIP17 expression in young seedlings.

CONCLUSIONS

Our results indicate that AGB1 functions in S1P and/or S2P-mediated proteolytic processing of bZIP17 under salt stress to regulate the induction of salinity-responsive gene expression.

Phytohormones in a universe of regulatory metabolites: lessons from jasmonate

Small-molecule phytohormones exert control over plant growth, development, and stress responses by coordinating the patterns of gene expression within and between cells. Increasing evidence indicates that currently recognized plant hormones are part of a larger group of regulatory metabolites that have acquired signaling properties during the evolution of land plants. This rich assortment of chemical signals reflects the tremendous diversity of plant secondary metabolism, which offers evolutionary solutions to the daunting challenges of sessility and other unique aspects of plant biology. A major gap in our current understanding of plant regulatory metabolites is the lack of insight into the direct targets of these compounds. Here, we illustrate the blurred distinction between classical phytohormones and other bioactive metabolites by highlighting the major scientific advances that transformed the view of jasmonate from an interesting floral scent to a potent transcriptional regulator. Lessons from jasmonate research generally apply to other phytohormones and thus may help provide a broad understanding of regulatory metabolite-protein interactions. In providing a framework that links small-molecule diversity to transcriptional plasticity, we hope to stimulate future research to explore the evolution, functions, and mechanisms of perception of a broad range of plant regulatory metabolites.

Volatile communication in plants relies on a KAI2-mediated signaling pathway

Plants are constantly exposed to volatile organic compounds (VOCs) that are released during plant-plant communication, within-plant self-signaling, and plant-microbe interactions. Therefore, understanding VOC perception and downstream signaling is vital for unraveling the mechanisms behind information exchange in plants, which remain largely unexplored. Using the hormone-like function of volatile terpenoids in reproductive organ development as a system with a visual marker for communication, we demonstrate that a petunia karrikin-insensitive receptor, PhKAI2ia, stereospecifically perceives the (-)-germacrene D signal, triggering a KAI2-mediated signaling cascade and affecting plant fitness. This study uncovers the role(s) of the intermediate clade of KAI2 receptors, illuminates the involvement of a KAI2ia-dependent signaling pathway in volatile communication, and provides new insights into plant olfaction and the long-standing question about the nature of potential endogenous KAI2 ligand(s).

Heterotrimeric G protein signaling without GPCRs: The Gα-binding-and-activating (GBA) motif

Heterotrimeric G proteins (Gαβγ) are molecular switches that relay signals from 7-transmembrane receptors located at the cell surface to the cytoplasm. The function of these receptors is so intimately linked to heterotrimeric G proteins that they are named G protein-coupled receptors (GPCRs), showcasing the interdependent nature of this archetypical receptor-transducer axis of transmembrane signaling in eukaryotes. It is generally assumed that activation of heterotrimeric G protein signaling occurs exclusively by the action of GPCRs, but this idea has been challenged by the discovery of alternative mechanisms by which G proteins can propagate signals in the cell. This review will focus on a general principle of G protein signaling that operates without the direct involvement of GPCRs. The mechanism of G protein signaling reviewed here is mediated by a class of G protein regulators defined by containing an evolutionarily conserved sequence named the Gα-binding-and-activating (GBA) motif. Using the best characterized proteins with a GBA motif as examples, Gα-interacting vesicle-associated protein (GIV)/Girdin and dishevelled-associating protein with a high frequency of leucine residues (DAPLE), this review will cover (i) the mechanisms by which extracellular cues not relayed by GPCRs promote the coupling of GBA motif-containing regulators with G proteins, (ii) the structural and molecular basis for how GBA motifs interact with Gα subunits to facilitate signaling, (iii) the relevance of this mechanism in different cellular and pathological processes, including cancer and birth defects, and (iv) strategies to manipulate GBA-G protein coupling for experimental therapeutics purposes, including the development of rationally engineered proteins and chemical probes.

Arabidopsis G-protein β subunit AGB1 represses abscisic acid signaling via attenuation of the MPK3-VIP1 phosphorylation cascade

Heterotrimeric G proteins play key roles in cellular processes. Although phenotypic analyses of Arabidopsis Gβ (AGB1) mutants have implicated G proteins in abscisic acid (ABA) signaling, the AGB1-mediated modules involved in ABA responses remain unclear. We found that a partial AGB1 protein was localized to the nucleus where it interacted with ABA-activated VirE2-interacting protein 1 (VIP1) and mitogen-activated protein kinase 3 (MPK3). AGB1 acts as an upstream negative regulator of VIP1 activity by initiating responses to ABA and drought stress, and VIP1 regulates the ABA signaling pathway in an MPK3-dependent manner in Arabidopsis. AGB1 outcompeted VIP1 for interaction with the C-terminus of MPK3, and prevented phosphorylation of VIP1 by MPK3. Importantly, ABA treatment reduced AGB1 expression in the wild type, but increased in vip1 and mpk3 mutants. VIP1 associates with ABA response elements present in the AGB1 promoter, forming a negative feedback regulatory loop. Thus, our study defines a new mechanism for fine-tuning ABA signaling through the interplay between AGB1 and MPK3-VIP1. Furthermore, it suggests a common G protein mechanism to receive and transduce signals from the external environment.

Phosphorylation Dynamics in a flg22-Induced, G Protein-Dependent Network Reveals the AtRGS1 Phosphatase

The microbe-associated molecular pattern flg22 is recognized in a flagellin-sensitive 2-dependent manner in root tip cells. Here, we show a rapid and massive change in protein abundance and phosphorylation state of the Arabidopsis root cell proteome in WT and a mutant deficient in heterotrimeric G-protein-coupled signaling. flg22-induced changes fall on proteins comprising a subset of this proteome, the heterotrimeric G protein interactome, and on highly-populated hubs of the immunity network. Approximately 95% of the phosphorylation changes in the heterotrimeric G-protein interactome depend, at least partially, on a functional G protein complex. One member of this interactome is ATBα, a substrate-recognition subunit of a protein phosphatase 2A complex and an interactor to Arabidopsis thaliana Regulator of G Signaling 1 protein (AtRGS1), a flg22-phosphorylated, 7-transmembrane spanning modulator of the nucleotide-binding state of the core G-protein complex. A null mutation of ATBα strongly increases basal endocytosis of AtRGS1. AtRGS1 steady-state protein level is lower in the atbα mutant in a proteasome-dependent manner. We propose that phosphorylation-dependent endocytosis of AtRGS1 is part of the mechanism to degrade AtRGS1, thus sustaining activation of the heterotrimeric G protein complex required for the regulation of system dynamics in innate immunity. The PP2A(ATBα) complex is a critical regulator of this signaling pathway.

Conserved Role of Heterotrimeric G Proteins in Plant Defense and Cell Death Progression

Programmed cell death (PCD) is a critical process in plant immunity, enabling the targeted elimination of infected cells to prevent the spread of pathogens. The tight regulation of PCD within plant cells is well-documented; however, specific mechanisms remain elusive or controversial. Heterotrimeric G proteins are multifunctional signaling elements consisting of three distinct subunits, Gα, Gβ, and Gγ. In Arabidopsis, the Gβγ dimer serves as a positive regulator of plant defense. Conversely, in species such as rice, maize, cotton, and tomato, mutants deficient in Gβ exhibit constitutively active defense responses, suggesting a contrasting negative role for Gβ in defense mechanisms within these plants. Using a transient overexpression approach in addition to knockout mutants, we observed that Gβγ enhanced cell death progression and elevated the accumulation of reactive oxygen species in a similar manner across Arabidopsis, tomato, and Nicotiana benthamiana, suggesting a conserved G protein role in PCD regulation among diverse plant species. The enhancement of PCD progression was cooperatively regulated by Gβγ and one Gα, XLG2. We hypothesize that G proteins participate in two distinct mechanisms regulating the initiation and progression of PCD in plants. We speculate that G proteins may act as guardees, the absence of which triggers PCD. However, in Arabidopsis, this G protein guarding mechanism appears to have been lost in the course of evolution.

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.

Heterotrimeric G Protein-Mediated Signaling Is Involved in Stress-Mediated Growth Inhibition in Arabidopsis thaliana

Heterotrimeric G protein-mediated signaling plays a vital role in physiological and developmental processes in eukaryotes. On the other hand, because of the absence of a G protein-coupled receptor and self-activating mechanism of the Gα subunit, plants appear to have different regulatory mechanisms, which remain to be elucidated, compared to canonical G protein signaling established in animals. Here we report that Arabidopsis heterotrimeric G protein subunits, such as Gα (GPA1) and Gβ (AGB1), regulate plant growth under stress conditions through the analysis of heterotrimeric G protein mutants. Flg22-mediated growth inhibition in wild-type roots was found to be caused by a defect in the elongation zone, which was partially blocked in agb1-2 but not gpa1-4. These results suggest that AGB1 may negatively regulate plant growth under biotic stress conditions. In addition, GPA1 and AGB1 exhibited genetically opposite effects on FCA-mediated growth inhibition under heat stress conditions. Therefore, these results suggest that plant G protein signaling is probably related to stress-mediated growth regulation for developmental plasticity in response to biotic and abiotic stress conditions.

Light intensity-mediated auxin homeostasis in spikelets links carbohydrate metabolism enzymes with grain filling rate in rice

Low light (LL) stress during the grain-filling stage acutely impairs the quality and quantity of starch accumulation in rice grains. Here, we observed that LL-induced poor starch biosynthesis is modulated by auxin homeostasis, which regulates the activities of major carbohydrate metabolism enzymes such as starch synthase (SS) and ADP-glucose pyrophosphorylase (AGPase) in rice. Further, during the grain-filling period under LL, the starch/sucrose ratio increased in leaves but significantly decreased in the developing spikelets. This suggests poor sucrose biosynthesis in leaves and starch in the grains of the rice under LL. A lower grain starch was found to be correlated with the depleted AGPase and SS activities in the developing rice grains under LL. Further, under LL, the endogenous auxin (IAA) level in the spikelets was found to be synchronized with the expression of a heteromeric G protein gene, RGB1. Interestingly, under LL, the expression of OsYUC11 was significantly downregulated, which subsequently resulted in reduced IAA in the developing rice spikelets, followed by poor activation of grain-filling enzymes. This resulted in lowered grain starch accumulation, grain weight, panicle number, spikelet fertility, and eventually grain yield, which was notably higher in the LL-susceptible (GR4, IR8) than in the LL-tolerant (Purnendu, Swarnaprabha) rice genotypes. Therefore, we hypothesize that depletion in auxin biosynthesis under LL stress is associated with the downregulation of RBG1, which discourages the expression and activities of grain-filling enzymes, resulting in lower starch production, panicle formation, and grain yield in rice.

Model of ligand-triggered information transmission in G-protein coupled receptor complexes

We present a model for the effects of ligands on information transmission in G-Protein Coupled Receptor (GPCR) complexes. The model is built ab initio entirely on principles of statistical mechanics and tenets of information transmission theory and was validated in part using agonist-induced effector activity and signaling bias for the angiotensin- and adrenergic-mediated signaling pathways, with in vitro observations of phosphorylation sites on the C tail of the GPCR complex, and single-cell information-transmission experiments. The model extends traditional kinetic models that form the basis for many existing models of GPCR signaling. It is based on maximizing the rates of entropy production and information transmission through the GPCR complex. The model predicts that (1) phosphatase-catalyzed reactions, as opposed to kinase-catalyzed reactions, on the C-tail and internal loops of the GPCR are responsible for controlling the signaling activity, (2) signaling favors the statistical balance of the number of switches in the ON state and the number in the OFF state, and (3) biased-signaling response depends discontinuously on ligand concentration.

G-Protein β-Subunit Gene TaGB1-B Enhances Drought and Salt Resistance in Wheat

In the hexaploid wheat genome, there are three Gα genes, three Gβ and twelve Gγ genes, but the function of Gβ in wheat has not been explored. In this study, we obtained the overexpression of TaGB1 Arabidopsis plants through inflorescence infection, and the overexpression of wheat lines was obtained by gene bombardment. The results showed that under drought and NaCl treatment, the survival rate of Arabidopsis seedlings' overexpression of TaGB1-B was higher than that of the wild type, while the survival rate of the related mutant agb1-2 was lower than that of the wild type. The survival rate of wheat seedlings with TaGB1-B overexpression was higher than that of the control. In addition, under drought and salt stress, the levels of superoxide dismutase (SOD) and proline (Pro) in the wheat overexpression of TaGB1-B were higher than that of the control, and the concentration of malondialdehyde (MDA) was lower than that of the control. This indicates that TaGB1-B could improve the drought resistance and salt tolerance of Arabidopsis and wheat by scavenging active oxygen. Overall, this work provides a theoretical basis for wheat G-protein β-subunits in a further study, and new genetic resources for the cultivation of drought-tolerant and salt-tolerant wheat varieties.

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids - including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids - also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.

The interplay of GTP-binding protein AGB1 with ER stress sensors IRE1a and IRE1b modulates Arabidopsis unfolded protein response and bacterial immunity

In eukaryotic cells, the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) results in ER stress that induces a cascade of reactions called the unfolded protein response (UPR). In Arabidopsis, the most conserved UPR sensor, Inositol-requiring enzyme 1 (IRE1), responds to both abiotic- and biotic-induced ER stress. Guanine nucleotide-binding proteins (G proteins) constitute another universal and conserved family of signal transducers that have been extensively investigated due to their ubiquitous presence and diverse nature of action. Arabidopsis GTP-binding protein β1 (AGB1) is the only G-protein β-subunit encoded by the Arabidopsis genome that is involved in numerous signaling pathways. Mounting evidence suggests the existence of a crosstalk between IRE1 and G protein signaling during ER stress. AGB1 has previously been shown to control a distinct UPR pathway independently of IRE1 when treated with an ER stress inducer tunicamycin. Our results obtained with combinatorial knockout mutants support the hypothesis that both IRE1 and AGB1 synergistically contribute to ER stress responses chemically induced by dithiothreitol (DTT) as well as to the immune responses against a phytopathogenic bacterium Pseudomonas syringae pv. tomato strain DC3000. Our study highlights the crosstalk between the plant UPR transducers under abiotic and biotic stress.

Mitochondrial GPAT-derived LPA controls auxin-dependent embryonic and postembryonic development

Membrane properties are emerging as important cues for the spatiotemporal regulation of hormone signaling. Lysophosphatidic acid (LPA) evokes multiple biological responses by activating G protein-coupled receptors in mammals. In this study, we demonstrated that LPA derived from the mitochondrial glycerol-3-phosphate acyltransferases GPAT1 and GPAT2 is a critical lipid-based cue for auxin-controlled embryogenesis and plant growth in Arabidopsis thaliana. LPA levels decreased, and the polarity of the auxin efflux carrier PIN-FORMED1 (PIN1) at the plasma membrane (PM) was defective in the gpat1 gpat2 mutant. As a consequence of distribution defects, instructive auxin gradients and embryonic and postembryonic development are severely compromised. Further cellular and genetic analyses revealed that LPA binds directly to PIN1, facilitating the vesicular trafficking of PIN1 and polar auxin transport. Our data support a model in which LPA provides a lipid landmark that specifies membrane identity and cell polarity, revealing an unrecognized aspect of phospholipid patterns connecting hormone signaling with development.

G protein controls stress readiness by modulating transcriptional and metabolic homeostasis in Arabidopsis thaliana and Marchantia polymorpha

The core G protein signaling module, which consists of Gα and extra-large Gα (XLG) subunits coupled with the Gβγ dimer, is a master regulator of various stress responses. In this study, we compared the basal and salt stress-induced transcriptomic, metabolomic and phenotypic profiles in Gα, Gβ, and XLG-null mutants of two plant species, Arabidopsis thaliana and Marchantia polymorpha, and showed that G protein mediates the shift of transcriptional and metabolic homeostasis to stress readiness status. We demonstrated that such stress readiness serves as an intrinsic protection mechanism against further stressors through enhancing the phenylpropanoid pathway and abscisic acid responses. Furthermore, WRKY transcription factors were identified as key intermediates of G protein-mediated homeostatic shifts. Statistical and mathematical model comparisons between A. thaliana and M. polymorpha revealed evolutionary conservation of transcriptional and metabolic networks over land plant evolution, whereas divergence has occurred in the function of plant-specific atypical XLG subunit. Taken together, our results indicate that the shifts in transcriptional and metabolic homeostasis at least partially act as the mechanisms of G protein-coupled stress responses that are conserved between two distantly related plants.

The caleosin CLO7 and its role in the heterotrimeric G-protein signalling network

The investigation of the caleosin CLO7 in relation to heterotrimeric G-protein signalling in Arabidopsis showed that the gene plays a role in seed germination and embryo viability. The caleosin CLO7 belongs to a multi-gene family of calcium-binding proteins which are characterized by single EF-hand motifs. Other members of the caleosin gene family have been shown to affect transpiration and seed germination as well as play a role in both abiotic and biotic stress responses. The proteins are associated with lipid droplets/oil bodies and some members of the gene family have been shown to have peroxygenase activity. Members of the gene family have also been shown to interact with the α subunit of the heterotrimeric G protein complex. In this study, we further expand on the diversity of physiological responses in which members of this gene family play regulatory roles. Utilizing BiFC and Y2H protein-protein interaction assays, CLO7 is identified as an interactor of the heterotrimeric G protein α subunit, GPA1. The full-length CLO7 is shown to interact with both the wild-type GPA1 and its constitutively active form, GPA1^QL, at the plasma membrane. Point mutations to critical amino acids for calcium binding in the EF-hand of CLO7 indicate that the interaction with GPA1 is calcium-dependent and that the interaction with GPA1^QL is enhanced by calcium. Protein-protein interaction assays also show that CLO7 interacts with Pirin1, a member of the cupin gene superfamily and a known downstream effector of GPA1, and this interaction is calcium-dependent. The N-terminal portion of CLO7 is responsible for these interactions. GFP-tagged CLO7 protein localizes to the endoplasmic reticulum (ER) and to lipid bodies. Characterization of the clo7 mutant line has shown that CLO7 is implicated in the abscisic acid (ABA) and mannitol-mediated inhibition of seed germination, with the clo7 mutant displaying higher germination rates in response to osmotic stress and ABA hormone treatment. These results provide insight into the role of CLO7 in seed germination in response to abiotic stress as well as its interaction with GPA1 and Pirin1. CLO7 also plays a role in embryo viability with the clo7gpa1 double mutant displaying embryo lethality, and therefore the double mutant cannot be recovered.

Study on exogenous application of thidiazuron on seed size of Brassica napus L

Thidiazuron (TDZ) is a novel and efficient cytokinin commonly used in tissue culture, and numerous studies have demonstrated that TDZ can increase berry size. However, no study to date has explored the effect of TDZ on seed size of Brassica napus and the mechanism. To shed light on the effect of TDZ on the seed size of B. napus, four different concentrations of TDZ were applied to B. napus. Results indicated that TDZ treatment could increase the seed diameter and silique length of B. napus to varying degrees and 100 and 200 μmol/L TDZ treatments were the most effective with a 3.6 and 4.6% increase in seed diameter, respectively. In addition, the yield of B. napus was also substantially increased under TDZ treatment. On the other hand, confocal micrographs of embryos and cotyledon cells suggested that embryos and their cotyledon epidermal cells treated with 200 μmol/L TDZ were obviously larger in size than the control. Furthermore, TDZ promoted the upregulation of some key maternal tissue growth-related genes, including two G-protein signaling genes (AGG3 and RGA1) and two transcriptional regulators (ANT and GS2). The expression analysis of genes related to the auxin metabolic pathways, G-protein signaling, endosperm growth and transcriptional regulators confirmed that treatment with TDZ negatively regulated the key genes ABI5, AGB1, AP2, ARF2, and ARF18 during bud development stage and florescence. The results strongly suggested that TDZ might regulate the transcriptional levels of key genes involved in auxin metabolic pathways, G-protein signaling, endosperm growth and transcriptional regulators, which resulted in bigger cotyledon epidermal cells and seed size in B. napus. This study explored the mechanism of TDZ treatment on the seed size of B. napus and provided an important reference for improving rapeseed yield.

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.

G-Protein Phosphorylation: Aspects of Binding Specificity and Function in the Plant Kingdom

Plant survival depends on adaptive mechanisms that constantly rely on signal recognition and transduction. The predominant class of signal discriminators is receptor kinases, with a vast member composition in plants. The transduction of signals occurs in part by a simple repertoire of heterotrimeric G proteins, with a core composed of α-, β-, and γ-subunits, together with a 7-transmembrane Regulator G Signaling (RGS) protein. With a small repertoire of G proteins in plants, phosphorylation by receptor kinases is critical in regulating the active state of the G-protein complex. This review describes the in vivo detected phosphosites in plant G proteins and conservation scores, and their in vitro corresponding kinases. Furthermore, recently described outcomes, including novel arrestin-like internalization of RGS and a non-canonical phosphorylation switching mechanism that drives G-protein plasticity, are discussed.

FFGA1 Protein Is Essential for Regulating Vegetative Growth, Cell Wall Integrity, and Protection against Stress in Flammunina filiformis

Flammulina filiformis is a popular mushroom which has been regarded as a potential model fungus for mycelium growth, fruiting body development, and stress response studies. Based on a genome-wide search, four genes encoding heterotrimeric G protein α subunits were identified in F. filiformis. The data of conserved domain analysis showed that these genes contain only one subgroup I of Gα subunit (Gαi), similar to many other fungi. To explore the function of Gαi, FfGa1 over-expression (OE) and RNA interference (RNAi) strains were generated using the Agrobacterium tumefaciens-mediated transformation (ATMT) approach. RNAi strains showed remarkably reduced growth on PDA medium and sensitivity to cell wall-perturbing agents, with maximum growth inhibition, but showed better growth in response to hypertonic stress-causing agents, while OE strains exhibited more resistance to thermal stress and mycoparasite Trichoderma as compared to the wild-type and RNAi strains. Taken together, our results indicated that FfGa1 positively regulates hyphal extension, and is crucial for the maintenance of cell wall integrity and protection against biotic and abiotic (hypertonic and thermal) stress.

Rice extra-large G proteins play pivotal roles in controlling disease resistance and yield-related traits

To better explore the potential of rice extra-large G (XLG) proteins in future breeding, we characterised the function of OsXLG1, OsXLG2 and OsXLG3 in disease resistance. Loss-of-function Osxlg2 and Osxlg3 mutants showed reduced resistance to the fungal pathogen Magnaporthe oryzae, whereas Osxlg1 mutants were specifically compromised in resistance to the bacterial pathogen Xanthomonas oryzae pv oryzae. Consistent with their effects on rice blast resistance, mutations in OsXLG2 and OsXLG3 caused greater defects than did mutations in OsXLG1 for chitin-induced defence responses. All three OsXLGs interacted with components of a surface immune receptor complex composed of OsCERK1, OsRLCK176 and OsRLCK185. Further characterisation of yield-related traits showed that the Osxlg3 mutants displayed reduced plant height, panicle length and 1000grain weight, whereas Osxlg1 mutants exhibited increased plant height, panicle length and 1000-grain weight. Together the study shows the differential contributions of the three OsXLG proteins to disease resistance to fungal and bacterial pathogens, their yield-related traits and provides insights for future improvement of rice production.

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.

Natural variation in Glume Coverage 1 causes naked grains in sorghum

One of the most critical steps in cereal threshing is the ease with which seeds are detached from sticky glumes. Naked grains with low glume coverage have dramatically increased threshing efficiency and seed quality. Here, we demonstrate that GC1 (Glume Coverage 1), encoding an atypical G protein γ subunit, negatively regulates sorghum glume coverage. Naturally truncated variations of GC1 C-terminus accumulate at higher protein levels and affect the stability of a patatin-related phospholipase SbpPLAII-1. A strong positive selection signature around the GC1 genic region is found in the naked sorghum cultivars. Our findings reveal a crucial event during sorghum domestication through a subtle regulation of glume development by GC1 C-terminus variation, and establish a strategy for future breeding of naked grains.

Low glume coverage is the preferred for easy threshing in grain production, but the genetic basis remains unclear. Here, the authors report the gene GC1, which encodes an atypical G protein γ subunit, negatively regulates sorghum glume coverage and the naturally truncated alleles can be useful in the naked grain breeding.

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.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

The Role of Heterotrimeric G-Protein Beta Subunits During Nodulation in Medicago truncatula Gaertn and Pisum sativum L

Heterotrimeric G-proteins regulate plant growth and development as master regulators of signaling pathways. In legumes with indeterminate nodules (e.g., Medicago truncatula and Pisum sativum), the role of heterotrimeric G-proteins in symbiosis development has not been investigated extensively. Here, the involvement of heterotrimeric G-proteins in M. truncatula and P. sativum nodulation was evaluated. A genome-based search for G-protein subunit-coding genes revealed that M. truncatula and P. sativum harbored only one gene each for encoding the canonical heterotrimeric G-protein beta subunits, MtG beta 1 and PsG beta 1, respectively. RNAi-based suppression of MtGbeta1 and PsGbeta1 significantly decreased the number of nodules formed, suggesting the involvement of G-protein beta subunits in symbiosis in both legumes. Analysis of composite M. truncatula plants carrying the pMtGbeta1:GUS construct showed β-glucuronidase (GUS) staining in developing nodule primordia and young nodules, consistent with data on the role of G-proteins in controlling organ development and cell proliferation. In mature nodules, GUS staining was the most intense in the meristem and invasion zone (II), while it was less prominent in the apical part of the nitrogen-fixing zone (III). Thus, MtG beta 1 may be involved in the maintenance of meristem development and regulation of the infection process during symbiosis. Protein-protein interaction studies using co-immunoprecipitation revealed the possible composition of G-protein complexes and interaction of G-protein subunits with phospholipase C (PLC), suggesting a cross-talk between G-protein- and PLC-mediated signaling pathways in these legumes. Our findings provide direct evidence regarding the role of MtG beta 1 and PsG beta 1 in symbiosis development regulation.

Tomato and cotton G protein beta subunit mutants display constitutive autoimmune responses

Heterotrimeric G protein Gβ-deficient mutants in rice and maize display constitutive immune responses, whereas Arabidopsis Gβ mutants show impaired defense, suggesting the existence of functional differences between monocots and dicots. Using CRISPR/Cas9, we produced one hemizygous tomato line with a mutated SlGB1 Gβ gene. Homozygous slgb1 knockout mutants exhibit all the hallmarks of autoimmune mutants, including development of necrotic lesions, constitutive expression of defense-related genes, and high endogenous levels of salicylic acid (SA) and reactive oxygen species, resulting in early seedling lethality. Virus-induced silencing of Gβ in cotton reproduced the symptoms observed in tomato mutants, confirming that the autoimmune phenotype is not limited to monocot species but is also shared by dicots. Even though multiple genes involved in SA and ethylene signaling are highly induced by Gβ silencing in tomato and cotton, co-silencing of SA or ethylene signaling components in cotton failed to suppress the lethal phenotype, whereas co-silencing of the oxidative burst oxidase RbohD can repress lethality. Despite the autoimmune response observed in slgb1 mutants, we show that SlGB1 is a positive regulator of the pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) response in tomato. We speculate that the phenotypic differences observed between Arabidopsis and tomato/cotton/rice/maize Gβ knockouts do not necessarily reflect divergences in G protein-mediated defense mechanisms.

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.

Genome‑wide characterization of the Gα subunit gene family in Rosaceae and expression analysis of PbrGPAs under heat stress

The Gα subunit is an important component of the heterotrimeric G-protein complex and an integral component of several signal transduction pathways. It plays crucial roles in the diverse processes of plant growth and development, including the response to abiotic stress, regulation of root development, involvement in stomatal movement, and participation in hormone responses, which have been well investigated in many species. However, no comprehensive analysis has identified and explored the evolution, expression pattern characteristics and heat stress response of the Gα subunit genes in Rosaceae. In this study, 52 Gα subunit genes were identified in eight Rosaceae species; these genes were divided into three subfamilies (I, II, and III) based on their phylogenetic, conserved motif, and structural characteristics. Whole genome and dispersed duplication events were found to have contributed significantly to the expansion of the Gα subunit gene family, and purifying selection to have played a key role in the evolution of Gα subunit genes. An expression analysis identified some PbrGPA genes that were highly expressed in leaf, root, and fruit, and exhibited diverse spatiotemporal expression models in pear. Under abiotic stress conditions, the mRNA transcript levels of PbrGPA genes were up-regulated in response to high temperature treatment in leaves. Furthermore, three Gα subunit genes were shown to be located in the plasma membrane and nucleus in pear. In conclusion, the study of the Gα subunit gene family will help us to better understand its evolutionary history and expression patterns, while facilitating further investigations into the function of the Gα subunit gene in response to heat stress.

Alternative Splicing of TaGS3 Differentially Regulates Grain Weight and Size in Bread Wheat

The heterotrimeric G-protein mediates growth and development by perceiving and transmitting signals in multiple organisms. Alternative splicing (AS), a vital process for regulating gene expression at the post-transcriptional level, plays a significant role in plant adaptation and evolution. Here, we identified five splicing variants of G_γ subunit gene TaGS3 (TaGS3.1 to TaGS3.5), which showed expression divergence during wheat polyploidization, and differential function in grain weight and size determination. TaGS3.1 overexpression significantly reduced grain weight by 5.89% and grain length by 5.04%, while TaGS3.2–3.4 overexpression did not significantly alter grain size compared to wild type. Overexpressing TaGS3.5 significantly increased the grain weight by 5.70% and grain length by 4.30%. Biochemical assays revealed that TaGS3 isoforms (TaGS3.1–3.4) with an intact OSR domain interact with WGB1 to form active G_βγ heterodimers that further interact with WGA1 to form inactive G_αβγ heterotrimers. Truncated isoforms TaGS3.2–3.4 , which lack the C-terminal Cys-rich region but have enhanced binding affinity to WGB1, antagonistically compete with TaGS3.1 to bind WGB1, while TaGS3.5 with an incomplete OSR domain does not interact with WGB1. Taking these observations together, we proposed that TaGS3 differentially regulates grain size via AS, providing a strategy by which the grain size is fine-tuned and regulated at the post-transcriptional level.

Rice G protein γ subunit qPE9-1 modulates root elongation for phosphorus uptake by involving 14-3-3 protein OsGF14b and plasma membrane H+ -ATPase

Heterotrimeric G protein is involved in plant growth and development, while the role of rice (Oryza sativa) G protein γ subunit qPE9-1 in response to low-phosphorus (LP) conditions remains unclear. The gene expression of qPE9-1 was significantly induced in rice roots under LP conditions. Rice varieties carrying the qPE9-1 allele showed a stronger primary root response to LP than the varieties carrying the qpe9-1 allele (mutant of the qPE9-1 allele). Transgenic rice plants with the qPE9-1 allele had longer primary roots and higher P concentrations than those with the qpe9-1 allele under LP conditions. The plasma membrane (PM) H⁺ -ATPase was important for the qPE9-1-mediated response to LP. Furthermore, OsGF14b, a 14-3-3 protein that acts as a key component in activating PM H⁺ -ATPase for root elongation, is also involved in the qPE9-1 mediation. Moreover, the overexpression of OsGF14b in WYJ8 (carrying the qpe9-1 allele) partially increased primary root length under LP conditions. Experiments using R18 peptide (a 14-3-3 protein inhibitor) showed that qPE9-1 is important for primary root elongation and H⁺ efflux under LP conditions by involving the 14-3-3 protein. In addition, rhizosheath weight, total P content, and the rhizosheath soil Olsen-P concentration of qPE9-1 lines were higher than those of qpe9-1 lines under soil drying and LP conditions. These results suggest that the G protein γ subunit qPE9-1 in rice plants modulates root elongation for phosphorus uptake by involving the 14-3-3 protein OsGF14b and PM H⁺ -ATPase, which is required for rice P use.

Arabidopsis G-Protein β Subunit AGB1 Negatively Regulates DNA Binding of MYB62, a Suppressor in the Gibberellin Pathway

Plant G proteins are versatile components of transmembrane signaling transduction pathways. The deficient mutant of heterotrimeric G protein leads to defects in plant growth and development, suggesting that it regulates the GA pathway in Arabidopsis. However, the molecular mechanism of G protein regulation of the GA pathway is not understood in plants. In this study, two G protein β subunit (AGB1) mutants, agb1-2 and N692967, were dwarfed after exogenous application of GA₃. AGB1 interacts with the DNA-binding domain MYB62, a GA pathway suppressor. Transgenic plants were obtained through overexpression of MYB62 in two backgrounds including the wild-type (MYB62/WT Col-0) and agb1 mutants (MYB62/agb1) in Arabidopsis. Genetic analysis showed that under GA₃ treatment, the height of the transgenic plants MYB62/WT and MYB62/agb1 was lower than that of WT. The height of MYB62/agb1 plants was closer to MYB62/WT plants and higher than that of mutants agb1-2 and N692967, suggesting that MYB62 is downstream of AGB1 in the GA pathway. qRT-PCR and competitive DNA binding assays indicated that MYB62 can bind MYB elements in the promoter of GA2ox7, a GA degradation gene, to activate GA2ox7 transcription. AGB1 affected binding of MYB62 on the promoter of GA2ox7, thereby negatively regulating th eactivity of MYB62.

G protein γ subunit qPE9-1 is involved in rice adaptation under elevated CO2 concentration by regulating leaf photosynthesis

G protein γ subunit qPE9-1 plays multiple roles in rice growth and development. However, the role of qPE9-1 in rice exposed to elevated carbon dioxide concentration (eCO₂) is unknown. Here, we investigated its role in the regulation of rice growth under eCO₂ conditions using qPE9-1 overexpression (OE) lines, RNAi lines and corresponding WT rice. Compared to atmospheric carbon dioxide concentration (aCO₂), relative expression of qPE9-1 in rice leaf was approximately tenfold higher under eCO₂. Under eCO₂, the growth of WT and qPE9-1-overexpressing rice was significantly higher than under aCO₂. Moreover, there was no significant effect of eCO₂ on the growth of qPE9-1 RNAi lines. Furthermore, WT and qPE9-1-overexpressing rice showed higher net photosynthetic rate and carbohydrate content under eCO₂ than under aCO₂. Moreover, the relative expression of some photosynthesis related genes in WT, but not in RNAi3 line, showed significant difference under eCO₂ in RNA-seq analysis. Compared to WT and RNAi lines, the rbcL gene expression and Rubisco content of rice leaves in qPE9-1-overexpressors were higher under eCO₂. Overall, these results suggest that qPE9-1 is involved in rice adaptation under elevated CO₂ concentration by regulating leaf photosynthesis via moderating rice photosynthetic light reaction and Rubisco content.

Lipid Signaling through G Proteins

N-Acylethanolamine (NAE) signaling has received considerable attention in vertebrates as part of the endocannabinoid signaling system, where anandamide acts as a ligand for G protein-coupled cannabinoid receptors. Recent studies indicate that G proteins also are required for some types of NAE signaling in plants. The genetic ablation of the Gβγ dimer or loss of the full set of extra-large G proteins strongly attenuated NAE-induced chloroplast responses in seedlings. Intriguing parallels and distinct differences have emerged between plants and animals in NAE signaling, despite the conserved use of these lipid mediators to modulate cellular processes. Here we compare similarities and differences and identify open questions in a fundamental lipid signaling pathway in eukaryotes with components that are both conserved and diverged in plants.

Heterotrimeric G protein signalling in plant biotic and abiotic stress response

Heterotrimeric G proteins act as molecular switches to participate in transmitting various stimuli signals from outside of cells. G proteins have three subunits, Gα, Gβ and Gγ, which function mutually to modulate many biological processes in plants, including plant growth and development, as well as biotic and abiotic stress responses. In plants, the number of Gγ subunits is larger than that of the α and β subunits. Based on recent breakthroughs in studies of plant G protein signal perception, transduction and downstream effectors, this review summarizes and analyses the connections between different subunits and the interactions of G proteins with other signalling pathways, especially in plant biotic and abiotic stress responses. Based on current progress and unresolved questions in the field, we also suggest future research directions on G proteins in plants.

Plant RABs: Role in Development and in Abiotic and Biotic Stress Responses

Endosomal trafficking plays an integral role in various eukaryotic cellular activities and is vital for higher-order functions in multicellular organisms. RAB GTPases are important proteins that influence various aspects of membrane traffic, which consequently influence many cellular functions and responses. Compared to yeast and mammals, plants have evolved a unique set of plant-specific RABs that play a significant role in their development. RABs form the largest family of small guanosine triphosphate (GTP)-binding proteins, and are divided into eight sub-families named RAB1, RAB2, RAB5, RAB6, RAB7, RAB8, RAB11 and RAB18. Recent studies on different species suggest that RAB proteins play crucial roles in intracellular trafficking and cytokinesis, in autophagy, plant microbe interactions and in biotic and abiotic stress responses. This review recaptures and summarizes the roles of RABs in plant cell functions and in enhancing plant survival under stress conditions.

Reported differences in the flg22 response of the null mutation of AtRGS1 correlates with fixed genetic variation in the background of Col-0 isolates

A role for the heterotrimeric G protein complex in the induction of a transient burst of reactive oxygen species (ROS) by the Microbial-Associated Molecular Pattern, flg22, a 22-amino acid peptide derived from bacterial flagella, is well established. However, the evidence for a negative or positive role for one component of the Arabidopsis G protein complex, namely, Regulator of G Signaling 1 (AtRGS1) leads to opposing conclusions. We show that the reason for this difference is due to the isolate of Col-0 ecotype used as the wildtype control in flg22-induced ROS and our data further support the idea that AtRGS1 is a negative regulator of the flg22-induced ROS response. Whole-genome genotyping led to the identification and validation of polymorphism in five genes between two Col-0 isolates that are candidates for the different ROS response relative to the rgs1 null mutant.

Heterotrimeric G-protein γ subunits regulate ABA signaling in response to drought through interacting with PP2Cs and SnRK2s in mulberry (Morus alba L.)

ABA signaling plays a central role in regulating plants respond to drought. Although much progress has been made in understanding the functions of ABA signaling in drought response, very little information is available regarding woody plants. In this study, the components of ABA signaling pathway were identified in mulberry which has excellent adaptation to drought, including three PYLs, two PP2Cs, two SnRK2s, four ABFs, and an ABA responsive gene MaRD29B. The gene expression of ABA signaling components exhibited significant response to ABA and drought, and their roles in drought response were revealed using a transient transformation system in mulberry seedlings. We discovered the ABA signaling components, MaABI1/2 and MaSnRK2.1/2.4, could directly interact with G-protein γ subunits, MaGγ1 and MaGγ2, which indicated that G-protein γ subunits may mediate the signal crosstalk between G-proteins and ABA signaling. Transient activation assay in tobacco and RNAi silencing assay in mulberry further demonstrated that MaGγ1 and MaGγ2 regulated drought response by enhancing ABA signaling. This study expands the repertoire of ABA signaling controlling drought responses in plants and provides the direct evidence about the crosstalk between ABA signaling and G-proteins for the first time.

RGB1 Regulates Grain Development and Starch Accumulation Through Its Effect on OsYUC11-Mediated Auxin Biosynthesis in Rice Endosperm Cells

RGB1, a subunit of heterotrimeric G protein, plays important roles in regulating grain size and weight of rice. However, the molecular mechanisms underlying controlling grain filling process by G protein are still largely unclear. In the present study, we show that RGB1 controls not only the grain size but also the grain filling process. Knock-down of RGB1 significantly delayed grain development and reduced starch accumulation and grain weight, which was closely related to the delayed and the lower expression of genes encoding sucrose metabolism and starch biosynthesis related enzymes during grain filling stage. Suppression of RGB1 expression also resulted in the lower auxin content in grains, which was correlated with the lower expression of OsNF-YB1 and OsYUC11 during grain filling stage. Further biochemical evidence showed that OsYUC11 expression was under control of OsNF-YB1 by its interaction with promoter of OsYUC11. Taken together, we propose that RGB1 controls rice grain development and grain filling process by changing auxin homeostasis in endosperm cells. OsNF-YB1, which acts as a key downstream effector of RGB1, interacts directly with the promoter of OsYUC11 and stimulates the OsYUC11 expression, thereby regulating auxin biosynthesis and starch accumulation and grain size.

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.