Isolation of cDNAs encoding guanine nucleotide-binding protein beta-subunit homologues from maize (ZGB1) and Arabidopsis (AGB1)

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.

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.

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.

GCR1 Positively Regulates UV-B- and Ethylene-Induced Stomatal Closure via Activating GPA1-Dependent ROS and NO Production

Heterotrimeric G proteins function as key players in guard cell signaling to many stimuli, including ultraviolet B (UV-B) and ethylene, but whether guard cell G protein signaling is activated by the only one potential G protein-coupled receptor, GCR1, is still unclear. Here, we found that gcr1 null mutants showed defects in UV-B- and ethylene-induced stomatal closure and production of reactive oxygen species (ROS) and nitric oxide (NO) in guard cells, but these defects could be rescued by the application of a Gα activator or overexpression of a constitutively active form of Gα subunit GPA1 (cGPA1). Moreover, the exogenous application of hydrogen peroxide (H₂O₂) or NO triggered stomatal closure in gcr1 mutants and cGPA1 transgenic plants in the absence or presence of UV-B or ethylene, but exogenous ethylene could not rescue the defect of gcr1 mutants in UV-B-induced stomatal closure, and gcr1 mutants did not affect UV-B-induced ethylene production in Arabidopsis leaves. These results indicate that GCR1 positively controls UV-B- and ethylene-induced stomatal closure by activating GPA1-dependent ROS and NO production in guard cells and that ethylene acts upstream of GCR1 to transduce UV-B guard cell signaling, which establishes the existence of a classic paradigm of G protein signaling in guard cell signaling to UV-B and ethylene.

Creativity comes from interactions: modules of protein interactions in plants

Protein interactions are the foundation of cell biology. For robust signal transduction to occur, proteins interact selectively and modulate their behavior to direct specific biological outcomes. Frequently, modular protein interaction domains are central to these processes. Some of these domains bind proteins bearing post-translational modifications, such as phosphorylation, whereas other domains recognize and bind to specific amino acid motifs. Other modules act as diverse protein interaction scaffolds or can be multifunctional, forming head-to-head homodimers and binding specific peptide sequences or membrane phospholipids. Additionally, the so-called head-to-tail oligomerization domains (SAM, DIX, and PB1) can form extended polymers to regulate diverse aspects of biology. Although the mechanism and structures of these domains are diverse, they are united by their modularity. Together, these domains are versatile and facilitate the evolution of complex protein interaction networks. In this review, we will highlight the role of select modular protein interaction domains in various aspects of plant biology.

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.

Arabidopsis cryptochrome 1 promotes stomatal development through repression of AGB1 inhibition of SPEECHLESS DNA-binding activity

Cryptochromes are blue light photoreceptors that mediate various light responses in plants and mammals. The heterotrimeric G-protein is known to regulate various physiological processes in plants and mammals. In Arabidopsis, cryptochrome 1 (CRY1) and the G-protein β subunit AGB1 act antagonistically to regulate stomatal development. The molecular mechanism by which CRY1 and AGB1 regulate this process remains unknown. Here, we show that Arabidopsis CRY1 acts partially through AGB1, and AGB1 acts through SPEECHLESS (SPCH), a master transcription factor that drives stomatal initiation and proliferation, to regulate stomatal development. We demonstrate that AGB1 physically interacts with SPCH to block the bHLH DNA-binding domain of SPCH and inhibit its DNA-binding activity. Moreover, we demonstrate that photoexcited CRY1 represses the interaction of AGB1 with SPCH to release AGB1 inhibition of SPCH DNA-binding activity, leading to the expression of SPCH-target genes promoting stomatal development. Taken together, our results suggest that the mechanism by which CRY1 promotes stomatal development involves positive regulation of the DNA-binding activity of SPCH mediated by CRY1 inhibition of the AGB1-SPCH interaction. We propose that the antagonistic regulation of SPCH DNA-binding activity by CRY1 and AGB1 may allow plants to balance light and G-protein signaling and optimize stomatal density and pattern.

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.

GTP binding by Arabidopsis extra-large G protein 2 is not essential for its functions

The extra-large guanosine-5'-triphosphate (GTP)-binding protein 2, XLG2, is an unconventional Gα subunit of the Arabidopsis (Arabidopsis thaliana) heterotrimeric GTP-binding protein complex with a major role in plant defense. In vitro biochemical analyses and molecular dynamic simulations show that affinity of XLG2 for GTP is two orders of magnitude lower than that of the conventional Gα, AtGPA1. Here we tested the physiological relevance of GTP binding by XLG2. We generated an XLG2(T476N) variant with abolished GTP binding, as confirmed by in vitro GTPγS binding assay. Yeast three-hybrid, bimolecular fluorescence complementation, and split firefly-luciferase complementation assays revealed that the nucleotide-depleted XLG2(T476N) retained wild-type XLG2-like interactions with the Gβγ dimer and defense-related receptor-like kinases. Both wild-type and nucleotide-depleted XLG2(T476N) restored the defense responses against Fusarium oxysporum and Pseudomonas syringae compromised in the xlg2 xlg3 double mutant. Additionally, XLG2(T476N) was fully functional restoring stomatal density, root growth, and sensitivity to NaCl, but failed to complement impaired germination and vernalization-induced flowering. We conclude that XLG2 is able to function in a GTP-independent manner and discuss its possible mechanisms of action.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Oligomerization of A. thaliana Heterotrimeric G Protein Subunits AGB1 and AGG2 In Vitro

Plant heterotrimeric G proteins are a major group of signaling molecules involved in regulation of critical processes including stress adaptation, seed size, grain quality and immune responses. Despite an abundance of in situ functional studies; purification of the individual subunits of the plant heterotrimer for biophysical and structural characterization and for studies on their interactions are lacking. In this study cloning of the genes encoding the β subunit AGB1 of A. thaliana and its γ-subunits AGG1 and AGG2 using different E. coli expression vectors and screening of expression in several strains are reported. AGB1 could be expressed albeit at very low levels and in all cases it was accompanied by overexpression of E. coli chaperone proteins. AGG1 could only be detected in inclusion body fractions, whereas AGG2 was obtained in soluble fractions and was purified. Purified AGB1 and AGG2 subunits were shown to dimerize in vitro. Further characterization of AGG2 by small angle X-ray scattering measurements and by dynamic light scattering revealed that AGG2 formed homodimers with extended shape in solution. These results are also consistent with those from circular dichroism spectroscopy which yielded 39.4% helical and 50% random coil content for AGG2. This is the first study showing heterologous expression of a plant heterotrimeric G protein β subunit individually and presenting its interaction with a plant γ-subunit in vitro. Results also show that the AGG2 subunit has a disordered structure, which would account for its role in diverse interactions for establishing selectivity in signal propagation.

Whole genome sequence of Diaporthe capsici, a new pathogen of walnut blight

Many fungi in the Diaporthe genus across the world are pathogenic. Diaporthe capsici. is a pathogenic fungus that can infect peppers and walnuts, causing their death. The aim of this study was to develop a genomic resource to provide substantial data and a theoretical basis for research on molecular pathogenesis, transcriptome, proteome, and metabonome of D. capsici. The whole genome of D. capsici was sequenced using the PacBio RSII sequencing platform, and functional annotation was performed using different public databases. The genome was found to be 57.56 Mb in size, with an N50 contig size of 5,171,887 bp, and encodes 14,425 putative genes. This is the first genome-scale assembly and annotation for D. capsici, which is the eighth species in Diaporthe to be sequenced.

JA-Induced Endocytosis of AtRGS1 Is Involved in G-Protein Mediated JA Responses

Arabidopsis heterotrimeric G proteins regulate diverse plant growth and defense processes by coupling to 7TM AtRGS1 proteins. Although G protein mutants display alterations in response to multiple plant hormones, the underlying mechanism by which G proteins participate in the regulation of hormone responses remains elusive. Here, we show that genetic disruption of Gα and Gβ subunits results in reduced sensitivity to JA treatment. Furthermore, using confocal microscopy, VA-TIRFM, and FRET-FLIM, we provide evidence that stimulation by JA induces phosphorylation- and C-terminus-dependent endocytosis of AtRGS1, which then promotes dissociation of AtRGS1 from AtGPA1. In addition, SPT analysis reveals that JA treatment affects the diffusion dynamics of AtRGS1 and AtRGS1-ΔCt. Taken together, these findings suggest that the JA signal activates heterotrimeric G proteins through the endocytosis of AtRGS1 and dissociation of AtRGS1 from AtGPA1, thus providing valuable insight into the mechanisms of how the G protein system perceives and transduces phytohormone signals.

The Arabidopsis heterotrimeric G-protein β subunit, AGB1, is required for guard cell calcium sensing and calcium-induced calcium release

Cytosolic calcium concentration ([Ca²⁺ ]_cyt ) and heterotrimeric G-proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca_o ) is as strong a stimulus for stomatal closure as the phytohormone abscisic acid (ABA), but underlying mechanisms remain elusive. Here, we report that the sole Arabidopsis heterotrimeric Gβ subunit, AGB1, is required for four guard cell Ca_o responses: induction of stomatal closure; inhibition of stomatal opening; [Ca²⁺ ]_cyt oscillation; and inositol 1,4,5-trisphosphate (InsP3) production. Stomata in wild-type Arabidopsis (Col) and in mutants of the canonical Gα subunit, GPA1, showed inhibition of stomatal opening and promotion of stomatal closure by Ca_o . By contrast, stomatal movements of agb1 mutants and agb1/gpa1 double-mutants, as well as those of the agg1agg2 Gγ double-mutant, were insensitive to Ca_o . These behaviors contrast with ABA-regulated stomatal movements, which involve GPA1 and AGB1/AGG3 dimers, illustrating differential partitioning of G-protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus-specific encoding. AGB1 knockouts retained reactive oxygen species and NO production, but lost YC3.6-detected [Ca²⁺ ]_cyt oscillations in response to Ca_o , initiating only a single [Ca²⁺ ]_cyt spike. Experimentally imposed [Ca²⁺ ]_cyt oscillations restored stomatal closure in agb1. Yeast two-hybrid and bimolecular complementation fluorescence experiments revealed that AGB1 interacts with phospholipase Cs (PLCs), and Ca_o induced InsP3 production in Col but not in agb1. In sum, G-protein signaling via AGB1/AGG1/AGG2 is essential for Ca_o -regulation of stomatal apertures, and stomatal movements in response to Ca_o apparently require Ca²⁺ -induced Ca²⁺ release that is likely dependent on Gβγ interaction with PLCs leading to InsP3 production.

TaCOLD1 defines a new regulator of plant height in bread wheat

Plant height is among the most important agronomic traits that influence crop yield. However, in addition to the Rht-1 alleles, the molecular basis of plant height in bread wheat remains largely unclear. Based on wheat gene expression profiling analysis, we identify a light-regulated gene from bread wheat, designated as TaCOLD1, whose encoding protein is homologous to cold sensor COLD1 in rice. We show that TaCOLD1 protein is localized to the endoplasmic reticulum (ER) and plasma membrane. Phenotypic analyses show that overexpression of a mutated form of TaCOLD1 (M187K) in bread wheat cultivar Kenong199 (Rht-B1b) background resulted in an obvious reduction in plant height. Further, we demonstrate that the hydrophilic loop of TaCOLD1 (residues 178-296) can interact with TaGα-7A (the α subunit of heterotrimeric G protein) protein but not TaGα-1B, and the mutation (M187K) in TaCOLD1 remarkably enhances its interaction with TaGα-7A. Physical interaction analyses show that the C-terminal region of TaGα-7A, which is lacking in the TaGα-1B protein, is necessary for its interaction with TaCOLD1. Intriguingly, the C-terminal region of TaGα-7A is also physically associated with the TaDEP1 protein (an atypical Gγ subunit). Significantly, we discover that TaCOLD1 and mTaCOLD1 (M187K) can interfere with the physical association between TaGα-7A and TaDEP1. Together, this study reveals that TaCOLD1 acts as a novel regulator of plant height through interfering with the formation of heterotrimeric G protein complex in bread wheat and is a valuable target for the engineering of wheat plant architecture.

Phytochrome B and AGB1 Coordinately Regulate Photomorphogenesis by Antagonistically Modulating PIF3 Stability in Arabidopsis

Phytochrome B (phyB), the primary red light photoreceptor, promotes photomorphogenesis in Arabidopsis by interacting with the basic helix-loop-helix transcriptional factor PIF3 and inducing its phosphorylation and degradation. Heterotrimeric G proteins are known to regulate various developmental processes in plants and animals. In Arabidopsis, the G-protein β subunit AGB1 is known to repress photomorphogenesis. However, whether and how phyB and AGB1 coordinately regulate photomorphogenesis are largely unknown. Here we show that phyB physically interacts with AGB1 in a red light-dependent manner and that AGB1 interacts directly with PIF3. Moreover, we demonstrate that the AGB1-PIF3 interaction inhibits the association of PIF3 with phyB, leading to reduced phosphorylation and degradation of PIF3, whereas the phyB-AGB1 interaction represses the association of PIF3 with AGB1, resulting in enhanced phosphorylation and degradation of PIF3. Our results suggest that phyB and AGB1 antagonistically regulate PIF3 stability by dynamically interacting with each other and PIF3. This dynamic mechanism may allow plants to balance phyB and G-protein signaling to optimize photomorphogenesis.

Phenotypic and genome-wide association with the local environment of Arabidopsis

The environment imposes critical selective forces on all living organisms, and the sessile nature of plants makes them particularly useful for investigating the relationship between genetic variation and environmental adaptation. In the model plant Arabidopsis thaliana, extensive information on phenotypic and genotypic variation is available, but comparable information on environmental variation within the native range of the species is lacking. Here, we compile 204 geoclimatic variables to describe the local environments of Arabidopsis accessions with known collection sites encompassing a wide geo-environmental range, and fully sequenced genomes from the 1001 Genomes Project. We identify candidate adaptive genetic variation associated with these environmental variables, and validate this approach through comparison with previous experimental studies, and by targeted confirmation of a role of the heterotrimeric G-protein γ subunit, AGG3, in cold tolerance, as newly predicted from our environmental genome wide association study (GWAS). To facilitate identification of adaptive variation, we created Arabidopsis CLIMtools : interactive web-based databases of the environment × genome associations and correlations between the local environments and 131 phenotypes compiled from previous experimental GWASs. Our study presents an extensive analysis of the local environments, landscape genomics and phenotypic variation of Arabidopsis, and illustrates how 'in silico GWAS' approaches can inform and complement experimental phenomics studies.

Characterization of Heterotrimeric G Protein γ4 Subunit in Rice

Heterotrimeric G proteins are the molecule switch that transmits information from external signals to intracellular target proteins in mammals and yeast cells. In higher plants, heterotrimeric G proteins regulate plant architecture. Rice harbors one canonical α subunit gene (RGA1), four extra-large GTP-binding protein genes (XLGs), one canonical β-subunit gene (RGB1), and five γ-subunit genes (tentatively designated RGG1, RGG2, RGG3/GS3/Mi/OsGGC1, RGG4/DEP1/DN1/qPE9-1/OsGGC3, and RGG5/OsGGC2) as components of the heterotrimeric G protein complex. Among the five γ-subunit genes, RGG1 encodes the canonical γ-subunit, RGG2 encodes a plant-specific type of γ-subunit with additional amino acid residues at the N-terminus, and the remaining three γ-subunit genes encode atypical γ-subunits with cysteine-rich C-termini. We characterized the RGG4/DEP1/DN1/qPE9-1/OsGGC3 gene product Gγ4 in the wild type (WT) and truncated protein Gγ4∆Cys in the RGG4/DEP1/DN1/qPE9-1/OsGGC3 mutant, Dn1-1, as littele information regarding the native Gγ4 and Gγ4∆Cys proteins is currently available. Based on liquid chromatography-tandem mass spectrometry analysis, immunoprecipitated Gγ4 candidates were confirmed as actual Gγ4. Similar to α-(Gα) and β-subunits (Gβ), Gγ4 was enriched in the plasma membrane fraction and accumulated in the developing leaf sheath. As RGG4/DEP1/DN1/qPE9-1/OsGGC3 mutants exhibited dwarfism, tissues that accumulated Gγ4 corresponded to the abnormal tissues observed in RGG4/DEP1/DN1/qPE9-1/OsGGC3 mutants.

Identification of Heterotrimeric G Protein γ3 Subunit in Rice Plasma Membrane

Heterotrimeric G proteins are important molecules for regulating plant architecture and transmitting external signals to intracellular target proteins in higher plants and mammals. The rice genome contains one canonical α subunit gene (RGA1), four extra-large GTP-binding protein genes (XLGs), one canonical β subunit gene (RGB1), and five γ subunit genes (tentatively named RGG1, RGG2, RGG3/GS3/Mi/OsGGC1, RGG4/DEP1/DN1/OsGGC3, and RGG5/OsGGC2). RGG1 encodes the canonical γ subunit; RGG2 encodes the plant-specific type of γ subunit with additional amino acid residues at the N-terminus; and the remaining three γ subunit genes encode the atypical γ subunits with cysteine abundance at the C-terminus. We aimed to identify the RGG3/GS3/Mi/OsGGC1 gene product, Gγ3, in rice tissues using the anti-Gγ3 domain antibody. We also analyzed the truncated protein, Gγ3∆Cys, in the RGG3/GS3/Mi/OsGGC1 mutant, Mi, using the anti-Gγ3 domain antibody. Based on nano-liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, the immunoprecipitated Gγ3 candidates were confirmed to be Gγ3. Similar to α (Gα) and β subunits (Gβ), Gγ3 was enriched in the plasma membrane fraction, and accumulated in the flower tissues. As RGG3/GS3/Mi/OsGGC1 mutants show the characteristic phenotype in flowers and consequently in seeds, the tissues that accumulated Gγ3 corresponded to the abnormal tissues observed in RGG3/GS3/Mi/OsGGC1 mutants.

Inter-relationships between the heterotrimeric Gβ subunit AGB1, the receptor-like kinase FERONIA, and RALF1 in salinity response

Plant heterotrimeric G proteins modulate numerous developmental stress responses. Recently, receptor-like kinases (RLKs) have been implicated as functioning with G proteins and may serve as plant G-protein-coupled-receptors. The RLK FERONIA (FER), in the Catharantus roseus RLK1-like subfamily, is activated by a family of polypeptides called rapid alkalinization factors (RALFs). We previously showed that the Arabidopsis G protein β subunit, AGB1, physically interacts with FER, and that RALF1 regulation of stomatal movement through FER requires AGB1. Here, we investigated genetic interactions of AGB1 and FER in plant salinity response by comparing salt responses in the single and double mutants of agb1 and fer. We show that AGB1 and FER act additively or synergistically depending on the conditions of the NaCl treatments. We further show that the synergism likely occurs through salt-induced ROS production. In addition, we show that RALF1 enhances salt toxicity through increasing Na⁺ accumulation and decreasing K⁺ accumulation rather than by inducing ROS production, and that the RALF1 effect on salt response occurs in an AGB1-independent manner. Our results indicate that RLK epistatic relationships are not fixed, as AGB1 and FER display different genetic relationships to RALF1 in stomatal versus salinity responses.

TaNBP1, a guanine nucleotide-binding subunit gene of wheat, is essential in the regulation of N starvation adaptation via modulating N acquisition and ROS homeostasis

BACKGROUND

Nitrate (NO3-) is the major source of nitrogen (N) for higher plants aside from its function in transducing the N signaling. Improving N use efficiency of crops has been an effective strategy for promotion of the sustainable agriculture worldwide. The regulatory pathways associating with N uptake and the corresponding biochemical processes impact largely on plant N starvation tolerance. Thus, exploration of the molecular mechanism underlying nitrogen use efficiency (NUE) and the gene wealth will pave a way for molecular breeding of N starvation-tolerant crop cultivars.

RESULTS

In the current study, we characterized the function of TaNBP1, a guanine nucleotide-binding protein subunit beta gene of wheat (T. aestivum), in mediating the plant N starvation response. TaNBP1 protein harbors a conserved W40 domain and the TaNBP1-GFP (green fluorescence protein) signals concentrate at positions of cytoplasm membrane and cytosol. TaNBP1 transcripts are induced in roots and leaves upon N starvation stress and that this upregulated expression is recovered by N recovery treatment. TaNBP1 overexpression confers improved phenotype, enlarged root system architecture (RSA), and increased biomass for plants upon N deprivation relative to the wild type, associating with its role in enhancing N accumulation and improving reactive oxygen species (ROS) homeostasis. Nitrate transporter (NRT) gene NtNRT2.2 and antioxidant enzyme genes NtSOD1, NtSOD2, and NtCAT1 are transcriptionally regulated under TaNBP1 and contribute to the improved N acquisition and the increased AE activities of plants.

CONCLUSIONS

Altogether, TaNBP1 is transcriptional response to N starvation stress. Overexpression of this gene enhances plant N starvation adaptation via improvement of N uptake and cellular ROS homeostasis by modifying transcription of NRT gene NtNRT2.2 and antioxidant enzyme genes NtSOD1, NtSOD2, and NtCAT1, respectively. Our research helps to understand the mechanism underlying plant N starvation response and benefits to genetically engineer crop cultivars with improved NUE under the N-saving cultivation conditions.

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.

The G Protein β-Subunit, AGB1, Interacts with FERONIA in RALF1-Regulated Stomatal Movement

Heterotrimeric guanine nucleotide-binding (G) proteins are composed of Gα, Gβ, and Gγ subunits and function as molecular switches in signal transduction. In Arabidopsis (Arabidopsis thaliana), there are one canonical Gα (GPA1), three extra-large Gα (XLG1, XLG2, and XLG3), one Gβ (AGB1), and three Gγ (AGG1, AGG2, and AGG3) subunits. To elucidate AGB1 molecular signaling, we performed immunoprecipitation using plasma membrane-enriched proteins followed by mass spectrometry to identify the protein interactors of AGB1. After eliminating proteins present in the control immunoprecipitation, commonly identified contaminants, and organellar proteins, a total of 103 candidate AGB1-associated proteins were confidently identified. We identified all of the G protein subunits except XLG1, receptor-like kinases, Ca²⁺ signaling-related proteins, and 14-3-3-like proteins, all of which may couple with or modulate G protein signaling. We confirmed physical interaction between AGB1 and the receptor-like kinase FERONIA (FER) using bimolecular fluorescence complementation. The Rapid Alkalinization Factor (RALF) family of polypeptides have been shown to be ligands of FER. In this study, we demonstrate that RALF1 regulates stomatal apertures and does so in a G protein-dependent manner, inhibiting stomatal opening and promoting stomatal closure in Columbia but not in agb1 mutants. We further show that AGGs and XLGs, but not GPA1, participate in RALF1-mediated stomatal signaling. Our results suggest that FER acts as a G protein-coupled receptor for plant heterotrimeric G proteins.

The G-Protein β Subunit AGB1 Promotes Hypocotyl Elongation through Inhibiting Transcription Activation Function of BBX21 in Arabidopsis

Hypocotyl development in Arabidopsis thaliana is regulated by light and endogenous hormonal cues, making it an ideal model to study the interplay between light and endogenous growth regulators. BBX21, a B-box (BBX)-like zinc-finger transcription factor, integrates light and abscisic acid signals to regulate hypocotyl elongation in Arabidopsis. Heterotrimeric G-proteins are pivotal regulators of plant development. The short hypocotyl phenotype of the G-protein β-subunit (AGB1) mutant (agb1-2) has been previously identified, but the precise role of AGB1 in hypocotyl elongation remains enigmatic. Here, we show that AGB1 directly interacts with BBX21, and the short hypocotyl phenotype of agb1-2 is partially suppressed in agb1-2bbx21-1 double mutant. BBX21 functions in the downstream of AGB1 and overexpression of BBX21 in agb1-2 causes a more pronounced reduction in hypocotyl length, indicating that AGB1 plays an oppositional role in relation to BBX21 during hypocotyl development. Furthermore, we demonstrate that the C-terminal region of BBX21 is important for both its intracellular localization and its transcriptional activation activity that is inhibited by interaction with AGB1. ChIP assays showed that BBX21 specifically associates with its own promoter and with those of BBX22, HY5, and GA2ox1. which is not altered in agb1-2. These data suggest that the AGB1-BBX21 interaction only affects the transcriptional activation activity of BBX21 but has no effect on its DNA binding ability. Taken together, our data demonstrate that AGB1 positively promotes hypocotyl elongation through repressing BBX21 activity.

A photoperiod-responsive protein compendium and conceptual proteome roadmap outline in maize grown in growth chambers with controlled conditions

Maize (Zea mays L.) is one of the major staple food crops of the world. However, high photoperiod sensitivity, especially for tropical germplasms, impedes attempts to improve maize agronomical traits by integration of tropical and temperate maize germplasms. Physiological and phenotypic responses of maize to photoperiod have widely been investigated based on multi-site field observations; however, proteome-based responsive mechanisms under controlled photoperiod regimes, nutrient and moisture soils are not yet well understood. In the present study, we sequenced and analyzed six proteomes of tropically-adapted and photoperiod-sensitive M9 inbred line at the vegetative 3 stage and proteomes from tropically-adapted and photoperiod-sensitive Shuang M9 (SM9) inbred line at the vegetative-tasseling stage. All plants were grown in growth chambers with controlled soil and temperature and three photoperiod regimes, a short photoperiod (SP) of 10 h light/14 h dark, a control neutral photoperiod (NP) of 12 h light/12 h dark, and a long photoperiod (LP) of 16 h light/8 h dark for a daily cycle. We identified 4,395 proteins of which 401 and 425 differentially-expressed proteins (DPs) were found in abundance in M9 leaves and in SM9 leaves as per SP/LP vs. NP, respectively. Some DPs showed responses to both SP and LP while some only responded to either SP or LP, depending on M9 or SM9. Our study showed that the photoperiodic response pathway, circadian clock rhythm, and high light density/intensity crosstalk with each other, but apparently differ from dark signaling routes. Photoperiod response involves light-responsive or dark-responsive proteins or both. The DPs positioned on the signaling routes from photoperiod changes to RNA/DNA responses involve the mago nashi homolog and glycine-rich RNA-binding proteins. Moreover, the cell-to-cell movement of ZCN14 through plasmodesmata is likely blocked under a 16-h-light LP. Here, we propose a photoperiodic model based on our findings and those from previous studies.

Sporophyte Formation and Life Cycle Completion in Moss Requires Heterotrimeric G-Proteins

In this study, we report the functional characterization of heterotrimeric G-proteins from a nonvascular plant, the moss Physcomitrella patens. In plants, G-proteins have been characterized from only a few angiosperms to date, where their involvement has been shown during regulation of multiple signaling and developmental pathways affecting overall plant fitness. In addition to its unparalleled evolutionary position in the plant lineages, the P. patens genome also codes for a unique assortment of G-protein components, which includes two copies of Gβ and Gγ genes, but no canonical Gα Instead, a single gene encoding an extra-large Gα (XLG) protein exists in the P. patens genome. Here, we demonstrate that in P. patens the canonical Gα is biochemically and functionally replaced by an XLG protein, which works in the same genetic pathway as one of the Gβ proteins to control its development. Furthermore, the specific G-protein subunits in P. patens are essential for its life cycle completion. Deletion of the genomic locus of PpXLG or PpGβ2 results in smaller, slower growing gametophores. Normal reproductive structures develop on these gametophores, but they are unable to form any sporophyte, the only diploid stage in the moss life cycle. Finally, the mutant phenotypes of ΔPpXLG and ΔPpGβ2 can be complemented by the homologous genes from Arabidopsis, AtXLG2 and AtAGB1, respectively, suggesting an overall conservation of their function throughout the plant evolution.

Plant G-Proteins Come of Age: Breaking the Bond with Animal Models

G-proteins are universal signal transducers mediating many cellular responses. Plant G-protein signaling has been modeled on the well-established animal paradigm but accumulated experimental evidence indicates that G-protein-dependent signaling in plants has taken a very different evolutionary path. Here we review the differences between plant and animal G-proteins reported over past two decades. Most importantly, while in animal systems the G-protein signaling cycle is activated by seven transmembrane-spanning G-protein coupled receptors, the existence of these type of receptors in plants is highly controversial. Instead plant G-proteins have been proven to be functionally associated with atypical receptors such as the Arabidopsis RGS1 and a number of receptor-like kinases. We propose that, instead of the GTP/GDP cycle used in animals, plant G-proteins are activated/de-activated by phosphorylation/de-phosphorylation. We discuss the need of a fresh new look at these signaling molecules and provide a hypothetical model that departs from the accepted animal paradigm.

Type B Heterotrimeric G Protein γ-Subunit Regulates Auxin and ABA Signaling in Tomato

Heterotrimeric G proteins composed of α, β, and γ subunits are central signal transducers mediating the cellular response to multiple stimuli in most eukaryotes. Gγ subunits provide proper cellular localization and functional specificity to the heterotrimer complex. Plant Gγ subunits, divided into three structurally distinct types, are more diverse than their animal counterparts. Type B Gγ subunits, lacking a carboxyl-terminal isoprenylation motif, are found only in flowering plants. We present the functional characterization of type B Gγ subunit (SlGGB1) in tomato (Solanum lycopersicum). We show that SlGGB1 is the most abundant Gγ subunit in tomato and strongly interacts with the Gβ subunit. Importantly, the green fluorescent protein-SlGGB1 fusion protein as well as the carboxyl-terminal yellow fluorescent protein-SlGGB1/amino-terminal yellow fluorescent protein-Gβ heterodimer were localized in the plasma membrane, nucleus, and cytoplasm. RNA interference-mediated silencing of SlGGB1 resulted in smaller seeds, higher number of lateral roots, and pointy fruits. The silenced lines were hypersensitive to exogenous auxin, while levels of endogenous auxins were lower or similar to those of the wild type. SlGGB1-silenced plants also showed strong hyposensitivity to abscisic acid (ABA) during seed germination but not in other related assays. Transcriptome analysis of the transgenic seeds revealed abnormal expression of genes involved in ABA sensing, signaling, and response. We conclude that the type B Gγ subunit SlGGB1 mediates auxin and ABA signaling in tomato.

Ternary WD40 Repeat-Containing Protein Complexes: Evolution, Composition and Roles in Plant Immunity

Plants, like mammals, rely on their innate immune system to perceive and discriminate among the majority of their microbial pathogens. Unlike mammals, plants respond to this molecular dialog by unleashing a complex chemical arsenal of defense metabolites to resist or evade pathogen infection. In basal or non-host resistance, plants utilize signal transduction pathways to detect "non-self," "damaged-self," and "altered-self"- associated molecular patterns and translate these "danger" signals into largely inducible chemical defenses. The WD40 repeat (WDR)-containing proteins Gβ and TTG1 are constituents of two independent ternary protein complexes functioning at opposite ends of a plant immune signaling pathway. They are also encoded by single-copy genes that are ubiquitous in higher plants, implying the limited diversity and functional conservation of their respective complexes. In this review, we summarize what is currently known about the evolutionary history of these WDR-containing ternary complexes, their repertoire and combinatorial interactions, and their downstream effectors and pathways in plant defense.

G-protein α-subunit (GPA1) regulates stress, nitrate and phosphate response, flavonoid biosynthesis, fruit/seed development and substantially shares GCR1 regulation in A. thaliana

Heterotrimeric G-proteins are implicated in several plant processes, but the mechanisms of signal-response coupling and the roles of G-protein coupled receptors in general and GCR1 in particular, remain poorly understood. We isolated a knock-out mutant of the Arabidopsis G-protein α subunit (gpa1-5) and analysed its transcriptome to understand the genomewide role of GPA1 and compared it with that of our similar analysis of a GCR1 mutant (Chakraborty et al. 2015, PLoS ONE 10(2):e0117819). We found 394 GPA1-regulated genes spanning 79 biological processes, including biotic and abiotic stresses, development, flavonoid biosynthesis, transcription factors, transporters and nitrate/phosphate responses. Many of them are either unknown or unclaimed explicitly in other published gpa1 mutant transcriptome analyses. A comparison of all known GPA1-regulated genes (including the above 394) with 350 GCR1-regulated genes revealed 114 common genes. This can be best explained by GCR1-GPA1 coupling, or by convergence of their independent signaling pathways. Though the common genes in our GPA1 and GCR1 mutant datasets constitute only 26% of the GPA1-regulated and 30% of the GCR1-responsive genes, they belong to nearly half of all the processes affected in both the mutants. Thus, GCR1 and GPA1 regulate not only some common genes, but also different genes belonging to the same processes to achieve similar outcomes. Overall, we validate some known and report many hitherto unknown roles of GPA1 in plants, including agronomically important ones such as biotic stress and nutrient response, and also provide compelling genetic evidence to revisit the role of GCR1 in G-protein signalling.

Extra-Large G Proteins Expand the Repertoire of Subunits in Arabidopsis Heterotrimeric G Protein Signaling

Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, are a conserved signal transduction mechanism in eukaryotes. However, G protein subunit numbers in diploid plant genomes are greatly reduced as compared with animals and do not correlate with the diversity of functions and phenotypes in which heterotrimeric G proteins have been implicated. In addition to GPA1, the sole canonical Arabidopsis (Arabidopsis thaliana) Gα subunit, Arabidopsis has three related proteins: the extra-large GTP-binding proteins XLG1, XLG2, and XLG3. We demonstrate that the XLGs can bind Gβγ dimers (AGB1 plus a Gγ subunit: AGG1, AGG2, or AGG3) with differing specificity in yeast (Saccharomyces cerevisiae) three-hybrid assays. Our in silico structural analysis shows that XLG3 aligns closely to the crystal structure of GPA1, and XLG3 also competes with GPA1 for Gβγ binding in yeast. We observed interaction of the XLGs with all three Gβγ dimers at the plasma membrane in planta by bimolecular fluorescence complementation. Bioinformatic and localization studies identified and confirmed nuclear localization signals in XLG2 and XLG3 and a nuclear export signal in XLG3, which may facilitate intracellular shuttling. We found that tunicamycin, salt, and glucose hypersensitivity and increased stomatal density are agb1-specific phenotypes that are not observed in gpa1 mutants but are recapitulated in xlg mutants. Thus, XLG-Gβγ heterotrimers provide additional signaling modalities for tuning plant G protein responses and increase the repertoire of G protein heterotrimer combinations from three to 12. The potential for signal partitioning and competition between the XLGs and GPA1 is a new paradigm for plant-specific cell signaling.

Heterotrimeric G protein subunits differentially respond to endoplasmic reticulum stress in Arabidopsis

Canonical heterotrimeric G proteins in eukaryotes are major components that localize at plasma membrane and transmit extracellular stimuli into the cell. Genome of a seed plant Arabidopsis thaliana encodes at least one Gα (GPA1), one Gβ (AGB1), and 3 Gγ (AGG1, AGG2 and AGG3) subunits. The loss-of-function mutations of G protein subunit(s) cause multiple defects in development as well as biotic and abiotic stress responses. However, it remains elusive how these subunits differentially express these defects. Here, we report that Arabidopsis heterotrimeric G protein subunits differentially respond to the endoplasmic reticulum (ER) stress. An isolated homozygous mutant of AGB1, agb1-3, was more sensitive to the tunicamycin-induced ER stress compared to the wild type and the other loss-of-function mutants of G protein subunits. Moreover, ER stress responsive genes were highly expressed in the agb1-3 plant. Our results indicate that AGB1 positively contributes to ER stress tolerance in Arabidopsis.

Evidence for an unusual transmembrane configuration of AGG3, a class C Gγ subunit of Arabidopsis

Heterotrimeric G proteins are crucial for the perception of external signals and subsequent signal transduction in animal and plant cells. In both model systems, the complex comprises one Gα, one Gβ, and one Gγ subunit. However, in addition to the canonical Gγ subunits (class A), plants also possess two unusual, plant-specific classes of Gγ subunits (classes B and C) that have not yet been found in animals. These include Gγ subunits lacking the C-terminal CaaX motif (class B), which is important for membrane anchoring of the protein; the presence of such subunits gives rise to a flexible sub-population of Gβ/γ heterodimers that are not necessarily restricted to the plasma membrane. Plants also contain class C Gγ subunits, which are twice the size of canonical Gγ subunits, with a predicted transmembrane domain and a large cysteine-rich extracellular C-terminus. However, neither the presence of the transmembrane domain nor the membrane topology have been unequivocally demonstrated. Here, we provide compelling evidence that AGG3, a class C Gγ subunit of Arabidopsis, contains a functional transmembrane domain, which is sufficient but not essential for plasma membrane localization, and that the cysteine-rich C-terminus is extracellular.

A G-protein β subunit, AGB1, negatively regulates the ABA response and drought tolerance by down-regulating AtMPK6-related pathway in Arabidopsis

Heterotrimeric G-proteins are versatile regulators involved in diverse cellular processes in eukaryotes. In plants, the function of G-proteins is primarily associated with ABA signaling. However, the downstream effectors and the molecular mechanisms in the ABA pathway remain largely unknown. In this study, an AGB1 mutant (agb1-2) was found to show enhanced drought tolerance, indicating that AGB1 might negatively regulate drought tolerance in Arabidopsis. Data showed that AGB1 interacted with protein kinase AtMPK6 that was previously shown to phosphorylate AtVIP1, a transcription factor responding to ABA signaling. Our study found that transcript levels of three ABA responsive genes, AtMPK6, AtVIP1 and AtMYB44 (downstream gene of AtVIP1), were significantly up-regulated in agb1-2 lines after ABA or drought treatments. Other ABA-responsive and drought-inducible genes, such as RD29A (downstream gene of AtMYB44), were also up-regulated in agb1-2 lines. Furthermore, overexpression of AtVIP1 resulted in hypersensitivity to ABA at seed germination and seedling stages, and significantly enhanced drought tolerance in transgenic plants. These results suggest that AGB1 was involved in the ABA signaling pathway and drought tolerance in Arabidopsis through down-regulating the AtMPK6, AtVIP1 and AtMYB44 cascade.

Fusarium oxysporum infection assays in Arabidopsis

Increased susceptibility to Fusarium oxysporum is one of the most conspicuous characteristics of the Arabidopsis mutants lacking the heterotrimeric G protein β and γ1 subunits. The molecular mechanisms placing these G proteins in the plant innate immunity network are yet to be discovered. However, a method to test susceptibility to and disease progression of an important plant pathogen, such as F. oxysporum, is of central importance to many plant defense studies. The optimized protocol presented here allows the routine processing and analysis of symptom progression in young Arabidopsis soil-grown seedlings and yields highly reproducible results.

Isolation, in silico characterization, localization and expression analysis of abiotic stress-responsive rice G-protein β subunit (RGB1)

Heterotrimeric G-proteins constitute the classical signaling paradigm along with their cognate G-protein coupled receptors (GPCRs) and appropriate downstream effectors. G-protein complex is composed of highly conserved Gα, Gβ, and Gγ subunits. In the present study, we have characterized the cis-regulatory elements of the promoter, signature motifs, transcript profile in response to abiotic stresses, and sub-cellular localization of G-protein β subunit RGB1(I) from Indica rice. The RGB1(I) promoter sequence has various stress-related cis-regulatory elements suggesting its role in abiotic stress signaling. Presence of six WD-40 repeat signature motifs in RGB1(I) suggest its role in exchange of GDP by GTP in Gα subunit and receptor recognition. Presence of multiple N-myristoylation consensus sites in RGB1(I) protein sequence, which is necessary for membrane localization of protein, confirms the association of RGB1(I) in plasma membrane. Extrinsic association of RGB1(I) with plasma membrane seems essential for its role in regulation of signaling pathways and adaptation to high salt stress. We report the sub-cellular localization of RGB1(I) in plasma membrane, cytosol and nucleus. The localization of RGB1(I) in nucleus supports its possible interaction with transcription factors regulating the expression of salt stress responsive genes. The RGB1(I) transcript was upregulated under KCl, cold, dehydration and micronutrient (Mn (2+) and Zn (2+)) stress. However, transcript variation under elevated temperature, ABA, NaCl, and toxic heavy metals (viz. arsenite, arsenate, cadmium and lead) was not encouraging. These evidences indicate an active and significant role of RGB1(I) in the regulation of abiotic stresses in rice and propound its possible exploitation in the development of abiotic stress tolerance in crops.

Heterotrimeric G protein-coupled signaling in plants

Investigators studying G protein-coupled signaling--often called the best-understood pathway in the world owing to intense research in medical fields--have adopted plants as a new model to explore the plasticity and evolution of G signaling. Much research on plant G signaling has not disappointed. Although plant cells have most of the core elements found in animal G signaling, differences in network architecture and intrinsic properties of plant G protein elements make G signaling in plant cells distinct from the animal paradigm. In contrast to animal G proteins, plant G proteins are self-activating, and therefore regulation of G activation in plants occurs at the deactivation step. The self-activating property also means that plant G proteins do not need and therefore do not have typical animal G protein-coupled receptors. Targets of activated plant G proteins, also known as effectors, are unlike effectors in animal cells. The simpler repertoire of G signal elements in Arabidopsis makes G signaling easier to manipulate in a multicellular context.

Arabidopsis heterotrimeric G-proteins play a critical role in host and nonhost resistance against Pseudomonas syringae pathogens

Heterotrimeric G-proteins have been proposed to be involved in many aspects of plant disease resistance but their precise role in mediating nonhost disease resistance is not well understood. We evaluated the roles of specific subunits of heterotrimeric G-proteins using knock-out mutants of Arabidopsis Gα, Gβ and Gγ subunits in response to host and nonhost Pseudomonas pathogens. Plants lacking functional Gα, Gβ and Gγ1Gγ2 proteins displayed enhanced bacterial growth and disease susceptibility in response to host and nonhost pathogens. Mutations of single Gγ subunits Gγ1, Gγ2 and Gγ3 did not alter bacterial disease resistance. Some specificity of subunit usage was observed when comparing host pathogen versus nonhost pathogen. Overexpression of both Gα and Gβ led to reduced bacterial multiplication of nonhost pathogen P. syringae pv. tabaci whereas overexpression of Gβ, but not of Gα, resulted in reduced bacterial growth of host pathogen P. syringae pv. maculicola, compared to wild-type Col-0. Moreover, the regulation of stomatal aperture by bacterial pathogens was altered in Gα and Gβ mutants but not in any of the single or double Gγ mutants. Taken together, these data substantiate the critical role of heterotrimeric G-proteins in plant innate immunity and stomatal modulation in response to P. syringae.

The Arabidopsis adaptor protein AP-3μ interacts with the G-protein β subunit AGB1 and is involved in abscisic acid regulation of germination and post-germination development

Heterotrimeric G-proteins (G-proteins) have been implicated in ubiquitous signalling mechanisms in eukaryotes. In plants, G-proteins modulate hormonal and stress responses and regulate diverse developmental processes. However, the molecular mechanisms of their functions are largely unknown. A yeast two-hybrid screen was performed to identify interacting partners of the Arabidopsis G-protein β subunit AGB1. One of the identified AGB1-interacting proteins is the Arabidopsis adaptor protein AP-3µ. The interaction between AGB1 and AP-3µ was confirmed by an in vitro pull-down assay and bimolecular fluorescence complementation assay. Two ap-3µ T-DNA insertional mutants were found to be hyposensitive to abscisic acid (ABA) during germination and post-germination growth, whereas agb1 mutants were hypersensitive to ABA. During seed germination, agb1/ap-3µ double mutants were more sensitive to ABA than the wild type but less sensitive than agb1 mutants. However, in post-germination growth, the double mutants were as sensitive to ABA as agb1 mutants. These data suggest that AP-3µ positively regulates the ABA responses independently of AGB1 in seed germination, while AP-3µ does require AGB1 to regulate ABA responses during post-germination growth.

Heterotrimeric G protein signalling in the plant kingdom

In animals, heterotrimeric G proteins, comprising α-, β-and γ-subunits, perceive extracellular stimuli through cell surface receptors, and transmit signals to ion channels, enzymes and other effector proteins to affect numerous cellular behaviours. In plants, G proteins have structural similarities to the corresponding molecules in animals but transmit signals by atypical mechanisms and effector proteins to control growth, cell proliferation, defence, stomate movements, channel regulation, sugar sensing and some hormonal responses. In this review, we summarize the current knowledge on the molecular regulation of plant G proteins, their effectors and the physiological functions studied mainly in two model organisms: Arabidopsis thaliana and rice (Oryza sativa). We also look at recent progress on structural analyses, systems biology and evolutionary studies.

Heterotrimeric G protein signalling in the plant kingdom

In animals, heterotrimeric G proteins, comprising α-, β-and γ-subunits, perceive extracellular stimuli through cell surface receptors, and transmit signals to ion channels, enzymes and other effector proteins to affect numerous cellular behaviours. In plants, G proteins have structural similarities to the corresponding molecules in animals but transmit signals by atypical mechanisms and effector proteins to control growth, cell proliferation, defence, stomate movements, channel regulation, sugar sensing and some hormonal responses. In this review, we summarize the current knowledge on the molecular regulation of plant G proteins, their effectors and the physiological functions studied mainly in two model organisms: Arabidopsis thaliana and rice (Oryza sativa). We also look at recent progress on structural analyses, systems biology and evolutionary studies.

Signaling specificity provided by the Arabidopsis thaliana heterotrimeric G-protein γ subunits AGG1 and AGG2 is partially but not exclusively provided through transcriptional regulation

The heterotrimeric G-protein complex in Arabidopsis thaliana consists of one α, one ß and three γ subunits. While two of the γ subunits, AGG1 and AGG2 have been shown to provide functional selectivity to the Gßγ dimer in Arabidopsis, it is unclear if such selectivity is embedded in their molecular structures or conferred by the different expression patterns observed in both subunits. In order to study the molecular basis for such selectivity we tested genetic complementation of AGG1- and AGG2 driven by the respectively swapped gene promoters. When expressed in the same tissues as AGG1, AGG2 rescues some agg1 mutant phenotypes such as the hypersensitivity to Fusarium oxysporum and D-mannitol as well as the altered levels of lateral roots, but does not rescue the early flowering phenotype. Similarly, AGG1 when expressed in the same tissues as AGG2 rescues the osmotic stress and lateral-root phenotypes observed in agg2 mutants but failed to rescue the heat-stress induction of flowering. The fact that AGG1 and AGG2 are functionally interchangeable in some pathways implies that, at least for those pathways, signaling specificity resides in the distinctive spatiotemporal expression patterns exhibited by each γ subunit. On the other hand, the lack of complementation for some phenotypes indicates that there are pathways in which signaling specificity is provided by differences in the primary AGG1 and AGG2 amino acid sequences.

Rice heterotrimeric G-protein alpha subunit (RGA1): in silico analysis of the gene and promoter and its upregulation under abiotic stress

Heterotrimeric G-protein complexes (Gα, Gβ and Gγ) operate at the apex of diverse signal transduction systems along with their cognate transmembrane G-protein coupled receptors (GPCRs) and appropriate downstream effectors in the plant. Rice Gα in response to stress has not been well studied. Here, we report the in silico analysis of Gα subunit from Oryza sativa cv. Indica group Swarna [RGA1(I), accession number HQ634688], its promoter and its transcript upregulation in response to abiotic stresses. Genomic sequence of RGA1(I) contains thirteen exonic and twelve intronic segments. Phylogenetic analysis of RGA1(I) demonstrated high homology with Sorghum and maize and is distantly related to barley and wheat. Promoter sequence analysis of RGA1(I) confirms the presence of stress-related cis-regulatory elements viz. ABA, MeJAE, ARE, GT-1 boxes and LTR suggesting its active and possible independent roles in abiotic stress signalling. Expasy PROSITE database of protein families and domains revealed important motifs, patterns and biologically significant sites in RGA1(I). Three dimensional structure of RGA1(I) protein predicted by I-TASSER server and its stereochemical qualities were validated by PROCHECK and QMEAN server indicating the acceptability of the predicted model. The transcript profiling of RGA1(I) showed upregulation following NaCl, cold and drought stress. Under elevated temperature, its transcript was down regulated. Heavy metal(loid)s stress showed rhythmic and strong upregulation. It showed a rhythmic response in ABA stress. These findings provide a critical evidence for its active role in regulation of abiotic stresses in rice. These findings suggest its possible exploitation in the development of abiotic stress tolerance in crops.

The role of G-proteins in plant immunity

Heterotrimeric G-proteins play an important regulatory role in multiple physiological processes, including the plant immune response, and substantial progress has been made in elucidating the G-protein-mediated defense-signaling network. This mini-review discusses the importance of G-proteins in plant immunity. We also provide an overview of how G-proteins affect plant cell death and stomatal movement. Our recent studies demonstrated that G-proteins are involved in signal transduction and induction of stomatal closure and defense responses. We also discuss future directions for G-protein signaling studies involving plant immunity.

The DNA-binding activity of an AP2 protein is involved in transcriptional regulation of a stress-responsive gene, SiWD40, in foxtail millet

A differentially expressed transcript, encoding a putative WD protein (Setaria italica WD40; SiWD40), was identified in foxtail millet. Tertiary structure modeling revealed that its C-terminus possesses eight blade β-propeller architecture. Its N-terminal has three α-helices and two 3(10)-helices and was highly induced by different abiotic stresses. The SiWD40:GFP fusion protein was nuclear localized. Promoter analysis showed the presence of many cis-acting elements, including two dehydration responsive elements (DRE). A stress-responsive SiAP2 domain containing protein could specifically bind to these elements in the SiWD40 promoter. Thus, for the first time, we report that DREs probably regulate expression of SiWD40 during environmental stress. Molecular docking analysis revealed that the circumference of the β-propeller structure was involved in an interaction with a SiCullin4 protein, supporting the adaptability of SiWD40 to act as a scaffold. Our study thus provides a vital clue for near future research on the stress-regulation of WD proteins.

Rice heterotrimeric G-protein gamma subunits (RGG1 and RGG2) are differentially regulated under abiotic stress

Heterotrimeric G-proteins (α, β and γ subunits) are primarily involved in diverse signaling processes by transducing signals from an activated transmembrane G-protein coupled receptor (GPCR) to appropriate downstream effectors within cells. The role of α and β G-protein subunits in salinity and heat stress has been reported but the regulation of γ subunit of plant G-proteins in response to abiotic stress has not heretofore been described. In the present study we report the isolation of full-length cDNAs of two isoforms of Gγ [RGG1(I), 282 bp and RGG2(I), 453 bp] from rice (Oryza sativa cv Indica group Swarna) and described their transcript regulation in response to abiotic stresses. Protein sequence alignment and pairwise comparison of γ subunits of Indica rice [RGG(I)] with other known plant G-protein γ subunits demonstrated high homology to barley (HvGs) while soybean (GmG2) and Arabidopsis (AGG1) were least related. The numbers of the exons and introns were found to be similar between RGG1(I) and RGG2(I), but their sizes were different. Analyses of promoter sequences of RGG(I) confirmed the presence of stress-related cis-regulatory signature motifs suggesting their active and possible independent roles in abiotic stress signaling. The transcript levels of RGG1(I) and RGG2(I) were upregulated following NaCl, cold, heat and ABA treatments. However, in drought stress only RGG1(I) was upregulated. Strong support by transcript profiling suggests that γ subunits play a critical role via cross talk in signaling pathways. These findings provide first direct evidence for roles of Gγ subunits of rice G-proteins in regulation of abiotic stresses. These findings suggest the possible exploitation of γ subunits of G-protein machinery for promoting stress tolerance in plants.

Gγ1+Gγ2+Gγ3=Gβ: the search for heterotrimeric G-protein γ subunits in Arabidopsis is over

In Arabidopsis, heterotrimeric G-proteins consist of one Gα (GPA1), one Gβ (AGB1) and three Gγ (AGG1, AGG2 and AGG3) subunits. Gβ and Gγ subunits function as obligate heterodimers, therefore any phenotypes observed in Gβ-deficient mutants should be apparent in Gγ-deficient mutants. Nevertheless, the first two Gγ subunits discovered failed to explain many of the phenotypes shown by the agb1 mutants in Arabidopsis, prompting the search for additional Gγ subunits. The recent discovery of an additional, although quite atypical, Gγ subunit in Arabidopsis (AGG3) has helped to complete the picture and explains almost all of the missing agb1 'orphan' phenotypes. There is nevertheless still one unexplained phenotype, the reduction in rosette size reported for agb1, that has not been observed in any of the individual agg mutants or the double agg1agg2 mutant. We have now created a triple gamma mutant (agg1agg2agg3) in Arabidopsis and show that it recapitulates the remaining 'orphan'agb1 phenotypes. Triple agg1agg2agg3 mutants show the reduction in rosette size previously observed in agb1 mutants. In addition we show that small differences in flower and silique size observed between agb1 and agg3 mutants are also accounted for by the triple agg1agg2agg3 mutant. Our results strongly suggest that there are no additional members of the G-protein family remaining to be discovered in Arabidopsis.