Wide range of interacting partners of pea Gβ subunit of G-proteins suggests its multiple functions in cell signalling

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

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

Overexpression of heterotrimeric G protein beta subunit gene (OsRGB1) confers both heat and salinity stress tolerance in rice

Constitutive overexpression of the rice heterotrimeric G protein beta subunit gene (RGB1) in the commercial rice cultivar BRRI Dhan 55 resulted in improved tolerance to heat or salinity or their combination. Two independently in planta transformed plants with the gene confirmed to be integrated at T₂ by Southern hybridization and showing high expression at the T₃ seedling stage showed better physiological performance after 8 days in 120 mM salt stress than the wild type. The plants had significantly lower electrolyte leakage and malondialdehyde production, while showing higher levels of chlorophyll. Significantly higher germination at 48 °C or with combined stresses of 42/40 °C day/night stress in the presence of 120 mM salt for 2 days was also observed. Stress responsive genes such as OsAPX1, OsSOD, OsHKT1, OsHSP1, OsHSP2 and OsCOR47 showed higher expression in the RGB1 positive plants. These RGB1 transgenic plants can likely provide a strong defense against climate change.

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

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

Ectopic Expression of Mulberry G-Proteins Alters Drought and Salt Stress Tolerance in Tobacco

Heterotrimeric guanine-nucleotide-binding proteins (G-proteins) play key roles in responses to various abiotic stress responses and tolerance in plants. However, the detailed mechanisms behind these roles remain unclear. Mulberry (Morus alba L.) can adapt to adverse abiotic stress conditions; however, little is known regarding the associated molecular mechanisms. In this study, mulberry G-protein genes, MaGα, MaGβ, MaGγ1, and MaGγ2, were independently transformed into tobacco, and the transgenic plants were used for resistance identification experiments. The ectopic expression of MaGα in tobacco decreased the tolerance to drought and salt stresses, while the overexpression of MaGβ, MaGγ1, and MaGγ2 increased the tolerance. Further analysis showed that mulberry G-proteins may regulate drought and salt tolerances by modulating reactive oxygen species' detoxification. This study revealed the roles of each mulberry G-protein subunit in abiotic stress tolerance and advances our knowledge of the molecular mechanisms underlying G-proteins' regulation of plant abiotic stress tolerance.

Pattern of expression and interaction specificity of multiple G-protein beta (Gβ) subunit isoforms with their potential target proteins reveal functional dominance of BjuGβ1 in the allotetraploid Brassica juncea

Heterotrimeric G-protein, consisting Gα, Gβ and Gγ subunits, interacts with various upstream and downstream effector (target) proteins to regulate a large array of conserved and species-specific biological functions. The targets of G-protein components are recently reported in model plant Arabidopsis thaliana; however limited information is available from crop species. In this study, we utilized yeast two-hybrid (Y2H) assay to screen the diversity of interacting partners of multiple Gβ subunit isoforms from allotetraploid Brassica juncea, a globally important oilseed and vegetable crop. The three BjuGβ genes (BjuGβ1-3), resulted from whole genome triplication event in Brassica lineage, showed distinct expression profile during plant developmental stages with maximal transcript abundance during reproductive stages. Protein-protein interaction of three BjuGβ proteins (bait) against the Y2H cDNA library (prey) identified a total of 14 and 1 non-redundant targets for BjuGβ1 and BjuGβ2, whereas BjuGβ3 screening surprisingly did not yield any genuine target, thereby suggesting functional dominance of BjuGβ1. The triplicated BjuGβ isoforms showed a high degree of interaction strength and specificity with the identified target proteins, which are known to be involved in diverse biological functions in plants. qRT-PCR analysis further indicated that the expression of BjuGβ-target genes was developmentally regulated under various tissue types studied and showed a high degree of co-expression pattern with the BjuGβ genes, particularly during flower and silique development in B. juncea. Taken together, our data provides novel insights on pattern of expression and interaction specificity governing functional divergence of multiple Gβ subunit proteins in polyploid B. juncea.

Function of heterotrimeric G-protein γ subunit RGG1 in providing salinity stress tolerance in rice by elevating detoxification of ROS

The present study provides evidence of a unique function of RGG1 in providing salinity stress tolerance in transgenic rice without affecting yield. It also provides a good example for signal transduction from the external environment to inside for enhanced agricultural production that withstands the extreme climatic conditions and ensures food security. The role of heterotrimeric G-proteins functioning as signalling molecules has not been studied as extensively in plants as in animals. Recently, their importance in plant stress signalling has been emerging. In this study, the function of rice G-protein γ subunit (RGG1) in the promotion of salinity tolerance in rice (Oryza sativa L. cv. IR64) was investigated. The overexpression of RGG1 driven by the CaMV35S promoter in transgenic rice conferred high salinity tolerance even in the presence of 200 mM NaCl. Transcript levels of antioxidative genes, i.e., CAT, APX, and GR, and their enzyme activities increased in salinity-stressed transgenic rice plants suggesting a better antioxidant system to cope the oxidative-damages caused by salinity stress. The RGG1-induced signalling events that conferred tolerance to salinity was mediated by increased gene expression of the enzymes that scavenged reactive oxygen species. In salinity-stressed RGG1 transgenic lines, the transcript levels of RGG2, RGB, RGA, DEP1, and GS3 also increased in addition to RGG1. These observations suggest that most likely the stoichiometry of the G-protein complex was not disturbed under stress. Agronomic parameters, endogenous sugar content (glucose and fructose) and hormones (GA3, zeatin and IAA) were also higher in the transgenic plants compared with the wild-type plants. A BiFC assay confirmed the interaction of RGG1 with different stress-responsive proteins which play active roles in signalling and prevention of aggregation of proteins under stress-induced perturbation. The present study will help in understanding the G-protein-mediated stress tolerance in plants.

The wiring diagram for plant G signaling

Like electronic circuits, the modular arrangement of cell-signaling networks decides how inputs produce outputs. Animal heterotrimeric guanine nucleotide binding proteins (G-proteins) operate as switches in the circuits that signal between extracellular agonists and intracellular effectors. There still is no biochemical evidence for a receptor or its agonist in the plant G-protein pathways. Plant G-proteins deviate in many important ways from the animal paradigm. This review covers important discoveries from the last two years that enlighten these differences and ends describing alternative wiring diagrams for the plant signaling circuits regulated by G-proteins. We propose that plant G-proteins are integrated in the signaling circuits as variable resistor rather than switches, controlling the flux of information in response to the cell's metabolic state.

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.

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.

A new DEAD-box helicase ATP-binding protein (OsABP) from rice is responsive to abiotic stress

The DEAD-box RNA helicase family comprise enzymes that participate in every aspect of RNA metabolism, associated with a diverse range of cellular functions including response to abiotic stress. In the present study, we report on the identification of a new DEAD-box helicase ATP-binding protein (OsABP) from rice which is upregulated in response e to multiple abiotic stress treatments  including NaCl, dehydration, ABA, blue and red light. It possesses an ORF of 2772 nt, encoding a protein of 923 aa, which contains the DEAD and helicase C-terminal domains, along with the nine conserved motifs specific to DEAD-box helicases. The in silico putative interaction with other proteins showed that OsABP interacts with proteins involved in RNA metabolism, signal transduction or stress response. These results imply that OsABP might perform important functions in the cellular response to specific abiotic stress.