Cloning and characterisation of PGA1 and PGA2: two G protein alpha-subunits from pea that promote growth in the yeast Saccharomyces cerevisiae

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.

Heterotrimeric G-protein α subunit (LeGPA1) confers cold stress tolerance to processing tomato plants (Lycopersicon esculentum Mill)

BACKGROUND

Tomatoes (Lycopersicon esculentum Mill) are key foods, and their molecular biology and evolution have been well described. Tomato plants originated in the tropics and, thus, are cold sensitive.

RESULTS

Here, we generated LeGPA1 overexpressing and RNA-interference (RNAi) transgenic tomato plants, which we then used to investigate the function of LeGPA1 in response to cold stress. Functional LeGPA1 was detected at the plasma membrane, and endogenous LeGPA1 was highly expressed in the roots and leaves. Cold treatment positively induced the expression of LeGPA1. Overexpression of LeGPA1 conferred tolerance to cold conditions and regulated the expression of genes related to the INDUCER OF CBF EXPRESSION-C-REPEAT-BINDING FACTOR (ICE-CBF) pathway in tomato plants. In the LeGPA1-overexpressing transgenic plants, the superoxide dismutase, peroxidase, and catalase activities and soluble sugar and proline contents were increased, and the production of reactive oxygen species and membrane lipid peroxidation decreased under cold stress.

CONCLUSIONS

Our findings suggest that improvements in antioxidant systems can help plants cope with the oxidative damage caused by cold stress, thereby stabilizing cell membrane structures and increasing the rate of photosynthesis. The data presented here provide evidence for the key role of LeGPA1 in mediating cold signal transduction in plant cells. These findings extend our knowledge of the roles of G-proteins in plants and help to clarify the mechanisms through which growth and development are regulated in processing tomato plants.

Pseudomonas fluorescens and Trichoderma asperellum Enhance Expression of Gα Subunits of the Pea Heterotrimeric G-protein during Erysiphe pisi Infection

We investigated the transcript accumulation patterns of all three subunits of heterotrimeric G-proteins (Gα1 and 2, Gβ, and Gγ) in pea under stimulation of two soil-inhabiting rhizosphere microbes Pseudomonas fluorescens OKC and Trichoderma asperellum T42. The microbes were either applied individually or co-inoculated and the transcript accumulation patterns were also investigated after challenging the same plants with a fungal biotrophic pathogen Erysiphe pisi. We observed that mostly the transcripts of Gα 1 and 2 subunits were accumulated when the plants were treated with the microbes (OKC and T42) either individually or co-inoculated. However, transcript accumulations of Gα subunits were highest in the T42 treatment particularly under the challenge of the biotroph. Transcript accumulations of the other two subunits Gβ and Gγ were either basal or even lower than the basal level. There was an indication for involvement of JA-mediated pathway in the same situations as activation of LOX1 and COI1 were relatively enhanced in the microbe co-inoculated treatments. Non-increment of SA content as well as transcripts of SA-dependent PR1 suggested non-activation of the SA-mediated signal transduction in the interaction of pea with E. pisi under the stimuli of OKC and T42. Gα1 and 2 transcript accumulations were further correlated with peroxidases activities, H2O2 generation and accumulation in ABA in pea leaves under OKC and T42 stimulations and all these activities were positively correlated with stomata closure at early stage of the biotroph challenge. The microbe-induced physiological responses in pea leaves finally led to reduced E. pisi development particularly in OKC and T42 co-inoculated plants. We conclude that OKC and T42 pretreatment stimulate transcript accumulations of the Gα1 and Gα2 subunits of the heterotrimeric G protein, peroxidases activities and phenol accumulation in pea during infection by E. pisi. The signal transduction was possibly mediated through JA in pea under the stimulus of the microbes and the cumulative effect of the co-inoculated microbes had a suppressive effect on E. pisi conidial development on pea leaves.

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.

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.

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.

Silencing of G proteins uncovers diversified plant responses when challenged by three elicitors in Nicotiana benthamiana

Signalling through heterotrimeric G protein composed of α-, β- and γ-subunits is essential in numerous physiological processes. Here we show that this prototypical G protein complex acts mechanistically by controlling elicitor sensitivity towards hypersensitive response (HR) and stomatal closure in Nicotiana benthamiana. Gα-, Gβ1-, and Gβ2-silenced plants were generated using virus-induced gene silencing. All silenced plants were treated with Xanthomonas oryzae harpin, Magnaporthe oryzae Nep1 and Phytophthora boehmeriae boehmerin, respectively. HR was dramatically impaired in Gα- and Gβ2-silenced plants treated with harpin, indicating that harpin-, rather than Nep1- or boehmerin-triggered HR, is Gα- and Gβ2-dependent. Moreover, all Gα-, Gβ1- and Gβ2-silenced plants significantly impaired elicitor-induced stomatal closure, elicitor-promoted nitric oxide (NO) production and active oxygen species accumulation in guard cells. To our knowledge, this is the first report of Gα and Gβ subunits involvement in stomatal closure in response to elicitors. Furthermore, silencing of Gα, Gβ1 and Gβ2 has an effect on the transcription of plant defence-related genes when challenged by three elicitors. In conclusion, silencing of G protein subunits results in many interesting plant cell responses, revealing that plant immunity systems employ both conserved and distinct G protein pathways to sense elicitors from distinct phytopathogens formed during plant-microbe evolution.

Ggamma1 + Ggamma2 not equal to Gbeta: heterotrimeric G protein Ggamma-deficient mutants do not recapitulate all phenotypes of Gbeta-deficient mutants

Heterotrimeric G proteins are signaling molecules ubiquitous among all eukaryotes. The Arabidopsis (Arabidopsis thaliana) genome contains one Galpha (GPA1), one Gbeta (AGB1), and two Ggamma subunit (AGG1 and AGG2) genes. The Gbeta requirement of a functional Ggamma subunit for active signaling predicts that a mutant lacking both AGG1 and AGG2 proteins should phenotypically resemble mutants lacking AGB1 in all respects. We previously reported that Gbeta- and Ggamma-deficient mutants coincide during plant pathogen interaction, lateral root development, gravitropic response, and some aspects of seed germination. Here, we report a number of phenotypic discrepancies between Gbeta- and Ggamma-deficient mutants, including the double mutant lacking both Ggamma subunits. While Gbeta-deficient mutants are hypersensitive to abscisic acid inhibition of seed germination and are hyposensitive to abscisic acid inhibition of stomatal opening and guard cell inward K+ currents, none of the available Ggamma-deficient mutants shows any deviation from the wild type in these responses, nor do they show the hypocotyl elongation and hook development defects that are characteristic of Gbeta-deficient mutants. In addition, striking discrepancies were observed in the aerial organs of Gbeta- versus Ggamma-deficient mutants. In fact, none of the distinctive traits observed in Gbeta-deficient mutants (such as reduced size of cotyledons, leaves, flowers, and siliques) is present in any of the Ggamma single and double mutants. Despite the considerable amount of phenotypic overlap between Gbeta- and Ggamma-deficient mutants, confirming the tight relationship between Gbeta and Ggamma subunits in plants, considering the significant differences reported here, we hypothesize the existence of new and as yet unknown elements in the heterotrimeric G protein signaling complex.

Plant signaling in stress: G-protein coupled receptors, heterotrimeric G-proteins and signal coupling via phospholipases

Plant growth and development are coordinalely controlled by several internal factors and environmental signals. To sense these environmental signals, the higher plants have evolved a complex signaling network, which may also cross talk with each other. Plants can respond to the signals as individual cells and as whole organisms. Various receptors including phytochromes, G-proteins coupled receptors (GPCR), kinase and hormone receptors play important role in signal transduction but very few have been characterized in plant system. The heterotrimeric G-proteins mediate the coupling of signal transduction from activated GPCR to appropriate downstream effectors and thereby play an important role in signaling. In this review we have focused on some of the recent work on G-proteins and two of the effectors, PLC and PLD, which have been shown to interact with Galpha subunit and also discussed their role in abiotic stress tolerance.

Heterotrimeric G-protein complex and G-protein-coupled receptor from a legume (Pisum sativum): role in salinity and heat stress and cross-talk with phospholipase C

Heterotrimeric G-proteins transduce signals from activated G-protein-coupled receptors (GPCR) to appropriate downstream effectors and thereby play an important role in signaling. A role of G-proteins in salinity and heat stress tolerance has not heretofore been described. We report isolation of cDNAs of two isoforms of Galpha (Galpha1, 1152 bp; Galpha2, 1152 bp), one Gbeta (1134 bp), two isoforms of Ggamma (Ggamma1, 345 bp; Ggamma2, 303 bp) and a GPCR (1008 bp) from Pisum sativum, and purification of all the encoded recombinant proteins (Galpha, 44 kDa; Gbeta, 41 kDa; Ggamma, 14 kDa; GPCR, 35 kDa). The transcript levels of Galpha and Gbeta were upregulated following NaCl, heat and H(2)O(2) treatments. Protein-protein interaction studies using an in vitro yeast two-hybrid system and in planta co-immunoprecipitation showed that the Galpha subunit interacted with the pea Gbeta subunit and pea phospholipase C (PLCdelta) at the calcium-binding domain (fn1). The GTPase activity of the Galpha subunit increased after interaction with PLCdelta. The GPCR protein interacted with all the subunits of G-proteins and with itself. Transgenic tobacco plants (T(0) and T(1)) constitutively over-expressing Galpha showed tolerance to salinity and heat, while Gbeta-over-expressing plants showed only heat tolerance, as tested by leaf disk senescence assay and germination/growth of T(1) seeds/seedlings. These findings provide direct evidence for a novel role of Galpha and Gbeta subunits in abiotic stress tolerance and possible cross-talk between PLC- and G-protein-mediated signaling pathways.

The GCR1, GPA1, PRN1, NF-Y signal chain mediates both blue light and abscisic acid responses in Arabidopsis

Different classes of biotic (e.g. plant hormones) and abiotic (e.g. different wavelengths of light) signals act through specific signal transduction mechanisms to coordinate higher plant development. While a great deal of progress has been made, full signal transduction chains have not yet been described for most blue light- or abscisic acid-mediated events. Based on data derived from T-DNA insertion mutants and yeast (Saccharomyces cerevisiae) two-hybrid and coprecipitation assays, we report a signal transduction chain shared by blue light and abscisic acid leading to light-harvesting chlorophyll a/b-binding protein expression in etiolated Arabidopsis (Arabidopsis thaliana) seedlings. The chain consists of GCR1 (the sole Arabidopsis protein coding for a potential G-protein-coupled receptor), GPA1 (the sole Arabidopsis Galpha-subunit), Pirin1 (PRN1; one of four members of an iron-containing subgroup of the cupin superfamily), and a nuclear factor Y heterotrimer comprised of A5, B9, and possibly C9. We also demonstrate that this mechanism is present in imbibed seeds wherein it affects germination rate.

G-protein-coupled receptor 1, G-protein Galpha-subunit 1, and prephenate dehydratase 1 are required for blue light-induced production of phenylalanine in etiolated Arabidopsis

Different classes of plant hormones and different wavelengths of light act through specific signal transduction mechanisms to coordinate higher plant development. A specific prephenate dehydratase protein (PD1) was discovered to have a strong interaction with the sole canonical G-protein Galpha-subunit (GPA1) in Arabidopsis (Arabidopsis thaliana). PD1 is a protein located in the cytosol, present in etiolated seedlings, with a specific role in blue light-mediated synthesis of phenylpyruvate and subsequently of phenylalanine (Phe). Insertion mutagenesis confirms that GPA1 and the sole canonical G-protein-coupled receptor (GCR1) in Arabidopsis also have a role in this blue light-mediated event. In vitro analyses indicate that the increase in PD1 activity is the direct and specific consequence of its interaction with activated GPA1. Because of their shared role in the light-mediated synthesis of phenylpyruvate and Phe, because they are iteratively interactive, and because activated GPA1 is directly responsible for the activation of PD1; GCR1, GPA1, and PD1 form all of or part of a signal transduction mechanism responsible for the light-mediated synthesis of phenylpyruvate, Phe, and those metabolites that derive from that Phe. Data are also presented to confirm that abscisic acid can act through the same pathway. An additional outcome of the work is the confirmation that phenylpyruvate acts as the intermediate in the synthesis of Phe in etiolated plants, as it commonly does in bacteria and fungi.

Cloning and characterization of two full-length cDNAs, TaGA1 and TaGA2, encoding G-protein alpha subunits expressed differentially in wheat genome

In the present study, we identified and characterized two cDNAs, named TaGA1 and TaGA2, encoding alpha subunits of heterotrimeric G proteins synthesized from one-week-old seedling mRNAs of common wheat cv. S615 using RACE PCR and RT-PCR methods. The clone TaGA1 contained an open reading frame that encoded a protein consisting of 383 amino acid residues with a molecular mass of 51.3 kDa, whereas the clone TaGA2 contained an open reading frame encoding 390 amino acids with a molecular mass of 52.5 kDa. At the amino acid level, both cDNAs (TaGA1 and TaGA2) showed 70-96% and 30-40% homologies to plant and animal G-protein alpha (G alpha) subunits, respectively, and 97.7% homology to each other. The regions essential for binding to GTP were conserved among all G alpha subunits in higher plants and mammals examined. However, the C-terminal amino acid sequences of TaGA1 and TaGA2 were similar to those of cereal G alpha subunits (rice and barley) but were different from the analogous sequences of mammalian G alpha subunits as well as from those of the leguminous and Solanaeceous G alpha subunits. Southern analysis revealed that the hexaploid wheat genome contained three major copies of G alpha subunit gene with a few less homologous copies. The analysis of the expression for G alpha subunit genes in wheat showed that both TaGA1 and TaGA2 mRNAs were abundant in one-week-old seedlings, immature seeds harvested one-week after anthesis, young spikes and internodes, indicating constitutive expression patterns in all of the organs tested. Especially, young spikes and internodes exhibited increased levels of mRNA accumulation, suggesting that G alpha subunit gene is highly expressed in actively elongating and fast growing tissues. Moreover, both TaGA1 and TaGA2 showed genome-specific expressions in wheat and may participate in the light-regulated growth and development of the seedlings.

Structure and function of heterotrimeric G proteins in plants

Heterotrimeric G proteins are mediators that transmit the external signals via receptor molecules to effector molecules. The G proteins consist of three different subunits: alpha, beta, and gamma subunits. The cDNAs or genes for all the alpha, beta, and gamma subunits have been isolated from many plant species, which has contributed to great progress in the study of the structure and function of the G proteins in plants. In addition, rice plants lacking the alpha subunit were generated by the antisense method and a rice mutant, Daikoku d1, was found to have mutation in the alpha-subunit gene. Both plants show abnormal morphology such as dwarfism, dark green leaf, and small round seed. The findings revealed that the G proteins are functional molecules regulating some body plans in plants. There is evidence that the plant G proteins participate at least in signaling of gibberellin at low concentrations. In this review, we summarize the currently known information on the structure of plant heterotrimeric G proteins and discuss the possible functions of the G proteins in plants.

Cloning and direct G-protein regulation of phospholipase D from tobacco

Phospholipase D (PLD) and heterotrimeric G-proteins are involved in plant signal transduction pathways at the plasma membrane. There is evidence suggesting that PLD acts downstream from G-proteins, but a direct interaction of specific members has not been shown. In the present paper, a PLD cDNA clone was isolated from tobacco, expressed as a GST fusion in bacteria, and the recombinant protein was purified by glutathione affinity. Its enzymatic properties identified it as an alpha-type PLD. The alpha-subunit of a G-protein from tobacco was isolated in a similar way. Both proteins were functional in biochemical assays. When the G-protein was included in the PLD assay, a strong dosage-dependent inhibition of the PLD activity was observed. Different control proteins did not exhibit this inhibitory effect. When GST-NtGPalpha1 was activated by incubation with GTPgammaS the inhibitory activity was greatly reduced. These results provide a first indication for a direct regulation of PLDalpha by a heterotrimeric G-protein alpha-subunit in plants.