Isolation and analysis of the soybean SGA2 gene (cDNA), encoding a new member of the plant G-protein family of signal transducers

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.

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.

An elaborate heterotrimeric G-protein family from soybean expands the diversity of plant G-protein networks

The repertoire of heterotrimeric G-proteins in plant species analyzed thus far is simple, with the presence of only two possible canonical heterotrimers in Arabidopsis and rice vs hundreds in animal systems. We assessed whether genome duplication events have resulted in the multiplicity of G-protein in plant species like soybean that would increase the complexity of G-protein networks. We identified and amplified four Gα, four Gβ and two Gγ proteins, analyzed their expression profile by quantitative PCR during different developmental stages. We purified the four Gα proteins and analyzed their guanosine-5'-triphosphate (GTP)-binding and GTPase activity. We performed yeast-based interaction analysis to assess the interaction specificity of different G-protein subunits. Our results show that all 10 G-protein genes are retained in the soybean genome and ubiquitously expressed. The four Gα proteins seem to be plasma membrane-localized. The G-protein genes have interesting expression profiles during seed development and germination. The four Gα proteins form two distinct groups based on their GTPase activity. Yeast-based interaction analyses predict that the proteins interact in most of the possible combinations, with some degree of interaction specificity between duplicated gene pairs. This research identifies the most elaborate heterotrimeric G-protein network known to date in the plant kingdom.

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.

Proteomic analysis of rice embryo: an approach for investigating Galpha protein-regulated proteins

The rice dwarf1 (d1) mutant, which lacks the alpha subunit of a heterotrimeric G protein (Galpha protein), shows abnormal morphology due to shortened internodes, dark green leaves and grains that are small and round. Proteome analysis was used in this study to aid in determining the function of Galpha protein in rice embryos. Using 2-DE, seven seed embryo proteins were shown to be down-regulated in the d1 mutant as compared with its wild type. These seven proteins included a receptor for activated C-kinase (RACK) and six rice embryo globulin-2 proteins (REG2). The six REG2 have similar molecular masses with minor differences in pI. In addition to the reduced accumulation of RACK in the d1 mutant, the increase in QL/d1, in which a constitutively active form of the Galpha protein is expressed, was significantly higher as compared with wild type. The level of accumulation of these seven proteins during seed development and maturation did not change significantly until the 2nd wk after pollination. Reduced accumulation of these seven proteins started in the d1 mutant at the 3rd wk after pollination, and continued until seed maturation was complete. All seven proteins were completely absent 24 h after imbibition in both d1 mutant and its wild type. However, the phytohormone abscisic acid promoted the expression level of RACK after imbibition in the wild type as compared with d1 mutant. These results suggest that RACK is regulated by Galpha-protein and plays an important role in a basic cellular process as well as in rice embryogenesis and germination.

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.

Completing the heterotrimer: isolation and characterization of an Arabidopsis thaliana G protein gamma-subunit cDNA

Heterotrimeric G proteins consist of three subunits (alpha, beta, and gamma). alpha- and beta- subunits have been previously cloned in plants, but the gamma-subunit has remained elusive. To isolate the gamma-subunit of a plant heterotrimeric G protein an Arabidopsis thaliana yeast two-hybrid library was screened by using a tobacco G-beta-subunit as the bait protein. One positive clone (AGG1) was isolated several times; it displays significant homology to the conserved domains of mammalian gamma-subunits. The predicted AGG1 protein sequence contains all of the typical characteristics of mammalian gamma-subunits such as small size (98 amino acids, 10.8 kDa), presence of a C-terminal CAAX box to direct isoprenyl modification, and an N-terminal alpha-helix region capable of forming a coiled-coil interaction with the beta-subunit. Northern and Southern analyses showed that AGG1 is a single-copy gene in Arabidopsis with a similar expression pattern to the Arabidopsis beta-subunit, AGB1 [Weiss, C. A., Garnaat, C. W., Mukai, K., Hu, Y. & Ma, H. (1994) Proc. Natl. Acad. Sci. USA 91, 9554-9558]. By using the yeast two-hybrid system, we show that AGG1 strongly interacts with tobacco and Arabidopsis beta-subunits. The in vivo results have been confirmed by using in vitro methods to prove the interaction between AGG1 and the Arabidopsis beta-subunit. As previously observed in mammalian systems, both the coiled-coil domain and the WD repeat regions of the beta-subunit are essential for AGG1 interaction. Also in agreement with previous observations, the removal of the N-terminal alpha-helix of the AGG1 greatly reduces but does not completely block the interaction.

Rice dwarf mutant d1, which is defective in the alpha subunit of the heterotrimeric G protein, affects gibberellin signal transduction

Previously, we reported that the rice dwarf mutant, d1, is defective in the alpha subunit of the heterotrimeric G protein (Galpha). In the present study, gibberellin (GA) signaling in d1 and the role of the Galpha protein in the GA-signaling pathway were investigated. Compared with the wild type, GA induction of alpha-amylase activity in aleurone cells of d1 was greatly reduced. Relative to the wild type, the GA(3)-treated aleurone layer of d1 had lower expression of Ramy1A, which encodes alpha-amylase, and OsGAMYB, which encodes a GA-inducible transcriptional factor, and no increase in expression of Ca(2 +)-ATPase. However, in the presence of high GA concentrations, alpha-amylase induction occurred even in d1. The GA sensitivity of second leaf sheath elongation in d1 was similar to that of the wild type in terms of dose responsiveness, but the response of internode elongation to GA was much lower in d1. Furthermore, Os20ox expression was up-regulated, and the GA content was elevated in the stunted internodes of d1. All these results suggest that d1 affects a part of the GA-signaling pathway, namely the induction of alpha-amylase in the aleurone layer and internode elongation. In addition, a double mutant between d1 and another GA-signaling mutant, slr, revealed that SLR is epistatic to the D1, supporting that the Galpha protein is involved in GA signaling. However, the data also provide evidence for the presence of an alternative GA-signaling pathway that does not involve the Galpha protein. It is proposed that GA signaling via the Galpha protein may be more sensitive than that of the alternative pathway, as indicated by the low GA responsiveness of this Galpha-independent pathway.

Molecular cloning, spatial and temporal characterization of spinach SOGA1 cDNA, encoding an alpha subunit of G protein

Heterotrimeric G proteins are an important component of signal transduction pathway in animals. Although these proteins have been described in plants, their exact function and action mode are not clearly defined. In order to analyze the relationship between these proteins and the transduction of light signals in spinach, we have isolated by 5' and 3' RACE-PCR a 1660bp cDNA clone called SOGA1. This codes for a 383aa protein, which reveals a very strong homology with other plant Galpha subunit sequences. Genomic analysis suggested that SOGA1 belonged to a small multiple gene family. Northern blots and in-situ hybridization analyses showed that SOGA1 transcripts accumulate in all organs tested with a specific high level associated with the apex, roots and hypocotyls. Finally, a time-course analysis performed on the green tissues showed that accumulation of SOGA1 transcripts follows a circadian rhythm. However, in-situ hybridization analysis of the apex suggested the opposite behavior, while no variation was observed in the hypocotyl.

Molecular characterization of cDNAs encoding G protein alpha and beta subunits and study of their temporal and spatial expression patterns in Nicotiana plumbaginifolia Viv

We have isolated cDNA sequences encoding alpha and beta subunits of potential G proteins from a cDNA library prepared from somatic embryos of Nicotiana plumbaginifolia Viv. at early developmental stages. The predicted NPGPA1 and NPGPB1 gene products are 75-98% identical to the known respective plant alpha and beta subunits. Southern hybridizations indicate that NPGPA1 is probably a single-copy gene, whereas at least two copies of NPGPB1 exist in the N. plumbaginifolia genome. Northern analyses reveal that both NPGPA1 and NPGPB1 mRNA are expressed in all embryogenic stages and plant tissues examined and their expression is obviously regulated by the plant hormone auxin. Immunohistological localization of NPGPalpha1 and NPGPbeta1 preferentially on plasma and endoplasmic reticulum membranes and their immunochemical detection exclusively in microsomal cell fractions implicate membrane association of both proteins. The temporal and spatial expression patterns of NPGPA1 and NPGPB1 show conformity as well as differences. This could account for not only cooperative, but also individual activities of both subunits during embryogenesis and plant development.

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

We report here on the cloning and characterization of two G protein alpha-subunits from pea: PGA1 and PGA2. Based on DNA gel blot analysis, PGA1 and PGA2 are the only Galpha homologous sequences in pea. RT-PCR analysis reveals that PGA1 and PGA2 transcripts are present in a variety of adult pea tissues. However, PGA2 mRNA is consistently detected at a lower level than PGA1 and demonstrates some degree of tissue specificity relative to PGA1. In the apical bud of pea seedlings, PGA1 and PGA2 transcripts decrease in response to 24 h of white light following growth for 6 days in darkness. The G protein mediated, yeast mating pathway was used to analyse the function of PGA1 and PGA2 in vivo. PGA1 downregulates the mating pathway, but through a mechanism that is independent of Gbetagamma sequestration. Unexpectedly, both PGA1 and PGA2 promote growth through a mating pathway independent mechanism.

The G protein alpha subunit (GP alpha1) is associated with the ER and the plasma membrane in meristematic cells of Arabidopsis and cauliflower

Towards the elucidation of the cellular function(s) of GP alpha1, we have characterized its subcellular localization using immunofluorescence and cell fractionation. GP alpha1 is not present in nuclei or chloroplasts. It is a membrane-bound protein, and analysis of isolated endoplasmic and plasma membranes indicates a good correlation between GP alpha1 in both the plasma membrane and the ER compartment. Interestingly, these results may suggest more different functions for GP alpha1: it might be involved in transmission of extracellular signals across the plasma membrane and in the cytoplasm, and/or it may also be involved in regulating some aspects of the ER functions or membrane trafficking between both membranes.