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

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.

Characterization of the heterotrimeric G-protein family and its transmembrane regulator from capsicum (Capsicum annuum L.)

Throughout evolution, organisms have created numerous mechanisms to sense and respond to their environment. One such highly conserved mechanism involves regulation by heterotrimeric G-protein complex comprised of alpha (Gα), beta (Gβ) and gamma (Gγ) subunits. In plants, these proteins play important roles in signal transduction pathways related to growth and development including response to biotic and abiotic stresses and consequently affect yield. In this work, we have identified and characterized the complete heterotrimeric G-protein repertoire in the Capsicum annuum (Capsicum) genome which consists of one Gα, one Gβ and three Gγ genes. We have also identified one RGS gene in the Capsicum genome that acts as a regulator of the G-protein signaling. Biochemical activities of the proteins were confirmed by assessing the GTP-binding and GTPase activity of the recombinant Gα protein and its regulation by the GTPase acceleration activity of the RGS protein. Interaction between different subunits was established using yeast- and plant-based analyses. Gene and protein expression profiles of specific G-protein components revealed interesting spatial and temporal regulation patterns, especially during root development and during fruit development and maturation. This research thus details the characterization of the first heterotrimeric G-protein family from a domesticated, commercially important vegetable crop.

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.

Regulation of gibberellin 20-oxidase1 expression in spinach by photoperiod

The multifunctional gibberellin (GA) 20-oxidase [GA(53), 2-oxoglutarate:oxygen oxidoreductase (20-oxidizing), EC 1.14.11] has been extensively investigated in various species at the genetic and molecular levels, but not at the protein level. Here, we report on expression of GA20ox1 protein in spinach (Spinacia oleracea L.) in response to photoperiod. Polyclonal antibodies were raised against recombinant SoGA20ox1 in a chicken. These antibodies immuno-inhibited the enzymatic activity of the recombinant SoGA20ox1 and immuno-precipitated SoGA20ox1 (43 kDa) isolated from spinach shoot tips. Northern and western analyses showed that the levels of SoGA20ox1 transcript and protein increased in the blades, petioles, young leaves, and tips in response to long-day (LD) conditions. The transcript and protein levels of the SoGA20ox1 gene were up-regulated in the petioles and tips in a time-dependent manner. The estimated number of SoGA20ox1 protein molecules per cell was approximately 13-fold higher in tips grown in LD than in short-day (SD) conditions. The levels of SoGA20ox1 protein gradually decreased in tips when spinach plants grown in LD were transferred to SD conditions. SoGA20ox1 transcripts were detected by in situ hybridization in rapidly growing tissues--such as the shoot apical meristem, leaf and flower primordia, leaflets, and vascular tissues--but not in the expanding subapical region. In petioles, expression of SoGA20ox1 was detected in the companion cells.

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.

Heterotrimeric G-proteins in plant cell signaling

Heterotrimeric G-proteins, which couple cell surface receptors with internal effectors, are evident in all eukaryotes. Their operation involves receptor activation, GTP/GDP exchange and modulation of effector activity; deactivation occurs by an intrinsic GTPase activity. Structurally, G-proteins comprise three dissimilar subunits; Gα, Gβ and Gγ. The Gα subunit consists of an α-helical and a GTPase domain, the latter is responsible for interaction with Gβγ, receptor and effector. Gβ and Gγ form a tightly associated heterodimer which can also modulate effector activity when released by the activated Gα. Genome sequence and other data suggest that, in plants, there are several (~8-10?) Gα, one or two Gβ and one Gγ. These proteins are expressed throughout the plant, mainly in the plasma membrane and endoplasmic reticulum. In vivo, there is strong evidence for G-protein control of ion channels, particularly K⁺ , in the response pathways to fungal and bacterial pathogens as well as in some aspects of gibberellin, abscisic acid and auxin signaling pathways. Finally, future prospects for understanding plant G-protein linked signaling will rely on new and emerging technologies; these include antisense suppression, gene knockouts, yeast two-hybrid and phage display molecular approaches, intracellular immunization using recombinant single chain antibodies and expression of peptide encoding minigenes.