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

Site-directed mutagenesis of the Arabidopsis heterotrimeric G protein β subunit suggests divergent mechanisms of effector activation between plant and animal G proteins

Heterotrimeric G proteins are integral components of signal transduction in humans and other mammals and have been therefore extensively studied. However, while they are known to mediate many processes, much less is currently known about the effector pathways and molecular mechanisms used by these proteins to regulate effectors in plants. We designed a complementation strategy to study G protein signaling in Arabidopsis thaliana, particularly the mechanism of action of AGB1, the sole identified β subunit. We used biochemical and effector regulation data from human G protein studies to identify four potentially important residues for site-directed mutagenesis (T65, M111, D250 and W361 of AGB1). Each residue was individually mutated and the resulting mutated protein introduced in the agb1-2 mutant background under the control of the native AGB1 promoter. Interestingly, even though these mutations have been shown to have profound effects on effector signaling in humans, all the mutated subunits were able to restore thirteen of the fifteen Gβ-deficient phenotypes characterized in this study. Only one mutated protein, T65A was unable to complement the hypersensitivity to mannitol during germination observed in agb1 mutants; while only D250A failed to restore lateral root numbers in the agb1 mutant to wild-type levels. Our results suggest that the mechanisms used in mammalian G protein signaling are not well conserved in plant G protein signaling, and that either the effectors used by plant G proteins, or the mechanisms used to activate them, are at least partially divergent from the well-studied mammalian G proteins.

Ca2+-dependent GTPase, extra-large G protein 2 (XLG2), promotes activation of DNA-binding protein related to vernalization 1 (RTV1), leading to activation of floral integrator genes and early flowering in Arabidopsis

Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein α subunit, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3, of largely unknown function. Each extra-large G (XLG) protein has a C-terminal Gα-like region and a ∼400 amino acid N-terminal extension. Here we show that the three XLG proteins specifically bind and hydrolyze GTP, despite the fact that these plant-specific proteins lack key conserved amino acid residues important for GTP binding and hydrolysis of GTP in mammalian Gα proteins. Moreover, unlike other known Gα proteins, these activities require Ca(2+) instead of Mg(2+) as a cofactor. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RTV1 (related to vernalization 1). Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence-specific manner and that GTP-bound XLG2 promotes the DNA binding activity of RTV1. Overexpression of RTV1 results in early flowering. Combined overexpression of XLG2 and RTV1 enhances this early flowering phenotype and elevates expression of the floral pathway integrator genes, FT and SOC1, but does not repress expression of the floral repressor, FLC. Chromatin immunoprecipitation assays show that XLG2 increases RTV1 binding to FT and SOC1 promoters. Thus, a Ca(2+)-dependent G protein, XLG2, promotes RTV1 DNA binding activity for a subset of floral integrator genes and contributes to floral transition.

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.

Two G-protein-coupled-receptor candidates, Cand2 and Cand7, are involved in Arabidopsis root growth mediated by the bacterial quorum-sensing signals N-acyl-homoserine lactones

Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) as quorum sensing (QS) signaling molecules to coordinate their group behavior. Recently, it was shown that plants can perceive and respond to these bacterial AHLs. However, little is known about the molecular mechanism underlying the response of plants to bacterial QS signals. In this study, we show that the promotion of root elongation in wild type Arabidopsis thaliana induced by the AHLs N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL) or N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) was completely abolished in plants with loss-of-function mutations in two candidate G-protein Coupled Receptors (GPCRs), Cand2 and Cand7. Furthermore, real-time PCR analysis revealed that the expression levels of Cand2 and Cand7 were elevated in plants treated with 3OC6-HSL or 3OC8-HSL. These results suggest that Cand2 and Cand7 are involved in the regulation of root growth by bacterial AHLs and that GPCRs play a role in mediating interactions between plants and microbes.

An atypical heterotrimeric G-protein γ-subunit is involved in guard cell K⁺-channel regulation and morphological development in Arabidopsis thaliana

Currently, there are strong inconsistencies in our knowledge of plant heterotrimeric G-proteins that suggest the existence of additional members of the family. We have identified a new Arabidopsis G-protein γ-subunit (AGG3) that modulates morphological development and ABA-regulation of stomatal aperture. AGG3 strongly interacts with the Arabidopsis G-protein β-subunit in vivo and in vitro. Most importantly, AGG3-deficient mutants account for all but one of the 'orphan' phenotypes previously unexplained by the two known γ-subunits in Arabidopsis. AGG3 has unique characteristics never before observed in plant or animal systems, such as its size (more than twice that of canonical γ-subunits) and the presence of a C-terminal Cys-rich domain. AGG3 thus represent a novel class of G-protein γ-subunits, widely spread throughout the plant kingdom but not present in animals. Homologues of AGG3 in rice have been identified as important quantitative trait loci for grain size and yield, but due to the atypical nature of the proteins their identity as G-protein subunits was thus far unknown. Our work demonstrates a similar trend in seeds of Arabidopsis agg3 mutants, and implicates G-proteins in such a crucial agronomic trait. The discovery of this highly atypical subunit reinforces the emerging notion that plant and animal G-proteins have distinct as well as shared evolutionary pathways.

Suppression of the rice heterotrimeric G protein β-subunit gene, RGB1, causes dwarfism and browning of internodes and lamina joint regions

In the present study, we investigated the function of the heterotrimeric G protein β-subunit (Gβ) gene (RGB1) in rice. RGB1 knock-down lines were generated in the wild type and d1-5, a mutant deficient for the heterotrimeric G protein α-subunit (Gα) gene (RGA1). Both transgenic lines showed browning of the lamina joint regions and nodes that could be attributed to a reduction of RGB1 function, as the abnormality was not observed in d1-5. The RGB1 knock-down lines generated in d1-5 were shorter, suggesting RGB1 to be a positive regulator of cellular proliferation, in addition to RGA1. The number of sterile seeds also increased in both RGB1 knock-down lines. These results suggest that Gβγ and Gα cooperatively function in cellular proliferation and seed fertility. We discuss the potential predominant role of RGB1 in G protein signaling in rice.

Roles of heterotrimeric G proteins in guard cell ion channel regulation

Stomata are formed by pairs of surrounding guard cells and perform important roles in photosynthesis, transpiration and innate immunity of terrestrial plants. Ionic solutes in the cytosol of guard cells are important for cell turgor and volume change. Consequently, transmembrane flux of ions such as K+, Cl-, and malate2- through K+ channels and anion channels of guard cells are a direct driving force for turgor change, while the opening of calcium permeable channels can serve as a trigger of cytosolic free calcium concentration elevations or oscillations, which play second messenger roles. In plants, heterotrimeric G proteins have fewer members than in animals, but they are well investigated and found to regulate these channels and to play fundamental roles in guard cell function. This mini-review focuses on the recent understanding of G-protein regulation of ion channels on the plasma membrane of guard cells and their participation in stomatal movements.

Functional reconstitution of an atypical G protein heterotrimer and regulator of G protein signaling protein (RGS1) from Arabidopsis thaliana

It has long been known that animal heterotrimeric Gαβγ proteins are activated by cell-surface receptors that promote GTP binding to the Gα subunit and dissociation of the heterotrimer. In contrast, the Gα protein from Arabidopsis thaliana (AtGPA1) can activate itself without a receptor or other exchange factor. It is unknown how AtGPA1 is regulated by Gβγ and the RGS (regulator of G protein signaling) protein AtRGS1, which is comprised of an RGS domain fused to a receptor-like domain. To better understand the cycle of G protein activation and inactivation in plants, we purified and reconstituted AtGPA1, full-length AtRGS1, and two putative Gβγ dimers. We show that the Arabidopsis Gα protein binds to its cognate Gβγ dimer directly and in a nucleotide-dependent manner. Although animal Gβγ dimers inhibit GTP binding to the Gα subunit, AtGPA1 retains fast activation in the presence of its cognate Gβγ dimer. We show further that the full-length AtRGS1 protein accelerates GTP hydrolysis and thereby counteracts the fast nucleotide exchange rate of AtGPA1. Finally, we show that AtGPA1 is less stable in complex with GDP than in complex with GTP or the Gβγ dimer. Molecular dynamics simulations and biophysical studies reveal that altered stability is likely due to increased dynamic motion in the N-terminal α-helix and Switch II of AtGPA1. Thus, despite profound differences in the mechanisms of activation, the Arabidopsis G protein is readily inactivated by its cognate RGS protein and forms a stable, GDP-bound, heterotrimeric complex similar to that found in animals.

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.

Identification of shared single copy nuclear genes in Arabidopsis, Populus, Vitis and Oryza and their phylogenetic utility across various taxonomic levels

BACKGROUND

Although the overwhelming majority of genes found in angiosperms are members of gene families, and both gene- and genome-duplication are pervasive forces in plant genomes, some genes are sufficiently distinct from all other genes in a genome that they can be operationally defined as 'single copy'. Using the gene clustering algorithm MCL-tribe, we have identified a set of 959 single copy genes that are shared single copy genes in the genomes of Arabidopsis thaliana, Populus trichocarpa, Vitis vinifera and Oryza sativa. To characterize these genes, we have performed a number of analyses examining GO annotations, coding sequence length, number of exons, number of domains, presence in distant lineages, such as Selaginella and Physcomitrella, and phylogenetic analysis to estimate copy number in other seed plants and to demonstrate their phylogenetic utility. We then provide examples of how these genes may be used in phylogenetic analyses to reconstruct organismal history, both by using extant coverage in EST databases for seed plants and de novo amplification via RT-PCR in the family Brassicaceae.

RESULTS

There are 959 single copy nuclear genes shared in Arabidopsis, Populus, Vitis and Oryza ["APVO SSC genes"]. The majority of these genes are also present in the Selaginella and Physcomitrella genomes. Public EST sets for 197 species suggest that most of these genes are present across a diverse collection of seed plants, and appear to exist as single or very low copy genes, though exceptions are seen in recently polyploid taxa and in lineages where there is significant evidence for a shared large-scale duplication event. Genes encoding proteins localized in organelles are more commonly single copy than expected by chance, but the evolutionary forces responsible for this bias are unknown.Regardless of the evolutionary mechanisms responsible for the large number of shared single copy genes in diverse flowering plant lineages, these genes are valuable for phylogenetic and comparative analyses. Eighteen of the APVO SSC single copy genes were amplified in the Brassicaceae using RT-PCR and directly sequenced. Alignments of these sequences provide improved resolution of Brassicaceae phylogeny compared to recent studies using plastid and ITS sequences. An analysis of sequences from 13 APVO SSC genes from 69 species of seed plants, derived mainly from public EST databases, yielded a phylogeny that was largely congruent with prior hypotheses based on multiple plastid sequences. Whereas single gene phylogenies that rely on EST sequences have limited bootstrap support as the result of limited sequence information, concatenated alignments result in phylogenetic trees with strong bootstrap support for already established relationships. Overall, these single copy nuclear genes are promising markers for phylogenetics, and contain a greater proportion of phylogenetically-informative sites than commonly used protein-coding sequences from the plastid or mitochondrial genomes.

CONCLUSIONS

Putatively orthologous, shared single copy nuclear genes provide a vast source of new evidence for plant phylogenetics, genome mapping, and other applications, as well as a substantial class of genes for which functional characterization is needed. Preliminary evidence indicates that many of the shared single copy nuclear genes identified in this study may be well suited as markers for addressing phylogenetic hypotheses at a variety of taxonomic levels.

Identification of shared single copy nuclear genes in Arabidopsis, Populus, Vitis and Oryza and their phylogenetic utility across various taxonomic levels

Background

Although the overwhelming majority of genes found in angiosperms are members of gene families, and both gene- and genome-duplication are pervasive forces in plant genomes, some genes are sufficiently distinct from all other genes in a genome that they can be operationally defined as 'single copy'. Using the gene clustering algorithm MCL-tribe, we have identified a set of 959 single copy genes that are shared single copy genes in the genomes of Arabidopsis thaliana, Populus trichocarpa, Vitis vinifera and Oryza sativa. To characterize these genes, we have performed a number of analyses examining GO annotations, coding sequence length, number of exons, number of domains, presence in distant lineages, such as Selaginella and Physcomitrella, and phylogenetic analysis to estimate copy number in other seed plants and to demonstrate their phylogenetic utility. We then provide examples of how these genes may be used in phylogenetic analyses to reconstruct organismal history, both by using extant coverage in EST databases for seed plants and de novo amplification via RT-PCR in the family Brassicaceae.

Results

There are 959 single copy nuclear genes shared in Arabidopsis, Populus, Vitis and Oryza ["APVO SSC genes"]. The majority of these genes are also present in the Selaginella and Physcomitrella genomes. Public EST sets for 197 species suggest that most of these genes are present across a diverse collection of seed plants, and appear to exist as single or very low copy genes, though exceptions are seen in recently polyploid taxa and in lineages where there is significant evidence for a shared large-scale duplication event. Genes encoding proteins localized in organelles are more commonly single copy than expected by chance, but the evolutionary forces responsible for this bias are unknown.

Regardless of the evolutionary mechanisms responsible for the large number of shared single copy genes in diverse flowering plant lineages, these genes are valuable for phylogenetic and comparative analyses. Eighteen of the APVO SSC single copy genes were amplified in the Brassicaceae using RT-PCR and directly sequenced. Alignments of these sequences provide improved resolution of Brassicaceae phylogeny compared to recent studies using plastid and ITS sequences. An analysis of sequences from 13 APVO SSC genes from 69 species of seed plants, derived mainly from public EST databases, yielded a phylogeny that was largely congruent with prior hypotheses based on multiple plastid sequences. Whereas single gene phylogenies that rely on EST sequences have limited bootstrap support as the result of limited sequence information, concatenated alignments result in phylogenetic trees with strong bootstrap support for already established relationships. Overall, these single copy nuclear genes are promising markers for phylogenetics, and contain a greater proportion of phylogenetically-informative sites than commonly used protein-coding sequences from the plastid or mitochondrial genomes.

Conclusions

Putatively orthologous, shared single copy nuclear genes provide a vast source of new evidence for plant phylogenetics, genome mapping, and other applications, as well as a substantial class of genes for which functional characterization is needed. Preliminary evidence indicates that many of the shared single copy nuclear genes identified in this study may be well suited as markers for addressing phylogenetic hypotheses at a variety of taxonomic levels.

Function and expression pattern of the alpha subunit of the heterotrimeric G protein in rice

The d1 mutant, which is deficient for the heterotrimeric G-protein alpha subunit (Galpha) gene of rice, shows dwarfism and sets small round seeds. To determine whether dwarfism in d1 is due to a reduction in cell number or to shortened cell length, the cell number of the leaf sheath, the internode, the root and the lemma was compared between Nipponbare, a wild-type rice and d1-5, a d1 allele derived from Nipponbare. Our results indicate that the cell number was reduced in all organs analyzed in d1-5. In addition, cell enlargement was found in roots and lemma of d1-5, although the organ length in d1-5 was shorter than that of wild-type rice. These results suggest that rice Galpha participates in cell proliferation in rice. Western blot analyses using anti-Galpha antibody and RT-PCR analyses indicate that Galpha is mostly expressed in the developing organs. Galpha promoter activity studies using the GUS reporter gene confirmed that the expression of Galpha was highest in developing organs. We conclude that rice Galpha participates in the regulation of cell number in a developmental stage-dependent manner.

Heterotrimeric G proteins regulate reproductive trait plasticity in response to water availability

Phenotypic plasticity is the ability of one genotype to display different phenotypes under different environmental conditions. Although variation for phenotypic plasticity has been documented in numerous species, little is known about the genetic mechanisms underlying phenotypic plasticity. Given their widespread roles in hormonal and environmental signaling, we examined whether genes which encode heterotrimeric G proteins are plasticity genes. We grew multiple alleles of heterotrimeric G-protein mutants, together with wild-type Arabidopsis thaliana, under different watering regimes to determine the contributions of G-protein genes to phenotypic plasticity for a number of developmental and reproduction-related traits. G-protein mutations did not affect significantly the amount of phenotypic variation within an environment for any trait, but did affect significantly the amount of phenotypic plasticity for certain traits. AGB1, which encodes the beta subunit of the heterotrimeric G protein in Arabidopsis, is a plasticity gene and regulates reproductive trait plasticity in response to water availability, resulting in increased fitness (defined as seed production) under drought stress.

Arabidopsis extra large G-protein 2 (XLG2) interacts with the Gbeta subunit of heterotrimeric G protein and functions in disease resistance

Heterotrimeric GTP-binding proteins, which consist of Galpha, Gbeta, and Ggamma subunits, play important roles in transducing extracellular signals perceived by cell surface receptors into intracellular physiological responses. In addition to a single prototypical Galpha protein (GPA1), Arabidopsis has three unique Galpha-like proteins, known as XLG1, XLG2, and XLG3, that have been found to be localized in nuclei, although their functions and mode of action remain largely unknown. Through a transcriptomic analysis, we found that XLG2 and XLG3 were rapidly induced by infection with the bacterial pathogen Pseudomonas syringae, whereas the XLG1 transcript level was not affected by pathogen infection. A reverse genetic screen revealed that the xlg2 loss-of-function mutation causes enhanced susceptibility to P. syringae. Transcriptome profiling revealed that the xlg2 mutation affects pathogen-triggered induction of a small set of defense-related genes. However, xlg1 and xlg3 mutants showed no difference from wild-type plants in resistance to P. syringae. In addition, the xlg2 xlg3 double mutant and the xlg1 xlg2 xlg3 triple mutant were not significantly different from the xlg2 single mutant in the disease resistance phenotype, suggesting that the roles of XLG1 and XLG3 in defense, if any, are less significant than for XLG2. Constitutive overexpression of XLG2 leads to the accumulation of abnormal transcripts from multiple defense-related genes. Through co-immunoprecipitation assays, XLG2 was found to interact with AGB1, the sole Gbeta subunit in Arabidopsis, which has previously been found to be a positive regulator in resistance to necrotrophic fungal pathogens. However, no significant difference was found between three xlg single mutants, the xlg2 xlg3 double mutant, the xlg triple mutant, and wild-type plants in resistance to the necrotrophic fungal pathogens Botrytis cinerea or Alternaria brassicicola. These results suggest that XLG2 and AGB1 are components of a G-protein complex different from the prototypical heterotrimeric G-protein and may have distinct functions in modulating defense responses.

Protein identification and mRNA analysis of phytochrome-regulated genes in Arabidopsis under red light

Phytochromes are a family of plant photoreceptors that mediate physiological and developmental responses to red and far-red light. According to the affymetrix ATH1 microarray, phytochrome A (phyA) and phytochrome B (phyB) together play a key role in transducing the Rc signals to light-responsive genes. In order to select those red light-responsive genes associated with phyA or phyB, a proteomic approach based on two-dimensional gel electrophoresis (2-DE) was used to compare the protein expression patterns of the phyAphyB double mutant and the wild type of Arabidopsis thaliana (col-4) which grew under constant red light conditions for 7 d. Thirty-two protein spots which exhibited differences in protein abundance were identified by matrix-assisted laser desorption/ionization-time of flight/time of flight mass spectrometry. The expression of ten genes corresponding to ten protein spots was analyzed by a semiquantitative reverse transcription-polymerase chain reaction. Two of the ten genes were confirmed by quantitative PCR (Q-PCR). The results showed that phytochromes may exert their function by regulating mRNA or protein expressions. Proteomic analysis may provide a novel pathway for identifying phytochrome-dependent genes.

Plant volatiles regulate the activities of Ca2+ -permeable channels and promote cytoplasmic calcium transients in Arabidopsis leaf cells

A variety of plant species emit volatile compounds in response to mechanical stresses such as herbivore attack. Although these volatile compounds promote gene expression leading to anti-herbivore responses, the underlying transduction mechanisms are largely unknown. While indirect evidence suggests that the cytoplasmic free Ca(2+) concentration ([Ca(2+)](c)) plays a crucial role in the volatile-sensing mechanisms in plants, these roles have not been directly demonstrated. In the present study, we used Arabidopsis leaves expressing apoaequorin, a Ca(2+)-sensitive luminescent protein, in combination with a luminometer, to monitor [Ca(2+)](c) transients that occur in response to a variety of volatile compounds and to characterized the pharmacological properties of the increase in [Ca(2+)](c). When leaves were exposed to volatiles, [Ca(2+)](c) was transiently raised. The [Ca(2+)](c) increases induced by acyclic compounds were disrupted by Ruthenium Red, a potential plasma-membrane and endo-membrane Ca(2+)-permeable channel inhibitor, but not by 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA), an extracellular Ca(2+)-chelator, suggesting that acyclic compounds promote Ca(2+)-release from intracellular stores. On the other hand, the electrophilic compound (E)-2-hexenal promoted Ca(2+)-influx via ROS production by natural oxidation at the aquarius phase. In a gpa1-2 mutant lacking a canonical Galpha subunit, the [Ca(2+)](c) transients induced by all tested volatiles were not attenuated, suggesting that G-protein coupled receptors are not involved in the volatile-induced [Ca(2+)](c) transients in Arabidopsis leaves.

The alpha-subunit of the heterotrimeric G-protein affects jasmonate responses in Arabidopsis thaliana

Heterotrimeric G-proteins have been implicated in having a role in many plant signalling pathways. To understand further the role of G-proteins, a preliminary experiment was performed to assess the impact of the G alpha subunit loss-of-function mutation gpa1-1 on the Arabidopsis transcriptome. The analysis indicated that the G alpha subunit may play a role in response to jasmonic acid (JA). Consistent with this, G alpha mutants showed a reduced response to JA in inhibition of chlorophyll accumulation and root growth, whilst G alpha gain-of-function plants overexpressing G alpha showed the opposite phenotype. The levels of JA and related compounds were unaffected in the gpa1-1 mutant, as was autoregulation of the Allene Oxide Synthase (AOS) gene that encodes a key enzyme for JA biosynthesis. In contrast, further analyses using G alpha loss- and gain-of-function Arabidopsis lines indicated that G alpha positively modulates the expression of the Vegetative Storage Protein (VSP) gene. This indicates that the G alpha subunit regulates a subset of JA-regulated genes defining a branch point in this signalling pathway in Arabidopsis. Further analysis of the impact of G alpha loss of function upon the JA-regulated transcriptome using Arabidopsis full genome arrays indicated that up to 29% of genes that are >2-fold regulated by JA in the wild type are misregulated in the G alpha mutant. This supports the observation that a significant proportion of, but not all, JA-regulated gene expression is mediated by G alpha.

Arabidopsis nucleoside diphosphate kinase-2 as a plant GTPase activating protein

Nucleoside diphosphate kinase (NDPK) is involved in multiple signaling pathways in mammalian systems, including G-protein signaling. Arabidopsis NDPK2, like its mammalian counterparts, is multifunctional despite its initial discovery phytochrome-interacting protein. This similarity raises the possibility that NDPK2 may play a role in G-protein signaling in plants. In the present study, we explore the potential relationship between NDPK2 and the small G proteins, Pra2 and Pra3, as well as the heterotrimeric G protein, GPA1. We report a physical interaction between NDPK2 and these small G proteins, and demonstrate that NDPK2 can stimulate their GTPase activities. Our results suggest that NDPK2 acts as a GTPase-activating protein for small G proteins in plants. We propose that NDPK2 might be a missing link between the phytochromemediated light signaling and G protein-mediated signaling.

Sugar sensing and signaling

Plants, restricted by their environment, need to integrate a wide variety of stimuli with their metabolic activity, growth and development. Sugars, generated by photosynthetic carbon fixation, are central in coordinating metabolic fluxes in response to the changing environment and in providing cells and tissues with the necessary energy for continued growth and survival. A complex network of metabolic and hormone signaling pathways are intimately linked to diverse sugar responses. A combination of genetic, cellular and systems analyses have uncovered nuclear HXK1 (hexokinase1) as a pivotal and conserved glucose sensor, directly mediating transcription regulation, while the KIN10/11 energy sensor protein kinases function as master regulators of transcription networks under sugar and energy deprivation conditions. The involvement of disaccharide signals in the regulation of specific cellular processes and the potential role of cell surface receptors in mediating sugar signals add to the complexity. This chapter gives an overview of our current insight in the sugar sensing and signaling network and describes some of the molecular mechanisms involved.

G protein βγ subunits: central mediators of G protein-coupled receptor signaling

G protein betagamma subunits are central participants in G protein-coupled receptor signaling pathways. They interact with receptors, G protein alpha subunits and downstream targets to coordinate multiple, different GPCR functions. Much is known about the biology of Gbetagamma subunits but mysteries remain. Here, we will review what is known about general aspects of structure and function of Gbetagamma as well as discuss emerging mechanisms for regulation of Gbetagamma signaling. Recent data suggest that Gbetagamma is a potential therapeutic drug target. Thus, a thorough understanding of the molecular and physiological functions of Gbetagamma has significant implications.

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.

Abscisic acid receptors: multiple signal-perception sites

BACKGROUND AND AIMS

The phytohormone abscisic acid (ABA) plays a vital role in various aspects of plant growth and development and in adaptation of plants to various environmental stresses. Cell response to ABA is initiated by ABA perception with a cell receptor. Recently, three distinct ABA receptors have been identified, opening a door to uncover the initial events of ABA signal transduction. The aim of this Botanical Briefing is to present a perspective of the ABA receptors identified.

SCOPE

This Briefing offers an introduction to the three ABA receptors identified and an analysis of the complexity and multiplicity of ABA receptors, and provides some viewpoints on future research.

Arabidopsis extra-large G proteins (XLGs) regulate root morphogenesis

As in mammalian systems, heterotrimeric G proteins, composed of alpha, beta and gamma subunits, are present in plants and are involved in the regulation of development and cell signaling. Besides the sole prototypical G protein alpha subunit gene, GPA1, the Arabidopsis thaliana genome has three extra-large GTP-binding protein (XLG)-encoding genes: XLG1 (At2g23460), XLG2 (At4g34390) and XLG3 (At1g31930). The C-termini of the XLGs are Galpha domains that are homologous to GPA1, whereas their N-termini each contain a cysteine-rich region and a putative nuclear localization signal (NLS). GFP fusions with each XLG confirmed nuclear localization. All three XLG genes are expressed in essentially all plant organs, with strong expression in vascular tissues, primary root meristems and lateral root primordia. Analysis of single, double and triple T-DNA insertional mutants of the XLG genes revealed redundancy in XLG function. Dark-grown xlg1-1 xlg2-1 xlg3-1 triple mutant plants showed markedly increased primary root length compared with wild-type plants. This phenotype was not observed in dark-grown xlg single mutants, and was suppressed upon complementation of the xlg triple mutant with each XLG. Root cell sizes of the xlg triple mutant and root morphology were highly similar to those of wild-type roots, suggesting that XLGs may regulate cell proliferation. Dark-grown roots of the xlg triple mutants also showed altered sensitivity to sugars, ABA hyposensitivity and ethylene hypersensitivity, whereas seed germination in xlg triple mutants was hypersensitive to osmotic stress and ABA. As plant-specific proteins, regulatory mechanisms of XLGs may differ from those of conventional Galphas.

Expression analysis and subcellular localization of the Arabidopsis thaliana G-protein beta-subunit AGB1

Heterotrimeric GTP-binding proteins (G-proteins), consisting of Galpha, Gbeta, and Ggamma subunits, function as molecular switches in many eukaryotic signal transduction pathways. In contrast to many eukaryotes, plants contain very few G-protein subunit isoforms that mediate a diverse array of signalling functions. We investigated the contribution of cell type-specific expression and subcellular localization to this multifunctional signalling capacity for the Arabidopsis thaliana Gbeta subunit, AGB1. Analysis of AGB1 promoter::beta-glucuronidase (GUS) fusions in germinating seeds, seedlings, and flowering plants revealed that AGB1 is widely expressed throughout development in a complex manner. As well as demonstrating similarities to existing Arabidopsis G-protein subunit expression data, several features of the AGB1 expression pattern align closely with known or proposed G-protein functions. A C-terminal AGB1-green fluorescent protein (GFP) fusion was localized at the plasma membrane and in the nucleus of leaf epidermal cells, trichomes and root cells, supporting previous evidence that plant G-protein functionality relies on subcellular compartmentalization.

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.

Heterotrimeric G protein gamma subunits provide functional selectivity in Gbetagamma dimer signaling in Arabidopsis

The Arabidopsis thaliana heterotrimeric G protein complex is encoded by single canonical Galpha and Gbeta subunit genes and two Ggamma subunit genes (AGG1 and AGG2), raising the possibility that the two potential G protein complexes mediate different cellular processes. Mutants with reduced expression of one or both Ggamma genes revealed specialized roles for each Ggamma subunit. AGG1-deficient mutants, but not AGG2-deficient mutants, showed impaired resistance against necrotrophic pathogens, reduced induction of the plant defensin gene PDF1.2, and decreased sensitivity to methyl jasmonate. By contrast, both AGG1- and AGG2-deficient mutants were hypersensitive to auxin-mediated induction of lateral roots, suggesting that Gbetagamma1 and Gbetagamma2 synergistically inhibit auxin-dependent lateral root initiation. However, the involvement of each Ggamma subunit in this root response differs, with Gbetagamma1 acting within the central cylinder, attenuating acropetally transported auxin signaling, while Gbetagamma2 affects the action of basipetal auxin and graviresponsiveness within the epidermis and/or cortex. This selectivity also operates in the hypocotyl. Selectivity in Gbetagamma signaling was also found in other known AGB1-mediated pathways. agg1 mutants were hypersensitive to glucose and the osmotic agent mannitol during seed germination, while agg2 mutants were only affected by glucose. We show that both Ggamma subunits form functional Gbetagamma dimers and that each provides functional selectivity to the plant heterotrimeric G proteins, revealing a mechanism underlying the complexity of G protein-mediated signaling in plants.

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.

Over-expression of a truncated Arabidopsis thaliana heterotrimeric G protein gamma subunit results in a phenotype similar to alpha and beta subunit knockouts

Heterotrimeric G proteins (G-proteins) are a diverse class of signal transducing proteins which have been implicated in a variety of important roles in plants. When G-proteins are activated, they dissociate into two functional subunits (alpha and the betagamma dimer) that effectively relay the signal to a multitude of effectors. In animal systems, the betagamma dimer is anchored to the plasma membrane by a prenyl group present in the gamma subunit and membrane localization has proven vital for heterotrimer function. A semi-dominant negative strategy was designed aiming to disrupt heterotrimer function in Arabidopsis thaliana (ecotype Columbia) plants by over-expressing a truncated gamma subunit lacking the isoprenylation motif (gamma()). Northern analysis shows that the levels of expression of the mutant gamma subunit in several transgenic lines (35S-gamma()) are orders of magnitude higher than that of the native subunits. In-depth characterization of the 35S-gamma() lines has been carried out, specifically focusing on a number of developmental characteristics and responses to several stimuli previously shown to be affected in alpha- and beta-deficient mutants. In all cases, the transgenic lines expressing the mutant gamma subunit behave in the same way as the alpha- and/or the beta-deficient mutants, albeit with reduced severity of the phenotype. Our data indicates that signaling from both functional subunits, alpha and the beta/gamma dimer, is disrupted in the transgenic plants. Even though physical association of the subunits has been previously reported, our research provides evidence of the functional association of alpha and beta with the gamma subunits in Arabidopsis, while also suggesting that plasma membrane localization may be critical for function of plant heterotrimeric G proteins.

Plant G protein heterotrimers require dual lipidation motifs of Gα and Gγ and do not dissociate upon activation

In plants one bona fide Gα subunit has been identified, as well as a single Gβ and two Gγ subunits. To study the roles of lipidation motifs in the regulation of subcellular location and heterotrimer formation in living plant cells, GFP-tagged versions of the Arabidopsis thaliana heterotrimeric G protein subunits were constructed. Mutational analysis showed that the Arabidopsis Gα subunit, GPα1, contains two lipidation motifs that were essential for plasma membrane localization. The Arabidopsis Gβ subunit, AGβ1, and the Gγ subunit, AGG1, were dependent upon each other for tethering to the plasma membrane. The second Gγ subunit, AGG2, did not require AGβ1 for localization to the plasma membrane. Like AGG1, AGG2 contains two putative lipidation motifs, both of which were necessary for membrane localization. Interaction between the subunits was studied using fluorescence resonance energy transfer (FRET) imaging by means of fluorescence lifetime imaging microscopy (FLIM). The results suggest that AGβ1 and AGG1 or AGβ1 and AGG2 can form heterodimers independent of lipidation. In addition, FLIM-FRET revealed the existence of GPα1-AGβ1-AGG1 heterotrimers at the plasma membrane. Importantly, rendering GPα1 constitutively active did not cause a FRET decrease in the heterotrimer, suggesting no dissociation upon GPα1 activation.

The plastid protein THYLAKOID FORMATION1 and the plasma membrane G-protein GPA1 interact in a novel sugar-signaling mechanism in Arabidopsis

Mutations in genes encoding components of the heterotrimeric G-protein complex were previously shown to confer altered sensitivity to increased levels of D-glucose. This suggests that G-protein coupling may be a novel sugar-signaling mechanism in Arabidopsis thaliana. THYLAKOID FORMATION1 (THF1) is here demonstrated in vivo as a Galpha interaction partner that functions downstream of the plasma membrane-delimited heterotrimeric G-protein (GPA1) in a D-glucose signaling pathway. THF1 is a plastid protein localized to both the outer plastid membrane and the stroma. Contact between root plastidic THF1 and GPA1 at the plasma membrane occurs at sites where the plastid membrane abuts the plasma membrane, as demonstrated by Förster resonance energy transfer (FRET). A probable role for THF1 in sugar signaling is demonstrated by both biochemical and genetic evidence. Root growth in the thf1-1 null mutant is hypersensitive to exogenous D-glucose, and THF1-overexpressing roots are resistant to inhibition of growth rate by high D-glucose. Additionally, THF1 levels are rapidly degraded by D-glucose but not L-glucose. The interaction between THF1 and GPA1 has been confirmed by in vitro and in vivo coimmunoprecipitation, FRET analysis, and genetic epistasis and provides evidence of a sugar-signaling mechanism between plastids and the plasma membrane.

Proteomic analysis of shoot-borne root initiation in maize (Zea mays L.)

Postembryonically formed shoot-borne roots make up the major backbone of the adult maize root stock. In this study the abundant soluble proteins of the first node (coleoptilar node) of wild-type and mutant rtcs seedlings, which do not initiate crown roots, were compared at two early stages of crown root formation. In Coomassie Bluestained 2-D gels, representing soluble proteins of coleoptilar nodes 5 and 10 days after germination, 146 and 203 proteins were detected, respectively. Five differentially accumulated proteins (> two-fold change; t-test: 95% significance) were identified in 5-day-old and 14 differentially accumulated proteins in 10-day-old coleoptilar nodes of wild-type versus rtcs. All 19 differentially accumulated proteins were identified via ESI MS/MS mass spectrometry. Five differentially accumulated proteins, including a regulatory G-protein and a putative auxin-binding protein, were further analyzed at the RNA expression level. These experiments confirmed differential gene expression and revealed subtle developmental regulation of these genes during early coleoptilar node development. This study represents the first proteomic analysis of shoot-borne root initiation in cereals and will contribute to a better understanding of the molecular basis of this developmental process unique to cereals.

Heterotrimeric G proteins facilitate Arabidopsis resistance to necrotrophic pathogens and are involved in jasmonate signaling

Heterotrimeric G proteins have been previously linked to plant defense; however a role for the Gbetagamma dimer in defense signaling has not been described to date. Using available Arabidopsis (Arabidopsis thaliana) mutants lacking functional Galpha or Gbeta subunits, we show that defense against the necrotrophic pathogens Alternaria brassicicola and Fusarium oxysporum is impaired in Gbeta-deficient mutants while Galpha-deficient mutants show slightly increased resistance compared to wild-type Columbia ecotype plants. In contrast, responses to virulent (DC3000) and avirulent (JL1065) strains of Pseudomonas syringae appear to be independent of heterotrimeric G proteins. The induction of a number of defense-related genes in Gbeta-deficient mutants were severely reduced in response to A. brassicicola infection. In addition, Gbeta-deficient mutants exhibit decreased sensitivity to a number of methyl jasmonate-induced responses such as induction of the plant defensin gene PDF1.2, inhibition of root elongation, seed germination, and growth of plants in sublethal concentrations of methyl jasmonate. In all cases, the behavior of the Galpha-deficient mutants is coherent with the classic heterotrimeric mechanism of action, indicating that jasmonic acid signaling is influenced by the Gbetagamma functional subunit but not by Galpha. We hypothesize that Gbetagamma acts as a direct or indirect enhancer of the jasmonate signaling pathway in plants.

Plants, symbiosis and parasites: a calcium signalling connection

A unique family of protein kinases has evolved with regulatory domains containing sequences that are related to Ca(2+)-binding EF-hands. In this family, the archetypal Ca(2+)-dependent protein kinases (CDPKs) have been found in plants and some protists, including the malarial parasite, Plasmodium falciparum. Recent genetic evidence has revealed isoform-specific functions for a CDPK that is essential for Plasmodium berghei gametogenesis, and for a related chimeric Ca(2+) and calmodulin-dependent protein kinase (CCaMK) that is essential to the formation of symbiotic nitrogen-fixing nodules in plants. In Arabidopsis thaliana, the analysis of 42 isoforms of CDPK and related kinases is expected to delineate Ca(2+) signalling pathways in all aspects of plant biology.

Genome-wide expression profiling and identification of gene activities during early flower development in Arabidopsis

We have used oligonucleotide microarrays to detect Arabidopsis gene expression during early flower development. Among the 22,746 genes represented on the Affymetrix ATH1 chip, approximately 14,660 (approximately 64.5%) genes were expressed with signal intensity at or more than 50 in each of the six organs/structures examined, including young inflorescences (floral stages 1-9), stage-12 floral buds, developing siliques, leaves, stems, and roots. 17,583 genes were expressed with an intensity at or above 50 in at least one tissue, including 12,245 genes that were expressed in all the six tissues. Comparison of genes expressed between young inflorescence or stage-12 floral buds with other tissues suggests that relatively large numbers of genes are expressed at similar levels in tissues that are related morphologically and/or developmentally, as supported by a cluster analysis with data from two other studies. Further analysis of the genes preferentially expressed in floral tissues has uncovered new genes potentially involved in Arabidopsis flower development. One hundred and four genes were determined to be preferentially expressed in young inflorescences, including 22 genes encoding putative transcription factors. We also identified 105 genes that were preferentially expressed in three reproductive structures (the young inflorescences, stage-12 floral buds and developing siliques), when compared with the vegetative tissues. RT-PCR results of selected genes are consistent with the results from these microarrays and suggest that the relative signal intensities detected with the Affymetrix microarray are reliable estimates of gene expression.

Plants: the latest model system for G-protein research

In humans, heterotrimeric G proteins couple stimulus perception by G-protein-coupled receptors (GPCRs) with numerous downstream effectors. By contrast, despite great complexity in their signal-transduction attributes, plants have a simpler repertoire of G-signalling components. Nonetheless, recent studies on Arabidopsis thaliana have shown the importance of plant G-protein signalling in such fundamental processes as cell proliferation, hormone perception and ion-channel regulation.

Plant G proteins, phytohormones, and plasticity: three questions and a speculation

Heterotrimeric guanine nucleotide-binding proteins (G proteins) composed of Galpha, Gbeta, and Ggamma subunits are important transducers of hormonal signals in organisms as evolutionarily distant as plants and humans. The genomes of diploid angiosperms, such as that of the model species Arabidopsis thaliana, encode only single canonical Galpha and Gbeta subunits, only two identified Ggamma subunits, and just one regulator of G protein signaling (RGS) protein. However, a wide range of processes-including seed germination, shoot and root growth, and stomatal regulation-are altered in Arabidopsis and rice plants with mutations in G protein components. Such mutants exhibit altered responsiveness to a number of plant hormones, including gibberellins, brassinosteroids, abscisic acid, and auxin. This review describes possible mechanisms by which such pleiotropic effects are generated and considers possible explanations for why G protein component mutations in plants fail to be lethal. A possible role of G protein signaling in the control of phenotypic plasticity, a hallmark of plant growth, is also discussed.

The Arabidopsis putative G protein-coupled receptor GCR1 interacts with the G protein alpha subunit GPA1 and regulates abscisic acid signaling

Heterotrimeric G proteins composed of alpha, beta, and gamma subunits link ligand perception by G protein-coupled receptors (GPCRs) with downstream effectors, providing a ubiquitous signaling mechanism in eukaryotes. The Arabidopsis thaliana genome encodes single prototypical Galpha (GPA1) and Gbeta (AGB1) subunits, and two probable Ggamma subunits (AGG1 and AGG2). One Arabidopsis gene, GCR1, encodes a protein with significant sequence similarity to nonplant GPCRs and a predicted 7-transmembrane domain structure characteristic of GPCRs. However, whether GCR1 actually interacts with GPA1 was unknown. We demonstrate by in vitro pull-down assays, by yeast split-ubiquitin assays, and by coimmunoprecipitation from plant tissue that GCR1 and GPA1 are indeed physically coupled. GCR1-GPA1 interaction depends on intracellular domains of GCR1. gcr1 T-DNA insertional mutants exhibit hypersensitivity to abscisic acid (ABA) in assays of root growth, gene regulation, and stomatal response. gcr1 guard cells are also hypersensitive to the lipid metabolite, sphingosine-1-phosphate (S1P), which is a transducer of the ABA signal upstream of GPA1. Because gpa1 mutants exhibit insensitivity in aspects of guard cell ABA and S1P responses, whereas gcr1 mutants exhibit hypersensitivity, GCR1 may act as a negative regulator of GPA1-mediated ABA responses in guard cells.

Characterization of heterotrimeric G protein complexes in rice plasma membrane

Two genes in the rice genome were identified as those encoding the gamma subunits, gamma1 and gamma2, of heterotrimeric G proteins. Using antibodies against the recombinant proteins for the alpha, beta, gamma1, and gamma2 subunits of the G protein complexes, all of the subunits were proven to be localized in the plasma membrane in rice. Gel filtration of solubilized plasma membrane proteins showed that all of the alpha subunits were present in large protein complexes (about 400 kDa) containing the other subunits, beta, gamma1, and gamma2, and probably also some other proteins, whereas large amounts of the beta and gamma (gamma1 and gamma2) subunits were freed from the large complexes and took a 60-kDa form. A yeast two-hybrid assay and co-immunoprecipitation experiments showed that the beta subunit interacted tightly with the gamma1 and gamma2 subunits, and so the beta and gamma subunits appeared to form dimers in rice cells. Some dimers were associated with the alpha subunit, because few beta, gamma1, and gamma2 subunits were present in the 400-kDa complexes in a rice mutant, d1, which was lacking in the alpha subunit. When a constitutively active form of the alpha subunit was prepared by the exchange of one amino acid residue and introduced into d1, the mutagenized subunit was localized in the plasma membrane of the transformants and took a free, and not the 400-kDa, form.

Arabidopsis phospholipase Dalpha1 interacts with the heterotrimeric G-protein alpha-subunit through a motif analogous to the DRY motif in G-protein-coupled receptors

Phospholipase D (PLD) and heterotrimeric G-protein both play important, diverse roles in cellular regulation and signal transduction. Here we have determined the physical interaction between plant PLD and the only canonical alpha-subunit (Galpha) of the G-protein in Arabidopsis thaliana and the molecular basis for the interaction. PLDalpha1 expressed in either Escherichia coli or Arabidopsis was co-precipitated with Galpha. PLDalpha1 contains a sequence motif analogous to the G alpha-interacting DRY motif normally conserved in G-protein-coupled receptors. Mutation of the central Lys residue PLD(K564A) of this motif abolished the PLDalpha1-Galpha binding, whereas mutation of the two flanking residues PLD(E563A) and PLD(F565A) decreased the binding. Addition of Galpha to PLDalpha1 inhibited PLDalpha1 activity, whereas the PLD(K564A) mutation that disrupted the Galpha-PLDalpha1 binding abolished the inhibition. GTP relieved the Galpha inhibition of PLDalpha1 activity and also inhibited the binding between PLDalpha1 and Galpha. Meanwhile, the PLDalpha1-Galpha interaction stimulated the intrinsic GTPase activity of Galpha. Therefore, these results have demonstrated the direct binding between Galpha and PLDalpha1, identified the DRY motif on PLDalpha1 as the site for the interaction, and indicated that the interaction modulates reciprocally the activities of PLDalpha1 and Galpha.

The Arabidopsis cupin domain protein AtPirin1 interacts with the G protein alpha-subunit GPA1 and regulates seed germination and early seedling development

Heterotrimeric G proteins are implicated in diverse signaling processes in plants, but the molecular mechanisms of their function are largely unknown. Finding G protein effectors and regulatory proteins can help in understanding the roles of these signal transduction proteins in plants. A yeast two-hybrid screen was performed to search for proteins that interact with Arabidopsis G protein alpha-subunit (GPA1). One of the identified GPA1-interacting proteins is the cupin-domain protein AtPirin1. Pirin is a recently defined protein found because of its ability to interact with a CCAAT box binding transcription factor. The GPA1-AtPirin1 interaction was confirmed in an in vitro binding assay. We characterized two atpirin1 T-DNA insertional mutants and established that they display a set of phenotypes similar to those of gpa1 mutants, including reduced germination levels in the absence of stratification and an abscisic acid-imposed delay in germination and early seedling development. These data indicate that AtPirin1 likely functions immediately downstream of GPA1 in regulating seed germination and early seedling development.

Receptor kinase signaling in plant development

The Arabidopsis genome sequence has revealed that plants contain a much larger complement of receptor kinase genes than other organisms. Early analysis of these genes revealed involvement in a diverse array of developmental and defense functions that included gametophyte development, pollen-pistil interactions, shoot apical meristem equilibrium, hormone perception, and cell morphogenesis. Amino acid sequence motifs and binding studies indicate that the ectodomains are capable of binding, either directly or indirectly, various classes of molecules including proteins, carbohydrates, and steroids. Genetic and biochemical approaches have begun to identify other components of several signal transduction pathways. Some receptor-like kinases (RLKs) appear to function with coreceptors lacking kinase domains, and genome analysis suggests this might be true for many RLKs. The KAPP protein phosphatase functions as a negative regulator of at least two RLK systems, and in vitro studies suggest it could be a common component of more. Whether plant signaling systems display a modularity similar to animal systems remains to be determined. Future efforts will reveal unknown functions of other RLKs and elucidate the relationships among their signaling networks.

The effect of pH and various cations on the GTP hydrolysis of rice heterotrimeric G-protein alpha subunit expressed in Escherichia coli

Previously, we reported the biochemical properties of RGA1 that is expressed in Escherichia coli (Seo et al., 1997). The activities of RGA1 that hydrolyzes and binds guanine nucleotide were dependent on the MgCl(2) concentration. The steady state rate constant (k(cat) ) for GTP hydrolysis of RGA1 at 2 mM MgCl(2) was 0.0075 +/- 0.0001 min(-1). Here, we examined the effects of pH and cations on the GTPase activity. The optimum pH at 2 mM MgCl(2) was approximately 6.0; whereas, the pH at 2 mM NH(4)Cl was approximately 4.0. The result from the cation dependence on the GTPase (guanosine 5'-triphosphatase) activity of RGA1 under the same condition showed that the GTP hydrolysis rate (k(cat)= 0.0353 min(-1)) under the condition of 2 mM NH(4)Cl at pH 4.0 was the highest. It corresponded to about 3.24-fold of the k(cat) value of 0.0109 min(-1) in the presence of 2 mM MgCl(2) at pH 6.0.

Secondary messengers and phospholipase A2 in auxin signal transduction

Despite recent progress auxin signal transduction remains largely scetchy and enigmatic. A good body of evidence supports the notion that the ABP1 could be a functional receptor or part of a receptor, respectively, but this is not generally accepted. Evidence for other functional receptors is lacking, as is any clearcut evidence for a function of G proteins. Protons may serve as second messengers in guard cells but the existing evidence for a role of calcium remains to be clearified. Phospholipases C and D seem not to have a function in auxin signal transduction whereas the indications for a role of phospholipase A2 in auxin signal transduction accumulated recently. Mitogen-activated protein kinase (MAPK) is modulated by auxin and the protein kinase PINOID has a role in auxin transport modulation even though their functional linkage to other signalling molecules is ill-defined. It is hypothesized that signal transduction precedes activation of early genes such as IAA genes and that ubiquitination and the proteasome are a mechanism to integrate signal duration and signal strength in plants and act as major regulators of hormone sensitivity.