GCR1 can act independently of heterotrimeric G-protein in response to brassinosteroids and gibberellins in Arabidopsis seed germination

A bZIP protein, VIP1, interacts with Arabidopsis heterotrimeric G protein β subunit, AGB1

Heterotrimeric G proteins (Gα, Gβ, Gγ) are signaling molecules conserved among eukaryotic species. The G proteins transmit signals via protein-protein interactions. By a yeast two-hybrid screen, we identified a bZIP protein, VIP1, as an Arabidopsis thaliana Gβ (AGB1)-interacting partner. The interaction between AGB1 and VIP1 was confirmed by an in vitro GST pull-down assay and a bimolecular fluorescence complementation (BiFC) assay. VIP1 was previously reported to be a regulator of osmosensory signaling. Interestingly, the BiFC pattern between AGB1 and VIP1 was speckled when cells were incubated in a hypotonic solution, but not when cells were incubated in a mannitol-containing hypertonic solution, suggesting that the subcellular localization of the AGB1-VIP1 complex is regulated by extracellular osmolarity and/or turgor pressure.

The role of Arabidopsis heterotrimeric G-protein subunits in MLO2 function and MAMP-triggered immunity

Heterotrimeric G-proteins, composed of Gα, Gβ, and Gγ subunits, regulate many fundamental processes in plants. In animals, ligand binding to seven transmembrane (7TM) cell surface receptors designated G-protein coupled receptors (GPCR) leads to heterotrimeric G-protein activation. Because the plant G-protein complex is constitutively active, the exact role of plant 7TM proteins in this process is unclear. Members of the mildew resistance locus O (MLO) family represent the best-characterized 7TM plant proteins. Although genetic ablation of either MLO2 or G-proteins alters susceptibility to pathogens in Arabidopsis thaliana, it is unknown whether G-proteins directly couple signaling through MLO2. Here, we exploited two well-documented phenotypes of mlo2 mutants, broad-spectrum powdery mildew resistance and spontaneous callose deposition in leaf mesophyll cells, to assess the relationship of MLO2 proteins to the G-protein complex. Although our data reveal modulation of antifungal defense responses by Gβ and Gγ subunits and a role for the Gγ1 subunit in mlo2-conditioned callose deposition, our findings overall are inconsistent with a role of MLO2 as a canonical GPCR. We discovered that mutants lacking the Gβ subunit show delayed accumulation of a subset of defense-associated genes following exposure to the microbe-associated molecular pattern flg22. Moreover, Gβ mutants were found to be hypersusceptible to spray inoculation with the bacterial pathogen Pseudomonas syringae.

Arabidopsis heterotrimeric G protein β subunit, AGB1, regulates brassinosteroid signalling independently of BZR1

The Arabidopsis thaliana heterotrimeric G protein β subunit, AGB1, is involved in both abscisic acid (ABA) signalling and brassinosteroid (BR) signalling, but it is unclear how AGB1 regulates these signalling pathways. A key transcription factor downstream of BR, BZR1, and its gain-of-function mutant, bzr1-1, were overexpressed in an AGB1-null mutant, agb1-1, to examine their effects on the BR hyposensitivity and the ABA hypersensitivity of agb1-1, and to examine whether AGB1 regulates the functions of BZR1. Because the amino acid sequence of AGB1 contains 17 putative modification motifs of glycogen synthase kinase 3/SHAGGY-like protein kinases (GSKs), which are known components of BR signalling, the interaction between AGB1 and one of the Arabidopsis GSKs, BIN2, was examined. Expression of bzr1-1 alleviated the effects of a BR biosynthesis inhibitor, brassinazole, in both the wild type and agb1-1, and overexpression of BZR1 alleviated the effects of ABA in both the wild type and agb1-1. AGB1 did not affect the phosphorylation state of BZR1 in vivo. AGB1 interacted with BIN2 in vitro, but did not affect the phosphorylation state of BIN2. The results suggest that AGB1 interacts with BIN2, but regulates the BR signalling in a BZR1-independent manner.

Mechanisms of signaling crosstalk between brassinosteroids and gibberellins

Brassinosteroids (BRs) and Gibberellins (GAs) are two principal groups of growth-promoting phytohormones. Accumulating evidence supports that there are crosstalks between BR and GA signaling pathways. However, a molecular mechanism for direct signaling crosstalk between BRs and GAs was not revealed until recently. Works from three different groups demonstrated that an interaction between BZR1/BES1 and DELLAs, two groups of key transcriptional regulators from the BR and GA signaling pathways, respectively, mediates the direct signaling crosstalk between BRs and GAs in controlling cell elongation in Arabidopsis. It was shown that DELLA proteins not only affect the protein stability but also inhibit the transcriptional activity of BZR1. Thus, GAs promote cell elongation, at least in part, through releasing DELLA-mediated inhibition of BZR1. This review aims to introduce these recent advances in our understanding of how BRs and GAs coordinate to regulate plant growth and development at the molecular level.

GK4, a G-protein-coupled receptor with a phosphatidylinositol phosphate kinase domain in Phytophthora infestans, is involved in sporangia development and virulence

For dispersal and host infection plant pathogens largely depend on asexual spores. Pathogenesis and sporulation are complex processes that are governed by cellular signalling networks including G-protein and phospholipid signalling. Oomycetes possess a family of novel proteins called GPCR-PIPKs (GKs) that are composed of a seven-transmembrane spanning (7-TM) domain fused to a phosphatidylinositol phosphate kinase (PIPK) domain. Based on this domain structure GKs are anticipated to link G-protein and phospholipid signal pathways; however, their functions are currently unknown. Expression analyses of the 12 GK genes in Phytophthora infestans and their orthologues in Phytophthora sojae, revealed differential expression during asexual development. PiGK1 and PiGK4 were fused to monomeric red fluorescent protein (mRFP) and ectopically expressed in P. infestans. In growing hyphae different subcellular distribution patterns were observed indicating that these two GKs act independently during development. We focused on the functional analyses of PiGK4. Its localization suggested involvement in cell differentiation and elongation and its 7-TM domain showed a canonical GPCR membrane topology. Silencing of GK4 and overexpression of full-length and truncated constructs in P. infestans revealed that PiGK4 is not only involved in spore germination and hyphal elongation but also in sporangia cleavage and infection.

Heterotrimeric G protein signalling in the plant kingdom

In animals, heterotrimeric G proteins, comprising α-, β-and γ-subunits, perceive extracellular stimuli through cell surface receptors, and transmit signals to ion channels, enzymes and other effector proteins to affect numerous cellular behaviours. In plants, G proteins have structural similarities to the corresponding molecules in animals but transmit signals by atypical mechanisms and effector proteins to control growth, cell proliferation, defence, stomate movements, channel regulation, sugar sensing and some hormonal responses. In this review, we summarize the current knowledge on the molecular regulation of plant G proteins, their effectors and the physiological functions studied mainly in two model organisms: Arabidopsis thaliana and rice (Oryza sativa). We also look at recent progress on structural analyses, systems biology and evolutionary studies.

Heterotrimeric G protein signalling in the plant kingdom

In animals, heterotrimeric G proteins, comprising α-, β-and γ-subunits, perceive extracellular stimuli through cell surface receptors, and transmit signals to ion channels, enzymes and other effector proteins to affect numerous cellular behaviours. In plants, G proteins have structural similarities to the corresponding molecules in animals but transmit signals by atypical mechanisms and effector proteins to control growth, cell proliferation, defence, stomate movements, channel regulation, sugar sensing and some hormonal responses. In this review, we summarize the current knowledge on the molecular regulation of plant G proteins, their effectors and the physiological functions studied mainly in two model organisms: Arabidopsis thaliana and rice (Oryza sativa). We also look at recent progress on structural analyses, systems biology and evolutionary studies.

Similarity between class A and class B G-protein-coupled receptors exemplified through calcitonin gene-related peptide receptor modelling and mutagenesis studies

Modelling class B G-protein-coupled receptors (GPCRs) using class A GPCR structural templates is difficult due to lack of homology. The plant GPCR, GCR1, has homology to both class A and class B GPCRs. We have used this to generate a class A–class B alignment, and by incorporating maximum lagged correlation of entropy and hydrophobicity into a consensus score, we have been able to align receptor transmembrane regions. We have applied this analysis to generate active and inactive homology models of the class B calcitonin gene-related peptide (CGRP) receptor, and have supported it with site-directed mutagenesis data using 122 CGRP receptor residues and 144 published mutagenesis results on other class B GPCRs. The variation of sequence variability with structure, the analysis of polarity violations, the alignment of group-conserved residues and the mutagenesis results at 27 key positions were particularly informative in distinguishing between the proposed and plausible alternative alignments. Furthermore, we have been able to associate the key molecular features of the class B GPCR signalling machinery with their class A counterparts for the first time. These include the [K/R]KLH motif in intracellular loop 1, [I/L]xxxL and KxxK at the intracellular end of TM5 and TM6, the NPXXY/VAVLY motif on TM7 and small group-conserved residues in TM1, TM2, TM3 and TM7. The equivalent of the class A DRY motif is proposed to involve Arg^2.39, His^2.43 and Glu^3.46, which makes a polar lock with T^6.37. These alignments and models provide useful tools for understanding class B GPCR function.

The pepper MLO gene, CaMLO2, is involved in the susceptibility cell-death response and bacterial and oomycete proliferation

Loss-of-function alleles of the mildew resistance locus O (MLO) gene provide broad-spectrum powdery mildew disease resistance. Here, we identified a pepper (Capsicum annuum) MLO gene (CaMLO2) that is transcriptionally induced by Xanthomonas campestris pv. vesicatoria (Xcv) infection. Topology and subcellular localization analyses reveal that CaMLO2 is a plasma membrane-anchored and amphiphilic Ca²⁺-dependent calmodulin-binding protein. CaMLO2 expression is up-regulated by Xcv and salicylic acid, as well as abiotic stresses. Silencing of CaMLO2 in pepper plants confers enhanced resistance against virulent Xcv, but not against avirulent Xcv. This resistance is accompanied by a compromised susceptibility cell-death response and reduced bacterial growth, as well as an accelerated reactive oxygen species burst. Virulent Xcv infection drastically induces expression of the salicylic acid-dependent defense marker gene CaPR1 in CaMLO2-silenced leaves. CaMLO2 over-expression in Arabidopsis enhances susceptibility to Pseudomonas syringae pv. tomato and Hyaloperonospora arabidopsidis. Leaves of plants over-expressing CaMLO2 exhibit a susceptibility cell-death response and high bacterial growth during virulent Pst DC3000 infection. These are accompanied by enhanced electrolyte leakage but compromised induction of some defense response genes and the reactive oxygen species. Together, our results suggest that CaMLO2 is involved in the susceptibility cell-death response and bacterial and oomycete proliferation in pepper and Arabidopsis.

A U-Box E3 ubiquitin ligase, PUB20, interacts with the Arabidopsis G-protein β subunit, AGB1

An Arabidopsis U-box E3 ubiquitin ligase Plant U-box 20 (PUB20; alternatively called AtCMPG1) was identified as a possible interactor of the Arabidopsis G-protein β subunit, AGB1, by yeast two-hybrid screening. A bimolecular fluorescence complementation (BiFC) assay showed that PUB20 interacted with AGB1 in the nuclei and the cytosol. The expression levels of PUB20 and its closest homolog, PUB21 were stable under many conditions. GUS driven by the PUB20 promoter was active in anthers, pollen, premature seeds and receptacles and GUS driven by the PUB21 promoter was active in anthers and funiculi. PUB20 was found to have autoubiquitination activity in vitro.

A mutation in Arabidopsis BSK5 encoding a brassinosteroid-signaling kinase protein affects responses to salinity and abscisic acid

As the most recently characterized group of plant hormones, brassinosteroids (BR) are involved in a number of physiological responses. Although many key components of the BR signaling pathway have been isolated and characterized, there is little information on detailed characterization of brassinosteroid-signaling kinase (BSK) proteins. In this study, Arabidopsis BSK5 was isolated and functionally analyzed. BSK5 transcripts were detected in various tissues, and were induced by abiotic stresses including salt and drought, as well as phytohormones of BR and abscisic acid (ABA). Arabidopsis loss-of-function mutant bsk5 exhibited sensitivity to salinity and ABA. Mutations of the BSK5 gene also altered the expression of several stress-regulated genes. We suggest that BSK5 responds to other signals as well as BR.

The GCR1 and GPA1 participate in promotion of Arabidopsis primary root elongation induced by N-acyl-homoserine lactones, the bacterial quorum-sensing signals

Many gram-negative bacteria use N-acyl-homoserine lactones (AHL) as quorum-sensing signals to coordinate their collective behaviors. Accumulating evidence indicates that plants can respond to AHL. However, little is known about the molecular mechanism of plants reacting to these bacterial signals. In this study, we show that the treatment of Arabidopsis roots with N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL) and N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) resulted in significant root elongation. The genetic analysis revealed that the T-DNA insertional mutants of gcr1, encoding a G-protein-coupled receptor GCR1, were insensitive to 3OC6-HSL or 3OC8-HSL in assays of root growth. The loss-of-function mutants of the sole canonical Gα subunit GPA1 showed no response to AHL promotion of root elongation whereas Gα gain-of-function plants overexpressing either the wild type or a constitutively active version of Arabidopsis Gα exhibited the exaggerated effect on root elongation caused by AHL. Furthermore, the expression of GCR1 and GPA1 were significantly upregulated after plants were contacted with both AHL. Taken together, our results suggest that GCR1 and GPA1 are involved in AHL-mediated elongation of Arabidopsis roots. This provides insight into the mechanism of plant responses to bacterial quorum-sensing signals.

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.

The mechanisms of brassinosteroids' action: from signal transduction to plant development

Brassinosteroids play diverse roles in plant growth and development. Plants deficient in brassinosteroid (BR) biosynthesis or defective in signal transduction show many abnormal developmental phenotypes, indicating the importance of both BR biosynthesis and the signaling pathway in regulating these biological processes. Recently, using genetics, proteomics, genomics, cell biology, and many other approaches, more components involved in the BR signaling pathway were identified. Furthermore, the physiological, cellular, and molecular mechanisms by which BRs regulate various aspects of plant development, are being discovered. These include root development, anther and pollen development and formation, stem elongation, vasculature differentiation, and cellulose biosynthesis, suggesting that the biological functions of BRs are far beyond promoting cell elongation. This review will focus on the up-to-date progresses about regulatory mechanisms of the BR signaling pathway and the physiological and molecular mechanisms whereby BRs regulate plant growth and development.

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.

Indolyl-3-butyric acid-induced Arabidopsis stomatal opening mediated by 3',5'-cyclic guanosine-monophosphate

It has been pharmacologically suggested that 3',5'-cyclic guanosine-monophosphate (cGMP) mediates indolyl-3-butyric acid (IBA)-induced stomatal opening. In Arabidopsis thaliana (L.) Heynh., such investigations compared the wild type (Columbia and Ws ecotypes) to mutants knockout for either GTP-binding protein (G protein) α subunit 1 (gpa1-4), putative G protein-coupled receptor 1 (gcr1-5), calcineurin B-like isoform 1 (cbl1) or 9 (cbl9), or the NADPH oxidases AtrbohD and AtrbohF (atrbohD/F). Stomatal opening to IBA or the permeant cGMP analogue, 8-bromo-cGMP (8-Br-cGMP) was abolished in the atrbohD/F mutant. The IBA response was fully or partially suppressed, respectively, in the gcr1-5 mutant, or the gpa1-4 and cbl1 mutants. In the cbl9 mutant, the response to IBA or 8-Br-cGMP, respectively, was partially or fully suppressed. Phenylarsine oxide (PAO) affected the IBA response, which the cbl1 mutant overlapped or the gpa1-4 and cbl9 mutants increased up to 100% inhibition. 6-anilino-5,8-quinolinedione, mas17, the (Rp)-diastereomer of 8-bromo-3',5'-cyclic guanosine monophosphorothioate (Rp-8-Br-cGMPS), nicotinamide, ruthenium red (RRed), 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), cyclosporine A (CsA) and FK506 converged to affect the IBA response, which the gpa1-4 and cbl9 mutants overlapped or the cbl1 mutant and PAO increased up to 100% inhibition. Rp-8-Br-cGMPS, nicotinamide, RRed, BAPTA, CsA or FK506 paralled the cbl9 and atrbohD/F mutants to abolish the 8-Br-cGMP response. Based on so far revealed features of these mutants and pharmacological compounds, these results confirmed cGMP as a Ca(2+)-mobilizing second messenger for apoplastic auxin whose perception and transduction would implicate a seven-transmembrane receptor - G protein - guanylyl cyclase unit at the guard cell plasma membrane.

Boolean modeling of transcriptome data reveals novel modes of heterotrimeric G-protein action

Classical mechanisms of heterotrimeric G-protein signaling are observed to function in regulation of the transcriptome. Conversely, many theoretical regulatory modes of the G-protein are not manifested in the transcriptomes we investigate.

A new mechanism of G-protein signaling is revealed, in which the β subunit regulates gene expression identically in the presence or absence of the α subunit.

We find evidence of cross-talk between G-protein-mediated and hormone-mediated transcriptional regulation.

We find evidence of system specificity in G-protein signaling.

Heterotrimeric G-proteins, composed of α, β, and γ subunits, participate in a wide range of signaling pathways in eukaryotes (Morris and Malbon, 1999). According to the typical, mammalian paradigm, in its inactive state, the G-protein exists as an associated heterotrimer. G-protein signaling begins with ligand binding that results in a conformational change in a G-protein-coupled receptor (GPCR). Once activated by the GPCR, the Gα separates from the associated Gβγ dimer and the freed Gα and Gβγ proteins can then interact with downstream effector molecules, alone or in combination, to transduce the signal. Subsequent to signal propagation, Gα re-associates with the Gβγ dimer to reform the G-protein complex.

There are several classical routes for signal propagation through heterotrimeric G-proteins that have been categorized in mammalian systems (Marrari et al, 2007; Dupre et al, 2009). One route, which we designate classical I, requires the presence of both subunits, and can invoke one of two distinct mechanisms. In one mechanism, on GPCR activation, freed Gα and Gβγ each interact with downstream effectors to elicit the downstream response. In a related mechanism, Gα but not Gβγ interacts with downstream effectors, but the Gβγ dimer is nevertheless required to facilitate coupling of Gα with the relevant GPCR (Marrari et al, 2007). In a second route, which we designate classical II, it is solely the Gβγ dimer that interacts with downstream effectors; in this case, sequestration of Gβγ within the heterotrimer prevents signal propagation. In addition, a few non-classical G-protein regulatory modes have also been implicated in some systems, for example signaling by the intact heterotrimer in yeast (Klein et al, 2000; Frank et al, 2005). Observations such as these lead to a fundamental question, namely, which of all the theoretical regulatory modes of G-protein signaling are realized biologically. Our study answers this question in the context of the model plant Arabidopsis thaliana, and in addition analyzes the manner in which G-protein signaling couples with signaling by the plant hormone abscisic acid. The Arabidopsis genome encodes only one canonical Gα subunit, GPA1, and one canonical Gβ subunit, AGB1, and knockout mutants are available for each of these, allowing clear dissection of Gα- and Gβ-related phenotypes.

Abscisic acid (ABA) is a major plant hormone, which inhibits growth and promotes tolerance of abiotic stresses such as drought, salinity, and cold. ABA signaling is known to interact with heterotrimeric G-protein signaling in both developmental and stress responses in a complex manner, causing, for example, ABA hyposensitivity of guard cell stomatal opening in gpa1 and agb1 single mutants as well as agb1 gpa1 double mutants (Fan et al, 2008), but ABA hypersensitivity of the inhibition of seed germination and post-germination seedling development in the same mutants (Pandey et al, 2006). These experimental observations implicate G-proteins as one of the components of ABA signaling, but to date no systematic study has been conducted in either plant or metazoan systems to define the co-regulatory modes of a G-protein and a hormone.

In this study, we conduct genome-wide gene expression profiling in G-protein subunit mutants of A. thaliana guard cells and leaves, with or without treatment with ABA. By introducing one or more mediators acting downstream of the G-protein and ABA to control transcript levels, we propose nine G-protein/ABA signaling pathways including ABA-independent G-protein signaling pathways, G-protein-independent ABA signaling pathways, and seven distinct ABA–G-protein-coupled signaling pathways (Figure 1). We develop a Boolean modeling framework to systematically enumerate 14 possible theoretical regulatory modes of the G-protein and 142 co-regulatory modes of the G-protein and ABA, and then use a pattern matching approach to associate target genes with theoretical regulatory modes.

Our analysis shows that the G-protein regulatory mode that requires the presence of both Gα and Gβγ subunits (consistent with classical I mechanisms), is well represented in both guard cells and leaves. The G-protein regulatory mode that requires a freed Gβγ subunit (classical II G-protein regulatory mechanism) is well supported in guard cells and somewhat less so in leaves. In addition, a G-protein regulatory mode representing a non-classical regulatory mechanism is prevalent in guard cells but less so in leaves (Figure 5). In this regulatory mode, signaling by Gβ(γ) occurs, and this signaling is not regulated in any way by Gα.

By relating the target genes with the nine proposed G-protein/ABA signaling pathways, we are able to gauge the plausibility of regulatory modes of the G-protein and ABA at the pathway level. We find that G-protein-independent ABA signaling pathways are prevalent in both guard cells and leaves. The existence of an ABA-independent regulatory activity of the G-protein is well supported in guard cells, but not supported in leaves. Additive regulation by G-protein signaling plus G-protein-independent ABA signaling is rare in both guard cells and leaves. In addition, combinatorial cross-talk between G-protein signaling and ABA signaling and additive cross-talk between ABA–G-protein signaling and G-protein-independent ABA signaling are observed in both guard cells and leaves. Our transcriptome analysis indicates that in some cases, ABA definitely does not influence G-protein signaling, though it may do so in some other cases.

To investigate whether previously observed hypersensitivity or hyposensitivity of developmental and dynamic transient responses to ABA in G-protein mutants is recapitulated at the level of transcriptional regulation, we compare gene regulation by ABA in guard cells and leaves of the G-protein mutants versus wild type. We find that in guard cells, equal ABA hyposensitivity of all mutants combined is significant, although hyposensitivity in individual mutants is not. There is also a separate group of genes in guard cells that show ABA hypersensitivity in the gpa1 mutant, suggesting complex interactions between ABA and G-protein signaling in gene regulation in this cell type. In leaves, ABA hyposensitivity of gene expression in the three individual mutants and equal hyposensitivity in all mutants are strongly supported. In addition, several of the functional categories identified by our analysis of G-protein regulatory modes have been implicated in previous physiological analyses of G-protein mutants, providing validation to the biological interpretation of our results.

In summary, by conducting a genome-wide gene expression profiling study in G-protein subunit mutants of A. thaliana guard cells and leaves and developing a Boolean modeling framework, we systematically evaluate the biological utilization of mechanisms of G-protein regulatory action and reveal novel regulatory modes of the G-protein. The results generate empirical evidence and insights regarding molecular events of G-protein signaling and response at the physiological level in both plants and mammals.

Heterotrimeric G-proteins mediate crucial and diverse signaling pathways in eukaryotes. Here, we generate and analyze microarray data from guard cells and leaves of G-protein subunit mutants of the model plant Arabidopsis thaliana, with or without treatment with the stress hormone, abscisic acid. Although G-protein control of the transcriptome has received little attention to date in any system, transcriptome analysis allows us to search for potentially uncommon yet significant signaling mechanisms. We describe the theoretical Boolean mechanisms of G-protein × hormone regulation, and then apply a pattern matching approach to associate gene expression profiles with Boolean models. We find that (1) classical mechanisms of G-protein signaling are well represented. Conversely, some theoretical regulatory modes of the G-protein are not supported; (2) a new mechanism of G-protein signaling is revealed, in which Gβ regulates gene expression identically in the presence or absence of Gα; (3) guard cells and leaves favor different G-protein modes in transcriptome regulation, supporting system specificity of G-protein signaling. Our method holds significant promise for analyzing analogous ‘switch-like' signal transduction events in any organism.

Comparative analysis of phytohormone-responsive phosphoproteins in Arabidopsis thaliana using TiO2-phosphopeptide enrichment and mass accuracy precursor alignment

Protein phosphorylation/dephosphorylation is a central post-translational modification in plant hormone signaling, but little is known about its extent and function. Although pertinent protein kinases and phosphatases have been predicted and identified for a variety of hormone responses, classical biochemical approaches have so far revealed only a few candidate proteins and even fewer phosphorylation sites. Here we performed a global quantitative analysis of the Arabidopsis phosphoproteome in response to a time course of treatments with various plant hormones using phosphopeptide enrichment and subsequent mass accuracy precursor alignment (MAPA). The use of three time points, 1, 3 and 6 h, in combination with five phytohormone treatments, abscisic acid (ABA), indole-3-acetic acid (IAA), gibberellic acid (GA), jasmonic acid (JA) and kinetin, resulted in 324,000 precursor ions from 54 LC-Orbitrap-MS analyses quantified and aligned in a data matrix with the dimension of 6000 x 54 using the ProtMax algorithm. To dissect the phytohormone responses, multivariate principal/independent components analysis was performed. In total, 152 phosphopeptides were identified as differentially regulated; these phosphopeptides are involved in a wide variety of signaling pathways. New phosphorylation sites were identified for ABA response element binding factors that showed a specific increase in response to ABA. New phosphorylation sites were also found for RLKs and auxin transporters. We found that different hormones regulate distinct amino acid residues of members of the same protein families. In contrast, tyrosine phosphorylation of the G alpha subunit appeared to be a common response for multiple hormones, demonstrating global cross-talk among hormone signaling pathways.

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.

Abscisic acid: emergence of a core signaling network

Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

Mechanisms of brassinosteroids interacting with multiple hormones

Various environmental and internal cues play essential roles in regulating diverse aspects of plant growth and development. Phytohormones usually coordinate multiple stimuli to directly regulate multiple developmental programs. Recent studies have provided progresses into the complexity of their cross talk. Particularly, the signaling pathways of various phytohormones have been revealed, leading to the discovery of the mechanisms of the interplay among different hormone signaling pathways. This review focuses on the recent advances of the signaling cross-talk between brassinosteroids and other hormones, including abscisic acid, auxin, gibberellins, ethylene and jasmonate.

Signaling through G protein coupled receptors

Heterotrimeric G proteins (Galpha, Gbeta/Ggamma subunits) constitute one of the most important components of cell signaling cascade. G Protein Coupled Receptors (GPCRs) perceive many extracellular signals and transduce them to heterotrimeric G proteins, which further transduce these signals intracellular to appropriate downstream effectors and thereby play an important role in various signaling pathways. GPCRs exist as a superfamily of integral membrane protein receptors that contain seven transmembrane alpha-helical regions, which bind to a wide range of ligands. Upon activation by a ligand, the GPCR undergoes a conformational change and then activate the G proteins by promoting the exchange of GDP/GTP associated with the Galpha subunit. This leads to the dissociation of Gbeta/Ggamma dimer from Galpha. Both these moieties then become free to act upon their downstream effectors and thereby initiate unique intracellular signaling responses. After the signal propagation, the GTP of Galpha-GTP is hydrolyzed to GDP and Galpha becomes inactive (Galpha-GDP), which leads to its re-association with the Gbeta/Ggamma dimer to form the inactive heterotrimeric complex. The GPCR can also transduce the signal through G protein independent pathway. GPCRs also regulate cell cycle progression. Till to date thousands of GPCRs are known from animal kingdom with little homology among them, but only single GPCR has been identified in plant system. The Arabidopsis GPCR was reported to be cell cycle regulated and also involved in ABA and in stress signaling. Here I have described a general mechanism of signal transduction through GPCR/G proteins, structure of GPCRs, family of GPCRs and plant GPCR and its role.

The heterotrimeric G-protein complex modulates light sensitivity in Arabidopsis thaliana seed germination

Release of dormancy and induction of seed germination are complex traits finely regulated by hormonal signals and environmental cues such as temperature and light. The Red (R):Far-Red (FR) phytochrome photoreceptors mediate light regulation of seed germination. We investigated the possible involvement of heterotrimeric G-protein complex in the phytochrome signaling pathways of Arabidopsis thaliana seed germination. Germination rates of null mutants of the alpha (Galpha) and beta (Gbeta) subunits of the G-protein (Atgpa1-4 and agb1-2, respectively) and the double mutant (agb1-2/gpa1-4) are lower than the wildtype (WT) under continuous or pulsed light. The Galpha and Gbeta subunits play a role in seed germination under hourly pulses of R lower than 0.1 micromol m(-2) s(-1) whereas the Gbeta subunit plays a role in higher R fluences. The germination of double mutants of G-protein subunits with phyA-211 and phyB-9 suggests that AtGPA1 seems to act as a positive regulator of phyA and probably phyB signaling pathways, while the role of AGB1 is ambiguous. The imbibition of seeds at 4 degrees C and 35 degrees C alters the R and FR light responsiveness of WT and G-protein mutants to a similar magnitude. Thus, Galpha and Gbeta subunits of the heterotrimeric G-protein complex modulate light induction of seed germination by phytochromes and are dispensable for the control of dormancy by low and high temperatures prior to irradiation. We discuss the possible indirect role of the G-protein complex on the phytochrome-regulated germination through hormonal signaling pathways.

AtMKK1 and AtMPK6 are involved in abscisic acid and sugar signaling in Arabidopsis seed germination

Abscisic acid (ABA) and sugars have been well established to be crucial factors controlling seed germination of Arabidopsis. Here we demonstrate that AtMKK1 and AtMPK6 are both critical signals involved in ABA and sugar-regulated seed germination. Wild type plants depended on stratification and after-ripening for seed germination, whereas this dependence on either stratification or after-ripening was not required for mutants of mkk1 and mpk6 as well as their double mutant mkk1 mpk6. While seed germination of wild type plants was sensitively inhibited by ABA and glucose, mkk1, mpk6 and mkk1 mpk6 were all strongly resistant to ABA or glucose treatments, and in contrast, plants overexpressing MKK1 or MPK6 were super-sensitive to ABA and glucose. Glucose treatment significantly induced increases in MKK1 and MPK6 activities. These results clearly indicate that MKK1 and MPK6 are involved in the ABA and sugar signaling in the process of seed germination. Further experiments showed that glucose was capable of inducing ABA biosynthesis by up-regulating NCED3 and ABA2, and furthermore, this up-regulation of NCED3 and ABA2 was arrested in the mkk1 mpk6 double mutant, indicating that the inhibition of seed germination by glucose is potentially resulted from sugar-induced up-regulation of the ABA level.

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.

GTPgammaS antagonizes the mastoparan-induced in vitro activity of PIP-phospholipase C from symbiotic root nodules of Phaseolus vulgaris

Phospholipase C (PLC) has been suggested to have a role in signal perception by Nod factors (NFs) in legume root hair cells. For instance, mastoparan, a well-described agonist of heterotrimeric G protein, induces nodulin expression after NFs treatment or Rhizobium inoculation. Furthermore, it has been recently demonstrated that mastoparan also mimics calcium oscillations induced by NFs, suggesting that PLC could play a key role during the nodulation process. In this study, we elucidate a biochemical relationship between PLC and heterotrimeric G proteins during NFs signaling in legumes. In particular, the effect of NFs on in vitro PLC activity from nodule membrane fractions in the presence of guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) and mastoparan was assayed. Our results indicate that for phosphatidylinositol 4,5 bisphosphate (PIP(2))-PLC, there is a specific activity of 20-27 nmol mg(-1) min(-1) in membrane fractions of nodules 18-20 days after inoculation with Rhizobium tropici. Interestingly, in the presence of 5 microM mastoparan, PIP(2)-PLC activity was almost double the basal level. In contrast, PIP(2)-PLC activity was downregulated by 1-10 microM GTPgammaS. Also, PLC activity was decreased by up to 64% in the presence of increasing concentrations of NFs (10(-8) to 10(-5) M). NFs are critical signaling molecules in rhizobia/legume symbiosis that can activate many of the plant's early responses during nodule development. Calcium spiking, kinases, PLC activity and possibly G proteins appear to be components downstream of the NFs perception pathway. Our results suggest the occurrence of a dual signaling pathway that could involve both G proteins and PLC in Phaseolus vulgaris during the development of root nodules.

The primary signaling outputs of brassinosteroids are regulated by abscisic acid signaling

Phytohormones have essential roles in coordinately regulating a large array of developmental processes. Studies have revealed that brassinosteroids (BRs) and abscisic acid (ABA) interact to regulate hundreds of expression in genes, governing many biological processes. However, whether their interaction is through modification or intersection of their primary signaling cascades, or by independent or parallel pathways remains a big mystery. Using biochemical and molecular markers of BR signaling and ABA biosynthetic mutants, we demonstrated that exogenous ABA rapidly inhibits BR signaling outputs as indicated by the phosphorylation status of BES1 and BR-responsive gene expression. Experiments using a bri1 null-allele, bri1-116, and analysis of subcellular localization of BKI1-YFP further revealed that the BR receptor complex is not required for ABA to act on BR signaling outputs. However, when the BR downstream signaling component BIN2 is inhibited by LiCl, ABA failed to inhibit BR signaling outputs. Also, using a set of ABA insensitive mutants, we found that regulation of ABA on the BR primary signaling pathway depends on the ABA early signaling components, ABI1 and ABI2. We propose that the signaling cascades of ABA and BR primarily cross-talk after BR perception, but before their transcriptional activation. This model provides a reasonable explanation for why a large proportion of BR-responsive genes are also regulated by ABA, and provides an insight into the molecular mechanisms by which BRs could interact with ABA.

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.

Function of the alpha subunit of rice heterotrimeric G protein in brassinosteroid signaling

The alpha subunit of plant heterotrimeric G proteins (Galpha) plays pivotal roles in multiple aspects of development and responses to plant hormones. Recently, several lines of evidence have shown that Galpha participates in brassinosteroid (BR) responses in Arabidopsis and rice plants. In this study, we conducted a comprehensive analysis of the roles of the rice Galpha in the responses to BR using a defective mutant of the Galpha gene, T65d1. Decreased sensitivity to 24-epi-brassinolide (24-epiBL) in the T65d1 mutant was observed in many processes examined, e.g. in the inhibition of root growth and the promotion of coleoptile elongation. The T65d1 mutant also showed similar phenotypes to those of BR-deficient mutants, such as the specifically shortened second internode and the constitutive photomorphogenic growth phenotype under dark conditions. However, a negative feedback effect by 24-epiBL on the expression of BR biosynthetic genes was observed in the T65d1 mutant, and the levels of BR intermediates did not fluctuate in this mutant. To determine the epistatic relationship between the T65d1 mutant and d61-7, a weak allele of a rice BR receptor mutant, the two mutants were crossed. The T65d1/d61-7 double mutant showed no epistasis in the elongation inhibition of the internodes, the internode elongation pattern, the leaf angle and the morphological abnormality of leaf, except for the vertical length of seed and the seed weight. Our results suggest that the rice Galpha affects the BR signaling cascade but the Galpha may not be a signaling molecule in BRI1-meditated perception/transduction.

Heterotrimeric G-protein signaling in Arabidopsis: Puzzling G-protein-coupled receptor

Seven transmembrane G-protein-coupled receptors (GPCRs) are commonly used by eukaryotes to sense extracellular signals to switch on cellular responses through the activation of cognate heterotrimeric G-proteins. In Arabidopsis thaliana, GCR2 has been proposed as a GPCR for the plant hormone abscisic acid. On the other hand, biochemical analysis demonstrates that the sole Arabidopsis heterotrimeric G-protein alpha subunit, GPA1, is in the activated state (GTP-bound) by default, suggesting that the heterotrimeric G-proteins may act without any GPCRs.

A sweet cycle for Arabidopsis G-proteins: Recent discoveries and controversies in plant G-protein signal transduction

Heterotrimeric G-proteins are a class of signal transduction proteins highly conserved throughout evolution that serve as dynamic molecular switches regulating the intracellular communication initiated by extracellular signals including sensory information. This property is achieved by a guanine nucleotide cycle wherein the inactive, signaling-incompetent Galpha subunit is normally bound to GDP; activation to signaling-competent Galpha occurs through the exchange of GDP for GTP (typically catalyzed via seven-transmembrane domain G-protein coupled receptors [GPCRs]), which dissociates the Gbetagamma dimer from Galpha-GTP and initiates signal transduction. The hydrolysis of GTP, greatly accelerated by "Regulator of G-protein Signaling" (RGS) proteins, returns Galpha to its inactive GDP-bound form and terminates signaling. Through extensive characterization of mammalian Galpha isoforms, the rate-limiting step in this cycle is currently considered to be the GDP/GTP exchange rate, which can be orders of magnitude slower than the GTP hydrolysis rate. However, we have recently demonstrated that, in Arabidopsis, the guanine nucleotide cycle appears to be limited by the rate of GTP hydrolysis rather than nucleotide exchange. This finding has important implications for the mechanism of sugar sensing in Arabidopsis. We also discuss these data on Arabidopsis G-protein nucleotide cycling in relation to recent reports of putative plant GPCRs and heterotrimeric G-protein effectors in Arabidopsis.

D-Glucose sensing by a plasma membrane regulator of G signaling protein, AtRGS1

Plants use sugars as signaling molecules and possess mechanisms to detect and respond to changes in sugar availability, ranging from the level of secondary signaling molecules to altered gene transcription. G-protein-coupled pathways are involved in sugar signaling in plants. The Arabidopsis thaliana regulator of G-protein signaling protein 1 (AtRGS1) combines a receptor-like seven transmembrane domain with an RGS domain, interacts with the Arabidopsis Galpha subunit (AtGPA1) in a d-glucose-regulated manner, and stimulates AtGPA1 GTPase activity. We determined that AtRGS1 interacts with additional components, genetically defined here, to serve as a plasma membrane sensor for d-glucose. This interaction between AtRGS1 and AtGPA1 involves, in part, the seven-transmembrane domain of AtRGS1.

Regulation of root-wave response by extra large and conventional G proteins in Arabidopsis thaliana

Heterotrimeric G proteins composed of alpha, beta and gamma subunits regulate a number of fundamental processes concerned with growth and development in plants. In addition to the canonical heterotrimeric G proteins, plants also contain a small family of extra large G proteins (XLGs) that show significant similarity to the G-protein alpha subunit in their C-terminal regions. In this paper we show that one of the three XLG genes, XLG3, and the Gbeta subunit (AGB1) of the Arabidopsis G-protein heterotrimer are specifically involved in the regulation of a subset of root morphological and growth responses. Based on analysis of T-DNA insertional mutant phenotypes, XLG3 and AGB1 each positively regulate root waving and root skewing. Since these responses are regulated by physical as well as physiological cues, we assessed the roles of AGB1 and XLG3 in gravitropism, thigmotropism and hormonal responses. Our data show that mutants lacking either XLG3 or AGB1 genes are hypersensitive to ethylene and show growth responses consistent with alterations in auxin transport, while maintaining an essentially wild-type response to the physical cues of gravity and touch. These results suggest that XLG3 and AGB1 proteins regulate the hormonal determinants of root-waving and root-skewing responses in plants and possibly interact in a tissue-specific or signal-specific manner. Because plants harboring knockout mutations in the Galpha subunit gene, GPA1, exhibit wild-type root waving and skewing, our results may indicate that the AGB1 subunit functions in these processes without formation of a classic Galphabetagamma heterotrimer.

Loss-of-function mutations in the Arabidopsis heterotrimeric G-protein alpha subunit enhance the developmental defects of brassinosteroid signaling and biosynthesis mutants

Loss-of-function alleles of the sole heterotrimeric G-protein alpha subunit in Arabidopsis, GPA1, display defects in cell proliferation throughout plant development. Previous studies indicated that GPA1 is involved in brassinosteroid (BR) response. Here we provide genetic evidence that loss-of-function mutations in GPA1, gpa1-2 and gpa1-4, enhance the developmental defects of bri1-5, a weak allele of a BR receptor mutant, and det2-1, a BR-deficient mutant in Arabidopsis. gpa1-2 bri1-5 and gpa1-4 det2-1 double mutants had shorter hypocotyls, shorter roots and fewer lateral roots, and displayed more severe dwarfism than bri1-5 and det2-1 single mutants, respectively. By using the Arabidopsis hypocotyl as a model system where the parameters of cell division and cell elongation can be simultaneously measured, we found that gpa1 can specifically enhance the cell division defects of bri1-5 and det2-1 mutants. Similarly, gpa1 specifically enhances cell division defects in the primary roots of bri1-5 and det2-1 mutants. Furthermore, an additive effect on cell division between gpa1 and bri1-5 or det2-1 mutations was observed in the hypocotyls, whereas a synergistic effect was observed in the roots. Taken together, these results provided the first genetic evidence that G-protein- and BR-mediated pathways may be converged to modulate cell proliferation in a cell/tissue-specific manner.

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.

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.

Genetic characterization reveals no role for the reported ABA receptor, GCR2, in ABA control of seed germination and early seedling development in Arabidopsis

Abscisic acid (ABA) is perceived by several different types of receptors in plant cells. At the cell surface, the ABA signal is proposed to be perceived by GCR2, which mediates ABA responses in seed germination, early seedling development and stomatal movement. GCR2 was also proposed to be a seven-transmembrane (7TM) G-protein-coupled receptor (GPCR). Here we characterize GCR2 and one of its two homologs, GCR2-LIKE 1 (GCL1), in ABA-mediated seed germination and early seedling development in Arabidopsis. We show that loss-of-function mutations in GCL1 did not confer ABA insensitivity. Similarly, we did not observe ABA insensitivity in three independent gcr2 alleles. Furthermore, we generated gcr2 gcl1 double mutants and found that the double mutants still had near wild-type responses to ABA. Consistent with this, we found that the transcription of ABA marker genes was induced by ABA to levels that were comparable in wild type and gcr2 and gcl1 single and double mutants. On the other hand, the loss-of-function alleles of the sole Arabidopsis heterotrimeric G protein alpha subunit, GPA1, were hypersensitive to ABA in the ABA-inhibition of seed germination and early seedling development, disfavoring a genetic coupling of GCR2 by GPA1. Using multiple robust transmembrane prediction systems, GCR2 was predicted not to be a 7TM protein, a structural hallmark of GPCRs. Taken together, our results do not support the notion that GCR2 is an ABA-signaling GPCR in seed germination and early seedling development.

GTPase acceleration as the rate-limiting step in Arabidopsis G protein-coupled sugar signaling

Heterotrimeric G protein signaling is important for cell-proliferative and glucose-sensing signal transduction pathways in the model plant organism Arabidopsis thaliana. AtRGS1 is a seven-transmembrane, RGS domain-containing protein that is a putative membrane receptor for d-glucose. Here we show, by using FRET, that d-glucose alters the interaction between the AtGPA1 and AtRGS1 in vivo. AtGPA1 is a unique heterotrimeric G protein alpha subunit that is constitutively GTP-bound given its high spontaneous nucleotide exchange coupled with slow GTP hydrolysis. Analysis of a point mutation in AtRGS1 that abrogates GTPase-accelerating activity demonstrates that the regulation of AtGPA1 GTP hydrolysis mediates sugar signal transduction during Arabidopsis development, in contrast to animals where nucleotide exchange is the limiting step in the heterotrimeric G protein nucleotide cycle.

Reactive oxygen species, ABA and nitric oxide interactions on the germination of warm-season C4-grasses

Hydrogen peroxide (H(2)O(2)) as a source of reactive oxygen species (ROS) significantly stimulated germination of switchgrass (Panicum virgatum L.) seeds with an optimal concentration of 20 mM at both 25 and 35 degrees C. For non-dormant switchgrass seeds exhibiting different levels of germination, treatment with H(2)O(2) resulted in rapid germination (<3 days) of all germinable seeds as compared to seeds placed on water. Exposure to 20 mM H(2)O(2) elicited simultaneous growth of the root and shoot system, resulting in more uniform seedling development. Seeds of big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash] also responded positively to H(2)O(2) treatment, indicating the universality of the effect of H(2)O(2) on seed germination in warm-season prairie grasses. For switchgrass seeds, abscisic acid (ABA) and the NADPH-oxidase inhibitor, diphenyleneiodonium (DPI) at 20 microM retarded germination (radicle emergence), stunted root growth and partially inhibited NADPH-oxidase activity in seeds. H(2)O(2) reversed the inhibitory effects of DPI and ABA on germination and coleoptile elongation, but did not overcome DPI inhibition of root elongation. Treatment with H(2)O(2) appeared to enhance endogenous production of nitric oxide, and a scavenger of nitric oxide abolished the peroxide-responsive stimulation of switchgrass seed germination. The activities and levels of several proteins changed earlier in seeds imbibed on H(2)O(2) as compared to seeds maintained on water or on ABA. These data demonstrate that seed germination of warm-season grasses is significantly responsive to oxidative conditions and highlights the complex interplay between seed redox status, ABA, ROS and NO in this system.

Rhizosphere communication of plants, parasitic plants and AM fungi

Plants use an array of secondary metabolites to defend themselves against harmful organisms and to attract others that are beneficial. However, the attraction of beneficial organisms could also lead to abuse by malevolent organisms. An exciting example of such abuse is the relationship between plants, beneficial mutualistic arbuscular mycorrhizal fungi and harmful parasitic plants. Signalling molecules called strigolactones, which are secreted by plant roots in low concentrations, induce the growth of both obligate biotrophs. Here, we review the importance of strigolactones for these two interactions and discuss possible developments that should further clarify the role of these signalling molecules in rhizosphere processes.

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.

The plant heterotrimeric G-protein complex

Heterotrimeric G-protein complexes couple extracellular signals via cell surface receptors to downstream enzymes called effectors. Heterotrimeric G-protein complexes, together with their cognate receptors and effectors, operate at the apex of signal transduction. In plants, the number of G-protein complex components is dramatically less than in other multicellular eukaryotes. An understanding of how multiple signals propagate transduction through the G-protein node can be found in the unique structural and kinetic properties of the plant heterotrimeric G-protein complex. This review addresses these unique features and speculates on why the repertoire of G-protein signaling elements is dramatically simpler than that in all other multicellular eukaryotes.

Gibberellin receptor and its role in gibberellin signaling in plants

Gibberellins (GAs) are a large family of tetracyclic, diterpenoid plant hormones that induce a wide range of plant growth responses. It has been postulated that plants have two types of GA receptors, including soluble and membrane-bound forms. Recently, it was determined that the rice GIBBERELLIN INSENSITIVE DWARF1 (GID1) gene encodes an unknown protein with similarity to the hormone-sensitive lipases that has high affinity only for biologically active GAs. Moreover, GID1 binds to SLR1, a repressor of GA signaling, in a GA-dependent manner in yeast cells. Based on these observations, it has been concluded that GID1 is a soluble receptor mediating GA signaling in rice. More recently, Arabidopsis thaliana was found to have three GID1 homologs, AtGID1a, b, and c, all of which bind GA and interact with the five Arabidopsis DELLA proteins.