"Round up the usual suspects": a comment on nonexistent plant G protein-coupled receptors

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins are a class of signal transduction complexes with broad roles in human health and agriculturally important plant traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. Using the Arabidopsis thaliana Gα subunit, GPA1, we developed a rapid StrepII-tag mediated purification method that facilitates isolation of protein with increased enzymatic activities as compared to conventional methods, and is demonstrably also applicable to mammalian Gα subunits. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate the instability of recombinant GPA1 is a function of the interaction between the Ras and helical domains, and can be partially uncoupled from the rapid nucleotide binding kinetics displayed by GPA1.

Moving beyond the arabidopsis-centric view of G-protein signaling in plants

Heterotrimeric G-protein-mediated signaling is a key mechanism to transduce a multitude of endogenous and environmental signals in diverse organisms. The scope and expectations of plant G-protein research were set by pioneering work in metazoans. Given the similarity of the core constituents, G-protein-signaling mechanisms were presumed to be universally conserved. However, because of the enormous diversity of survival strategies and endless forms among eukaryotes, the signal, its interpretation, and responses vary even among different plant groups. Earlier G-protein research in arabidopsis (Arabidopsis thaliana) has emphasized its divergence from Metazoa. Here, we compare recent evidence from diverse plant lineages with the available arabidopsis G-protein model and discuss the conserved and novel protein components, signaling mechanisms, and response regulation.

Life, death and resurrection of plant GPCRs

The activation of G-protein coupled receptors (GPCRs) by extracellular ligands constitutes the first step of heterotrimeric G-protein signalling in animals. In plants, canonical GPCRs have been known for over 25 years, often in association with agronomically important functions. But their role in plant G-protein signalling and even their annotation as GPCR was contested in the last decade, only to be revisited in the light of more recent evidences. In this first ever review on plant GPCRs, we catalogue all the plant GPCRs described to date and discuss the evidences for and against their role in plants in general and G-protein signalling in particular. We argue against writing off GPCRs and point to the missing links to be investigated to establish firm conclusions either way.

Cilia-derived vesicles: An ancient route for intercellular communication

Extracellular vesicles (EVs) provide a mechanism for intercellular communication that transports complex signals in membrane delimited structures between cells, tissues and organisms. Cells secrete EVs of various subtypes defined by the pathway leading to release and by the pathological condition of the cell. Cilia are evolutionarily conserved organelles that can act as sensory structures surveilling the extracellular environment. Here we discuss the secretory functions of cilia and their biological implications. Studies in multiple species - from the nematode Caenorhabditis elegans and the chlorophyte alga Chlamydomonas reinhardtii to mammals - have revealed that cilia shed bioactive EVs (ciliary EVs or ectosomes) by outward budding of the ciliary membrane. The content of ciliary EVs is distinct from that of other vesicles released by cells. Peptides regulate numerous aspects of metazoan physiology and development through evolutionarily conserved mechanisms. Intriguingly, cilia-derived vesicles have recently been found to mediate peptidergic signaling. C. reinhardtii releases the peptide α-amidating enzyme (PAM), bioactive amidated products and components of the peptidergic signaling machinery in ciliary EVs in a developmentally regulated manner. Considering the origin of cilia in early eukaryotes, it is likely that release of peptidergic signals in ciliary EVs represents an alternative and ancient mode of regulated secretion that cells can utilize in the absence of dedicated secretory granules.

GCR1 Positively Regulates UV-B- and Ethylene-Induced Stomatal Closure via Activating GPA1-Dependent ROS and NO Production

Heterotrimeric G proteins function as key players in guard cell signaling to many stimuli, including ultraviolet B (UV-B) and ethylene, but whether guard cell G protein signaling is activated by the only one potential G protein-coupled receptor, GCR1, is still unclear. Here, we found that gcr1 null mutants showed defects in UV-B- and ethylene-induced stomatal closure and production of reactive oxygen species (ROS) and nitric oxide (NO) in guard cells, but these defects could be rescued by the application of a Gα activator or overexpression of a constitutively active form of Gα subunit GPA1 (cGPA1). Moreover, the exogenous application of hydrogen peroxide (H₂O₂) or NO triggered stomatal closure in gcr1 mutants and cGPA1 transgenic plants in the absence or presence of UV-B or ethylene, but exogenous ethylene could not rescue the defect of gcr1 mutants in UV-B-induced stomatal closure, and gcr1 mutants did not affect UV-B-induced ethylene production in Arabidopsis leaves. These results indicate that GCR1 positively controls UV-B- and ethylene-induced stomatal closure by activating GPA1-dependent ROS and NO production in guard cells and that ethylene acts upstream of GCR1 to transduce UV-B guard cell signaling, which establishes the existence of a classic paradigm of G protein signaling in guard cell signaling to UV-B and ethylene.

Melatonin functions in priming of stomatal immunity in Panax notoginseng and Arabidopsis thaliana

Melatonin (MT) plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that MT functions in stomatal immunity in Panax notoginseng and Arabidopsis thaliana. Biochemical analyses showed that MT-induced stomatal closure plays a prominent role in preventing invasion of bacteria Pseudomonas syringe pv. tomato (Pst) DC3000 via activation of mitogen-activated protein kinase (MAPK) and NADPH oxidase-mediated reactive oxygen species production in P. notoginseng. The first putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving MT signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and MT-induced MAPK activation in a heterotrimeric GTP-binding protein Gα subunit GPA1-independent manner. GPA1 functions in the same genetic pathways of FLS2/BAK1 (Flagellin Sensing 2/Brassinosteroid Insensitive 1-associated kinase 1)- as well as PMTR1-mediated flg22 and MT signaling in stomatal closure. The stomata in pmtr1 are insensitive to MT and flg22, but the application of MT induces stomatal closure and reduces the bacterial growth in fls2 and bak1 plants, indicating that PMTR1 might be a downstream signaling component in FLS2- and BAK1-mediated stomatal immunity. In summary, our results (i) demonstrate that phytomelatonin functions in the priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.

Heterotrimeric G Proteins in Plants: Canonical and Atypical Gα Subunits

Heterotrimeric GTP-binding proteins (G proteins), consisting of Gα, Gβ and Gγ subunits, transduce signals from a diverse range of extracellular stimuli, resulting in the regulation of numerous cellular and physiological functions in Eukaryotes. According to the classic G protein paradigm established in animal models, the bound guanine nucleotide on a Gα subunit, either guanosine diphosphate (GDP) or guanosine triphosphate (GTP) determines the inactive or active mode, respectively. In plants, there are two types of Gα subunits: canonical Gα subunits structurally similar to their animal counterparts and unconventional extra-large Gα subunits (XLGs) containing a C-terminal domain homologous to the canonical Gα along with an extended N-terminal domain. Both Gα and XLG subunits interact with Gβγ dimers and regulator of G protein signalling (RGS) protein. Plant G proteins are implicated directly or indirectly in developmental processes, stress responses, and innate immunity. It is established that despite the substantial overall similarity between plant and animal Gα subunits, they convey signalling differently including the mechanism by which they are activated. This review emphasizes the unique characteristics of plant Gα subunits and speculates on their unique signalling mechanisms.

Research Advances in Heterotrimeric G-Protein α Subunits and Uncanonical G-Protein Coupled Receptors in Plants

As crucial signal transducers, G-proteins and G-protein-coupled receptors (GPCRs) have attracted increasing attention in the field of signal transduction. Research on G-proteins and GPCRs has mainly focused on animals, while research on plants is relatively rare. The mode of action of G-proteins is quite different from that in animals. The G-protein α (Gα) subunit is the most essential member of the G-protein signal cycle in animals and plants. The G-protein is activated when Gα releases GDP and binds to GTP, and the relationships with the GPCR and the downstream signal are also achieved by Gα coupling. It is important to study the role of Gα in the signaling pathway to explore the regulatory mechanism of G-proteins. The existence of a self-activated Gα in plants makes it unnecessary for the canonical GPCR to activate the G-protein by exchanging GDP with GTP. However, putative GPCRs have been found and proven to play important roles in G-protein signal transduction. The unique mode of action of G-proteins and the function of putative GPCRs in plants suggest that the same definition used in animal research cannot be used to study uncanonical GPCRs in plants. This review focuses on the different functions of the Gα and the mode of action between plants and animals as well as the functions of the uncanonical GPCR. This review employs a new perspective to define uncanonical GPCRs in plants and emphasizes the role of uncanonical GPCRs and Gα subunits in plant stress resistance and agricultural production.

Melatonin metabolism, signaling and possible roles in plants

Melatonin is a multifunctional biomolecule found in both animals and plants. In this review, the biosynthesis, levels, signaling, and possible roles of melatonin and its metabolites in plants is summarized. Tryptamine 5-hydroxylase (T5H), which catalyzes the conversion of tryptamine into serotonin, has been proposed as a target to create a melatonin knockout mutant presenting a lesion-mimic phenotype in rice. With a reduced anabolic capacity for melatonin biosynthesis and an increased catabolic capacity for melatonin metabolism, all plants generally maintain low melatonin levels. Some plants, including Arabidopsis and Nicotiana tabacum (tobacco), do not possess tryptophan decarboxylase (TDC), the first committed step enzyme required for melatonin biosynthesis. Major melatonin metabolites include cyclic 3-hydroxymelatonin (3-OHM) and 2-hydroxymelatonin (2-OHM). Other melatonin metabolites such as N¹ -acetyl-N² -formyl-5-methoxykynuramine (AFMK), N-acetyl-5-methoxykynuramine (AMK) and 5-methoxytryptamine (5-MT) are also produced when melatonin is applied to Oryza sativa (rice). The signaling pathways of melatonin and its metabolites act via the mitogen-activated protein kinase (MAPK) cascade, possibly with Cand2 acting as a melatonin receptor, although the integrity of Cand2 remains controversial. Melatonin mediates many important functions in growth stimulation and stress tolerance through its potent antioxidant activity and function in activating the MAPK cascade. The concentration distribution of melatonin metabolites appears to be species specific because corresponding enzymes such as M2H, M3H, catalases, indoleamine 2,3-dioxygenase (IDO) and N-acetylserotonin deacetylase (ASDAC) are differentially expressed among plant species and even among different tissues within species. Differential levels of melatonin and its metabolites can lead to differential physiological effects among plants when melatonin is either applied exogenously or overproduced through ectopic overexpression.

Pea Gβ subunit of G proteins has a role in nitric oxide-induced stomatal closure in response to heat and drought stress

Heterotrimeric G proteins consisting of Gα, Gβ and Gγ subunits act as downstream effectors to regulate multiple functions including abiotic stress tolerance. However, the mechanism of Gβ-mediated heat and drought tolerance is yet to be established. To explore the role of Pisum sativum Gβ subunit (PsGβ) in heat and drought stress, transgenic tobacco plants overexpressing (OEs) PsGβ were raised. Transgenic plants showing ectopic expression of PsGβ performed better under heat and drought stress in comparison with vector control plants. The seed germination, relative water content (RWC) and nitric oxide (NO) induction in the guard cells of transgenic plants were significantly higher in contrast to control plants. PsGβ promoter was isolated and several stress-responsive elements were identified. The change in Gβ expression in response to heat, methyl jasmonate (MeJA), abscisic acid (ABA), drought and salt confirms the presence of heat, low temperature and drought-responsive elements in the PsGβ promoter. Also, heat and drought stress caused the release of NO-induced stomatal closure in the leaves of transgenic tobacco plants OEs PsGβ. The better performance of transgenic plant OEs PsGβ is also attributed to the improved photosynthetic parameters as compared with control plants. These findings suggest a role of PsGβ in the signalling pathway leading to NO-induced stomatal closure during heat and drought stress.

The maize heterotrimeric G protein β subunit controls shoot meristem development and immune responses

Heterotrimeric G proteins are important transducers of receptor signaling, functioning in plants with CLAVATA receptors in controlling shoot meristem size and with pathogen-associated molecular pattern receptors in basal immunity. However, whether specific members of the heterotrimeric complex potentiate cross-talk between development and defense, and the extent to which these functions are conserved across species, have not yet been addressed. Here we used CRISPR/Cas9 to knock out the maize G protein β subunit gene (Gβ) and found that the mutants are lethal, differing from those in Arabidopsis, in which homologous mutants have normal growth and fertility. We show that lethality is caused not by a specific developmental arrest, but by autoimmunity. We used a genetic diversity screen to suppress the lethal Gβ phenotype and also identified a maize Gβ allele with weak autoimmune responses but strong development phenotypes. Using these tools, we show that Gβ controls meristem size in maize, acting epistatically with G protein α subunit gene (Gα), suggesting that Gβ and Gα function in a common signaling complex. Furthermore, we used an association study to show that natural variation in Gβ influences maize kernel row number, an important agronomic trait. Our results demonstrate the dual role of Gβ in immunity and development in a cereal crop and suggest that it functions in cross-talk between these competing signaling networks. Therefore, modification of Gβ has the potential to optimize the trade-off between growth and defense signaling to improve agronomic production.

The Nucleotide-Dependent Interactome of Rice Heterotrimeric G-Protein α -Subunit

The rice heterotrimeric G-protein complex, a guanine-nucleotide-dependent on-off switch, mediates vital cellular processes and responses to biotic and abiotic stress. Exchange of bound GDP (resting state) for GTP (active state) is spontaneous in plants including rice and thus there is no need for promoting guanine nucleotide exchange in vivo as a mechanism for regulating the active state of signaling as it is well known for animal G signaling. As such, a master regulator controlling the G-protein activation state is unknown in plants. Therefore, an ab initio approach is taken to discover candidate regulators. The rice Gα subunit (RGA1) is used as bait to screen for nucleotide-dependent protein partners. A total of 264 proteins are identified by tandem mass spectrometry of which 32 were specific to the GDP-bound inactive state and 22 specific to the transition state. Approximately, 10% are validated as previously identified G-protein interactors.

GCR1 and GPA1 coupling regulates nitrate, cell wall, immunity and light responses in Arabidopsis

G-protein signaling components have been attributed many biological roles in plants, but the extent of involvement of G-protein coupled receptor 1 (GCR1) with the Gα (GPA1) remained unknown. To address this, we have performed transcriptomic analyses on Arabidopsis gpa1-5gcr1-5 double mutant and identified 656 differentially expressed genes (DEGs). MapMan and Gene Ontology analyses revealed global transcriptional changes associated with external stimulus, cell wall organization/biogenesis and secondary metabolite process among others. Comparative transcriptomic analyses using the single and double mutants of gcr1-5 and gpa1-5 identified 194, 139 and 391 exclusive DEGs respectively, whereas 64 DEGs were common to all three mutants. Further, pair wise comparison of DEGs of double mutant with single mutants of gcr1-5 or gpa1-5 showed about one-third and over half common DEGs, respectively. Further analysis of the DEGs exclusive to the double mutant using protein-protein interaction networks revealed molecular complexes associated with nitrate and light signaling and plant-pathogen interactions among others. Physiological and molecular validation of nitrate-response revealed the sensitivity of germination to low N in the double mutant and differential expression of nitrate transporter (and nitrate reductase in all three mutants). Taken together, GCR1 and GPA1 work in partnership as well as independently to regulate different pathways.

Heterotrimeric G protein signaling in plant immunity

In animals, heterotrimeric guanine nucleotide-binding proteins (G proteins) transduce signals perceived by numerous G protein-coupled receptors (GPCRs). However, no canonical GPCRs with guanine nucleotide exchange factor (GEF) activity are present in plant genomes. Accumulated evidence indicates that, instead of GPCRs, the receptor-like kinases (RLKs) function upstream of G proteins in plants. Regulator of G protein signaling 1 (RGS1) functions to convert the GTP-bound Gα to the GDP-bound form through its GTPase-accelerating protein (GAP) activity. Because of the intrinsic differences in the biochemical properties between Arabidopsis and animal Gα, the actions of animal and Arabidopsis RGS1 result in contrasting outcomes in G signaling activation/deactivation. Animal RGSs accelerate the deactivation of the activated G signaling, whereas Arabidopsis RGS1 prevents the activation of G signaling in the resting state. Phosphorylation of Arabidopsis RGS1 triggered by ligand-RLK recognition results in the endocytosis or degradation of RGS1, leading to the separation of RGS1 from Gα and thus the derepression of G signaling. Here, we summarize the involvement of the G proteins in plant immunity, with a special focus on the molecular mechanism of G signaling activation/deactivation regulated by RLKs and RGS1. We also provide a brief perspective on the outstanding questions that need to be addressed to fully understand G signaling in plant immunity.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

G proteins sculp root architecture in response to nitrogen in rice and Arabidopsis

Nitrogen is a key nutrient for plant growth and development. Plants regulate nitrogen availability and uptake efficiency through controlling root architecture. While the heterotrimeric G protein complex is an important element to regulate root morphology, it remains unknown whether the G protein regulates the root architecture in response to nitrogen supply. We used rice and Arabidopsis G protein mutants to study the root architecture in response to different nitrogen concentrations. We found that nitrogen inhibits root horizontal projection area (network area), root perimeter, total length, but not root diameter (average root width). Nitrogen influenced bushiness and root spatial distribution by inhibiting horizontal growth and promoting vertical expansion. The dynamic changes of the rice G protein mutant DK22 at different concentrations of nitrogen from day 7 to day 9 were different from the wild type with regard to bushiness and spatial distribution. The agb1-2 mutant in Arabidopsis lacked the inhibitory effect of nitrate on root growth. The heterotrimeric G protein complex regulates the inhibitory effect on root growth caused by high nitrogen supply and root spatial distribution in response to different nitrogen concentrations.

A Novel G-Protein-Coupled Receptors Gene from Upland Cotton Enhances Salt Stress Tolerance in Transgenic Arabidopsis

Plants have developed a number of survival strategies which are significant for enhancing their adaptation to various biotic and abiotic stress factors. At the transcriptome level, G-protein-coupled receptors (GPCRs) are of great significance, enabling the plants to detect a wide range of endogenous and exogenous signals which are employed by the plants in regulating various responses in development and adaptation. In this research work, we carried out genome-wide analysis of target of Myb1 (TOM1), a member of the GPCR gene family. The functional role of TOM1 in salt stress tolerance was studied using a transgenic Arabidopsis plants over-expressing the gene. By the use of the functional domain PF06454, we obtained 16 TOM genes members in Gossypium hirsutum, 9 in Gossypium arboreum, and 11 in Gossypium raimondii. The genes had varying physiochemical properties, and it is significant to note that all the grand average of hydropathy (GRAVY) values were less than one, indicating that all are hydrophobic in nature. In all the genes analysed here, both the exonic and intronic regions were found. The expression level of Gh_A07G0747 (GhTOM) was significantly high in the transgenic lines as compared to the wild type; a similar trend in expression was observed in all the salt-related genes tested in this study. The study in epidermal cells confirmed the localization of the protein coded by the gene TOM1 in the plasma membrane. Analysis of anti-oxidant enzymes showed higher concentrations of antioxidants in transgenic lines and relatively lower levels of oxidant substances such as H₂O₂. The low malondialdehyde (MDA) level in transgenic lines indicated that the transgenic lines had relatively low level of oxidative damage compared to the wild types. The results obtained indicate that Gh_A07G0747 (GhTOM) can be a putative target gene for enhancing salt stress tolerance in plants and could be exploited in the future for the development of salt stress-tolerant cotton cultivars.

G protein signaling in plants: minus times minus equals plus

Heterotrimeric G proteins are key regulators in the transduction of extracellular signals both in animals and plants. In plants, heterotrimeric G protein signaling plays essential roles in development and in response to biotic and abiotic stress. However, over the last decade it has become clear that plants have unique mechanisms of G protein signaling. Although plants share most of the core components of heterotrimeric G proteins, some of them exhibit unusual properties compared to their animal counterparts. In addition, plants do not share functional GPCRs. Therefore the well-established paradigm of the animal G protein signaling cycle is not applicable in plants. In this review, we summarize recent insights into these unique mechanisms of G protein signaling in plants with special focus on the evident potential of G protein signaling as a target to modify developmental and physiological parameters important for yield increase.

Down-regulated Lotus japonicus GCR1 plants exhibit nodulation signalling pathways alteration

G Protein Coupled Receptor (GPCRs) are integral membrane proteins involved in various signalling pathways by perceiving many extracellular signals and transducing them to heterotrimeric G proteins, which further transduce these signals to intracellular downstream effectors. GCR1 is the only reliable plant candidate as a member of the GPCRs superfamily. In the legume/rhizobia symbiotic interaction, G proteins are involved in signalling pathways controlling different steps of the nodulation program. In order to investigate the putative hierarchic role played by GCR1 in these symbiotic pathways we identified and characterized the Lotus japonicus gene encoding the seven transmembrane GCR1 protein. The detailed molecular and topological analyses of LjGCR1 expression patterns that are presented suggest a possible involvement in the early steps of nodule organogenesis. Furthermore, phenotypic analyses of independent transgenic RNAi lines, showing a significant LjGCR1 expression down regulation, suggest an epistatic action in the control of molecular markers of nodulation pathways, although no macroscopic symbiotic phenotypes could be revealed.

Direct Modulation of Heterotrimeric G Protein-coupled Signaling by a Receptor Kinase Complex

Plants and some protists have heterotrimeric G protein complexes that activate spontaneously without canonical G protein-coupled receptors (GPCRs). In Arabidopsis, the sole 7-transmembrane regulator of G protein signaling 1 (AtRGS1) modulates the G protein complex by keeping it in the resting state (GDP-bound). However, it remains unknown how a myriad of biological responses is achieved with a single G protein modulator. We propose that in complete contrast to G protein activation in animals, plant leucine-rich repeat receptor-like kinases (LRR RLKs), not GPCRs, provide this discrimination through phosphorylation of AtRGS1 in a ligand-dependent manner. G protein signaling is directly activated by the pathogen-associated molecular pattern flagellin peptide 22 through its LRR RLK, FLS2, and co-receptor BAK1.

Plant G-Proteins Come of Age: Breaking the Bond with Animal Models

G-proteins are universal signal transducers mediating many cellular responses. Plant G-protein signaling has been modeled on the well-established animal paradigm but accumulated experimental evidence indicates that G-protein-dependent signaling in plants has taken a very different evolutionary path. Here we review the differences between plant and animal G-proteins reported over past two decades. Most importantly, while in animal systems the G-protein signaling cycle is activated by seven transmembrane-spanning G-protein coupled receptors, the existence of these type of receptors in plants is highly controversial. Instead plant G-proteins have been proven to be functionally associated with atypical receptors such as the Arabidopsis RGS1 and a number of receptor-like kinases. We propose that, instead of the GTP/GDP cycle used in animals, plant G-proteins are activated/de-activated by phosphorylation/de-phosphorylation. We discuss the need of a fresh new look at these signaling molecules and provide a hypothetical model that departs from the accepted animal paradigm.

Lipid signalling in plant responses to abiotic stress

Lipids are one of the major components of biological membranes including the plasma membrane, which is the interface between the cell and the environment. It has become clear that membrane lipids also serve as substrates for the generation of numerous signalling lipids such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, N-acylethanolamines, free fatty acids and others. The enzymatic production and metabolism of these signalling molecules are tightly regulated and can rapidly be activated upon abiotic stress signals. Abiotic stress like water deficit and temperature stress triggers lipid-dependent signalling cascades, which control the expression of gene clusters and activate plant adaptation processes. Signalling lipids are able to recruit protein targets transiently to the membrane and thus affect conformation and activity of intracellular proteins and metabolites. In plants, knowledge is still scarce of lipid signalling targets and their physiological consequences. This review focuses on the generation of signalling lipids and their involvement in response to abiotic stress. We describe lipid-binding proteins in the context of changing environmental conditions and compare different approaches to determine lipid-protein interactions, crucial for deciphering the signalling cascades.

Early eukaryotic origins for cilia-associated bioactive peptide-amidating activity

Ciliary axonemes and basal bodies were present in the last eukaryotic common ancestor and play crucial roles in sensing and responding to environmental cues. Peptidergic signaling, generally considered a metazoan innovation, is essential for organismal development and homeostasis. Peptidylglycine α-amidating monooxygenase (PAM) is crucial for the last step of bioactive peptide biosynthesis. However, identification of a complete PAM-like gene in green algal genomes suggests ancient evolutionary roots for bioactive peptide signaling. We demonstrate that the Chlamydomonas reinhardtii PAM gene encodes an active peptide-amidating enzyme (CrPAM) that shares key structural and functional features with the mammalian enzyme, indicating that components of the peptide biosynthetic pathway predate multicellularity. In addition to its secretory pathway localization, CrPAM localizes to cilia and tightly associates with the axonemal superstructure, revealing a new axonemal enzyme activity. This localization pattern is conserved in mammals, with PAM present in both motile and immotile sensory cilia. The conserved ciliary localization of PAM adds to the known signaling capabilities of the eukaryotic cilium and provides a potential mechanistic link between peptidergic signaling and endocrine abnormalities commonly observed in ciliopathies.

G-protein α-subunit (GPA1) regulates stress, nitrate and phosphate response, flavonoid biosynthesis, fruit/seed development and substantially shares GCR1 regulation in A. thaliana

Heterotrimeric G-proteins are implicated in several plant processes, but the mechanisms of signal-response coupling and the roles of G-protein coupled receptors in general and GCR1 in particular, remain poorly understood. We isolated a knock-out mutant of the Arabidopsis G-protein α subunit (gpa1-5) and analysed its transcriptome to understand the genomewide role of GPA1 and compared it with that of our similar analysis of a GCR1 mutant (Chakraborty et al. 2015, PLoS ONE 10(2):e0117819). We found 394 GPA1-regulated genes spanning 79 biological processes, including biotic and abiotic stresses, development, flavonoid biosynthesis, transcription factors, transporters and nitrate/phosphate responses. Many of them are either unknown or unclaimed explicitly in other published gpa1 mutant transcriptome analyses. A comparison of all known GPA1-regulated genes (including the above 394) with 350 GCR1-regulated genes revealed 114 common genes. This can be best explained by GCR1-GPA1 coupling, or by convergence of their independent signaling pathways. Though the common genes in our GPA1 and GCR1 mutant datasets constitute only 26% of the GPA1-regulated and 30% of the GCR1-responsive genes, they belong to nearly half of all the processes affected in both the mutants. Thus, GCR1 and GPA1 regulate not only some common genes, but also different genes belonging to the same processes to achieve similar outcomes. Overall, we validate some known and report many hitherto unknown roles of GPA1 in plants, including agronomically important ones such as biotic stress and nutrient response, and also provide compelling genetic evidence to revisit the role of GCR1 in G-protein signalling.

G-protein Signaling Components GCR1 and GPA1 Mediate Responses to Multiple Abiotic Stresses in Arabidopsis

G-protein signaling components have been implicated in some individual stress responses in Arabidopsis, but have not been comprehensively evaluated at the genetic and biochemical level. Stress emerged as the largest functional category in our whole transcriptome analyses of knock-out mutants of GCR1 and/or GPA1 in Arabidopsis (Chakraborty et al., 2015a,b). This led us to ask whether G-protein signaling components offer converging points in the plant's response to multiple abiotic stresses. In order to test this hypothesis, we carried out detailed analysis of the abiotic stress category in the present study, which revealed 144 differentially expressed genes (DEGs), spanning a wide range of abiotic stresses, including heat, cold, salt, light stress etc. Only 10 of these DEGs are shared by all the three mutants, while the single mutants (GCR1/GPA1) shared more DEGs between themselves than with the double mutant (GCR1-GPA1). RT-qPCR validation of 28 of these genes spanning different stresses revealed identical regulation of the DEGs shared between the mutants. We also validated the effects of cold, heat and salt stresses in all the 3 mutants and WT on % germination, root and shoot length, relative water content, proline content, lipid peroxidation and activities of catalase, ascorbate peroxidase and superoxide dismutase. All the 3 mutants showed evidence of stress tolerance, especially to cold, followed by heat and salt, in terms of all the above parameters. This clearly shows the role of GCR1 and GPA1 in mediating the plant's response to multiple abiotic stresses for the first time, especially cold, heat and salt stresses. This also implies a role for classical G-protein signaling pathways in stress sensitivity in the normal plants of Arabidopsis. This is also the first genetic and biochemical evidence of abiotic stress tolerance rendered by knock-out mutation of GCR1 and/or GPA1. This suggests that G-protein signaling pathway could offer novel common targets for the development of tolerance/resistance to multiple abiotic stresses.

TOR signalling in plants

Although the eukaryotic TOR (target of rapamycin) kinase signalling pathway has emerged as a key player for integrating nutrient-, energy- and stress-related cues with growth and metabolic outputs, relatively little is known of how this ancient regulatory mechanism has been adapted in higher plants. Drawing comparisons with the substantial knowledge base around TOR kinase signalling in fungal and animal systems, functional aspects of this pathway in plants are reviewed. Both conserved and divergent elements are discussed in relation to unique aspects associated with an autotrophic mode of nutrition and adaptive strategies for multicellular development exhibited by plants.

Signals fly when kinases meet Rho-of-plants (ROP) small G-proteins

Rho-type small GTP-binding plant proteins function as two-state molecular switches in cellular signalling. There is accumulating evidence that Rho-of-plants (ROP) signalling is positively controlled by plant receptor kinases, through the ROP guanine nucleotide exchange factor proteins. These signalling modules regulate cell polarity, cell shape, hormone responses, and pathogen defence, among other things. Other ROP-regulatory proteins might also be subjected to protein phosphorylation by cellular kinases (e.g., mitogen-activated protein kinases or calcium-dependent protein kinases), in order to integrate various cellular signalling pathways with ROP GTPase-dependent processes. In contrast to the role of kinases in upstream ROP regulation, much less is known about the potential link between ROP GTPases and downstream kinase signalling. In other eukaryotes, Rho-type G-protein-activated kinases are widespread and have a key role in many cellular processes. Recent data indicate the existence of structurally different ROP-activated kinases in plants, but their ROP-dependent biological functions still need to be validated. In addition to these direct interactions, ROPs may also indirectly control the activity of mitogen-activated protein kinases or calcium-dependent protein kinases. These kinases may therefore function as upstream as well as downstream kinases in ROP-mediated signalling pathways, such as the phosphatidylinositol monophosphate kinases involved in cell polarity establishment.

Transcriptome analysis of Arabidopsis GCR1 mutant reveals its roles in stress, hormones, secondary metabolism and phosphate starvation

The controversy over the existence or the need for G-protein coupled receptors (GPCRs) in plant G-protein signalling has overshadowed a more fundamental quest for the role of AtGCR1, the most studied and often considered the best candidate for GPCR in plants. Our whole transcriptome microarray analysis of the GCR1-knock-out mutant (gcr1-5) in Arabidopsis thaliana revealed 350 differentially expressed genes spanning all chromosomes. Many of them were hitherto unknown in the context of GCR1 or G-protein signalling, such as in phosphate starvation, storage compound and fatty acid biosynthesis, cell fate, etc. We also found some GCR1-responsive genes/processes that are reported to be regulated by heterotrimeric G-proteins, such as biotic and abiotic stress, hormone response and secondary metabolism. Thus, GCR1 could have G-protein-mediated as well as independent roles and regardless of whether it works as a GPCR, further analysis of the organism-wide role of GCR1 has a significance of its own.

The global phosphoproteome of Chlamydomonas reinhardtii reveals complex organellar phosphorylation in the flagella and thylakoid membrane

Chlamydomonas reinhardtii is the most intensively-studied and well-developed model for investigation of a wide-range of microalgal processes ranging from basic development through understanding triacylglycerol production. Although proteomic technologies permit interrogation of these processes at the protein level and efforts to date indicate phosphorylation-based regulation of proteins in C. reinhardtii is essential for its underlying biology, characterization of the C. reinhardtii phosphoproteome has been limited. Herein, we report the richest exploration of the C. reinhardtii proteome to date. Complementary enrichment strategies were used to detect 4588 phosphoproteins distributed among every cellular component in C. reinhardtii. Additionally, we report 18,160 unique phosphopeptides at <1% false discovery rate, which comprise 15,862 unique phosphosites - 98% of which are novel. Given that an estimated 30% of proteins in a eukaryotic cell are subject to phosphorylation, we report the majority of the phosphoproteome (23%) of C. reinhardtii. Proteins in key biological pathways were phosphorylated, including photosynthesis, pigment production, carbon assimilation, glycolysis, and protein and carbohydrate metabolism, and it is noteworthy that hyperphosphorylation was observed in flagellar proteins. This rich data set is available via ProteomeXchange (ID: PXD000783) and will significantly enhance understanding of a range of regulatory mechanisms controlling a variety of cellular process and will serve as a critical resource for the microalgal community.

Topology assessment, G protein-coupled receptor (GPCR) prediction, and in vivo interaction assays to identify plant candidate GPCRs

Genomic sequencing has provided a vast resource for identifying interesting genes, but often an exact "gene-of-interest" is unknown and is only described as putatively present in a genome by an observed phenotype, or by the known presence of a conserved signaling cascade, such as that facilitated by the heterotrimeric G-protein. The low sequence similarity of G protein-coupled receptors (GPCRs) and the absence of a known ligand with an associated high-throughput screening system in plants hampers their identification by simple BLAST queries or brute force experimental assays. Combinatorial bioinformatic analysis is useful in that it can reduce a large pool of possible candidates to a number manageable by medium or even low-throughput methods. Here we describe a method for the bioinformatic identification of candidate GPCRs from whole proteomes and their subsequent in vivo analysis for G-protein coupling using a membrane based yeast two-hybrid variant (Gookin et al., Genome Biol 9:R120, 2008). Rather than present the bioinformatic process in a format requiring scripts or computer programming knowledge, we describe procedures here in a simple, biologist-friendly outline that only utilizes the basic syntax of regular expressions.

Heterotrimeric G protein-coupled signaling in plants

Investigators studying G protein-coupled signaling--often called the best-understood pathway in the world owing to intense research in medical fields--have adopted plants as a new model to explore the plasticity and evolution of G signaling. Much research on plant G signaling has not disappointed. Although plant cells have most of the core elements found in animal G signaling, differences in network architecture and intrinsic properties of plant G protein elements make G signaling in plant cells distinct from the animal paradigm. In contrast to animal G proteins, plant G proteins are self-activating, and therefore regulation of G activation in plants occurs at the deactivation step. The self-activating property also means that plant G proteins do not need and therefore do not have typical animal G protein-coupled receptors. Targets of activated plant G proteins, also known as effectors, are unlike effectors in animal cells. The simpler repertoire of G signal elements in Arabidopsis makes G signaling easier to manipulate in a multicellular context.

Do plants contain g protein-coupled receptors?

Whether G protein-coupled receptors (GPCRs) exist in plants is a fundamental biological question. Interest in deorphanizing new GPCRs arises because of their importance in signaling. Within plants, this is controversial, as genome analysis has identified 56 putative GPCRs, including G protein-coupled receptor1 (GCR1), which is reportedly a remote homolog to class A, B, and E GPCRs. Of these, GCR2 is not a GPCR; more recently, it has been proposed that none are, not even GCR1. We have addressed this disparity between genome analysis and biological evidence through a structural bioinformatics study, involving fold recognition methods, from which only GCR1 emerges as a strong candidate. To further probe GCR1, we have developed a novel helix-alignment method, which has been benchmarked against the class A-class B-class F GPCR alignments. In addition, we have presented a mutually consistent set of alignments of GCR1 homologs to class A, class B, and class F GPCRs and shown that GCR1 is closer to class A and/or class B GPCRs than class A, class B, or class F GPCRs are to each other. To further probe GCR1, we have aligned transmembrane helix 3 of GCR1 to each of the six GPCR classes. Variability comparisons provide additional evidence that GCR1 homologs have the GPCR fold. From the alignments and a GCR1 comparative model, we have identified motifs that are common to GCR1, class A, B, and E GPCRs. We discuss the possibilities that emerge from this controversial evidence that GCR1 has a GPCR fold.

The role of ECL2 in CGRP receptor activation: a combined modelling and experimental approach

The calcitonin gene-related peptide (CGRP) receptor is a complex of a calcitonin receptor-like receptor (CLR), which is a family B G-protein-coupled receptor (GPCR) and receptor activity modifying protein 1. The role of the second extracellular loop (ECL2) of CLR in binding CGRP and coupling to Gs was investigated using a combination of mutagenesis and modelling. An alanine scan of residues 271–294 of CLR showed that the ability of CGRP to produce cAMP was impaired by point mutations at 13 residues; most of these also impaired the response to adrenomedullin (AM). These data were used to select probable ECL2-modelled conformations that are involved in agonist binding, allowing the identification of the likely contacts between the peptide and receptor. The implications of the most likely structures for receptor activation are discussed.

Can prolonged exposure to low VPD disturb the ABA signalling in stomatal guard cells?

The response of stomata to many environmental factors is well documented. Multiple signalling pathways for abscisic acid (ABA)-induced stomatal closure have been proposed over the last decades. However, it seems that exposure of a leaf for a long time (several days) to some environmental conditions generates a sort of memory in the guard cells that results in the loss of suitable responses of the stomata to closing stimuli, such as desiccation and ABA. In this review paper we discuss changes in the normal pattern of signal transduction that could account for disruption of guard cell signalling after long-term exposure to some environmental conditions, with special emphasis on long-term low vapour pressure deficit (VPD).

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.

Abscisic acid binds to recombinant Arabidopsis thaliana G-protein coupled receptor-type G-protein 1 in Sacaromycese cerevisiae and in vitro

The G-protein coupled receptor-type G-proteins (GTG) 1 and 2 from Arabidopsis thaliana have been proposed to function in the modulation of abscisic acid (ABA) mediated responses to stress and development. In particular it has been suggested that they function as ABA receptors based on in planta and in vitro analyses. However a recent independent report was inconsistent with this, suggesting that there is no link between the GTGs and ABA in planta. Here we provide an independent assessment of the ability of ABA to bind to recombinant GTG1 in vitro and in vivo in Sacaromycese cerevisiae. Radio-labelled binding assays on enriched lipid-reconstituted recombinant GTG1, demonstrated specific concentration dependent binding of [(3)H]-ABA with a dissociation constant (KD) of 80 nM, corroborating previous reports. Assessment of the binding of [(3)H]-ABA to intact GTG1 expressing yeast, showed GTG1-dependent binding in vivo, yielding a physiologically relevant KD of 0.6 μM. Together these results provide independent evidence of a binding-interaction between ABA and GTG1 in vitro and in vivo, in support of the previously proposed possibility of a biologically relevant interaction between GTG1 and ABA.

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.