Arabidopsis N-MYC DOWNREGULATED-LIKE1, a positive regulator of auxin transport in a G protein-mediated pathway

The Arabidopsis adaptor protein AP-3μ interacts with the G-protein β subunit AGB1 and is involved in abscisic acid regulation of germination and post-germination development

Heterotrimeric G-proteins (G-proteins) have been implicated in ubiquitous signalling mechanisms in eukaryotes. In plants, G-proteins modulate hormonal and stress responses and regulate diverse developmental processes. However, the molecular mechanisms of their functions are largely unknown. A yeast two-hybrid screen was performed to identify interacting partners of the Arabidopsis G-protein β subunit AGB1. One of the identified AGB1-interacting proteins is the Arabidopsis adaptor protein AP-3µ. The interaction between AGB1 and AP-3µ was confirmed by an in vitro pull-down assay and bimolecular fluorescence complementation assay. Two ap-3µ T-DNA insertional mutants were found to be hyposensitive to abscisic acid (ABA) during germination and post-germination growth, whereas agb1 mutants were hypersensitive to ABA. During seed germination, agb1/ap-3µ double mutants were more sensitive to ABA than the wild type but less sensitive than agb1 mutants. However, in post-germination growth, the double mutants were as sensitive to ABA as agb1 mutants. These data suggest that AP-3µ positively regulates the ABA responses independently of AGB1 in seed germination, while AP-3µ does require AGB1 to regulate ABA responses during post-germination growth.

N-MYC down-regulated-like proteins regulate meristem initiation by modulating auxin transport and MAX2 expression

Background

N-MYC DOWN-REGULATED-LIKE (NDL) proteins interact with the Gβ subunit (AGB1) of the heterotrimeric G protein complex and play an important role in AGB1-dependent regulation of lateral root formation by affecting root auxin transport, auxin gradients and the steady-state levels of mRNA encoding the PIN-FORMED 2 and AUXIN 1 auxin transport facilitators. Auxin transport in aerial tissue follows different paths and utilizes different transporters than in roots; therefore, in the present study, we analyzed whether NDL proteins play an important role in AGB1-dependent, auxin-mediated meristem development.

Methodology/Principal Findings

Expression levels of NDL gene family members need to be tightly regulated, and altered expression (both over-expression and down-regulation) confers ectopic growth. Over-expression of NDL1 disrupts vegetative and reproductive organ development. Reduced expression of the NDL gene family members results in asymmetric leaf emergence, twinning of rosette leaves, defects in leaf formation, and abnormal silique distribution. Reduced expression of the NDL genes in the agb1-2 (null allele) mutant rescues some of the abnormal phenotypes, such as silique morphology, silique distribution, and peduncle angle, suggesting that proper levels of NDL proteins are maintained by AGB1. We found that all of these abnormal aerial phenotypes due to altered NDL expression were associated with increases in basipetal auxin transport, altered auxin maxima and altered MAX2 expression within the inflorescence stem.

Conclusion/Significance

NDL proteins, together with AGB1, act as positive regulators of meristem initiation and branching. AGB1 and NDL1 positively regulate basipetal inflorescence auxin transport and modulate MAX2 expression in shoots, which in turn regulates organ and lateral meristem formation by the establishment and maintenance of auxin gradients.

Maize LAZY1 mediates shoot gravitropism and inflorescence development through regulating auxin transport, auxin signaling, and light response

Auxin is a plant hormone that plays key roles in both shoot gravitropism and inflorescence development. However, these two processes appear to be parallel and to be regulated by distinct players. Here, we report that the maize (Zea mays) prostrate stem1 mutant, which is allelic to the classic mutant lazy plant1 (la1), displays prostrate growth with reduced shoot gravitropism and defective inflorescence development. Map-based cloning identified maize ZmLA1 as the functional ortholog of LAZY1 in rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana). It has a unique role in inflorescence development and displays enriched expression in reproductive organs such as tassels and ears. Transcription of ZmLA1 responds to auxin and is repressed by light. Furthermore, ZmLA1 physically interacts with a putative auxin transport regulator in the plasma membrane and a putative auxin signaling protein in the nucleus. RNA-SEQ data showed that dozens of auxin transport, auxin response, and light signaling genes were differentially expressed in la1 mutant stems. Therefore, ZmLA1 might mediate the cross talk between shoot gravitropism and inflorescence development by regulating auxin transport, auxin signaling, and probably light response in maize.

Transcriptomic analysis highlights epigenetic and transcriptional regulation during zygotic embryo development of Pinus pinaster

BACKGROUND

It is during embryogenesis that the plant body plan is established and the meristems responsible for all post-embryonic growth are specified. The molecular mechanisms governing conifer embryogenesis are still largely unknown. Their elucidation may contribute valuable information to clarify if the distinct features of embryo development in angiosperms and gymnosperms result from differential gene regulation. To address this issue, we have performed the first transcriptomic analysis of zygotic embryo development in a conifer species (Pinus pinaster) focusing our study in particular on regulatory genes playing important roles during plant embryo development, namely epigenetic regulators and transcription factors.

RESULTS

Microarray analysis of P. pinaster zygotic embryogenesis was performed at five periods of embryo development from early developing to mature embryos. Our results show that most changes in transcript levels occurred in the first and the last embryo stage-to-stage transitions, namely early to pre-cotyledonary embryo and cotyledonary to mature embryo. An analysis of functional categories for genes that were differentially expressed through embryogenesis highlighted several epigenetic regulation mechanisms. While putative orthologs of transcripts associated with mechanisms that target transposable elements and repetitive sequences were strongly expressed in early embryogenesis, PRC2-mediated repression of genes seemed more relevant during late embryogenesis. On the other hand, functions related to sRNA pathways appeared differentially regulated across all stages of embryo development with a prevalence of miRNA functions in mid to late embryogenesis. Identification of putative transcription factor genes differentially regulated between consecutive embryo stages was strongly suggestive of the relevance of auxin responses and regulation of auxin carriers during early embryogenesis. Such responses could be involved in establishing embryo patterning. Later in development, transcripts with homology to genes acting on modulation of auxin flow and determination of adaxial-abaxial polarity were up-regulated, as were putative orthologs of genes required for meristem formation and function as well as establishment of organ boundaries. Comparative analysis with A. thaliana embryogenesis also highlighted genes involved in auxin-mediated responses, as well as epigenetic regulation, indicating highly correlated transcript profiles between the two species.

CONCLUSIONS

This is the first report of a time-course transcriptomic analysis of zygotic embryogenesis in a conifer. Taken together our results show that epigenetic regulation and transcriptional control related to auxin transport and response are critical during early to mid stages of pine embryogenesis and that important events during embryogenesis seem to be coordinated by putative orthologs of major developmental regulators in angiosperms.

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.

RNA-seq of Arabidopsis pollen uncovers novel transcription and alternative splicing

Pollen grains of Arabidopsis (Arabidopsis thaliana) contain two haploid sperm cells enclosed in a haploid vegetative cell. Upon germination, the vegetative cell extrudes a pollen tube that carries the sperm to an ovule for fertilization. Knowing the identity, relative abundance, and splicing patterns of pollen transcripts will improve our understanding of pollen and allow investigation of tissue-specific splicing in plants. Most Arabidopsis pollen transcriptome studies have used the ATH1 microarray, which does not assay splice variants and lacks specific probe sets for many genes. To investigate the pollen transcriptome, we performed high-throughput sequencing (RNA-Seq) of Arabidopsis pollen and seedlings for comparison. Gene expression was more diverse in seedling, and genes involved in cell wall biogenesis were highly expressed in pollen. RNA-Seq detected at least 4,172 protein-coding genes expressed in pollen, including 289 assayed only by nonspecific probe sets. Additional exons and previously unannotated 5' and 3' untranslated regions for pollen-expressed genes were revealed. We detected regions in the genome not previously annotated as expressed; 14 were tested and 12 were confirmed by polymerase chain reaction. Gapped read alignments revealed 1,908 high-confidence new splicing events supported by 10 or more spliced read alignments. Alternative splicing patterns in pollen and seedling were highly correlated. For most alternatively spliced genes, the ratio of variants in pollen and seedling was similar, except for some encoding proteins involved in RNA splicing. This study highlights the robustness of splicing patterns in plants and the importance of ongoing annotation and visualization of RNA-Seq data using interactive tools such as Integrated Genome Browser.

Regulation of cell division and expansion by sugar and auxin signaling

Plant growth and development are modulated by concerted actions of a variety of signaling molecules. In recent years, evidence has emerged on the roles of sugar and auxin signals network in diverse aspects of plant growth and development. Here, based on recent progress of genetic analyses and gene expression profiling studies, we summarize the functional similarities, diversities, and their interactions of sugar and auxin signals in regulating two major processes of plant development: cell division and cell expansion. We focus on roles of sugar and auxin signaling in both vegetative and reproductive tissues including developing seed.

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.

Functional analysis of heterotrimeric G proteins in chloroplast development in Arabidopsis

Functional analysis of G-proteins has been extensively carried out using their over-expressing lines and knockout mutants in plants. Since α subunit exists in an active or inactive form, overexpressing α subunit does not mean that G-protein signaling pathways are activated in the transgenic lines. Ectopic expression of the constitutively active form of the α subunit will magnify a role of G-protein signaling pathways in plant growth and development, and ultimately yield phenotypes. Here, we describe the method to study function of G-proteins in chloroplast development using the constitutively active form of the α subunit in Arabidopsis.

Visualizing and quantifying Fusarium oxysporum in the plant host

Host-specific forms of Fusarium oxysporum infect the roots of numerous plant species. I present a novel application of familiar methodology to visualize and quantify F. oxysporum in roots. Infection in the roots of Arabidopsis thaliana, tomato, and cotton was detected with colorimetric reagents that are substrates for Fusarium spp.-derived arabinofuranosidase and N-acetyl-glucosaminidase activities and without the need for genetic modification of either plant host or fungal pathogen. Similar patterns of blue precipitation were produced by treatment with 5-bromo-4-chloro-3-indoxyl-α-l-arabinofuranoside and 5-bromo-4-chloro-3-indoxyl-2-acetamido-2-deoxy-β-d-glucopyranoside, and these patterns were consistent with prior histological descriptions of F. oxysporum in roots. Infection was quantified in roots of wild-type and mutant Arabidopsis using 4-nitrophenyl-α-l-arabinofuranoside. In keeping with an expectation that disease severity above ground is correlated with F. oxysporum infection below ground, elevated levels of arabinofuranosidase activity were measured in the roots of susceptible agb1 and rfo1 while a reduced level was detected in the resistant eir1. In contrast, disease severity and F. oxysporum infection were uncoupled in tir3. The distribution of staining patterns in roots suggests that AGB1 and RFO1 restrict colonization of the vascular cylinder by F. oxysporum whereas EIR1 promotes colonization of root apices.

DICER-LIKE3 activity in Physcomitrella patens DICER-LIKE4 mutants causes severe developmental dysfunction and sterility

Trans-acting small interfering RNAs (ta-siRNAs) are plant-specific siRNAs released from TAS precursor transcripts after microRNA-dependent cleavage, conversion into double-stranded RNA, and Dicer-dependent phased processing. Like microRNAs (miRNAs), ta-siRNAs direct site-specific cleavage of target RNAs at sites of extensive complementarity. Here, we show that the DICER-LIKE 4 protein of Physcomitrella patens (PpDCL4) is essential for the biogenesis of 21 nucleotide (nt) ta-siRNAs. In ΔPpDCL4 mutants, off-sized 23 and 24-nt ta-siRNAs accumulated as the result of PpDCL3 activity. ΔPpDCL4 mutants display severe abnormalities throughout Physcomitrella development, including sterility, that were fully reversed in ΔPpDCL3/ΔPpDCL4 double-mutant plants. Therefore, PpDCL3 activity, not loss of PpDCL4 function per se, is the cause of the ΔPpDCL4 phenotypes. Additionally, we describe several new Physcomitrella trans-acting siRNA loci, three of which belong to a new family, TAS6. TAS6 loci are typified by sliced miR156 and miR529 target sites and are in close proximity to PpTAS3 loci.

Arabidopsis heterotrimeric G protein β subunit interacts with a plasma membrane 2C-type protein phosphatase, PP2C52

Heterotrimeric G proteins (Gα, Gβ, Gγ) play important roles in signal transduction among various eukaryotic species. G proteins transmit signals by regulating the activities of effector proteins, but only a few Gβ-interacting effectors have been identified in plants. Here we show by a yeast two-hybrid screen that a putative myristoylated 2C-type protein phosphatase, PP2C52, is an Arabidopsis Gβ (AGB1)-interacting partner. The interaction between AGB1 and PP2C52 was confirmed by an in vitro pull-down assay and a bimolecular fluorescence complementation assay. PP2C52 transcripts were detected in many tissues. PP2C52 was localized to the plasma membrane and a mutation in the putative myristoylation site of PP2C52 disrupted its plasma membrane localization. Our results suggest that PP2C52 interacts with AGB1 on the plasma membrane and transmits signals via dephosphorylation of other proteins.

Wide range of interacting partners of pea Gβ subunit of G-proteins suggests its multiple functions in cell signalling

Climate change is a major concern especially in view of the increasing global population and food security. Plant scientists need to look for genetic tools whose appropriate usage can contribute to sustainable food availability. G-proteins have been identified as some of the potential genetic tools that could be useful for protecting plants from various stresses. Heterotrimeric G-proteins consisting of three subunits Gα, Gβ and Gγ are important components of a number of signalling pathways. Their structure and functions are already well studied in animals but their potential in plants is now gaining attention for their role in stress tolerance. Earlier we have reported that over expressing pea Gβ conferred heat tolerance in tobacco plants. Here we report the interacting partners (proteins) of Gβ subunit of Pisum sativum and their putative role in stress and development. Out of 90 transformants isolated from the yeast-two-hybrid (Y2H) screening, seven were chosen for further investigation due to their recurrence in multiple experiments. These interacting partners were confirmed using β-galactosidase colony filter lift and ONPG (O-nitrophenyl-β-D-galactopyranoside) assays. These partners include thioredoxin H, histidine-containing phosphotransfer protein 5-like, pathogenesis-related protein, glucan endo-beta-1, 3-glucosidase (acidic isoform), glycine rich RNA binding protein, cold and drought-regulated protein (corA gene) and soluble inorganic pyrophosphatase 1. This study suggests the role of pea Gβ subunit in stress signal transduction and development pathways owing to its capability to interact with a wide range of proteins of multiple functions.

Dissecting Arabidopsis Gβ signal transduction on the protein surface

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) Gβ-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGB1-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of Gβ-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits.

Structural and functional analysis of VQ motif-containing proteins in Arabidopsis as interacting proteins of WRKY transcription factors

WRKY transcription factors are encoded by a large gene superfamily with a broad range of roles in plants. Recently, several groups have reported that proteins containing a short VQ (FxxxVQxLTG) motif interact with WRKY proteins. We have recently discovered that two VQ proteins from Arabidopsis (Arabidopsis thaliana), SIGMA FACTOR-INTERACTING PROTEIN1 and SIGMA FACTOR-INTERACTING PROTEIN2, act as coactivators of WRKY33 in plant defense by specifically recognizing the C-terminal WRKY domain and stimulating the DNA-binding activity of WRKY33. In this study, we have analyzed the entire family of 34 structurally divergent VQ proteins from Arabidopsis. Yeast (Saccharomyces cerevisiae) two-hybrid assays showed that Arabidopsis VQ proteins interacted specifically with the C-terminal WRKY domains of group I and the sole WRKY domains of group IIc WRKY proteins. Using site-directed mutagenesis, we identified structural features of these two closely related groups of WRKY domains that are critical for interaction with VQ proteins. Quantitative reverse transcription polymerase chain reaction revealed that expression of a majority of Arabidopsis VQ genes was responsive to pathogen infection and salicylic acid treatment. Functional analysis using both knockout mutants and overexpression lines revealed strong phenotypes in growth, development, and susceptibility to pathogen infection. Altered phenotypes were substantially enhanced through cooverexpression of genes encoding interacting VQ and WRKY proteins. These findings indicate that VQ proteins play an important role in plant growth, development, and response to environmental conditions, most likely by acting as cofactors of group I and IIc WRKY transcription factors.

Microarray profiling of HepG2 cells ectopically expressing NDRG2

Previous studies have demonstrated that N-Myc downstream-regulated gene 2 (NDRG2) is a tumor suppressor that is downregulated in many human cancers and when overexpressed, can inhibit tumor growth and metastasis. However, its molecular function, its modulatory targets, and signaling pathways associated with it remain unclear. Here, in an effort to identify the genes modulated by NDRG2 expression, a microarray study was conducted to detect the expression profile of HepG2 cells overexpressing NDRG2 or LacZ. Gene Ontology (GO) biological process analysis revealed that genes related to G protein signaling pathway were upregulated. Five of them were selected and verified by real-time PCR. Gene sets related to M phase of cell cycle were downregulated. This was in agreement with cell cycle analysis. Signaling pathway analysis demonstrated apparent augmented hematopoietic cell lineage pathway and cell adhesion, but reduced glycosylphosphatidylinositol (GPI)-anchor biosynthesis, protein degradation and SNARE interactions. Furthermore, through motif analysis and experimental validation, we found that the p38 phosphorylation can be increased by NDRG2. Our research provides the molecular basis for understanding the role of NDRG2 in tumor cells and raises interesting questions about its mechanisms and potential use in cancer therapy.

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

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

AtIRE1A/AtIRE1B and AGB1 independently control two essential unfolded protein response pathways in Arabidopsis

The endoplasmic reticulum (ER) has the ability to maintain the balance between demand for and synthesis of secretory proteins. To ensure protein-folding homeostasis in the ER, cells invoke signaling pathways known as the unfolded protein response (UPR). To initiate UPR, yeasts largely rely on a conserved sensor, IRE1. In metazoans, there are at least three independent UPR signalling pathways. Some UPR transducers have been identified in plants, but no genetic interaction among them has yet been examined. The Arabidopsis genome encodes two IRE1 sequence homologs, AtIRE1A and AtIRE1B. Here we provide evidence that AtIRE1A and AtIRE1B have overlapping functions that are essential for the plant UPR. A double mutant of AtIRE1A and AtIRE1B, atire1a atire1b, showed reduced ER stress tolerance and a compromised UPR activation phenotype. We have also established that Arabidopsis AGB1, a subunit of the ubiquitous heterotrimeric GTP-binding protein family, and AtIRE1A/AtIRE1B independently control two essential plant UPR pathways. By demonstrating that atire1a atire1b has a short root phenotype that is enhanced by an agb1 loss-of-function mutation, we have identified a role for UPR transducers in organ growth regulation.

Arabidopsis G-protein interactome reveals connections to cell wall carbohydrates and morphogenesis

Yeast two-hybrid technology is used to build a high-quality protein interaction network centered on Arabidopsis G-protein coupled signaling. The interactions uncovered are without precedent in animals and fungi and help identify new cellular roles for G-protein signaling in plants.

The heterotrimeric G-protein complex is minimally composed of Gα, Gβ, and Gγ subunits. In the classic scenario, the G-protein complex is the nexus in signaling from the plasma membrane, where the heterotrimeric G-protein associates with heptahelical G-protein-coupled receptors (GPCRs), to cytoplasmic target proteins called effectors. Although a number of effectors are known in metazoans and fungi, none of these are predicted to exist in their canonical forms in plants. To identify ab initio plant G-protein effectors and scaffold proteins, we screened a set of proteins from the G-protein complex using two-hybrid complementation in yeast. After deep and exhaustive interrogation, we detected 544 interactions between 434 proteins, of which 68 highly interconnected proteins form the core G-protein interactome. Within this core, over half of the interactions comprising two-thirds of the nodes were retested and validated as genuine in planta. Co-expression analysis in combination with phenotyping of loss-of-function mutations in a set of core interactome genes revealed a novel role for G-proteins in regulating cell wall modification.

Acireductone dioxygenase 1 (ARD1) is an effector of the heterotrimeric G protein beta subunit in Arabidopsis

Heterotrimeric G protein complexes are conserved from plants to mammals, but the complexity of each system varies. Arabidopsis thaliana contains one Gα, one Gβ (AGB1), and at least three Gγ subunits, allowing it to form three versions of the heterotrimer. This plant model is ideal for genetic studies because mammalian systems contain hundreds of unique heterotrimers. The activation of these complexes promotes interactions between both the Gα subunit and the Gβγ dimer with enzymes and scaffolds to propagate signaling to the cytoplasm. However, although effectors of Gα and Gβ are known in mammals, no Gβ effectors were previously known in plants. Toward identifying AGB1 effectors, we genetically screened for dominant mutations that suppress Gβ-null mutant (agb1-2) phenotypes. We found that overexpression of acireductone dioxygenase 1 (ARD1) suppresses the 2-day-old etiolated phenotype of agb1-2. ARD1 is homologous to prokaryotic and eukaryotic ARD proteins; one function of ARDs is to operate in the methionine salvage pathway. We show here that ARD1 is an active metalloenzyme, and AGB1 and ARD1 both control embryonic hypocotyl length by modulating cell division; they also may contribute to the production of ethylene, a product of the methionine salvage pathway. ARD1 physically interacts with AGB1, and ARD enzymatic activity is stimulated by AGB1 in vitro. The binding interface on AGB1 was deduced using a comparative evolutionary approach and tested using recombinant AGB1 mutants. A possible mechanism for AGB1 activation of ARD1 activity was tested using directed mutations in a loop near the substrate-binding site.

Stress induced β subunit of heterotrimeric G-proteins from Pisum sativum interacts with mitogen activated protein kinase

We here report in Pisum sativum system a novel protein-protein interaction of β-subunit of heterotrimeric G-proteins (PsGβ) with a Mitogen activated protein kinase (PsMPK3) during cDNA library screening by yeast-two-hybrid assay. The transcript of these two genes also showed co-regulation under abscisic acid (ABA) and methyl jasmonate (MeJA) treatments. The protein-protein interaction was further validated by performing one-to-one interaction and β-galactosidase assay in yeast system. β-subunit of G-proteins from a heterologous system Oryzae sativa also showed interaction with PsMPK3. The interaction between PsGβ and PsMPK3 was further confirmed by in vitro protein-protein interaction. This suggested that MPK3 function as effector molecule for Gβ, which may helps in the regulation of stomatal functioning. These findings also provide an evidence for a possible cross-talk between MPK3 and G-protein-mediated signaling pathways in plants.

Glucose attenuation of auxin-mediated bimodality in lateral root formation is partly coupled by the heterotrimeric G protein complex

BACKGROUND

Auxin and glucose are both essential elements in normal root development. The heterotrimeric G protein complex in Arabidopsis thaliana, defined as containing alpha (AtGPA1), beta (AGB1), and gamma (AGG) subunits and a GTPase accelerating protein called Regulator of G Signaling 1 protein (AtRGS1), are involved in glucose signaling and regulate auxin transport.

METHODOLOGY/PRINCIPAL FINDINGS

A systems approach was used to show that formation of lateral roots, a process requiring coordinated cell division followed by targeted cell expansion, involves a signaling interaction between glucose and auxin. We dissected the relationship between auxin and glucose action using lateral root formation as the biological context. We found that auxin and glucose act synergistically to yield a complex output involving both stimulatory and antagonist glucose effects on auxin responsiveness. Auxin-induced, lateral-root formation becomes bimodal with regard to auxin dose in the presence of glucose. This bimodality is mediated, in part, by the G protein complex defined above.

CONCLUSION/SIGNIFICANCE

Auxin and glucose are essential signals controlling the rate of cell proliferation and expansion in roots. Auxin promotes the formation of lateral roots and is consequently essential for proper root architecture. Glucose affects the activation state of the heterotrimeric G protein complex which regulates auxin distribution in the root. The bimodality of auxin-induced, lateral-root formation becomes prominent in the presence of glucose and in roots lacking the G protein complex. Bimodality is apparent without added glucose in all loss-of-function mutants for these G protein components, suggesting that the heterotrimeric G protein complex attenuates the bimodality and that glucose inhibits this attenuation through the complex. The bimodality can be further resolved into the processes of lateral root primordia formation and lateral root emergence, from which a model integrating these signals is proposed.

Necrotroph attacks on plants: wanton destruction or covert extortion?

Necrotrophic pathogens cause major pre- and post-harvest diseases in numerous agronomic and horticultural crops inflicting significant economic losses. In contrast to biotrophs, obligate plant parasites that infect and feed on living cells, necrotrophs promote the destruction of host cells to feed on their contents. This difference underpins the divergent pathogenesis strategies and plant immune responses to biotrophic and necrotrophic infections. This chapter focuses on Arabidopsis immunity to necrotrophic pathogens. The strategies of infection, virulence and suppression of host defenses recruited by necrotrophs and the variation in host resistance mechanisms are highlighted. The multiplicity of intraspecific virulence factors and species diversity in necrotrophic organisms corresponds to variations in host resistance strategies. Resistance to host-specific necrotophs is monogenic whereas defense against broad host necrotrophs is complex, requiring the involvement of many genes and pathways for full resistance. Mechanisms and components of immunity such as the role of plant hormones, secondary metabolites, and pathogenesis proteins are presented. We will discuss the current state of knowledge of Arabidopsis immune responses to necrotrophic pathogens, the interactions of these responses with other defense pathways, and contemplate on the directions of future research.

NDR proteins: lessons learned from Arabidopsis and animal cells prompt a testable hypothesis

N-myc Down Regulated (NDR) genes were discovered more than fifteen years ago. Indirect evidence support a role in tumor progression and cellular differentiation, but their biochemical function is still unknown. Our detailed analyses on Arabidopsis NDL proteins show their involvement in altering auxin transport, local auxin gradients and expression level of auxin transport proteins. Animal NDL proteins may be involved in membrane recycling of E-cadherin and effector for the small GTPase. In light of these findings, we hypothesize that NDL proteins regulate vesicular trafficking of auxin transport facilitator PIN proteins by biochemically alterating the local lipid environment of PIN proteins.

Auxin control of root development

A plant's roots system determines both the capacity of a sessile organism to acquire nutrients and water, as well as providing a means to monitor the soil for a range of environmental conditions. Since auxins were first described, there has been a tight connection between this class of hormones and root development. Here we review some of the latest genetic, molecular, and cellular experiments that demonstrate the importance of generating and maintaining auxin gradients during root development. Refinements in the ability to monitor and measure auxin levels in root cells coupled with advances in our understanding of the sources of auxin that contribute to these pools represent important contributions to our understanding of how this class of hormones participates in the control of root development. In addition, we review the role of identified molecular components that convert auxin gradients into local differentiation events, which ultimately defines the root architecture.