Heterotrimeric G proteins serve as a converging point in plant defense signaling activated by multiple receptor-like kinases

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.

Dynamic Protein S-Acylation in Plants

Lipid modification is an important post-translational modification. S-acylation is unique among lipid modifications, as it is reversible and has thus attracted much attention. We summarize some proteins that have been shown experimentally to be S-acylated in plants. Two of these S-acylated proteins have been matched to the S-acyl transferase. More importantly, the first protein thioesterase with de-S-acylation activity has been identified in plants. This review shows that S-acylation is important for a variety of different functions in plants and that there are many unexplored aspects of S-acylation in plants.

WD40 Repeat Proteins: Signalling Scaffold with Diverse Functions

The WD40 domain is one of the most abundant and interacting domains in the eukaryotic genome. In proteins the WD domain folds into a β-propeller structure, providing a platform for the interaction and assembly of several proteins into a signalosome. WD40 repeats containing proteins, in lower eukaryotes, are mainly involved in growth, cell cycle, development and virulence, while in higher organisms, they play an important role in diverse cellular functions like signal transduction, cell cycle control, intracellular transport, chromatin remodelling, cytoskeletal organization, apoptosis, development, transcriptional regulation, immune responses. To play the regulatory role in various processes, they act as a scaffold for protein-protein or protein-DNA interaction. So far, no WD40 domain has been identified with intrinsic enzymatic activity. Several WD40 domain-containing proteins have been recently characterized in prokaryotes as well. The review summarizes the vast array of functions performed by different WD40 domain containing proteins, their domain organization and functional conservation during the course of evolution.

NADPH oxidase activity is required for ER stress survival in plants

In all eukaryotes, the unfolded protein response (UPR) relieves endoplasmic reticulum (ER) stress, which is a potentially lethal condition caused by the accumulation of misfolded proteins in the ER. In mammalian and yeast cells, reactive oxygen species (ROS) generated during ER stress attenuate the UPR, negatively impacting cell survival. In plants, the relationship between the UPR and ROS is less clear. Although ROS develop during ER stress, the sources of ROS linked to ER stress responses and the physiological impact of ROS generation on the survival from proteotoxic stress are yet unknown. Here we show that in Arabidopsis thaliana the respiratory burst oxidase homologs, RBOHD and RBOHF, contribute to the production of ROS during ER stress. We also demonstrate that during ER stress RBOHD and RBOHF are necessary to properly mount the adaptive UPR and overcome temporary and chronic ER stress situations. These results ascribe a cytoprotective role to RBOH-generated ROS in the defense from proteotoxic stress in an essential organelle, and support a plant-specific feature of the UPR management among eukaryotes.

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.

Plant Hormone Signaling Crosstalks between Biotic and Abiotic Stress Responses

In the natural environment, plants are often bombarded by a combination of abiotic (such as drought, salt, heat or cold) and biotic (necrotrophic and biotrophic pathogens) stresses simultaneously. It is critical to understand how the various response pathways to these stresses interact with one another within the plants, and where the points of crosstalk occur which switch the responses from one pathway to another. Calcium sensors are often regarded as the first line of response to external stimuli to trigger downstream signaling. Abscisic acid (ABA) is a major phytohormone regulating stress responses, and it interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to channel resources into mitigating the effects of abiotic stresses versus defending against pathogens. The signal transduction in these pathways are often carried out via GTP-binding proteins (G-proteins) which comprise of a large group of proteins that are varied in structures and functions. Deciphering the combined actions of these different signaling pathways in plants would greatly enhance the ability of breeders to develop food crops that can thrive in deteriorating environmental conditions under climate change, and that can maintain or even increase crop yield.

Inter-relationships between the heterotrimeric Gβ subunit AGB1, the receptor-like kinase FERONIA, and RALF1 in salinity response

Plant heterotrimeric G proteins modulate numerous developmental stress responses. Recently, receptor-like kinases (RLKs) have been implicated as functioning with G proteins and may serve as plant G-protein-coupled-receptors. The RLK FERONIA (FER), in the Catharantus roseus RLK1-like subfamily, is activated by a family of polypeptides called rapid alkalinization factors (RALFs). We previously showed that the Arabidopsis G protein β subunit, AGB1, physically interacts with FER, and that RALF1 regulation of stomatal movement through FER requires AGB1. Here, we investigated genetic interactions of AGB1 and FER in plant salinity response by comparing salt responses in the single and double mutants of agb1 and fer. We show that AGB1 and FER act additively or synergistically depending on the conditions of the NaCl treatments. We further show that the synergism likely occurs through salt-induced ROS production. In addition, we show that RALF1 enhances salt toxicity through increasing Na⁺ accumulation and decreasing K⁺ accumulation rather than by inducing ROS production, and that the RALF1 effect on salt response occurs in an AGB1-independent manner. Our results indicate that RLK epistatic relationships are not fixed, as AGB1 and FER display different genetic relationships to RALF1 in stomatal versus salinity responses.

Autoimmunity in plants

Attenuation in the activity of the negative regulators or the hyperactivity of plant innate immune receptors often causes ectopic defense activation manifested in severe growth retardation and spontaneous lesion formations, referred to as autoimmunity. In this review, we have described the cellular and molecular basis of the development of autoimmune responses for their useful applications in plant defense. Plants are exposed to diverse disease-causing pathogens, which bring infections by taking over the control on host immune machineries. To counter the challenges of evolving pathogenic races, plants recruit specific types of intracellular immune receptors that mostly belong to the family of polymorphic nucleotide-binding oligomerization domain-containing leucine-rich repeat (NLR) proteins. Upon recognition of effector molecules, NLR triggers hyperimmune signaling, which culminates in the form of a typical programmed cell death, designated hypersensitive response. Besides, few plant NLRs also guard certain host proteins known as 'guardee' that are modified by effector proteins. However, this fine-tuned innate immune system can be lopsided upon knock-out of the alleles that correspond to the host guardees, which mimick the presence of pathogen. The absence of pathogens causes inappropriate activation of the respective NLRs and results in the constitutive activation of plant defense and exhibiting autoimmunity. In plants, autoimmune mutants are readily scorable due to their dwarf phenotype and development of characteristic macroscopic disease lesions. Here, we summarize recent reports on autoimmune response in plants, how it is triggered, and phenotypic consequences associated with this phenomenon.

A kinase-dead version of FERONIA receptor-like kinase has dose-dependent impacts on rosette morphology and RALF1-mediated stomatal movements

The receptor-like kinase FERONIA (FER) pleiotropically affects plant reproduction, development, and stress tolerance. We recently showed that the FER ligand RALF1 promotes stomatal closure and inhibits stomatal opening in a G-protein-dependent manner. FER responses have been designated as kinase-dependent or kinase-independent, based largely on fer complementation assays employing a kinase-dead FERK565R. Our quantification of FERK565R-GFP transcript and FERK565R-GFP protein in fer complementation lines reveal that, even within individual complementation lines, different levels of FERK565R expression prevail. FERK565R-GFP expression comparable to that of FER in Col-0 plants fail to elicit complementation of either fer rosette phenotypes or RALF1-elicited stomatal movements, whereas overexpression levels of FERK565R-GFP result in complementation. These results suggest possible alternative interpretations of previous conclusions regarding kinase-independent FER signaling.

On plant defense signaling networks and early land plant evolution

All land plants must cope with phytopathogens. Algae face pathogens, too, and it is reasonable to assume that some of the strategies for dealing with pathogens evolved prior to the origin of embryophytes – plant terrestrialization simply changed the nature of the plant-pathogen interactions. Here we highlight that many potential components of the angiosperm defense toolkit are i) found in streptophyte algae and non-flowering embryophytes and ii) might be used in non-flowering plant defense as inferred from published experimental data. Nonetheless, the common signaling networks governing these defense responses appear to have become more intricate during embryophyte evolution. This includes the evolution of the antagonistic signaling pathways of jasmonic and salicylic acid, multiple independent expansions of resistance genes, and the evolution of resistance gene-regulating microRNAs. Future comparative studies will illuminate which modules of the streptophyte defense signaling network constitute the core and which constitute lineage- and/or environment-specific (peripheral) signaling circuits.

Integrated mRNA, sRNA, and degradome sequencing reveal oilseed rape complex responses to Sclerotinia sclerotiorum (Lib.) infection

Sclerotinia stem rot (SSR), caused by the fungal pathogen Sclerotinia sclerotiorum, is a devastating disease resulting in yield losses and decreases in seed quality in oilseed rape (Brassica napus) worldwide. However, the molecular mechanisms underlying the response of oilseed rape to S. sclerotiorum infection at the transcriptional and post-transcriptional levels are poorly understood. Here, we used an integrated omics approach (transcriptome, sRNAome, and degradome sequencing) on the Illumina platform to compare the RNA expression and post-transcriptional profiles of oilseed rape plants inoculated or not with S. sclerotiorum. In total, 7,065 differentially expressed genes (DEGs) compared with the mock-inoculated control at 48 hours post inoculation were identified. These DEGs were associated with protein kinases, signal transduction, transcription factors, hormones, pathogenesis-related proteins, secondary metabolism, and transport. In the sRNA-Seq analysis, 77 known and 176 novel miRNAs were identified; however, only 10 known and 41 novel miRNAs were differentially expressed between the samples inoculated or not with S. sclerotiorum. Degradome sequencing predicted 80 cleavage sites with 64 miRNAs. Integrated mRNA, sRNA and degradome sequencing analysis reveal oilseed rape complex responses to S. sclerotiorum infection. This study provides a global view of miRNA and mRNA expression profiles in oilseed rape following S. sclerotiorum infection.

SOBIR1 and AGB1 independently contribute to nonhost resistance to Pyricularia oryzae (syn. Magnaporthe oryzae) in Arabidopsis thaliana

Rice blast caused by Pyricularia oryzae (syn. Magnaporthe oryzae) is a disease devastating to rice. We have studied the Arabidopsis-P. oryzae pathosystem as a model system for nonhost resistance (NHR) and found that SOBIR1, but not BAK1, is a positive regulator of NHR to P. oryzae in Arabidopsis. AGB1 is also involved in NHR. However, the genetic interactions between SOBIR1, BAK1, and AGB1 are uncharacterized. In this study, we delineated the genetic interactions between SOBIR1, BAK1, and AGB1 in NHR to P. oryzae in Arabidopsis and found SOBIR1 and AGB1 independently control NHR to P. oryzae in Arabidopsis pen2-1 mutant plants. Furthermore, XLG2, but not TMM, has a positive role in penetration resistance to P. oryzae in Arabidopsis pen2-1 mutant plants. Our study characterized genetic interactions in Arabidopsis NHR. Abbreviations: PRR: pattern recognition receptor, RLK: receptor-like kinase, RLP: receptor-like protein, BAK1: BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1, BIR1: BAK1-INTERACTING RECEPTOR-LIKE KINASE 1, SOBIR1: SUPPRESSOR OF BIR1-1-1, AGB1: ARABIDOPSIS G PROTEIN ß-SUBUNIT 1, XLG2: EXTRA-LARGE G PROTEIN 2.

Plant G proteins interact with endoplasmic reticulum luminal protein receptors to regulate endoplasmic reticulum retrieval

Maintaining endoplasmic reticulum (ER) homeostasis is essential for the production of biomolecules. ER retrieval, i.e., the retrograde transport of compounds from the Golgi to the ER, is one of the pathways that ensures ER homeostasis. However, the mechanisms underlying the regulation of ER retrieval in plants remain largely unknown. Plant ERD2-like proteins (ERD2s) were recently suggested to function as ER luminal protein receptors that mediate ER retrieval. Here, we demonstrate that heterotrimeric G protein signaling is involved in ERD2-mediated ER retrieval. We show that ERD2s interact with the heterotrimeric G protein Gα and Gγ subunits at the Golgi. Silencing of Gα, Gβ, or Gγ increased the retention of ER luminal proteins. Furthermore, overexpression of Gα, Gβ, or Gγ caused ER luminal proteins to escape from the ER, as did the co-silencing of ERD2a and ERD2b. These results suggest that G proteins interact with ER luminal protein receptors to regulate ER retrieval.

Emerging themes in heterotrimeric G-protein signaling in plants

Heterotrimeric G-proteins are key signaling components involved during the regulation of a multitude of growth and developmental pathways in all eukaryotes. Although the core proteins (Gα, Gβ, Gγ subunits) and their basic biochemistries are conserved between plants and non-plant systems, seemingly different inherent properties of specific components, altered wirings of G-protein network architectures, and the presence of novel receptors and effector proteins make plant G-protein signaling mechanisms somewhat distinct from the well-established animal paradigm. G-protein research in plants is getting a lot of attention recently due to the emerging roles of these proteins in controlling many agronomically important traits. New findings on both canonical and novel G-protein components and their conserved and unique signaling mechanisms are expected to improve our understanding of this important module in affecting critical plant growth and development pathways and eventually their utilization to produce plants for the future needs. In this review, we briefly summarize what is currently known in plant G-protein research, describe new findings and how they are changing our perceptions of the field, and discuss important issues that still need to be addressed.

Receptor-Like Cytoplasmic Kinases: Central Players in Plant Receptor Kinase-Mediated Signaling

Receptor kinases (RKs) are of paramount importance in transmembrane signaling that governs plant reproduction, growth, development, and adaptation to diverse environmental conditions. Receptor-like cytoplasmic kinases (RLCKs), which lack extracellular ligand-binding domains, have emerged as a major class of signaling proteins that regulate plant cellular activities in response to biotic/abiotic stresses and endogenous extracellular signaling molecules. By associating with immune RKs, RLCKs regulate multiple downstream signaling nodes to orchestrate a complex array of defense responses against microbial pathogens. RLCKs also associate with RKs that perceive brassinosteroids and signaling peptides to coordinate growth, pollen tube guidance, embryonic and stomatal patterning, floral organ abscission, and abiotic stress responses. The activity and stability of RLCKs are dynamically regulated not only by RKs but also by other RLCK-associated proteins. Analyses of RLCK-associated components and substrates have suggested phosphorylation relays as a major mechanism underlying RK-mediated signaling.

YODA MAP3K kinase regulates plant immune responses conferring broad-spectrum disease resistance

Mitogen-activated protein kinases (MAPKs) cascades play essential roles in plants by transducing developmental cues and environmental signals into cellular responses. Among the latter are microbe-associated molecular patterns perceived by pattern recognition receptors (PRRs), which trigger immunity. We found that YODA (YDA) - a MAPK kinase kinase regulating several Arabidopsis developmental processes, like stomatal patterning - also modulates immune responses. Resistance to pathogens is compromised in yda alleles, whereas plants expressing the constitutively active YDA (CA-YDA) protein show broad-spectrum resistance to fungi, bacteria, and oomycetes with different colonization modes. YDA functions in the same pathway as ERECTA (ER) Receptor-Like Kinase, regulating both immunity and stomatal patterning. ER-YDA-mediated immune responses act in parallel to canonical disease resistance pathways regulated by phytohormones and PRRs. CA-YDA plants exhibit altered cell-wall integrity and constitutively express defense-associated genes, including some encoding putative small secreted peptides and PRRs whose impairment resulted in enhanced susceptibility phenotypes. CA-YDA plants show strong reprogramming of their phosphoproteome, which contains protein targets distinct from described MAPKs substrates. Our results suggest that, in addition to stomata development, the ER-YDA pathway regulates an immune surveillance system conferring broad-spectrum disease resistance that is distinct from the canonical pathways mediated by described PRRs and defense hormones.

Convergent and Divergent Signaling in PAMP-Triggered Immunity and Effector-Triggered Immunity

Plants use diverse immune receptors to sense pathogen attacks. Recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors localized on the plasma membrane leads to PAMP-triggered immunity (PTI). Detection of pathogen effectors by intracellular or plasma membrane-localized immune receptors results in effector-triggered immunity (ETI). Despite the large variations in the magnitude and duration of immune responses triggered by different PAMPs or pathogen effectors during PTI and ETI, plasma membrane-localized immune receptors activate similar downstream molecular events such as mitogen-activated protein kinase activation, oxidative burst, ion influx, and increased biosynthesis of plant defense hormones, indicating that defense signals initiated at the plasma membrane converge at later points. On the other hand, activation of ETI by immune receptors localized to the nucleus appears to be more directly associated with transcriptional regulation of defense gene expression. Here, we review recent progress in signal transductions downstream of different groups of plant immune receptors, highlighting the converging and diverging molecular events.

Ligand-triggered de-repression of Arabidopsis heterotrimeric G proteins coupled to immune receptor kinases

Arabidopsis heterotrimeric G proteins regulate diverse processes by coupling to single-transmembrane receptors. One such receptor is the FLS2 receptor kinase, which perceives bacterial flagellin epitope flg22 to activate immunity through a class of cytoplasmic kinases called BIK1/PBLs. Unlike animal and fungal heterotrimeric G proteins that are activated by a ligand-induced guanine nucleotide exchange activity of seven-transmembrane G protein-coupled receptors (GPCRs), plant heterotrimeric G proteins are self-activating. How plant receptors regulate heterotrimeric G proteins in response to external ligands remains unknown. Here we show that RGS1, a GTPase accelerating protein, maintains Arabidopsis G proteins in an inactive state in complex with FLS2. Activation of FLS2 by flg22 induces a BIK1/PBL-mediated phosphorylation of RGS1 at Ser428 and Ser431 and that promotes RGS1 dissociation from the FLS2-G protein complex. This relieves G proteins from the RGS1-mediated repression and enables positive regulation of immune signaling. We additionally show that RGS1 is similarly regulated by multiple immune receptors. Our results uncover ligand-induced de-repression as a mechanism for G protein signaling in plants that is distinct from previously reported mechanism underlying the activation of heterotrimeric G proteins in other systems.

G Protein α Subunit GpaB is Required for Asexual Development, Aflatoxin Biosynthesis and Pathogenicity by Regulating cAMP Signaling in Aspergillus flavus

The heterotrimeric G proteins are critical for signal transduction and function in numerous biological processes including vegetative growth, asexual development and fungal virulence in fungi. Here, we identified four G protein alpha subunits (GanA, GpaB, FadA and GaoC) in the notorious Aflatoxin-producing fungus Aspergillus flavus. GanA, GpaB and FadA have homologues in other fungal species, while GaoC is a novel one. Here, we showed that the loss function of gpaB displayed a defect in conidiophore formation and considerably reduced expression levels of conidia-specific genes brlA and abaA. A decreased viability of cell wall integrity stress and oxidative stress were also found in the ∆gpaB mutant. More importantly, aflatoxin (AF) biosynthesis and infection on crop seeds were severely impaired in the gpaB-deficient mutant. Further analyses demonstrated that the intracellular cAMP levels significantly reduced in the gpaB-deficient mutant compared to wildtype strains. Additionally, an alteration of PKA activities in the ∆gpaB mutant was also found. Overall, our results indicated that GpaB played diverse roles in asexual sporulation, AF biosynthesis and virulence by regulating cAMP signaling in Aspergillus flavus.

Genomic Analyses Yield Markers for Identifying Agronomically Important Genes in Potato

Wild potato species have substantial phenotypic and physiological diversity. Here, we report a comprehensive assessment of wild and cultivated potato species based on genomic analyses of 201 accessions of Solanum section Petota. We sequenced the genomes of these 201 accessions and identified 6 487 006 high-quality single nucleotide polymorphisms (SNPs) from 167 accessions in clade 4 of Solanum section Petota, including 146 wild and 21 cultivated diploid potato accessions with a broad geographic distribution. Genome-wide genetic variation analysis showed that the diversity of wild potatoes is higher than that of cultivated potatoes, and much higher genetic diversity in the agronomically important disease resistance genes was observed in wild potatoes. Furthermore, by exploiting information about known quantitative trait loci (QTL), we identified 609 genes under selection, including those correlated with the loss of bitterness in tubers and those involved in tuberization, two major domesticated traits of potato. Phylogenetic analyses revealed a north-south division of all species in clade 4, not just those in the S. brevicaule complex, and further supported S. candolleanum as the progenitor of cultivated potato and the monophyletic origin of cultivated potato in southern Peru. In addition, we analyzed the genome of S. candolleanum and identified 529 genes lost in cultivated potato. Collectively, the molecular markers generated in this study provide a valuable resource for the identification of agronomically important genes useful for potato breeding.

The G Protein β-Subunit, AGB1, Interacts with FERONIA in RALF1-Regulated Stomatal Movement

Heterotrimeric guanine nucleotide-binding (G) proteins are composed of Gα, Gβ, and Gγ subunits and function as molecular switches in signal transduction. In Arabidopsis (Arabidopsis thaliana), there are one canonical Gα (GPA1), three extra-large Gα (XLG1, XLG2, and XLG3), one Gβ (AGB1), and three Gγ (AGG1, AGG2, and AGG3) subunits. To elucidate AGB1 molecular signaling, we performed immunoprecipitation using plasma membrane-enriched proteins followed by mass spectrometry to identify the protein interactors of AGB1. After eliminating proteins present in the control immunoprecipitation, commonly identified contaminants, and organellar proteins, a total of 103 candidate AGB1-associated proteins were confidently identified. We identified all of the G protein subunits except XLG1, receptor-like kinases, Ca²⁺ signaling-related proteins, and 14-3-3-like proteins, all of which may couple with or modulate G protein signaling. We confirmed physical interaction between AGB1 and the receptor-like kinase FERONIA (FER) using bimolecular fluorescence complementation. The Rapid Alkalinization Factor (RALF) family of polypeptides have been shown to be ligands of FER. In this study, we demonstrate that RALF1 regulates stomatal apertures and does so in a G protein-dependent manner, inhibiting stomatal opening and promoting stomatal closure in Columbia but not in agb1 mutants. We further show that AGGs and XLGs, but not GPA1, participate in RALF1-mediated stomatal signaling. Our results suggest that FER acts as a G protein-coupled receptor for plant heterotrimeric G proteins.

Duplicated RGS (Regulator of G-protein signaling) proteins exhibit conserved biochemical but differential transcriptional regulation of heterotrimeric G-protein signaling in Brassica species

G-alpha (Gα) and ‘Regulator of G-protein Signaling (RGS)’ proteins are the two key components primarily involved in regulation of heterotrimeric G-proteins signaling across phyla. Unlike Arabidopsis thaliana, our knowledge about G-protein regulation in polyploid Brassica species is sparse. In this study, we identified one Gα and two RGS genes each from three species of Brassica ‘U’ triangle and assessed the effects of whole genome triplication on the divergence of gene sequence and structure, protein-protein interaction, biochemical activities, and gene expression. Sequence and phylogenetic analysis revealed that the deduced Gα and RGS proteins are evolutionarily conserved across Brassica species. The duplicated RGS proteins of each Brassica species interacted with their cognate Gα but displayed varying levels of interaction strength. The Gα and the duplicated RGS proteins of Brassica species exhibited highly conserved G-protein activities when tested under in-vitro conditions. Expression analysis of the B. rapa RGS genes revealed a high degree of transcriptional differentiation across the tested tissue types and in response to various elicitors, particularly under D-glucose, salt and phytohormone treatments. Taken together, our results suggest that the RGS-mediated regulation of G-protein signaling in Brassica species is predominantly governed by stage and condition-specific expression differentiation of the duplicated RGS genes.

Plant cell wall-mediated immunity: cell wall changes trigger disease resistance responses

Plants have evolved a repertoire of monitoring systems to sense plant morphogenesis and to face environmental changes and threats caused by different attackers. These systems integrate different signals into overreaching triggering pathways which coordinate developmental and defence-associated responses. The plant cell wall, a dynamic and complex structure surrounding every plant cell, has emerged recently as an essential component of plant monitoring systems, thus expanding its function as a passive defensive barrier. Plants have a dedicated mechanism for maintaining cell wall integrity (CWI) which comprises a diverse set of plasma membrane-resident sensors and pattern recognition receptors (PRRs). The PRRs perceive plant-derived ligands, such as peptides or wall glycans, known as damage-associated molecular patterns (DAMPs). These DAMPs function as 'danger' alert signals activating DAMP-triggered immunity (DTI), which shares signalling components and responses with the immune pathways triggered by non-self microbe-associated molecular patterns that mediate disease resistance. Alteration of CWI by impairment of the expression or activity of proteins involved in cell wall biosynthesis and/or remodelling, as occurs in some plant cell wall mutants, or by wall damage due to colonization by pathogens/pests, activates specific defensive and growth responses. Our current understanding of how these alterations of CWI are perceived by the wall monitoring systems is scarce and few plant sensors/PRRs and DAMPs have been characterized. The identification of these CWI sensors and PRR-DAMP pairs will help us to understand the immune functions of the wall monitoring system, and might allow the breeding of crop varieties and the design of agricultural strategies that would enhance crop disease resistance.

Tyrosine phosphorylation switching of a G protein

Heterotrimeric G protein complexes are molecular switches relaying extracellular signals sensed by G protein-coupled receptors (GPCRs) to downstream targets in the cytoplasm, which effect cellular responses. In the plant heterotrimeric GTPase cycle, GTP hydrolysis, rather than nucleotide exchange, is the rate-limiting reaction and is accelerated by a receptor-like regulator of G signaling (RGS) protein. We hypothesized that posttranslational modification of the Gα subunit in the G protein complex regulates the RGS-dependent GTPase cycle. Our structural analyses identified an invariant phosphorylated tyrosine residue (Tyr¹⁶⁶ in the Arabidopsis Gα subunit AtGPA1) located in the intramolecular domain interface where nucleotide binding and hydrolysis occur. We also identified a receptor-like kinase that phosphorylates AtGPA1 in a Tyr¹⁶⁶-dependent manner. Discrete molecular dynamics simulations predicted that phosphorylated Tyr¹⁶⁶ forms a salt bridge in this interface and potentially affects the RGS protein-accelerated GTPase cycle. Using a Tyr¹⁶⁶ phosphomimetic substitution, we found that the cognate RGS protein binds more tightly to the GDP-bound Gα substrate, consequently reducing its ability to accelerate GTPase activity. In conclusion, we propose that phosphorylation of Tyr¹⁶⁶ in AtGPA1 changes the binding pattern with AtRGS1 and thereby attenuates the steady-state rate of the GTPase cycle. We coin this newly identified mechanism "substrate phosphoswitching."

Fats and function: protein lipid modifications in plant cell signalling

The post-translational lipid modifications N-myristoylation, prenylation and S-acylation are traditionally associated with increasing protein membrane affinity and localisation. However this is an over-simplification, with evidence now implicating these modifications in a variety of roles such as membrane microdomain partitioning, protein trafficking, protein complex assembly and polarity maintenance. Evidence for a regulatory role is also emerging, with changes or manipulation of lipid modifications offering a means of directly controlling various aspects of protein function. Proteomics advances have revealed an enrichment of signalling proteins in the lipid-modified proteome, potentially indicating an important role for these modifications in responding to stimuli. This review highlights some of the key themes and possible functions of lipid modification during signalling processes in plants.

Alteration of cell wall xylan acetylation triggers defense responses that counterbalance the immune deficiencies of plants impaired in the β-subunit of the heterotrimeric G-protein

Arabidopsis heterotrimeric G-protein complex modulates pathogen-associated molecular pattern-triggered immunity (PTI) and disease resistance responses to different types of pathogens. It also plays a role in plant cell wall integrity as mutants impaired in the Gβ- (agb1-2) or Gγ-subunits have an altered wall composition compared with wild-type plants. Here we performed a mutant screen to identify suppressors of agb1-2 (sgb) that restore susceptibility to pathogens to wild-type levels. Out of the four sgb mutants (sgb10-sgb13) identified, sgb11 is a new mutant allele of ESKIMO1 (ESK1), which encodes a plant-specific polysaccharide O-acetyltransferase involved in xylan acetylation. Null alleles (sgb11/esk1-7) of ESK1 restore to wild-type levels the enhanced susceptibility of agb1-2 to the necrotrophic fungus Plectosphaerella cucumerina BMM (PcBMM), but not to the bacterium Pseudomonas syringae pv. tomato DC3000 or to the oomycete Hyaloperonospora arabidopsidis. The enhanced resistance to PcBMM of the agb1-2 esk1-7 double mutant was not the result of the re-activation of deficient PTI responses in agb1-2. Alteration of cell wall xylan acetylation caused by ESK1 impairment was accompanied by an enhanced accumulation of abscisic acid, the constitutive expression of genes encoding antibiotic peptides and enzymes involved in the biosynthesis of tryptophan-derived metabolites, and the accumulation of disease resistance-related secondary metabolites and different osmolites. These esk1-mediated responses counterbalance the defective PTI and PcBMM susceptibility of agb1-2 plants, and explain the enhanced drought resistance of esk1 plants. These results suggest that a deficient PTI-mediated resistance is partially compensated by the activation of specific cell-wall-triggered immune responses.

E3 ligase SAUL1 serves as a positive regulator of PAMP-triggered immunity and its homeostasis is monitored by immune receptor SOC3

In both plants and animals, intracellular nucleotide-binding leucine-rich repeat proteins (NLRs; or Nod-like receptors) serve as immune receptors to recognize pathogen-derived molecules and mount effective immune responses against microbial infections. Plant NLRs often guard the presence or activity of other host proteins, which are the direct virulence targets of pathogen effectors. These guardees are sometimes immune-promoting components such as those in a mitogen-activated protein kinase cascade. Plant E3 ligases serve many roles in immune regulation, but it is unclear whether they can also be guarded by NLRs. Here, we report on an immune-regulating E3 ligase SAUL1, whose homeostasis is monitored by a Toll interleukin 1 receptor (TIR)-type NLR (TNL), SOC3. SOC3 can associate with SAUL1, and either loss or overexpression of SAUL1 triggers autoimmunity mediated by SOC3. By contrast, SAUL1 functions redundantly with its close homolog PUB43 to promote PAMP-triggered immunity (PTI). Taken together, the E3 ligase SAUL1 serves as a positive regulator of PTI and its homeostasis is monitored by the TNL SOC3.

Pattern of expression and interaction specificity of multiple G-protein beta (Gβ) subunit isoforms with their potential target proteins reveal functional dominance of BjuGβ1 in the allotetraploid Brassica juncea

Heterotrimeric G-protein, consisting Gα, Gβ and Gγ subunits, interacts with various upstream and downstream effector (target) proteins to regulate a large array of conserved and species-specific biological functions. The targets of G-protein components are recently reported in model plant Arabidopsis thaliana; however limited information is available from crop species. In this study, we utilized yeast two-hybrid (Y2H) assay to screen the diversity of interacting partners of multiple Gβ subunit isoforms from allotetraploid Brassica juncea, a globally important oilseed and vegetable crop. The three BjuGβ genes (BjuGβ1-3), resulted from whole genome triplication event in Brassica lineage, showed distinct expression profile during plant developmental stages with maximal transcript abundance during reproductive stages. Protein-protein interaction of three BjuGβ proteins (bait) against the Y2H cDNA library (prey) identified a total of 14 and 1 non-redundant targets for BjuGβ1 and BjuGβ2, whereas BjuGβ3 screening surprisingly did not yield any genuine target, thereby suggesting functional dominance of BjuGβ1. The triplicated BjuGβ isoforms showed a high degree of interaction strength and specificity with the identified target proteins, which are known to be involved in diverse biological functions in plants. qRT-PCR analysis further indicated that the expression of BjuGβ-target genes was developmentally regulated under various tissue types studied and showed a high degree of co-expression pattern with the BjuGβ genes, particularly during flower and silique development in B. juncea. Taken together, our data provides novel insights on pattern of expression and interaction specificity governing functional divergence of multiple Gβ subunit proteins in polyploid B. juncea.

The integration of Gβ and MAPK signaling cascade in zygote development

Cells respond to many signals with a limited number of signaling components. Heterotrimeric G proteins and MAPK cascades are universally used by eukaryotic cells to transduce signals in various developmental processes or stress responses by activating different effectors. MAPK cascade is integrated with G proteins by scaffold protein during plant immunity. However, the molecular relationship between G proteins and MAPK modules in plant development is still unclear. In this study, we demonstrate that Arabidopsis Gβ protein AGB1 interacts with MPK3 and 6, MKK4 and 5, as well as the regulatory domains of YODA (YDA), the upstream MEKK of MKK4/5. Remarkably, YDA interacts with the plasma membrane associated SHORT SUSPENSOR (SSP) through its N- and C-terminal region in vitro and in vivo. Additionally, genetic analysis shows that AGB1 functions together with MPK3/6 signaling cascade during the asymmetric division of the zygote. These data indicate that Gβ may function likely as a scaffold, through direct physical interaction with the components of the MPK signaling module in plant development. Our results provide new insights into the molecular functions of G protein and will advance the understanding of the complex mechanism of kinase signaling cascades.

From Chaos to Harmony: Responses and Signaling upon Microbial Pattern Recognition

Pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) are detected as nonself by host pattern recognition receptors (PRRs) and activate pattern-triggered immunity (PTI). Microbial invasions often trigger the production of host-derived endogenous signals referred to as danger- or damage-associated molecular patterns (DAMPs), which are also perceived by PRRs to modulate PTI responses. Collectively, PTI contributes to host defense against infections by a broad range of pathogens. Remarkable progress has been made toward demonstrating the cellular and physiological responses upon pattern recognition, elucidating the molecular, biochemical, and genetic mechanisms of PRR activation, and dissecting the complex signaling networks that orchestrate PTI responses. In this review, we present an update on the current understanding of how plants recognize and respond to nonself patterns, a process from which the seemingly chaotic responses form into a harmonic defense.

Apoplastic ROS signaling in plant immunity

Reactive oxygen species (ROS) are widely produced in different cellular compartments under both biotic and abiotic stress conditions. ROS play a central role in plant signaling and regulate diverse cellular processes. Recent advances are shedding new light on sophisticated mechanisms controlling ROS biogenesis and signaling in plant immunity. In this review, we summarize our current understanding of the regulation of apoplastic ROS production in response to microbial molecular patterns and draw comparison with abscisic acid (ABA)-induced apoplastic ROS. We also discuss how ROS act as signal molecules to regulate cellular activities using stomatal movement as an example.

A look at plant immunity through the window of the multitasking coreceptor BAK1

Recognition of microbe- and danger-associated molecular patterns (MAMPs and DAMPs, respectively) by pattern recognition receptors (PRRs) is central to innate immunity in both plants and animals. The plant PRRs described to date are all cell surface-localized receptors. According to their ligand-binding ectodomains, each PRR engages a specific coreceptor or adaptor kinase in its signaling complexes to regulate defense signaling. With a focus on the coreceptor RLK BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) and related SOMATIC EMBRYOGENESIS RECEPTOR KINASEs (SERKs), here we review the increasing inventory of BAK1 partners and their functions in plant immunity. We also discuss the significance of autoimmunity triggered by BAK1/SERK4 disintegration in shaping the strategies for attenuation of PRR signaling by infectious microbes and host plants.

Dynamic G protein alpha signaling in Arabidopsis innate immunity

Heterotrimeric G proteins composed of Gα, Gβ and Gγ subunits are evolutionarily conserved signaling modules involved in diverse biological processes in plants and animals. The role and action of Gα remain largely enigmatic in plant innate immunity. We have recently demonstrated that Arabidopsis Gα (GPA1) is a key component of a new immune signaling pathway activated by bacteria-secreted proteases. Here we show that GPA1 is also involved in the signaling network of Arabidopsis in response to the bacterial flagellin epitope flg22. Specifically, GPA1 plays a pivotal role in an immune pathway involving the flg22 receptor FLS2, co-receptor BAK1, Regulator of G Signaling 1 (RGS1), and Arabidopsis Gβ (AGB1), in which flg22 elicits GPA1/AGB1 dissociation from the FLS2/BAK1/RGS1 receptor complex. Consequently, we observed flg22-induced degradation of FLS2, BAK1 and RGS1 but not GPA1 or AGB1. We also found that GPA1 constitutively interacts with the NADPH oxidase RbohD to potentiate flg22-induced ROS burst independently of the central cytoplasmic kinase BIK1. Taken together, our work sheds multiple novel insights into the functions and regulatory mechanisms of GPA1 in Arabidopsis innate immunity.

Identification and Characterization of Phospholipase D Genes Putatively Involved in Internal Browning of Pineapple during Postharvest Storage

Phospholipase D (PLD) in plants plays vital roles in growth, development, and stress responses. However, the precise role of PLDs in pineapple remains poorly understood. In this study, 10 putative PLD genes, designated as AcPLD1-AcPLD10, were identified based on the pineapple genome database. The 10 AcPLDs could be clustered into five of the six known PLD families according to sequence characterization. Their deduced amino acid sequences displayed similarities to PLDs from other plant species. Expression analyses of PLD mRNAs from pineapple pulp were performed. The 10 PLDs exhibited differential expression patterns during storage periods of fruits treated with hexaldehyde (a specific PLD inhibitor) which could alleviate internal browning (IB) of pineapple after harvest. Functional subcellular localization signaling assays of two PLD proteins (AcPLD2 and AcPLD9) were performed by fluorescence microscopy. To further detect the potential action mechanism underlying PLD involved in the IB defense response, PLD, hydrogen peroxide (H₂O₂) and H₂O₂ associated with antioxidative enzymes such as superoxide dismutase, catalase, NADPH, and ascorbate peroxidase were quantified by enzyme-linked immunosorbent assay. This report is the first to provide a genome-wide description of the pineapple PLD gene family, and the results should expand knowledge of this family.

Intracellular Ca2+ is important for flagellin-triggered defense in Arabidopsis and involves inositol polyphosphate signaling

Cytosolic Ca2+ increase is a crucial and early step of plant immunity evoked by pathogen-associated molecular patterns (PAMPs) such as flagellin (flg). Components responsible for this increase are still not uncovered, although current models of plant immune signaling portray extracellular Ca2+ influx as paramount to flg activation of defense pathways. Work presented here provides new insights into cytosolic Ca2+ increase associated with flg-induced defense responses. We show that extracellular Ca2+ contributes more to immune responses evoked by plant elicitor peptide (Pep3) than that evoked by flg, indicating an intracellular Ca2+ source responsible for immune responses evoked by flg. Genetic impairment of the inositol polyphosphate (InsP) and G-protein signal associated with flg perception reduced flg-dependent immune responses. Previous work indicates that prior exposure of Arabidopsis plants to flg leads to an immune response reflected by less vigorous growth of a pathogenic microbe. We found that this immune response to flg was compromised in mutants lacking the ability to generate an InsP or G-protein signal. We conclude that the recruitment of intracellular Ca2+ stores by flg may involve InsP and G-protein signaling. We also found a notable difference in contribution of intracellular stores of Ca2+ to the immune signaling evoked by another PAMP, elf18 peptide, which had a very different response profile to impairment of InsP signaling. Although Ca2+ signaling is at the core of the innate immune as well as hypersensitive response to plant pathogens, it appears that the molecular mechanisms generating the Ca2+ signal in response to different PAMPs are different.

Heterotrimeric G-protein regulatory circuits in plants: Conserved and novel mechanisms

ARTICLE ADDENDUM Efficient activation and deactivation of Gα protein is critical for the regulation of heterotrimeric G-protein mediated signaling pathways. While the core G-protein components and their activation/deactivation chemistries are broadly conserved throughout the eukaryotic evolution, their regulatory mechanisms seem to have been rewired in plants to meet specific needs. Plants such as Arabidopsis, which have a limited number of G-protein components and their regulators, offer a unique opportunity to dissect the mechanistic details of distinct signaling pathways. We have recently established an interaction between the regulator of G-protein signaling 1 (RGS1) and phospholipase Dα1 (PLDα1); 2 of the GTPase activity accelerating proteins (GAPs) of the Arabidopsis Gα protein, GPA1. We now show that phosphatidic acid (PA), a key product of PLDα1 activity, can bind with and modulate the GAP activity of RGS1, uncovering a molecular link between lipid and G-protein signaling and its role in providing the specificity of response regulation.

Functional analysis of oxidative burst in sugarcane smut-resistant and -susceptible genotypes

Smut pathogen induced an early modulation of the production and scavenging of reactive oxygen species during defence responses in resistant sugarcane that coincided with the developmental stages of fungal growth. Sporisorium scitamineum is the causal agent of sugarcane smut disease. In this study, we characterized sugarcane reactive oxygen species (ROS) metabolism in response to the pathogen in smut-resistant and -susceptible genotypes. Sporisorium scitamineum teliospore germination and appressorium formation coincided with H₂O₂ accumulation in resistant plants. The superoxide dismutase (SOD) activity was not responsive in any of the genotypes; however, a higher number of isoenzymes were detected in resistant plants. In addition, related to resistance were lipid peroxidation, a decrease in catalase (CAT), and an increase in glutathione S-transferase (GST) activities and an earlier transcript accumulation of ROS marker genes (CAT3, CATA, CATB, GST31, GSTt3, and peroxidase 5-like). Furthermore, based on proteomic data, we suggested that the source of the increased hydrogen peroxide (H₂O₂) may be due to a protein of the class III peroxidase, which was inhibited in the susceptible genotype. H₂O₂ is sensed and probably transduced through overlapping systems related to ascorbate-glutathione and thioredoxin to influence signalling pathways, as revealed by the presence of thioredoxin h-type, ascorbate peroxidase, and guanine nucleotide-binding proteins in the infected resistant plants. Altogether, our data depicted the balance of the oxidative burst and antioxidant enzyme activity in the outcome of this interaction.

Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition

Receptor-like kinases (RLKs) and Receptor-like proteins (RLPs) play crucial roles in plant immunity, growth, and development. Plants deploy a large number of RLKs and RLPs as pattern recognition receptors (PRRs) that detect microbe- and host-derived molecular patterns as the first layer of inducible defense. Recent advances have uncovered novel PRRs, their corresponding ligands, and mechanisms underlying PRR activation and signaling. In general, PRRs associate with other RLKs and function as part of multiprotein immune complexes at the cell surface. Innovative strategies have emerged for the rapid identification of microbial patterns and their cognate PRRs. Successful pathogens can evade or block host recognition by secreting effector proteins to "hide" microbial patterns or inhibit PRR-mediated signaling. Furthermore, newly identified pathogen effectors have been shown to manipulate RLKs controlling growth and development by mimicking peptide hormones of host plants. The ongoing studies illustrate the importance of diverse plant RLKs in plant disease resistance and microbial pathogenesis.

Ligand-induced dynamics of heterotrimeric G protein-coupled receptor-like kinase complexes

Background

Arabidopsis, 7-transmembrane Regulator of G signaling protein 1 (AtRGS1) modulates canonical G protein signaling by promoting the inactive state of heterotrimeric G protein complex on the plasma membrane. It is known that plant leucine-rich repeat receptor–like kinases (LRR RLKs) phosphorylate AtRGS1 in vitro but little is known about the in vivo interaction, molecular dynamics, or the cellular consequences of this interaction.

Methods

Therefore, a subset of the known RLKs that phosphorylate AtRGS1 were selected for elucidation, namely, BAK1, BIR1, FLS2. Several microscopies for both static and dynamic protein-protein interactions were used to follow in vivo interactions between the RLKs and AtRGS1 after the presentation of the Pathogen-associated Molecular Pattern, Flagellin 22 (Flg22). These microscopies included Förster Resonance Energy Transfer, Bimolecular Fluoresence Complementation, and Cross Number and Brightness Fluorescence Correlation Spectroscopy. In addition, reactive oxygen species and calcium changes in living cells were quantitated using luminometry and R-GECO1 microscopy.

Results

The LRR RLKs BAK1 and BIR1, interact with AtRGS1 at the plasma membrane. The RLK ligand flg22 sets BAK1 in motion toward AtRGS1 and BIR1 away, both returning to the baseline orientations by 10 minutes. The C-terminal tail of AtRGS1 is important for the interaction with BAK1 and for the tempo of the AtRGS1/BIR1 dynamics. This window of time corresponds to the flg22-induced transient production of reactive oxygen species and calcium release which are both attenuated in the rgs1 and the bak1 null mutants.

Conclusions

A temporal model of these interactions is proposed. flg22 binding induces nearly instantaneous dimerization between FLS2 and BAK1. Phosphorylated BAK1 interacts with and enables AtRGS1 to move away from BIR1 and AtRGS1 becomes phosphorylated leading to its endocytosis thus leading to de-repression by permitting AtGPA1 to exchange GDP for GTP. Finally, the G protein complex becomes dissociated thus AGB1 interacts with its effector proteins leading to changes in reactive oxygen species and calcium.

EXTRA-LARGE G PROTEINs Interact with E3 Ligases PUB4 and PUB2 and Function in Cytokinin and Developmental Processes

Heterotrimeric GTP-binding proteins (G proteins) composed of Gα, Gβ, and Gγ subunits are conserved signal transduction molecules in animals and plants. In Arabidopsis (Arabidopsis thaliana), there are three Gα-like proteins named EXTRA-LARGE G PROTEINs (XLGs) in addition to the canonical Gα protein GPA1. XLGs have been reported to be implicated in multiple pathways, although the underlying mechanisms of their action remain elusive. Here, we report that all three XLGs interact with two closely related plant U-box (PUB) E3 ligases, PUB2 and PUB4. Three XLGs are predominantly localized at the plasma membrane, whereas XLG2 and XLG3 also show nuclear localization. The interactions between PUB2/4 and XLGs suggest that they might function in the same pathways. Indeed, we found that a newly obtained xlg1/2/3 triple knockout mutant, the pub4 mutant, and the pub2/4 double mutant all exhibited defects in cytokinin responses, stamen development, tapetum development, and male fertility. However, the xlg single mutants and the pub2 mutant did not exhibit an obvious defect in these processes, which suggests functional redundancy among the three XLGs and between PUB2 and PUB4. Overexpressing ARR10 to enhance the cytokinin response in pub4 or in the xlg1/2/3 triple mutant partially restored several phenotypes caused by the pub4 and xlg1/2/3 mutations. Our findings reveal that the XLGs and PUB2/4 are components in the complex cytokinin signaling networks regulating many developmental and physiological processes.

Heterotrimeric G Protein-Regulated Ca2+ Influx and PIN2 Asymmetric Distribution Are Involved in Arabidopsis thaliana Roots' Avoidance Response to Extracellular ATP

Extracellular ATP (eATP) has been reported to be involved in plant growth as a primary messenger in the apoplast. Here, roots of Arabidopsis thaliana seedlings growing in jointed medium bent upon contact with ATP-containing medium to keep away from eATP, showing a marked avoidance response. Roots responded similarly to ADP and bz-ATP but did not respond to AMP and GTP. The eATP avoidance response was reduced in loss-of-function mutants of heterotrimeric G protein α subunit (Gα) (gpa1-1 and gpa1-2) and enhanced in Gα-over-expression (OE) lines (wGα and cGα). Ethylenebis(oxyethylenenitrilo) tetraacetic acid (EGTA) and Gd³⁺ remarkably suppressed eATP-induced root bending. ATP-stimulated Ca²⁺ influx was impaired in Gα null mutants and increased in its OE lines. DR5-GFP and PIN2 were asymmetrically distributed in ATP-stimulated root tips, this effect was strongly suppressed by EGTA and diminished in Gα null mutants. In addition, some eATP-induced genes' expression was also impaired in Gα null mutants. Based on these results, we propose that heterotrimeric Gα-regulated Ca²⁺ influx and PIN2 distribution may be key signaling events in eATP sensing and avoidance response in Arabidopsis thaliana roots.

Predicted Functional Implications of Phosphorylation of Regulator of G Protein Signaling Protein in Plants

Heterotrimeric G proteins function in development, biotic, and abiotic stress responses, hormone signaling as well as sugar sensing. We previously proposed that discrimination of these various external signals in the G protein pathway is accomplished in plants by membrane-localized receptor-like kinases (RLKs) rather than G-protein-coupled receptors. Arabidopsis thaliana Regulator of G Signaling protein 1 (AtRGS1) modulates G protein activation and is phosphorylated by several RLKs and by WITH-NO-LYSINE kinases (WNKs). Here, a combination of in vitro kinase assays, mass spectrometry, and computational bioinformatics identified and functionally prioritized phosphorylation sites in AtRGS1. Phosphosites for two more RLKs (BRL3 and PEPR1) were identified and added to the AtRGS1 phosphorylation profile. Bioinformatics analyses revealed that RLKs and WNK kinases phosphorylate plant RGS proteins within regions that are conserved across eukaryotes and at a high frequency. Four phospho-sites among 14 identified are proximal to equivalent mammalian phosphosites that impact RGS function, including: pS437 and pT267 in GmRGS2, and pS339 and pS436 in AtRGS1. Based on these analyses, we propose that pS437 and pS436 regulate GmRGS2 and AtRGS1 protein interactions and/or localization, whereas pT267 is important for modulation of GmRGS2 GAP activity and localization. Moreover, pS339 most likely affects AtRGS1 activation.

Loss-of-function of Arabidopsis receptor-like kinase BIR1 activates cell death and defense responses mediated by BAK1 and SOBIR1

The Arabidopsis receptor-like kinase (RLK) BIR1 (BAK1-INTERACTING RECEPTOR-LIKE KINASE 1) functions as a negative regulator of plant immunity. Previous work showed that loss-of-function of BIR1 leads to constitutive activation of cell death and defense responses. These autoimmune phenotypes are partially dependent on another RLK, SOBIR1. In order to identify additional components involved in the BIR1-regulated plant defense signaling pathway, a suppressor screen was carried out in the bir1-1 pad4-1 mutant background. Mutations in the suppressor mutants were identified by genetic mapping and re-sequencing of the mutant genomes. A number of suppressor mutants were found to carry mutations in an additional RLK, BAK1, indicating that BAK1 is required for activation of cell death and defense responses in bir1-1. Co-immunoprecipitation analysis revealed that BAK1 and SOBIR1 associate with each other in planta when the function of BIR1 is compromised. Although BAK1 was previously characterized as a negative regulator of cell death, our study highlights a novel role of BAK1 in promoting cell death and defense responses in conjunction with SOBIR1.

Plant TRAF Proteins Regulate NLR Immune Receptor Turnover

In animals, Tumor necrosis factor receptor-associated factor (TRAF) proteins are molecular adaptors that regulate innate and adaptive immunity, development, and abiotic stress responses. Although gene families encoding TRAF domain-containing proteins exhibit enriched diversity in higher plants, their biological roles are poorly defined. Here, we report the identification of two redundant TRAF proteins, Mutant, snc1-enhancing 13 (MUSE13) and MUSE14, that contribute to the turnover of nucleotide-binding domain and leucine-rich repeat-containing (NLR) immune receptors SNC1 and RPS2. Loss of both MUSE13 and MUSE14 leads to enhanced pathogen resistance, NLR accumulation, and autoimmunity, while MUSE13 overexpression results in reduced NLR levels and activity. In planta, MUSE13 associates with SNC1, RPS2, and the E3 ubiquitin ligase SCF(CPR1). Taken together, we speculate that MUSE13 and MUSE14 associate with the SCF E3 ligase complex to form a plant-type TRAFasome, which modulates ubiquitination and subsequent degradation of NLR immune sensors to maintain their homeostasis.

Heterotrimeric G-proteins facilitate resistance to plant pathogenic viruses in Arabidopsis thaliana (L.) Heynh

Heterotrimeric G-proteins, consisting of Gα, Gβ and Gγ subunits, are important signal transducers in eukaryotes. In plants, G-protein-mediated signaling contributes to defense against a range of fungal and bacterial pathogens. Here we studied response of G-protein-deficient mutants to ssRNA viruses representing 2 different families: Cucumber mosaic virus (CMV) (Bromoviridae) and Turnip mosaic virus (TuMV) (Potyviridae). We found that development of spreading necrosis on infected plants was suppressed in the Gβ-deficient mutant (agb1-2) compared to wild type and Gα-deficient mutant (gpa1-4). In accordance, ion leakage caused by viral infection was also significantly reduced in agb1-2 compared to wild type and gpa1-4. Nevertheless, both viruses replicated better in agb1-2 plants, while gpa1-4 was similar to wild type. Analysis of pathogenesis-related genes showed that Gβ negatively regulated salicylic acid, jasmonic acid and abscisic acid marker genes during CMV and TuMV infections. Interestingly, analysis of salicylic acid deficient transgenic plants indicated that salicylic acid did not affect resistance against these viruses and did not influence the Gβ-mediated defense response. We conclude that heterotrimeric G-proteins play a positive role in defense against viral pathogens probably by promoting cell death.