Large-scale analysis of the N-terminal regulatory elements of the kinase domain in plant Receptor-like kinase family

BACKGROUND

The N-terminal regulatory element (NRE) of Receptor-like kinases (RLKs), consisting of the juxtamembrane segment in receptor kinases (RKs) and the N-terminal extension segment in RLCKs, is a crucial component that regulates the activities of these proteins. However, the features and functions of the NRE have remained largely unexplored. Herein, we comprehensively analyze 510,233 NRE sequences in RLKs from 528 plant species, using information theory and data mining techniques to unravel their common characteristics and diversity. We also use recombinant RKs to investigate the function of the NRE in vitro.

RESULTS

Our findings indicate that the majority of NRE segments are around 40-80 amino acids in length and feature a serine-rich region and a 14-amino-acid consensus sequence, 'FSYEELEKAT[D/N]NF[S/D]', which contains a characteristic α-helix and ST motif that connects to the core kinase domain. This conserved signature sequence is capable of suppressing FERONIA's kinase activity. A motif discovery algorithm identifies 29 motifs with highly conserved phosphorylation sites in RK and RLCK classes, especially the motif 'VGPWKpTGLpSGQLQKAFVTGVP' in LRR-VI-2 class. Phosphorylation of an NRE motif in an LRR-VI-2 member, MDIS1, modulates the auto-phosphorylation of its co-receptor, MIK1, indicating the potential role of NRE as a 'kinase switch' in RLK activation. Furthermore, the characterization of phosphorylatable NRE motifs improves the accuracy of predicting phosphorylatable sites.

CONCLUSIONS

Our study provides a comprehensive dataset to investigate NRE segments from individual RLKs and enhances our understanding of the underlying mechanisms of RLK signal transduction and kinase activation processes in plant adaptation.

SERKs and NIKs: Coreceptors or signaling hubs in a complex crosstalk between growth and defense?

SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES (SERKs) and NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASES (NIKs) belong to superfamily II of leucine-rich repeat receptor-like kinases, which share cytosolic kinase conservation and a similar ectodomain configuration. SERKs have been extensively demonstrated to function as coreceptors of receptor-like kinases, which sense biotic or developmental signals to initiate specific responses. NIKs, on the other hand, have emerged as downstream components in signaling cascades, not functioning as coreceptors but rather serving as hubs that converge information from both biotic and abiotic signals, resulting in a unified response. Like SERKs, NIKs play a crucial role as information spreaders in plant cells, forming hubs of high centrality. However, unlike SERKs, which function as coreceptors and assemble paired receptor-specific responses, NIKs employ a shared signaling circuit to transduce diverse biotic and abiotic signals into the same physiological response. Therefore, this review highlights the concept of signaling hubs that differ from coreceptors in signaling pathways.

L-type lectin receptor-like kinase OsCORK1 as an important negative regulator confers copper stress tolerance in rice

Copper (Cu) is vital for plant growth but becomes toxic in excess, posing potential threats to human health. Although receptor-like kinases (RLKs) have been studied in plant response to abiotic stresses, their roles in Cu stress response remain poorly understood. Therefore, we aimed to evaluate Cu toxicity effects on rice and elucidate its potential molecular mechanisms. Specifically, rice lectin-type RLK OsCORK1 (Copper-response receptor-like kinase 1) function in Cu stress response was investigated. RNA sequencing and expression assays revealed that OsCORK1 is mainly expressed in roots and leaves, and its expression was significantly induced by Cu stress time- and dose-dependently. Kinase activity assays demonstrated OsCORK1 as a Mn2+-preferred functional kinase. Genetically, OsCORK1 gene-edited mutants exhibited increased tolerance to Cu stress and reduced Cu accumulation compared to the wild type (WT). Conversely, OsCORK1 overexpression compromised the Cu stress tolerance observed in OsCORK1 gene-edited mutants. OsCORK1 gene-edited mutants slightly damaged the root tips compared to the WT under Cu stress. Furthermore, OsCORK1 was demonstrated to modulate Cu stress tolerance by mainly altering cell wall components, particularly lignin, in rice. Overall, OsCORK1 is an important negative regulator of Cu stress tolerance, providing a potential gene target to reduce Cu pollution in rice production.

Cross-kingdom analyses of transmembrane protein kinases show their functional diversity and distinct origins in protists

Transmembrane kinases (TMKs) are important mediators of cellular signaling cascades. The kinase domains of most metazoan and plant TMKs belong to the serine/threonine/tyrosine kinase (S/T/Y-kinase) superfamily. They share a common origin with prokaryotic kinases and have diversified into distinct subfamilies. Diverse members of the eukaryotic crown radiation such as amoebae, ciliates, and red and brown algae (grouped here under the umbrella term "protists") have long diverged from higher eukaryotes since their ancient common ancestry, making them ideal organisms for studying TMK evolution. Here, we developed an accurate and high-throughput pipeline to predict TMKomes in cellular organisms. Cross-kingdom analyses revealed distinct features of TMKomes in each grouping. Two-transmembrane histidine kinases constitute the main TMKomes of bacteria, while metazoans, plants, and most protists have a large proportion of single-pass TM S/T/Y-kinases. Phylogenetic analyses classified most protist S/T/Y-kinases into three clades, with clades II and III specifically expanded in amoebae and oomycetes, respectively. In contrast, clade I kinases were widespread in all protists examined here, and likely shared a common origin with other eukaryotic S/T/Y-kinases. Functional annotation further showed that most non-kinase domains were grouping-specific, suggesting that their recombination with the more conserved kinase domains led to the divergence of S/T/Y-kinases. However, we also found that protist leucine-rich repeat (LRR)- and G-protein-coupled receptor (GPCR)-type TMKs shared similar sensory domain architectures with respective plant and animal TMKs, despite that they belong to distinct kinase subfamilies. Collectively, our study revealed the functional diversity of TMKomes and the distinct origins of S/T/Y-kinases in protists.

The Arabidopsis thaliana onset of leaf death 12 mutation in the lectin receptor kinase P2K2 results in an autoimmune phenotype

BACKGROUND

Plant immunity relies on the perception of immunogenic signals by cell-surface and intracellular receptors and subsequent activation of defense responses like programmed cell death. Under certain circumstances, the fine-tuned innate immune system of plants results in the activation of autoimmune responses that cause constitutive defense responses and spontaneous cell death in the absence of pathogens.

RESULTS

Here, we characterized the onset of leaf death 12 (old12) mutant that was identified in the Arabidopsis accession Landsberg erecta. The old12 mutant is characterized by a growth defect, spontaneous cell death, plant-defense gene activation, and early senescence. In addition, the old12 phenotype is temperature reversible, thereby exhibiting all characteristics of an autoimmune mutant. Mapping the mutated locus revealed that the old12 phenotype is caused by a mutation in the Lectin Receptor Kinase P2-TYPE PURINERGIC RECEPTOR 2 (P2K2) gene. Interestingly, the P2K2 allele from Landsberg erecta is conserved among Brassicaceae. P2K2 has been implicated in pathogen tolerance and sensing extracellular ATP. The constitutive activation of defense responses in old12 results in improved resistance against Pseudomonas syringae pv. tomato DC3000.

CONCLUSION

We demonstrate that old12 is an auto-immune mutant and that allelic variation of P2K2 contributes to diversity in Arabidopsis immune responses.

Peptide signaling in anther development and pollen-stigma interactions

Polypeptides play irreplaceable roles in cell-cell communication by binding to receptor-like kinases. Various types of peptide-receptor-like kinase-mediated signaling have been identified in anther development and male-female interactions in flowering plants. Here, we provide a comprehensive summary of the biological functions and signaling pathways of peptides and receptors involved in anther development, self-incompatibility, pollen tube growth and pollen tube guidance.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

The function of the plant cell wall in plant-microbe interactions

The plant cell wall is an interface of plant-microbe interactions. The ability of microbes to decompose cell wall polysaccharides contributes to microbial pathogenicity. Plants have evolved mechanisms to prevent cell wall degradation. However, the role of the cell wall in plant-microbe interactions is not well understood. Here, we discuss four functions of the plant cell wall-physical defence, storage of antimicrobial compounds, production of cell wall-derived elicitors, and provision of carbon sources-in the context of plant-microbe interactions. In addition, we discuss the four families of cell surface receptors associated with plant cell walls (malectin-like receptor kinase family, wall-associated kinase family, leucine-rich repeat receptor-like kinase family, and lysin motif receptor-like kinase family) that have been the subject of several important studies in recent years. This review summarises the findings on both plant cell wall and plant immunity, improving our understanding and may provide impetus to various researchers.

Liguleless narrow and narrow odd dwarf act in overlapping pathways to regulate maize development and metabolism

Narrow odd dwarf (nod) and Liguleless narrow (Lgn) are pleiotropic maize mutants that both encode plasma membrane proteins, cause similar developmental patterning defects, and constitutively induce stress signaling pathways. To investigate how these mutants coordinate maize development and physiology, we screened for protein interactors of NOD by affinity purification. LGN was identified by this screen as a strong candidate interactor, and we confirmed the NOD-LGN molecular interaction through orthogonal experiments. We further demonstrated that LGN, a receptor-like kinase, can phosphorylate NOD in vitro, hinting that they could act in intersecting signal transduction pathways. To test this hypothesis, we generated Lgn-R;nod mutants in two backgrounds (B73 and A619), and found that these mutations enhance each other, causing more severe developmental defects than either single mutation on its own, with phenotypes including very narrow leaves, increased tillering, and failure of the main shoot. Transcriptomic and metabolomic analyses of the single and double mutants in the two genetic backgrounds revealed widespread induction of pathogen defense genes and a shift in resource allocation away from primary metabolism in favor of specialized metabolism. These effects were similar in each single mutant and heightened in the double mutant, leading us to conclude that NOD and LGN act cumulatively in overlapping signaling pathways to coordinate growth-defense tradeoffs in maize.

A. thaliana Hybrids Develop Growth Abnormalities through Integration of Stress, Hormone and Growth Signaling

Hybrids between Arabidopsis thaliana accessions are important in revealing the consequences of epistatic interactions in plants. F1 hybrids between the A. thaliana accessions displaying either defense or developmental phenotypes have been revealing the roles of the underlying epistatic genes. The interaction of two naturally occurring alleles of the OUTGROWTH-ASSOCIATED KINASE (OAK) gene in Sha and Lag2-2, previously shown to cause a similar phenotype in a different allelic combination in A. thaliana, was required for the hybrid phenotype. Outgrowth formation in the hybrids was associated with reduced levels of salicylic acid, jasmonic acid and abscisic acid in petioles and the application of these hormones mitigated the formation of the outgrowths. Moreover, different abiotic stresses were found to mitigate the outgrowth phenotype. The involvement of stress and hormone signaling in outgrowth formation was supported by a global transcriptome analysis, which additionally revealed that TCP1, a transcription factor known to regulate leaf growth and symmetry, was downregulated in the outgrowth tissue. These results demonstrate that a combination of natural alleles of OAK regulates growth and development through the integration of hormone and stress signals and highlight the importance of natural variation as a resource to discover the function of gene variants that are not present in the most studied accessions of A. thaliana.

The plasma membrane as a mechanotransducer in plants

The plasma membrane is a physical boundary made of amphiphilic lipid molecules, proteins and carbohydrates extensions. Its role in mechanotransduction generates increasing attention in animal systems, where membrane tension is mainly induced by cortical actomyosin. In plant cells, cortical tension is of osmotic origin. Yet, because the plasma membrane in plant cells has comparable physical properties, findings from animal systems likely apply to plant cells too. Recent results suggest that this is indeed the case, with a role of membrane tension in vesicle trafficking, mechanosensitive channel opening or cytoskeleton organization in plant cells. Prospects for the plant science community are at least three fold: (i) to develop and use probes to monitor membrane tension in tissues, in parallel with other biochemical probes, with implications for protein activity and nanodomain clustering, (ii) to develop single cell approaches to decipher the mechanisms operating at the plant cell cortex at high spatio-temporal resolution, and (iii) to revisit the role of membrane composition at cell and tissue scale, by considering the physical implications of phospholipid properties and interactions in mechanotransduction.

PERK13 modulates phosphate deficiency-induced root hair elongation in Arabidopsis

Phosphate starvation (-Pi)-induced root hair is crucial for enhancing plants' Pi absorption. Proline-rich extensin-like receptor kinase 13 (PERK13) is transcriptionally induced by -Pi and co-expressed with genes associated with root hair growth. However, how PERK13 participates in -Pi-induced root hair growth remains unclear. Here, we found that PERK13 was transcriptionally responsive to Pi, nitrogen, and iron deficiencies. Loss of PERK13 function (perk13) enhanced root hair growth under Pi/nitrogen limitation. Similar phenotype was also observed in transgenic lines overexpressing PERK13 (PERK13ox). Under -Pi, both perk13 and PERK13ox showed prolonged root hair elongation and increased reactive oxygen species (ROS). Deletion analysis showed, in PERK13ox, the extracellular domain was indispensable for PERK13 in -Pi-induced root hair growth. Different transcription profiles were observed under -Pi between perk13 and PERK13ox with the jasmonate zim-domain genes being repressed in perk13 and genes involved in cell wall remodeling being increased in PERK13ox. Taken together, we demonstrated that PERK13 participates in -Pi-induced root hair growth probably via regulating root hair elongation and the generation of ROS. Our study also suggested PERK13 probably being a vital hub coupling the environmental cues and root hair growth, and might play dual roles in -Pi-induced root hair growth via different processes.

Beyond gene ontology (GO): using biocuration approach to improve the gene nomenclature and functional annotation of rice S-domain kinase subfamily

The S-domain subfamily of receptor-like kinases (SDRLKs) in plants is poorly characterized. Most members of this subfamily are currently assigned gene function based on the S-locus Receptor Kinase from Brassica that acts as the female determinant of self-incompatibility (SI). However, Brassica like SI mechanisms does not exist in most plants. Thus, automated Gene Ontology (GO) pipelines are not sufficient for functional annotation of SDRLK subfamily members and lead to erroneous association with the GO biological process of SI. Here, we show that manual bio-curation can help to correct and improve the gene annotations and association with relevant biological processes. Using publicly available genomic and transcriptome datasets, we conducted a detailed analysis of the expansion of the rice (Oryza sativa) SDRLK subfamily, the structure of individual genes and proteins, and their expression.The 144-member SDRLK family in rice consists of 82 receptor-like kinases (RLKs) (67 full-length, 15 truncated),12 receptor-like proteins, 14 SD kinases, 26 kinase-like and 10 GnK2 domain-containing kinases and RLKs. Except for nine genes, all other SDRLK family members are transcribed in rice, but they vary in their tissue-specific and stress-response expression profiles. Furthermore, 98 genes show differential expression under biotic stress and 98 genes show differential expression under abiotic stress conditions, but share 81 genes in common.Our analysis led to the identification of candidate genes likely to play important roles in plant development, pathogen resistance, and abiotic stress tolerance. We propose a nomenclature for 144 SDRLK gene family members based on gene/protein conserved structural features, gene expression profiles, and literature review. Our biocuration approach, rooted in the principles of findability, accessibility, interoperability and reusability, sets forth an example of how manual annotation of large-gene families can fill in the knowledge gap that exists due to the implementation of automated GO projections, thereby helping to improve the quality and contents of public databases.

Receptor-like kinase OsCR4 controls leaf morphogenesis and embryogenesis by fixing the distribution of auxin in rice

Cell differentiation is a key event in organ development; it involves auxin gradient formation, cell signaling, and transcriptional regulation. Yet, how these processes are orchestrated during leaf morphogenesis is poorly understood. Here, we demonstrate an essential role for the receptor-like kinase OsCR4 in leaf development. oscr4 loss-of-function mutants displayed short shoots and roots, with tiny, crinkly, or even dead leaves. The delayed outgrowth of the first three leaves and seminal root in oscr4 was due to defects in plumule and radicle formation during embryogenesis. The deformed epidermal, mesophyll, and vascular tissues observed in oscr4 leaves arose at the postembryo stage; the corresponding expression pattern of proOsCR4:GUS in embryos and young leaves suggests that OsCR4 functions in these tissues. Signals from the auxin reporter DR5rev:VENUS were found to be altered in oscr4 embryos and disorganized in oscr4 leaves, in which indole-3-acetic acid accumulation was further revealed by immunofluorescence. OsWOX3A, which is auxin responsive and related to leaf development, was activated extensively and ectopically in oscr4 leaves, partially accounting for the observed lack of cell differentiation. Our data suggest that OsCR4 plays a fundamental role in leaf morphogenesis and embryogenesis by fixing the distribution of auxin.

MINI SEED 2 (MIS2) Encodes a Receptor-like Kinase that Controls Grain Size and Shape in Rice

BACKGROUND

Grain size is a key agronomic trait that is directly associated with grain yield in rice. Although several genes related to grain size in rice have been identified, our understanding of the mechanism of grain development is still limited.

RESULTS

In this study, we reported the characterization of a novel seed size mutant mini seed 2 (mis2), in which the grain showed reduced length, width and thickness along with wrinkled surface. Microscopic analysis revealed that the spikelet epidermal cell size was reduced but the cell number was increased in the mis2 mutant, suggesting that MIS2 controls grain size by coordinately regulating epidermal cell size and cell number. Map-based cloning revealed that MIS2 encodes a receptor-like kinase CRINKLY4 (CR4) which showed the highest expression in developing panicles. The MIS2 protein is localized primarily on the plasma membrane along with the endosome. However, the Arg258Gln mutation located in extracellular domain in the mis2 mutant disturbed its subcellular localization. Additionally, three major haplotypes of MIS2 were identified in the japonica, indica and aus rice cultivars. The 18-bp InDel (insertion and deletion) in the 5'-UTR (untranslated region) caused different expression level of MIS2 in haplotypes.

CONCLUSIONS

We reported a key role of OsCR4 in controlling grain size and shape by coordinately regulating epidermal cell size and cell number. The Arg258 in the extracellular seven-repeat domain is essential for the correct subcellular behavior and function of the OsCR4 protein.

Plant cell surface molecular cypher: Receptor-like proteins and their roles in immunity and development

Plant receptor-like proteins (RLPs) are a family of transmembrane receptors which are distinguished from receptor-like kinases (RLKs) by their lack of a cytoplasmic kinase domain. RLPs continue to be implicated in a broad range of plant immunological and developmental processes as critical sensors or participants in receptor complexes on the plasma membrane. RLPs often associate with RLKs to activate or attenuate signal perception and relay. Some RLPs also physically cluster with RLKs and bear similar expression patterns. Here, we discuss the characteristics, function, and expression of characterized RLPs in the context of their associated RLKs in plant immunity and development.

The role of receptor-like kinases in regulating plant male reproduction

RLKs in anther development. The cell-to-cell communication is essential for specifying different cell types during plant growth, development and adaption to the ever-changing environment. Plant male reproduction, in particular, requires the exquisitely synchronized development of different cell layers within the male tissue, the anther. Receptor-like kinases (RLKs) belong to a large group of kinases localized on the cell surfaces, perceiving extracellular signals and thereafter regulating intracellular processes. Here we update the role of RLKs in early anther development by defining the cell fate and anther patterning, responding to the changing environment and controlling anther carbohydrate metabolism. We provide speculation of the poorly characterized ligands and substrates of these RLKs. The conserved and diversified aspects underlying the function of RLKs in anther development are discussed.

Plant cell surface receptor-mediated signaling - a common theme amid diversity

Sessile plants employ a diverse array of plasma membrane-bound receptors to perceive endogenous and exogenous signals for regulation of plant growth, development and immunity. These cell surface receptors include receptor-like kinases (RLKs) and receptor-like proteins (RLPs) that harbor different extracellular domains for perception of distinct ligands. Several RLK and RLP signaling pathways converge at the somatic embryogenesis receptor kinases (SERKs), which function as shared co-receptors. A repertoire of receptor-like cytoplasmic kinases (RLCKs) associate with the receptor complexes to relay intracellular signaling. Downstream of the receptor complexes, mitogen-activated protein kinase (MAPK) cascades are among the key signaling modules at which the signals converge, and these cascades regulate diverse cellular and physiological responses through phosphorylation of different downstream substrates. In this Review, we summarize the emerging common theme that underlies cell surface receptor-mediated signaling pathways in Arabidopsisthaliana: the dynamic association of RLKs and RLPs with specific co-receptors and RLCKs for signal transduction. We further discuss how signaling specificities are maintained through modules at which signals converge, with a focus on SERK-mediated receptor signaling.

Post genomics era for orchid research

Among 300,000 species in angiosperms, Orchidaceae containing 30,000 species is one of the largest families. Almost every habitats on earth have orchid plants successfully colonized, and it indicates that orchids are among the plants with significant ecological and evolutionary importance. So far, four orchid genomes have been sequenced, including Phalaenopsis equestris, Dendrobium catenatum, Dendrobium officinale, and Apostaceae shengen. Here, we review the current progress and the direction of orchid research in the post genomics era. These include the orchid genome evolution, genome mapping (genome-wide association analysis, genetic map, physical map), comparative genomics (especially receptor-like kinase and terpene synthase), secondary metabolomics, and genome editing.

The cell wall-localized atypical β-1,3 glucanase ZERZAUST controls tissue morphogenesis in Arabidopsis thaliana

Orchestration of cellular behavior in plant organogenesis requires integration of intercellular communication and cell wall dynamics. The underlying signaling mechanisms are poorly understood. Tissue morphogenesis in Arabidopsis depends on the receptor-like kinase STRUBBELIG. Mutations in ZERZAUST were previously shown to result in a strubbelig-like mutant phenotype. Here, we report on the molecular identification and functional characterization of ZERZAUST We show that ZERZAUST encodes a putative GPI-anchored β-1,3 glucanase suggested to degrade the cell wall polymer callose. However, a combination of in vitro, cell biological and genetic experiments indicate that ZERZAUST is not involved in the regulation of callose accumulation. Nonetheless, Fourier-transformed infrared-spectroscopy revealed that zerzaust mutants show defects in cell wall composition. Furthermore, the results indicate that ZERZAUST represents a mobile apoplastic protein, and that its carbohydrate-binding module family 43 domain is required for proper subcellular localization and function whereas its GPI anchor is dispensable. Our collective data reveal that the atypical β-1,3 glucanase ZERZAUST acts in a non-cell-autonomous manner and is required for cell wall organization during tissue morphogenesis.

Ligand Receptor-Mediated Regulation of Growth in Plants

Growth and development of multicellular organisms are coordinately regulated by various signaling pathways involving the communication of inter- and intracellular components. To form the appropriate body patterns, cellular growth and development are modulated by either stimulating or inhibiting these pathways. Hormones and second messengers help to mediate the initiation and/or interaction of the various signaling pathways in all complex multicellular eukaryotes. In plants, hormones include small organic molecules, as well as larger peptides and small proteins, which, as in animals, act as ligands and interact with receptor proteins to trigger rapid biochemical changes and induce the intracellular transcriptional and long-term physiological responses. During the past two decades, the availability of genetic and genomic resources in the model plant species, Arabidopsis thaliana, has greatly helped in the discovery of plant hormone receptors and the components of signal transduction pathways and mechanisms used by these immobile but highly complex organisms. Recently, it has been shown that two of the most important plant hormones, auxin and abscisic acid (ABA), act through signaling pathways that have not yet been recognized in animals. For example, auxins stimulate cell elongation by bringing negatively acting transcriptional repressor proteins to the proteasome to be degraded, thus unleashing the gene expression program required for increasing cell size. The "dormancy" inducing hormone, ABA, binds to soluble receptor proteins and inhibits a specific class of protein phosphatases (PP2C), which activates phosphorylation signaling leading to transcriptional changes needed for the desiccation of the seeds prior to entering dormancy. While these two hormone receptors have no known animal counterparts, there are also many similarities between animal and plant signaling pathways. For example, in plants, the largest single gene family in the genome is the protein kinase family (approximately 5% of the protein coding genes), although the specific function for only a few dozen of these kinases is clearly established. Recent comparative genomics studies have revealed that parasitic nematodes and pathogenic microbes produce plant peptide hormone mimics that target specific plant plasma membrane receptor-like protein kinases, thus usurping endogenous signaling pathways for their own pathogenic purposes. With biochemical, genetic, and physiological analyses of the regulation of hormone receptor signal pathways, we are thus just now beginning to understand how plants optimize the development of their body shape and cope with constantly changing environmental conditions.

Comparative proteomic analysis reveals a dynamic pollen plasma membrane protein map and the membrane landscape of receptor-like kinases and transporters important for pollen tube growth and interaction with pistils in rice

BACKGROUND

The coordination of pollen tube (PT) growth, guidance and timely growth arrest and rupture mediated by PT-pistil interaction is crucial for the PT to transport sperm cells into ovules for double fertilization. The plasma membrane (PM) represents an important interface for cell-cell interaction, and PM proteins of PTs are pioneers for mediating PT integrity and interaction with pistils. Thus, understanding the mechanisms underlying these events is important for proteomics.

RESULTS

Using the efficient aqueous polymer two-phase system and alkali buffer treatment, we prepared high-purity PM from mature and germinated pollen of rice. We used iTRAQ quantitative proteomic methods and identified 1,121 PM-related proteins (PMrPs) (matched to 899 loci); 192 showed differential expression in the two pollen cell types, 119 increased and 73 decreased in abundance during germination. The PMrP and differentially expressed PMrP sets all showed a functional skew toward signal transduction, transporters, wall remodeling/metabolism and membrane trafficking. Their genomic loci had strong chromosome bias. We found 37 receptor-like kinases (RLKs) from 8 kinase subfamilies and 209 transporters involved in flux of diversified ions and metabolites. In combination with the rice pollen transcriptome data, we revealed that in general, the protein expression of these PMrPs disagreed with their mRNA expression, with inconsistent mRNA expression for 74% of differentially expressed PMrPs.

CONCLUSIONS

This study identified genome-wide pollen PMrPs, and provided insights into the membrane profile of receptor-like kinases and transporters important for pollen tube growth and interaction with pistils. These pollen PMrPs and their mRNAs showed discordant expression. This work provides resource and knowledge to further dissect mechanisms by which pollen or the PT controls PMrP abundance and monitors interactions and ion and metabolite exchanges with female cells in rice.

Immunoprecipitation of Plasma Membrane Receptor-Like Kinases for Identification of Phosphorylation Sites and Associated Proteins

Membrane proteins are difficult to study for numerous reasons. The surface of membrane proteins is relatively hydrophobic and sometimes very unstable, additionally requiring detergents for their extraction from the membrane. This leads to challenges at all levels, including expression, solubilization, purification, identification of associated proteins, and the identification of post-translational modifications. However, recent advances in immunoprecipitation technology allow to isolate membrane proteins efficiently, facilitating the study of protein-protein interactions, the identification of novel associated proteins, and to identify post-translational modifications, such as phosphorylation. Here, we describe an optimized immunoprecipitation protocol for plant plasma membrane receptor-like kinases.

The receptor-like kinase SlSOBIR1 is differentially modulated by virus infection but its overexpression in tobacco has no significant impact on virus accumulation

KEY MESSAGE

The role of the tomato receptor-like kinase SlSOBIR1 in antiviral defense was investigated. SlSOBIR1 was transcriptionally modulated by unrelated viruses but its ectopic expression had no effect on virus accumulation. Leucine-rich repeat receptor-like kinases (LRR-RLK) constitute a diverse group of proteins allowing the cell to recognize and respond to the extracellular environment. In the present study we focused on a gene encoding a tomato LRR-RLK (named SlSOBIR1) involved in the host defense against fungal pathogens. Curiously, SlSOBIR1 has been previously reported to be down-regulated by Pepper yellow mosaic virus (PepYMV) infection. Here, we show that SlSOBIR1 is responsive to wounding and differentially modulated by unrelated virus infection, i.e., down-regulated by PepYMV and up-regulated by Tomato chlorotic spot virus (TCSV). Despite these divergent expression profiles, SlSOBIR1 overexpression in transgenic tobacco plants had no evident effect on TCSV and PepYMV accumulation. On the other hand, overexpression of SlSOBIR1 significantly increased the expression of selected defense genes (PR-1a and PR-6) and exacerbated superoxide production in wounded leaves. Our data indicate that the observed modulation of SlSOBIR1 expression is probably triggered by secondary effects of the virus infection process and suggest that SlSOBIR1 is not directly involved in antiviral signaling response.

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

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

A Receptor-Like Kinase, Related to Cell Wall Sensor of Higher Plants, is Required for Sexual Reproduction in the Unicellular Charophycean Alga, Closterium peracerosum-strigosum-littorale Complex

Here, we cloned the CpRLK1 gene, which encodes a receptor-like protein kinase expressed during sexual reproduction, from the heterothallic Closterium peracerosum-strigosum-littorale complex, one of the closest unicellular alga to land plants. Mating-type plus (mt(+)) cells with knockdown of CpRLK1 showed reduced competence for sexual reproduction and formed an abnormally enlarged conjugation papilla after pairing with mt(-) cells. The knockdown cells were unable to release a naked gamete, which is indispensable for zygote formation. We suggest that the CpRLK1 protein is an ancient cell wall sensor that now functions to regulate osmotic pressure in the cell to allow proper gamete release.

An update on receptor-like kinase involvement in the maintenance of plant cell wall integrity

BACKGROUND

Plant cell walls form the interface between the cells and their environment. They perform different functions, such as protecting cells from biotic and abiotic stress and providing structural support during development. Maintenance of the functional integrity of cell walls during these different processes is a prerequisite that enables the walls to perform their particular functions. The available evidence suggests that an integrity maintenance mechanism exists in plants that is capable of both detecting wall integrity impairment caused by cell wall damage and initiating compensatory responses to maintain functional integrity. The responses involve 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonic acid, reactive oxygen species and calcium-based signal transduction cascades as well as the production of lignin and other cell wall components. Experimental evidence implicates clearly different signalling molecules, but knowledge regarding contributions of receptor-like kinases to this process is less clear. Different receptor-like kinase families have been considered as possible sensors for perception of cell wall damage; however, strong experimental evidence that provides insights into functioning exists for very few kinases.

SCOPE AND CONCLUSIONS

This review examines the involvement of cell wall integrity maintenance in different biological processes, defines what constitutes plant cell wall damage that impairs functional integrity, clarifies which stimulus perception and signal transduction mechanisms are required for integrity maintenance and assesses the available evidence regarding the functions of receptor-like kinases during cell wall integrity maintenance. The review concludes by discussing how the plant cell wall integrity maintenance mechanism could form an essential component of biotic stress responses and of plant development, functions that have not been fully recognized to date.

The endoplasmic reticulum-quality control component SDF2 is essential for XA21-mediated immunity in rice

Plant genomes contain large number of plasma membrane (PM)-localized immune receptors, also called pattern recognition receptors (PRRs). PRRs are synthesized in the endoplasmic reticulum (ER) and then translocated to the PM, where they recognize conserved pathogen-associated molecular patterns (PAMPs) and activate innate immune response. The rice XA21 immune receptor confers resistance to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). To identify components that mediate XA21-mediated signaling, we performed co-purification experiments using C-terminal GFP tagged XA21 protein. Several endoplasmic reticulum-quality control (ER-QC) proteins including stromal-derived factor 2 (SDF2) co-purified with XA21. Silencing of the SDF2 genes in the XA21 rice genetic background compromises resistance to Xoo but does not affect plant growth and development.

Autophosphorylation of gatekeeper tyrosine by symbiosis receptor kinase

Plant receptor-like kinases (RLKs) share their evolutionary origin with animal interleukin-1 receptor-associated kinase (IRAK)/Pelle family of soluble kinases and are distinguished by having tyrosine as 'gatekeeper'. This position is adjacent to the hinge region and is hidden in a hydrophobic pocket of the catalytic cleft of protein kinases and is therefore least probable to be a target for any modification. This communication illustrates the accessibility of the gatekeeper site (Y670) towards both autophosphorylation and dephosphorylation in the recombinant cytoplasmic domain of symbiosis receptor kinase from Arachis hypogaea (AhSYMRK). Autophosphorylation on gatekeeper tyrosine was detected prior to extraction but never under in vitro conditions. We hypothesize gatekeeper phosphorylation to be associated with synthesis/maturation of AhSYMRK and this phenomenon may be prevalent among RLKs.