Receptor Kinases and Signal Pathway in the Arbuscular Mycorrhizal Symbiosis

Most terrestrial plants establish symbiotic interactions with arbuscular mycorrhizal fungi (AMF) to acquire phosphorus and nitrogen nutrients. The current understanding regarding how plants recognize symbiotic signals has now been updated. Plant Lysin-Motif receptor kinases, that is, rice OsCERK1 and OsMYR1 or orthologs from other plants, perceive Myc factor, a lipochitooligosaccharide from AMF, to initiate symbiotic signaling pathway. The Myc factor receptor model is quite similar to the known Nod factor receptors required for rhizobial symbiosis and chitin receptors for chitin-triggered immunity. Thus, the open question is how plants use similar receptor complexes to recognize structurally similar molecules to induce different signaling pathways. Upon recognition of Myc/Nod factors signaling, LysM receptors could activate the symbiosis receptor kinase (SymRK), which is an essential component of common symbiotic signaling pathway (CSSP) for both mycorrhizal symbiosis and rhizobial symbiosis. Downstream of SymRK, a clear module in the CSSP by CCaMK-CYCLOPS-DELLA was identified to promote both mycorrhizal symbiosis by activating the expression of RAM1, and rhizobial symbiosis by forming a complex with NSP1/NSP2 to regulate the expression of NIN. In this chapter, we discussed the roles of receptor kinases and CSSP in mycorrhizal symbiosis, as well as in rhizobial symbiosis

Bradyrhizobium diazoefficiens USDA 110–glycine max interactome provides candidate proteins associated with symbiosis

Although the legume−rhizobium symbiosis is a most-important biological process, there is a limited knowledge about the protein interaction network between host and symbiont. Using interolog- and domain-based approaches, we constructed an interspecies protein interactome containing 5115 protein−protein interactions between 2291 Glycine max and 290 Bradyrhizobium diazoefficiens USDA 110 proteins. The interactome was further validated by the expression pattern analysis in nodules, gene ontology term semantic similarity, co-expression analysis, and luciferase complementation image assay. In the G. max−B. diazoefficiens interactome, bacterial proteins are mainly ion channel and transporters of carbohydrates and cations, while G. max proteins are mainly involved in the processes of metabolism, signal transduction, and transport. We also identified the top 10 highly interacting proteins (hubs) for each species. Kyoto Encyclopedia of Genes and Genomes pathway analysis for each hub showed that a pair of 14-3-3 proteins (SGF14g and SGF14k) and 5 heat shock proteins in G. max are possibly involved in symbiosis, and 10 hubs in B. diazoefficiens may be important symbiotic effectors. Subnetwork analysis showed that 18 symbiosis-related soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins may play roles in regulating bacterial ion channels, and SGF14g and SGF14k possibly regulate the rhizobium dicarboxylate transport protein DctA. The predicted interactome provide a valuable basis for understanding the molecular mechanism of nodulation in soybean

Extracellular ATP acts as a damage-associated molecular pattern (DAMP) signal in plants

As sessile organisms, plants have evolved effective mechanisms to protect themselves from environmental stresses. Damaged (i.e., wounded) plants recognize a variety of endogenous molecules as danger signals, referred to as damage-associated molecular patterns (DAMPs). ATP is among the molecules that are released by cell damage, and recent evidence suggests that ATP can serve as a DAMP. Although little studied in plants, extracellular ATP is well known for its signaling roles in animals, including acting as a DAMP during the inflammatory response and wound healing. If ATP acts outside the cell, then it is reasonable to expect that it is recognized by a plasma membrane-localized receptor. Recently, DORN1, a lectin receptor kinase, was shown to recognize extracellular ATP in Arabidopsis . DORN1 is the founding member of a new purinoceptor subfamily, P2K (P2 receptor kinase), which is plant-specific. P2K1 (DORN1) is required for ATP-induced cellular responses (e.g., cytosolic Ca 2+ elevation, MAPK phosphorylation, and gene expression). Genetic analysis of loss-of-function mutants and overexpression lines showed that P2K1 participates in the plant wound response, consistent with the role of ATP as a DAMP. In this review, we summarize past research on the roles and mechanisms of extracellular ATP signaling in plants, and discuss the direction of future research on extracellular ATP as a DAMP signal

The Perspective of Arbuscular Mycorrhizal Symbiosis in Rice Domestication and Breeding.

In nature, symbiosis with arbuscular mycorrhizal (AM) fungi contributes to sustainable acquisition of phosphorus and other elements in over 80% of plant species; improving interactions with AM symbionts may mitigate some of the environmental problems associated with fertilizer application in grain crops such as rice. Recent developments of high-throughput genome sequencing projects of thousands of rice cultivars and the discovery of the molecular mechanisms underlying AM symbiosis suggest that interactions with AM fungi might have been an overlooked critical trait in rice domestication and breeding. In this review, we discuss genetic variation in the ability of rice to form AM symbioses and how this might have affected rice domestication. Finally, we discuss potential applications of AM symbiosis in rice breeding for more sustainable agriculture

Role of LysM receptors in chitin-triggered plant innate immunity

Recent research findings clearly indicate that lysin motif (LysM)-containing cell surface receptors are involved in the recognition of specific oligosaccharide elicitors (chitin and peptidoglycan), which trigger an innate immunity response in plants. These receptors are either LysM-containing receptor-like kinases (LYKs) or LysM-containing receptor proteins (LYPs). In Arabidopsis, five LYKs (AtCERK1/AtLYK1 and AtLYK2-5) and three LYPs (AtLYP1-3) are likely expressed on the plasma membrane. In this review, we summarize recent research results on the role of these receptors in plant innate immunity, including the recent structural characterization of AtCERK1 and composition of the various receptor complexes in Arabidopsis

LIK1, A CERK1-Interacting Kinase, Regulates Plant Immune Responses in <i>Arabidopsis</i>

<div><p>Chitin, an integral component of the fungal cell wall, is one of the best-studied microbe-associated molecular patterns. Previous work identified a LysM receptor-like kinase (LysM-RLK1/CERK1) as the primary chitin receptor in <i>Arabidopsis</i>. In order to identify proteins that interact with CERK1, we conducted a yeast two-hybrid screen using the intracellular kinase domain of CERK1 as the bait. This screen identified 54 putative CERK1-interactors. Screening mutants defective in 43 of these interacting proteins identified only two, a calmodulin like protein (At3g10190) and a leucine-rich repeat receptor like kinase (At3g14840), which differed in their response to pathogen challenge. In the present work, we focused on characterizing the LRR-RLK gene where mutations altered responses to chitin elicitation. This LRR-RLK was named LysM RLK1-interacting kinase 1 (LIK1). The interaction between CERK1 and LIK1 was confirmed by co-immunoprecipitation using protoplasts and transgenic plants. <i>In vitro</i> experiments showed that LIK1 was directly phosphorylated by CERK1. <i>In vivo</i> phosphorylation assays showed that Col-0 wild-type plants have more phosphorylated LIK1 than <i>cerk1</i> mutant plants, suggesting that LIK1 may be directly phosphorylated by CERK1. <i>Lik1</i> mutant plants showed an enhanced response to both chitin and flagellin elicitors. In comparison to the wild-type plants, <i>lik1</i> mutant plants were more resistant to the hemibiotrophic pathogen <i>Pseudomonas syringae</i>, but more susceptible to the necrotrophic pathogen <i>Sclerotinia sclerotiorum</i>. Consistent with the enhanced susceptibility to necrotrophs, <i>lik1</i> mutants showed reduced expression of genes involved in jasmonic acid and ethylene signaling pathways. These data suggest that LIK1 directly interacts with CERK1 and regulates MAMP-triggered innate immunity.</p></div

Probing the Arabidopsis Flagellin Receptor: FLS2-FLS2 Association and the Contributions of Specific Domains to Signaling Function[W][OA]

Transmembrane LRR-RLKs are a major class of plant proteins. This study investigates the functional contributions of multiple FLS2 protein domains and modifications to provide insight into structure-function relationships of LRR-RLK proteins in general

Computational Analysis of the Ligand Binding Site of the Extracellular ATP Receptor, DORN1

DORN1 (also known as P2K1) is a plant receptor for extracellular ATP, which belongs to a large gene family of legume-type (L-type) lectin receptor kinases. Extracellular ATP binds to DORN1 with strong affinity through its lectin domain, and the binding triggers a variety of intracellular activities in response to biotic and abiotic stresses. However, information on the tertiary structure of the ligand binding site of DORN1is lacking, which hampers efforts to fully elucidate the mechanism of receptor action. Available data of the crystal structures from more than 50 L-type lectins enable us to perform an in silico study of molecular interaction between DORN1 and ATP. In this study, we employed a computational approach to develop a tertiary structure model of the DORN1 lectin domain. A blind docking analysis demonstrated that ATP binds to a cavity made by four loops (defined as loops A B, C and D) of the DORN1 lectin domain with high affinity. In silico target docking of ATP to the DORN1 binding site predicted interaction with 12 residues, located on the four loops, via hydrogen bonds and hydrophobic interactions. The ATP binding pocket is structurally similar in location to the carbohydrate binding pocket of the canonical L-type lectins. However, four of the residues predicted to interact with ATP are not conserved between DORN1 and the other carbohydrate-binding lectins, suggesting that diversifying selection acting on these key residues may have led to the ATP binding activity of DORN1. The in silico model was validated by in vitro ATP binding assays using the purified extracellular lectin domain of wild-type DORN1, as well as mutated DORN1 lacking key ATP binding residues

<i>Lik1</i> mutant plants are more resistant to <i>Pseudomonas syringae</i> pv. <i>tomato</i> DC3000.

<p>A. Ten-day-old mutant and wild-type seedlings were inoculated with <i>P. syringae</i> pv. <i>Tomato</i> DC3000 at the concentration of 0.5×10⁴ cfu/ml by soaking in a bacterial suspension for 3 hours. The bacterial solution was then removed and the seedlings were washed three times with H₂O prior to incubation. Bacterial growth was measured by grinding the seedlings and then plating the resulting extracts on YPD medium with rifampicin and kanamycin as selection. The data are shown as the log10 of colony forming units (3 hours and 48 hours after inoculation) per seedling. The data are the average of 18 seedlings. Bars represent standard deviations. Student T-test (**) P<0.01. The experiment was done in triplicate, each with similar results. B. Leaf populations of <i>P. syringae</i> pv. <i>tomato</i> strain DC3000 from 4-week-old. Data are mean ±SE for three separate experiments. Student T-test (*) P<0.05.</p