OsBSK1-2, an Orthologous of AtBSK1, Is Involved in Rice Immunity

The receptor-like cytoplasmic kinase OsBSK1-2 regulates immunity via an HLH/bHLH complex

Plants need to fine-tune defense responses to maintain a robust but flexible host barrier to various pathogens. Helix-loop-helix/basic helix-loop-helix (HLH/bHLH) complexes play important roles in fine-tuning plant development. However, the function of these genes in plant immunity and how they are regulated remain obscure. Here, we identified an atypical bHLH transcription factor, Oryza sativa (Os)HLH46, that interacts with rice receptor-like cytoplasmic kinase (RLCK) Os BRASSINOSTEROID-SIGNALING KINASE1-2 (OsBSK1-2), which plays a key role in rice blast resistance. OsBSK1-2 stabilized OsHLH46 both in vivo and in vitro. In addition, OsHLH46 positively regulates rice blast resistance, which depends on OsBSK1-2. OsHLH46 has no transcriptional activation activity and interacts with a typical bHLH protein, OsbHLH6, which negatively regulates rice blast resistance. OsbHLH6 binds to the promoter of OsWRKY45 and inhibits its expression, while OsHLH46 suppresses the function of OsbHLH6 by blocking its DNA binding and transcriptional inhibition of OsWRKY45. Consistent with these findings, OsWRKY45 was up-regulated in OsHLH46-overexpressing plants. In addition, the oshlh46 mutant overexpressing OsbHLH6 is more susceptible to Magnaporthe oryzae than is the wild type, suggesting that OsHLH46 suppresses OsbHLH6-mediated rice blast resistance. Our results not only demonstrated that OsBSK1-2 regulates rice blast resistance via the OsHLH46/OsbHLH6 complex, but also uncovered a new mechanism for plants to fine-tune plant immunity by regulating the HLH/bHLH complex via RLCKs.

Receptor-like cytoplasmic kinases: orchestrating plant cellular communication

The receptor-like kinase (RLK) family of receptors and the associated receptor-like cytoplasmic kinases (RLCKs) have expanded in plants because of selective pressure from environmental stress and evolving pathogens. RLCKs link pathogen perception to activation of coping mechanisms. RLK-RLCK modules regulate hormone synthesis and responses, reactive oxygen species (ROS) production, Ca2+ signaling, activation of mitogen-activated protein kinase (MAPK), and immune gene expression, all of which contribute to immunity. Some RLCKs integrate responses from multiple receptors recognizing distinct ligands. RLKs/RLCKs and nucleotide-binding domain, leucine-rich repeats (NLRs) were found to synergize, demonstrating the intertwined genetic network in plant immunity. Studies in arabidopsis (Arabidopsis thaliana) have provided paradigms about RLCK functions, but a lack of understanding of crop RLCKs undermines their application. In this review, we summarize current understanding of the diverse functions of RLCKs, based on model systems and observations in crop species, and the emerging role of RLCKs in pathogen and abiotic stress response signaling.

Brassinosteroid-signaling kinase ZmBSK7 enhances salt stress tolerance in maize

Salinity has become a crucial environmental factor that restricts plant growth, development, and productivity. Nevertheless, the mechanisms by which plants react to salt stress remain inadequately comprehended. In this study, we identified maize brassinosteroid-signaling kinase gene ZmBSK7 which is homologous to AtBSK1. Our results showed that ZmBSK7 is induced by salt stress and ZmBSK7 localizes in the plasma membrane. ZmBSK7 overexpression increases salt tolerance, while its knockdown decreases salt tolerance in maize. ZmBSK7 reduces the malondialdehyde (MDA) content and the percentage of electrolyte leakage, and also elevates the activities of antioxidant enzymes. Furthermore, ZmBSK7 promotes K+ content accumulation and reduces Na+/K+ ratio. Further found that ZmBSK7 physically interacts with K+ efflux antiporter 2 (ZmKEA2) in vivo and in vitro. Salt stress also increased the expression of ZmKEA2. Thus, ZmBSK7 improves salt tolerance in maize by affecting ZmKEA2 expression to promote K+ content accumulation and reduce Na+/K+ ratio. This study enhances the comprehension of BSK proteins and establishes a theoretical foundation for investigating salt stress tolerance in plants.

Carbohydrate elicitor-induced plant immunity: Advances and prospects

The perceived negative impacts of synthetic agrochemicals gave way to alternative, biological plant protection strategies. The deployment of induced resistance, comprising boosting the natural defense responses of plants, is one of those. Plants developed multi-component defense mechanisms to defend themselves against biotic and abiotic stresses. These are activated upon recognition of stress signatures via membrane-localized receptors. The induced immune responses enable plants to tolerate and limit the impact of stresses. A systemic cascade of signals enables plants to prime un-damaged tissues, which is crucial during secondary encounters with stress. Comparable stress tolerance mechanisms can be induced in plants by the application of carbohydrate elicitors such as chitin/chitosan, β-1,3-glucans, oligogalacturonides, cellodextrins, xyloglucans, alginates, ulvans, and carrageenans. Treating plants with carbohydrate-derived elicitors enable the plants to develop resistance appliances against diverse stresses. Some carbohydrates are also known to have been involved in promoting symbiotic signaling. Here, we review recent progresses on plant resistance elicitation effect of various carbohydrate elicitors and the molecular mechanisms of plant cell perception, cascade signals, and responses to cascaded cues. Besides, the molecular mechanisms used by plants to distinguish carbohydrate-induced immunity signals from symbiotic signals are discussed. The structure-activity relationships of the carbohydrate elicitors are also described. Furthermore, we forwarded future research outlooks that might increase the utilization of carbohydrate elicitors in agriculture in order to improve the efficacy of plant protection strategies.

BRASSINOSTEROID-SIGNALING KINASE1 associates with and is required for cysteine protease RESPONSE TO DEHYDRATION 19-mediated disease resistance in Arabidopsis

The receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALING KINASE1 (BSK1) interacts with pattern recognition receptor (PRR) FLAGELLIN SENSING2 (FLS2) and positively regulates plant innate immunity in Arabidopsis thaliana. However, the molecular components involved in BSK1-mediated immune signaling remain largely unknown. To further explore the molecular mechanism underlying BSK1-mediated disease resistance, we screened two cysteine proteases, RESPONSE TO DEHYDRATION 19 (RD19) and RD19-LIKE 2 (RDL2), as BSK1-binding partners. Overexpression of RD19, but not RDL2, displayed an autoimmune phenotype, presenting programmed cell death and enhanced resistance to multiple pathogens. Interestingly, RD19-mediated immune activation depends on BSK1, as knockout of BSK1 in RD19-overexpressing plants rescued their autoimmunity and abolished the increased resistance. Furthermore, we found that BSK1 plays a positive role in maintaining RD19 protein abundance in Arabidopsis. Our results provide new insights into BSK1-mediated immune signaling and reveal a potential mechanism by which BSK1 stabilizes RD19 to promote effective immune output.

Transcriptomic and functional analyzes reveal that the brassinosteroid insensitive 1 receptor (OsBRI1) regulates cold tolerance in rice

Brassinosteroids (BR) play crucial roles in plant development and abiotic stresses in plants. Exogenous application of BR can significantly enhance cold tolerance in rice. However, the regulatory relationship between cold tolerance and the BR signaling pathway in rice remains largely unknown. Here, we characterized functions of the BR receptor OsBRI1 in response to cold tolerance in rice using its loss-of-function mutant (d61-1). Our results showed that mutant d61-1 was less tolerant to cold stress than wild-type (WT). Besides, d61-1 had lower levels than WT for some physiological parameters, including catalase activity (CAT), superoxide dismutase activity (SOD), peroxidase activity (POD), peroxidase activity (PRO), soluble protein, and soluble sugar content, while malondialdehyde content (MDA) and relative electrical conductivity (REC) levels in d61-1 were higher than those in WT plants. These results indicated that the loss of OsBRI1 function resulted in decreased cold tolerance in rice. In addition, we performed RNA sequencing (RNA-seq) of WT and d61-1 mutant under cold stress. Numerous common and unique differentially expressed genes (DEGs) with up- and down-regulation were observed in WT and d61-1 mutant. Some DEGs were expressed to various degrees, even opposite, between CK1 vs. T1 (WT) and CK2 vs. T2 (d61-1). Among these specific DEGs, some typical genes are involved in plant tolerance to cold stress. Through weighted correlation network analysis (WGCNA), 50 hub genes were screened in the turquoise and blue module. Many genes were involved in cold stress and plant hormone, such as Os01g0279800 (BRI1-associated receptor kinase 1 precursor), Os10g0513200 (Dwarf and tiller-enhancing 1, DTE1), Os02g0706400 (MYB-related transcription factor, OsRL3), etc. Differential expression levels of some genes were verified in WT and d61-1 under cold stress using qRT-PCR. These valuable findings and gene resources will be critical for understanding the regulatory relationships between cold stress tolerance and the BR signaling pathways in rice.

Molecular analysis of broad-spectrum induced resistance in rice by the green leaf volatile Z-3-hexenyl acetate

Green leaf volatiles (GLVs), volatile organic compounds released by plants upon tissue damage, are key signaling molecules in plant immunity. The ability of exogenous GLV application to trigger an induced resistance (IR) phenotype against arthropod pests has been widely reported, but its effectiveness against plant pathogens is less well understood. In this study, we combined mRNA sequencing-based transcriptomics and phytohormone measurements with multispectral imaging-based precision phenotyping to gain insights into the molecular basis of Z-3-hexenyl acetate-induced resistance (Z-3-HAC-IR) in rice. Furthermore, we evaluated the efficacy of Z-3-HAC-IR against a panel of economically significant rice pathogens: Pyricularia oryzae, Rhizoctonia solani, Xanthomonas oryzae pv. oryzae, Cochliobolus miyabeanus, and Meloidogyne graminicola. Our data revealed rapid induction of jasmonate metabolism and systemic induction of plant immune responses upon Z-3-HAC exposure, as well as a transient allocation cost due to accelerated chlorophyll degradation and nutrient remobilization. Z-3-HAC-IR proved effective against all tested pathogens except for C. miyabeanus, including against the (hemi)biotrophs M. graminicola, X. oryzae pv. oryzae, and P. oryzae. The Z-3-HAC-IR phenotype was lost in the jasmonate (JA)-deficient hebiba mutant, which confirms the causal role of JA in Z-3-HAC-IR. Together, our results show that GLV exposure in rice induces broad-spectrum, JA-mediated disease resistance with limited allocation costs, and may thus be a promising alternative crop protection approach.

Disruption of the Novel Small Protein RBR7 Leads to Enhanced Plant Resistance to Blast Disease

Plant disease is a threat to global food security. Breeding crops carrying broad-spectrum resistance loci is an effective way to control infectious disease. Disease-resistant mutants are valuable resources for deciphering the underlying mechanisms of plant immunity and could provide genetic loci to generate disease-resistant crops. Here, we identified a rice mutant, rbr7 (rice blast resistance 7), that confers resistance against different strains of Magnaporthe oryzae. Disease-mimicking necrotic lesions started to appear on the leaves of rbr7 four weeks after sowing. Histochemical analysis revealed reactive oxygen species accumulation and cell death accompanied by spontaneous lesion formation in rbr7. Map-based cloning and bulk segregation analysis showed a 2855 bp fragment deletion on chromosome 5, leading to the disruption of the LOC_Os05g28480-coding protein. Transgenic rbr7 complementation plants showed compromised resistance to rice blast, indicating that LOC_Os05g28480, or Rbr7, regulates the rice immune response. Rbr7 encodes a small protein of unknown function with 85 amino acids. Transcriptomic analysis revealed that disruption of RBR7 led to the upregulation of genes responding to salicylic acid, systemic acquired resistance and pathogenesis-related genes. Taken together, our findings reveal insights into a novel small protein involved in regulating plant resistance to rice blast and provide a potential target for crop breeding.

Characterization and gene expression patterns analysis implies BSK family genes respond to salinity stress in cotton

Identification, evolution, and expression patterns of BSK (BR signaling kinase) family genes revealed that BSKs participated in the response of cotton to abiotic stress and maintained the growth of cotton in extreme environment. The steroidal hormone brassinosteroids (BR) play important roles in different plant biological processes. This study focused on BSK which were downstream regulatory element of BR, in order to help to decipher the functions of BSKs genes from cotton on growth development and responses to abiotic stresses and lean the evolutionary relationship of cotton BSKs. BSKs are a class of plant-specific receptor-like cytoplasmic kinases involved in BR signal transduction. In this study, bioinformatics methods were used to identify the cotton BSKs gene family at the cotton genome level, and the gene structure, promoter elements, protein structure and properties, gene expression patterns and candidate interacting proteins were analyzed. In the present study, a total of 152 BSKs were identified by a genome-wide search in four cotton species and other 11 plant species, and phylogenetic analysis revealed three evolutionary clades. It was identified that BSKs contain typical PKc and TPR domains, the N-terminus is composed of extended chains and helical structures. Cotton BSKs genes show different expression patterns in different tissues and organs. The gene promoter contains numerous cis-acting elements induced by hormones and abiotic stress, the hormone ABA and Cold-inducing related elements have the highest count, indicating that cotton BSK genes may be regulated by various hormones at different growth stages and involved in the response regulation of cotton to various stresses. The expression analysis of BSKs in cotton showed that the expression levels of GhBSK06, GhBSK10, GhBSK21 and GhBSK24 were significantly increased with salt-inducing. This study is helpful to analyze the function of cotton BSKs genes in growth and development and in response to stress.

BRASSINOSTEROID-SIGNALING KINASE1-1, a positive regulator of brassinosteroid signalling, modulates plant architecture and grain size in rice

Brassinosteroids (BRs) are a crucial class of plant hormones that regulate plant growth and development, thus affecting many important agronomic traits in crops. However, there are still significant gaps in our understanding of the BR signalling pathway in rice. In this study, we provide multiple lines of evidence to indicate that BR-SIGNALING KINASE1-1 (OsBSK1-1) likely represents a missing component in the BR signalling pathway in rice. We showed that knockout mutants of OsBSK1-1 are less sensitive to BR and exhibit a pleiotropic phenotype, including lower plant height, less tiller number and shortened grain length, whereas transgenic plants overexpressing a gain-of-function dominant mutant form of OsBSK1-1 (OsBSK1-1A295V) are hypersensitive to BR, and exhibit some enhanced BR-responsive phenotypes. We found that OsBSK1-1 physically interacts with the BR receptor BRASSINOSTEROID INSENSITIVE1 (OsBRI1), and GLYCOGEN SYNTHASE KINASE2 (OsGSK2), a downstream component crucial for BR signalling. Moreover, we showed that OsBSK1-1 can be phosphorylated by OsBRI1 and can inhibit OsGSK2-mediated phosphorylation of BRASSINOSTEROID RESISTANT1 (OsBZR1). We further demonstrated that OsBSK1-1 genetically acts downstream of OsBRI1, but upstream of OsGSK2. Together, our results suggest that OsBSK1-1 may serve as a scaffold protein directly bridging OsBRI1 and OsGSK2 to positively regulate BR signalling, thus affecting plant architecture and grain size in rice.

Identification, evolution, and expression analysis of OsBSK gene family in Oryza sativa Japonica

BACKGROUND

As an essential component of the BR (brassinosteroid) signaling pathway, BSK (BR-signalling kinases) plays a vital role in plant growth, development, and stress regulation. There have been sporadic reports on the functions of members of this family in monocotyledonous model plant rice, but few reports have been reported on the phylogenetic analysis and gene expression profiling of the family genes.

RESULTS

In this study, a total of 6 OsBSK members were identified at the genomic level by bioinformatics methods, distributed on four rice chromosomes. Through the evolution analysis of 74 BSK proteins from 22 species, it was found that BSKs originated from higher plants, were highly conserved, and could be divided into six subgroups. Among them, OsBSKs belonged to four subgroups or two significant groups. OsBSK family gene promoters contained a large number of light, abscisic acid (ABA), and methyl jasmonate (MeJA) response-related elements. At the same time, the qRT-PCR test also showed that the genes of this family were involved in response to a variety of hormones, biotic and abiotic stress treatments, and expression patterns of the family gene can be roughly divided into two categories, which were similar to the tissue expression patterns of genes in different growth stages. OsBSK1-1, OsBSK1-2, and OsBSK3 were mostly up-regulated. OsBSK2, OsBSK4, and OsBSK5 were mostly down-regulated or had little change in expression.

CONCLUSIONS

This study revealed the origin and evolution of the BSK family and the farm-out of BSKs in rice growth, development, and stress response. It provides the theoretical reference for in-depth analysis of BR hormone, signal transduction, and molecular breeding design for resistance.

OsBSK2, a putative brassinosteroid-signalling kinase, positively controls grain size in rice

Grain size is an important trait that directly affects grain yield in rice; however, the genetic and molecular mechanisms regulating grain size remain unclear. In this study, we identified a mutant, grain length and grain weight 10 (glw10), which exhibited significantly reduced grain length and grain weight. Histological analysis demonstrated that GLW10 affects cell expansion, which regulates grain size. MutMap-based gene mapping and transgenic experiments demonstrated that GLW10 encodes a putative brassinosteroid (BR) signalling kinase, OsBSK2. OsBSK2 is a plasma membrane protein, and an N-myristoylation site is needed for both membrane localization and function. OsBSK2 directly interacts with the BR receptor kinase OsBRI1; however, genetic experiments have demonstrated that OsBSK2 may regulate grain size independent of the BR signalling pathway. OsBSK2 can form a homodimer or heterodimer with OsBSK3 and OsBSK4, and silencing OsBSK2, OsBSK3, and OsBSK4 reduce grain size. This indicates that OsBSKs seem to function as homodimers or heterodimers to positively regulate grain size in rice. OsBSK2/3/4 are all highly expressed in young panicles and spikelet hulls, suggesting that they control grain size. In summary, our results provide novel insights into the function of BSKs in rice, and identify novel targets for improving grain size during crop breeding.

Comparative transcriptome analysis of rice cultivars resistant and susceptible to Rhizoctonia solani AG1-IA

BACKGROUND

Rice sheath blight, which is caused by Rhizoctonia solani, is the most destructive disease affecting rice production, but the resistance mechanism to this pathogen has not been fully elucidated.

RESULTS

In this study, we selected two rice cultivars based on their resistance to the pathogen and analyzed and compared the transcriptomic profiles of two cultivars, the moderately resistant variety Gangyuan8 and the highly susceptible variety Yanfeng47, at different time points after inoculation. The comparative transcriptome profiling showed that the expression of related genes gradually increased after pathogen inoculation. The number of differentially expressed genes (DEGs) in Yanfeng47 was higher than that in Gangyuan8, and this result revealed that Yanfeng47 was more susceptible to fungal attack. At the early stage (24 and 48 h), the accumulation of resistance genes and a resistance metabolism occurred earlier in Ganguan8 than in Yanfeng47, and the resistance enrichment entries were more abundant in Ganguan8 than in Yanfeng47.

CONCLUSIONS

Based on the GO and KEGG enrichment analyses at five infection stages, we concluded that phenylalanine metabolism and the jasmonic acid pathway play a crucial role in the resistance of rice to sheath blight. Through a comparative transcriptome analysis, we preliminarily analyzed the molecular mechanism responsible for resistance to sheath blight in rice, and the results lay the foundation for the development of gene mining and functional research on rice resistance to sheath blight.

ZmBSK1 positively regulates BR-induced H2O2 production via NADPH oxidase and functions in oxidative stress tolerance in maize

Brassinosteroid (BR) has been indicated to induce the production of hydrogen peroxide (H₂O₂) in plants in response to various environmental stimuli. However, it remains largely unknown how BR induces H₂O₂ production. In this study, we found that BR treatment significantly raised the kinase activity of maize (Zea mays L.) brassinosteroid-signaling kinase 1 (ZmBSK1) using the immunoprecipitation kinase assay. ZmBSK1 could modulate the gene expressions and activities of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (EC 1.6.3.1) to modulate BR-induced H₂O₂ production. BR could enhance the interaction between ZmBSK1 and maize calcium/calmodulin-dependent protein kinase (ZmCCaMK), a previously identified substrate of ZmBSK1. The BR-induced phosphorylation and kinase activity of ZmCCaMK are dependent on ZmBSK1. Moreover, we showed that ZmBSK1 regulated the NADPH oxidase gene expression and activity via directly phosphorylating ZmCCaMK. Genetic analysis suggested that ZmBSK1-ZmCCaMK module strengthened plant tolerance to oxidative stress induced by exogenous application of H₂O₂ through improving the activities of antioxidant defense enzyme and alleviating the malondialdehyde (MDA) accumulation and electrolyte leakage rate. In conclusion, these findings provide the new insights of ZmBSK1 functioning in BR-induced H₂O₂ production and the theoretical supports for breeding stress-tolerant crops.

Genome-wide identification and expression analysis reveals spinach brassinosteroid-signaling kinase (BSK) gene family functions in temperature stress response

Background

Brassinosteroid (BR)- signaling kinase (BSK) is a critical family of receptor-like cytoplasmic kinase for BR signal transduction, which plays important roles in plant development, immunity, and abiotic stress responses. Spinach (Spinacia oleracea) is cold- tolerant but heat- sensitive green leafy vegetable. A study on BSK family members and BSKs- mediated metabolic processes in spinach has not been performed.

Results

We identified and cloned seven SoBSKs in spinach. Phylogenetic and collinearity analyses suggested that SoBSKs had close relationship with dicotyledonous sugar beet (Beta vulgaris) rather than monocotyledons. The analyses of gene structure and conserved protein domain/ motif indicated that most SoBSKs were relative conserved, while SoBSK6 could be a truncated member. The prediction of post-translation modification (PTM) sites in SoBSKs implied their possible roles in signal transduction, redox regulation, and protein turnover of SoBSKs, especially the N-terminal myristoylation site was critical for BSK localization to cell periphery. Cis-acting elements for their responses to light, drought, temperature (heat and cold), and hormone distributed widely in the promoters of SoBSKs, implying the pivotal roles of SoBSKs in response to diverse abiotic stresses and phytohormone stimuli. Most SoBSKs were highly expressed in leaves, except for SoBSK7 in roots. Many SoBSKs were differentially regulated in spinach heat- sensitive variety Sp73 and heat- tolerant variety Sp75 under the treatments of heat, cold, as well as exogenous brassinolide (BL) and abscisic acid (ABA). The bsk134678 mutant Arabidopsis seedlings exhibited more heat tolerance than wild- type and SoBSK1- overexpressed seedlings.

Conclusions

A comprehensive genome- wide analysis of the BSK gene family in spinach presented a global identification and functional prediction of SoBSKs. Seven SoBSKs had relatively- conserved gene structure and protein function domains. Except for SoBSK6, all the other SoBSKs had similar motifs and conserved PTM sites. Most SoBSKs participated in the responses to heat, cold, BR, and ABA. These findings paved the way for further functional analysis on BSK- mediated regulatory mechanisms in spinach development and stress response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08684-5.

OsBSK3 Positively Regulates Grain Length and Weight by Inhibiting the Phosphatase Activity of OsPPKL1

Brassinosteroids (BRs) are a crucial class of plant hormones that regulate many important agronomic traits in rice (Oryza sativa L.); thus, the BR signaling pathway is a very important tool for breeders to improve the grain yield and quantity of rice. Contrary to the well-established BR signaling pathway in Arabidopsis, there are significant gaps in the rice BR signaling pathway, especially the regulation mechanism from OsBSK3 to OsPPKLs and OsGSKs. In this study, we report how OsBSK3 knockout mutants confer shorter and lighter grains and exhibit a typical BR-insensitive phenotype, suggesting OsBSK3 plays a positive role in BR signaling without genetic redundancy with homologs. Furthermore, OsBSK3 could physically interact with OsPPKL1 and OsGSK3, the downstream components in BR signaling, as a scaffold protein, and inhibit the phosphatase activity of OsPPKL1 on the dephosphorylation of OsGSK3. In addition, the genetic evidence showed OsBSK3 acts upstream of OsPPKL1 in regulating grain length and weight. Our results clarify the role of OsBSK3 and provide new insights into BR-signaling mechanisms, leading to potential new targets for the genetic improvement of rice.

Overexpression of a Zea mays Brassinosteroid-Signaling Kinase Gene ZmBSK1 Confers Salt Stress Tolerance in Maize

Salinity has become a crucial environmental factor seriously restricting maize (Zea mays L.) growth, development and productivity. However, how plants respond to salt stress is still poorly understood. In this study, we report that a maize brassinosteroid-signaling kinase gene ZmBSK1 plays a significant role in salt stress response. Expression pattern analysis revealed that the transcript level of ZmBSK1 was upregulated by NaCl treatment both in maize leaves, roots, and stems. Phenotypic and physiological analysis showed that overexpression of ZmBSK1 in maize improved salt tolerance by reducing the malondialdehyde (MDA) content, the percentage of electrolyte leakage, O₂ ^- and H₂O₂ accumulation under salt stress, relying on the increases of antioxidant defense enzyme activities and proline content. qRT-PCR analysis showed that overexpression of ZmBSK1 also positively modulated the expression levels of reactive oxygen species (ROS)-scavenging and proline biosynthesis-related genes under salt stress. Moreover, immunoprecipitation-mass spectrometry (IP-MS) assay and firefly luciferase complementation imaging (LCI) assay showed that ZmBSK1 could associate with heat shock protein ZmHSP8 and 14-3-3-like protein ZmGF14-6, and their gene expression levels could be significantly induced by NaCl treatment in different maize tissues. Our findings unravel the new function of ZmBSK1 in salt stress response, which provides the theoretical bases for the improvement of maize salt resistance.

Molecular mechanisms of early plant pattern-triggered immune signaling

All eukaryotic organisms have evolved sophisticated immune systems to appropriately respond to biotic stresses. In plants and animals, a key part of this immune system is pattern recognition receptors (PRRs). Plant PRRs are cell-surface-localized receptor kinases (RKs) or receptor proteins (RPs) that sense microbe- or self-derived molecular patterns to regulate pattern-triggered immunity (PTI), a robust form of antimicrobial immunity. Remarkable progress has been made in understanding how PRRs perceive their ligands, form active protein complexes, initiate cell signaling, and ultimately coordinate the cellular reprogramming that leads to PTI. Here, we discuss the critical roles of PRR complex formation and phosphorylation in activating PTI signaling, as well as the emerging paradigm in which receptor-like cytoplasmic kinases (RLCKs) act as executors of signaling downstream of PRR activation.

BRASSINOSTEROID-SIGNALING KINASE 1 phosphorylating CALCIUM/CALMODULIN-DEPENDENT PROTEIN KINASE functions in drought tolerance in maize

Drought stress seriously limits crop productivity. Although studies have been carried out, it is still largely unknown how plants respond to drought stress. Here we find that drought treatment can enhance the phosphorylation activity of brassinosteroid-signaling kinase 1 (ZmBSK1) in maize (Zea mays). Our genetic studies reveal that ZmBSK1 positively affects drought tolerance in maize plants. ZmBSK1 localizes in plasma membrane, interacts with calcium/calmodulin (Ca²⁺ /CaM)-dependent protein kinase (ZmCCaMK), and phosphorylates ZmCCaMK. Ser-67 is a crucial phosphorylation site of ZmCCaMK by ZmBSK1. Drought stress enhances not only the interaction between ZmBSK1 and ZmCCaMK but also the phosphorylation of Ser-67 in ZmCCaMK by ZmBSK1. Furthermore, Ser-67 phosphorylation in ZmCCaMK regulates its Ca²⁺ /CaM binding, autophosphorylation and transphosphorylation activity, and positively affects its function in drought tolerance in maize. Our results reveal an important role for ZmBSK1 in drought tolerance and suggest a direct regulatory mode of ZmBSK1 phosphorylating ZmCCaMK.

Receptors in the Induction of the Plant Innate Immunity

Plants adjust amplitude and duration of immune responses via different strategies to maintain growth, development, and resistance to pathogens. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) play vital roles. Pattern recognition receptors, comprising a large number of receptor-like protein kinases and receptor-like proteins, recognize related ligands and trigger immunity. PTI is the first layer of the innate immune system, and it recognizes PAMPs at the plasma membrane to prevent infection. However, pathogens exploit effector proteins to bypass or directly inhibit the PTI immune pathway. Consistently, plants have evolved intracellular nucleotide-binding domain and leucine-rich repeat-containing proteins to detect pathogenic effectors and trigger a hypersensitive response to activate ETI. PTI and ETI work together to protect plants from infection by viruses and other pathogens. Diverse receptors and the corresponding ligands, especially several pairs of well-studied receptors and ligands in PTI immunity, are reviewed to illustrate the dynamic process of PTI response here.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Transcriptome analysis reveals underlying immune response mechanism of fungal (Penicillium oxalicum) disease in Gastrodia elata Bl. f. glauca S. chow (Orchidaceae)

BACKGROUND

Gastrodia elata Bl. f. glauca S. Chow is a medicinal plant. G. elata f. glauca is unavoidably infected by pathogens in their growth process. In previous work, we have successfully isolated and identified Penicillium oxalicum from fungal diseased tubers of G. elata f. glauca. As a widespread epidemic, this fungal disease seriously affected the yield and quality of G. elata f. glauca. We speculate that the healthy G. elata F. glauca might carry resistance genes, which can resist against fungal disease. In this study, healthy and fungal diseased mature tubers of G. elata f. glauca from Changbai Mountain area were used as experimental materials to help us find potential resistance genes against the fungal disease.

RESULTS

A total of 7540 differentially expressed Unigenes (DEGs) were identified (FDR < 0.01, log2FC > 2). The current study screened 10 potential resistance genes. They were attached to transcription factors (TFs) in plant hormone signal transduction pathway and plant pathogen interaction pathway, including WRKY22, GH3, TIFY/JAZ, ERF1, WRKY33, TGA. In addition, four of these genes were closely related to jasmonic acid signaling pathway.

CONCLUSIONS

The immune response mechanism of fungal disease in G. elata f. glauca is a complex biological process, involving plant hormones such as ethylene, jasmonic acid, salicylic acid and disease-resistant transcription factors such as WRKY, TGA.

OsExo70B1 Positively Regulates Disease Resistance to Magnaporthe oryzae in Rice

The exocyst, an evolutionarily conserved octameric protein complex, mediates tethering of vesicles to the plasma membrane in the early stage of exocytosis. Arabidopsis Exo70, a subunit of the exocyst complex, has been found to be involved in plant immunity. Here, we characterize the function of OsExo70B1 in rice. OsExo70B1 mainly expresses in leaf and shoot and its expression is induced by pathogen-associated molecular patterns (PAMPs) and rice blast fungus Magnaporthe oryzae (M. oryzae). Knocking out OsExo70B1 results in significantly decreased resistance and defense responses to M. oryzae compared to the wild type, including more disease lesions and enhanced fungal growth, downregulated expression of pathogenesis-related (PR) genes, and decreased reactive oxygen species accumulation. In contrast, the exo70B1 mutant does not show any defects in growth and development. Furthermore, OsExo70B1 can interact with the receptor-like kinase OsCERK1, an essential component for chitin reception in rice. Taken together, our data demonstrate that OsExo70B1 functions as an important regulator in rice immunity.

Hide-and-Seek: Chitin-Triggered Plant Immunity and Fungal Counterstrategies

Fungal pathogens are major destructive microorganisms for land plants and pose growing challenges to global crop production. Chitin is a vital building block for fungal cell walls and also a broadly effective elicitor of plant immunity. Here we review the rapid progress in understanding chitin perception and signaling in plants and highlight similarities and differences of these processes between arabidopsis and rice. We also outline moonlight functions of CERK1, an indispensable chitin coreceptor conserved across the plant kingdom, which imply potential crosstalk between chitin signaling and symbiotic or biotic/abiotic stress signaling in plants via CERK1. Moreover, we summarize current knowledge about fungal counterstrategies for subverting chitin-triggered plant immunity and propose open questions and future directions in this field.

Mai1 Protein Acts Between Host Recognition of Pathogen Effectors and Mitogen-Activated Protein Kinase Signaling

The molecular mechanisms acting between host recognition of pathogen effectors by nucleotide-binding leucine-rich repeat receptor (NLR) proteins and mitogen-activated protein kinase (MAPK) signaling cascades are unknown. MAPKKKα (M3Kα) activates MAPK signaling leading to programmed cell death (PCD) associated with NLR-triggered immunity. We identified a tomato M3Kα-interacting protein, SlMai1, that has 80% amino acid identity with Arabidopsis brassinosteroid kinase 1 (AtBsk1). SlMai1 has a protein kinase domain and a C-terminal tetratricopeptide repeat domain that interacts with the kinase domain of M3Kα. Virus-induced gene silencing of Mai1 homologs in Nicotiana benthamiana increased susceptibility to Pseudomonas syringae and compromised PCD induced by four NLR proteins. PCD was restored by expression of a synthetic SlMai1 gene that resists silencing. Expression of AtBsk1 did not restore PCD in Mai1-silenced plants, suggesting SlMai1 is functionally divergent from AtBsk1. PCD caused by overexpression of M3Kα or MKK2 was unaffected by Mai1 silencing, suggesting Mai1 acts upstream of these proteins. Coexpression of Mai1 with M3Kα in leaves enhanced MAPK phosphorylation and accelerated PCD. These findings suggest Mai1 is a molecular link acting between host recognition of pathogens and MAPK signaling.

Metabolic Basis of Pathogenesis and Host Adaptation in Rice Blast

The blast disease, caused by the ascomycete Magnaporthe oryzae, poses a great threat to rice production worldwide. Increasing use of fungicides and/or blast-resistant varieties of rice (Oryza sativa) has proved to be ineffective in long-term control of blast disease under field conditions. To develop effective and durable resistance to blast, it is important to understand the cellular mechanisms underlying pathogenic development in M. oryzae. In this review, we summarize the latest research in phototropism, autophagy, nutrient and redox signaling, and intrinsic phytohormone mimics in M. oryzae for cellular and metabolic adaptation(s) during its interactions with the host plants.

BRASSINOSTEROID-SIGNALING KINASE5 Associates with Immune Receptors and Is Required for Immune Responses

Plants utilize cell surface-localized pattern recognition receptors (PRRs) to detect pathogen- or damage-associated molecular patterns (PAMP/DAMPs) and initiate pattern-triggered immunity (PTI). Here, we investigated the role of Arabidopsis (Arabidopsis thaliana) BRASSINOSTEROID-SIGNALING KINASE5 (BSK5), a member of the receptor-like cytoplasmic kinase subfamily XII, in PRR-initiated immunity. BSK5 localized to the plant cell periphery, interacted in yeast and in planta with multiple receptor-like kinases, including the ELONGATION FACTOR-TU RECEPTOR (EFR) and PEP1 RECEPTOR1 (PEPR1) PRRs, and was phosphorylated in vitro by PEPR1 and EFR in the kinase activation loop. Consistent with a role in PTI, bsk5 mutant plants displayed enhanced susceptibility to the bacterial pathogen Pseudomonas syringae and to the fungus Botrytis cinerea Furthermore, bsk5 mutant plants were impaired in several immune responses induced by the elf18, pep1, and flg22 PAMP/DAMPs, including resistance to P. syringae and B. cinerea, production of reactive oxygen species, callose deposition at the cell wall, and enhanced PATHOGENESIS-RELATED1 gene expression. However, bsk5 plants were not affected in PAMP/DAMP activation of mitogen-activated protein kinases and expression of the FLG22-INDUCED RECEPTOR-LIKE KINASE1 or the WRKY domain-containing gene WRKY29 BSK5 variants mutated in the BSK5 myristoylation site, ATP-binding site, and kinase activation loop failed to complement defective PTI phenotypes of bsk5 mutant plants, suggesting that localization to the cell periphery, kinase activity, and phosphorylation by PRRs are critical for the function of BSK5 in PTI. These findings demonstrate that BSK5 plays a role in PTI by interacting with multiple immune receptors.

Genome-Wide Identification, Expression Profile, and Alternative Splicing Analysis of the Brassinosteroid-Signaling Kinase (BSK) Family Genes in Arabidopsis

Brassinosteroids (BRs) are steroid hormones essential for different biological processes, ranging from growth to environmental adaptation in plants. The plant brassinosteroid-signaling kinase (BSK) proteins belong to a family of receptor-like cytoplasmic kinases, which have been reported to play an important role in BR signal transduction. However, the knowledge of BSK genes in plants is still quite limited. In the present study, a total of 143 BSK proteins were identified by a genome-wide search in 17 plant species. A phylogenetic analysis showed that the BSK gene originated in embryophytes, with no BSK found in green algae, and these BSK genes were divided into six groups by comparison with orthologs/paralogs. A further study using comparative analyses of gene structure, expression patterns and alternative splicing of BSK genes in Arabidopsis revealed that all BSK proteins shared similar protein structure with some exception and post-translation modifications including sumolyation and ubiquitination. An expression profile analysis showed that most Arabidopsis BSK genes were constitutively expressed in different tissues; of these, several BSK genes were significantly expressed in response to some hormones or abiotic stresses. Furthermore, reverse transcription-polymerase chain reaction (RT-PCR) assays showed that BSK5, BSK7, and BSK9 underwent alternative splicing in specific stress induced and tissue-dependent patterns. Collectively, these results lay the foundation for further functional analyses of these genes in plants.

Constitutive signaling activity of a receptor-associated protein links fertilization with embryonic patterning in Arabidopsis thaliana

In flowering plants, the asymmetrical division of the zygote is the first hallmark of apical-basal polarity of the embryo and is controlled by a MAP kinase pathway that includes the MAPKKK YODA (YDA). In Arabidopsis, YDA is activated by the membrane-associated pseudokinase SHORT SUSPENSOR (SSP) through an unusual parent-of-origin effect: SSP transcripts accumulate specifically in sperm cells but are translationally silent. Only after fertilization is SSP protein transiently produced in the zygote, presumably from paternally inherited transcripts. SSP is a recently diverged, Brassicaceae-specific member of the BRASSINOSTEROID SIGNALING KINASE (BSK) family. BSK proteins typically play broadly overlapping roles as receptor-associated signaling partners in various receptor kinase pathways involved in growth and innate immunity. This raises two questions: How did a protein with generic function involved in signal relay acquire the property of a signal-like patterning cue, and how is the early patterning process activated in plants outside the Brassicaceae family, where SSP orthologs are absent? Here, we show that Arabidopsis BSK1 and BSK2, two close paralogs of SSP that are conserved in flowering plants, are involved in several YDA-dependent signaling events, including embryogenesis. However, the contribution of SSP to YDA activation in the early embryo does not overlap with the contributions of BSK1 and BSK2. The loss of an intramolecular regulatory interaction enables SSP to constitutively activate the YDA signaling pathway, and thus initiates apical-basal patterning as soon as SSP protein is translated after fertilization and without the necessity of invoking canonical receptor activation.

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.