A renaissance of elicitors: perception of microbe-associated molecular patterns and danger signals by pattern-recognition receptors

Evaluation of PSP1 biostimulant on Fusarium graminearum-wheat pathosystem: impact on disease parameters, grain yield, and grain quality

BACKGROUND

Plant defense elicitors are valuable tools in sustainable agriculture, providing an environmentally friendly and effective means of enhancing plant defense and promoting plant health. Fusarium head blight (FHB) is one of the most important fungal diseases of cereal crops worldwide. The PSP1 is a novel biopesticide formulated based on an elicitor, the extracellular protein AsES, from the fungus Sarocladium strictum. The present work aimed to evaluate the effectiveness of PSP1 in controlling FHB under field conditions. Experiments were conducted during three consecutive growing seasons (2019, 2020, and 2021). Three biostimulant treatments were tested in different physiological stages (from late tillering to heading stage), and FHB inoculations were performed at anthesis. Disease parameters, seed parameters, grain yield, and grain quality parameters were evaluated.

RESULTS

Depending on the year and the genotype, reductions in disease incidence (up to 11%) and disease severity (up to 5%) were reported, although these differences could not be attributed to the use of the PSP1 biostimulant. Occasional improvements in seed parameters and grain quality were observed, suggesting that early treatments could work better than late treatments, probably due to early activation/priming of defense response mechanisms. However, more studies are deemed necessary.

CONCLUSION

The use of PSP1 biostimulant in commercial wheat crops could be a biological alternative or complement to traditional chemical fungicides to manage FHB. The reduced environmental impact and the potential benefits in grain yield and quality are other reasons that can generate new adherents of this technology in worldwide agriculture systems in the coming years. © 2024 Society of Chemical Industry.

CERK1 compromises Fusarium solani resistance by reducing jasmonate level and undergoes a negative feedback regulation via the MMK2-WRKY71 module in apple

Fusarium spp., a necrotrophic soil-borne pathogen, causes root rot disease on many crops. CERK1, as a typical pattern recognition receptor, has been widely studied. However, the function of CERK1 during plant-Fusarium interaction has not been well described. We determined that MdCERK1 is a susceptibility gene in the apple-Fusarium solani (Fs) interaction, and jasmonic acid (JA) plays a crucial role in this process. MdCERK1 directly targets and phosphorylates the lipoxygenase MdLOX2.1, an enzyme initiating the JA biosynthesis, at positions Ser326 and Thr327. These phosphorylations inhibit its translocation from the cytosol to the chloroplasts, leading to a compromised JA biosynthesis. Fs upregulates MdCERK1 expression during infection. In turn, when the JA level is low, the apple MdWRKY71, a transcriptional repressor of MdCERK1, is markedly upregulated and phosphorylated at Thr99 and Thr102 residues by the MAP kinase MdMMK2. The phosphorylation of MdWRKY71 enhances its transcription inhibition on MdCERK1. Taken together, MdCERK1 plays a novel role in limiting JA biosynthesis. There seems to be an arms race between apple and Fs, in which Fs activates MdCERK1 expression to reduce the JA level, while apple senses the low JA level and activates the MdMMK2-MdWRKY71 module to elevate JA level by inhibiting MdCERK1 expression.

The RXLR effector PpE18 of Phytophthora parasitica is a virulence factor and suppresses peroxisome membrane-associated ascorbate peroxidase NbAPX3-1-mediated plant immunity

Phytophthora parasitica causes diseases on a broad range of host plants. It secretes numerous effectors to suppress plant immunity. However, only a few virulence effectors in P. parasitica have been characterized. Here, we highlight that PpE18, a conserved RXLR effector in P. parasitica, was a virulence factor and suppresses Nicotiana benthamiana immunity. Utilizing luciferase complementation, co-immunoprecipitation, and GST pull-down assays, we determined that PpE18 targeted NbAPX3-1, a peroxisome membrane-associated ascorbate peroxidase with reactive oxygen species (ROS)-scavenging activity and positively regulates plant immunity in N. benthamiana. We show that the ROS-scavenging activity of NbAPX3-1 was critical for its immune function and was hindered by the binding of PpE18. The interaction between PpE18 and NbAPX3-1 resulted in an elevation of ROS levels in the peroxisome. Moreover, we discovered that the ankyrin repeat-containing protein NbANKr2 acted as a positive immune regulator, interacting with both NbAPX3-1 and PpE18. NbANKr2 was required for NbAPX3-1-mediated disease resistance. PpE18 competitively interfered with the interaction between NbAPX3-1 and NbANKr2, thereby weakening plant resistance. Our results reveal an effective counter-defense mechanism by which P. parasitica employed effector PpE18 to suppress host cellular defense, by suppressing biochemical activity and disturbing immune function of NbAPX3-1 during infection.

Comparative RNA-seq analysis of Arabidopsis thaliana response to AtPep1 and flg22, reveals the identification of PP2-B13 and ACLP1 as new members in pattern-triggered immunity

In this research, a high-throughput RNA sequencing-based transcriptome analysis technique (RNA-Seq) was used to evaluate differentially expressed genes (DEGs) in the wild type Arabidopsis seedlings in response to AtPep1, a well-known peptide representing an endogenous damage-associated molecular pattern (DAMP), and flg22, a well-known microbe-associated molecular pattern (MAMP). We compared and dissected the global transcriptional landscape of Arabidopsis thaliana in response to AtPep1 and flg22 and could identify shared and unique DEGs in response to these elicitors. We found that while a remarkable number of flg22 up-regulated genes were also induced by AtPep1, 256 genes were exclusively up-regulated in response to flg22, and 328 were exclusively up-regulated in response to AtPep1. Furthermore, among down-regulated DEGs upon flg22 treatment, 107 genes were exclusively down-regulated by flg22 treatment, while 411 genes were exclusively down-regulated by AtPep1. We found a number of hitherto overlooked genes to be induced upon treatment with either flg22 or with AtPep1, indicating their possible involvement general pathways in innate immunity. Here, we characterized two of them, namely PP2-B13 and ACLP1. pp2-b13 and aclp1 mutants showed increased susceptibility to infection by the virulent pathogen Pseudomonas syringae DC3000 and its mutant Pst DC3000 hrcC (lacking the type III secretion system), as evidenced by increased proliferation of the two pathogens in planta. Further, we present evidence that the aclp1 mutant is deficient in ethylene production upon flg22 treatment, while the pp2-b13 mutant is deficient in the production of reactive oxygen species (ROS). The results from this research provide new information for a better understanding of the immune system in Arabidopsis.

LORE receptor homomerization is required for 3-hydroxydecanoic acid-induced immune signaling and determines the natural variation of immunosensitivity within the Arabidopsis genus

The S-domain-type receptor-like kinase (SD-RLK) LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) from Arabidopsis thaliana is a pattern recognition receptor that senses medium-chain 3-hydroxy fatty acids, such as 3-hydroxydecanoic acid (3-OH-C10:0), to activate pattern-triggered immunity. Here, we show that LORE homomerization is required to activate 3-OH-C10:0-induced immune signaling. Fluorescence lifetime imaging in Nicotiana benthamiana demonstrates that AtLORE homomerizes via the extracellular and transmembrane domains. Co-expression of AtLORE truncations lacking the intracellular domain exerts a dominant negative effect on AtLORE signaling in both N. benthamiana and A. thaliana, highlighting that homomerization is essential for signaling. Screening for 3-OH-C10:0-induced reactive oxygen species production revealed natural variation within the Arabidopsis genus. Arabidopsis lyrata and Arabidopsis halleri do not respond to 3-OH-C10:0, although both possess a putative LORE ortholog. Both LORE orthologs have defective extracellular domains that bind 3-OH-C10:0 to a similar level as AtLORE, but lack the ability to homomerize. Thus, ligand binding is independent of LORE homomerization. Analysis of AtLORE and AlyrLORE chimera suggests that the loss of AlyrLORE homomerization is caused by several amino acid polymorphisms across the extracellular domain. Our findings shed light on the activation mechanism of LORE and the loss of 3-OH-C10:0 perception within the Arabidopsis genus.

Arginine methylation of SM-LIKE PROTEIN 4 antagonistically affects alternative splicing during Arabidopsis stress responses

Arabidopsis (Arabidopsis thaliana) PROTEIN ARGININE METHYLTRANSFERASE5 (PRMT5) post-translationally modifies RNA-binding proteins by arginine (R) methylation. However, the impact of this modification on the regulation of RNA processing is largely unknown. We used the spliceosome component, SM-LIKE PROTEIN 4 (LSM4), as a paradigm to study the role of R-methylation in RNA processing. We found that LSM4 regulates alternative splicing (AS) of a suite of its in vivo targets identified here. The lsm4 and prmt5 mutants show a considerable overlap of genes with altered AS raising the possibility that splicing of those genes could be regulated by PRMT5-dependent LSM4 methylation. Indeed, LSM4 methylation impacts AS, particularly of genes linked with stress response. Wild-type LSM4 and an unmethylable version complement the lsm4-1 mutant, suggesting that methylation is not critical for growth in normal environments. However, LSM4 methylation increases with abscisic acid and is necessary for plants to grow under abiotic stress. Conversely, bacterial infection reduces LSM4 methylation, and plants that express unmethylable-LSM4 are more resistant to Pseudomonas than those expressing wild-type LSM4. This tolerance correlates with decreased intron retention of immune-response genes upon infection. Taken together, this provides direct evidence that R-methylation adjusts LSM4 function on pre-mRNA splicing in an antagonistic manner in response to biotic and abiotic stress.

Plant-Entomopathogenic Fungi Interaction: Recent Progress and Future Prospects on Endophytism-Mediated Growth Promotion and Biocontrol

Entomopathogenic fungi, often acknowledged primarily for their insecticidal properties, fulfill diverse roles within ecosystems. These roles encompass endophytism, antagonism against plant diseases, promotion of the growth of plants, and inhabitation of the rhizosphere, occurring both naturally and upon artificial inoculation, as substantiated by a growing body of contemporary research. Numerous studies have highlighted the beneficial aspects of endophytic colonization. This review aims to systematically organize information concerning the direct (nutrient acquisition and production of phytohormones) and indirect (resistance induction, antibiotic and secondary metabolite production, siderophore production, and mitigation of abiotic and biotic stresses) implications of endophytic colonization. Furthermore, a thorough discussion of these mechanisms is provided. Several challenges, including isolation complexities, classification of novel strains, and the impact of terrestrial location, vegetation type, and anthropogenic reluctance to use fungal entomopathogens, have been recognized as hurdles. However, recent advancements in biotechnology within microbial research hold promising solutions to many of these challenges. Ultimately, the current constraints delineate potential future avenues for leveraging endophytic fungal entomopathogens as dual microbial control agents.

Row1, a member of a new family of conserved fungal proteins involved in infection, is required for appressoria functionality in Ustilago maydis

The appressorium of phytopathogenic fungi is a specific structure with a crucial role in plant cuticle penetration. Pathogens with melanized appressoria break the cuticle through cell wall melanization and intracellular turgor pressure. However, in fungi with nonmelanized appressorium, the mechanisms governing cuticle penetration are poorly understood. Here we characterize Row1, a previously uncharacterized appressoria-specific protein of Ustilago maydis that localizes to membrane and secretory vesicles. Deletion of row1 decreases appressoria formation and plant penetration, thereby reducing virulence. Specifically, the Δrow1 mutant has a thicker cell wall that is more resistant to glucanase degradation. We also observed that the Δrow1 mutant has secretion defects. We show that Row1 is functionally conserved at least among Ustilaginaceae and belongs to the Row family, which consists of five other proteins that are highly conserved among Basidiomycota fungi and are involved in U. maydis virulence. We observed similarities in localization between Row1 and Row2, which is also involved in cell wall remodelling and secretion, suggesting similar molecular functions for members of this protein family. Our data suggest that Row1 could modify the chitin-glucan matrix of the fungal cell wall and may be involved in unconventional protein secretion, thereby promoting both appressoria maturation and penetration.

Arabidopsis SBT5.2 and SBT1.7 subtilases mediate C-terminal cleavage of flg22 epitope from bacterial flagellin

Plants initiate specific defense responses by recognizing conserved epitope peptides within the flagellin proteins derived from bacteria. Proteolytic cleavage of epitope peptides from flagellin by plant apoplastic proteases is thought to be crucial for the perception of the epitope by the plant receptor. However, the identity of the plant proteases involved in this process remains unknown. Here, we establish an efficient identification system for the target proteases in Arabidopsis apoplastic fluid; the method employs native two-dimensional electrophoresis followed by an in-gel proteolytic assay using a fluorescence-quenching peptide substrate. We designed a substrate to specifically detect proteolytic activity at the C-terminus of the flg22 epitope in flagellin and identified two plant subtilases, SBT5.2 and SBT1.7, as specific proteases responsible for the C-terminal cleavage of flg22. In the apoplastic fluid of Arabidopsis mutant plants deficient in these two proteases, we observe a decrease in the C-terminal cleavage of the flg22 domain from flagellin, leading to a decrease in the efficiency of flg22 epitope liberation. Consequently, defensive reactive oxygen species (ROS) production is delayed in sbt5.2 sbt1.7 double-mutant leaf disks compared to wild type following flagellin exposure.

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.

An effector SsCVNH promotes the virulence of Sclerotinia sclerotiorum through targeting class III peroxidase AtPRX71

Many plant pathogens secrete effector proteins into the host plant to suppress host immunity and facilitate pathogen colonization. The necrotrophic pathogen Sclerotinia sclerotiorum causes severe plant diseases and results in enormous economic losses, in which secreted proteins play a crucial role. SsCVNH was previously reported as a secreted protein, and its expression is significantly upregulated at 3 h after inoculation on the host plant. Here, we further demonstrated that deletion of SsCVNH leads to attenuated virulence. Heterologous expression of SsCVNH in Arabidopsis enhanced pathogen infection, inhibited the host PAMP-triggered immunity (PTI) response and increased plant susceptibility to S. sclerotiorum. SsCVNH interacted with class III peroxidase AtPRX71, a positive regulator of innate immunity against plant pathogens. SsCVNH could also interact with other class III peroxidases, thus reducing peroxidase activity and suppressing plant immunity. Our results reveal a new infection strategy employed by S. sclerotiorum in which the fungus suppresses the function of class III peroxidases, the major component of PTI to promote its own infection.

Inhibition of PbeXTH1 and PbeSEOB1 is required for the Valsa canker resistance contributed by Wall-associated kinase gene MbWAK1

Wall-associated kinases (WAKs) have been determined to recognize pathogenic signals and initiate plant immune responses. However, the roles of the family members in host resistance against Valsa canker, a serious fungal disease of apples and pears, are largely unknown. Here, we identified MbWAK1 in Malus baccata, a resistant germplasm differentially expressed during infection by Valsa mali (Vm). Over-expression of MbWAK1 enhanced the Valsa canker resistance of apple and pear fruits and 'Duli-G03' (Pyrus betulifolia) suspension cells. A large number of phloem, cell wall, and lipid metabolic process-related genes were differentially expressed in overexpressed suspension cell lines in response to Valsa pyri (Vp) signals. Among these, the expression of xyloglucan endotransglucosylase/hydrolase (XTH) gene PbeXTH1 and sieve element occlusion B-like (SEOB) gene PbeSEOB1 were significantly inhibited. Transient expression of PbeXTH1 or PbeSEOB1 compromised the expressional induction of MbWAK1 and the resistance contributed by MbWAK1. In addition, PbeXTH1 and PbeSEOB1 suppressed the immune response induced by MbWAK1. Our results enriched the molecular mechanisms for MbWAK1 against Valsa canker and resistant breeding.

The Truncated Peptide AtPEP1(9-23) Has the Same Function as AtPEP1(1-23) in Inhibiting Primary Root Growth and Triggering of ROS Burst

Currently, the widely used active form of plant elicitor peptide 1 (PEP1) from Arabidopsis thaliana is composed of 23 amino acids, hereafter AtPEP1(1-23), serving as an immune elicitor. The relatively less conserved N-terminal region in AtPEP family indicates that the amino acids in this region may be unrelated to the function and activity of AtPEP peptides. Consequently, we conducted an investigation to determine the necessity of the nonconserved amino acids in AtPEP1(1-23) peptide for its functional properties. By assessing the primary root growth and the burst of reactive oxygen species (ROS), we discovered that the first eight N-terminal amino acids of AtPEP1(1-23) are not crucial for its functionality, whereas the conserved C-terminal aspartic acid plays a significant role in its functionality. In this study, we identified a truncated peptide, AtPEP1(9-23), which exhibits comparable activity to AtPEP1(1-23) in inhibiting primary root growth and inducing ROS burst. Additionally, the truncated peptide AtPEP1(13-23) shows similar ability to induce ROS burst as AtPEP1(1-23), but its inhibitory effect on primary roots is significantly reduced. These findings are significant as they provide a novel approach to explore and understand the functionality of the AtPEP1(1-23) peptide. Moreover, exogenous application of AtPEP1(13-23) may enhance plant resistance to pathogens without affecting their growth and development. Therefore, AtPEP1(13-23) holds promise for development as a potentially applicable biopesticides.

Pattern recognition receptors as potential therapeutic targets for developing immunological engineered plants

There is an urgent need to combat pathogen infestations in crop plants to ensure food security worldwide. To counter this, plants have developed innate immunity mediated by Pattern Recognition Receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and damage- associated molecular patterns (DAMPs). PRRs activate Pattern-Triggered Immunity (PTI), a defence mechanism involving intricate cell-surface and intracellular receptors. The diverse ligand-binding ectodomains of PRRs, including leucine-rich repeats (LRRs) and lectin domains, facilitate the recognition of MAMPs and DAMPs. Pathogen resistance is mediated by a variety of PTI responses, including membrane depolarization, ROS production, and the induction of defence genes. An integral part of intracellular immunity is the Nucleotide-binding Oligomerization Domain, Leucine-rich Repeat proteins (NLRs) which recognize and respond to effectors in a potent manner. Enhanced understanding of PRRs, their ligands, and downstream signalling pathways has contributed to the identification of potential targets for genetically modified plants. By transferring PRRs across plant species, it is possible to create broad-spectrum resistance, potentially offering innovative solutions for plant protection and global food security. The purpose of this chapter is to provide an update on PRRs involved in disease resistance, clarify the mechanisms by which PRRs recognize ligands to form active receptor complexes and present various applications of PRRs and PTI in disease resistance management for plants.

The conceptual foundations of innate immunity: Taking stock 30 years later

While largely neglected over decades during which adaptive immunity captured most of the attention, innate immune mechanisms have now become central to our understanding of immunology. Innate immunity provides the first barrier to infection in vertebrates, and it is the sole mechanism of host defense in invertebrates and plants. Innate immunity also plays a critical role in maintaining homeostasis, shaping the microbiota, and in disease contexts such as cancer, neurodegeneration, metabolic syndromes, and aging. The emergence of the field of innate immunity has led to an expanded view of the immune system, which is no longer restricted to vertebrates and instead concerns all metazoans, plants, and even prokaryotes. The study of innate immunity has given rise to new concepts and language. Here, we review the history and definition of the core concepts of innate immunity, discussing their value and fruitfulness in the long run.

The MRE11-ATM-SOG1 DNA damage signaling pathway confers rice immunity to Xanthomonas oryzae

Plants are constantly exposed to microbial pathogens in the environment. One branch of innate plant immunity is mediated by cell-membrane-localized receptors, but less is known about associations between DNA damage and plant immune responses. Here, we show that rice (Oryza sativa) mesophyll cells are prone to DNA double-stranded breaks (DSBs) in response to ZJ173, a strain of Xanthomonas oryzae pv. oryzae (Xoo). The DSB signal transducer ataxia telangiectasia mutated (ATM), but not the ATM and Rad3-related branch, confers resistance against Xoo. Mechanistically, the MRE11-ATM module phosphorylates suppressor of gamma response 1 (SOG1), which activates several phenylpropanoid pathway genes and prompts downstream phytoalexin biosynthesis during Xoo infection. Intriguingly, overexpression of the topoisomerase gene TOP6A3 causes a switch from the classic non-homologous end joining (NHEJ) pathway to the alternative NHEJ and homologous recombination pathways at Xoo-induced DSBs. The enhanced ATM signaling of the alternative NHEJ pathway strengthens the SOG1-regulated phenylpropanoid pathway and thereby boosts Xoo-induced phytoalexin biosynthesis in TOP6A3-OE1 overexpression lines. Overall, the MRE11-ATM-SOG1 pathway serves as a prime example of plant-pathogen interactions that occur via host non-specific recognition. The function of TOP6-facilitated ATM signaling in the defense response makes it a promising target for breeding of rice germplasm that exhibits resistance to bacterial blight disease without a growth penalty.

Releasing hidden MAMPs from precursor proteins in plants

The recognition of pathogens by plants at the cell surface is crucial for activating plant immunity. Plants employ pattern recognition receptors (PRRs) to detect microbe-associated molecular patterns (MAMPs). However, our knowledge of the release of peptide MAMPs from their precursor proteins is very limited. Here, we explore seven protein precursors of well-known MAMP peptides and discuss the likelihood of processing being required for their recognition based on structural models and public knowledge. This analysis indicates the existence of multiple extracellular events that are likely pivotal for pathogen perception but remain to be uncovered.

Elevating plant immunity by translational regulation of a rice WRKY transcription factor

Plants have intricate mechanisms that tailor their defence responses to pathogens. WRKY transcription factors play a pivotal role in plant immunity by regulating various defence signalling pathways. Many WRKY genes are transcriptionally activated upon pathogen attack, but how their functions are regulated after transcription remains elusive. Here, we show that OsWRKY7 functions as a crucial positive regulator of rice basal immunity against Xanthomonas oryzae pv. oryzae (Xoo). The activity of OsWRKY7 was regulated at both translational and post-translational levels. Two translational products of OsWRKY7 were generated by alternative initiation. The full-length OsWRKY7 protein is normally degraded by the ubiquitin-proteasome system but was accumulated following elicitor or pathogen treatment, whereas the alternate product initiated from the downstream in-frame start codon was stable. Both the full and alternate OsWRKY7 proteins have transcriptional activities in yeast and rice cells, and overexpression of each form enhanced resistance to Xoo infection. Furthermore, disruption of the main AUG in rice increased the endogenous translation of the alternate stabilized form of OsWRKY7 and enhanced bacterial blight resistance. This study provides insights into the coordination of alternative translation and protein stability in the regulation of plant growth and basal defence mediated by the OsWRKY7 transcription factor, and also suggests a promising strategy to breed disease-resistant rice by translation initiation control.

Bacterial indole-3-acetic acid: A key regulator for plant growth, plant-microbe interactions, and agricultural adaptive resilience

Indole-3-acetic acid (IAA), a fundamental phytohormone categorized under auxins, not only influences plant growth and development but also plays a critical role in plant-microbe interactions. This study reviews the role of IAA in bacteria-plant communication, with a focus on its biosynthesis, regulation, and the subsequent effects on host plants. Bacteria synthesize IAA through multiple pathways, which include the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and several other routes, whose full mechanisms remain to be fully elucidated. The production of bacterial IAA affects root architecture, nutrient uptake, and resistance to various abiotic stresses such as drought, salinity, and heavy metal toxicity, enhancing plant resilience and thus offering promising routes to sustainable agriculture. Bacterial IAA synthesis is regulated through complex gene networks responsive to environmental cues, impacting plant hormonal balances and symbiotic relationships. Pathogenic bacteria have adapted mechanisms to manipulate the host's IAA dynamics, influencing disease outcomes. On the other hand, beneficial bacteria utilize IAA to promote plant growth and mitigate abiotic stresses, thereby enhancing nutrient use efficiency and reducing dependency on chemical fertilizers. Advancements in analytical methods, such as liquid chromatography-tandem mass spectrometry, have improved the quantification of bacterial IAA, enabling accurate measurement and analysis. Future research focusing on molecular interactions between IAA-producing bacteria and host plants could facilitate the development of biotechnological applications that integrate beneficial bacteria to improve crop performance, which is essential for addressing the challenges posed by climate change and ensuring global food security. This integration of bacterial IAA producers into agricultural practice promises to revolutionize crop management strategies by enhancing growth, fostering resilience, and reducing environmental impact.

The OsCPK17-OsPUB12-OsRLCK176 module regulates immune homeostasis in rice

Plant immunity is fine-tuned to balance growth and defense. However, little is yet known about molecular mechanisms underlying immune homeostasis in rice (Oryza sativa). In this study, we reveal that a rice calcium-dependent protein kinase (CDPK), OsCPK17, interacts with and stabilizes the receptor-like cytoplasmic kinase (RLCK) OsRLCK176, a close homolog of Arabidopsis thaliana BOTRYTIS-INDUCED KINASE 1 (AtBIK1). Oxidative burst and pathogenesis-related gene expression triggered by pathogen-associated molecular patterns are significantly attenuated in the oscpk17 mutant. The oscpk17 mutant and OsCPK17-silenced lines are more susceptible to bacterial diseases than the wild-type plants, indicating that OsCPK17 positively regulates rice immunity. Furthermore, the plant U-box (PUB) protein OsPUB12 ubiquitinates and degrades OsRLCK176. OsCPK17 phosphorylates OsRLCK176 at Ser83, which prevents the ubiquitination of OsRLCK176 by OsPUB12 and thereby enhances the stability and immune function of OsRLCK176. The phenotypes of the ospub12 mutant in defense responses and disease resistance show that OsPUB12 negatively regulates rice immunity. Therefore, OsCPK17 and OsPUB12 reciprocally maintain OsRLCK176 homeostasis and function as positive and negative immune regulators, respectively. This study uncovers positive cross talk between CDPK- and RLCK-mediated immune signaling in plants and reveals that OsCPK17, OsPUB12, and OsRLCK176 maintain rice immune homeostasis.

Functional analysis of reactive oxygen species-driven stress systemic signalling, interplay and acclimation

Reactive oxygen species (ROS) play a critical role in plant development and stress responses, acting as key components in rapid signalling pathways. The 'ROS wave' triggers essential acclimation processes, ultimately ensuring plant survival under diverse challenges. This review explores recent advances in understanding the composition and functionality of the ROS wave within plant cells. During their initiation and propagation, ROS waves interact with other rapid signalling pathways, hormones and various molecular compounds. Recent research sheds light on the intriguing lack of a rigid hierarchy governing these interactions, highlighting a complex interplay between diverse signals. Notably, ROS waves culminate in systemic acclimation, a crucial outcome for enhanced stress tolerance. This review emphasizes the versatility of ROS, which act as flexible players within a network of short- and long-term factors contributing to plant stress resilience. Unveiling the intricacies of these interactions between ROS and various signalling molecules holds immense potential for developing strategies to augment plant stress tolerance, contributing to improved agricultural practices and overall ecosystem well-being.

Bacterial host adaptation through sequence and structural variations of a single type III effector gene

Molecular mechanisms underlying quantitative variations of pathogenicity remain elusive. Here, we identified the Xanthomonas campestris XopJ6 effector that triggers disease resistance in cauliflower and Arabidopsis thaliana. XopJ6 is a close homolog of the Ralstoniapseudosolanacearum PopP2 YopJ family acetyltransferase. XopJ6 is recognized by the RRS1-R/RPS4 NLR pair that integrates a WRKY decoy domain mimicking effector targets. We identified a XopJ6 natural variant carrying a single residue substitution in XopJ6 WRKY-binding site that disrupts interaction with WRKY proteins. This mutation allows XopJ6 to evade immune perception while retaining some XopJ6 virulence functions. Interestingly, xopJ6 resides in a Tn3-family transposon likely contributing to xopJ6 copy number variation (CNV). Using synthetic biology, we demonstrate that xopJ6 CNV tunes pathogen virulence on Arabidopsis through gene dosage-mediated modulation of xopJ6 expression. Together, our findings highlight how sequence and structural genetic variations restricted at a particular effector gene contribute to bacterial host adaptation.

Signals and Machinery for Mycorrhizae and Cereal and Oilseed Interactions towards Improved Tolerance to Environmental Stresses

In the quest for sustainable agricultural practices, there arises an urgent need for alternative solutions to mineral fertilizers and pesticides, aiming to diminish the environmental footprint of farming. Arbuscular mycorrhizal fungi (AMF) emerge as a promising avenue, bestowing plants with heightened nutrient absorption capabilities while alleviating plant stress. Cereal and oilseed crops benefit from this association in a number of ways, including improved growth fitness, nutrient uptake, and tolerance to environmental stresses. Understanding the molecular mechanisms shaping the impact of AMF on these crops offers encouraging prospects for a more efficient use of these beneficial microorganisms to mitigate climate change-related stressors on plant functioning and productivity. An increased number of studies highlighted the boosting effect of AMF on grain and oil crops' tolerance to (a)biotic stresses while limited ones investigated the molecular aspects orchestrating the different involved mechanisms. This review gives an extensive overview of the different strategies initiated by mycorrhizal cereal and oilseed plants to manage the deleterious effects of environmental stress. We also discuss the molecular drivers and mechanistic concepts to unveil the molecular machinery triggered by AMF to alleviate the tolerance of these crops to stressors.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

Structural and biochemical basis of FLS2-mediated signal activation and transduction in rice

The receptor-like kinase FLAGELLIN-SENSITIVE 2 (FLS2) functions as a bacterial flagellin receptor localized on the cell membrane of plants. In Arabidopsis, the co-receptor BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) cooperates with FLS2 to detect the flagellin epitope flg22, resulting in formation of a signaling complex that triggers plant defense responses. However, the co-receptor responsible for recognizing and signaling the flg22 epitope in rice remains to be determined, and the precise structural mechanism underlying FLS2-mediated signal activation and transduction has not been clarified. This study presents the structural characterization of a kinase-dead mutant of the intracellular kinase domain of OsFLS2 (OsFLS2-KDD1013A) in complex with ATP or ADP, resolved at resolutions of 1.98 Å and 2.09 Å, respectively. Structural analysis revealed that OsFLS2 can adopt an active conformation in the absence of phosphorylation, although it exhibits only weak basal catalytic activity for autophosphorylation. Subsequent investigations demonstrated that OsSERK2 effectively phosphorylates OsFLS2, which reciprocally phosphorylates OsSERK2, leading to complete activation of OsSERK2 and rapid phosphorylation of the downstream substrate receptor-like cytoplasmic kinases OsRLCK176 and OsRLCK185. Through mass spectrometry experiments, we successfully identified critical autophosphorylation sites on OsSERK2, as well as sites transphosphorylated by OsFLS2. Furthermore, we demonstrated the interaction between OsSERK2 and OsFLS2, which is enhanced in the presence of flg22. Genetic evidence suggests that OsRLCK176 and OsRLCK185 may function downstream of the OsFLS2-mediated signaling pathway. Our study reveals the molecular mechanism by which OsFLS2 mediates signal transduction pathways in rice and provides a valuable example for understanding RLK-mediated signaling pathways in plants.

Metformin blocks BIK1-mediated CPK28 phosphorylation and enhances plant immunity

INTRODUCTION

Metformin (MET), derived from Galega officinalis, stands as the primary first-line medication for the treatment of type 2 diabetes (T2D). Despite its well-documented benefits in mammalian cellular processes, its functions and underlying mechanisms in plants remain unclear.

OBJECTIVES

This study aimed to elucidate MET's role in inducing plant immunity and investigate the associated mechanisms.

METHODS

To investigate the impact of MET on enhancing plant immune responses, we conducted assays measuring defense gene expression, reactive oxygen species (ROS) accumulation, mitogen-activated protein kinase (MAPK) phosphorylation, and pathogen infection. Additionally, surface plasmon resonance (SPR) and microscale thermophoresis (MST) techniques were employed to identify MET targets. Protein-protein interactions were analyzed using a luciferase complementation assay and a co-immunoprecipitation assay.

RESULTS

Our findings revealed that MET boosts plant disease resistance by activating MAPKs, upregulating the expression of downstream defense genes, and fortifying the ROS burst. CALCIUM-DEPENDENT PROTEIN KINASE 28 (CPK28) was identified as a target of MET. It inhibited the interaction between BOTRYTIS-INDUCED KINASE 1 (BIK1) and CPK28, blocking CPK28 threonine 76 (T76) transphosphorylation by BIK1, and alleviating the negative regulation of immune responses by CPK28. Moreover, MET enhanced disease resistance in tomato, pepper, and soybean plants.

CONCLUSION

Collectively, our data suggest that MET enhances plant immunity by blocking BIK1-mediated CPK28 phosphorylation.

Lipid mediated plant immunity in susceptible and tolerant soybean cultivars in response to Phytophthora sojae colonization and infection

BACKGROUND

Soybean is one of the most cultivated crops globally and a staple food for much of the world's population. The annual global crop losses due to infection by Phytophthora sojae is currently estimated at $20B USD, yet we have limited understanding of the role of lipid mediators in the adaptative strategies used by the host plant to limit infection. Since root is the initial site of this infection, we examined the infection process in soybean root infected with Phytophthora sojae using scanning electron microscopy to observe the changes in root morphology and a multi-modal lipidomics approach to investigate how soybean cultivars remodel their lipid mediators to successfully limit infection by Phytophthora sojae.

RESULTS

The results reveal the presence of elevated biogenic crystals and more severe damaged cells in the root morphology of the infected susceptible cultivar compared to the infected tolerant cultivars. Furthermore, induced accumulation of stigmasterol was observed in the susceptible cultivar whereas, induced accumulation of phospholipids and glycerolipids occurred in tolerant cultivar.

CONCLUSION

The altered lipidome reported in this study suggest diacylglycerol and phosphatidic acid mediated lipid signalling impacting phytosterol anabolism appears to be a strategy used by tolerant soybean cultivars to successfully limit infection and colonization by Phytophthora sojae.

Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

Nonpathogenic Pseudomonas syringae derivatives and its metabolites trigger the plant "cry for help" response to assemble disease suppressing and growth promoting rhizomicrobiome

Plants are capable of assembling beneficial rhizomicrobiomes through a "cry for help" mechanism upon pathogen infestation; however, it remains unknown whether we can use nonpathogenic strains to induce plants to assemble a rhizomicrobiome against pathogen invasion. Here, we used a series of derivatives of Pseudomonas syringae pv. tomato DC3000 to elicit different levels of the immune response to Arabidopsis and revealed that two nonpathogenic DC3000 derivatives induced the beneficial soil-borne legacy, demonstrating a similar "cry for help" triggering effect as the wild-type DC3000. In addition, an increase in the abundance of Devosia in the rhizosphere induced by the decreased root exudation of myristic acid was confirmed to be responsible for growth promotion and disease suppression of the soil-borne legacy. Furthermore, the "cry for help" response could be induced by heat-killed DC3000 and flg22 and blocked by an effector triggered immunity (ETI) -eliciting derivative of DC3000. In conclusion, we demonstrate the potential of nonpathogenic bacteria and bacterial elicitors to promote the generation of disease-suppressive soils.

HvMPK4 phosphorylates HvWRKY1 to enhance its suppression of barley immunity to powdery mildew fungus

Mitogen-activated protein kinase (MAPK) cascades play important roles in disease resistance in model plant species. However, the functions of MAPK signaling pathways in crop disease resistance are largely unknown. Here we report the function of HvMKK1-HvMPK4-HvWRKY1 module in barley immune system. HvMPK4 is identified to play a negative role in barley immune response against Bgh, as virus-induced gene silencing of HvMPK4 results in enhanced disease resistance whilst stably overexpressing HvMPK4 leads to super-susceptibility to Bgh infection. Furthermore, the barley MAPK kinase HvMKK1 is found to specifically interact with HvMPK4, and the activated HvMKK1DD variant specifically phosphorylates HvMPK4 in vitro. Moreover, the transcription factor HvWRKY1 is identified to be a downstream target of HvMPK4 and phosphorylated by HvMPK4 in vitro in the presence of HvMKK1DD. Phosphorylation assay coupled with mutagenesis analyses identifies S122, T284, and S347 in HvWRKY1 as the major residues phosphorylated by HvMPK4. HvWRKY1 is phosphorylated in barley at the early stages of Bgh infection, which enhances its suppression on barley immunity likely due to enhanced DNA-binding and transcriptional repression activity. Our data suggest that the HvMKK1-HvMPK4 kinase pair acts upstream of HvWRKY1 to negatively regulate barley immunity against powdery mildew.

Long small RNA76113 targets CYCLIC NUCLEOTIDE-GATED ION CHANNEL 5 to repress disease resistance in rice

Small RNAs are widely involved in plant immune responses. However, the role of long small RNAs (25 to 40 nt) in monocot plant disease resistance is largely unknown. Here, we identified a long small RNA (lsiR76113) from rice (Oryza sativa) that is downregulated by Magnaporthe oryzae infection and targets a gene encoding CYCLIC NUCLEOTIDE-GATED CHANNEL 5 (CNGC5). The cngc5 mutant lines were more susceptible to M. oryzae than the wild type, while knocking down lsiR76113 in transgenic rice plants promoted pathogen resistance. A protoplast transient expression assay showed that OsCNGC5 promotes Ca2+ influx. These results demonstrate that OsCNGC5 enhances rice resistance to rice blast by increasing the cytosolic Ca2+ concentration. Importantly, exogenous Ca2+ application enhanced rice M. oryzae resistance by affecting reactive oxygen species (ROS) production. Moreover, cngc5 mutants attenuated the PAMP-triggered immunity response, including chitin-induced and flg22-induced ROS bursts and protein phosphorylation in the mitogen-activated protein kinase cascade, indicating that OsCNGC5 is essential for PAMP-induced calcium signaling in rice. Taken together, these results suggest that lsiR76113-mediated regulation of Ca2+ influx is important for PTI responses and disease resistance in rice.

Receptor-like kinases OsRLK902-1 and OsRLK902-2 form immune complexes with OsRLCK185 to regulate rice blast resistance

Receptor-like kinases (RLKs) are major regulators of the plant immune response and play important roles in the perception and transmission of immune signals. RECEPTOR LIKE KINASE 902 (RLK902) is at the key node in leucine-rich repeat receptor-like kinase interaction networks and positively regulates resistance to the bacterial pathogen Pseudomonas syringae in Arabidopsis. However, the function of RLK902 in fungal disease resistance remains obscure. In this study, we found that the expression levels of OsRLK902-1 and OsRLK902-2, encoding two orthologues of RLK902 in rice, were induced by Magnaporthe oryzae, chitin, and flg22 treatment. osrlk902-1 and osrlk902-2 knockout mutants displayed enhanced susceptibility to M. oryzae. Interestingly, the osrlk902-1 rlk902-2 double mutant exhibited similar disease susceptibility, hydrogen peroxide production, and callose deposition to the two single mutants. Further investigation showed that OsRLK902-1 interacts with and stabilizes OsRLK902-2. The two OsRLKs form a complex with OsRLCK185, a key regulator in chitin-triggered immunity, and stabilize it. Taken together, our data demonstrate that OsRLK902-1 and OsRLK902-2, as well as OsRLCK185 function together in regulating disease resistance to M. oryzae in rice.

Genome-Wide Comprehensive Identification and In Silico Characterization of Lectin Receptor-Like Kinase Gene Family in Barley (Hordeum vulgare L.)

Lectin receptor-like kinases (LecRLKs) are a significant subgroup of the receptor-like kinases (RLKs) protein family. They play crucial roles in plant growth, development, immune responses, signal transduction, and stress tolerance. However, the genome-wide identification and characterization of LecRLK genes and their regulatory elements have not been explored in a major cereal crop, barley (Hordeum vulgare L.). Therefore, in this study, integrated bioinformatics tools were used to identify and characterize the LecRLK gene family in barley. Based on the phylogenetic tree and domain organization, a total of 113 LecRLK genes were identified in the barley genome (referred to as HvlecRLK) corresponding to the LecRLK genes of Arabidopsis thaliana. These putative HvlecRLK genes were classified into three groups: 62 G-type LecRLKs, 1 C-type LecRLK, and 50 L-type LecRLKs. They were unevenly distributed across eight chromosomes, including one unknown chromosome, and were predominantly located in the plasma membrane (G-type HvlecRLK (96.8%), C-type HvlecRLK (100%), and L-type HvlecRLK (98%)). An analysis of motif composition and exon-intron configuration revealed remarkable homogeneity with the members of AtlecRLK. Notably, most of the HvlecRLKs (27 G-type, 43 L-type) have no intron, suggesting their rapid functionality. The Ka/Ks and syntenic analysis demonstrated that HvlecRLK gene pairs evolved through purifying selection and gene duplication was the major factor for the expansion of the HvlecRLK gene family. Exploration of gene ontology (GO) enrichment indicated that the identified HvlecRLK genes are associated with various cellular processes, metabolic pathways, defense mechanisms, kinase activity, catalytic activity, ion binding, and other essential pathways. The regulatory network analysis identified 29 transcription factor families (TFFs), with seven major TFFs including bZIP, C2H2, ERF, MIKC_MADS, MYB, NAC, and WRKY participating in the regulation of HvlecRLK gene functions. Most notably, eight TFFs were found to be linked to the promoter region of both L-type HvleckRLK64 and HvleckRLK86. The promoter cis-acting regulatory element (CARE) analysis of barley identified a total of 75 CARE motifs responsive to light responsiveness (LR), tissue-specific (TS), hormone responsiveness (HR), and stress responsiveness (SR). The maximum number of CAREs was identified in HvleckRLK11 (25 for LR), HvleckRLK69 (17 for TS), and HvleckRLK80 (12 for HR). Additionally, HvleckRLK14, HvleckRLK16, HvleckRLK33, HvleckRLK50, HvleckRLK52, HvleckRLK56, and HvleckRLK110 were predicted to exhibit higher responses in stress conditions. In addition, 46 putative miRNAs were predicted to target 81 HvlecRLK genes and HvlecRLK13 was the most targeted gene by 8 different miRNAs. Protein-protein interaction analysis demonstrated higher functional similarities of 63 HvlecRLKs with 7 Arabidopsis STRING proteins. Our overall findings provide valuable information on the LecRLK gene family which might pave the way to advanced research on the functional mechanism of the candidate genes as well as to develop new barley cultivars in breeding programs.

Antiviral Mechanisms of Anisomycin Produced by Streptomyces albulus SN40 on Potato Virus Y

Microbial secondary metabolites produced by Streptomyces have diverse application prospects in the control of plant diseases. Herein, the fermentation filtrate of Streptomyces SN40 effectively inhibited the infection of tobacco mosaic virus (TMV) in Nicotiana glutinosa and systemic infection of potato virus Y (PVY) in Nicotiana benthamiana. Additionally, metabolomic analysis indicated that anisomycin (C14H19NO4) and trans-3-indoleacrylic acid (C11H9NO2) were highly abundant in the crude extract and that anisomycin effectively suppressed the infection of TMV as well as PVY. Subsequently, transcriptomic analysis was conducted to elucidate its mechanisms on the induction of host defense responses. Furthermore, the results of molecular docking suggested that anisomycin can potentially bind with the helicase domain (Hel) of TMV replicase, TMV coat protein (CP), and PVY helper component proteinase (HC-Pro). This study demonstrates new functions of anisomycin in virus inhibition and provides important theoretical significance for the development of new biological pesticides to control diverse plant viruses.

Plant elicitor Peptides regulate root hair development in Arabidopsis

Plant Elicitor Peptides (Peps) induce plant immune responses and inhibit root growth through their receptors PEPR1 and PEPR2, two receptor-like kinases. In our study, we found a previously unknown function of Peps that enhance root hair growth in a PEPRs-independent manner. When we characterized the expression patterns of PROPEP genes, we found several gene promoters of PROPEP gene family were particularly active in root hairs. Furthermore, we observed that PROPEP2 is vital for root hair development, as disruption of PROPEP2 gene led to a significant reduction in root hair density and length. We also discovered that PROPEP2 regulates root hair formation via the modulation of CPC and GL2 expression, thereby influencing the cell-fate determination of root hairs. Additionally, calcium signaling appeared to be involved in PROPEP2/Pep2-induced root hair growth. These findings shed light on the function of Peps in root hair development.

An organismal understanding of C. elegans innate immune responses, from pathogen recognition to multigenerational resistance

The nematode Caenorhabditis elegans has been a model for studying infection since the early 2000s and many major discoveries have been made regarding its innate immune responses. C. elegans has been found to utilize some key conserved aspects of immune responses and signaling, but new interesting features of innate immunity have also been discovered in the organism that might have broader implications in higher eukaryotes such as mammals. Some of the distinctive features of C. elegans innate immunity involve the mechanisms this bacterivore uses to detect infection and mount specific immune responses to different pathogens, despite lacking putative orthologs of many important innate immune components, including cellular immunity, the inflammasome, complement, or melanization. Even when orthologs of known immune factors exist, there appears to be an absence of canonical functions, most notably the lack of pattern recognition by its sole Toll-like receptor. Instead, recent research suggests that C. elegans senses infection by specific pathogens through contextual information, including unique products produced by the pathogen or infection-induced disruption of host physiology, similar to the proposed detection of patterns of pathogenesis in mammalian systems. Interestingly, C. elegans can also transfer information of past infection to their progeny, providing robust protection for their offspring in face of persisting pathogens, in part through the RNAi pathway as well as potential new mechanisms that remain to be elucidated. Altogether, some of these strategies employed by C. elegans share key conceptual features with vertebrate adaptive immunity, as the animal can differentiate specific microbial features, as well as propagate a form of immune memory to their offspring.

Magnaporthe oryzae effector AvrPik-D targets a transcription factor WG7 to suppress rice immunity

Rice blast, caused by the fungal pathogen Magnaporthe oryzae, is one of the most devastating diseases for rice crops, significantly affecting crop yield and quality. During the infection process, M. oryzae secretes effector proteins that help in hijacking the host's immune responses to establish infection. However, little is known about the interaction between the effector protein AvrPik-D and the host protein Pikh, and how AvrPik-D increases disease severity to promote infection. In this study, we show that the M. oryzae effector AvrPik-D interacts with the zinc finger-type transcription factor WG7 in the nucleus and promotes its transcriptional activity. Genetic removal (knockout) of the gene WG7 in transgenic rice enhances resistance to M. oryzae and also results in an increased burst of reactive oxygen species after treatments with chitin. In addition, the hormone level of SA and JA, is increased and decreased respectively in WG7 KO plants, indicating that WG7 may negatively mediate resistance through salicylic acid pathway. Conversely, WG7 overexpression lines reduce resistance to M. oryzae. However, WG7 is not required for the Pikh-mediated resistance against rice blast. In conclusion, our results revealed that the M. oryzae effector AvrPik-D targets and promotes transcriptional activity of WG7 to suppress rice innate immunity to facilitate infection.

OsLRR-RLP2 Gene Regulates Immunity to Magnaporthe oryzae in Japonica Rice

Rice is an important cereal crop worldwide, the growth of which is affected by rice blast disease, caused by the fungal pathogen Magnaporthe oryzae. As climate change increases the diversity of pathogens, the disease resistance genes (R genes) in plants must be identified. The major blast-resistance genes have been identified in indica rice varieties; therefore, japonica rice varieties with R genes now need to be identified. Because leucine-rich repeat (LRR) domain proteins possess R-gene properties, we used bioinformatics analysis to identify the rice candidate LRR domain receptor-like proteins (OsLRR-RLPs). OsLRR-RLP2, which contains six LRR domains, showed differences in the DNA sequence, containing 43 single-nucleotide polymorphisms (SNPs) in indica and japonica subpopulations. The results of the M. oryzae inoculation analysis indicated that indica varieties with partial deletion of OsLRR-RLP2 showed susceptibility, whereas japonica varieties with intact OsLRR-RLP2 showed resistance. The oslrr-rlp2 mutant, generated using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), showed increased pathogen susceptibility, whereas plants overexpressing this gene showed pathogen resistance. These results indicate that OsLRR-RLP2 confers resistance to rice, and OsLRR-RLP2 may be useful for breeding resistant cultivars.

Increasing vineyard sustainability: innovating a targeted chitosan-derived biocontrol solution to induce grapevine resistance against downy and powdery mildews

The European Green Deal aims to reduce the pesticide use, notably by developing biocontrol products to protect crops from diseases. Indeed, the use of significant amounts of chemicals negatively impact the environment such as soil microbial biodiversity or groundwater quality, and human health. Grapevine (Vitis vinifera) was selected as one of the first targeted crop due to its economic importance and its dependence on fungicides to control the main damaging diseases worldwide: grey mold, downy and powdery mildews. Chitosan, a biopolymer extracted from crustacean exoskeletons, has been used as a biocontrol agent in many plant species, including grapevine, against a variety of cryptogamic diseases such as downy mildew (Plasmopara viticola), powdery mildew (Erysiphe necator) and grey mold (Botrytis cinerea). However, the precise molecular mechanisms underlying its mode of action remain unclear: is it a direct biopesticide effect or an indirect elicitation activity, or both? In this study, we investigated six chitosans with diverse degrees of polymerization (DP) ranging from low to high DP (12, 25, 33, 44, 100, and 470). We scrutinized their biological activities by evaluating both their antifungal properties and their abilities to induce grapevine immune responses. To investigate their elicitor activity, we analyzed their ability to induce MAPKs phosphorylation, the activation of defense genes and metabolite changes in grapevine. Our results indicate that the chitosans with a low DP are more effective in inducing grapevine defenses and possess the strongest biopesticide effect against B. cinerea and P. viticola. We identified chitosan with DP12 as the most efficient resistance inducer. Then, chitosan DP12 has been tested against downy and powdery mildews in the vineyard trials performed during the last three years. Results obtained indicated that a chitosan-based biocontrol product could be sufficiently efficient when the amount of pathogen inoculum is quite low and could be combined with only two fungicide treatments during whole season programs to obtain a good protection efficiency. On the whole, a chitosan-based biocontrol product could become an interesting alternative to meet the chemicals reduction targeted in sustainable viticulture.

E3 Ubiquitin Ligase PUB23 in Kiwifruit Interacts with Trihelix Transcription Factor GT1 and Negatively Regulates Immune Responses against Pseudomonas syringae pv. actinidiae

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) is the most serious disease threatening kiwifruit production. Our previous study found genes encoding the U-box containing proteins were significantly regulated by Psa infection. Here, we report a U-box type E3 ubiquitin ligase PUB23 in kiwifruit which acts as a negative regulator of immune responses against Psa. PUB23 was found to physically interact with GT1, a trihelix transcription factor, in vitro and in vivo. The expression of GT1 was up-regulated in PUB23-silenced plants, indicating that interacting with PUB23 may directly or indirectly suppress GT1 expression. The silencing of PUB23 led to enhanced immune responses of PAMP-triggered immunity (PTI), including a higher expression level of defense marker genes PR1 and RIN4, and increased accumulation of hydrogen peroxide and superoxide anion. Our results reveal a negative role PUB23 plays in kiwifruit immune responses against Psa and may regulate gene expression by interacting with GT1.

Additive and Specific Effects of Elicitor Treatments on the Metabolic Profile of Arabidopsis thaliana

Several elicitors of plant defense have been identified and numerous efforts to use them in the field have been made. Exogenous elicitor treatments mimic the in planta activation of pattern-triggered immunity (PTI), which relies on the perception of pathogen-associated molecular patterns (PAMPs) such as bacterial flg22 or fungal chitins. Early transcriptional responses to distinct PAMPs are mostly overlapping, regardless of the elicitor being used. However, it remains poorly known if the same patterns are observed for metabolites and proteins produced later during PTI. In addition, little is known about the impact of a combination of elicitors on PTI and the level of induced resistance to pathogens. Here, we monitored Arabidopsis thaliana resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pto DC3000) following application of flg22 and chitosan elicitors, used individually or in combination. A slight, but not statistically significant increase in induced resistance was observed when the elicitors were applied together when compared with individual treatments. We investigated the effect of these treatments on the metabolome by using an untargeted analysis. We found that the combination of flg22 and chitosan impacted a higher number of metabolites and deregulated specific metabolic pathways compared with the elicitors individually. These results contribute to a better understanding of plant responses to elicitors, which might help better rationalize their use in the field. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Cocoa Apoplastome Contains Defense Proteins Against Pathogens

The apoplast performs important functions in the plant, such as defense against stress, and compounds present form the apoplastic washing fluid (AWF). The fungus Moniliophthora perniciosa, the causal agent of witches' broom disease (WBD) in Theobroma cacao, initially colonizes the apoplast in its biotrophic phase. In this period, the fungus can remain for approximately 60 days, until it changes to its second phase, causing tissue death and consequently large loss in the production of beans. To better understand the importance of the apoplast in the T. cacao-M. perniciosa interaction, we performed the first apoplastic proteomic mapping of two contrasting genotypes for WBD resistance (CCN51-resistant and Catongo-susceptible). Based on two-dimensional gel analysis, we identified 36 proteins in CCN-51 and 15 in Catongo. We highlight PR-proteins, such as peroxidases, β-1,3-glucanases, and chitinases. A possible candidate for a resistance marker of the CCN-51 genotype, osmotin, was identified. The antioxidative metabolism of the superoxide dismutase (SOD) enzyme showed a significant increase (P < 0.05) in the AWF of the two genotypes under field conditions (FD). T. cacao AWF inhibited the germination of M. perniciosa basidiospores (>80%), in addition to causing morphological changes. Our results shed more light on the nature of the plant's defense performed by the apoplast in the T. cacao-M. perniciosa interaction in the initial (biotrophic) phase of fungal infection and therefore make it possible to expand WBD control strategies based on the identification of potential targets for resistance markers and advance scientific knowledge of the disease.

A Sorghum F-Box Protein Induces an Oxidative Burst in the Defense Against Colletotrichum sublineola

The hemibiotrophic fungal pathogen Colletotrichum sublineola is the causal agent of anthracnose in sorghum (Sorghum bicolor), resulting in leaf blight, stalk rot, and head blight in susceptible genotypes, with yield losses of up to 50%. The development of anthracnose-resistant cultivars can reduce reliance on fungicides and provide a more sustainable and economical means for disease management. A previous genome-wide association study of the sorghum association panel identified the candidate resistance gene Sobic.005G172300 encoding an F-box protein. To better understand the role of this gene in the defense against C. sublineola, gene expression following infection with C. sublineola was monitored by RNA sequencing in seedlings of sorghum accession SC110, which harbored the resistance allele, and three accessions that harbored a susceptible allele. Only in SC110 did the expression of Sobic.005G172300 increase during the biotrophic phase of infection. Subsequent transcriptome analysis, gene co-expression networks, and gene regulatory networks of inoculated and mock-inoculated seedlings of resistant and susceptible accessions suggest that the increase in expression of Sobic.005G172300 induces an oxidative burst by lowering the concentration of ascorbic acid during the biotrophic phase of infection. Based on gene regulatory network analysis, the protein encoded by Sobic.005G172300 is proposed to target proteins involved in the biosynthesis of ascorbic acid for polyubiquitination through the SCF E3 ubiquitin ligase, causing their degradation via the proteasome.

Genome-wide analysis and characterization of the TaTLP gene family in wheat and functional characterization of the TaTLP44 in response to Rhizoctonia cerealis

Wheat sharp eyespot is a soil-borne disease caused by Rhizoctonia cerealis, which occurs in many countries worldwide and significantly reduces the yield. Thaumatin-like protein (TLP), also known as PR5, is a member of the pathogen response protein family and plays an essential role in plant resistance to pathogen infection. In this study, 131 TaTLP genes were identified from the wheat genome, of which 38 TaTLPs were newly discovered. The TaTLP gene family contains many tandem duplications and fragment duplications, which is a major pathway for gene amplification. Besides, we also analyzed the physicochemical properties, gene structure and promoter cis-acting regulatory elements of all the TaTLP genes. In addition, the expression patterns of nine TaTLPs in response to R. cerealis were analyzed by RT-qPCR. Six TaTLP proteins expressed in vitro had no significant inhibitory effect on R. cerealis, suggesting that these TaTLP proteins may function in other ways. Finally, we performed gene silencing of TaTLP44 in wheat, which increased the expression of some defense-associated genes and improved resistance to R. cerealis. In summary, we systematically analyzed TaTLP family members and demonstrated that TaTLP44 negatively regulates the resistance to R. cerealis by controlling expression of defense-associated genes. These results provide new insights into the functional mechanism of TaTLP proteins.

Oomycete effector AVRblb2 targets cyclic nucleotide-gated channels through calcium sensors to suppress pattern-triggered immunity

Transient and rapid increase in cytosolic Ca2+ plays a crucial role in plant-pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Cyclic nucleotide-gated channels (CNGCs) have been implicated in mediating this Ca2+ influx; however, their regulatory mechanisms remain poorly understood. Here, we have found that AVRblb2 requires the calmodulin (CaM) and calmodulin-like (CML) proteins as co-factors to interact with the NbCNGCs, resulting in the formation of AVRblb2-CaM/CML-NbCNGCs complex. Furthermore, CaM and CML are dissociated from NbCNGC18 during PTI response to increase Ca2+ influx; however, Avrblb2 inhibits calcium channel activation by disrupting the release of CaM and CML from NbCNGC18. Following recognition of PAMP, NbCNGC18 forms active heteromeric channels with other NbCNGCs, which may give selectivity of CNGC complex against diverse signals for fine-tuning of cytosolic Ca2+ level to mediate appropriate responses. Silencing of multiple NbCNGCs compromised the function of AVRblb2 on the pathogenicity of Phytophthora infestans, confirming that AVRblb2 contributes to pathogen virulence by targeting CNGCs. Our findings provide new insights into the regulation of CNGCs in PTI and the role of pathogen effectors in manipulating host cell physiology to promote infection.

Commensal lifestyle regulated by a negative feedback loop between Arabidopsis ROS and the bacterial T2SS

Despite the plant health-promoting effects of plant microbiota, these assemblages also comprise potentially detrimental microbes. How plant immunity controls its microbiota to promote plant health under these conditions remains largely unknown. We find that commensal bacteria isolated from healthy Arabidopsis plants trigger diverse patterns of reactive oxygen species (ROS) production dependent on the immune receptors and completely on the NADPH oxidase RBOHD that selectively inhibited specific commensals, notably Xanthomonas L148. Through random mutagenesis, we find that L148 gspE, encoding a type II secretion system (T2SS) component, is required for the damaging effects of Xanthomonas L148 on rbohD mutant plants. In planta bacterial transcriptomics reveals that RBOHD suppresses most T2SS gene expression including gspE. L148 colonization protected plants against a bacterial pathogen, when gspE was inhibited by ROS or mutation. Thus, a negative feedback loop between Arabidopsis ROS and the bacterial T2SS tames a potentially detrimental leaf commensal and turns it into a microbe beneficial to the host.

Phytosulfokine peptides, their receptors, and functions

Phytosulfokines (PSKs) are a class of disulfated pentapeptides and are regarded as plant peptide hormones. PSK-α, -γ, -δ, and -ϵ are four bioactive PSKs that are reported to have roles in plant growth, development, and immunity. In this review, we summarize recent advances in PSK biosynthesis, signaling, and function. PSKs are encoded by precursor genes that are widespread in higher plants. PSKs maturation from these precursors requires a sulfation step, which is catalyzed by a tyrosylprotein sulfotransferase, as well as proteolytic cleavage by subtilisin serine proteases. PSK signaling is mediated by plasma membrane-localized receptors PSKRs that belong to the leucine-rich repeat receptor-like kinase family. Moreover, multiple biological functions can be attributed to PSKs, including promoting cell division and cell growth, regulating plant reproduction, inducing somatic embryogenesis, enhancing legume nodulation, and regulating plant resistance to biotic and abiotic stress. Finally, we propose several research directions in this field. This review provides important insights into PSKs that will facilitate biotechnological development and PSK application in agriculture.

Identification of NADPH Oxidase Genes Crucial for Rice Multiple Disease Resistance and Yield Traits

Reactive oxygen species (ROS) act as a group of signaling molecules in rice functioning in regulation of development and stress responses. Respiratory burst oxidase homologues (Rbohs) are key enzymes in generation of ROS. However, the role of the nine Rboh family members was not fully understood in rice multiple disease resistance and yield traits. In this study, we constructed mutants of each Rboh genes and detected their requirement in rice multiple disease resistance and yield traits. Our results revealed that mutations of five Rboh genes (RbohA, RbohB, RbohE, RbohH, and RbohI) lead to compromised rice blast disease resistance in a disease nursery and lab conditions; mutations of five Rbohs (RbohA, RbohB, RbohC, RbohE, and RbohH) result in suppressed rice sheath blight resistance in a disease nursery and lab conditions; mutations of six Rbohs (RbohA, RbohB, RbohC, RbohE, RbohH and RbohI) lead to decreased rice leaf blight resistance in a paddy yard and ROS production induced by PAMPs and pathogen. Moreover, all Rboh genes participate in the regulation of rice yield traits, for all rboh mutants display one or more compromised yield traits, such as panicle number, grain number per panicle, seed setting rate, and grain weight, resulting in reduced yield per plant except rbohb and rbohf. Our results identified the Rboh family members involved in the regulation of rice resistance against multiple pathogens that caused the most serious diseases worldwide and provide theoretical supporting for breeding application of these Rbohs to coordinate rice disease resistance and yield traits.

Analysis of Plant Virus-Induced Immunity by Using Viral-Derived Double-Stranded RNA in Arabidopsis thaliana

Pattern-triggered immunity is the first line of defense against infection by pathogens such as bacteria and fungi in plants, and this mechanism remains poorly defined in plant viruses. Double-stranded RNA (dsRNA) is an intermediate in the replication of plant RNA viruses, and is considered to be a conserved structure of plant viruses similar to pathogen-associated molecular pattern. Whether dsRNA is the elicitor that activates plant immunity in response to virus infection remains obscure. In this method, we use the cDNA of turnip mosaic virus genome as the template to in vitro synthesis of viral dsRNA and examine whether viral dsRNA could activate plant immunity in Arabidopsis thaliana, including MAPK kinase cascade and reactive oxygen burst. In order to provide some references for researchers studying dsRNA in terms of research methodology and experimental methods, we use western blot to measure MAPK kinase cascade and luminol-based assay to measure ROS burst in Arabidopsis thaliana treated by viral dsRNA.

In-vivo Cross-linking of Biotinylated Peptide Ligands to Cell Surface Receptors

In-vivo cross-linking of biotinylated peptides is a technique to analyze the interaction of small proteins or peptide ligands with their corresponding receptors. Here, we describe an in-vivo method in which leaves of living plants, transiently expressing receptor proteins, are infiltrated with biotinylated peptides. The interaction between ligand and receptor is irreversibly fixed by the infiltration of a cross-linking agent. Subsequently, co-immunoprecipitation is used to pull down the receptor-ligand pair. After western blotting, the biotin tag of the ligand peptide cross-linked to the receptor can be detected by streptavidin-AP conjugate on the membrane.