Of PAMPs and effectors: the blurred PTI-ETI dichotomy

The role of ribosomal protein StRPS5 in mediating resistance of Solanum tuberosum plants to Phytophthora infestans

Potato late blight caused by Phytophthora infestans is a devastating disease in potato production. Effectors secreted by P. infestans can target plant proteins and disrupt plant immune responses. The research on plant target proteins has mainly focused on ubiquitination, immune-related proteases, MAPK signal transduction pathways, and transcription factors. The question of whether plants possess novel disease resistance-related proteins or pathways remains unanswered. In this study, we identified a potato ribosomal protein, StRPS5, as a target of the P. infestans RxLR effector, Pi16275. Subcellular co-localization of StRPS5 and Pi16275 was observed in the nucleus, cytoplasm and cell membrane of Nicotiana benthamiana. The expression of StRPS5 was induced and up-regulated in the early stage of P. infestans infection. Furthermore, transient overexpression of StRPS5 in tobacco leaves was observed to inhibit the infection. We also observed a significant accumulation of H2O2 at the site of StRPS5 overexpression, indicating a role of StRPS5 in promoting the outbreak of reactive oxygen species (ROS) burst after pathogen infection. Silencing of StRPS5 in potato exhibited a marked increase in susceptibility towards P. infestans, whereas overexpression of the gene led to an enhancement of disease resistance. As ROS are key signaling molecules in plant immune responses against pathogens, we investigated flg22-triggered ROS accumulation in transgenic potatoes and the results showed that ROS accumulation in StRPS5-silenced leaves was significantly depressed, while the accumulation was increased in StRPS5-overexpressing leaves. Collectively, our findings demonstrate that potato ribosomal protein StRPS5 serves as a target for the P. infestans effector Pi16275 and that StRPS5 positively regulates potato resistance to P. infestans by increasing the accumulation of ROS.

Enhancing plant resistance to tobacco mosaic virus through the combined application of Verticillium dahliae Aspf2-like protein and microelements

BACKGROUND

Tobacco mosaic virus (TMV) poses a significant threat to global agriculture, infecting economically vital crops such as tobacco, tomato, pepper, and potato. Previous studies have suggested that the Verticillium dahliae Aspf2-like protein (VDAL) enhances plant resistance to TMV. This study investigated the preventive and therapeutic effects of VDAL, with and without microelements, on TMV resistance by analyzing plant hormone levels, defense related enzyme activities, and transcriptomic responses.

RESULTS

Plants were subjected to six experimental treatments: CK0 (untreated control, no TMV or VDAL treatment), CK (TMV inoculated control), T1 (preventive VDAL treatment), T2 (preventive VDAL + microelements), CT1 (therapeutic VDAL treatment), and CT2 (therapeutic VDAL + microelements). TMV inoculation (CK) significantly increased (P < 0.05) TMV content, jasmonic acid (JA), salicylic acid (SA) levels, and activities of defense related enzymes, including benzoic acid 2-hydroxylase (BA2H), peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD), compared to CK0. Both preventive treatments (T1 and T2) effectively reduced TMV content and enhanced JA, SA, and defense related enzyme activities. Notably, the microelement-supplemented preventive treatment (T2) showed 37.73% greater reduction in TMV content compared to T1. Similarly, the therapeutic applications, CT2 reduced the TMV content by 32.50% than CT1. Treatments T2 and CT2 also increased the contents of JA by 5.48% and 2.88%, respectively compared to their respective controls. Transcriptomic analysis revealed that these treatments activated plant-pathogen interaction pathways and pathogen-associated molecular pattern-triggered immunity (PTI), with significant upregulation of key defense related genes (e.g., CALM, BAK1, PTI6, and WRKY33), indicating a robust antiviral defense response.

CONCLUSION

Overall, we conclude that the synergistic application of VDAL and microelements significantly enhances plant resistance to TMV through coordinated activation of phytohormone signaling, defense enzymes, and immune-related gene expression. This combined approach offers an effective, eco-friendly alternative for sustainable management of viral diseases in agricultural crops. © 2025 Society of Chemical Industry.

Transcriptomic Profiling Reveals Regulatory Pathways of Tomato in Resistance to Verticillium Wilt Triggered by VdR3e

Tomatoes are important horticultural crops worldwide. Verticillium wilt is a disease caused by Verticillium dahliae that causes serious tomato yield losses. V. dahliae can be classified into three distinct races in tomatoes. We identified the specific VdR3e gene of V. dahliae race 3 and found that VdR3e triggered immune responses in the resistant tomato cultivar IVF6384. We confirmed that VdR3e triggers immune responses in the parents of IVF6384 plants and conducted transcriptome sequencing between male and female IVF6384 plants after VdR3e infiltration to analyze the potential regulatory network response to VdR3e. We found that both parents had a series of detoxification and stress resistance responses to VdR3e, but those of the male IVF6384 parent were concentrated in disease resistance-related signaling pathways. Moreover, several vital differentially expressed genes involved in functional annotation related to plant-pathogen interactions and plant hormone signaling stimulated immune responses in Nicotiana benthamiana. This study provides a new and comprehensive perspective on tomato resistance to Verticillium wilt.

Specific selection on XEG1 and XLP1 genes correlates with host range and adaptability in Phytophthora

In diverse Phytophthora-plant pathosystems, Phytophthora secretes XLP1 (PsXEG1-Like Protein), a non-enzymatic paralog that functions as a decoy to protect XEG1 (Xyloglucan-specific Endoglucanase) from host inhibitors. Here, we show that the genus-specific selection pressures on the XEG1/XLP1 gene pair are crucial for host adaptation and are closely linked to Phytophthora host range. Our findings reveal that the XEG1/XLP1 gene pair originated within Phytophthora and subsequently evolved into genus-specific genes, undergoing functional divergence driven by preferential selection. Positive selection sites within the XEG1/XLP1 gene pair in Phytophthora contribute to this functional divergence and are associated with the host range variability of Phytophthora as evidenced by multivariate statistical analyses. Furthermore, mutations at key selection sites in Phytophthora sojae and Phytophthora capsici significantly impair their pathogenicity, with P. capsici exhibiting almost no colonization expansion on tobacco and pea. Notably, natural Phytophthora populations harbor mutations at the positive selection sites, indicating ongoing evolutionary pressures on the XEG1/XLP1 gene pair.

The cell death-inducing protein BcPlp1 from Botrytis cinerea contributes to pathogenicity and modulates plant resistance

Botrytis cinerea is a necrotrophic plant pathogen fungus with a broad host range, causing grey mould and rot diseases in many important crops, leading to significant economic losses in agriculture. Cell death-inducing proteins (CDIPs) secreted by necrotrophic phytopathogens promote plant tissue death and play important roles in infection. However, the mechanisms by which CDIPs induce cell death in B. cinerea-plants interactions remain unclear. Here, we demonstrate that the B. cinerea CDIP BcPlp1 is secreted into the plant apoplast where it induces cell death. BcPlp1 is a cysteine-rich protein, and four out of the 8 cysteine residues and a conserved N-terminal α-helix structure are essential for its cell death-inducing activity. A purified GST-tagged BcPlp1 fusion protein triggered cell death in multiple plant species, up-regulated expression of defense-related genes and enhanced plant resistance to B. cinerea. Additionally, the cell death-inducing activity of BcPlp1 was mediated by leucine-rich repeat (LRR) receptor-like kinases BAK1 and SOBIR1. Furthermore, BcPlp1 was not necessary for colony morphology, conidial production, growth rate, and stress tolerance. Although deletion of BcPlp1 did not affect virulence, its overexpression led to larger disease lesion, highlighting its contribution to B. cinerea pathogenicity when upregulated.

Polyploidy-mediated variations in glutamate receptor proteins linked to Fusarium wilt resistance in upland cotton

Cotton production in the US faces a serious threat from Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4), a soil-borne fungus causing Fusarium wilt by infecting the roots and vascular system of susceptible cotton, leading to rapid wilting and death. Here, we investigate genetic mechanisms of resistance to FOV4 in the highly resistant upland cotton genotype "U1" using an early-generation segregating biparental population ("U1" × "CSX8308") with comprehensive genomic resources. Reference-grade genomic assemblies of the parents revealed minor structural variations between "U1" haplotypes, a high degree of collinearity at chromosome synteny and micro-synteny levels, and significant divergence from "CSX8308" with 8.9 million SNPs. QTL analysis identified significant markers on chromosomes D03 and A02 linked to reduced Fusarium wilt severity. Within these regions, two glutamate-receptor-like (GLR) genes showed structural variation and overlapped between translocated segments on A02 and D03, suggesting a rare but important reinforcing effect of parallel evolution between susceptible and resistant genotypes. Transcriptome profiles of "U1" under FOV4 infection reveal activation of calcium-binding proteins and transcription factors regulating plant hormones (ethylene, abscisic acid, jasmonic acid, and salicylic acid), along with enzymes involved in cell wall remodeling and phytoalexin production. Advancing cotton improvement depends on incorporating durable genetic disease resistance into high-yielding, high-quality cultivars.

Symbiosis vs pathogenesis in plants: Reflections and perspectives

Plant-microbe partnerships constitute a complex and intricately woven network of connections that have evolved over countless centuries, involving both cooperation and antagonism. In various contexts, plants and microorganisms engage in mutually beneficial partnerships that enhance crop health and maintain balance in ecosystems. However, these associations also render plants susceptible to a range of pathogens. Understanding the fundamental molecular mechanisms governing these associations is crucial, given the notable susceptibility of plants to external environmental influences. Based on quorum sensing signals, phytohormone, and volatile organic carbon (VOC) production and others molecules, microorganisms influence plant growth, health, and defense responses. This review explores the multifaceted relationships between plants and their associated microorganisms, encompassing mutualism, commensalism, and antagonism. The molecular mechanisms of symbiotic and pathogenic interactions share similarities but lead to different outcomes. While symbiosis benefits both parties, pathogenesis harms the host. Genetic adaptations optimize these interactions, involving coevolution driving process. Environmental factors influence outcomes, emphasizing the need for understanding and manipulation of microbial communities for beneficial results. Research directions include employing multi-omics techniques, functional studies, investigating environmental factors, understanding evolutionary trajectories, and harnessing knowledge to engineer synthetic microbial consortia for sustainable agriculture and disease management.

Technical Advances Drive the Molecular Understanding of Effectors from Wheat and Barley Powdery Mildew Fungi

Pathogens manipulate host physiology through the secretion of virulence factors (effectors) to invade and proliferate on the host. The molecular functions of effectors inside plant hosts have been of interest in the field of molecular plant-microbe interactions. Obligate biotrophic pathogens, such as rusts and powdery mildews, cannot proliferate outside of plant hosts. In addition to the inhibition of the plant's immune components, these pathogens are under particular pressure to extract nutrients efficiently from the host. Understanding the molecular basis of infections mediated by obligate biotrophic pathogens is significant because of their impact in modern agriculture. In addition, powdery mildews serve as excellent models for obligate biotrophic cereal pathogens. Here, we summarize the current knowledge on the effectorome of the barley and wheat powdery mildews and putative molecular virulence functions of effectors. We emphasize the availability of comprehensive genomic, transcriptomic, and proteomic resources and discuss the methodological approaches used for identifying candidate effectors, assessing effector virulence traits, and identifying effector targets in the host. We highlight established and more recently employed methodologies, discuss limitations, and suggest additional strategies. We identify open questions and discuss how addressing them with currently available resources will enhance our understanding of Blumeria candidates for secretor effector proteins. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of in planta bioprotectants against Fusarium wilt in Medicago sativa L. (lucerne) from a collection of bacterial isolates derived from Medicago seeds

Fusarium wilt caused by Fusarium oxysporum f. sp. medicaginis (Fom) is an important disease affecting lucerne/alfalfa cultivations worldwide. Medicago sativa L. (lucerne) is one of the major legume crops in global forage industry. This study aimed to identify bacteria capable of biologically controlling the wilt pathogen through a comprehensive screening of bacterial isolates obtained from domesticated and wild growing Medicago seeds. Using a multi-tiered evaluation pipeline, including in vitro, soil-free and potting mix-based pathogenicity and bioprotection assay systems, the bioprotection efficacy of 34 bacterial isolates derived from Medicago seeds was initially evaluated against six Fusarium strains in vitro. Fusarium oxysporum (Fo) F5189, which has previously been characterized as a Fusarium oxysporum f. sp. medicaginis isolate causing Fusarium wilt in lucerne was selected for in planta assays. Lucerne cultivars Grazer and Sequel, representing susceptible and resistant genotypes were chosen to assess the disease progression. Pathogenicity and bioprotection time-course studies were conducted to understand the temporal dynamics of host-pathogen interactions and efficacy of the bioprotectants. The disease symptoms were scored using a disease rating index developed in this study. The results indicated variability in bioprotection efficacy across bacterial isolates, with some strains suppressing disease in both soil-free and potting mix-based systems. Paenibacillus sp. (Lu_MgY_007; NCBI: PQ756884) and Pseudomonas sp. (Lu_LA164_018; NCBI: PQ756887) were identified as promising bioprotectants against Fusarium wilt under tested growth conditions. The time-course studies highlighted the critical role of persistent biocontrol activity and precise timing of biocontrol application for achieving long-term disease suppression. Overall, the observed reduction in disease severity underscores the potential of these bioprotectants as sustainable strategies for managing Fusarium wilt in lucerne cultivars. However, comprehensive molecular-level analyses are warranted to elucidate the underlying pathogenicity and bioprotection mechanisms, offering valuable insights for the development of more precise and effective future biocontrol strategies in agricultural systems.

Transcriptomic analysis of early stages of 'Candidatus Liberibacter asiaticus' infection in susceptible and resistant species after inoculation by Diaphorina citri feeding on young shoots

Huanglongbing (HLB) is a devastating disease of citrus plants caused by the non-culturable phloem-inhabiting bacterium Candidatus Liberibacter ssp., being Ca. Liberibacter asiaticus (CLas) the most aggressive species. CLas is vectored by the psyllid Diaphorina citri and introduced into sieve cells, establishing a successful infection in all Citrus species. Partial or complete resistance has been documented in the distant relatives Murraya paniculata and Bergera koenigii, respectively, providing excellent systems to investigate the molecular basis of HLB-resistance. It has been shown previously that the first weeks after bacterial release into the phloem are critical for the establishment of the bacterium. In this study, a thorough transcriptomic analysis of young flushes exposed to CLas-positive and negative psyllids has been performed in Citrus × sinensis, as well as in the aforementioned resistant species, along the first eight weeks after exposure. Our results indicate that the resistant species do not deploy a classical immunity response upon CLas recognition. Instead, transcriptome changes are scarce and only a few genes are differentially expressed when flushes exposed to CLas-positive and negative psyllid are compared. Functional analysis suggests that primary metabolism and other basic cellular functions could be rewired in the resistant species to limit infection. Transcriptomes of young flushes of the three species are very different, supporting the existence of distinct biochemical niches for the bacterium. These findings suggest that both intrinsic metabolic inadequacies to CLas survival, as well as inducible reprogramming of physiological functions upon CLas recognition, could orchestrate together restriction of bacterial multiplication in these resistant hosts.

Effect of AM fungi on the growth and powdery mildew development of Astragalus sinicus L. under water stress

Arbuscular mycorrhizal (AM) fungi are widely existing soil microorganisms that form symbiotic relationships with most terrestrial plants. They are important for enhancing adversity resistance, including resistance to disease and water stresses. Nevertheless, it is not clear whether the benefits can be maintained in regulating the occurrence of plant diseases under drought, flooding stress and during water restoration. In this study, we investigated the effect of AM fungus (Glomus versiforme) on the development of powdery mildew in Chinese milk vetch (Astragalus sinicus) under drought, flooding, and water recovery. The results showed that AM fungal symbiosis promoted the growth of Chinese milk vetch under water stress conditions. It increased the accumulation of ethylene (ET) and jasmonic acid (JA), enhanced the activities of antioxidant enzymes, and decreased the accumulation of salicylic acid (SA) and abscisic acid (ABA). The differentially expressed genes (DEGs) obtained from transcriptome sequencing under each stress were subjected to weighted gene co-expression network analysis (WGCNA), and a total of 12 gene co-expression modules were obtained. The analysis of the relationship between the co-expressed genes in the 12 modules and plant physiological traits showed that the magent, grey60 and darkturquoise modules were significantly associated with ET, SA, JA, ABA, plant defence enzyme activities, malondialdehyde (MDA) and H2O2 content. Water stress and disease were related with the up-regulated expression of genes in the flavonoid biosynthesis and oxidative phosphorylation, plant hormone signal transduction and plant-pathogen interaction pathways. Importantly, inoculation with AM fungus reduced the incidence of powdery mildew under drought stress by 16.54%. In summary, the results of this study showed that inoculation with AM had a positive effect on powdery mildew development tolerance in Chinese milk vetch under drought and flooding stresses and stress recovery. This provides a good basis for field management and sustainable growth of green manure crop Chinese milk vetch.

A Novel Secreted Protein of Fusarium oxysporum Promotes Infection by Inhibiting PR-5 Protein in Plant

Fusarium oxysporum, an important soilborne fungal pathogen that causes serious Fusarium wilt disease, secretes diverse effectors during the infection. In this study, we identified a novel secreted cysteine-rich protein, FolSCP1, which contains unknown protein functional domain. Here, we characterized FolSCP1 as a secreted virulence factor that promotes the pathogen infection of host plants by inhibiting diverse plant defence responses. FolSCP1 interacted with the pathogenesis-related 5 (PR-5) protein SlPR5, a positive regulator of tomato plant immunity against multiple tomato pathogens, and effectively attenuated the antifungal activity of the tomato PR-5 protein. FoSCP1, a homologue of FolSCP1, was secreted by a F. oxysporum isolate from infected tobacco and targeted the tobacco PR-5 protein NtPR5 to suppress plant defence for further infection. In summary, our study revealed a fungal virulence strategy in which F. oxysporum secrete effectors that interfere with plant immunity by binding to the PR-5 protein of the host plant and inhibiting its biological activity, thereby promoting fungal infection.

Mannose-binding lectin 1.1A interacts with hypersensitive-induced response 4 to promote hypersensitive cell death and defense responses in cotton upon Verticillium dahliae infection

Increasing evidence indicates that mannose-binding lectins (MBLs) act as a lectin-receptor-like protein in plant immune responses, yet the functional basis remains elusive. In this study, we dissected the functional mechanism of GbMBL1.1A in defense against Verticillium dahliae infection in sea island cotton (Gossypium barbadense). GbMBL1.1A expressed preferentially in cotton roots and significantly upregulated upon V. dahliae infection. Transgenic expression of GbMBL1.1A in upland cotton (Gossypium hirsutum) and Arabidopsis remarkably improved the disease resistance of the plants, while silencing of GbMBL1.1A resulted in an increased susceptibility of cotton plants in response to V. dahliae attack. Protein interaction assays revealed that GbMBL1.1A interacted with the cotton hypersensitive-induced response protein 4 (GbHIR4, a scaffold protein for immune signaling in the plasma membrane microdomain) through PAN domain. GbHIR4 expression was upregulated in response to V. dahliae invasion, and silencing of GbHIR4 seriously attenuated the disease tolerance of cotton plants. GbMBL1.1A enhanced the cell death phenotype induced by the transient expression of GbHIR4, and meanwhile promoted HR-PCD and GbHIR4-dependent resistance upon V. dahliae infection. The results suggested that GbMBL1.1A employed GbHIR4 as a downstream component to trigger the hypersensitive responses and therefore contributed to cotton resistance against V. dahliae. In addition, we observed that GbMBL1.1A overexpression could alter the phytohormone-mediated defense and growth signaling under both normal and V. dahliae infection conditions. Collectively, these results demonstrated that the lectin receptor-like protein GbMBL1.1A interacts with GbHIR4 in cotton immunity to induce the hypersensitive response, which is associated with phytohormone-mediated defense and growth signaling.

Biological control of bacterial leaf blight (BLB) in rice-A sustainable approach

Bacterial leaf blight (BLB) in rice, caused by the pathogen Xanthomonas oryzae pv. oryzae, is a significant agricultural problem managed through chemical control and cultivating rice varieties with inherent resistance to the bacterial pathogen. Research has highlighted the potential of using antagonistic microbes which can suppress the BLB pathogen through the production of secondary metabolites like siderophores, rhamnolipids, and hydroxy-alkylquinolines offering a sustainable alternative for BLB management. Additionally, the induction of plant immunity and defense-related enzymes in rice further enhances the resistance against the disease. Therefore, implementation of biological controls can complement chemical treatments in contributing towards the sustainability of rice production systems by aiming at host immunity improvement and killing of pathogen. It is crucial to continue exploring and understanding the complex interactions between various beneficial microbes, the rice plants, and the BLB pathogen to optimize and implement effective biocontrol strategies in future.

From Recognition to Response: Resistance-Effector Gene Interactions in the Brassica napus and Leptosphaeria maculans Patho-System

Blackleg disease, caused by the hemibiotrophic fungal pathogen Leptosphaeria maculans, poses a serious threat to Brassica crops and requires a broad understanding of the plant defence mechanisms. The Brassica. napus-L. maculans pathosystem provides a useful model to understand plant resistance response to hemibiotrophs. This review aims to explain the mechanisms underlying R-Avr interaction, signalling cascades, and the hypersensitive response (HR) produced by B. napus towards L. maculans, causing local cell death that restricts the pathogen to the site of infection. The role of transcription factors is pivotal to the process of HR, coordinating the regulation of genes involved in pathogen recognition and the activation of SA responsive genes and production of secondary metabolites. The R-Avr interaction signalling cascade involves production of reactive oxygen species (ROS), calcium ion influx, Salicylic acid (SA) hormonal signalling and mitogen activated protein kinases (MAPKs), which are critical in the HR in B. napus. The in-depth understanding of molecular signalling pathway of the R-Avr interaction between B. napus-L. maculans pathosystem provides valuable information for future research endeavours regarding enhancing disease resistance in Brassica crops.

Transcriptional time-course analysis during ash dieback infection revealed different responses in tolerant and susceptible Fraxinus excelsior genotypes

Hymenoscyphus fraxineus, the causal agent of Ash Dieback (ADB), has been introduced to eastern Europe in the 1990s from where it spread causing decline in European ash populations. However, the genetic basis of the molecular response in tolerant and susceptible ash trees to this disease is still largely unknown. We performed RNA-sequencing to study the transcriptomic response to the disease in four ash genotypes (ADB-tolerant FAR3 and FS36, and ADB-susceptible UW1 and UW2), during a time-course of 7, 14, 21, and 28 days post-inoculation, including mock-inoculated trees as control samples for each sampling time point. The analysis yielded 395 and 500 Differentially Expressed Genes (DEGs) along the response for ADB-tolerant FAR3 and FS36, respectively, while ADB-susceptible UW1 and UW2 revealed 194 and 571 DEGs, respectively, with most DEGs found exclusively in just one of the genotypes. DEGs shared between tolerant genotypes FAR3 and FS36, included genes involved in the production of phytoalexins and other secondary metabolites with roles in plant defense. Moreover, we identified an earlier expression of genes involved in both pattern- and effector-triggered immunity (PTI and ETI) in ADB-tolerant genotypes, while in ADB-susceptible genotypes both responses were delayed (late response). Overall, these results revealed different transcriptomic expression patterns not only between ADB-tolerant and ADB-susceptible genotypes, but also within these two groups. This hints to individual responses in the natural tolerance to ADB, possibly revealing diversified strategies across ash genotypes.

Genomic and mutational analysis of Pseudomonas syringae pv. tagetis EB037 pathogenicity on sunflower

BACKGROUND

Pseudomonas syringae pv. tagetis (Pstag) causes apical chlorosis on sunflower and various other plants of the Asteraceae family. Whole genome sequencing of Pstag strain EB037 and transposon-mutant derivatives, no longer capable of causing apical chlorosis, was conducted to improve understanding of the molecular basis of disease caused by this pathogen.

RESULTS

A tripartite pathogenicity island (TPI) for a Type III secretion system (T3SS) with the complete hrp-hrc gene cluster and conserved effector locus was detected in the Pstag genome. The exchange effector region of the TPI contained genes potentially functioning in detoxification of the environment as well as two integrases, but no previously described T3SS effector homologues. In all, the Pstag EB037 genome contained homologues for at least 44 T3SS effectors with 30 having known functions. Plasmids similar with pTagA and pTagB of P. syringae pv. tagetis ICMP 4091 were also identified in the Pstag genome. The pTagA-like plasmid contained a complete Type IV secretion system (T4SS) with associated putative killer protein. Mutational analysis using transposon insertions within genes functioning in the T3SS and T4SS confirmed the role of both secretion systems and these plasmids in apical chlorosis. Transposon mutagenesis identified an additional 22 genes in loci, including two more plasmid-bound loci, involved in apical chlorosis on sunflower; some with known importance in other plant or animal pathosystems.

CONCLUSIONS

Apical chlorosis disease caused by Pstag EB037 is the result of a complex set of mechanisms. This study identified a TPI and homologues for at least 44 T3SS effectors, 30 of which with known functions in disease, and another 20 genes in loci correlated with apical chlorosis on sunflower. Two plasmids were detected that were correlated with apical chlorosis disease, one of which contained a complete T4SS that was correlated with disease. To our knowledge, we provide the first direct evidence for a T4SS functioning in disease by a pathogenic P. syringae strain.

Importance of OsRac1 in Signalling of Pigm-1 Mediated Resistance to Rice Blast Disease

In rice, leucine-rich repeat nucleotide-binding site (NLR) proteins are pivotal immune receptors in combating Magnaporthe oryzae-triggered rice blast. However, the precise molecular mechanism underlying how NLR proteins regulate downstream signalling remains elusive due to the lack of knowledge regarding their direct downstream targets. The NLR protein Pigm-1 was cloned from Shuangkang 77009 in our laboratory. This study shows that the nucleotide-binding site (NBS) domain of Pigm-1 facilitates its binding to and activation of OsRac1 while the coiled-coil (CC) domain enables its binding to and activation of RAI1, ultimately inducing cell death. At the same time, after knocking out OsRac1 in the background of Shuangkang 77009 containing Pigm-1, two knockout lines showed susceptibility to rice blast. This study reveals OsRac1, a GTPase, as a signalling molecule involved in Pigm-1-mediated blast resistance, suggesting its potential as a common downstream effector of rice NLR proteins. Additionally, a transcriptional activator, RAI1, acts as an essential Pigm-1 interactor for blast resistance. Furthermore, a novel material 9311(Pigm-1) was prepared by using two-line restorer line 9311 as receptor and Shuangkang 77009 as donor with molecular marker-assisted technology, which improved blast resistance and yield. This research demonstrates that molecular marker-assisted selection technology enhances both resistance and yield in the crucial two-line restorer 9311(Pigm-1). This study offers crucial insights into how Pigm-1 protein activates downstream molecules and serves as a valuable reference for the molecular breeding of rice blast resistance genes, particularly Pigm-1.

Enhancing Fruit Resistance against Fungal Pathogens Using a Pathogen-Associated Molecular Pattern PdEIX

Fruit is an essential part of the human diet, and postharvest fungal diseases are the major cause of fruit postharvest losses worldwide. Pathogen-associated molecular patterns (PAMPs) are important elicitors from microbes, and the recognition between microbial PAMPs and plant receptors leads to PAMP-triggered immunity. Here, we identified a PAMP, PdEIX, that is an important protein elicitor with plant immunity-inducing activity, from the citrus green mold pathogen Penicillium digitatum. PdEIX showed an apoplastic location similar to that of known PAMPs, and plant receptor-like protein NbEIX2, receptor-like kinase BAK1, and other signaling components of plant immunity were required for PdEIX-triggered plant cell death in the model plant Nicotiana benthamiana. Moreover, PdEIX triggered a series of immune responses in citrus fruit and enhanced the resistance of citrus and other fruit against fungal pathogens. Our results indicate that application of a microbial PAMP as the plant immunity inducer is an effective strategy for controlling postharvest fungal diseases of fruit.

OsBRW1, a novel blast-resistant gene, coded a NBS-LRR protein to interact with OsSRFP1 to balance rice growth and resistance

It is urgent to mine novel blast-resistant genes in rice and develop new rice varieties with pyramiding blast-resistant genes. In this study, a new blast-resistant gene, OsBRW1, was screened from a set of rice near-isogenic lines (NILs) with different blast-resistant ability. Under the infection of Magnaporthe oryzae (M. oryzae), OsBRW1 in the resistant NIL Pi-4b was highly induced than that in the susceptible NIL Pi-1 and their parent line CO39, and the blast-resistant ability of OsBRW1 was further confirmed by using CRISPR/Cas9 knockout and over-expression methods. The protein encoded by OsBRW1 was a typical NBS-LRR with NB-ARC domain and localized in both cytoplasm and nucleus, and the transient expression of OsBRW1 was capable of triggering hypersensitive response in tobacco leaves. Protein interaction experiments showed that OsBRW1 protein directly interacted with OsSRFP1. At the early infection stage of M. oryzae, OsBRW1 gene induced OsSRFP1 to highly expression level and accumulated H2O2, up-regulated the defence responsive signalling transduction genes and the pathogenesis-related genes and increased JA and SA content in the resistant NIL Pi-4b. By contrary, lower content of endogenous JA and SA in osbrw1 mutants was found at the same stage. After that, OsSRFP1 was down-regulated to constitution abundance to balance the growth of the resistant NIL Pi-4b. In summary, OsBRW1 solicited OsSRFP1 to resist the infection of blast fungus in rice by inducing the synergism of induced systemic resistance (ISR) and system acquired resistance (SAR) and to balance the growth of rice plants.

A novel type III effector RipBU from Ralstonia solanacearum suppresses plant immunity and promotes peanut susceptibility

A predicted peanut R. solanacearum T3E RS_T3E_Hyp6 was identified as a definite T3E and renamed as RipBU. It is relative conserved in 31 R. solanacearum strains. Deletion of RipBU in R. solanacearum HA4-1 strain caused the attenuate pathogenicity in peanut, and complementarity of RipBU recovered the virulence of ΔRipBU mutant strain. Transient expression of RipBU decreased the level of chlorophyll, resulting in leaf chlorosis and suppressed flg22-triggered reactive oxygen species (ROS) burst and the expression of pattern-triggered immunity (PTI) marker genes in the leaves of Nicotiania benthamiana. Subcellular localization observation showed that RipBU localizes to chloroplasts in tobacco cells. RipBU significantly increased the jasmonic acid (JA) content and the expressions of JA-signaling marker genes in tobacco leaves, while significantly decreased the salicylic acid (SA) level and the expressions of SA-signaling marker genes. RipBU contained a putative lipase domain, and mutation of which abolished the ability of RipBU to induce tobacco leaf chlorosis and peanut wilt, while still localized to chloroplasts. Our study reveals the virulence function of RipBU that suppresses plant immunity by inhibiting PTI and SA signaling, and promoting JA signaling.

Immune signaling: receptor-like proteins make the difference

To resist biotic attacks, plants have evolved a sophisticated, receptor-based immune system. Cell-surface immune receptors, which are either receptor-like kinases (RLKs) or receptor-like proteins (RLPs), form the front line of the plant defense machinery. RLPs lack a cytoplasmic kinase domain for downstream immune signaling, and leucine-rich repeat (LRR)-containing RLPs constitutively associate with the RLK SOBIR1. The RLP/SOBIR1 complex was proposed to be the bimolecular equivalent of genuine RLKs. However, it appears that the molecular mechanisms by which RLP/SOBIR1 complexes and RLKs mount immunity show some striking differences. Here, we summarize the differences between RLP/SOBIR1 and RLK signaling, focusing on the way these receptors recruit the BAK1 co-receptor and elaborating on the negative crosstalk taking place between the two signaling networks.

Subcellular spatial regulation of immunity-induced phosphorylation of RIN4 links PAMP-triggered immunity to Exo70B1

RIN4 is a crucial regulator of plant immunity, playing a role in both PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI). While the impact of post-translational modifications (PTMs) on RIN4 has been extensively studied, their specific effects on plant immune response regulation and the underlying mechanisms have remained unclear. In this study, we investigated the phosphorylation of RIN4 at threonine-166 (RIN4T166) in Arabidopsis transgenic lines expressing various RIN4 variants. Our pathological and molecular genetic analyses reveal that RIN4T166 phosphorylation disrupts its localization to the plasma membrane (PM) and represses plant defense activation. We found that RIN4's PM tethering relies on Exo70B1-mediated exocytosis and the integrity of the host cytoskeletal actin network. Phosphorylation at RIN4T166 disrupts its PM localization due to reduced binding affinity with Exo70B1. This disruption was further evidenced by the 35S::RIN4T166D/rin124 transgenic line, which exhibited suppressed PTI responses similar to the exo70b1 mutant. Our findings demonstrate that RIN4's subcellular localization is regulated by phosphorylation, suggesting that plants use a sophisticated network of signaling processes to precisely control the timing and localization of immune signaling activation. This study uncovers a mechanism by which PTI is repressed through RIN4 phosphorylation, providing new insights into the spatial regulation of RIN4 within plant immune signaling pathways.

Harnessing Single-Cell and Spatial Transcriptomics for Crop Improvement

Single-cell and spatial transcriptomics technologies have significantly advanced our understanding of the molecular mechanisms underlying crop biology. This review presents an update on the application of these technologies in crop improvement. The heterogeneity of different cell populations within a tissue plays a crucial role in the coordinated response of an organism to its environment. Single-cell transcriptomics enables the dissection of this heterogeneity, offering insights into the cell-specific transcriptomic responses of plants to various environmental stimuli. Spatial transcriptomics technologies complement single-cell approaches by preserving the spatial context of gene expression profiles, allowing for the in situ localization of transcripts. Together, single-cell and spatial transcriptomics facilitate the discovery of novel genes and gene regulatory networks that can be targeted for genetic manipulation and breeding strategies aimed at enhancing crop yield, quality, and resilience. This review highlights significant findings from recent studies, discusses the expanding roles of these technologies, and explores future opportunities for their application in crop improvement.

Overexpression of GmSRC2 confers resistance to Phytophthora sojae in soybean

Phytophthora root and stem rot caused by Phytophthora sojae (P. sojae) is one of the most destructive diseases to affect soybean (Glycine max (L.) Merr) production. GmSRC2 that encodes a C2 domain-containing protein can respond to various stresses, however, the molecular mechanism of GmSRC2 in resistance of soybean to P. sojae is yet to be fully elucidated. In this study, GmSRC2 was found to be significantly up-regulated under P. sojae treatment; GmSRC2-overexpression (OE) transgenic lines and GmSRC2-silencing transient plants were generated via Agrobacterium tumefaciens mediated transformation and virus-induced gene silencing (VIGS) system, respectively. Infected leaves and cotyledons of OE-GmSRC2-1 and OE-GmSRC2-2 lines showed significant decreases in the disease symptoms and P. sojae biomass than those of wild type (WT); the activities of superoxide dismutase (SOD) and peroxidase (POD) confirmed the accumulation of reactive oxygen species (ROS) in overexpressed transgenic lines. Whereas, silencing of GmSRC2 severely increased the disease symptoms and the biomass of P. sojae. Further, we confirmed that GmSRC2 interacted with the effector PsAvh23 of P. sojae, and the C2 domain was crucial for the interaction. Overexpression of GmSRC2 upregulated the ADA2/GCN5 module upon P. sojae. The aforementioned results demonstrated that GmSRC2 played vital roles in regulating soybean resistance to oomycetes.

A conserved extracellular effector protein Ssh1296 from Scleromitrula shiraiana triggers cell death and regulates plant immunity

The plant apoplast is a key battleground in the initial stages of interaction between the plant and pathogen. Despite its importance, few apoplastic effectors have been characterized to date. Here, we identified Ssh1296, a conserved apoplastic effector from Scleromitrula shiraiana. Ssh1296 and its homologous proteins, prevalent among fungi and oomycetes, possess the ability to induce cell death and enhance resistance against pathogens in Nicotiana benthamiana. Fragments containing conserved motifs 1-3 elicit more pronounced cell death responses than the full-length Ssh1296 protein. Furthermore, cysteine residues at positions C38, C52, C84, and C89 are essential for inducing sufficient cell death. The cell death response mediated by Ssh1296 depends on the RECEPTOR-LIKE PROTEIN REQUIRED FOR CSP22 RESPONSIVENESS (NbCSPR/RE02), SUPPRESSOR OF BIR1-1 (SOBIR1), and BRI1-ASSOCIATED KINASE-1 (BAK1). Constitutive expression of Ssh1296 and its homologous protein in Arabidopsis thaliana activates plant immunity but concurrently inhibits growth and development. These findings suggest that Ssh1296 and its homologous proteins can be recognized by plants as pathogen-associated molecular patterns (PAMPs). In conclusion, Ssh1296 holds promise as a potential inducer of plant immunity.

DNA methylation analysis reveals local changes in resistant and susceptible soybean lines in response to Phytophthora sansomeana

Phytophthora sansomeana is an emerging oomycete pathogen causing root rot in many agricultural species including soybean. However, as of now, only one potential resistance gene has been identified in soybean, and our understanding of how genetic and epigenetic regulation in soybean contributes to responses against this pathogen remains largely unknown. In this study, we performed whole genome bisulfite sequencing (WGBS) on two soybean lines, Colfax (resistant) and Williams 82 (susceptible), in response to P. sansomeana at two time points: 4 and 16 hours post-inoculation to compare their methylation changes. Our findings revealed that there were no significant changes in genome-wide CG, CHG (H = A, T, or C), and CHH methylation. However, we observed local methylation changes, specially an increase in CHH methylation around genes and transposable elements (TEs) after inoculation, which occurred earlier in the susceptible line and later in the resistant line. After inoculation, we identified differentially methylated regions (DMRs) in both Colfax and Williams 82, with a predominant presence in TEs. Notably, our data also indicated that more TEs exhibited changes in their methylomes in the susceptible line compared to the resistant line. Furthermore, we discovered 837 DMRs within or flanking 772 differentially expressed genes (DEGs) in Colfax and 166 DMRs within or flanking 138 DEGs in Williams 82. These DEGs had diverse functions, with Colfax primarily showing involvement in metabolic process, defense response, plant and pathogen interaction, anion and nucleotide binding, and catalytic activity, while Williams 82 exhibited a significant association with photosynthesis. These findings suggest distinct molecular responses to P. sansomeana infection in the resistant and susceptible soybean lines.

Alternaria solani core effector Aex59 is a new member of the Alt a 1 protein family and is recognized as a PAMP

Early blight caused by Alternaria solani is a destructive disease in potato production. Here, through systematically screening of an effector protein pool consisting of 115 small cysteine-containing candidate Aex (Alternariaextracellular proteins) in A. solani, we identified a core effector protein named Aex59, a pathogen-associated molecular pattern (PAMP) molecule. Aex59 is uniquely present in the Ascomycota of fungi and can activate defense responses in multiple plants. Targeted gene disruption showed that Aex59 is a virulence factor and participates in spore development. Perception of Aex59 in Nicotiana benthamiana does not depend on the receptor-like kinases Brassinosteroid-associated kinase1 (BAK1) and Suppressor of BIR1-1 (SOBIR1), which are required for multiple pattern recognition receptors (PRR) pathways. Sequence analysis revealed that Aex59 is a new member of the Alt a 1 protein family and is a potential molecular marker capable of aiding in the classification of the fungi Alternaria spp.

Friendly fire: recognition of self by the innate immune system

The innate immune system employs two different strategies to detect pathogens: first, it recognizes microbial components as ligands of pattern recognition receptors (pattern-triggered immunity [PTI]), and second, it detects the activities of pathogen-encoded effectors (effector-triggered immunity [ETI]). Recently, these pathogen-centric concepts were expanded to include sensing of self-derived signals during cellular distress or damage (damage-triggered immunity [DTI]). This extension relied on broadening the PTI model to include damage-associated molecular patterns (DAMPs). However, applying the pattern recognition framework of PTI to DTI overlooks the critical role of sterile activation of ETI pathways. We argue that both PTI and ETI pathways are prone to erroneous detection of self, which is largely attributable to 'friendly fire' rather than protective immune activation. This erroneous activation is inherent to the trade-off between sensitivity and specificity of immune sensing and might be tolerated because its detrimental effects emerge late in life, a phenomenon known as antagonistic pleiotropy.

Salivary Protein Sfapyrase of Spodoptera frugiperda Stimulates Plant Defence Response

Plants have developed various resistance mechanisms against herbivorous insects through prolonged coevolution. Plant defence responses can be triggered by specific compounds present in insect saliva. Apyrase, a known enzyme that catalyzes the hydrolysis of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) into adenosine monophosphate (AMP) and inorganic phosphorus, has recently been identified in some herbivorous insects. However, whether insect salivary apyrase induces or inhibits plant responses remains poorly understood. In this study, we identified an apyrase-like protein in the salivary proteome of the fall armyworm, Spodoptera frugiperda, named Sfapyrase. Sfapyrase was primarily expressed in the salivary gland and secreted into plants during insect feeding. Transient expression of Sfapyrase in tobacco and maize enhanced plant resistance and resulted in decreased insect feeding. Knockdown of Sfapyrase through RNA interference led to increased growth and feeding of S. frugiperda. Furthermore, we showed that Sfapyrase activates the jasmonic acid signalling pathway and promotes the synthesis of secondary metabolites, especially benzoxazinoids, thereby enhancing resistance to S. frugiperda. In summary, our findings demonstrated that Sfapyrase acts as a salivary elicitor, inducing maize jasmonic acid defence responses and the production of insect-resistant benzoxazinoids. This study provides valuable insights into plant-insect interactions and offers potential targets for developing innovative insect pest management strategies.

Effector Cs02526 from Ciboria shiraiana induces cell death and modulates plant immunity

Sclerotinia disease is one of the most devastating fungal diseases worldwide, as it reduces the yields of many economically important crops. Pathogen-secreted effectors play crucial roles in infection processes. However, key effectors of Ciboria shiraiana, the pathogen primarily responsible for sclerotinia disease in mulberry (Morus spp.), remain poorly understood. In this study, we identified and functionally characterized the effector Cs02526 in C. shiraiana and found that Cs02526 could induce cell death in a variety of plants. Moreover, Cs02526-induced cell death was mediated by the central immune regulator brassinosteroid insensitive 1-associated receptor kinase 1, dependent on a 67-amino acid fragment. Notably, Cs02526 homologs were widely distributed in hemibiotrophic and necrotrophic phytopathogenic fungi, but the homologs failed to induce cell death in plants. Pretreatment of plants with recombinant Cs02526 protein enhanced resistance against both C. shiraiana and Sclerotinia sclerotiorum. Furthermore, the pathogenicity of C. shiraiana was diminished upon spraying plants with synthetic dsRNA-Cs02526. In conclusion, our findings highlight the cell death-inducing effector Cs02526 as a potential target for future biological control strategies against plant diseases.

Changes in planta K nutrient content altered the interaction pattern between Nicotiana benthamiana and Alternaria longipes

Potassium (K) fertilisation has frequently been shown to enhance plant resistance against pathogens, though the mechanisms remain elusive. This study investigates the interaction dynamics between Nicotiana benthamiana and the pathogen Alternaria longipes under different planta K levels. On the host side, adding K activated the expressions of three NLR (nucleotide-binding domain and leucine-rich repeat-containing proteins) resistance genes, including NbRPM1, NbR1B23 and NbNBS12. Silencing these NLRs attenuated resistance in high-K (HK, 40.8 g/kg) plant, whereas their overexpression strengthened resistance in low-K (LK, 23.9 g/kg) plant. Typically, these NLRs mainly strengthened plant resistance via promoting the expression of pathogenesis-related genes (PRs), ROS burst and synthesis of antifungal metabolites in HK plant. On the pathogen side, the expression of effectors HKCSP1, HKCSP2 and LKCSP were shown to be related to planta K content. A. longipes mainly expressed effectors HKCSP1 and HKCSP2 in HK plant to interfere host resistance. HKCSP1 physically interacted with NbRPM1 to promote the degradation of NbRPM1, then attenuated related resistance in HK N. benthamiana. Meanwhile, HKCSP2 directly interacted with NbPR5 to suppress resistance in HK plant. In LK plant, A. longipes mainly deployed LKCSP that interacted with NbR1B23 to interfere reduce resistance in N. benthamiana. Overall, our research insights that both pathogen and host mobilise distinct strategies to outcompete each other during interactions in different K nutrient environments.

Comparing Apples and Oranges: Advances in Disease Resistance Breeding of Woody Perennial Fruit Crops

Apple and citrus are perennial tree fruit crops that are vital for nutritional security and agricultural economy and to achieve the Sustainable Development Goals of the United Nations. Apple scab and fire blight, along with Huanglongbing, canker, and tristeza virus, stand out as their most notorious diseases and annually destabilize fruit supply. An environmentally sound approach to managing these diseases is improving tree resistance through breeding and biotechnology. Perennial fruit tree germplasm collections are distributed globally and offer untapped potential as sources of resistance. However, long juvenility, specific pollination and flowering habits, and extensive outcrossing hinder apple and citrus breeding. Advances in breeding approaches include trans- and cis-genesis, genome editing, and rapid-cycle breeding, which, in addition to conventional crossbreeding, can all facilitate accelerated integration of resistance into elite germplasm. In addition, the global pool of available sources of resistance can be characterized by the existing genetic mapping and gene expression studies for accurate discovery of associated loci, genes, and markers to efficiently include these sources in breeding efforts. We discuss and propose a multitude of approaches to overcome the challenges of breeding for resistance in woody perennials and outline a technical path to reduce the time required for the ultimate deployment of disease-resistant cultivars.

Ripening and rot: How ripening processes influence disease susceptibility in fleshy fruits

Fleshy fruits become more susceptible to pathogen infection when they ripen; for example, changes in cell wall properties related to softening make it easier for pathogens to infect fruits. The need for high-quality fruit has driven extensive research on improving pathogen resistance in important fruit crops such as tomato (Solanum lycopersicum). In this review, we summarize current progress in understanding how changes in fruit properties during ripening affect infection by pathogens. These changes affect physical barriers that limit pathogen entry, such as the fruit epidermis and its cuticle, along with other defenses that limit pathogen growth, such as preformed and induced defense compounds. The plant immune system also protects ripening fruit by recognizing pathogens and initiating defense responses involving reactive oxygen species production, mitogen-activated protein kinase signaling cascades, and jasmonic acid, salicylic acid, ethylene, and abscisic acid signaling. These phytohormones regulate an intricate web of transcription factors (TFs) that activate resistance mechanisms, including the expression of pathogenesis-related genes. In tomato, ripening regulators, such as RIPENING INHIBITOR and NON_RIPENING, not only regulate ripening but also influence fruit defenses against pathogens. Moreover, members of the ETHYLENE RESPONSE FACTOR (ERF) family play pivotal and distinct roles in ripening and defense, with different members being regulated by different phytohormones. We also discuss the interaction of ripening-related and defense-related TFs with the Mediator transcription complex. As the ripening processes in climacteric and non-climacteric fruits share many similarities, these processes have broad applications across fruiting crops. Further research on the individual contributions of ERFs and other TFs will inform efforts to diminish disease susceptibility in ripe fruit, satisfy the growing demand for high-quality fruit and decrease food waste and related economic losses.

Plant microRNAs regulate the defense response against pathogens

MicroRNAs (miRNAs) are a class of small non-coding RNAs, typically 20-25 nucleotides in length, that play a crucial role in regulating gene expression post-transcriptionally. They are involved in various biological processes such as plant growth, development, stress response, and hormone signaling pathways. Plants interact with microbes through multiple mechanisms, including mutually beneficial symbiotic relationships and complex defense strategies against pathogen invasions. These defense strategies encompass physical barriers, biochemical defenses, signal recognition and transduction, as well as systemic acquired resistance. MiRNAs play a central role in regulating the plant's innate immune response, activating or suppressing the transcription of specific genes that are directly involved in the plant's defense mechanisms against pathogens. Notably, miRNAs respond to pathogen attacks by modulating the balance of plant hormones such as salicylic acid, jasmonic acid, and ethylene, which are key in activating plant defense mechanisms. Moreover, miRNAs can cross boundaries into fungal and bacterial cells, performing cross-kingdom RNA silencing that enhances the plant's disease resistance. Despite the complex and diverse roles of miRNAs in plant defense, further research into their function in plant-pathogen interactions is essential. This review summarizes the critical role of miRNAs in plant defense against pathogens, which is crucial for elucidating how miRNAs control plant defense mechanisms.

Plasmodesmata Function and Callose Deposition in Plant Disease Defense

Callose, found in the cell walls of higher plants such as β-1,3-glucan with β-1,6 branches, is pivotal for both plant development and responses to biotic and abiotic stressors. Plasmodesmata (PD), membranous channels linking the cytoplasm, plasma membrane, and endoplasmic reticulum of adjacent cells, facilitate molecular transport, crucial for developmental and physiological processes. The regulation of both the structural and transport functions of PD is intricate. The accumulation of callose in the PD neck is particularly significant for the regulation of PD permeability. This callose deposition, occurring at a specific site of pathogenic incursion, decelerates the invasion and proliferation of pathogens by reducing the PD pore size. Scholarly investigations over the past two decades have illuminated pathogen-induced callose deposition and the ensuing PD regulation. This gradual understanding reveals the complex regulatory interactions governing defense-related callose accumulation and protein-mediated PD regulation, underscoring its role in plant defense. This review systematically outlines callose accumulation mechanisms and enzymatic regulation in plant defense and discusses PD's varied participation against viral, fungal, and bacterial infestations. It scrutinizes callose-induced structural changes in PD, highlighting their implications for plant immunity. This review emphasizes dynamic callose calibration in PD constrictions and elucidates the implications and potential challenges of this intricate defense mechanism, integral to the plant's immune system.

Ss4368: Pathogen-Associated Molecular Pattern for Inducing Plant Cell Death and Resistance to Phytophthora capsici

Plant recognition of pathogen-associated molecular patterns (PAMPs) is pivotal in triggering immune responses, highlighting their potential as inducers of plant immunity. However, the number of PAMPs identified and applied in such contexts remains limited. In this study, we characterize a novel PAMP, designated Ss4368, which is derived from Scleromitrula shiraiana. Ss4368 is specifically distributed among a few fungal genera, including Botrytis, Monilinia, and Botryotinia. The transient expression of Ss4368 elicits cell death in a range of plant species. The signaling peptides, three conserved motifs, and cysteine residues (C46, C88, C112, C130, and C148) within Ss4368 are crucial for inducing robust cell death. Additionally, these signaling peptides are essential for the protein's localization to the apoplast. The cell death induced by Ss4368 and its homologous protein, Bc4368, is independent of the SUPPRESSOR OF BIR1-1 (SOBIR1), BRI1-ASSOCIATED KINASE-1 (BAK1), and salicylic acid (SA) pathways. Furthermore, the immune responses triggered by Ss4368 and Bc4368 significantly enhance the resistance of Nicotiana benthamiana to Phytophthora capsici. Therefore, we propose that Ss4368, as a novel PAMP, holds the potential for developing strategies to enhance plant resistance against P. capsici.

Vitis rotundifolia Genes Introgressed with RUN1 and RPV1: Poor Recombination and Impact on V. vinifera Berry Transcriptome

Thanks to several Vitis vinifera backcrosses with an initial V. vinifera L. × V. rotundifolia (previously Muscadinia rotundifolia) interspecific cross, the MrRUN1/MrRPV1 locus (resistance to downy and powdery mildews) was introgressed in genotypes phenotypically close to V. vinifera varieties. To check the consequences of introgressing parts of the V. rotundifolia genome on gene expression during fruit development, we conducted a comparative RNA-seq study on single berries from different V. vinifera cultivars and V. vinifera × V. rotundifolia hybrids, including 'G5' and two derivative microvine lines, 'MV102' (resistant) and 'MV32' (susceptible) segregating for the MrRUN1/RPV1 locus. RNA-Seq profiles were analyzed on a comprehensive set of single berries from the end of the herbaceous plateau to the ripe stage. Pair-end reads were aligned both on V. vinifera PN40024.V4 reference genome, V. rotundifolia cv 'Trayshed' and cv 'Carlos', and to the few resistance genes from the original V. rotundifolia cv '52' parent available at NCBI. Weighted Gene Co-expression Network Analysis (WGCNA) led to classifying the differentially expressed genes into 15 modules either preferentially correlated with resistance or berry phenology and composition. Resistance positively correlated transcripts predominantly mapped on the 4-5 Mb distal region of V. rotundifolia chromosome 12 beginning with the MrRUN1/MrRPV1 locus, while the negatively correlated ones mapped on the orthologous V. vinifera region, showing this large extremity of LG12 remained recalcitrant to internal recombination during the successive backcrosses. Some constitutively expressed V. rotundifolia genes were also observed at lower densities outside this region. Genes overexpressed in developing berries from resistant accessions, either introgressed from V. rotundifolia or triggered by these in the vinifera genome, spanned various functional groups, encompassing calcium signal transduction, hormone signaling, transcription factors, plant-pathogen-associated interactions, disease resistance proteins, ROS and phenylpropanoid biosynthesis. This transcriptomic insight provides a foundation for understanding the disease resistance inherent in these hybrid cultivars and suggests a constitutive expression of NIR NBS LRR triggering calcium signaling. Moreover, these results illustrate the magnitude of transcriptomic changes caused by the introgressed V. rotundifolia background in backcrossed hybrids, on a large number of functions largely exceeding the ones constitutively expressed in single resistant gene transformants.

CaWRKY22b Plays a Positive Role in the Regulation of Pepper Resistance to Ralstonia solanacearum in a Manner Associated with Jasmonic Acid Signaling

As important transcription factors, WRKYs play a vital role in the defense response of plants against the invasion of multiple pathogens. Though some WRKY members have been reported to participate in pepper immunity in response to Ralstonia solanacearum infection, the functions of the majority of WRKY members are still unknown. Herein, CaWRKY22b was cloned from the pepper genome and its function against R. solanacearum was analyzed. The transcript abundance of CaWRKY22b was significantly increased in response to the infection of R. solanacearum and the application of exogenous methyl jasmonate (MeJA). Subcellular localization assay in the leaves of Nicotiana benthamiana showed that CaWRKY22b protein was targeted to the nuclei. Agrobacterium-mediated transient expression in pepper leaves indicated that CaWRKY22b overexpression triggered intensive hypersensitive response-like cell death, H2O2 accumulation, and the up-regulation of defense- and JA-responsive genes, including CaHIR1, CaPO2, CaBPR1, and CaDEF1. Virus-induced gene silencing assay revealed that knock-down of CaWRKY22b attenuated pepper's resistance against R. solanacearum and the up-regulation of the tested defense- and jasmonic acid (JA)-responsive genes. We further assessed the role of CaWRKY22b in modulating the expression of JA-responsive CaDEF1, and the result demonstrated that CaWRKY22b trans-activated CaDEF1 expression by directly binding to its upstream promoter. Collectively, our results suggest that CaWRKY22b positively regulated pepper immunity against R. solanacearum in a manner associated with JA signaling, probably by modulating the expression of JA-responsive CaDEF1.

Cultivation of Fungal Endophytes with Tissue Culture Grapevine Seedlings Reprograms Metabolism by Triggering Defence Responses

In this study, the transcriptome profiles of tissue-cultured grapevine (Vitis vinifera L. × Vitis labrusca L.: Rose Honey) seedlings inoculated with fungal endophytes Epicoccum layuense R2-21 (Epi R2-21) and Alternaria alternata XHYN2 (Alt XHYN2), were analyzed at three different time points (6 h, 6 d, and 15 d). A total of 4783 differentially expressed genes (DEGs) was found, of which 1853 (6 h), 3878 (6 d), and 4732 (15 d) were differentially expressed relative to those of the control in endophyte Epi R2-21 treatments, while a total of 5898 DEGs, of which 2726 (6 h), 4610 (6 d), and 3938 (15 d) were differentially expressed in endophyte Alt XHYN2 treatments. DEGs enriched in secondary metabolic pathways, plant-pathogen interaction, and hormone signalling were further analysed. The upregulated DEGs in the Epi R2-21 and Alt XHYN2 treatments, both enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG), were mainly involved in flavonoid biosynthesis, phenylpropanoid biosynthesis, stilbenoid, diarylheptanoid and gingerol biosynthesis, phenylalanine metabolism, and circadian rhythms-plant and plant-pathogen interactions, similar to the trend observed in our previous study conducted on the cultivar 'Cabernet Sauvignon' (Vitis vinifera L.). Taken together with the results obtained from the cultivar 'Cabernet Sauvignon', it was found that tissue-cultured seedlings of the cultivar 'Rose Honey' induced a stronger defence response to fungal endophyte infection than that of the cultivar 'Cabernet Sauvignon', and inoculation with the endophyte Alt XHYN2 triggered a stronger response than inoculation with the endophyte Epi R2-21. In addition, the protein-protein interaction (PPI) network revealed that the genes VIT_16s0100g00910, encoding CHS, and VIT_11s0065g00350, encoding CYP73A, were involved in secondary metabolism and thus mediated in the resistance mechanism of grapevine on both the cultivars. The results showed that inoculation with the endophytes Epi R2-21 and Alt XHYN2 had a great ability to induce defence responses and reprogram the gene expression profiles in different grapevine cultivars, which deepens our knowledge of the interaction between fungal endophytes and grapevine and gives hints for grape quality management in viticulture using candidate fungal endophytes.

Enhancing Pseudomonas syringae pv. Actinidiae sensitivity in kiwifruit by repressing the NBS-LRR genes through miRNA-215-3p and miRNA-29-3p identification

Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. actinidiae (PSA), poses a grave threat to the global kiwifruit industry. In this study, we examined the role of microRNAs (miRNAs) in kiwifruit's response to PSA. Kiwifruit seedlings subjected to PSA treatment showed significant changes in both miRNA and gene expression compared to the control group. We identified 364 differentially expressed miRNAs (DEMs) and 7170 differentially expressed genes (DEGs). Further analysis revealed 180 miRNAs negatively regulating 641 mRNAs. Notably, two miRNAs from the miRNA482 family, miRNA-215-3p and miRNA-29-3p, were found to increase kiwifruit's sensitivity to PSA when overexpressed. These miRNAs were linked to the regulation of NBS-LRR target genes, shedding light on their role in kiwifruit's defence against PSA. This study offers insights into the miRNA482-NBS-LRR network as a crucial component in enhancing kiwifruit bioresistance to PSA infestation and provides promising candidate genes for further research.

From molecule to cell: the expanding frontiers of plant immunity

In recent years, the field of plant immunity has witnessed remarkable breakthroughs. During the co-evolution between plants and pathogens, plants have developed a wealth of intricate defense mechanisms to safeguard their survival. Newly identified immune receptors have added unexpected complexity to the surface and intracellular sensor networks, enriching our understanding of the ongoing plant-pathogen interplay. Deciphering the molecular mechanisms of resistosome shapes our understanding of these mysterious molecules in plant immunity. Moreover, technological innovations are expanding the horizon of the plant-pathogen battlefield into spatial and temporal scales. While the development provides new opportunities for untangling the complex realm of plant immunity, challenges remain in uncovering plant immunity across spatiotemporal dimensions from both molecular and cellular levels.

Chloroplast protein StFC-II was manipulated by a Phytophthora effector to enhance host susceptibility

Oomycete secretes a range of RxLR effectors into host cells to manipulate plant immunity by targeting proteins from several organelles. In this study, we report that chloroplast protein StFC-II is hijacked by a pathogen effector to enhance susceptibility. Phytophthora infestans RxLR effector Pi22922 is activated during the early stages of P. infestans colonization. Stable overexpression of Pi22922 in plants suppresses flg22-triggered reactive oxygen species (ROS) burst and enhances leaf colonization by P. infestans. A potato ferrochelatase 2 (FC-II, a nuclear-encoded chloroplast-targeted protein), a key enzyme for heme biosynthesis in chloroplast, was identified as a target of Pi22922 in the cytoplasm. The pathogenicity of Pi22922 in plants is partially dependent on FC-II. Overexpression of StFC-II decreases resistance of potato and Nicotiana benthamiana against P. infestans, and silencing of NbFC-II in N. benthamiana reduces P. infestans colonization. Overexpression of StFC-II increases heme content and reduces chlorophyll content and photosynthetic efficiency in potato leaves. Moreover, ROS accumulation both in chloroplast and cytoplasm is attenuated and defense-related genes are down-regulated in StFC-II overexpression transgenic potato and N. benthamiana leaves. Pi22922 inhibits E3 ubiquitin ligase StCHIP-mediated StFC-II degradation in the cytoplasm and promotes its accumulation in chloroplasts. In summary, this study characterizes a new mechanism that an oomycete RxLR effector suppresses host defenses by promoting StFC-II accumulation in chloroplasts, thereby compromising the host immunity and promoting susceptibility.

Using RNA sequencing to unravel molecular changes underlying the defense response in chickpea induced by Phytophthora medicaginis

Phytophthora root rot (PRR), caused by Phytophthora medicaginis, is a major soil-borne disease of chickpea in Australia. Breeding for PRR resistance is an effective approach to avoid significant yield loss. Genetic resistance has been identified in cultivated chickpea (Cicer arietinum) and in the wild relative C. echinospermum, with previous studies identifying independent genetic loci associated with each of these sources. However, the molecular mechanisms associated with PRR resistance are not known. RNA sequencing analysis employed in this study identified changes in gene expression in roots of three chickpea genotypes grown hydroponically, early post-infection with P. medicaginis zoospores. Analyses of differentially expressed genes (DEG) identified the activation of a higher number of non-specific R-genes in a PRR-susceptible variety than in the resistant genotypes, suggesting a whole plant resistance response occurring in chickpea against the pathogen. Contrasting molecular changes in signaling profiles, proteolysis and transcription factor pathways were observed in the cultivated and wild Cicer-derived resistant genotypes. DEG patterns supported a hypothesis that increased root elongation and reduced adventitious root formation limit the pathogen entry points in the genotype containing the wild Cicer source of PRR resistance. Candidate resistance genes, including an aquaporin and a maltose transporter in the wild Cicer source and GDSL esterases/lipases in the cultivated source of resistance, were oppositely regulated. Increased knowledge of these genes and pathways will improve our understanding of molecular mechanisms controlling PRR resistance in chickpea, and support the development of elite chickpea varieties through molecular breeding approaches.

CaMAPK1 Plays a Vital Role in the Regulation of Resistance to Ralstonia solanacearum Infection and Tolerance to Heat Stress

As an important member of mitogen-activated protein kinase (MAPK) cascades, MAPKs play an important role in plant defense response against biotic and abiotic stresses; however, the involvement of the majority of the MAPK family members against Ralstonia solanacearum and heat stress (HS) remains poorly understood. In the present study, CaMAPK1 was identified from the genome of pepper and its function against R. solanacearum and HS was analyzed. The transcript accumulations of CaMAPK1 and the activities of its native promoter were both significantly induced by R. solanacearum inoculation, HS, and the application of exogenous hormones, including SA, MeJA, and ABA. Transient expression of CaMAPK1 showed that CaMAPK1 can be targeted throughout the whole cells in Nicotiana benthamiana and triggered chlorosis and hypersensitive response-like cell death in pepper leaves, accompanied by the accumulation of H2O2, and the up-regulations of hormones- and H2O2-associated marker genes. The knock-down of CaMAPK1 enhanced the susceptibility to R. solanacearum partially by down-regulating the expression of hormones- and H2O2-related genes and impairing the thermotolerance of pepper probably by attenuating CaHSFA2 and CaHSP70-1 transcripts. Taken together, our results revealed that CaMAPK1 is regulated by SA, JA, and ABA signaling and coordinates responses to R. solanacearum infection and HS in pepper.

Logistics of defense: The contribution of endomembranes to plant innate immunity

Phytopathogens cause plant diseases that threaten food security. Unlike mammals, plants lack an adaptive immune system and rely on their innate immune system to recognize and respond to pathogens. Plant response to a pathogen attack requires precise coordination of intracellular traffic and signaling. Spatial and/or temporal defects in coordinating signals and cargo can lead to detrimental effects on cell development. The role of intracellular traffic comes into a critical focus when the cell sustains biotic stress. In this review, we discuss the current understanding of the post-immune activation logistics of plant defense. Specifically, we focus on packaging and shipping of defense-related cargo, rerouting of intracellular traffic, the players enabling defense-related traffic, and pathogen-mediated subversion of these pathways. We highlight the roles of the cytoskeleton, cytoskeleton-organelle bridging proteins, and secretory vesicles in maintaining pathways of exocytic defense, acting as sentinels during pathogen attack, and the necessary elements for building the cell wall as a barrier to pathogens. We also identify points of convergence between mammalian and plant trafficking pathways during defense and highlight plant unique responses to illustrate evolutionary adaptations that plants have undergone to resist biotic stress.

Characterization of a Small Cysteine-Rich Secreted Effector, TcSCP_9014, in Tilletia controversa

Tilletia controversa J. G. Kühn is the causal agent of wheat dwarf bunt (DB), a destructive disease causing tremendous economic losses. Small cysteine-rich secreted proteins (SCPs) of plant fungi are crucial in modulating host immunity and promoting infection. Little is known about the virulence effectors of T. controversa. Here, we characterized TcSCP_9014, a novel effector of SCPs, in T. controversa which suppressed programmed cell death triggered by BAX without relying on its signal peptide (SP). The SP in the N-terminus of TcSCP_9014 was functional in the secretory process. Live-cell imaging in the epidermal cells of Nicothiana benthamiana suggested that TcSCP_9014 localized to the plasma membrane, cytoplasm, and nucleus. Furthermore, yeast cDNA library screening was performed to obtain the interacting proteins in wheat. Yeast two-hybrid and BiFC assays were applied to validate the interaction of TcSCP_9014 with TaMTAN and TaGAPDH. Our work revealed that the novel effector TcSCP_9014 is vital in modulating plant immunity, which opens up new avenues for plant-pathogen interactions in the T. controversa infection process.

Transcriptome analysis of Gossypium hirsutum cultivar Zhongzhimian No.2 uncovers the gene regulatory networks involved in defense against Verticillium dahliae

BACKGROUND

Cotton is globally important crop. Verticillium wilt (VW), caused by Verticillium dahliae, is the most destructive disease in cotton, reducing yield and fiber quality by over 50% of cotton acreage. Breeding resistant cotton cultivars has proven to be an efficient strategy for improving the resistance of cotton to V. dahliae. However, the lack of understanding of the genetic basis of VW resistance may hinder the progress in deploying elite cultivars with proven resistance.

RESULTS

We planted the VW-resistant Gossypium hirsutum cultivar Zhongzhimian No.2 (ZZM2) in an artificial greenhouse and disease nursery. ZZM2 cotton was subsequently subjected to transcriptome sequencing after Vd991 inoculation (6, 12, 24, 48, and 72 h post-inoculation). Several differentially expressed genes (DEGs) were identified in response to V. dahliae infection, mainly involved in resistance processes, such as flavonoid and terpenoid quinone biosynthesis, plant hormone signaling, MAPK signaling, phenylpropanoid biosynthesis, and pyruvate metabolism. Compared to the susceptible cultivar Junmian No.1 (J1), oxidoreductase activity and reactive oxygen species (ROS) production were significantly increased in ZZM2. Furthermore, gene silencing of cytochrome c oxidase subunit 1 (COX1), which is involved in the oxidation-reduction process in ZZM2, compromised its resistance to V. dahliae, suggesting that COX1 contributes to VW resistance in ZZM2.

CONCLUSIONS

Our data demonstrate that the G. hirsutum cultivar ZZM2 responds to V. dahliae inoculation through resistance-related processes, especially the oxidation-reduction process. This enhances our understanding of the mechanisms regulating the ZZM2 defense against VW.

Extracellular perception of multiple novel core effectors from the broad host-range pear anthracnose pathogen Colletotrichum fructicola in the nonhost Nicotiana benthamiana

Colletotrichum fructicola is emerging as a devastating pathogenic fungus causing anthracnose in a wide range of horticultural crops, particularly fruits. Exploitation of nonhost resistance (NHR) represents a robust strategy for plant disease management. Perception of core effectors from phytopathogens frequently leads to hypersensitive cell death and resistance in nonhost plants; however, such core effectors in C. fructicola and their signaling components in non-hosts remain elusive. Here, we found a virulent C. fructicola strain isolated from pear exhibits non-adaptation in the model plant Nicotiana benthamiana. Perception of secreted molecules from C. fructicola appears to be a dominant factor in NHR, and four novel core effectors-CfCE4, CfCE25, CfCE61, and CfCE66-detected by N. benthamiana were, accordingly, identified. These core effectors exhibit cell death-inducing activity in N. benthamiana and accumulate in the apoplast. With a series of CRISPR/Cas9-edited mutants or gene-silenced plants, we found the coreceptor BAK1 and helper NLRs including ADR1, NRG1, and NRCs mediate perceptions of these core effectors in N. benthamiana. Concurrently, multiple N. benthamiana genes encoding cell surface immune receptors and intracellular immune receptors were greatly induced by C. fructicola. This work represents the first characterization of the repertoire of C. fructicola core effectors responsible for NHR. Significantly, the novel core effectors and their signaling components unveiled in this study offered insights into a continuum of layered immunity during NHR and will be helpful for anthracnose disease management in diverse horticultural crops.