Suppress Plant Defenses Research Articles

Salivary effector DcE1 suppresses plant defense and facilitates the successful feeding of Asian citrus psyllid, Diaphorina citri.

Piercing-sucking insects secrete diverse repertoires of effectors into their hosts to weaken host defenses and promote infestation. The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most destructive insect pest in citrus orchards because of its role as a vector for the huanglongbing pathogen, Candidatus Liberibacter asiaticus (CLas). However, specific effector proteins and their functions in D. citri remain unclear. We demonstrate that DcE1, a salivary protein gene from D. citri, is predominantly expressed in the salivary gland tissues and is delivered into host plants during feeding. Transient expression in tobacco leaves revealed that DcE1 was subcellularly localized in the cytoplasm and plasma membrane, where it inhibited BAX- and INF1-induced cell death, suppressed callose deposition, and activated the salicylic acid pathway by upregulating the expression of endo-β-1,3-glucanase NtBGL2 and regulatory protein NtNPR1. Further, DcE1 knockdown by double-stranded RNA (dsRNA) injection decreased the survival rates of D. citri and interrupted D. citri phloem-feeding on host plants. These results indicate that DcE1 is a novel effector that promotes plant susceptibility and enables D. citri feeding. These findings enhance our understanding of D. citri-plant interactions and offer a potential new target gene for the development of citrus protection strategies. © 2024 Society of Chemical Industry.

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.

Molecular Dialogue During Host Manipulation by the Vascular Wilt Fungus Fusarium oxysporum.

Vascular wilt fungi are a group of hemibiotrophic phytopathogens that infect diverse crop plants. These pathogens have adapted to thrive in the nutrient-deprived niche of the plant xylem. Identification and functional characterization of effectors and their role in the establishment of compatibility across multiple hosts, suppression of plant defense, host reprogramming, and interaction with surrounding microbes have been studied mainly in model vascular wilt pathogens Fusarium oxysporum and Verticillium dahliae. Comparative analysis of genomes from fungal isolates has accelerated our understanding of genome compartmentalization and its role in effector evolution. Also, advances in recent years have shed light on the cross talk of root-infecting fungi across multiple scales from the cellular to the ecosystem level, covering their interaction with the plant microbiome as well as their interkingdom signaling. This review elaborates on our current understanding of the cross talk between vascular wilt fungi and the host plant, which eventually leads to a specialized lifestyle in the xylem. We particularly focus on recent findings in F. oxysporum, including multihost associations, and how they have contributed to understanding the biology of fungal adaptation to the xylem. In addition, we discuss emerging research areas and highlight open questions and future challenges.

A Putative Effector Pst-18220, from Puccinia striiformis f. sp. tritici, Participates in Rust Pathogenicity and Plant Defense Suppression.

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), stands out as one of the most devastating epidemics impacting wheat production worldwide. Resistant wheat varieties had swiftly been overcome due to the emergence of new virulent Pst strains. Effectors secreted by Pst interfere with plant immunity, and verification of their biological function is extremely important for controlling wheat stripe rust. In this study, we identified an effector, Pst-18220, from Puccinia striiformis f. sp. tritici (Pst), which was induced during the early infection stage of Pst. Silencing the expression of Pst-18220 through virus-mediated host-induced gene silencing (HIGS) resulted in a decreased number of rust pustules. In Nicotiana benthamiana, it significantly suppressed cell death induced by Pseudomonas syringae pv. tomato (Pto) DC3000. In Arabidopsis, plants with stable overexpression of Pst-18220 showed increased susceptibility to Pto DC3000, accompanied by a decrease in the expression level of pattern-triggered immunity (PTI)/effector-triggered immunity (ETI)-related genes, namely, AtPCRK1, AtPCRK2, and AtBIK1. These results emphasize the significant role of the Pst candidate effector, Pst-18220, in rust pathogenicity and the suppression of plant defense mechanisms. This broadens our understanding of effectors without any known motif.

Plant viruses exploit insect salivary GAPDH to modulate plant defenses

Salivary proteins of insect herbivores can suppress plant defenses, but the roles of many remain elusive. One such protein is glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from the saliva of the Recilia dorsalis (RdGAPDH) leafhopper, which is known to transmit rice gall dwarf virus (RGDV). Here we show that RdGAPDH was loaded into exosomes and released from salivary glands into the rice phloem through an exosomal pathway as R. dorsalis fed. In infected salivary glands of R. dorsalis, the virus upregulated the accumulation and subsequent release of exosomal RdGAPDH into the phloem. Once released, RdGAPDH consumed H2O2 in rice plants owing to its –SH groups reacting with H2O2. This reduction in H2O2 of rice plant facilitated R. dorsalis feeding and consequently promoted RGDV transmission. However, overoxidation of RdGAPDH could cause potential irreversible cytotoxicity to rice plants. In response, rice launched emergency defense by utilizing glutathione to S-glutathionylate the oxidization products of RdGAPDH. This process counteracts the potential cellular damage from RdGAPDH overoxidation, helping plant to maintain a normal phenotype. Additionally, salivary GAPDHs from other hemipterans vectors similarly suppressed H2O2 burst in plants. We propose a strategy by which plant viruses exploit insect salivary proteins to modulate plant defenses, thus enabling sustainable insect feeding and facilitating viral transmission.

Salivary-secreted vitellogenin suppresses H2O2 burst of plants facilitating Recilia dorsalis leafhopper feeding.

Vitellogenin (Vg), known as the yolk protein precursor for oocyte development in female insects, can be secreted to plant host from salivary glands of hemipterans, including rice leafhopper Recilia dorsalis. The aim of this study was to investigate the function of salivary-secreted Vg of R. dorsalis (RdVg) in rice host. We propose that RdVg possibly regulates the rice defense against insects, benefiting R. dorsalis feeding. RdVg was released into rice phloem along with saliva during R. dorsalis feeding. Knocking down RdVg increased the level of H2O2 and improved H2O2 metabolism in rice plants, making it difficult for R. dorsalis to feed. The transient expression or overexpression of the lipoprotein N-terminal domain of RdVg (RdVg2) significantly reduced hydrogen peroxide (H2O2) metabolism in plants. This suggests that salivary-secreted RdVg acts as an effector suppressing the H2O2 burst in rice plants, and RdVg2 is the key domain. RdVg2 could interact with rice sulfite oxidase (OsSO), which catalyzes the oxidation of SO3 2- and produces H2O2. Exposure of rice plants to R. dorsalis, overexpression of RdVg2 or knocking out OsSO reduced OsSO accumulation and SO3 2- oxidation, benefiting R. dorsalis feeding. However overexpression of OsSO increased SO3 2- oxidation and H2O2 metabolism, inhibiting R. dorsalis feeding. RdVg inhibits H2O2 generation via suppressing OsSO accumulation, ultimately benefiting R. dorsalis feeding. These findings identify RdVg as an effector that suppresses plant defense to insects, and provide insights into the function of salivary-secreted Vg in other Hemiptera insects. © 2024 Society of Chemical Industry.

Comparative Transcriptomic Analysis of Soybean Cyst Nematode Inbred Populations Non-adapted or Adapted on Soybean rhg1-a/Rhg4-Mediated Resistance.

Soybean cyst nematode (SCN, Heterodera glycines) is most effectively managed through planting resistant soybean cultivars, but the repeated use of the same resistance sources has led to a widespread emergence of virulent SCN populations that can overcome soybean resistance. Resistance to SCN HG type 0 (Race 3) in soybean cultivar Forrest is mediated by an epistatic interaction between the soybean resistance genes rhg1-a and Rhg4. We previously developed two SCN inbred populations by mass-selecting SCN HG type 0 (Race 3) on susceptible and resistant recombinant inbred lines, derived from a cross between Forrest and the SCN-susceptible cultivar Essex, which differ for Rhg4. To identify SCN genes potentially involved in overcoming rhg1-a/Rhg4-mediated resistance, we conducted RNA sequencing on early parasitic juveniles of these two SCN inbred populations infecting their respective hosts, only to discover a handful of differentially expressed genes (DEGs). However, in a comparison with early parasitic juveniles of an avirulent SCN inbred population infecting a resistant host, we discovered 59 and 171 DEGs uniquely up- or downregulated in virulent parasitic juveniles adapted on the resistant host. Interestingly, the proteins coded by these 59 DEGs included vitamin B-associated proteins (reduced folate carrier, biotin synthase, and thiamine transporter) and nematode effectors known to play roles in plant defense suppression, suggesting that virulent SCN may exert a heightened transcriptional response to cope with enhanced plant defenses and an altered nutritional status of a resistant soybean host. [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.

A root-knot nematode effector targets the Arabidopsis cysteine protease RD21A for degradation to suppress plant defense and promote parasitism.

Meloidogyne incognita is one of the most widely distributed plant-parasitic nematodes and causes severe economic losses annually. The parasite produces effector proteins that play essential roles in successful parasitism. Here, we identified one such effector named MiCE108, which is exclusively expressed within the nematode subventral esophageal gland cells and is upregulated in the early parasitic stage of M. incognita. A yeast signal sequence trap assay showed that MiCE108 contains a functional signal peptide for secretion. Virus-induced gene silencing of MiCE108 impaired the parasitism of M. incognita in Nicotiana benthamiana. The ectopic expression of MiCE108 in Arabidopsis suppressed the deposition of callose, the generation of reactive oxygen species, and the expression of marker genes for bacterial flagellin epitope flg22-triggered immunity, resulting in increased susceptibility to M. incognita, Botrytis cinerea, and Pseudomonas syringae pv. tomato (Pst) DC3000. The MiCE108 protein physically associates with the plant defense protease RD21A and promotes its degradation via the endosomal-dependent pathway, or 26S proteasome. Consistent with this, knockout of RD21A compromises the innate immunity of Arabidopsis and increases its susceptibility to a broad range of pathogens, including M. incognita, strongly indicating a role in defense against this nematode. Together, our data suggest that M. incognita deploys the effector MiCE108 to target Arabidopsis cysteine protease RD21A and affect its stability, thereby suppressing plant innate immunity and facilitating parasitism.

Integrative systems biology of wheat susceptibility to Fusarium graminearum uncovers a conserved gene regulatory network and identifies master regulators targeted by fungal core effectors

BackgroundPlant diseases are driven by an intricate set of defense mechanisms counterbalanced by the expression of host susceptibility factors promoted through the action of pathogen effectors. In spite of their central role in the establishment of the pathology, the primary components of plant susceptibility are still poorly understood and challenging to trace especially in plant-fungal interactions such as in Fusarium head blight (FHB) of bread wheat. Designing a system-level transcriptomics approach, we leveraged the analysis of wheat responses from a susceptible cultivar facing Fusarium graminearum strains of different aggressiveness and examined their constancy in four other wheat cultivars also developing FHB.ResultsIn this study, we describe unexpected differential expression of a conserved set of transcription factors and an original subset of master regulators were evidenced using a regulation network approach. The dual-integration with the expression data of pathogen effector genes combined with database mining, demonstrated robust connections with the plant molecular regulators and identified relevant candidate genes involved in plant susceptibility, mostly able to suppress plant defense mechanisms. Furthermore, taking advantage of wheat cultivars of contrasting susceptibility levels, a refined list of 142 conserved susceptibility gene candidates was proposed to be necessary host’s determinants for the establishment of a compatible interaction.ConclusionsOur findings emphasized major FHB determinants potentially controlling a set of conserved responses associated with susceptibility in bread wheat. They provide new clues for improving FHB control in wheat and also could conceivably leverage further original researches dealing with a broader spectrum of plant pathogens.

Glutathione peroxidase LtGPX3 contributes to oxidative stress tolerance, virulence, and plant defense suppression in the peach gummosis fungus Lasiodiplodia theobromae

The notorious woody plant-degrading pathogen Lasiodiplodia theobromae is a major causal agent of peach gummosis, one of the prevalent and devastating trunk diseases to peach production; however, its pathogenesis is largely unknown. Our previous study showed that L. theobromae LtGPX3, which encodes a glutathione peroxidase resembling yeast GPX3/HYR1-like, was constantly and dramatically upregulated at the infectious stages. Here, we functionally characterized LtGPX3 using the CRISPR-Cas9-aided split marker approach. The ΔLtgpx3 deletion mutants displayed increased sensitivity to the osmotic stress agent KCl and less sensitivity to the cell wall-damaging agent calcofluor white. Exogenous oxidants highly induced the expression of LtGPX3, and the ΔLtgpx3 mutants displayed increased sensitivity to ROS-generating oxidants. Pathogenicity assays revealed that ΔLtgpx3 mutants showed compromised virulence in peach shoots, which was partially restored when peach shoots were pretreated with an NADPH oxidase inhibitor before inoculation. Moreover, ROS levels were strongly boosted, and transcripts of plant defense-related genes were highly induced in the ΔLtgpx3 mutants-infected peach shoots compared with the wild-type-inoculated. Overall, our results showed the essential roles of LtGPX3 in the oxidative stress response and tolerance and pathological functions in L. theobromae. These findings deepen our understanding of the survival strategies of the woody plant-degrading pathogen L. theobromae and provide new insights into developing new strategies for peach gummosis disease control.

Leafhopper salivary vitellogenin mediates virus transmission to plant phloem

Salivary effectors of piercing-sucking insects can suppress plant defense to promote insect feeding, but it remains largely elusive how they facilitate plant virus transmission. Leafhopper Nephotettix cincticeps transmits important rice reovirus via virus-packaging exosomes released from salivary glands and then entering the rice phloem. Here, we report that intact salivary vitellogenin of N. cincticeps (NcVg) is associated with the GTPase Rab5 of N. cincticeps (NcRab5) for release from salivary glands. In virus-infected salivary glands, NcVg is upregulated and packaged into exosomes mediated by virus-induced NcRab5, subsequently entering the rice phloem. The released NcVg inherently suppresses H2O2 burst of rice plants by interacting with rice glutathione S-transferase F12, an enzyme catalyzing glutathione-dependent oxidation, thus facilitating leafhoppers feeding. When leafhoppers transmit virus, virus-upregulated NcVg thus promotes leafhoppers feeding and enhances viral transmission. Taken together, the findings provide evidence that viruses exploit insect exosomes to deliver virus-hijacked effectors for efficient transmission.

Identification of salivary proteins of the cowpea aphid Aphis craccivora by transcriptome and LC-MS/MS analyses

Aphid salivary proteins mediate the interaction between aphids and their host plants. Moreover, these proteins facilitate digestion, detoxification of secondary metabolites, as well as activation and suppression of plant defenses. The cowpea aphid, Aphis craccivora, is an important sucking pest of leguminous crops worldwide. Although aphid saliva plays an important role in aphid plant interactions, knowledge of the cowpea aphid salivary proteins is limited. In this study, we performed transcriptomic and LC-MS/MS analyses to identify the proteins present in the salivary glands and saliva of A. craccivora. A total of 1,08,275 assembled transcripts were identified in the salivary glands of aphids. Of all these assembled transcripts, 53,714 (49.11%) and 53,577 (49.48%) transcripts showed high similarity to known proteins in the Nr and UniProt databases, respectively. A total of 2159 proteins were predicted as secretory proteins from the salivary gland transcriptome dataset, which contain digestive enzymes, detoxification enzymes, previously known effectors and elicitors, and potential proteins whose functions have yet to be determined. The proteomic analysis of aphid saliva resulted in the identification of 171 proteins. Tissue-specific expression of selected genes using RT-PCR showed that three genes were expressed only in the salivary glands. Overall, our results provide a comprehensive repertoire of cowpea aphid salivary proteins from the salivary gland and saliva, which will be a good resource for future effector functional studies and might also be useful for sustainable aphid management.

Transcriptional and epigenetic changes during tomato yellow leaf curl virus infection in tomato

BackgroundGeminiviruses are DNA plant viruses that cause highly damaging diseases affecting crops worldwide. During the infection, geminiviruses hijack cellular processes, suppress plant defenses, and cause a massive reprogramming of the infected cells leading to major changes in the whole plant homeostasis. The advances in sequencing technologies allow the simultaneous analysis of multiple aspects of viral infection at a large scale, generating new insights into the molecular mechanisms underlying plant-virus interactions. However, an integrative study of the changes in the host transcriptome, small RNA profile and methylome during a geminivirus infection has not been performed yet. Using a time-scale approach, we aim to decipher the gene regulation in tomato in response to the infection with the geminivirus, tomato yellow leaf curl virus (TYLCV).ResultsWe showed that tomato undergoes substantial transcriptional and post-transcriptional changes upon TYLCV infection and identified the main altered regulatory pathways. Interestingly, although the principal plant defense-related processes, gene silencing and the immune response were induced, this cannot prevent the establishment of the infection. Moreover, we identified extra- and intracellular immune receptors as targets for the deregulated microRNAs (miRNAs) and established a network for those that also produced phased secondary small interfering RNAs (phasiRNAs). On the other hand, there were no significant genome-wide changes in tomato methylome at 14 days post infection, the time point at which the symptoms were general, and the amount of viral DNA had reached its maximum level, but we were able to identify differentially methylated regions that could be involved in the transcriptional regulation of some of the differentially expressed genes.ConclusionWe have conducted a comprehensive and reliable study on the changes at transcriptional, post-transcriptional and epigenetic levels in tomato throughout TYLCV infection. The generated genomic information is substantial for understanding the genetic, molecular and physiological changes caused by TYLCV infection in tomato.

Functional Analysis of a Salicylate Hydroxylase in Sclerotinia sclerotiorum.

Salicylic acid plays a crucial role during plant defense to Sclerotinia sclerotiorum. Some bacteria and a few fungi can produce salicylate hydroxylase to degrade SA to suppress plant defense and increase their virulence. But there has been no single salicylate hydroxylase in Sclerotinia sclerotiorum identified until now. In this study, we found that SS1G_02963 (SsShy1), among several predicted salicylate hydroxylases in S. sclerotiorum, was induced approximately 17.6-fold during infection, suggesting its potential role in virulence. SsShy1 could catalyze the conversion of SA to catechol when heterologous expression in E. coli. Moreover, overexpression of SsShy1 in Arabidopsis thaliana decreased the SA concentration and the resistance to S. sclerotiorum, confirming that SsShy1 is a salicylate hydroxylase. Deletion mutants of SsShy1 (∆Ssshy1) showed slower growth, less sclerotia production, more sensitivity to exogenous SA, and lower virulence to Brassica napus. The complemented strain with a functional SsShy1 gene recovered the wild-type phenotype. These results indicate that SsShy1 plays an important role in growth and sclerotia production of S. sclerotiorum, as well as the ability to metabolize SA affects the virulence of S. sclerotiorum.

Glucose Oxidase of a Crucifer-Specialized Insect: A Potential Role in Suppressing Plant Defense via Modulating Antagonistic Plant Hormones.

Glucose oxidase (GOX) is a representative compound found in most insect saliva that can suppress plant-defensive responses. However, little is known about the origin and role of GOX in the crucifer-specialized pest Plutella xylostella. In this study, we showed obvious regurgitation from the larval gut of P. xylostella and identified abundant peptides highly similar to known GOX. Three PxGOX genes were verified with PxGOX2 preferentially expressed in the gut. The heterologously expressed PxGOX2 confirmed its function to be a GOX, and it was detected in plant wounds together with the gut regurgitant. Further experiments revealed that PxGOX2 functioned as an effector and may suppress defensive responses in plant through the production of H2O2, which modulates levels of antagonistic salicylic acid and jasmonic acid. However, excessive H2O2 in the host plant may be neutralized by peroxidase, thus forming defensive feedback. Our findings provided new insights into understanding the GOX-mediated insect-plant interactions.

The Role of Mycotoxins in Interactions between Fusarium graminearum and F. verticillioides Growing in Saprophytic Cultures and Co-Infecting Maize Plants.

Fusarium graminearum (FG) and Fusarium verticillioides (FV) co-occur in infected plants and plant residues. In maize ears, the growth of FV is stimulated while FG is suppressed. To elucidate the role of mycotoxins in these effects, we used FG mutants with disrupted synthesis of nivalenol (NIV) and deoxynivalenol (DON) and a FV mutant with disrupted synthesis of fumonisins to monitor fungal growth in mixed cultures in vitro and in co-infected plants by real-time PCR. In autoclaved grains as well as in maize ears, the growth of FV was stimulated by FG regardless of the production of DON or NIV by the latter, whereas the growth of FG was suppressed. In autoclaved grains, fumonisin-producing FV suppressed FG more strongly than a fumonisin-nonproducing strain, indicating that fumonisins act as interference competition agents. In co-infected maize ears, FG suppression was independent of fumonisin production by FV, likely due to heterogeneous infection and a lower level of fumonisins in planta. We conclude that (i) fumonisins are agents of interference competition of FV, and (ii) trichothecenes play no role in the interaction between FG and FV. We hypothesize the following: (i) In vitro, FG stimulates the FV growth by secreting hydrolases that mobilize nutrients. In planta, suppression of plant defense by FG may additionally play a role. (ii) The biological function of fumonisin production in planta is to protect kernels shed on the ground by accumulating protective metabolites before competitors become established. Therefore, to decipher the biological function of mycotoxins, the entire life history of mycotoxin producers must be considered.

Ustilago maydis PR-1-like protein has evolved two distinct domains for dual virulence activities

The diversification of effector function, driven by a co-evolutionary arms race, enables pathogens to establish compatible interactions with hosts. Structurally conserved plant pathogenesis-related PR-1 and PR-1-like (PR-1L) proteins are involved in plant defense and fungal virulence, respectively. It is unclear how fungal PR-1L counters plant defense. Here, we show that Ustilago maydis UmPR-1La and yeast ScPRY1, with conserved phenolic resistance functions, are Ser/Thr-rich region mediated cell-surface localization proteins. However, UmPR-1La has gained specialized activity in sensing phenolics and eliciting hyphal-like formation to guide fungal growth in plants. Additionally, U. maydis hijacks maize cathepsin B-like 3 (CatB3) to release functional CAPE-like peptides by cleaving UmPR-1La’s conserved CNYD motif, subverting plant CAPE-primed immunity and promoting fungal virulence. Surprisingly, CatB3 avoids cleavage of plant PR-1s, despite the presence of the same conserved CNYD motif. Our work highlights that UmPR-1La has acquired additional dual roles to suppress plant defense and sustain the infection process of fungal pathogens.

Salivary protein expression profiles of five species of Pentatomidae (Hemiptera)

Abstract Stink bug (Hemiptera: Pentatomidae) development typically requires feeding on a diversity of plant species and various plant tissues. During feeding, stink bugs discharge salivary enzymes with roles in extraoral digestion and countering plant defense responses. Although previous research has described digestive salivary proteins from stink bugs, less is known of the salivary proteins involved in the suppression of plant defenses. We sequenced the transcriptomes of salivary glands dissected from five stink bug species collected from non-crop habitats in Washington: Halyomorpha halys (Stål), Nezara viridula L., Euschistus conspersus (Uhler), Thyanta pallidovirens (Stål), and Chlorochroa ligata (Say). We identified a total of 677 candidate secreted proteins from the salivary glands of the five species. Based on work from other insects, many of the proteins have potential functions in the suppression of plant defense signaling and deactivation of plant defense molecules. We also identified salivary proteins with potential roles in the extraoral digestion of plant tissues, protection from entomopathogens, and deposition of salivary sheaths. This report provides a curation of putative salivary effector genes for further functional analysis.

Suppressing plant defense: Scavenge the ROS!

Reactive oxygen species (ROS)-mediated defense against fungal pathogens is an essential arm of plant immunity. As a counter defense, these pathogens synthesize antioxidant enzymes that scavenge the ROS produced by plants. The molecular mechanism behind the upregulation of these enzymes in fungal pathogens was unknown. A recent study by Zhang et al. (2023, https://doi.org/10.15252/embj.2022112634) has shed light on the mechanism, and it has been shown that deacetylation of FolSrpk1 protein on the K304 residue following oxidative stress is an important event in the signaling cascade leading to ROS detoxification in Fusarium oxysporum f. sp. lycopersici. Deacetylated FolSrpk1 moves to the nucleus where it hyperphosphorylates FolSr1, which further regulates the transcription of antioxidant enzymes. This mechanism of ROS detoxification is conserved in Botrytis cinerea as well. Several other phytopathogenic fungi have a corresponding K304 site and NLS present in their Srpk1, suggesting a similar mechanism of ROS detoxification and suppression of plant defense. In this article, we have presented our views on how future studies can be synthesized based on the pathway deciphered by Zhang et al. (2023, https://doi.org/10.15252/embj.2022112634).

HexR Transcription Factor Contributes to Pseudomonas cannabina pv. alisalensis Virulence by Coordinating Type Three Secretion System Genes.

Pseudomonas cannabina pv. alisalensis (Pcal) causes bacterial blight on cabbage. We previously conducted a screening for reduced virulence using Tn5 transposon mutants and identified one of the transcriptional factors, HexR, as a potential Pcal virulence factor. However, the role of HexR in plant pathogenic Pseudomonas virulence has not been investigated well. Here, we demonstrated that the Pcal hexR mutant showed reduced disease symptoms and bacterial populations on cabbage, indicating that HexR contributes to Pcal virulence. We used RNA-seq analysis to characterize the genes regulated by HexR. We found that several type three secretion system (T3SS)-related genes had lower expression of the Pcal hexR mutant. Five genes were related to T3SS machinery, two genes were related to type three helper proteins, and three genes encoded type three effectors (T3Es). We also confirmed that T3SS-related genes, including hrpL, avrPto, hopM1, and avrE1, were also down-regulated in the Pcal hexR mutant both in culture and in vivo by using RT-qPCR. T3SS functions to suppress plant defense in host plants and induce hypersensitive response (HR) cell death in non-host plants. Therefore, we investigated the expression profiles of cabbage defense-related genes, including PR1 and PR5, and found that the expressions of these genes were greater in the Pcal hexR mutant. We also demonstrated that the hexR mutant did not induce HR cell death in non-host plants, indicating that HexR contributes in causing HR in nonhost plants. Together, these results indicate that the mutation in hexR leads to a reduction in the T3SS-related gene expression and thus an impairment in plant defense suppression, reducing Pcal virulence.