Arabidopsis is susceptible to infection by a downy mildew fungus

A powdery mildew core effector protein targets the host endosome tethering complexes HOPS and CORVET in barley

Powdery mildew fungi are serious pathogens affecting many plant species. Their genomes encode extensive repertoires of secreted effector proteins that suppress host immunity. Here, we revised and analyzed the candidate secreted effector protein (CSEP) effectome of the powdery mildew fungus, Blumeria hordei (Bh). We identified seven putative effectors that are broadly conserved in powdery mildew species, suggesting that they are core effectors of these phytopathogens. We showed that one of these effectors, CSEP0214, interacts with the barley (Hordeum vulgare) vacuolar protein-sorting 18 (VPS18) protein, a shared component of the class C core vacuole/endosome tethering (CORVET) and homotypic fusion and protein-sorting (HOPS) endosomal tethering complexes that mediate fusion of early endosomes and multivesicular bodies, respectively, with the central vacuole. Overexpression of CSEP0214 and knockdown of either VPS18, HOPS-specific VPS41, or CORVET-specific VPS8 blocked the vacuolar pathway and the accumulation of the fluorescent vacuolar marker protein (SP)-RFP-AFVY in the endoplasmic reticulum. Moreover, CSEP0214 inhibited the interaction between VPS18 and VPS16, which are both shared components of CORVET as well as HOPS. Additionally, introducing CSEP0214 into barley leaf cells blocked the hypersensitive cell death response associated with resistance gene-mediated immunity, indicating that endomembrane trafficking is required for this process. CSEP0214 expression also prevented callose deposition in cell wall appositions at attack sites and encasements of fungal infection structures. Our results indicate that the powdery mildew core effector CSEP0214 is an essential suppressor of plant immunity.

Pathogen challenge in Arabidopsis cotyledons induces enhanced disease resistance at newly formed rosette leaves via sustained upregulation of WRKY70

Pathogenic microorganisms often target seedlings shortly after germination. If plants exhibit resistance or resilience to pathogens, those exposed to pathogen challenge may grow further and form new unchallenged leaves. The purpose of this study was to examine disease resistance in the newly formed leaves of plants subjected to pathogen challenge. We used Arabidopsis thaliana and the oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) as the model pathosystem. We found that Arabidopsis seedlings primarily challenged with the avirulent isolate Hpa exhibited enhanced disease resistance against the virulent isolate Hpa in newly formed rosette leaves (NFRLs). Our observations indicated that the transcript levels of the transcription factor gene WRKY70, which is essential for full resistance to the virulent isolate HpaNoco2, were elevated and maintained at high levels in the NFRLs. In contrast, the transcript levels of the salicylic acid marker gene PR1 and systemic acquired resistance-related genes did not exhibit sustained elevation. The maintenance of increased transcript levels of WRKY70 operated independently of non-expressor of pathogenesis-related gene 1. These findings suggest that prolonged upregulation of WRKY70 represents a defensive state synchronized with plant development to ensure survival against subsequent infections.

Combined metabolome and transcriptome reveal HmF6'H1 regulating simple coumarin accumulation against powdery mildew infection in Heracleum moellendorffii Hance

BACKGROUND

Powdery mildew, caused by Eeysiphe heraclei, seriously threatens Heracleum moellendorffii Hance. Plant secondary metabolites are essential to many activities and are necessary for defense against biotic stress. In order to clarify the functions of these metabolites in response to the pathogen, our work concentrated on the variations in the accumulation of secondary metabolites in H. moellendorffii during E. heraclei infection.

RESULTS

Following E. heraclei infection, a significant upregulation of coumarin metabolites-particularly simple coumarins and associated genes was detected by RNA-seq and UPLC-MS/MS association analysis. Identifying HmF6'H1, a Feruloyl CoA 6'-hydroxylase pivotal in the biosynthesis of the coumarin basic skeleton through ortho-hydroxylation, was a significant outcome. The cytoplasmic HmF6'H1 protein was shown to be able to catalyze the ortho-hydroxylation of p-coumaroyl-CoA and caffeoyl-CoA, resulting in the formation of umbelliferone and esculetin, respectively. Over-expression of the HmF6'H1 gene resulted in increased levels of simple coumarins, inhibiting the biosynthesis of furanocoumarins and pyranocoumarins by suppressing PT gene expression, enhancing H. moellendorffii resistance to powdery mildew.

CONCLUSIONS

These results established HmF6'H1 as a resistance gene aiding H. moellendorffii in combatting E. heraclei infection, offering additional evidence of feruloyl-CoA 6'-hydroxylase role in catalyzing various types of simple coumarins. Therefore, this work contributes to our understanding of the function of simple coumarins in plants' defense against powdery mildew infection.

Powdery mildew effectors AVRA1 and BEC1016 target the ER J-domain protein HvERdj3B required for immunity in barley

The barley powdery mildew fungus, Blumeria hordei (Bh), secretes hundreds of candidate secreted effector proteins (CSEPs) to facilitate pathogen infection and colonization. One of these, CSEP0008, is directly recognized by the barley nucleotide-binding leucine-rich-repeat (NLR) receptor MLA1 and therefore is designated AVRA1. Here, we show that AVRA1 and the sequence-unrelated Bh effector BEC1016 (CSEP0491) suppress immunity in barley. We used yeast two-hybrid next-generation interaction screens (Y2H-NGIS), followed by binary Y2H and in planta protein-protein interactions studies, and identified a common barley target of AVRA1 and BEC1016, the endoplasmic reticulum (ER)-localized J-domain protein HvERdj3B. Silencing of this ER quality control (ERQC) protein increased Bh penetration. HvERdj3B is ER luminal, and we showed using split GFP that AVRA1 and BEC1016 translocate into the ER signal peptide-independently. Overexpression of the two effectors impeded trafficking of a vacuolar marker through the ER; silencing of HvERdj3B also exhibited this same cellular phenotype, coinciding with the effectors targeting this ERQC component. Together, these results suggest that the barley innate immunity, preventing Bh entry into epidermal cells, requires ERQC. Here, the J-domain protein HvERdj3B appears to be essential and can be regulated by AVRA1 and BEC1016. Plant disease resistance often occurs upon direct or indirect recognition of pathogen effectors by host NLR receptors. Previous work has shown that AVRA1 is directly recognized in the cytosol by the immune receptor MLA1. We speculate that the AVRA1 J-domain target being inside the ER, where it is inapproachable by NLRs, has forced the plant to evolve this challenging direct recognition.

Wheat powdery mildew resistance gene Pm13 encodes a mixed lineage kinase domain-like protein

Wheat powdery mildew is one of the most destructive diseases threatening global wheat production. The wild relatives of wheat constitute rich sources of diversity for powdery mildew resistance. Here, we report the map-based cloning of the powdery mildew resistance gene Pm13 from the wild wheat species Aegilops longissima. Pm13 encodes a mixed lineage kinase domain-like (MLKL) protein that contains an N-terminal-domain of MLKL (MLKL_NTD) domain in its N-terminus and a C-terminal serine/threonine kinase (STK) domain. The resistance function of Pm13 is validated by mutagenesis, gene silencing, transgenic assay, and allelic association analyses. The development of introgression lines with significantly reduced chromosome segments of Ae. longissima encompassing Pm13 enables widespread deployment of this gene into wheat cultivars. The cloning of Pm13 may provide valuable insights into the molecular mechanisms underlying Pm13-mediated powdery mildew resistance and highlight the important roles of kinase fusion proteins (KFPs) in wheat immunity.

Role of PMR4 and PDLP1 in priming of early acting penetration defense by resistance-inducing β-amino acids

R-β-homoserine (RBH) and β-aminobutyric acid (BABA) induce resistance against the oomycete Hyaloperonospora arabidopsidis (Hpa) in Arabidopsis, which is based on priming of multiple defense layers, including early acting penetration resistance at the cell wall. Here, we have examined the molecular basis of RBH- and BABA-primed defense by cell wall papillae against Hpa. Three-dimensional reconstruction of Hpa-induced papillae by confocal microscopy revealed no structural differences between control-, RBH-, and BABA-treated plants after Hpa challenge. However, mutations affecting POWDERY MILDEW RESISTANCE 4 or PLASMODESMATA LOCATED PROTEINs (PDLPs) only impaired BABA-induced penetration resistance and not RBH-induced penetration resistance. Furthermore, PDLP1 over-expression mimicked primed penetration resistance, while the intensity of GFP-tagged PDLP1 at germinating Hpa conidiospores was increased in BABA-primed plants but not RBH-primed plants. Our study reveals new regulatory layers of immune priming by β-amino acids and supports the notion that penetration resistance is a multifaceted defense layer that can be achieved through seperate pathways.

Inhibition of NPR1 Leads to Shoot Growth Improvement under Low-Calcium Conditions in Arabidopsis

Under low-Ca conditions, plants accumulate salicylic acid (SA) and induce SA-responsive genes. However, the relationship between SA and low-Ca tolerance remains unclear. Here, we demonstrated that the inhibition or suppression of nonexpressor of pathogenesis-related 1 (NPR1) activity, a major regulator of the SA signaling pathway in the defense response, improves shoot growth under low-Ca conditions. Furthermore, mutations in phytoalexin-deficient 4 (PAD4) or enhanced disease susceptibility 1 (EDS1), which are upstream regulators of NPR1, improved shoot growth under low-Ca conditions, suggesting that NPR1 suppressed growth under low-Ca conditions. In contrast, growth of SA induction-deficient 2-2 (sid2-2), which is an SA-deficient mutant, was sensitive to low Ca levels, suggesting that SA accumulation by SID2 was not related to growth inhibition under low-Ca conditions. Additionally, npr1-1 showed low-Ca tolerance, and the application of tenoxicam-an inhibitor of the NPR1-mediated activation of gene expression-also improved shoot growth under low Ca conditions. The low-Ca tolerance of double mutants pad4-1, npr1-1 and eds1-22 npr1-1 was similar to that of the single mutants, suggesting that PAD4 and EDS1 are involved in the same genetic pathway in suppressing growth under low-Ca conditions as NPR1. Cell death and low-Ca tolerance did not correlate among the mutants, suggesting that growth improvement in the mutants was not due to cell death inhibition. In conclusion, we revealed that NPR1 suppresses plant growth under low-Ca conditions and that the other SA-related genes influence plant growth and cell death.

Brassica Cover Crops and Natural Spongospora subterranea Infestation of Peat-Based Potting Mix May Increase Powdery Scab Risk on Potato

Spongospora subterranea is a soilborne plasmodiophorid that causes powdery scab and root gall formation in potato. In this study, 18 cover crops suitable for use in dry, high-altitude potato production regions were assessed in potting mix trials to determine whether these cover crops altered S. subterranea population levels. Although S. subterranea appeared to invade roots of all plant species tested, the pathogen was unable to complete its life cycle on 11 of 18 cover crops based on postharvest qPCR and microscopy results. Buckwheat, legumes, and scarlet barley do not appear to support pathogen replication, but the pathogen may be able to complete its life cycle in some mustards. High variability occurred in the experiments and part of this may be due to the natural infestations of peat-based potting mix with S. subterranea. A tomato bioassay was used to confirm that commercial sources of peat-based potting mix were infested with S. subterranea. Dry heat and autoclaving were tested as sanitation methods and multiple rounds of autoclaving were required to reduce viable S. subterranea in potting mix. A second cover crop experiment with autoclaved potting mix was conducted and it confirmed that buckwheat, legumes, and barley do not support S. subterranea replication but that some brassica crops may be hosts of this pathogen. The results suggest that buckwheat, legumes, and barley pose the least risk as cover crops in S. subterranea infested fields and show that peat-based potting mix should not be used in seed potato production.

Lesion mimic mutant 8 balances disease resistance and growth in rice

Lesion-mimic mutants (LMM) spontaneously produce necrotic spots, a process not affected by environmental stress or pathogen infection. In this study, we identified a LMM, lesion mimic mutant 8 (lmm8) in rice (Oryza sativa). The lmm8 mutant produces brown and off-white lesions on its leaves during the second- and third-leaf stages. The lesion mimic phenotype of the lmm8 mutant was enhanced by light. At the mature stage, lmm8 mutant are shorter and exhibit inferior agronomic traits than the wild type. Contents of photosynthetic pigments and chloroplast fluorescence were significantly reduced in lmm8 leaves, along with increased production of reactive oxygen species and programmed cell death compared to the wild type. The mutated gene was identified as LMM8 (LOC_Os01g18320) by map-based cloning. A point mutation occurred in LMM8, causing a Leu to Arg mutation of the 146th amino acid of LMM8. It is an allele of SPRL1, encoding a protoporphyrinogen IX oxidase (PPOX) located in chloroplasts and involved in the biosynthesis of tetrapyrrole in chloroplasts. The lmm8 mutant showed enhanced resistance and broad-spectrum resistance. Together, our results demonstrate the importance of rice LMM8 protein in defense responses and plant growth in rice, and provides theoretical support for resistance breeding to improve rice yield.

The Arabidopsis thaliana onset of leaf death 12 mutation in the lectin receptor kinase P2K2 results in an autoimmune phenotype

BACKGROUND

Plant immunity relies on the perception of immunogenic signals by cell-surface and intracellular receptors and subsequent activation of defense responses like programmed cell death. Under certain circumstances, the fine-tuned innate immune system of plants results in the activation of autoimmune responses that cause constitutive defense responses and spontaneous cell death in the absence of pathogens.

RESULTS

Here, we characterized the onset of leaf death 12 (old12) mutant that was identified in the Arabidopsis accession Landsberg erecta. The old12 mutant is characterized by a growth defect, spontaneous cell death, plant-defense gene activation, and early senescence. In addition, the old12 phenotype is temperature reversible, thereby exhibiting all characteristics of an autoimmune mutant. Mapping the mutated locus revealed that the old12 phenotype is caused by a mutation in the Lectin Receptor Kinase P2-TYPE PURINERGIC RECEPTOR 2 (P2K2) gene. Interestingly, the P2K2 allele from Landsberg erecta is conserved among Brassicaceae. P2K2 has been implicated in pathogen tolerance and sensing extracellular ATP. The constitutive activation of defense responses in old12 results in improved resistance against Pseudomonas syringae pv. tomato DC3000.

CONCLUSION

We demonstrate that old12 is an auto-immune mutant and that allelic variation of P2K2 contributes to diversity in Arabidopsis immune responses.

Genome-wide identification, evolution, and expression analysis of MLO gene family in melon (Cucumis melo L.)

Powdery mildew (PM) is one of the main fungal diseases that appear during the cultivation of the melon fruit crop. Mildew Resistance Locus "O" (MLO) is known as a gene family and has seven conserved transmembrane domains. An induced functional loss of a specific MLO gene could mainly confer PM resistance to melons. However, the genomic structure of MLO genes and its main role in PM resistance still remain unclear in melon. In this study, bioinformatic analysis identified a total of 14 MLO gene family members in the melon genome sequence, and these genes were distributed in an uneven manner on eight chromosomes. The phylogenetic analysis divided the CmMLO genes into five different clades, and gene structural analysis showed that genes in the same clade had similar intron and exon distribution patterns. In addition, by cloning the CmMLO gene sequence in four melon lines, analyzing the CmMLO gene expression pattern after infection, and making microscopic observations of the infection pattern of PM, we concluded that the CmMLO5 (MELO3C012438) gene plays a negative role in regulating PM-resistance in the susceptible melon line (Topmark), and the critical time point for gene function was noticed at 24 and 72 hours after PM infection. The mutational analysis exhibited a single base mutation at 572 bp, which further results in loss of protein function, thus conferring PM resistance in melon. In summary, our research evidence provides a thorough understanding of the CmMLO gene family and demonstrates their potential role in disease resistance, as well as a theoretical foundation for melon disease resistance breeding.

Bacillus vallismortis TU-Orga21 blocks rice blast through both direct effect and stimulation of plant defense

Beneficial microorganisms are an important strategy for sustainable plant production processes such as stimulate root exudation, stress tolerance, and yield improvement. This study investigated various microorganisms isolated from the rhizosphere of Oryza sativa L. in order to inhibit Magnaporthe oryzae cause of rice blast, by direct and indirect mode of action. The results indicated that Bacillus vallismortis strain TU-Orga21 significantly reduced M. oryzae mycelium growth and deformed the hyphal structures. The effects of biosurfactant TU-Orga21 was studied against M. oryzae spore development. The dose of ≥5% v/v biosurfactant significantly inhibited the germ tubes and appressoria formation. The biosurfactants were evaluated as surfactin and iturin A by Matrix-assisted laser desorption ionization dual time-of-flight tandem mass spectrometry. Under greenhouse conditions, priming the biosurfactant three times before M. oryzae infection significantly accumulated endogenous salicylic acid, phenolic compounds, and hydrogen peroxide (H₂O₂) during the infection process of M. oryzae. The SR-FT-IR spectral changes from the mesophyll revealed higher integral area groups of lipids, pectins, and proteins amide I and amide II in the elicitation sample. Furthermore, scanning electron microscope revealed appressorium and hyphal enlargement in un-elicitation leaves whereas appressorium formation and hyphal invasion were not found in biosurfactant-elicitation at 24 h post inoculation. The biosurfactant treatment significantly mitigated rice blast disease severity. Therefore, B. vallismortis can be a promising novel biocontrol agent which contains the preformed active metabolites for a rapid control of rice blast by a direct action against pathogen and by boosting plant immunity.

The grapevine LysM receptor-like kinase VvLYK5-1 recognizes chitin oligomers through its association with VvLYK1-1

The establishment of defense reactions to protect plants against pathogens requires the recognition of invasion patterns (IPs), mainly detected by plasma membrane-bound pattern recognition receptors (PRRs). Some IPs, also termed elicitors, are used in several biocontrol products that are gradually being developed to reduce the use of chemicals in agriculture. Chitin, the major component of fungal cell walls, as well as its deacetylated derivative, chitosan, are two elicitors known to activate plant defense responses. However, recognition of chitooligosaccharides (COS) in Vitis vinifera is still poorly understood, hampering the improvement and generalization of protection tools for this important crop. In contrast, COS perception in the model plant Arabidopsis thaliana is well described and mainly relies on a tripartite complex formed by the cell surface lysin motif receptor-like kinases (LysM-RLKs) AtLYK1/CERK1, AtLYK4 and AtLYK5, the latter having the strongest affinity for COS. In grapevine, COS perception has for the moment only been demonstrated to rely on two PRRs VvLYK1-1 and VvLYK1-2. Here, we investigated additional players by overexpressing in Arabidopsis the two putative AtLYK5 orthologs from grapevine, VvLYK5-1 and VvLYK5-2. Expression of VvLYK5-1 in the atlyk4/5 double mutant background restored COS sensitivity, such as chitin-induced MAPK activation, defense gene expression, callose deposition and conferred non-host resistance to grapevine downy mildew (Erysiphe necator). Protein-protein interaction studies conducted in planta revealed a chitin oligomer-triggered interaction between VvLYK5-1 and VvLYK1-1. Interestingly, our results also indicate that VvLYK5-1 mediates the perception of chitin but not chitosan oligomers showing a part of its specificity.

The plant trans-Golgi network component ECHIDNA regulates defense, cell death, and endoplasmic reticulum stress

The trans-Golgi network (TGN) acts as a central platform for sorting and secreting various cargoes to the cell surface, thus being essential for the full execution of plant immunity. However, the fine-tuned regulation of TGN components in plant defense and stress response has been not fully elucidated. Our study revealed that despite largely compromising penetration resistance, the loss-of-function mutation of the TGN component protein ECHIDNA (ECH) induced enhanced postinvasion resistance to powdery mildew in Arabidopsis thaliana. Genetic and transcriptome analyses and hormone profiling demonstrated that ECH loss resulted in salicylic acid (SA) hyperaccumulation via the ISOCHORISMATE SYNTHASE 1 biosynthesis pathway, thereby constitutively activating SA-dependent innate immunity that was largely responsible for the enhanced postinvasion resistance. Furthermore, the ech mutant displayed accelerated SA-independent spontaneous cell death and constitutive POWDERY MILDEW RESISTANCE 4-mediated callose depositions. In addition, ECH loss led to a chronically prolonged endoplasmic reticulum stress in the ech mutant. These results provide insights into understanding the role of TGN components in the regulation of plant immunity and stress responses.

A fijiviral nonstructural protein triggers cell death in plant and bacterial cells via its transmembrane domain

Southern rice black-streaked dwarf virus (SRBSDV; Fijivirus, Reoviridae) has become a threat to cereal production in East Asia in recent years. Our previous cytopathologic studies have suggested that SRBSDV induces a process resembling programmed cell death in infected tissues that results in distinctive growth abnormalities. The viral product responsible for the cell death, however, remains unknown. Here P9-2 protein, but not its RNA, was shown to induce cell death in Escherichia coli and plant cells when expressed either locally with a transient expression vector or systemically using a heterologous virus. Both computer prediction and fluorescent assays indicated that the viral nonstructural protein was targeted to the plasma membrane (PM) and further modification of its subcellular localization abolished its ability to induce cell death, indicating that its PM localization was required for the cell death induction. P9-2 was predicted to harbour two transmembrane helices within its central hydrophobic domain. A series of mutation assays further showed that its central transmembrane hydrophobic domain was crucial for cell death induction and that its conserved F90, Y101, and L103 amino acid residues could play synergistic roles in maintaining its ability to induce cell death. Its homologues in other fijiviruses also induced cell death in plant and bacterial cells, implying that the fijiviral nonstructural protein may trigger cell death by targeting conserved cellular factors or via a highly conserved mechanism.

Barley endosomal MONENSIN SENSITIVITY1 is a target of the powdery mildew effector CSEP0162 and plays a role in plant immunity

Encasements formed around haustoria and biotrophic hyphae as well as hypersensitive reaction (HR) cell death are essential plant immune responses to filamentous pathogens. In this study we examine the components that may contribute to the absence of these responses in susceptible barley attacked by the powdery mildew fungus. We find that the effector CSEP0162 from this pathogen targets plant MONENSIN SENSITIVITY1 (MON1), which is important for the fusion of multivesicular bodies to their target membranes. Overexpression of CSEP0162 and silencing of barley MON1 both inhibit encasement formation. We find that the Arabidopsis ecotype No-0 has resistance to powdery mildew, and that this is partially dependent on MON1. Surprisingly, we find the MON1-dependent resistance in No-0 not only includes an encasement response, but also an effective HR. Similarly, silencing of MON1 in barley also blocks Mla3-mediated HR-based powdery mildew resistance. Our results indicate that MON1 is a vital plant immunity component, and we speculate that the barley powdery mildew fungus introduces the effector CSEP0162 to target MON1 and hence reduce encasement formation and HR.

A recessive gene Cmpmr2F confers powdery mildew resistance in melon (Cucumis melo L.)

Identified a recessive gene (Cmpmr2F) associated with resistance to infection by the powdery mildew causing agent Podosphaera xanthii race 2F. Powdery mildew (PM) is one of the most destructive fungal diseases of melon, which significantly reduces the crop yield and quality. Multiple studies are being performed for in-depth genetic understandings of PM-susceptibility or -resistance mechanisms in melon plants, but the holistic knowledge of the precise genetic basis of PM-resistance is unexplored. In this study, we characterized the recessive gene "Cmpmr2F" and found its association with resistance against the PM causative agent "Podosphaera xanthii race 2F." Fine genetic mapping revealed the major-effect region of a 26.25-kb interval on chromosome 12, which harbored the Cmpmr2F gene corresponding to the MELO3C002403, encoding allantoate amidohydrolase. The functional gene annotation, expression pattern, and sequence alignment analyses were carried out using two contrast parent lines of melon "X055" PM-susceptible and "PI 124112" PM-resistant. Further, gene silencing of Cmpmr2F using virus-induced gene silencing (VIGS) significantly increased PM-resistance in the susceptible plant. In contrast to the previously reported studies, we identified that Cmpmr2F-silenced plants showed no impairment in growth due to less apparent negative effects in silenced melon plants. So, it is believed that the Cmpmr2F gene has great potential for further breeding studies to increase the P. xanthii race 2F resistance in melon. In short, our study provides new genetic resources and a solid foundation for further functional analysis of PM-resistance genes in melon, as well as powerful molecular markers for marker-assisted breeding aimed at developing new melon varieties resistant to PM infection.

Glutathione imparts stress tolerance against Alternaria brassicicola infection via miRNA mediated gene regulation

Glutathione (GSH) is well known to play a crucial role in imparting resistance against various pathogen invasions. Nevertheless, the role of GSH in regulating miRNA-mediated defense response is yet to be explored. To decipher the GSH-mediated regulation of miRNA expression during necrotrophic infection in Arabidopsis thaliana, wild-type Col-0 and AtECS1, the transgenic line exhibiting enhanced GSH content, were infected with necrotrophic pathogen Alternaria brassicicola. AtECS1 plants exhibited enhanced resistance as compared to wild-type. MiRNA next-generation sequencing (NGS) was performed to compare the miRNA expression in Col-0 and AtECS1 leaves. Under control condition, differentially expressed 96 known miRNAs and 17 novel miRNAs viz. ath-miR8167f, ath-miR1886.3, ath-miR3932b-5p, etc. were identified. However, under infected condition, 73 known and 43 novel differentially expressed miRNAs viz. ath-miR5652, ath-miR160b, ath-miR865-5p, etc. were identified. Functional annotation and enrichment analysis revealed that several miRNAs that target defense-related genes like leucine-rich repeat protein kinase, MYB transcription factors, TCP8, etc. were down regulated in the AtECS1 line, which, in turn, relieves the repression of their target gene expression, leading to resistance against infection. Together, the present investigation suggests that GSH plays a decisive role in modulating the miRNA-mediated regulation of defense-related genes during pathogen invasion.

Antagonistic and plant growth promotion of rhizobacteria against Phytophthora colocasiae in taro

Taro leaf blight caused by Phytophthora colocasiae adversely affects the growth and yield of taro. The management of this disease depends heavily on synthetic fungicides. These compounds, however, pose potential hazards to human health and the environment. The present study aimed to investigate an alternative approach for plant growth promotion and disease control by evaluating seven different bacterial strains (viz., Serratia plymuthica, S412; S. plymuthica, S414; S. plymuthica, AS13; S. proteamaculans, S4; S. rubidaea, EV23; S. rubidaea, AV10; Pseudomonas fluorescens, SLU-99) and their different combinations as consortia against P. colocasiae. Antagonistic tests were performed in in vitro plate assays and the effective strains were selected for detached leaf assays and greenhouse trials. Plant growth-promoting and disease prevention traits of selected bacterial strains were also investigated in vitro. Our results indicated that some of these strains used singly (AV10, AS13, S4, and S414) and in combinations (S4+S414, AS13+AV10) reduced the growth of P. colocasiae (30-50%) in vitro and showed disease reduction ability when used singly or in combinations as consortia in greenhouse trials (88.75-99.37%). The disease-suppressing ability of these strains may be related to the production of enzymes such as chitinase, protease, cellulase, and amylase. Furthermore, all strains tested possessed plant growth-promoting traits such as indole-3-acetic acid production, siderophore formation, and phosphate solubilization. Overall, the present study revealed that bacterial strains significantly suppressed P. colocasiae disease development using in vitro, detached leaf, and greenhouse assays. Therefore, these bacterial strains can be used as an alternative strategy to minimize the use of synthetic fungicides and fertilizers to control taro blight and improve sustainable taro production.

Plastid methylerythritol phosphate pathway participates in the hypersensitive response-related cell death in Nicotiana benthamiana

Programmed cell death (PCD), a characteristic feature of hypersensitive response (HR) in plants, is an important cellular process often associated with the defense response against pathogens. Here, the involvement of LytB, a gene encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase that participates in the final step of the plastid methylerythritol phosphate (MEP) pathway, in plant HR cell death was studied. In Nicotiana benthmiana plants, silencing of the NbLytB gene using virus-induced gene silencing (VIGS) caused plant growth retardation and albino leaves with severely malformed chloroplasts. In NbLytB-silenced plants, HR-related cell death mediated by the expression of either the human proapoptotic protein gene Bax or an R gene with its cognate Avr effector gene was inhibited, whereas that induced by the nonhost pathogen Pseudomonas syringae pv. syringae 61 was enhanced. To dissect the isoprenoid pathway and avoid the pleiotropic effects of VIGS, chemical inhibitors that specifically inhibit isoprenoid biosynthesis in plants were employed. Treatment of N. benthamiana plants with fosmidomycin, a specific inhibitor of the plastid MEP pathway, effectively inhibited HR-related PCD, whereas treatment with mevinolin (a cytoplasmic mevalonate pathway inhibitor) and fluridone (a carotenoid biosynthesis inhibitor) did not. Together, these results suggest that the MEP pathway as well as reactive oxygen species (ROS) generation in the chloroplast play an important role in HR-related PCD, which is not displaced by the cytosolic isoprenoid biosynthesis pathway.

Stb6 mediates stomatal immunity, photosynthetic functionality, and the antioxidant system during the Zymoseptoria tritici-wheat interaction

This study offers new perspectives on the biochemical and physiological changes that occur in wheat following a gene-for-gene interaction with the fungal pathogen Zymoseptoria tritici. The Z. tritici isolate IPO323, carries AvrStb6, while ΔAvrStb6#33, lacks AvrStb6. The wheat cultivar (cv.) Shafir, bears the corresponding resistance gene Stb6. Inoculation of cv. Shafir with these isolates results in two contrasted phenotypes, offering a unique opportunity to study the immune response caused by the recognition of AvrStb6 by Stb6. We employed a variety of methodologies to dissect the physiological and biochemical events altered in cv. Shafir, as a result of the AvrStb6-Stb6 interaction. Comparative analysis of stomatal conductance demonstrated that AvrStb6-Stb6 mediates transient stomatal closures to restrict the penetration of Zymoseptoria tritici. Tracking photosynthetic functionality through chlorophyll fluorescence imaging analysis demonstrated that AvrStb6-Stb6 retains the functionality of photosynthesis apparatus by promoting Non-Photochemical Quenching (NPQ). Furthermore, the PlantCV image analysis tool was used to compare the H₂O₂ accumulation and incidence of cell death (2, 4, 8, 12, 16, and 21 dpi), over Z. tritici infection. Finally, our research shows that the AvrStb6-Stb6 interaction coordinates the expression and activity of antioxidant enzymes, both enzymatic and non-enzymatic, to counteract oxidative stress. In conclusion, the Stb6-AvrStb6 interaction in the Z. tritici-wheat pathosystem triggers transient stomatal closure and maintains photosynthesis while regulating oxidative stress.

ANGUSTIFOLIA negatively regulates resistance to Sclerotinia sclerotiorum via modulation of PTI and JA signalling pathways in Arabidopsis thaliana

Sclerotinia sclerotiorum is a devastating pathogen that infects a broad range of host plants. The mechanism underlying plant defence against fungal invasion is still not well characterized. Here, we report that ANGUSTIFOLIA (AN), a CtBP family member, plays a role in the defence against S. sclerotiorum attack. Arabidopsis an mutants exhibited stronger resistance to S. sclerotiorum at the early stage of infection than wild-type plants. Accordingly, an mutants exhibited stronger activation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) responses, including mitogen-activated protein kinase activation, reactive oxygen species accumulation, callose deposition, and the expression of PTI-responsive genes, upon treatment with PAMPs/microbe-associated molecular patterns. Moreover, Arabidopsis lines overexpressing AN were more susceptible to S. sclerotiorum and showed defective PTI responses. Our luminometry, bimolecular fluorescence complementation, coimmunoprecipitation, and in vitro pull-down assays indicate that AN interacts with allene oxide cyclases (AOC), essential enzymes involved in jasmonic acid (JA) biosynthesis, negatively regulating JA biosynthesis in response to S. sclerotiorum infection. This work reveals AN is a negative regulator of the AOC-mediated JA signalling pathway and PTI activation.

Indole-3-Carboxylic Acid From the Endophytic Fungus Lasiodiplodia pseudotheobromae LPS-1 as a Synergist Enhancing the Antagonism of Jasmonic Acid Against Blumeria graminis on Wheat

The discovery of natural bioactive compounds from endophytes or medicinal plants against plant diseases is an attractive option for reducing the use of chemical fungicides. In this study, three compounds, indole-3-carbaldehyde, indole-3-carboxylic acid (3-ICA), and jasmonic acid (JA), were isolated from the EtOAc extract of the culture filtrate of the endophytic fungus Lasiodiplodia pseudotheobromae LPS-1, which was previously isolated from the medicinal plant, Ilex cornuta. Some experiments were conducted to further determine the antifungal activity of these compounds on wheat powdery mildew. The results showed that JA was much more bioactive than indole-3-carbaldehyde and 3-ICA against Blumeria graminis, and the disease severity caused by B. graminis decreased significantly with the concentration increase of JA treatment. The assay of the interaction of 3-ICA and JA indicated that there was a significant synergistic effect between the two compounds on B. graminis in each of the ratios of 3-ICA to JA (3-ICA:JA) ranging from 1:9 to 9:1. When the compound ratio of 3-ICA to JA was 2:8, the synergistic coefficient was the highest as 22.95. Meanwhile, a histological investigation indicated that, under the treatment of JA at 500 μg/ml or 3-ICA:JA (2:8) at 40 μg/ml, the appressorium development and haustorium formation of B. graminis were significantly inhibited. Taken together, we concluded that JA plays an important role in the infection process of B. graminis and that 3-ICA as a synergist of JA enhances the antagonism against wheat powdery mildew.

TuRLK1, a leucine-rich repeat receptor-like kinase, is indispensable for stripe rust resistance of YrU1 and confers broad resistance to multiple pathogens

BACKGROUND

YrU1 is a nucleotide-binding site (NBS) and leucine-rich repeat (LRR) protein (NLR), with additional ankyrin-repeat and WRKY domains and confers effective resistance to stripe rust fungus Puccinia striiformis f. sp. Tritici (Pst). YrU1 was positionally cloned in the progenitor species of the A genome of bread wheat, Tricicum urartu, recently. However, the molecular mechanism and components involved in YrU1-mediated resistance are not clear.

RESULTS

In this study, we found that the transcript level of TuRLK1, which encodes a novel leucine-rich repeat receptor-like kinase, was up-regulated after inoculation with Pst in the presence of YrU1, through RNA-seq analysis in T. urartu accession PI428309. TuRLK1 contained only a small number of LRR motifs, and was localized in the plasma-membrane. Transient expression of TuRLK1 induced hypersensitive cell death response in N. benthamiana leaves. Silencing of TuRLK1, using barley stripe mosaic virus (BSMV)-induced gene silencing (VIGS) system in PI428309 that contains YrU1, compromised the resistance against stripe rust caused by Pst CY33, indicating that TuRLK1 was required for YrU1-activated plant immunity. Furthermore, overexpression of TuRLK1 could enhance powdery mildew resistance in bread wheat and Arabidopsis thaliana after inoculating with the corresponding pathogens.

CONCLUSIONS

Our study indicates that TuRLK1 is required for immune response mediated by the unique NLR protein YrU1, and likely plays an important role in disease resistance to other pathogens.

Pathogen resistance in Sphagneticola trilobata (Singapore daisy): molecular associations and differentially expressed genes in response to disease from a widespread fungus

Understanding the molecular associations underlying pathogen resistance in invasive plant species is likely to provide useful insights into the effective control of alien plants, thereby facilitating the conservation of native biodiversity. In the current study, we investigated pathogen resistance in an invasive clonal plant, Sphagneticola trilobata, at the molecular level. Sphagneticola trilobata (i.e., Singapore daisy) is a noxious weed that affects both terrestrial and aquatic ecosystems, and is less affected by pathogens in the wild than co-occurring native species. We used Illumina sequencing to investigate the transcriptome of S. trilobata following infection by a globally distributed generalist pathogen (Rhizoctonia solani). RNA was extracted from leaves of inoculated and un-inoculated control plants, and a draft transcriptome of S. trilobata was generated to examine the molecular response of this species following infection. We obtained a total of 49,961,014 (94.3%) clean reads for control (un-inoculated plants) and 54,182,844 (94.5%) for the infected treatment (inoculated with R. solani). Our analyses facilitated the discovery of 117,768 de novo assembled contigs and 78,916 unigenes. Of these, we identified 3506 differentially expressed genes and 60 hormones associated with pathogen resistance. Numerous genes, including candidate genes, were associated with plant-pathogen interactions and stress response in S. trilobata. Many recognitions, signaling, and defense genes were differentially regulated between treatments, which were confirmed by qRT-PCR. Overall, our findings improve our understanding of the genes and molecular associations involved in plant defense of a rapidly spreading invasive clonal weed, and serve as a valuable resource for further work on mechanism of disease resistance and managing invasive plants.

The elemental defense effect of cadmium on Alternaria brassicicola in Brassica juncea

BACKGROUND

The elemental defense hypothesis states a new defensive strategy that hyperaccumulators defense against herbivores or pathogens attacks by accumulating heavy metals. Brassica juncea has an excellent ability of cadmium (Cd) accumulation. However, the elemental defense effect and its regulation mechanism in B. juncea remain unclear.

RESULTS

In this study, we profiled the elemental defense effect and the molecular regulatory mechanism in Cd-accumulated B. juncea after Alternaria brassicicola infection. B. juncea treated with 180 mg Kg^- 1 DW CdCl₂ 2.5H₂O exhibited obvious elemental defense effect after 72 h of infection with A. brassicicola. The expression of some defense-related genes including BjNPR1, BjPR12, BjPR2, and stress-related miRNAs (miR156, miR397, miR398a, miR398b/c, miR408, miR395a, miR395b, miR396a, and miR396b) were remarkably elevated during elemental defense in B. juncea.

CONCLUSIONS

The results indicate that Cd-accumulated B. juncea may defend against pathogens by coordinating salicylic acid (SA) and jasmonic acid (JA) mediated systemic acquired resistance (SAR) and elemental defense in a synergistic joint effect. Furthermore, the expression of miRNAs related to heavy metal stress response and disease resistance may regulate the balance between pathogen defense and heavy metal stress-responsive in B. juncea. The findings provide experimental evidence for the elemental defense hypothesis in plants from the perspectives of phytohormones, defense-related genes, and miRNAs.

Cultivar-biased regulation of HSFA7 and HSFB4a govern high-temperature tolerance in tomato

Cultivar-biased regulation of HSFB4a and HSFA7 mediates heat stress tolerance/sensitivity in tomato. Reduced HSFB4a repressor levels and enhanced HSFA7 activator levels govern thermo-tolerance in tolerant cultivars. Heat shock factors (HSFs) are at the core of heat stress (HS) response in plants. However, the contribution of HSFs governing the inherent thermo-tolerance mechanism in tomato from sub-tropical hot climates is poorly understood. With the above aim, comparative expression profiles of the HSF family in a HS-tolerant (CLN1621L) and -sensitive cultivars (CA4 and Pusa Ruby) of tomato under HS revealed cultivar-biased regulation of an activator (HSFA7) and a repressor (HSFB4a) class HSF. HSFA7 exhibited strong upregulation while HSFB4a showed downregulation in tolerant tomato cultivar upon HS. Functional characterization of HSFA7 and HSFB4a in a tolerant-sensitive cultivar pair by virus-induced gene silencing (VIGS)-based silencing and transient overexpression established them as a positive and a negative regulator of HS tolerance, respectively. Promoter:GUS reporter assays and promoter sequence analyses suggest heat-mediated transcriptional control of both the HSF genes in the contrasting cultivars. Moreover, degradome data highlighted HSFB4a is a probable target of microRNA Sly-miR4200. Transient in-planta Sly-MIR4200-effector:HSFB4a-reporter assays showed miRNA-dependent target down-regulation. Chelation of miRNA by short-tandem-target-mimic of Sly-miR4200 increased target abundance, highlighting a link between Sly-miR4200 and HSFB4a. This miRNA has induced several folds upon HS in the tolerant cultivar where HSFB4a levels are reduced, thus exhibiting the inverse miR:target expression. Thus, we speculate that the alleviation of HSFB4a and increased HSFA7 levels govern thermo-tolerance in the tolerant cultivar by regulating downstream heat stress-responsive genes.

Enhanced Flavonoid Accumulation Reduces Combined Salt and Heat Stress Through Regulation of Transcriptional and Hormonal Mechanisms

Abiotic stresses, such as salt and heat stress, coexist in some regions of the world and can have a significant impact on agricultural plant biomass and production. Rice is a valuable crop that is susceptible to salt and high temperatures. Here, we studied the role of flavanol 3-hydroxylase in response to combined salt and heat stress with the aim of better understanding the defensive mechanism of rice. We found that, compared with wild-type plants, the growth and development of transgenic plants were improved due to higher biosynthesis of kaempferol and quercetin. Furthermore, we observed that oxidative stress was decreased in transgenic plants compared with that in wild-type plants due to the reactive oxygen species scavenging activity of kaempferol and quercetin as well as the modulation of glutathione peroxidase and lipid peroxidase activity. The expression of high-affinity potassium transporter (HKT) and salt overly sensitive (SOS) genes was significantly increased in transgenic plants compared with in control plants after 12 and 24 h, whereas sodium-hydrogen exchanger (NHX) gene expression was significantly reduced in transgenic plants compared with in control plants. The expression of heat stress transcription factors (HSFs) and heat shock proteins (HSPs) in the transgenic line increased significantly after 6 and 12 h, although our understanding of the mechanisms by which the F3H gene regulates HKT, SOS, NHX, HSF, and HSP genes is limited. In addition, transgenic plants showed higher levels of abscisic acid (ABA) and lower levels of salicylic acid (SA) than were found in control plants. However, antagonistic cross talk was identified between these hormones when the duration of stress increased; SA accumulation increased, whereas ABA levels decreased. Although transgenic lines showed significantly increased Na+ ion accumulation, K+ ion accumulation was similar in transgenic and control plants, suggesting that increased flavonoid accumulation is crucial for balancing Na+/K+ ions. Overall, this study suggests that flavonoid accumulation increases the tolerance of rice plants to combined salt and heat stress by regulating physiological, biochemical, and molecular mechanisms.

Spray-Induced Gene Silencing as a Potential Tool to Control Potato Late Blight Disease

Phytophthora infestans causes late blight disease on potato and tomato and is currently controlled by resistant cultivars or intensive fungicide spraying. Here, we investigated an alternative means for late blight control by spraying potato leaves with double-stranded RNAs (dsRNA) that target the P. infestans genes essential for infection. First, we showed that the sporangia of P. infestans expressing green fluorescent protein (GFP) can take up in vitro synthesized dsRNAs homologous to GFP directly from their surroundings, including leaves, which led to the reduced relative expression of GFP. We further demonstrate the potential of spray-induced gene silencing (SIGS) in controlling potato late blight disease by targeting developmentally important genes in P. infestans such as guanine-nucleotide binding protein β-subunit (PiGPB1), haustorial membrane protein (PiHmp1), cutinase (PiCut3), and endo-1,3(4)-β-glucanase (PiEndo3). Our results demonstrate that SIGS can potentially be used to mitigate potato late blight; however, the degree of disease control is dependent on the selection of the target genes.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Over-Expression of Chorismate Mutase Enhances the Accumulation of Salicylic Acid, Lignin, and Antioxidants in Response to the White-Backed Planthopper in Rice Plants

The white-backed planthopper (WBPH) is a serious pest of rice crop and causes sever yield loss each year, especially in Asian countries. In this study, we used chorismate mutase (CM) transgenic line to examine the defense mechanism of rice plants against WBPH. The survival rate of WBPHs, infestation rate of plants, lignin biosynthesis, transcriptional regulation of related genes, salicylic acid (SA) accumulation and signaling and antioxidants regulation were investigated. The WBPH population decreased by 67% in OxCM-t, and the plant infestation rate was 3.5-fold higher in wild-type plants compared with transgenic plants. A substantial increase in lignin was found in the transgenic line (742%) and wild-type (417%) plants. Additionally, CM, phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), and chalcone isomerase (CHI) showed significant increases in their relative expression level in the transgenic line. Salicylic acid was significantly enhanced in the transgenic line compared with WBPH infestation. SA can activate pathogenesis related proteins-1 (PR1), PR2, antioxidants, and the expression of their related genes: superoxide dismutase (SOD) and catalase (CAT). WBPH infestation reduced the chlorophyll contents of both transgenic and wild-type plants, but the reduction was great in wild-type than transgenic plants. The sugar content was only significantly increased in the transgenic line, indicating that sugars are not heavily involved in WBPH stress. Phenylalanine, proline, aspartic acid, and total amino acids were increased in the transgenic line and reduced in the wild-type plants. Taken together, all the results suggest that overexpression of CM gene regulates the defense mechanisms and enhances the rice toward WBPH stress.

Grapevine Rpv3-, Rpv10- and Rpv12-mediated defense responses against Plasmopara viticola and the impact of their deployment on fungicide use in viticulture

BACKGROUND

The high susceptibility of European grapevine cultivars (Vitis vinifera) to downy mildew (Plasmopara viticola) leads to the intensive use of fungicides in viticulture. To reduce this input, breeding programs have introgressed resistance loci from wild Vitis species into V. vinifera, resulting in new fungus-resistant grapevine cultivars (FRC). However, little is known about how these different resistance loci confer resistance and what the potential reduction in fungicide applications are likely to be if these FRCs are deployed. To ensure a durable and sustainable resistance management and breeding, detailed knowledge about the different defense mechanisms mediated by the respective Rpv (Resistance to P. viticola) resistance loci is essential.

RESULTS

A comparison of the resistance mechanisms mediated by the Rpv3-1, Rpv10 and/or Rpv12-loci revealed an early onset of programmed cell death (PCD) at 8 hours post infection (hpi) in Rpv12-cultivars and 12 hpi in Rpv10-cultivars, whereas cell death was delayed in Rpv3-cultivars and was not observed until 28 hpi. These temporal differences correlated with an increase in the trans-resveratrol level and the formation of hydrogen peroxide shortly before onset of PCD. The differences in timing of onset of Rpv-loci specific defense reactions following downy mildew infection could be responsible for the observed differences in hyphal growth, sporulation and cultivar-specific susceptibility to this pathogen in the vineyard. Hereby, Rpv3- and Rpv12/Rpv3-cultivars showed a potential for a significant reduction of fungicide applications, depending on the annual P. viticola infection pressure and the Rpv-loci. Furthermore, we report on the discovery of a new P. viticola isolate that is able to overcome both Rpv3- and Rpv12-mediated resistance.

CONCLUSION

This study reveals that differences in the timing of the defense reaction mediated by the Rpv3-, Rpv10- and Rpv12-loci, result in different degrees of natural resistance to downy mildew in field. Vineyard trials demonstrate that Rpv12/Rpv3- and Rpv3-cultivars are a powerful tool to reduce the dependence of grape production on fungicide applications. Furthermore, this study indicates the importance of sustainable breeding and plant protection strategies based on resistant grapevine cultivars to reduce the risk of new P. viticola isolates that are able to overcome the respective resistance mechanism.

An ABHD17-like hydrolase screening system to identify de-S-acylation enzymes of protein substrates in plant cells

Protein S-acylation is an important post-translational modification in eukaryotes, regulating the subcellular localization, trafficking, stability, and activity of substrate proteins. The dynamic regulation of this reversible modification is mediated inversely by protein S-acyltransferases and de-S-acylation enzymes, but the de-S-acylation mechanism remains unclear in plant cells. Here, we characterized a group of putative protein de-S-acylation enzymes in Arabidopsis thaliana, including 11 members of Alpha/Beta Hydrolase Domain-containing Protein 17-like acyl protein thioesterases (ABAPTs). A robust system was then established for the screening of de-S-acylation enzymes of protein substrates in plant cells, based on the effects of substrate localization and confirmed via the protein S-acylation levels. Using this system, the ABAPTs, which specifically reduced the S-acylation levels and disrupted the plasma membrane localization of five immunity-related proteins, were identified respectively in Arabidopsis. Further results indicated that the de-S-acylation of RPM1-Interacting Protein 4, which was mediated by ABAPT8, resulted in an increase of cell death in Arabidopsis and Nicotiana benthamiana, supporting the physiological role of the ABAPTs in plants. Collectively, our current work provides a powerful and reliable system to identify the pairs of plant protein substrates and de-S-acylation enzymes for further studies on the dynamic regulation of plant protein S-acylation.

Jasmonic Acid and Ethylene Participate in the Gibberellin-Induced Ovule Programmed Cell Death Process in Seedless Pear '1913' (Pyrus hybrid)

Seedless fruit is a feature appreciated by consumers. The ovule abortion process is highly orchestrated and controlled by numerous environmental and endogenous signals. However, the mechanisms underlying ovule abortion in pear remain obscure. Here, we found that gibberellins (GAs) have diverse functions during ovules development between seedless pear '1913' and seeded pear, and that GA₄₊₇ activates a potential programmed cell death process in '1913' ovules. After hormone analyses, strong correlations were determined among jasmonic acid (JA), ethylene and salicylic acid (SA) in seedless and seeded cultivars, and GA₄₊₇ treatments altered the hormone accumulation levels in ovules, resulting in significant correlations between GA and both JA and ethylene. Additionally, SA contributed to ovule abortion in '1913'. Exogenously supplying JA, SA or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid promoted 'Bartlett' seed death. The regulatory mechanism in which ethylene controls ovule death has been demonstrated; therefore, JA's role in regulating '1913' ovule abortion was investigated. A further study identified that the JA signaling receptor MYC2 bound the SENESCENCE-ASSOCIATED 39 promoter and triggered its expression to regulate ovule abortion. Thus, we established ovule abortion-related relationships between GA and the hormones JA, ethylene and SA, and we determined their synergistic functions in regulating ovule death.

Jasmonic Acid-Dependent MYC Transcription Factors Bind to a Tandem G-Box Motif in the YUCCA8 and YUCCA9 Promoters to Regulate Biotic Stress Responses

The indole-3-pyruvic acid pathway is the main route for auxin biosynthesis in higher plants. Tryptophan aminotransferases (TAA1/TAR) and members of the YUCCA family of flavin-containing monooxygenases catalyze the conversion of l-tryptophan via indole-3-pyruvic acid to indole-3-acetic acid (IAA). It has been described that jasmonic acid (JA) locally produced in response to mechanical wounding triggers the de novo formation of IAA through the induction of two YUCCA genes, YUC8 and YUC9. Here, we report the direct involvement of a small number of basic helix-loop-helix transcription factors of the MYC family in this process. We show that the JA-mediated regulation of the expression of the YUC8 and YUC9 genes depends on the abundance of MYC2, MYC3, and MYC4. In support of this observation, seedlings of myc knockout mutants displayed a strongly reduced response to JA-mediated IAA formation. Furthermore, transactivation assays provided experimental evidence for the binding of MYC transcription factors to a particular tandem G-box motif abundant in the promoter regions of YUC8 and YUC9, but not in the promoters of the other YUCCA isogenes. Moreover, we demonstrate that plants that constitutively overexpress YUC8 and YUC9 show less damage after spider mite infestation, thereby underlining the role of auxin in plant responses to biotic stress signals.

Characterization of PmHHXM, a New Broad-Spectrum Powdery Mildew Resistance Gene in Chinese Wheat Landrace Honghuaxiaomai

Powdery mildew, caused by fungal pathogen Blumeria graminis f. sp. tritici, is an agronomically important and widespread wheat disease causing severe yield losses. Deployment of broad-spectrum disease resistance genes is the preferred strategy to prevent this pathogen. Chinese wheat landrace Honghuaxiaomai (HHXM) was resistant to all 23 tested B. graminis f. sp. tritici isolates at the seedling stage. The F₁, F₂, and F_2:3 progenies derived from the cross HHXM × Yangmai 158 were used in this study, and genetic analysis revealed that a single dominant gene, designated PmHHXM, conferred resistance to B. graminis f. sp. tritici isolate E09. Bulked segregant analysis and molecular mapping initially located PmHHXM to the distal region of chromosome 4AL. To fine map PmHHXM, we identified two critical recombinants from 592 F₂ plants and delimited PmHHXM to a 0.18-cM Xkasp475200 to Xhnu552 interval covering 1.77 Mb, in which a number of disease resistance-related gene clusters were annotated. Comparative mapping of this interval revealed a perturbed synteny among Triticeae species. This study reports the new powdery mildew resistance gene PmHHXM, which seems different from three known quantitative trait loci/genes identified on chromosome 4AL and has significant values for further genetic improvement. Analysis of the polymorphisms of 13 cosegregating markers between HHXM and 170 modern wheat cultivars indicates that Xhnu227 and Xsts478700 developed here are ideal for marker-assisted introgression of this locus in wheat breeding.

Salicylic acid and nitric oxide cross-talks to improve innate immunity and plant vigor in tomato against Fusarium oxysporum stress

Foliar application of SA cross-talks and induce endogenous nitric oxide and reactive oxygen species to improve innate immunity and vigor of tomato plant against Fusarium oxysporum stress. The present investigation was aimed to demonstrate the efficacy of salicylic acid (SA), as a powerful elicitor or plant growth regulator (PGR) and its cross-talk with nitric oxide (NO) in tomato against the biotic stress caused by wilt pathogen, Fusarium oxysporum f. sp. lycopersici. Different defense-related enzymes and gene expression, phenol, flavonoid, and phenolic acid content along with NO generation and other physiological characters have been estimated after foliar application of SA. Total chlorophyll content was steadily maintained and the amount of death of cells was negligible after 72 h of SA treatment. Significant reduction of disease incidence was also recorded in SA treated sets. Simultaneously, NO generation was drastically improved at this stage, which has been justified by both spectrophotometrically and microscopically. A direct correlation between reactive oxygen species (ROS) generation and NO has been established. Production of defense enzymes, gene expressions, different phenolic acids was positively influenced by SA treatment. However, tomato plants treated with SA along with NO synthase (NOS) inhibitor or NO scavenger significantly reduce all those parameters tested. On the other hand, NO donor-treated plants showed the same inductive effect like SA. Furthermore, SA treated seeds of tomato also showed improved physiological parameters like higher seedling vigor index, shoot and root length, mean trichome density, etc. It is speculated that the cross-talk between SA and endogenous NO have tremendous ability to improve defense responses and growth of the tomato plant. It can be utilized in future sustainable agriculture for bimodal action.

H3K27me3 demethylases alter HSP22 and HSP17.6C expression in response to recurring heat in Arabidopsis

Acclimation to high temperature increases plants' tolerance of subsequent lethal high temperatures. Although epigenetic regulation of plant gene expression is well studied, how plants maintain a memory of environmental changes over time remains unclear. Here, we show that JUMONJI (JMJ) proteins, demethylases involved in histone H3 lysine 27 trimethylation (H3K27me3), are necessary for Arabidopsis thaliana heat acclimation. Acclimation induces sustained H3K27me3 demethylation at HEAT SHOCK PROTEIN22 (HSP22) and HSP17.6C loci by JMJs, poising the HSP genes for subsequent activation. Upon sensing heat after a 3-day interval, JMJs directly reactivate these HSP genes. Finally, jmj mutants fail to maintain heat memory under fluctuating field temperature conditions. Our findings of an epigenetic memory mechanism involving histone demethylases may have implications for environmental adaptation of field plants.

Opposing roles of plant laticifer cells in the resistance to insect herbivores and fungal pathogens

More than 12,000 plant species (ca. 10% of flowering plants) exude latex when their tissues are injured. Latex is produced and stored in specialized cells named "laticifers". Laticifers form a tubing system composed of rows of elongated cells that branch and create an internal network encompassing the entire plant. Laticifers constitute a recent evolutionary achievement in ecophysiological adaptation to specific natural environments; however, their fitness benefit to the plant still remains to be proven. The identification of Euphorbia lathyris mutants (pil mutants) deficient in laticifer cells or latex metabolism, and therefore compromised in latex production, allowed us to test the importance of laticifers in pest resistance. We provided genetic evidence indicating that laticifers represent a cellular adaptation for an essential defense strategy to fend off arthropod herbivores with different feeding habits, such as Spodoptera exigua and Tetranychus urticae. In marked contrast, we also discovered that a lack of laticifer cells causes complete resistance to the fungal pathogen Botrytis cinerea. Thereafter, a latex-derived factor required for conidia germination on the leaf surface was identified. This factor promoted disease susceptibility enhancement even in the non-latex-bearing plant Arabidopsis. We speculate on the role of laticifers in the co-evolutionary arms race between plants and their enemies.

Histological Methods to Detect Early-stage Plant Defense Responses during Artificial Inoculation of Lolium perenne with Epichloë festucae

Epichloë species form agriculturally important symbioses with many cool season grasses. To study these symbioses, such as the interaction of Epichloë festucae with perennial ryegrass (Lolium perenne), host plants can be infected by artificial inoculation of etiolated seedlings. This inoculation is performed by placing mycelium into an incision in the meristem, as previously described by Latch and Christensen (1985). In recent years, this method has been broadly used to study this interaction at the molecular level using different Epichloë festucae mutants that can cause incompatible interactions. We have developed and adapted methods to study four of the most important host plant responses to infection, including cell death, callose deposition, lignin production, and hydrogen peroxide (H2O2) production, which are useful in defining the host response to infection at a very early time point.

Tenuazonic Acid-Triggered Cell Death Is the Essential Prerequisite for Alternaria alternata (Fr.) Keissler to Infect Successfully Host Ageratina adenophora

The necrotrophic fungus Alternaria alternata contains different pathotypes that produce different mycotoxins. The pathotype Ageratina adenophora secretes the non-host-selective toxin tenuazonic acid (TeA), which can cause necrosis in many plants. Although TeA is thought to be a central virulence factor of the A. adenophora pathotype, the precise role of TeA in different stages of host infection by pathogens remains unclear. Here, an A. alternata wild-type and the toxin-deficient mutant ΔHP001 with a 75% reduction in TeA production were used. It was observed that wild-type pathogens could induce the reactive oxygen species (ROS) bursts in host leaves and killed photosynthetic cells before invading hyphae. The ROS interceptor catalase remarkably inhibited hyphal penetration and invasive hyphal growth and expansion in infected leaves and suppressed necrotic leaf lesion. This suggests that the production of ROS is critical for pathogen invasion and proliferation and disease symptom formation during infection. It was found that the mutant pathogens did not cause the formation of ROS and cell death in host leaves, showing an almost complete loss of disease susceptibility. In addition, the lack of TeA resulted in a significant reduction in the ability of the pathogen to penetrate invasive hyphal growth and spread. The addition of exogenous TeA, AAL-toxin, and bentazone to the mutant ΔHP001 pathogens during inoculation resulted in a significant restoration of pathogenicity by increasing the level of cell death, frequency of hyphal penetration, and extent of invasive hyphal spread. Our results suggest that cell death triggered by TeA is the essential requirement for successful colonization and disease development in host leaves during infection with A. adenophora pathogens.

EDS1-interacting J protein 1 is an essential negative regulator of plant innate immunity in Arabidopsis

Plants have evolved precise mechanisms to optimize immune responses against pathogens. ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) plays a vital role in plant innate immunity by regulating basal resistance and effector-triggered immunity. Nucleocytoplasmic trafficking of EDS1 is required for resistance reinforcement, but the molecular mechanism remains elusive. Here, we show that EDS1-INTERACTING J PROTEIN1 (EIJ1), which acts as a DnaJ protein-like chaperone in response to pathogen infection, functions as an essential negative regulator of plant immunity by interacting with EDS1. The loss-of-function mutation of EIJ1 did not affect plant growth but significantly enhanced pathogen resistance. Upon pathogen infection, EIJ1 relocalized from the chloroplast to the cytoplasm, where it interacted with EDS1, thereby restricting pathogen-triggered trafficking of EDS1 to the nucleus and compromising resistance at an early infection stage. During disease development, EIJ1 was gradually degraded, allowing the nuclear accumulation of EDS1 for transcriptional resistance reinforcement. The avirulent strain Pst DC3000 (AvrRps4) abolished the repressive action of EIJ1 by rapidly inducing its degradation in the effector-triggered immunity response. Thus, our findings show that EIJ1 is an essential EDS1-dependent negative regulator of innate plant immunity and provide a mechanistic understanding of how the nuclear versus cytoplasmic distribution of EDS1 is regulated during the immune response.

An Arabidopsis downy mildew non-RxLR effector suppresses induced plant cell death to promote biotroph infection

Our understanding of obligate biotrophic pathogens is limited by lack of knowledge concerning the molecular function of virulence factors. We established Arabidopsis host-induced gene silencing (HIGS) to explore gene functions of Hyaloperonospora arabidopsidis, including CYSTEINE-RICH PROTEIN (HaCR)1, a potential secreted effector gene of this obligate biotrophic pathogen. HaCR1 HIGS resulted in H. arabidopsidis-induced local plant cell death and reduced pathogen reproduction. We functionally characterized HaCR1 by ectopic expression in Nicotiana benthamiana. HaCR1 was capable of inhibiting effector-triggered plant cell death. Consistent with this, HaCR1 expression in N. benthamiana led to stronger disease symptoms caused by the hemibiotrophic oomycete pathogen Phytophthora capsici, but reduced disease symptoms caused by the necrotrophic fungal pathogen Botrytis cinerea. Expressing HaCR1 in transgenic Arabidopsis confirmed higher susceptibility to H. arabidopsidis and to the bacterial hemibiotrophic pathogen Pseudomonas syringae. Increased H. arabidopsidis infection was in accordance with reduced PATHOGENESIS RELATED (PR)1 induction. Expression of full-length HaCR1 was required for its function, which was lost if the signal peptide was deleted, suggesting its site of action in the plant apoplast. This study provides phytopathological and molecular evidence for the importance of this widespread, but largely unexplored class of non-RxLR effectors in biotrophic oomycetes.

Isolate-Specific Responses of the Nonhost Grass Brachypodium distachyon to the Fungal Pathogen Zymoseptoria tritici Compared with Wheat

Septoria tritici blotch (STB) is an important foliar disease of wheat that is caused by the fungal pathogen Zymoseptoria tritici. The grass Brachypodium distachyon has been used previously as a model system for cereal-pathogen interactions. In this study, we examined the nonhost resistance (NHR) response of B. distachyon to two different Z. tritici isolates in comparison with wheat. These isolates vary in aggressiveness on wheat cultivar Remus, displaying significant differences in disease and pycnidia coverage. Using microscopy, we found that similar isolate-specific responses were observed for hydrogen peroxide accumulation and cell death in both wheat and B. distachyon. Despite this, induction of isolate-specific patterns of defense gene expression by Z. tritici did differ between B. distachyon and wheat. Our results suggest that expression of the phenylalanine ammonia lyase PAL gene may be important for NHR in B. distachyon, while pathogenesis-related PR genes and expression of genes regulating reactive oxygen species may be important to limit disease in wheat. Future studies of the B. distachyon-Z. tritici interaction may allow identification of conserved plant immunity targets that are responsible for the isolate-specific responses observed in both plant species.

Tryptophan-derived metabolites and BAK1 separately contribute to Arabidopsis postinvasive immunity against Alternaria brassicicola

Nonhost resistance of Arabidopsis thaliana against the hemibiotrophic fungus Colletotrichum tropicale requires PEN2-dependent preinvasive resistance and CYP71A12 and CYP71A13-dependent postinvasive resistance, which both rely on tryptophan (Trp) metabolism. We here revealed that CYP71A12, CYP71A13 and PAD3 are critical for Arabidopsis' postinvasive basal resistance toward the necrotrophic Alternaria brassicicola. Consistent with this, gene expression and metabolite analyses suggested that the invasion by A. brassicicola triggered the CYP71A12-dependent production of indole-3-carboxylic acid derivatives and the PAD3 and CYP71A13-dependent production of camalexin. We next addressed the activation of the CYP71A12 and PAD3-dependent postinvasive resistance. We found that bak1-5 mutation significantly reduced postinvasive resistance against A. brassicicola, indicating that pattern recognition contributes to activation of this second defense-layer. However, the bak1-5 mutation had no detectable effects on the Trp-metabolism triggered by the fungal penetration. Together with this, further comparative gene expression analyses suggested that pathogen invasion in Arabidopsis activates (1) CYP71A12 and PAD3-related antifungal metabolism that is not hampered by bak1-5, and (2) a bak1-5 sensitive immune pathway that activates the expression of antimicrobial proteins.

Protection of Photosynthesis by Halotolerant Staphylococcus sciuri ET101 in Tomato (Lycoperiscon esculentum) and Rice (Oryza sativa) Plants During Salinity Stress: Possible Interplay Between Carboxylation and Oxygenation in Stress Mitigation

Tomato (Lycoperiscon esculentum) and rice (Oryza sativa) are the two most important agricultural crops whose productivity is severely impacted by salinity stress. Soil salinity causes an irreversible damage to the photosynthetic apparatus in plants at all developmental stages leading to significant reduction in agricultural productivity. Reduction in photosynthesis is the primary response that is observed in all glycophytic plants during salt stress. Employment of salt-tolerant plant growth-promoting bacteria (PGPB) is an economical and viable approach for the remediation of saline soils and improvement of plant growth. The current study is aimed towards investigating the growth patterns and photosynthetic responses of rice and tomato plants upon inoculation with halotolerant PGPB Staphylococcus sciuri ET101 under salt stress conditions. Tomato and rice plants inoculated with PGPB showed increased growth rate and stimulated root growth, along with higher transpiration rates (E), stomatal conductance (g s ), and intracellular CO2 accumulation (Ci). Additionally, correlation of relative water content (RWC) to electrolyte leakage (EL) in tomato and rice plants showed decreased EL in inoculated plants during salt stress conditions, along with higher proline and glycine betaine content. Energy dissipation by non-photochemical quenching (NPQ) and increased photorespiration of 179.47% in tomato and 264.14% in rice plants were observed in uninoculated plants subjected to salinity stress. Furthermore, reduced photorespiration with improved salinity tolerance is observed in inoculated plants. The higher rates of photosynthesis in inoculated plants during salt stress were accompanied by increased quantum efficiency (ΦPSII) and maximum quantum yield (F v /F m ) of photosystem II. Furthermore, inoculated plants showed increased carboxylation efficiency of RuBisCO, along with higher photosynthetic electron transport rate (ETR) (J) during salinity stress. Although the total cellular ATP levels are drastically affected by salt stress in tomato and rice plants along with increased reactive oxygen species (ROS) accumulation, the restoration of cellular ATP levels in leaves of inoculated plants along with decreased ROS accumulation suggests the protective role of PGPB. Our results reveal the beneficial role of S. sciuri ET101 in protection of photosynthesis and amelioration of salinity stress responses in rice and tomato plants.

HOS1 activates DNA repair systems to enhance plant thermotolerance

Plants possess an astonishing capability of effectively adapting to a wide range of temperatures, ranging from freezing to near-boiling temperatures^1,2. Yet, heat is a critical obstacle to plant survival. The deleterious effects of heat shock on cell function include misfolding of cellular proteins, disruption of cytoskeletons and membranes, and disordering of RNA metabolism and genome integrity^3-5. Plants stimulate diverse heat shock response pathways in response to abrupt temperature increases. While it is known that stressful high temperatures disturb genome integrity by causing nucleotide modifications and strand breakages or impeding DNA repair⁶, it is largely unexplored how plants cope with heat-induced DNA damages. Here, we demonstrated that high expression of osmotically reponsive genes 1 (HOS1) induces thermotolerance by activating DNA repair components. Thermotolerance and DNA repair capacity were substantially reduced in HOS1-deficient mutants, in which thermal induction of genes encoding DNA repair systems, such as the DNA helicase RECQ2, was markedly decreased. Notably, HOS1 proteins were thermostabilized in a heat shock factor A1/heat shock protein 90 (HSP90)-dependent manner. Our data indicate that the thermoresponsive HSP90-HOS1-RECQ2 module contributes to sustaining genome integrity during the acquisition of thermotolerance, providing a distinct molecular link between DNA repair and thermotolerance.

Ceratocystis fimbriata Employs a Unique Infection Strategy Targeting Peltate Glandular Trichomes of Sweetpotato (Ipomoea batatas) Plants

The infection processes of Ceratocystis fimbriata BMPZ13 (BMPZ13) was elucidated on vegetative tissues of sweetpotato plants employing light and scanning electron microscopy. Vegetative tissues infected with C. fimbriata BMPZ13 by either wounding or nonwounding inoculation methods developed typical disease symptoms, establishing black rot in stems and necrosis on buds, young leaves, and stems of sprouts, in addition to wilt on leaves and shoot cuttings, typical of vascular associated diseases. The runner hyphae of C. fimbriata BMPZ13 formed from germinated conidia were able to directly penetrate the epidermal cuticle for initial infection and invade sweetpotato peltate glandular trichomes, specialized secretory structures to store and secrete metabolites. A two-step biotrophic phase was observed with nonwounding inoculation on leaves and stems, featuring both intercellular and intracellular invasive hyphae, with the latter found within living cells of the leaf epidermis. Subsequent to the biotrophic phase was a necrotrophic phase displaying cell death in infected leaves and veins. Additionally, this cell death was an iron-associated ferroptosis, supporting the notion that iron is involved in the necrotrophic phase of C. fimbriata BMPZ13 infection. Significantly, we establish that C. fimbriata employs a unique infection strategy: the targeting of peltate glandular trichomes. Collectively, our findings show that C. fimbriata is a plant fungal pathogen with a hemibiotrophic infection style in sweetpotato vegetative tissues.

The mode of action of plant associated Burkholderia against grey mould disease in grapevine revealed through traits and genomic analyses

Plant-associated Burkholderia spp. have been shown to offer a promising alternative method that may address concerns with ecological issue associated with pesticide overuse in agriculture. However to date, little work has studied the role of Burkholderia species as biocontrol agents for grapevine pathogens. To this end, two Burkholderia strains, BE17 and BE24 isolated from the maize rhizosphere in France, were investigated to determine their biocontrol potential and their ability to induce systemic resistance against grey mould disease in grapevine. Results showed the capacity of both strains to inhibit spore germination and mycelium growth of Botrytis cinerea. Experimental inoculation with BE17 and BE24 showed a significant protection of bacterized-plantlets against grey mould compared to the non-bacterized control. BE17 and BE24-bacterized plants accumulated more reactive oxygen species and an increased callose deposition was observed in leaves of bacterized plantlets compared to the control plantlets. In bacterized plants, gene expression analysis subsequent to B. cinerea challenge showed that strains BE17 and BE24 significantly increased the relative transcript level of pathogenesis-related (PR) proteins PR5 and PR10, two markers involved in the Salicylic acid (SA)-signaling pathway. Furthermore, in silico analysis of strains revealed the presence of genes involved in plant growth promotion and biocontrol highlighting the attractiveness of these strains for sustainable agricultural applications.

Differential Expression of Fungal Genes Determines the Lifestyle of Plectosphaerella Strains During Arabidopsis thaliana Colonization

The fungal genus Plectosphaerella comprises species and strains with different lifestyles on plants, such as P. cucumerina, which has served as model for the characterization of Arabidopsis thaliana basal and nonhost resistance to necrotrophic fungi. We have sequenced, annotated, and compared the genomes and transcriptomes of three Plectosphaerella strains with different lifestyles on A. thaliana, namely, PcBMM, a natural pathogen of wild-type plants (Col-0), Pc2127, a nonpathogenic strain on Col-0 but pathogenic on the immunocompromised cyp79B2 cyp79B3 mutant, and P0831, which was isolated from a natural population of A. thaliana and is shown here to be nonpathogenic and to grow epiphytically on Col-0 and cyp79B2 cyp79B3 plants. The genomes of these Plectosphaerella strains are very similar and do not differ in the number of genes with pathogenesis-related functions, with the exception of secreted carbohydrate-active enzymes (CAZymes), which are up to five times more abundant in the pathogenic strain PcBMM. Analysis of the fungal transcriptomes in inoculated Col-0 and cyp79B2 cyp79B3 plants at initial colonization stages confirm the key role of secreted CAZymes in the necrotrophic interaction, since PcBMM expresses more genes encoding secreted CAZymes than Pc2127 and P0831. We also show that P0831 epiphytic growth on A. thaliana involves the transcription of specific repertoires of fungal genes, which might be necessary for epiphytic growth adaptation. Overall, these results suggest that in-planta expression of specific sets of fungal genes at early stages of colonization determine the diverse lifestyles and pathogenicity of Plectosphaerella strains.

Characterization of novel regulators for heat stress tolerance in tomato from Indian sub-continent

The footprint of tomato cultivation, a cool region crop that exhibits heat stress (HS) sensitivity, is increasing in the tropics/sub-tropics. Knowledge of novel regulatory hot spots from varieties growing in the Indian sub-continent climatic zones could be vital for developing HS-resilient crops. Comparative transcriptome-wide signatures of a tolerant (CLN1621L) and sensitive (CA4) cultivar pair shortlisted from a pool of varieties exhibiting variable thermo-sensitivity using physiological-, survival- and yield-related traits revealed redundant to cultivar-specific HS regulation. The antagonistically expressing genes encode enzymes and proteins that have roles in plant defence and abiotic stresses. Functional characterization of three antagonistic genes by overexpression and silencing established Solyc09g014280 (Acylsugar acyltransferase) and Solyc07g056570 (Notabilis) that are up-regulated in tolerant cultivar, as positive regulators of HS tolerance and Solyc03g020030 (Pin-II proteinase inhibitor), that are down-regulated in CLN1621L, as negative regulator of thermotolerance. Transcriptional assessment of promoters of these genes by SNPs in stress-responsive cis-elements and promoter swapping experiments in opposite cultivar background showed inherent cultivar-specific orchestration of transcription factors in regulating transcription. Moreover, overexpression of three ethylene response transcription factors (ERF.C1/F4/F5) also improved HS tolerance in tomato. This study identifies several novel HS tolerance genes and provides proof of their utility in tomato thermotolerance.