Biosynthesis and metabolism of salicylic acid

Bacillus velezensis Isolate X5 Stimulates the Resistance of Resistant and Susceptible Banana Varieties to Foc Through Different Mechanisms

Banana wilt disease is an important disease in banana production, causing significant losses. Herein, we investigated the mechanism by which Bacillus velezensis isolate X5 enhances the resistance of different resistant banana cultivars to Fusarium oxysporum f. sp. cubense race 4 (Foc4). From the perspectives of metabolism, transcriptome, and key genes in important pathways, this study analyzed the composition and content changes of other types of signaling molecules, such as free amino acids and soluble sugars, in resistant/susceptible varieties. The results indicate that under pathogen stress, the contents of root-secreted metabolite components in both resistant and susceptible varieties increase significantly overall, and the increase in susceptible varieties is generally higher. For example, the free amino acid components in susceptible varieties are significantly more than those in resistant varieties. However, the addition of biocontrol bacteria can inhibit this increase. Exogenous addition experiments prove that differential metabolites can either promote or inhibit Foc4 and X5 at certain concentrations. The results of KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment and GO (Gene Ontology) annotation show that resistant varieties have more defense pathways compared to susceptible varieties. Under X5 treatment, more defense genes in resistant varieties are activated or their expression is enhanced, promoting the plant roots to secrete more substances related to plant resistance, such as phenylpropanoids and lignin. This research revealed the effects of Bacillus velezensis on bananas and pathogens, allowing valuable conclusions to be drawn. The results have good application potential to understand the relationships among the three species, defining the biocontrol effect and mechanism of Bacillus velezensis, and providing a theoretical basis for the biological control of soil-borne diseases such as banana wilt disease.

Disruption of allene oxide cyclase in maize reveals the necessity of enzymatically produced 12-OPDA for the induction of jasmonic acid during herbivory

Allene oxide cyclase (AOC) catalyzes the formation of 12-oxo-phytodienoic acid (12-OPDA) and represents an understudied step in jasmonate biosynthesis. Here the effects of eliminating AOC function in maize (Zea mays) are investigated. Gene editing was used to disrupt a pair of redundant AOC-coding genes, and mutants were analyzed with targeted metabolomics in a biochemical characterization of jasmonate deficiency. Our findings confirm essential roles for AOC in male flower development and resistance to biotic stresses. Metabolomic examinations show that AOC deficiency leads to a 90% reduction in 12-OPDA and a 99% reduction in jasmonic acid (JA) and JA-Isoleucine after treatment with fall armyworm. The presence of 12-OPDA in equal proportions of cis-(+) and cis-(-) stereochemical isomers indicates nonenzymatic allene oxide cyclization in the absence of functional AOC. This residual 12-OPDA is not converted into JA or other downstream jasmonates during herbivory, revealing the necessity of enzymatic cyclization of allene oxide by AOC for insect-induced JA and JA-dependent defense responses. The AOC-deficient mutants developed here provide a new tool for investigating the roles of jasmonates in maize.

Integrating Transcriptomics and Metabolomics to Comprehensively Analyze Phytohormone Regulatory Mechanisms in Rhododendron chrysanthum Pall. Under UV-B Radiation

In order to fully elucidate the roles and systems of phytohormones in UV-B radiation (UV-B) leaves of the Rhododendron chrysanthum Pall. (R. chrysanthum), we conducted a comprehensive analysis of how R. chrysanthum protects itself against UV-B using transcriptomic and metabolomic data. Transcript and metabolite profiles were generated by a combination of deep sequencing and LC-MS/MS (liquid chromatography-tandem mass spectrometry), respectively. Combined with physiological and biochemical assays, we studied compound accumulation, biosynthesis and expression of signaling genes of seven hormones and the effects of hormones on plant photosynthesis. The findings indicate that during leaf defense against UV-B, photosynthesis declined, the photosynthetic system was impaired and the concentration of salicylic acid (SA) hormones increased, whereas the contents of cytokinin (CK), abscisic acid (ABA), ethylene, auxin, jasmonic acid (JA) and gibberellins (GAs) continued to decrease. Finally, correlation tests between hormone content and genes were analyzed, and genes closely related to leaf resistance to UV-B were identified in seven pathways. These results will expand our understanding of the hormonal regulatory mechanisms of plant resistance to UV-B and at the same time lay the foundation for plant resistance to adversity stress.

Application of a Cell-Free Synthetic Biology Platform for the Reconstitution of Teleocidin B and UK-2A Precursor Biosynthetic Pathways

We report the successful cell-free reconstitution of two natural product biosynthetic pathways of divergent complexity and structural classes. We first constructed the teleocidin biosynthetic pathway using our BY-2 (tobacco) cell-free protein synthesis (CFPS) system. We discovered a direct interaction between TleA and MbtH, and showed that the BY-2 system is capable of producing more than 80 mg/L teleocidin B-3 with cofactor supplementation and ∼20 mg/L with no cofactors supplemented, demonstrating the high metabolic activity of the system. We then extended our methodology and report the first successful cell-free biosynthesis of UK-2 diol (precursor to the commercially valuable secondary metabolite UK-2A) from simple building blocks by refactoring a complex pathway of 10 proteins in the wheat germ CFPS system. We show that plant CFPS systems are suitable for reconstructing pathways and identifying the functions of uncharacterized genes linked to biosynthetic gene clusters and rate-limiting biosynthetic steps.

Viral proteins resolve the virus-vector conundrum during hemipteran-mediated transmission by subverting salicylic acid signaling pathway

Hemipteran insects transmit viruses when infesting plants, during which vectors activate salicylic acid (SA)-regulated antiviral defenses. How vector-borne plant viruses circumvent these antiviral defenses is largely unexplored. During co-infections of begomoviruses and betasatellites in plants, betasatellite-encoded βC1 proteins interfere with SA signaling and reduce the activation of antiviral resistance. βC1 inhibits SA-induced degradation of NbNPR3 (Nicotiana benthamiana nonexpressor of pathogenesis-related genes 3), a negative regulator of SA signaling. βC1 does not bind directly to NbNPR3, but regulates NbNPR3 degradation via heat shock protein 90s (NbHSP90s). NbHSP90s bind to both NbNPR3 and βC1 and suppress SA signaling. This viral success strategy appears to be conserved as it is also documented for viral proteins encoded by two aphid-borne viruses. Our findings reveal an exquisite mechanism that facilitates the persistence of vector-borne plant viruses and provide important insights into the intricacies of the virus life cycle.

The miR7125-MdARF1 module enhances the resistance of apple to Colletotrichum gloeosporioides by promoting lignin synthesis in response to salicylic acid signalling

Apple is an important cash crop in China, and it is susceptible to fungal infections that have deleterious effects on its yield. Apple bitter rot caused by Colletorichum gloeosporioides is one of the most severe fungal diseases of apple. Salicylic acid (SA) is a key signalling molecule in the plant disease resistance signalling pathways. Lignin synthesis also plays a key role in conferring disease resistance. However, few studies have clarified the relationship between the SA disease resistance signalling pathway and the lignin disease resistance pathway in apple. MdMYB46 has previously been shown to promote lignin accumulation in apple and enhance salt and osmotic stress tolerance. Here, we investigated the relationship between MdMYB46 and biological stress; we found that MdMYB46 overexpression enhances the resistance of apple to C. gloeosporioides. We also identified MdARF1, a transcription factor upstream of MdMYB46, via yeast library screening and determined that MdARF1 was regulated by miR7125 through psRNATarget prediction. This regulatory relationship was confirmed through LUC and qRT-PCR experiments, demonstrating that miR7125 negatively regulates MdARF1. Analysis of the miR7125 promoter revealed that miR7125 responds to SA signals. The accumulation of SA level will result in the decrease of miR7125 expression level. In sum, the results of our study provide novel insights into the molecular mechanisms underlying the resistance of apple to C. gloeosporioides and reveal a new pathway that enhances lignin accumulation in apple in response to SA signals. These findings provide valuable information for future studies aimed at breeding apple for disease resistance.

Molecular Investigations to Improve Fusarium Head Blight Resistance in Wheat: An Update Focusing on Multi-Omics Approaches

Fusarium head blight (FHB) is mainly caused by Fusarium graminearum (Fg) and is a very widespread disease throughout the world, leading to severe damage to wheat with losses in both grain yield and quality. FHB also leads to mycotoxin contamination in the infected grains, being toxic to humans and animals. In spite of the continuous advancements to elucidate more and more aspects of FHB host resistance, to date, our knowledge about the molecular mechanisms underlying wheat defense response to this pathogen is not comprehensive, most likely due to the complex wheat-Fg interaction. Recently, due to climate changes, such as high temperature and heavy rainfall, FHB has become more frequent and severe worldwide, making it even more urgent to completely understand wheat defense mechanisms. In this review, after a brief description of the first wheat immune response to Fg, we discuss, for each FHB resistance type, from Type I to Type V resistances, the main molecular mechanisms involved, the major quantitative trait loci (QTLs) and candidate genes found. The focus is on multi-omics research helping discover crucial molecular pathways for each resistance type. Finally, according to the emerging examined studies and results, a wheat response model to Fg attack, showing the major interactions in the different FHB resistance types, is proposed. The aim is to establish a useful reference point for the researchers in the field interested to adopt an interdisciplinary omics approach.

An endophytic fungus interacts with the defensin-like protein OsCAL1 to regulate cadmium allocation in rice

Defensin-like proteins are conserved in multicellular organisms and contribute to innate immune responses against fungal pathogens. In rice, defensins play a novel role in regulating cadmium (Cd) efflux from the cytosol. However, whether the antifungal activity of defensins correlates with Cd-efflux function remains unknown. In this study, we isolated an endophytic Fusarium, designed Fo10, by a comparative microbiome analysis of rice plants grown in a paddy contaminated with Cd. Fo10 is tolerant to high levels of Cd, but is sensitive to the defensin-like protein OsCAL1, which mediates Cd efflux to the apoplast. We found that Fo10 symbiosis in rice is regulated by OsCAL1 dynamics, and Fo10 coordinates multiple plant processes, including Cd uptake, vacuolar sequestration, efflux to the environment, and formation of Fe plaques in the rhizosphere. These processes are dependent on the salicylic acid signaling pathway to keep Cd levels low in the cytosol of rice cells and to decrease Cd levels in rice grains without any yield penalty. Fo10 also plays a role in Cd tolerance in the poaceous crop maize and wheat, but has no observed effects in the eudicot plants Arabidopsis and tomato. Taken together, these findings provide insights into the mechanistic basis underlying how a fungal endophyte and host plant interact to control Cd accumulation in host plants by adapting defense responses to promote the establishment of a symbiosis that permits adaptation to high-Cd environments.

Essential oil-grafted copper nanoparticles as a potential next-generation fungicide for holistic disease management in maize

Sustainable food production is necessary to meet the demand of the incessantly growing human population. Phytopathogens pose a major constraint in food production, and the use of conventional fungicides to manage them is under the purview of criticism due to their numerous setbacks. In the present study, essential oil-grafted copper nanoparticles (EGC) were generated, characterized, and evaluated against the maize fungal pathogens, viz., Bipolaris maydis, Rhizoctonia solani f. sp. sasakii, Macrophomina phaseolina, Fusarium verticillioides, and Sclerotium rolfsii. The ED₅₀ for the fungi under study ranged from 43 to 56 μg ml^-1, and a significant inhibition was observed at a low dose of 20 μg ml^-1 under in vitro conditions. Under net house conditions, seed treatment + foliar spray at 250 and 500 mg L^-1 of EGC performed remarkably against maydis leaf blight (MLB), with reduced percent disease index (PDI) by 27.116 and 25.292%, respectively, in two Kharif seasons (May-Sep, 2021, 2022). The activity of enzymatic antioxidants, viz., β-1, 3-glucanase, PAL, POX, and PPO, and a non-enzymatic antioxidant (total phenolics) was increased in treated maize plants, indicating host defense was triggered. The optimum concentrations of EGC (250 mg L^-1 and 500 mg L^-1) exhibited improved physiological characteristics such as photosynthetic activity, shoot biomass, plant height, germination percentage, vigor index, and root system traits. However, higher concentrations of 1,000 mg L^-1 rendered phytotoxicity, reducing growth, biomass, and copper bioaccumulation to high toxic levels, mainly in the foliar-sprayed maize leaves. In addition, EGC and copper nanoparticles (CuNPs) at 1,000 mg L^-1 reduced the absorption and concentration of manganese and zinc indicating a negative correlation between Cu and Mn/Zn. Our study proposes that the CuNPs combined with EO (Clove oil) exhibit astounding synergistic efficacy against maize fungal pathogens and optimized concentrations can be used as an alternative to commercial fungicides without any serious impact on environmental health.

Genome-Wide Identification of the NPR1-like Gene Family in Solanum tuberosum and Functional Characterization of StNPR1 in Resistance to Ralstonia solanacearum

The NPR1 (nonexpressor of pathogenesis-related genes 1) gene is an activator of the systemic acquisition of resistance (SAR) in plants and is one of the central factors in their response to pathogenic bacterial infestation, playing an important role in plant disease resistance. Potato (Solanum tuberosum) is a crucial non-grain crop that has been extensively studied. However, the identification and analysis of the NPR1-like gene within potato have not been understood well. In this study, a total of six NPR1-like proteins were identified in potato, and phylogenetic analysis showed that the six NPR1-like proteins in Solanum tuberosum could be divided into three major groups with NPR1-related proteins from Arabidopsis thaliana and other plants. Analysis of the exon-intron patterns and protein domains of the six NPR1-like genes from potato showed that the exon-intron patterns and protein domains of the NPR1-like genes belonging to the same Arabidopsis thaliana subfamily were similar. By performing quantitative real-time PCR (qRT-PCR) analysis, we found that six NPR1-like proteins have different expression patterns in different potato tissues. In addition, the expression of three StNPR1 genes was significantly downregulated after being infected by Ralstonia solanacearum (RS), while the difference in the expression of StNPR2/3 was insignificant. We also established potato StNPR1 overexpression lines that showed a significantly increased resistance to R. solanacearum and elevated activities of chitinase, β-1,3-glucanase, and phenylalanine deaminase. Increased peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) activities, as well as decreased hydrogen peroxide, regulated the dynamic balance of reactive oxygen species (ROS) in the StNPR1 overexpression lines. The transgenic plants activated the expression of the genes associated with the Salicylic acid (SA) defense response but suppressed the expression of the genes associated with Jasmonic acid (JA) signaling. This resulted in resistance to Ralstonia solanacearum.

Salicylic acid biosynthesis is not from phenylalanine in Arabidopsis

The phytohormone salicylic acid (SA) regulates biotic and abiotic stress responses in plants. Two distinct biosynthetic pathways for SA have been well documented in plants: the isochorismate (IC) pathway in the chloroplast and the phenylalanine ammonia-lyase (PAL) pathway in the cytosol. However, there has been no solid evidence that the PAL pathway contributes to SA biosynthesis. Here, we report that feeding Arabidopsis thaliana with Ring-¹³ C-labeled phenylalanine (¹³ C₆ -Phe) resulted in incorporation of the ¹³ C label not into SA, but into its isomer 4-hydroxybenzoic acid (4-HBA) instead. We obtained similar results when feeding ¹³ C₆ -Phe to the SA-deficient ics1 ics2 mutant and the SA-hyperaccumulating mutant s3h s5h. Notably, we detected ¹³ C₆ -SA when ¹³ C₆ -benzoic acid (BA) was provided, suggesting that SA can be synthesized from BA. Furthermore, despite the substantial accumulation of SA upon pathogen infection, we did not observe incorporation of ¹³ C label from Phe into SA. We also did not detect ¹³ C₆ -SA in PAL-overexpressing lines in the kfb01 kfb02 kfb39 kfb50 background after being fed ¹³ C₆ -Phe, although endogenous PAL levels were dramatically increased. Based on these combined results, we propose that SA biosynthesis is not from Phe in Arabidopsis. These results have important implications for our understanding of the SA biosynthetic pathway in land plants.

Molecular regulation of immunity in tea plants

Tea, which is mainly produced using the young leaves and buds of tea plants (Camellia sinensis (L.) O. Kuntze), is one of the most common non-alcoholic beverages consumed in the world. The standard of tea mostly depends on the variety and quality of tea plants, which generally grow in subtropical areas, where the warm and humid conditions are also conducive to the occurrence of diseases. In fighting against pathogens, plants rely on their sophisticated innate immune systems which has been extensively studied in model plants. Many components involved in pathogen associated molecular patterns (PAMPs) triggered immunity (PTI) and effector triggered immunity (ETI) have been found. Nevertheless, the molecular regulating network against pathogens (e.g., Pseudopestalotiopsis sp., Colletotrichum sp. and Exobasidium vexans) causing widespread disease (such as grey blight disease, anthracnose, and blister blight) in tea plants is still unclear. With the recent release of the genome data of tea plants, numerous genes involved in tea plant immunity have been identified, and the molecular mechanisms behind tea plant immunity is being studied. Therefore, the recent achievements in identifying and cloning functional genes/gene families, in finding crucial components of tea immunity signaling pathways, and in understanding the role of secondary metabolites have been summarized and the opportunities and challenges in the future studies of tea immunity are highlighted in this review.

A family of methyl esterases converts methyl salicylate to salicylic acid in ripening tomato fruit

Methyl salicylate imparts a potent flavor and aroma described as medicinal and wintergreen that is undesirable in tomato (Solanum lycopersicum) fruit. Plants control the quantities of methyl salicylate through a variety of biosynthetic pathways, including the methylation of salicylic acid to form methyl salicylate and subsequent glycosylation to prevent methyl salicylate emission. Here, we identified a subclade of tomato methyl esterases, SALICYLIC ACID METHYL ESTERASE1-4, responsible for demethylation of methyl salicylate to form salicylic acid in fruits. This family was identified by proximity to a highly significant methyl salicylate genome-wide association study locus on chromosome 2. Genetic mapping studies in a biparental population confirmed a major methyl salicylate locus on chromosome 2. Fruits from SlMES1 knockout lines emitted significantly (P < 0,05, t test) higher amounts of methyl salicylate than wild-type fruits. Double and triple mutants of SlMES2, SlMES3, and SlMES4 emitted even more methyl salicylate than SlMES1 single knockouts-but not at statistically distinguishable levels-compared to the single mutant. Heterologously expressed SlMES1 and SlMES3 acted on methyl salicylate in vitro, with SlMES1 having a higher affinity for methyl salicylate than SlMES3. The SlMES locus has undergone major rearrangement, as demonstrated by genome structure analysis in the parents of the biparental population. Analysis of accessions that produce high or low levels of methyl salicylate showed that SlMES1 and SlMES3 genes expressed the highest in the low methyl salicylate lines. None of the MES genes were appreciably expressed in the high methyl salicylate-producing lines. We concluded that the SlMES gene family encodes tomato methyl esterases that convert methyl salicylate to salicylic acid in ripe tomato fruit. Their ability to decrease methyl salicylate levels by conversion to salicylic acid is an attractive breeding target to lower the level of a negative contributor to flavor.

Genome-wide studies of PAL genes in sorghum and their responses to aphid infestation

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.25) plays a crucial role in plant adaptation to biotic and abiotic stresses. However, the current knowledge about PAL proteins in sorghum is essentially lacking. Thus, in this study we aimed to analyze the PAL family genes in sorghum using a genome-wide approach and to explore the role of PAL genes in host plant resistance to aphids via SA-mediated defense signaling. Here, we report gene structural features of 8 PAL (SbPAL) genes in sorghum (Sorghum bicolor), their phylogeny, protein motifs and promoter analysis. Furthermore, we demonstrated that the SbPAL genes were induced by sugarcane aphid (SCA) infestation and SbPAL exhibited differential gene expression in susceptible and resistant genotypes. PAL activity assays further validated upregulated expression of the SbPAL genes in a resistant genotype. In addition, exogenous application of SA reduced plant damage and suppressed aphid population growth and fecundity in susceptible genotype, suggesting that those SbPAL genes act as positive regulator of the SA-mediated defense signaling pathway to combat aphid pests in sorghum. This study provides insights for further examination of the defense role of PAL in sorghum against other pests and pathogens.

Rice β-Glucosidase 4 (Os1βGlu4) Regulates the Hull Pigmentation via Accumulation of Salicylic Acid

Salicylic acid (SA) is a stress hormone synthesized in phenylalanine ammonia-lyase (PAL) and the branching acid pathway. SA has two interconvertible forms in plants: SAG (SA O-β-glucoside) and SA (free form). The molecular mechanism of conversion of SA to SAG had been reported previously. However, which genes regulate SAG to SA remained unknown. Here, we report a cytoplasmic β-glucosidase (β-Glu) which participates in the SA pathway and is involved in the brown hull pigmentation in rice grain. In the current study, an EMS-generated mutant brown hull 1 (bh1) displayed decreased contents of SA in hulls, a lower photosynthesis rate, and high-temperature sensitivity compared to the wild type (WT). A plaque-like phenotype (brown pigmentation) was present on the hulls of bh1, which causes a significant decrease in the seed setting rate. Genetic analysis revealed a mutation in LOC_Os01g67220, which encodes a cytoplasmic Os1βGlu4. The knock-out lines displayed the phenotype of brown pigmentation on hulls and decreased seed setting rate comparable with bh1. Overexpression and complementation lines of Os1βGlu4 restored the phenotype of hulls and normal seed setting rate comparable with WT. Subcellular localization revealed that the protein of Os1βGlu4 was localized in the cytoplasm. In contrast to WT, bh1 could not hydrolyze SAG into SA in vivo. Together, our results revealed the novel role of Os1βGlu4 in the accumulation of flavonoids in hulls by regulating the level of free SA in the cellular pool.

Biochar stimulates tomato roots to recruit a bacterial assemblage contributing to disease resistance against Fusarium wilt

Biochar amendment is acknowledged to favor plant resistance against soil-borne diseases. Although plant-beneficial bacteria enrichment in the rhizosphere is often proposed to be associated with this protection, the mechanism behind this stimulating effect remains unelucidated. Here, we tested whether biochar promotes plants to recruit beneficial bacteria to the rhizosphere, and thus develop a disease-suppressive rhizosphere microbiome. In a pot experiment, biochar amendment decreased tomato Fusarium wilt disease severity. Using a transplanting rhizosphere microbiome experiment, we showed that biochar enhanced the suppressiveness of tomato rhizosphere microbiome against Fusarium wilt disease. High-throughput sequencing of 16S ribosomal RNA gene and in vitro cultures further indicated that the recruited suppressive rhizosphere microbiome was associated with the increase of plant-beneficial bacteria, such as Pseudomonas sp. This amendment also enhanced the in vitro chemoattraction and biofilm promotion activity of tomato root exudates. Collectively, our results demonstrate that biochar amendment induces tomato seedlings to efficiently recruit a disease-suppressive rhizosphere microbiome against Fusarium wilt.

Primary root response to combined drought and heat stress is regulated via salicylic acid metabolism in maize

The primary root is the first organ to perceive the stress signals for abiotic stress. In this study, maize plants subjected to drought, heat and combined stresses displayed a significantly reduced primary root length. Metabolic and transcriptional analyses detected 72 and 5,469 differentially expressed metabolites and genes in response to stress conditions, respectively. The functional annotation of differentially expressed metabolites and genes indicated that primary root development was mediated by pathways involving phenylalanine metabolism, hormone metabolism and signaling under stress conditions. Furthermore, we found that the concentration of salicylic acid and two precursors, shikimic acid and phenylalanine, showed rapid negative accumulation after all three stresses. The expression levels of some key genes involved in salicylic acid metabolism and signal transduction were differentially expressed under stress conditions. This study extends our understanding of the mechanism of primary root responses to abiotic stress tolerance in maize.

Hydrogel-extraction technique for non-invasive detection of blue fluorescent substances in plant leaves

This paper reports a new hydrogel extraction technique for detecting blue fluorescent substances in plant leaves. These blue fluorescent substances were extracted by placing a hydrogel film on the leaf of a cherry tomato plant infected with Ralstonia solanacearum; herein, chlorogenic acid was confirmed to be a blue fluorescent substance. The wavelength at the maximum fluorescence intensity of the film after the hydrogel extraction was similar to that of the methanolic extract obtained from the infected cherry tomato leaves. Chlorophyll was not extracted from the hydrogel film because no fluorescence peak was observed at 680 nm. Accordingly, the blue fluorescence of the substances extracted from the hydrogel film was not quenched by the strong absorption of chlorophyll in the blue light region. This hydrogel extraction technique can potentially detect small amounts of blue fluorescent substances and the changes in its amount within the leaves of infected plants. These changes in the amount of blue fluorescent substances in the early stages of infection can be used to detect presymptomatic infections. Therefore, hydrogel extraction is a promising technique for the noninvasive detection of infections before onset.

OsTGAL1 suppresses the resistance of rice to bacterial blight disease by regulating the expression of salicylic acid glucosyltransferase OsSGT1

Many TGA transcription factors participate in immune responses in the SA-mediated signaling pathway in Arabidopsis. This study identified a transcription factor OsTGAL1, which is induced upon infection by Xoo. Overexpression of OsTGAL1 increased the susceptibility of rice to Xoo. Plants overexpressing OsTGAL1 could affect the expression of many SA signaling-related genes. OsTGAL1 was able to interact with the promoter of OsSGT1, which encodes a key enzyme for SA metabolism. The transcript of OsSGT1 was induced by Xoo and this responsive expression was further increased in plants overexpressing OsTGAL1. OsSGT1 knockout lines had enhanced resistance to Xoo, and knocking out OsSGT1 in plants overexpressing OsTGAL1 blocked the susceptibility caused by OsTGAL1. Altered expression levels of several OsPRs in all the transgenic plants demonstrated that SA-mediated signaling had been affected. Furthermore, we identified an oxidoreductase of CC-type glutaredoxin, OsGRX17, which interacted with OsTGAL1. OsGRX17 reduced the regulation of OsTGAL1 on OsSGT1, and this may be due to its redox modulation. Thus, our results demonstrate that OsTGAL1 negatively regulates resistance to Xoo by its effects on SA metabolism via the activation of OsSGT1, which provides valuable targets for plant breeders in developing new cultivars that are resistant to Xoo.

Salicylic acid carboxyl glucosyltransferase UGT87E7 regulates disease resistance in Camellia sinensis

Plant immune response following pathogenic infection is regulated by plant hormones, and salicylic acid (SA) and its sugar conjugates play important roles in establishing basal resistance. Here, the important pathogen Pseudopestalotiopsis camelliae-sinensis (Pcs) was isolated from tea gray blight, one of the most destructive diseases in tea plantations. Transcriptomic analysis led to the discovery of the putative Camellia sinensis UDP-glucosyltransferase CsUGT87E7 whose expression was significantly induced by SA application and Pcs infection. Recombinant CsUGT87E7 glucosylates SA with a Km value of 12 µM to form SA glucose ester (SGE). Downregulation reduced the accumulation of SGE, and CsUGT87E7-silenced tea plants exhibited greater susceptibility to pathogen infection than control plants. Similarly, CsUGT87E7-silenced tea leaves accumulated significantly less SA after infection and showed reduced expression of pathogenesis-related genes. These results suggest that CsUGT87E7 is an SA carboxyl glucosyltransferase that plays a positive role in plant disease resistance by modulating SA homeostasis through a mechanism distinct from that described in Arabidopsis (Arabidopsis thaliana). This study provides insight into the mechanisms of SA metabolism and highlights the role of SGE in the modulation of plant disease resistance.

Linking Multi-Omics to Wheat Resistance Types to Fusarium Head Blight to Reveal the Underlying Mechanisms

Fusarium head blight (FHB) caused by Fusarium graminearum is a worldwide disease which has destructive effects on wheat production, resulting in severe yield reduction and quality deterioration, while FHB-infected wheat grains are toxic to people and animals due to accumulation of fungal toxins. Although impressive progress towards understanding host resistance has been achieved, our knowledge of the mechanism underlying host resistance is still quite limited due to the complexity of wheat-pathogen interactions. In recent years, disease epidemics, the resistance germplasms and components, the genetic mechanism of FHB, and disease management and control, etc., have been well reviewed. However, the resistance mechanism of FHB is quite complex with Type I, II to V resistances. In this review, we focus on the potential resistance mechanisms by linking different resistance types to multi-omics and emphasize the pathways or genes that may play significant roles in the different types of resistance. Deciphering the complicated mechanism of FHB resistance types in wheat at the integral levels based on multi-omics may help discover the genes or pathways that are critical for different FHB resistance, which could then be utilized and manipulated to improve FHB resistance in wheat breeding programs by using transgenic approaches, gene editing, or marker assisted selection strategies.

Amplification of early drought responses caused by volatile cues emitted from neighboring plants

Plants have developed sophisticated mechanisms to survive in dynamic environments. Plants can communicate via volatile organic compounds (VOCs) to warn neighboring plants of threats. In most cases, VOCs act as positive regulators of plant defense. However, the communication and role of volatiles in response to drought stress are poorly understood. Here, we showed that tea plants release numerous VOCs. Among them, methyl salicylate (MeSA), benzyl alcohol, and phenethyl alcohol markedly increased under drought stress. Interestingly, further experiments revealed that drought-induced MeSA lowered the abscisic acid (ABA) content in neighboring plants by reducing 9-cis-epoxycarotenoid dioxygenase (NCED) gene expression, resulting in inhibition of stomatal closure and ultimately decreasing early drought tolerance in neighboring plants. Exogenous application of ABA reduced the wilting of tea plants caused by MeSA exposure. Exposure of Nicotiana benthamiana to MeSA also led to severe wilting, indicating that the ability of drought-induced MeSA to reduce early drought tolerance in neighboring plants may be conserved in other plant species. Taken together, these results provide evidence that drought-induced volatiles can reduce early drought tolerance in neighboring plants and lay a novel theoretical foundation for optimizing plant density and spacing.

Fatal attraction of Caenorhabditis elegans to predatory fungi through 6-methyl-salicylic acid

Salicylic acid is a phenolic phytohormone which controls plant growth and development. A methyl ester (MSA) derivative thereof is volatile and involved in plant-insect or plant-plant communication. Here we show that the nematode-trapping fungus Duddingtonia flagrans uses a methyl-salicylic acid isomer, 6-MSA as morphogen for spatiotemporal control of trap formation and as chemoattractant to lure Caenorhabditis elegans into fungal colonies. 6-MSA is the product of a polyketide synthase and an intermediate in the biosynthesis of arthrosporols. The polyketide synthase (ArtA), produces 6-MSA in hyphal tips, and is uncoupled from other enzymes required for the conversion of 6-MSA to arthrosporols, which are produced in older hyphae. 6-MSA and arthrosporols both block trap formation. The presence of nematodes inhibits 6-MSA and arthrosporol biosyntheses and thereby enables trap formation. 6-MSA and arthrosporols are thus morphogens with some functions similar to quorum-sensing molecules. We show that 6-MSA is important in interkingdom communication between fungi and nematodes.

Impact of the female and hermaphrodite forms of Opuntia robusta on the plant defence hypothesis

The optimal defence hypothesis predicts that increased plant defence capabilities, lower levels of damage, and lower investment in vegetative biomass will occur more frequently in sexual forms with higher resource-demanding tissue production and/or younger plant parts. We aimed to examine the effects of sexual form, cladode, and flower age on growth rate, herbivore damage, and 4-hydroxybenzoic acid (4-HBA), chlorogenic acid, and quercetin (QUE) concentrations in Opuntia robusta plants in central Mexico. Our findings demonstrated that hermaphrodite flowers showed faster growth and lesser damage than female flowers. The effect of cladode sexual forms on 4-HBA and QUE occurrence was consistent with the predictions of the optimal defence hypothesis. However, chlorogenic acid occurrences were not significantly affected by sexual forms. Old cladodes exhibited higher QUE and 4-HBA occurrences than young cladodes, and hermaphrodites exhibited higher 4-HBA concentrations than females. Resource allocation for reproduction and secondary metabolite production, and growth was higher and lower, respectively, in females, compared to hermaphrodites, indicating a trade-off between investment in reproduction, growth, and secondary metabolite production. Secondary metabolite concentrations in O. robusta plants were not negatively correlated with herbivore damage, and the two traits were not accurate predictors of plant reproductive output.

Microfluidic device for multiplexed detection of fungal infection biomarkers in grape cultivars

Early diagnosis of fungal infections, which have seen an increase due to different environmental factors, is essential to an appropriate treatment of the plant by avoiding proliferation of the pathogen without excessive fungicide applications. In this work, we propose a microfluidic based approach to a multiplexed, point-of-need detection system capable of identifying infected grape cultivars. The system relies on the simultaneous detection of three plant hormones: salicylic, azelaic and jasmonic acids with a total assay time under 7 minutes, with LODs of 15 μM, 10 μM and 4.4 nM respectively. The three detection assays are based on optical transduction, with the detection of salicylic and azelaic acids using transmission measurements, while the detection of jasmonic acid is a fluorescence-based assay. The molecular recognition event for each metabolite is different: nanoparticle conjugation for salicylic acid, enzymatic reaction for azelaic acid and antibody-antigen recognition for jasmonic acid. In this work, two cultivars, Trincadeira and Carignan, presented infections with two fungal pathogens, Botrytis cinerea and Erysiphe necator. The grapes were tested using the microfluidic system alongside the benchmark techniques such as, high-performance liquid chromatography and enzyme-linked immunosorbent assay. The microfluidic system was not only capable of distinguishing infected from healthy samples, but also capable of distinguishing between different infection types.

Arabidopsis UGT76B1 glycosylates N-hydroxy-pipecolic acid and inactivates systemic acquired resistance in tomato

Systemic acquired resistance (SAR) is a mechanism that plants utilize to connect a local pathogen infection to global defense responses. N-hydroxy-pipecolic acid (NHP) and a glycosylated derivative are produced during SAR, yet their individual roles in this process are currently unclear. Here, we report that Arabidopsis thaliana UGT76B1 generated glycosylated NHP (NHP-Glc) in vitro and when transiently expressed alongside Arabidopsis NHP biosynthetic genes in two Solanaceous plants. During infection, Arabidopsis ugt76b1 mutants did not accumulate NHP-Glc and accumulated less glycosylated salicylic acid (SA-Glc) than wild-type plants. The metabolic changes in ugt76b1 plants were accompanied by enhanced defense to the bacterial pathogen Pseudomonas syringae, suggesting that glycosylation of the SAR molecules NHP and salicylic acid by UGT76B1 plays an important role in modulating defense responses. Transient expression of Arabidopsis UGT76B1 with the Arabidopsis NHP biosynthesis genes ALD1 and FMO1 in tomato (Solanum lycopersicum) increased NHP-Glc production and reduced NHP accumulation in local tissue and abolished the systemic resistance seen when expressing NHP-biosynthetic genes alone. These findings reveal that the glycosylation of NHP by UGT76B1 alters defense priming in systemic tissue and provide further evidence for the role of the NHP aglycone as the active metabolite in SAR signaling.

Production of salicylic acid; a potent pharmaceutically active agent and its future prospects

Salicylic acid is one of the potent pharmaceutical organic acids that have various applications in the medical field. It acts as a plant hormone and helps in plant's growth & defence against pathogens. Beyond its numerous functions in plants, SA has great pharmaceutical importance since it acts as an intermediate for the synthesis of various drugs and dyes e.g. aspirin. At the industrial scale, chemical methods are used for the synthesis of SA but presently, several other sources are available that have the capability to alternate the chemical process which will be a step forward toward green synthesis. Aim of this paper is to provide comprehensive knowledge of SA production and its biological application.

Brachypodium Phenylalanine Ammonia Lyase (PAL) Promotes Antiviral Defenses against Panicum mosaic virus and Its Satellites

Brachypodium distachyon has recently emerged as a premier model plant for monocot biology, akin to Arabidopsis thaliana We previously reported genome-wide transcriptomic and alternative splicing changes occurring in Brachypodium during compatible infections with Panicum mosaic virus (PMV) and its satellite virus (SPMV). Here, we dissected the role of Brachypodium phenylalanine ammonia lyase 1 (PAL1), a key enzyme for phenylpropanoid and salicylic acid (SA) biosynthesis and the induction of plant defenses. Targeted metabolomics profiling of PMV-infected and PMV- plus SPMV-infected (PMV/SPMV) Brachypodium plants revealed enhanced levels of multiple defense-related hormones and metabolites such as cinnamic acid, SA, and fatty acids and lignin precursors during disease progression. The virus-induced accumulation of SA and lignin was significantly suppressed upon knockdown of B. distachyon PAL1 (BdPAL1) using RNA interference (RNAi). The compromised SA accumulation in PMV/SPMV-infected BdPAL1 RNAi plants correlated with weaker induction of multiple SA-related defense gene markers (pathogenesis related 1 [PR-1], PR-3, PR-5, and WRKY75) and enhanced susceptibility to PMV/SPMV compared to that of wild-type (WT) plants. Furthermore, exogenous application of SA alleviated the PMV/SPMV necrotic disease phenotypes and delayed plant death caused by single and mixed infections. Together, our results support an antiviral role for BdPAL1 during compatible host-virus interaction, perhaps as a last resort attempt to rescue the infected plant.IMPORTANCE Although the role of plant defense mechanisms against viruses are relatively well studied in dicots and in incompatible plant-microbe interactions, studies of their roles in compatible interactions and in grasses are lagging behind. In this study, we leveraged the emerging grass model Brachypodium and genetic resources to dissect Panicum mosaic virus (PMV)- and its satellite virus (SPMV)-compatible grass-virus interactions. We found a significant role for PAL1 in the production of salicylic acid (SA) in response to PMV/SPMV infections and that SA is an essential component of the defense response preventing the plant from succumbing to viral infection. Our results suggest a convergent role for the SA defense pathway in both compatible and incompatible plant-virus interactions and underscore the utility of Brachypodium for grass-virus biology.

Induction of priming by cold stress via inducible volatile cues in neighboring tea plants

Plants have evolved sophisticated defense mechanisms to overcome their sessile nature. However, if and how volatiles from cold-stressed plants can trigger interplant communication is still unknown. Here, we provide the first evidence for interplant communication via inducible volatiles in cold stress. The volatiles, including nerolidol, geraniol, linalool, and methyl salicylate, emitted from cold-stressed tea plants play key role(s) in priming cold tolerance of their neighbors via a C-repeat-binding factors-dependent pathway. The knowledge will help us to understand how plants respond to volatile cues in cold stress and agricultural ecosystems.

Transcriptome Analysis of Early Defenses in Rice against Fusarium fujikuroi

BACKGROUND

Bakanae is a seedborne disease caused by Fusarium fujikuroi. Rice seedlings emerging from infected seeds can show diverse symptoms such as elongated and slender stem and leaves, pale coloring, a large leaf angle, stunted growth and even death. Little is known about rice defense mechanisms at early stages of disease development.

RESULTS

This study focused on investigating early defenses against F. fujikuroi in a susceptible cultivar, Zerawchanica karatals (ZK), and a resistant cultivar, Tainung 67 (TNG67). Quantitative PCR revealed that F. fujikuroi colonizes the root and stem but not leaf tissues. Illumina sequencing was conducted to analyze the stem transcriptomes of F. fujikuroi-inoculated and mock-inoculated ZK and TNG67 plants collected at 7 days post inoculation (dpi). More differentially expressed genes (DEGs) were identified in ZK (n = 169) than TNG67 (n = 118), and gene ontology terms related to transcription factor activity and phosphorylation were specifically enriched in ZK DEGs. Among the complex phytohormone biosynthesis and signaling pathways, only DEGs involved in the jasmonic acid (JA) signaling pathway were identified. Fourteen DEGs encoding pattern-recognition receptors, transcription factors, and JA signaling pathway components were validated by performing quantitative reverse transcription PCR analysis of individual plants. Significant repression of jasmonate ZIM-domain (JAZ) genes (OsJAZ9, OsJAZ10, and OsJAZ13) at 3 dpi and 7 dpi in both cultivars, indicated the activation of JA signaling during early interactions between rice and F. fujikuroi. Differential expression was not detected for salicylic acid marker genes encoding phenylalanine ammonia-lyase 1 and non-expressor of pathogenesis-related genes 1. Moreover, while MeJA did not affect the viability of F. fujikuroi, MeJA treatment of rice seeds (prior to or after inoculation) alleviated and delayed bakanae disease development in susceptible ZK.

CONCLUSIONS

Different from previous transcriptome studies, which analyzed the leaves of infected plants, this study provides insights into defense-related gene expression patterns in F. fujikuroi-colonized rice stem tissues. Twelve out of the 14 selected DEGs were for the first time shown to be associated with disease resistance, and JA-mediated resistance was identified as a crucial component of rice defense against F. fujikuroi. Detailed mechanisms underlying the JA-mediated bakanae resistance and the novel defense-related DEGs are worthy of further investigation.

Integrating transcriptomic and metabolomic analysis of hormone pathways in Acer rubrum during developmental leaf senescence

BACKGROUND

To fully elucidate the roles and mechanisms of plant hormones in leaf senescence, we adopted an integrated analysis of both non-senescing and senescing leaves from red maple with transcriptome and metabolome data.

RESULTS

Transcription and metabolite profiles were generated through a combination of deep sequencing, third-generation sequencing data analysis, and ultrahigh-performance liquid chromatograph Q extractive mass spectrometry (UHPLC-QE-MS), respectively. We investigated the accumulation of compounds and the expression of biosynthesis and signaling genes for eight hormones. The results revealed that ethylene and abscisic acid concentrations increased during the leaf senescence process, while the contents of cytokinin, auxin, jasmonic acid, and salicylic acid continued to decrease. Correlation tests between the hormone content and transcriptional changes were analyzed, and in six pathways, genes closely linked with leaf senescence were identified.

CONCLUSIONS

These results will enrich our understanding of the mechanisms of plant hormones that regulate leaf senescence in red maple, while establishing a foundation for the genetic modification of Acer in the future.

Metabolism of airborne methyl salicylate in adjacent plants

Salicylic acid (SA) and methyl salicylate (MeSA) are synthesized in many plants and are crucial components that establish their disease responses. The metabolism of airborne MeSA to SA has been previously reported. In this report, it was found that SA glucose ester (SAGE), ether (SAG), and salicyloyl-L-aspartic acid (SA-Asp) are metabolites of airborne MeSA. Furthermore, it was found that airborne MeSA was able to increase the endogenous amount of rosmarinic acid in Perilla frutescens, which is known as one of the functional components that contributes to the maintenance of human health.

Genome-wide identification of the NPR1-like gene family in Brassica napus and functional characterization of BnaNPR1 in resistance to Sclerotinia sclerotiorum

The BnaNPR1-like gene family was identified in B. napus, and it was revealed that repression of BnaNPR1 significantly reduces resistance toS. sclerotiorum, intensifies ROS accumulation, and changes the expression of genes associated with SA and JA/ET signaling in response to this pathogen. The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) and related NPR1-like genes play an important role in regulating plant defense. Oilseed rape (Brassica napus L.) is an important oilseed crop; however, little is known about the B. napus (Bna) NPR1-like gene family. Here, a total of 19 BnaNPR1-like genes were identified in the B. napus genome, and then named according to their respective best match in Arabidopsis thaliana (At), which led to the determination of B. napus homologs of every AtNPR1-like gene. Analysis of important protein domains and functional motifs indicated the conservation and variation among these homologs. Phylogenetic analysis of these BnaNPR1-like proteins and their Arabidopsis homologs revealed six distinct sub-clades, consequently indicating that their name classification totally conformed to their phylogenetic relationships. Further, B. napus transcriptomic data showed that the expression of three BnaNPR1s was significantly down-regulated in response to infection with Sclerotinia sclerotiorum, the most important pathogen of this crop, whereas BnaNPR2/3/4/5/6s did not show the expression differences in general. Further, we generated B. napus BnaNPR1-RNAi lines to interpret the effect of the down-regulated expression of BnaNPR1s on resistance to S. sclerotiorum. The results showed that BnaNPR1-RNAi significantly decreased this resistance. Further experiments revealed that BnaNPR1-RNAi intensified ROS production and changed defense responses in the interaction of plants with this pathogen. These results indicated that S. sclerotiorum might use BnaNPR1 to regulate specific physiological processes of B. napus, such as ROS production and SA defense response, for the infection.

Arabidopsis GDSL1 overexpression enhances rapeseed Sclerotinia sclerotiorum resistance and the functional identification of its homolog in Brassica napus

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum is a devastating disease of rapeseed (Brassica napus L.). To date, the genetic mechanisms of rapeseed' interactions with S. sclerotiorum are not fully understood, and molecular-based breeding is still the most effective control strategy for this disease. Here, Arabidopsis thaliana GDSL1 was characterized as an extracellular GDSL lipase gene functioning in Sclerotinia resistance. Loss of AtGDSL1 function resulted in enhanced susceptibility to S. sclerotiorum. Conversely, overexpression of AtGDSL1 in B. napus enhanced resistance, which was associated with increased reactive oxygen species (ROS) and salicylic acid (SA) levels, and reduced jasmonic acid levels. In addition, AtGDSL1 can cause an increase in lipid precursor phosphatidic acid levels, which may lead to the activation of downstream ROS/SA defence-related pathways. However, the rapeseed BnGDSL1 with highest sequence similarity to AtGDSL1 had no effect on SSR resistance. A candidate gene association study revealed that only one AtGDSL1 homolog from rapeseed, BnaC07g35650D (BnGLIP1), significantly contributed to resistance traits in a natural B. napus population, and the resistance function was also confirmed by a transient expression assay in tobacco leaves. Moreover, genomic analyses revealed that BnGLIP1 locus was embedded in a selected region associated with SSR resistance during the breeding process, and its elite allele type belonged to a minor allele in the population. Thus, BnGLIP1 is the functional equivalent of AtGDSL1 and has a broad application in rapeseed S. sclerotiorum-resistance breeding.

An update on salicylic acid biosynthesis, its induction and potential exploitation by plant viruses

Salicylic acid (SA) is a plant hormone essential for effective resistance to viral and non-viral pathogens. SA biosynthesis increases rapidly in resistant hosts when a dominant host resistance gene product recognizes a pathogen. SA stimulates resistance to viral replication, intercellular spread and systemic movement. However, certain viruses stimulate SA biosynthesis in susceptible hosts. This paradoxical effect limits virus titer and prevents excessive host damage, suggesting that these viruses exploit SA-induced resistance to optimize their accumulation. Recent work showed that SA production in plants does not simply recapitulate bacterial SA biosynthetic mechanisms, and that the relative contributions of the shikimate and phenylpropanoid pathways to the SA pool differ markedly between plant species.

MdWRKY15 improves resistance of apple to Botryosphaeria dothidea via the salicylic acid-mediated pathway by directly binding the MdICS1 promoter

Isochorismate synthase (ICS) plays an essential role in the accumulation of salicylic acid (SA) and plant disease resistance. Diseases caused by Botryosphaeria dothidea affect apple yields. Thus, it is important to understand the role of ICS1 in disease resistance to B. dothidea in apple. In this study, SA treatment enhanced the resistance to B. dothidea. MdICS1 was induced by B. dothidea and enhanced the resistance to B. dothidea. MdICS1 promoter analysis indicated that the W-box was vital for the response to B. dothidea treatment. MdWRKY15 was found to interact with the W-box using yeast one-hybrid screening. Subsequently, the interaction was confirmed by EMSA, yeast one-hybrid, ChIP-PCR, and quantitative PCR assays. Moreover, luciferase and GUS analysis further indicated that MdICS1 was transcriptionally activated by MdWRKY15. Finally, we found the function of MdWRKY15 in the resistance to B. dothidea was partially dependent on MdICS1 from the phenotype of transgenic apples and calli. In summary, we revealed that MdWRKY15 activated the transcription of MdICS1 by directly binding to its promoter to increase the accumulation of SA and the expression of disease-related genes, thereby resulting in the enhanced resistance to B. dothidea in the SA biosynthesis pathway.

Two SLENDER AND CRINKLY LEAF dioxygenases play an essential role in rice shoot development

It is clear that 2-oxoglutarate-dependent dioxygenases have critical functions in salicylic acid (SA) metabolism in plants, yet their role in SA biosynthesis is poorly understood. Here, we report that two dioxygenase-encoding genes, SLENDER AND CRINKLY LEAF1 (SLC1) and SLC2, play essential roles in shoot development and SA production in rice. Overexpression of SLC1 (SLC1-OE) or SLC2 (SLC2-OE) in rice produced infertile plants with slender and crinkly leaves. Disruption of SLC1 or SLC2 led to dwarf plants, while simultaneous down-regulation of SLC1 and SLC2 resulted in a severe defect in early leaf development. Enhanced SA levels in SLC1-OE plants and decreased SA levels in slc1 and slc2 mutants were observed. Accordingly, these lines all showed altered expression of a set of SA-related genes. We demonstrated that SLC1 interacts with homeobox1 (OSH1), and that either the knotted1-like homeobox (KNOX1) or glutamate, leucine, and lysine (ELK) domain of OSH1 is sufficient for accomplishing this interaction. Collectively, our data reveal the importance of SLC1 and SLC2 in rice shoot development.

Genome-Wide Association Analysis of the Genetic Basis for Sheath Blight Resistance in Rice

BACKGROUND

Sheath blight (ShB), caused by Rhizoctonia solani Kühn, is one of the most destructive rice diseases. Developing ShB-resistant rice cultivars represents the most economical and environmentally sound strategy for managing ShB.

RESULTS

To characterize the genetic basis for ShB resistance in rice, we conducted association studies for traits related to ShB resistance, namely culm length (CL), lesion height (LH), and relative lesion height (RLH). Combined a single locus genome-wide scan and a multi-locus method using 2,977,750 single-nucleotide polymorphisms to analyse 563 rice accessions, we detected 134, 562, and 75 suggestive associations with CL, LH, and RLH, respectively. The adjacent signals associated with RLH were merged into 27 suggestively associated loci (SALs) based on the estimated linkage disequilibrium blocks. More than 44% of detected RLH-SALs harboured multiple QTLs/genes associated with ShB resistance, while the other RLH-SALs were putative novel ShB resistance loci. A total of 261 ShB resistance putative functional genes were screened from 23 RLH-SALs according to bioinformatics and haplotype analyses. Some of the annotated genes were previously reported to encode defence-related and pathogenesis-related proteins, suggesting that quantitative resistance to ShB in rice is mediated by SA- and JA-dependent signalling pathways.

CONCLUSIONS

Our findings may improve the application of germplasm resources as well as knowledge-based ShB management and the breeding of ShB-resistant rice cultivars.

Exercise plasma metabolomics and xenometabolomics in obese, sedentary, insulin-resistant women: impact of a fitness and weight loss intervention

Insulin resistance has wide-ranging effects on metabolism, but there are knowledge gaps regarding the tissue origins of systemic metabolite patterns and how patterns are altered by fitness and metabolic health. To address these questions, plasma metabolite patterns were determined every 5 min during exercise (30 min, ∼45% of V̇o_2peak, ∼63 W) and recovery in overnight-fasted sedentary, obese, insulin-resistant women under controlled conditions of diet and physical activity. We hypothesized that improved fitness and insulin sensitivity following a ∼14-wk training and weight loss intervention would lead to fixed workload plasma metabolomics signatures reflective of metabolic health and muscle metabolism. Pattern analysis over the first 15 min of exercise, regardless of pre- versus postintervention status, highlighted anticipated increases in fatty acid tissue uptake and oxidation (e.g., reduced long-chain fatty acids), diminution of nonoxidative fates of glucose [e.g., lowered sorbitol-pathway metabolites and glycerol-3-galactoside (possible glycerolipid synthesis metabolite)], and enhanced tissue amino acid use (e.g., drops in amino acids; modest increase in urea). A novel observation was that exercise significantly increased several xenometabolites ("non-self" molecules, from microbes or foods), including benzoic acid-salicylic acid-salicylaldehyde, hexadecanol-octadecanol-dodecanol, and chlorogenic acid. In addition, many nonannotated metabolites changed with exercise. Although exercise itself strongly impacted the global metabolome, there were surprisingly few intervention-associated differences despite marked improvements in insulin sensitivity, fitness, and adiposity. These results and previously reported plasma acylcarnitine profiles support the principle that most metabolic changes during submaximal aerobic exercise are closely tethered to absolute ATP turnover rate (workload), regardless of fitness or metabolic health status.

MdWRKY46-Enhanced Apple Resistance to Botryosphaeria dothidea by Activating the Expression of MdPBS3.1 in the Salicylic Acid Signaling Pathway

Salicylic acid (SA) is closely related to disease resistance of plants. WRKY transcription factors have been linked to the growth and development of plants, especially under stress conditions. However, the regulatory mechanism of WRKY proteins involved in SA production and disease resistance in apple is not clear. In this study, MdPBS3.1 responded to Botryosphaeria dothidea and enhanced resistance to B. dothidea. Electrophoretic mobility shift assays, yeast one-hybrid assays, and chromatin immunoprecipitation and quantitative PCR demonstrated that MdWRKY46 can directly bind to a W-box motif in the promoter of MdPBS3.1. Glucuronidase transactivation and luciferase analysis further showed that MdWRKY46 can activate the expression of MdPBS3.1. Finally, B. dothidea inoculation in transgenic apple calli and fruits revealed that MdWRKY46 improved resistance to B. dothidea by the transcriptional activation of MdPBS3.1. Viral vector-based transformation assays indicated that MdWRKY46 elevates SA content and transcription of SA-related genes, including MdPR1, MdPR5, and MdNPR1 in an MdPBS3.1-dependent way. These findings provide new insights into how MdWRKY46 regulates plant resistance to B. dothidea through the SA signaling pathway.

Maize phenylalanine ammonia-lyases contribute to resistance to Sugarcane mosaic virus infection, most likely through positive regulation of salicylic acid accumulation

Sugarcane mosaic virus (SCMV) is a pathogen of worldwide importance that causes dwarf mosaic disease on maize (Zea mays). Until now, few maize genes/proteins have been shown to be involved in resistance to SCMV. In this study, we characterized the role of maize phenylalanine ammonia-lyases (ZmPALs) in accumulation of the defence signal salicylic acid (SA) and in resistance to virus infection. SCMV infection significantly increased SA accumulation and expression of SA-responsive pathogenesis-related protein genes (PRs). Interestingly, exogenous SA treatment decreased SCMV accumulation and enhanced resistance. Both reverse transcription-coupled quantitative PCR and RNA-Seq data confirmed that expression levels of at least four ZmPAL genes were significantly up-regulated upon SCMV infection. Knockdown of ZmPAL expression led to enhanced SCMV infection symptom severity and virus multiplication, and simultaneously resulted in decreased SA accumulation and PR gene expression. Intriguingly, application of exogenous SA to SCMV-infected ZmPAL-silenced maize plants decreased SCMV accumulation, showing that ZmPALs are required for SA-mediated resistance to SCMV infection. In addition, lignin measurements and metabolomic analysis showed that ZmPALs are also involved in SCMV-induced lignin accumulation and synthesis of other secondary metabolites via the phenylpropanoid pathway. In summary, our results indicate that ZmPALs are required for SA accumulation in maize and are involved in resistance to virus infection by limiting virus accumulation and moderating symptom severity.

Identification of Regulatory Networks of MicroRNAs and Their Targets in Response to Colletotrichum gloeosporioides in Tea Plant (Camellia sinensis L.)

Anthracnose disease is caused by Colletotrichum gloeosporioides, and is common in leaves of the tea plant (Camellia sinensis). MicroRNAs (miRNAs) have been known as key modulators of gene expression in response to environmental stresses, disease resistance, defense responses, and plant immunity. However, the role of miRNAs in responses to C. gloeosporioides remains unexplored in tea plant. Therefore, in the present study, six miRNA sequencing data sets and two degradome data sets were generated from C. gloeosporioides-inoculated and control tea leaves. A total of 485 conserved and 761 novel miRNAs were identified. Of those, 239 known and 369 novel miRNAs exhibited significantly differential expression under C. gloeosporioides stress. One thousand one hundred thirty-four and 596 mRNAs were identified as targets of 389 conserved and 299 novel miRNAs by degradome analysis, respectively. Based on degradome analysis, most of the predicted targets are negatively correlated with their corresponding conserved and novel miRNAs. The expression levels of 12 miRNAs and their targets were validated by quantitative real-time PCR. A negative correlation between expression profiles of five miRNAs (PC-5p-80764_22, csn-miR160c, csn-miR828a, csn-miR164a, and csn-miR169e) and their targets (WRKY, ARF, MYB75, NAC, and NFY transcription factor) was observed. The predicted targets of five interesting miRNAs were further validated through 5'RLM-RACE. Furthermore, Gene Ontology and metabolism pathway analysis revealed that most of the target genes were involved in the regulation of auxin pathway, ROS scavenging pathway, salicylic acid mediated pathway, receptor kinases, and transcription factors for plant growth and development as well as stress responses in tea plant against C. gloeosporioides stress. This study enriches the resources of stress-responsive miRNAs and their targets in C. sinensis and thus provides novel insights into the miRNA-mediated regulatory mechanisms, which could contribute to the enhanced susceptibility of C. gloeosporioides in tea plant.

Time-dependent effects of Pochonia chlamydosporia endophytism on gene expression profiles of colonized tomato roots

A transcriptome analysis was produced from tomato roots inoculated with the hyphomycete Pochonia chlamydosporia at three different times. Gene expression data were also yielded from fungus grown in vitro or endophytic. A next-generation sequencing (NGS) and network analysis approach were applied. We identified 3.676 differentially expressed tomato genes (DEG), highlighting a core of 93 transcripts commonly down- or upregulated at every time point, shedding light on endophytism process. Functional categories related to plant information-processing system, which recognizes, percepts, and transmits signals, were associated with gene upregulated early in time, with higher representations in processes such as plant defense regulation later in time. Network analysis of a DEG subset showed dominance of MAP kinase hubs in the uninoculated control samples, replaced by an increased centrality of WRKY transcription factor and ETR-ethylene response factor genes in the colonized roots. Fungus genes expressed during progression of plant colonization, therefore related to the host colonization process or endophytism persistence, were also identified. Data provided a high-resolution insight on tomato transcriptome changes as induced by endophytism, highlighting a specific modulation of stress-responsive transcripts, related to a selective activation of defense pathways, likely required by the fungus to establish a persistent endophytic lifestyle.

Impaired PSII Proteostasis Promotes Retrograde Signaling via Salicylic Acid

Photodamage of the PSII reaction center (RC) is an inevitable process in an oxygen-rich environment. The damaged PSII RC proteins (Dam-PSII) undergo degradation via the thylakoid membrane-bound FtsH metalloprotease, followed by posttranslational assembly of PSII. While the effect of Dam-PSII on gene regulation is described for cyanobacteria, its role in land plants is largely unknown. In this study, we reveal an intriguing retrograde signaling pathway by using the Arabidopsis (Arabidopsis thaliana) yellow variegated2-9 mutant, which expresses a mutated FtsH2 (FtsH2G267D) metalloprotease, specifically impairing its substrate-unfolding activity. This lesion leads to the perturbation of PSII protein homeostasis (proteostasis) and the accumulation of Dam-PSII. Subsequently, this results in an up-regulation of salicylic acid (SA)-responsive genes, which is abrogated by inactivation of either an SA transporter in the chloroplast envelope membrane or extraplastidic SA signaling components as well as by removal of SA. These results suggest that the stress hormone SA, which is mainly synthesized via the chloroplast isochorismate pathway in response to the impaired PSII proteostasis, mediates the retrograde signaling. These findings reinforce the emerging view of chloroplast function toward plant stress responses and suggest SA as a potential plastid factor mediating retrograde signaling.

Identification of Fusarium graminearum-responsive miRNAs and their targets in wheat by sRNA sequencing and degradome analysis

Fusarium head blight (FHB), a prevalent disease of bread wheat (Triticum aestivum L.) caused by Fusarium graminearum, leads to considerable losses of yield and quality in wheat production. MicroRNAs (miRNAs) are important regulators of plant defense responses. Here, to better understand the F. graminearum-responsive miRNAs, we constructed sRNA libraries for wheat cultivar Sumai 3 challenged with F. graminearum and sterile water, respectively. As a result, a total of 203 known miRNAs from 46 families and 68 novel miRNAs were identified. Among them, 18 known and six novel miRNAs were found to be differentially expressed between the F. graminearum-infected samples and the controls and thus were considered to be responsive to F. graminearum. The expression patterns of eight miRNAs were further validated by stem-loop qRT-PCR. Meanwhile, target genes were validated by degradome sequencing. Integrative analysis of the differentially expressed miRNAs and their targets revealed complex miRNA-mediated regulatory networks involved in the response of wheat to F. graminearum infection. Our findings are expected to facilitate a better understanding of the miRNA regulation in wheat-F. graminearum interaction.

trans-Cinnamic acid-induced leaf expansion involves an auxin-independent component

The phenylpropanoid pathway, the source of a large array of compounds with diverse functions, starts with the synthesis of trans-cinnamic acid (t-CA) that is converted by cinnamate-4-hydroxylase (C4H) into p-coumaric acid. We have recently shown that in Arabidopsis, exogenous t-CA promotes leaf growth by increasing cell expansion and that this response requires auxin signaling. We have also shown that cell expansion is increased in C4H loss-of-function mutants. Here we provide further evidence that leaf growth is enhanced by either t-CA or a t-CA derivative that accumulates upstream of C4H. We also show that this growth response pathway has two components: one that requires auxin signaling and another which employs a currently unknown mechanism.

Rhizosphere microbiomes diverge among Populus trichocarpa plant-host genotypes and chemotypes, but it depends on soil origin

BACKGROUND

Plants have developed defense strategies for phytopathogen and herbivore protection via coordinated metabolic mechanisms. Low-molecular weight metabolites produced within plant tissues, such as salicylic acid, represent one such mechanism which likely mediates plant - microbe interactions above and below ground. Salicylic acid is a ubiquitous phytohormone at low levels in most plants, yet are concentrated defense compounds in Populus, likely acting as a selective filter for rhizosphere microbiomes. We propagated twelve Populus trichocarpa genotypes which varied an order of magnitude in salicylic acid (SA)-related secondary metabolites, in contrasting soils from two different origins. After four months of growth, plant properties (leaf growth, chlorophyll content, and net photosynthetic rate) and plant root metabolomics specifically targeting SA metabolites were measured via GC-MS. In addition, rhizosphere microbiome composition was measured via Illumina MiSeq sequencing of 16S and ITS2 rRNA-genes.

RESULTS

Soil origin was the primary filter causing divergence in bacterial/archaeal and fungal communities with plant genotype secondarily influential. Both bacterial/archaeal and fungal evenness varied between soil origins and bacterial/archaeal diversity and evenness correlated with at least one SA metabolite (diversity: populin; evenness: total phenolics). The production of individual salicylic acid derivatives that varied by host genotype resulted in compositional differences for bacteria /archaea (tremuloidin) and fungi (salicylic acid) within one soil origin (Clatskanie) whereas soils from Corvallis did not illicit microbial compositional changes due to salicylic acid derivatives. Several dominant bacterial (e.g., Betaproteobacteria, Acidobacteria, Verrucomicrobia, Chloroflexi, Gemmatimonadete, Firmicutes) and one fungal phyla (Mortierellomycota) also correlated with specific SA secondary metabolites; bacterial phyla exhibited more negative interactions (declining abundance with increasing metabolite concentration) than positive interactions.

CONCLUSIONS

These results indicate microbial communities diverge most among soil origin. However, within a soil origin, bacterial/archaeal communities are responsive to plant SA production within greenhouse-based rhizosphere microbiomes. Fungal microbiomes are impacted by root SA-metabolites, but overall to a lesser degree within this experimental context. These results suggest plant defense strategies, such as SA and its secondary metabolites, may partially drive patterns of both bacterial/archaeal and fungal taxa-specific colonization and assembly.

Plant Hormones in Phytoplasma Infected Plants

Phytoplasmas are bacterial plant pathogens that need a plant host and an insect vector for their spread and survival. In plants, the physiological responses that phytoplasmas trigger result in symptom development through effects on hormonal, nutritional, and stress signaling pathways, and the interactions between these. In this review, recent advances on the involvement of plant hormones together with their known and deduced roles in plants infected with phytoplasmas are discussed. Several studies have directly, or in many cases indirectly, addressed plant hormone systems in phytoplasma-infected plants. These have provided accumulating evidence that phytoplasmas extensively affect plant hormone pathways. Phytoplasmas thus, with disturbing complex plant hormone networks, suppress plant immunity and modify plant structure, while optimizing their nutrient acquisition and facilitating their colonization of the plants, and their dissemination among plants by their insect vectors.

Plant defense signals: Players and pawns in plant-virus-vector interactions

Plant viruses face an array of host defenses. Well-studied responses that protect against viruses include effector-triggered immunity, induced resistance (such as systemic acquired resistance mediated by salicylic acid), and RNA silencing. Recent work shows that viruses are also affected by non-host resistance mechanisms; previously thought to affect only bacteria, oomycetes and fungi. However, an enduring puzzle is how viruses are inhibited by several inducible host resistance mechanisms. Many viruses have been shown to encode factors that inhibit antiviral silencing. A number of these, including the cucumoviral 2b protein, the poytviral P1/HC-Pro and, respectively, geminivirus or satellite DNA-encoded proteins such as the C2 or βC1, also inhibit defensive signaling mediated by salicylic acid and jasmonic acid. This helps to explain how viruses can, in some cases, overcome host resistance. Additionally, interference with defensive signaling provides a means for viruses to manipulate plant-insect interactions. This is important because insects, particularly aphids and whiteflies, transmit many viruses. Indeed, there is now substantial evidence that viruses can enhance their own transmission through their effects on hosts. Even more surprisingly, it appears that viruses may be able to manipulate plant interactions with beneficial insects by, for example, 'paying back' their hosts by attracting pollinators.

BnaMPK3 Is a Key Regulator of Defense Responses to the Devastating Plant Pathogen Sclerotinia sclerotiorum in Oilseed Rape

The disease caused by Sclerotinia sclerotiorum has traditionally been difficult to control, resulting in tremendous economic losses in oilseed rape (Brassica napus). Identification of important genes in the defense responses is critical for molecular breeding, an important strategy for controlling the disease. Here, we report that a B. napus mitogen-activated protein kinase gene, BnaMPK3, plays an important role in the defense against S. sclerotiorum in oilseed rape. BnaMPK3 is highly expressed in the stems, flowers and leaves, and its product is localized in the nucleus. Furthermore, BnaMPK3 is highly responsive to infection by S. sclerotiorum and treatment with jasmonic acid (JA) or the biosynthesis precursor of ethylene (ET), but not to treatment with salicylic acid (SA) or abscisic acid. Moreover, overexpression (OE) of BnaMPK3 in B. napus and Nicotiana benthamiana results in significantly enhanced resistance to S. sclerotiorum, whereas resistance is diminished in RNAi transgenic plants. After S. sclerotiorum infection, defense responses associated with ET, JA, and SA signaling are intensified in the BnaMPK3-OE plants but weakened in the BnaMPK3-RNAi plants when compared to those in the wild type plants; by contrast the level of both H₂O₂ accumulation and cell death exhibits a reverse pattern. The candidate gene association analyses show that the BnaMPK3-encoding BnaA06g18440D locus is a cause of variation in the resistance to S. sclerotiorum in natural B. napus population. These results suggest that BnaMPK3 is a key regulator of multiple defense responses to S. sclerotiorum, which may guide the resistance improvement of oilseed rape and related economic crops.