Integrated Analysis of Transcriptome and Metabolome Reveals Differential Responses to Alternaria brassicicola Infection in Cabbage (Brassica oleracea var. capitata)

Black spot, caused by Alternaria brassicicola (Ab), poses a serious threat to crucifer production, and knowledge of how plants respond to Ab infection is essential for black spot management. In the current study, combined transcriptomic and metabolic analysis was employed to investigate the response to Ab infection in two cabbage (Brassica oleracea var. capitata) genotypes, Bo257 (resistant to Ab) and Bo190 (susceptible to Ab). A total of 1100 and 7490 differentially expressed genes were identified in Bo257 (R_mock vs. R_Ab) and Bo190 (S_mock vs. S_Ab), respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that "metabolic pathways", "biosynthesis of secondary metabolites", and "glucosinolate biosynthesis" were the top three enriched KEGG pathways in Bo257, while "metabolic pathways", "biosynthesis of secondary metabolites", and "carbon metabolism" were the top three enriched KEGG pathways in Bo190. Further analysis showed that genes involved in extracellular reactive oxygen species (ROS) production, jasmonic acid signaling pathway, and indolic glucosinolate biosynthesis pathway were differentially expressed in response to Ab infection. Notably, when infected with Ab, genes involved in extracellular ROS production were largely unchanged in Bo257, whereas most of these genes were upregulated in Bo190. Metabolic profiling revealed 24 and 56 differentially accumulated metabolites in Bo257 and Bo190, respectively, with the majority being primary metabolites. Further analysis revealed that dramatic accumulation of succinate was observed in Bo257 and Bo190, which may provide energy for resistance responses against Ab infection via the tricarboxylic acid cycle pathway. Collectively, this study provides comprehensive insights into the Ab-cabbage interactions and helps uncover targets for breeding Ab-resistant varieties in cabbage.

Comparative transcriptome profiling reveals differential defense responses among Alternaria brassicicola resistant Sinapis alba and susceptible Brassica rapa

Alternaria blight is a devastating disease that causes significant crop losses in oilseed Brassicas every year. Adoption of conventional breeding to generate disease-resistant varieties has so far been unsuccessful due to the lack of suitable resistant source germplasms of cultivated Brassica spp. A thorough understanding of the molecular basis of resistance, as well as the identification of defense-related genes involved in resistance responses in closely related wild germplasms, would substantially aid in disease management. In the current study, a comparative transcriptome profiling was performed using Illumina based RNA-seq to detect differentially expressed genes (DEGs) specifically modulated in response to Alternaria brassicicola infection in resistant Sinapis alba, a close relative of Brassicas, and the highly susceptible Brassica rapa. The analysis revealed that, at 48 hpi (hours post inoculation), 3396 genes were upregulated and 23239 were downregulated, whereas at 72 hpi, 4023 genes were upregulated and 21116 were downregulated. Furthermore, a large number of defense response genes were detected to be specifically regulated as a result of Alternaria infection. The transcriptome data was validated using qPCR-based expression profiling for selected defense-related DEGs, that revealed significantly higher fold change in gene expression in S. alba when compared to B. rapa. Expression of most of the selected genes was elevated across all the time points under study with significantly higher expression towards the later time point of 72 hpi in the resistant germplasm. S. alba activates a stronger defense response reaction against the disease by deploying an array of genes and transcription factors involved in a wide range of biological processes such as pathogen recognition, signal transduction, cell wall modification, antioxidation, transcription regulation, etc. Overall, the study provides new insights on resistance of S. alba against A. brassicicola, which will aid in devising strategies for breeding resistant varieties of oilseed Brassica.

Conserved copper regulation of the antimicrobial isocyanide brassicicolin A in Alternaria brassicicola

Phytopathogenic Alternaria species are renown for production of toxins that contribute to virulence on host plants. Typically, these toxins belong to well-known secondary metabolite chemical classes including polyketides, non-ribosomal peptides and terpenes. However, the purported host toxin brassicicolin A produced by A. brassicicola is an isocyanide, a chemical class whose genetics and encoding gene structure is largely unknown. The chemical structure of brassicicolin A shows it to have similarity to the recently characterized fumicicolins derived from the Aspergillus fumigatus isocyanide synthase CrmA. Examination of the A. brassicicola genome identified AbcrmA, a putative homolog with 64% identity to A. fumigatus CrmA. Deletion of AbcrmA resulted in loss of production of brassicicolin A. Contrary to reports that brassicicolin A is a host-specific toxin, the ΔAbcrmA mutants were equally virulent as the wildtype on Brassica hosts. However, in line with results of A. fumigatus CrmA generated metabolites, we find that brassicicolin A increased 360-fold under copper limited conditions. Also, like A. fumigatus CrmA derived metabolites, we find brassicicolin A to be a broad-spectrum antimicrobial. We speculate that CrmA-like isocyanide synthase products provide the producing fungi a fitness advantage in copper depleted environments.

Three new amide derivatives from the fungus Alternaria brassicicola

Three new amide derivatives (alteralkaloids A-C, 1-3) and three known alkaloids (4-6) were afforded after phytochemical investigation of fungus Alternaria brassicicola. The structures of these compounds were confirmed by NMR spectroscopic and HRESIMS data. Furthermore, the absolute configuration of 1 was determined using the single-crystal X-ray diffraction analysis. Compounds 1-3 belong to a class of amide derivatives that have not been found in nature before, sharing the same characteristic signals of the butyl moiety and amide group. These isolated compounds mentioned above were tested for the cytotoxic activity.

Comparative Transcriptome and Targeted Metabolome Profiling Unravel the Key Role of Phenylpropanoid and Glucosinolate Pathways in Defense against Alternaria brassicicola in Broccoli

Alternaria brassicicola (Ab) can cause a major yield and quality-limiting disease of Brassica oleracea called black spot, and the genetic resources conferring complete resistance against Ab have not been identified to date. Here, comparative transcriptome and targeted metabolome analysis were performed utilizing a newly identified resistant (R) line and a broccoli susceptible (S) line at 6, 24, and 72 h post-inoculation (hpi). Kyoto encyclopedia of genes and genomes pathway enrichment and the weighted gene co-expression network analyses showed that the phenylpropanoid pathway regulates the resistance to Ab in broccoli. One metabolite, cinnamic acid, was significantly upregulated in the Ab_inoculated R line compared with the mock treatment but no significant difference in the S line, indicating that the cinnamic acid may cause the resistance difference between R and S lines. Our results also revealed that three indolic glucosinolates of I3G, 4MI3G, and 1MI3G were significantly increased in the Ab_inoculated R line compared with the mock treatment, and some related genes were differentially expressed between the R and S lines. These results provided new insights into the mechanism of Ab defense in B. oleracea and have laid a theoretical foundation for effectively utilizing resistant germplasm resources in broccoli breeding.

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.

The Significance of Mycoparasitism by Streptomyces sp. MBCN152-1 for Its Biocontrol Activity against Alternaria brassicicola

Streptomyces sp. strain MBCN152-1, isolated from cabbage, has potential as a biocontrol agent for Alternaria brassicicola on cabbage seedlings. The present study examined its mode of action. Light microscopy showed that appressorium formation by A. brassicicola was significantly suppressed on cabbage seedlings bacterized with MBCN152-1. Furthermore, scanning electron microscopy revealed that the mycelia of MBCN152-1, which were epiphytically growing on the cotyledon leaves of cabbage seedlings, intensively coiled around the germinating conidia of A. brassicicola. In vitro co-culture experiments demonstrated that MBCN152-1 is an aggressive mycoparasite of A. brassicicola, but not of A. brassicae or Colletotrichum higginsianum. Biocontrol experiments indicated that MBCN152-1 did not control diseases caused by A. brassicae or C. higginsianum. These results suggest that mycoparasitism is the primary mode of action for MBCN152-1. This is the first study to clearly demonstrate the significance of mycoparasitism in the biocontrol efficacy of endophytic Streptomyces.

The Ca2+ sensor proteins CML37 and CML42 antagonistically regulate plant stress responses by altering phytohormone signals

Calmodulin-like-proteins (CML) belong to a family of calcium-sensing proteins that are unique for plants and involved in many different developmental and stress-related reactions. In defense against herbivory, some pathogens and drought, CML37 acts as a positive and CML42 as a negative regulator, respectively. We provide evidence that both CMLs act antagonistically in the regulation of induced defense responses. A double knock-out line, cml37 x cml42, thus shows wild-type phenotypes upon all kind of stresses we used. A transient increase in the cytosolic calcium concentration is one of the first reactions that can be measured in plant cells upon abiotic as well as biotic stress treatments. These calcium signals are sensed by calcium binding proteins such as calmodulin-like proteins (CMLs), which transduce the sensed information into appropriate stress responses by interacting with downstream target proteins. In previous studies, CML37 has been shown to positively regulate the plants' defense against both the insect herbivore Spodoptera littoralis and the response to drought stress. In contrast, CML42 is known to negatively regulate those two stress responses. Here, we provide evidence that these two CMLs act antagonistically in the regulation of induced responses directed against drought and herbivory stress as well as in the defense against the necrotrophic pathogen Alternaria brassicicola. Both CMLs shape the plant reactions by altering the phytohormone signaling. Consequently, the phytohormone-regulated production of defensive compounds like glucosinolates is also antagonistically mediated by both CMLs. The finding that CML37 and CML42 have antagonistic roles in diverse stress-related responses suggests that these calcium sensor proteins represent important tools for the plant to balance and fine-tune the signaling and downstream reactions upon environmental stress.

WRKY33-mediated indolic glucosinolate metabolic pathway confers resistance against Alternaria brassicicola in Arabidopsis and Brassica crops

The tryptophan (Trp)-derived plant secondary metabolites, including camalexin, 4-hydroxy-indole-3-carbonylnitrile, and indolic glucosinolate (IGS), show broad-spectrum antifungal activity. However, the distinct regulations of these metabolic pathways among different plant species in response to fungus infection are rarely studied. In this study, our results revealed that WRKY33 directly regulates IGS biosynthesis, notably the production of 4-methoxyindole-3-ylmethyl glucosinolate (4MI3G), conferring resistance to Alternaria brassicicola, an important pathogen which causes black spot in Brassica crops. WRKY33 directly activates the expression of CYP81F2, IGMT1, and IGMT2 to drive side-chain modification of indole-3-ylmethyl glucosinolate (I3G) to 4MI3G, in both Arabidopsis and Chinese kale (Brassica oleracea var. alboglabra Bailey). However, Chinese kale showed a more severe symptom than Arabidopsis when infected by Alternaria brassicicola. Comparative analyses of the origin and evolution of Trp metabolism indicate that the loss of camalexin biosynthesis in Brassica crops during evolution might attenuate the resistance of crops to Alternaria brassicicola. As a result, the IGS metabolic pathway mediated by WRKY33 becomes essential for Chinese kale to deter Alternaria brassicicola. Our results highlight the differential regulation of Trp-derived camalexin and IGS biosynthetic pathways in plant immunity between Arabidopsis and Brassica crops.

Synthesis and antifungal activity of the proposed structure of a volatile compound isolated from the edible mushroom Hypsizygus marmoreus

We synthesized the proposed structure of an antifungal compound detected in the culture broth of the edible mushroom Hypsizygus marmoreus. Using the Evans aldol and Abiko-Masamune aldol reactions as the key steps, we synthesized all of the stereoisomers of the compound with high stereoselectivity. The GC retention times and the fragmentation patterns in the mass spectra of the synthesized isomers did not match those of the natural product. Therefore, this result may imply that it is necessary to reisolate the natural product and reconsider its structure. All of the synthesized isomers were found to exhibit antifungal activity against the phytopathogenic fungus Alternaria brassicicola. Due to their simple structures, the obtained isomers could be lead compounds for new pesticides.

New α-pyrone derivatives with herbicidal activity from the endophytic fungus Alternaria brassicicola

Three pairs of undescribed enantiomeric α-pyrone derivatives (1a/1b-3a/3b) and six undescribed congeners (4-9), were obtained from the fungus Alternaria brassicicola that was isolated from the fresh leaves of Siegesbeckia pubescens Makino (Compositae). The structures of these new compounds were characterized by extensive NMR spectroscopic and HRESIMS data, and their absolute configurations were further elucidated by a modified Mosher's method, chemical conversion, single-crystal X-ray diffraction analysis, and ECD calculations. This is the first report of three pairs of enantiomeric α-pyrone derivatives from the fungus A. brassicicola, and these enantiomers were successfully acquired from scalemic mixtures via chiral HPLC. Compounds 1a/1b-3a/3b and 4-9 were evaluated for the herbicidal activity against Echinochloa crusgalli, Setaria viridis, Portulaca oleracea, and Taraxacum mongolicum. At a concentration of 100 μg/mL, compounds 1a and 1b could significantly inhibit the germination of monocotyledon weed seeds (E. crusgalli and S. viridis) with inhibitory ratios ranging from 68.6 ± 6.4% to 84.2 ± 5.1%, which was equivalent to that of the positive control (glyphosate). The potential structure-herbicidal activity relationship of these compounds was also discussed. To a certain extent, the results of this study will attract great interest for the potential practical application of promising fungal metabolites, α-pyrone derivatives, as ecofriendly herbicides.

Electrohydraulic Streamer Discharge Plasma-Enhanced Alternaria brassicicola Disinfection in Seed Sterilization

As emerging chemical-free and eco-friendly technologies, nonthermal (gas discharge) plasma and (liquid phase) plasma-activated water (PAW) offer exceptional microbial disinfection solutions for biological, medical, environmental, and agricultural applications. Herein, we present electrohydraulic streamer discharge plasma (ESDP), which combines streamer discharge plasma (SDP) and PAW generated at a gas-liquid interface, to sterilize Chinese kale (Brassica oleracea var. alboglabra) seeds contaminated with Alternaria brassicicola (A. brassicicola). The results showed that the ESDP treatment of A. brassicicola-inoculated seeds provides a ∼75% reduction of A. brassicicola (incident percentage) compared with nontreated seeds. Likewise, the healthy seedling percentage of the plasma-treated seeds was significantly improved to ∼70%, while that of the nontreated seeds remained at ∼15%. A microscopic examination was performed, and it confirmed that ESDP can damage the A. brassicicola spores attached to Chinese kale seeds and lead to severe morphological abnormalities after treatment. Also, an electric field simulation was performed, and it indicated that the strongly localized electric field at the liquid-gas interface on the seed surface boundary had initiated local breakdown of the gas at the air-liquid interface, resulting in exceptional physical-chemical reactions for antimicrobial efficacy beyond typical plasma treatments. Moreover, the optical emission spectra and physicochemical properties (pH, conductivity, and oxidation-reduction potential) showed that inactivation is mainly associated with the reactive oxygen-nitrogen species in the liquid and gas phases. We believe that this work is of great interest when using electrical discharge plasma on liquid interfaces in food, agricultural, and medical industries.

Overexpression of LYK4, a lysin motif receptor with non-functional kinase domain, enhances tolerance to Alternaria brassicicola and increases trichome density in Brassica juncea

Lysin motif receptor-like kinases (LYKs) are involved in the recognition of chitin and activation of plant immune response. In this study, we found LYK4 to be strongly induced in resistant Sinapis alba compared with susceptible Brassica juncea on challenge with Alternaria brassicicola. In silico analysis and in vitro kinase assay revealed that despite the presence of canonical protein kinase fold, B.juncea LYK4 (BjLYK4) lacks several key residues of a prototype protein kinase which renders it catalytically inactive. Transient expression analysis confirmed that fluorescently tagged BjLYK4 localizes specifically to the plasma membrane. Overexpression (OE) of BjLYK4 in B. juncea enhanced tolerance against A. brassicicola. Interestingly, the OE lines also exhibited a novel trichome dense phenotype and increased jasmonic acid (JA) responsiveness. We further showed that many chitin responsive WRKY transcription factors and JA biosynthetic genes were strongly induced in the OE lines on challenge with the pathogen. Moreover, several JA inducible trichome developmental genes constituting the WD-repeat/bHLH/MYB activator complex were also upregulated in the OE lines compared with vector control and RNA interference line. These results suggest that BjLYK4 plays an essential role in chitin-dependent activation of defense response and chitin independent trichome development likely by influencing the JA signaling pathway.

Suppression of Alternaria brassicicola infection by volatile compounds from spent mushroom substrates

Most commercially circulating mushrooms are produced via cultivation using artificially produced mushroom substrates. However, after mushroom harvesting, the disposal of spent mushroom substrates (SMSs) is a serious problem for the mushroom industry owing to the need for a disposal site and the cost involved. Thus, in view of the possibility of recycling SMSs as a soil modifier, we examined the effect of soil mixed with SMSs on the infection of Arabidopsis leaves by Alternaria brassicicola, the causal agent of cabbage leaf spot. The mixing of SMSs used for Hypsizygus marmoreus, Pholiota microspora, Lyophyllum decastes, and Auricularia polytricha into culture soil suppressed the lesion formation caused by A. brassicicola. The defense responses of Arabidopsis were not induced by the culturing of these seedlings in soils containing SMSs. Suppressed lesion formation was observed after the seedlings were treated with volatiles emitted from SMSs that were incubated with soil for 7 days and used for H. marmoreus, P. microspora, L. decastes, A. polytricha, Lentinula edodes, and Cyclocybe cylindracea. The volatiles from the SMSs reduced the elongation of A. brassicicola hyphae. GC-MS analyses of extracts from the SMS containing soils led to the detection of various volatile compounds; among these, skatole, 2,4-di-tert-butylphenol, γ-dodecalactone, butyric acid, guaiacol, 6-amyl-2-pyrone, and 1-octen-3-ol were examined for inhibitory activity on A. brassicicola and found to suppress hyphae elongation. These findings indicate that the antifungal volatile compounds emitted by the SMSs suppress A. brassicicola infection.

Metapopulation Structure Predicts Population Dynamics in the Cakile maritima-Alternaria brassicicola Host-Pathogen Interaction

AbstractIn symbiotic interactions, spatiotemporal variation in the distribution or population dynamics of one species represents spatial and temporal heterogeneity of the landscape for the other. Such interdependent demographic dynamics result in situations where the relative importance of biotic and abiotic factors in determining ecological processes is complicated to decipher. Using a detailed survey of three metapopulations of the succulent plant Cakile maritima and the necrotrophic fungus Alternaria brassicicola located along the southeastern Australian coast, we developed a series of statistical analyses-namely, synchrony analysis, patch occupancy dynamics, and a spatially explicit metapopulation model-to understand how habitat quality, weather conditions, dispersal, and spatial structure determine metapopulation dynamics. Climatic conditions are important drivers, likely explaining the high synchrony among populations. Host availability, landscape features facilitating dispersal, and habitat conditions also impact the occurrence and spread of disease. Overall, we show that the collection of extensive data on host and pathogen population dynamics, in combination with spatially explicit epidemiological modeling, makes it possible to accurately predict disease dynamics-even when there is extreme variability in host population dynamics. Finally, we discuss the importance of genetic information for predicting demographic dynamics in this pathosystem.

Sample Preparation for Culture-Independent Profiling and Isolation of Phyllosphere Bacteria to Identify Potential Biopesticides

Studying the plant phyllosphere to understand inhibition patterns to the growth of fungal foliar pathogens by using the Arabidopsis thaliana pathosystem offers unique opportunities for evaluating strategies for plant protection against foliar diseases. The wide array of bacteria inhabiting the phylloplane of plants has been researched to a much lesser extent compared to the bacteria in the rhizosphere. This difference is especially evident as bacteria derived from the aerial section of plants are rarely used in formulations of foliage sprays against pathogens and pests. In this chapter we outline easy and reliable methods for sample preparation to profile phyllosphere bacteria using high throughput amplicon sequencing and isolate/characterize potentially beneficial phyllosphere bacteria from Arabidopsis thaliana that inhibit in vitro the growth of foliar pathogens such as Alternaria brassicicola. The use of the described methods for profiling and screening phyllosphere bacteria may provide tangible progress on the discovery of new potential biological control agents against agriculturally important pathogens.

Prior exposure of Arabidopsis seedlings to mechanical stress heightens jasmonic acid-mediated defense against necrotrophic pathogens

BACKGROUND

Prolonged mechanical stress (MS) causes thigmomorphogenesis, a stress acclimation response associated with increased disease resistance. What remains unclear is if; 1) plants pre-exposed to a short period of repetitive MS can prime defence responses upon subsequent challenge with necrotrophic pathogens, 2) MS mediates plant immunity via jasmonic acid (JA) signalling, and 3) a short period of repetitive MS can cause long-term changes in gene expression resembling a stress-induced memory. To address these points, 10-days old juvenile Arabidopsis seedlings were mechanically stressed for 7-days using a soft brush and subsequently challenged with the necrotrophic pathogens, Alternaria brassicicola, and Botrytis cinerea. Here we assessed how MS impacted structural cell wall appositions, disease symptoms and altered gene expression in response to infection.

RESULTS

The MS-treated plants exhibited enhanced cell wall appositions and jasmonic acid (JA) accumulation that correlated with a reduction in disease progression compared to unstressed plants. The expression of genes involved in JA signalling, callose deposition, peroxidase and phytoalexin biosynthesis and reactive oxygen species detoxification were hyper-induced 4-days post-infection in MS-treated plants. The loss-of-function in JA signalling mediated by the JA-insensitive coronatine-insensitive 1 (coi1) mutant impaired the hyper-induction of defense gene expression and promoted pathogen proliferation in MS-treated plants subject to infection. The basal expression level of PATHOGENESIS-RELATED GENE 1 and PLANT DEFENSIN 1.2 defense marker genes were constitutively upregulated in rosette leaves for 5-days post-MS, as well as in naïve cauline leaves that differentiated from the inflorescence meristem well after ceasing MS.

CONCLUSION

This study reveals that exposure of juvenile Arabidopsis plants to a short repetitive period of MS can alter gene expression and prime plant resistance upon subsequent challenge with necrotrophic pathogens via the JA-mediated COI1 signalling pathway. MS may facilitate a stress-induced memory to modulate the plant's response to future stress encounters. These data advance our understanding of how MS primes plant immunity against necrotrophic pathogens and how that could be utilised in sustainable agricultural practices.

Complexity of Brassica oleracea-Alternaria brassicicola Susceptible Interaction Reveals Downregulation of Photosynthesis at Ultrastructural, Transcriptional, and Physiological Levels

Black spot disease, caused by Alternaria brassicicola in Brassica species, is one of the most devastating diseases all over the world, especially since there is no known fully resistant Brassica cultivar. In this study, the visualization of black spot disease development on Brassica oleracea var. capitata f. alba (white cabbage) leaves and subsequent ultrastructural, molecular and physiological investigations were conducted. Inter- and intracellular hyphae growth within leaf tissues led to the loss of host cell integrity and various levels of organelle disintegration. Severe symptoms of chloroplast damage included the degeneration of chloroplast envelope and grana, and the loss of electron denseness by stroma at the advanced stage of infection. Transcriptional profiling of infected leaves revealed that photosynthesis was the most negatively regulated biological process. However, in infected leaves, chlorophyll and carotenoid content did not decrease until 48 hpi, and several chlorophyll a fluorescence parameters, such as photosystem II quantum yield (F_v/F_m), non-photochemical quenching (NPQ), or plant vitality parameter (Rdf) decreased significantly at 24 and 48 hpi compared to control leaves. Our results indicate that the initial stages of interaction between B. oleracea and A. brassicicola are not uniform within an inoculation site and show a complexity of host responses and fungal attempts to overcome host cell defense mechanisms. The downregulation of photosynthesis at the early stage of this susceptible interaction suggests that it may be a part of a host defense strategy, or, alternatively, that chloroplasts are targets for the unknown virulence factor(s) of A. brassicicola. However, the observed decrease of photosynthetic efficiency at the later stages of infection is a result of the fungus-induced necrotic lesion expansion.

Fusicoccane-derived diterpenoids with bridgehead double-bond-containing tricyclo[9.2.1.03,7]tetradecane ring systems from Alternaria brassicicola

Fusicoccane-derived diterpenoids bearing a unique bridgehead double-bond-containing tricyclo[9.2.1.0^3,7]tetradecane (5-9-5 ring system) core skeleton represent a rarely reported class of rearranged terpenoids, which traced back to fusicoccanes with a classical dicyclopenta[a,d]cyclooctane (5-8-5 ring system) core skeleton via a crucial Wagner-Meerwein rearrangement reaction. In this research, alterbrassicenes B-D (1-3), three new rearranged fusicoccane diterpenoids bearing a rare bridgehead double-bond-containing tricyclo[9.2.1.0^3,7]tetradecane core skeleton, together with two known congeners, brassicicenes O and K (4 and 5), were isolated from the modified cultures of fungus Alternaria brassicicola. Their structures were elucidated by comprehensive analyses of the NMR and HRESIMS data, and the absolute configurations of 1 and 4 were further confirmed via a combination of ¹³C NMR and ECD calculations and single-crystal X-ray diffraction analysis (Cu Kα). Interestingly, alterbrassicene B (1) represented the second case of bridgehead C-10-C-11 double-bond-containing natural products with a bicyclo[6.2.1]undecane core skeleton, and also featured an undescribed oxygen bridge between C-6 and C-14 to construct an unprecedentedly caged tetracyclic system. Alterbrassicenes B-D showed moderate cytotoxic activity against certain human tumor cell lines with IC₅₀ values in the range of 15.87-36.85 μM.

Overexpression of a Monocot Acyl-CoA-Binding Protein Confers Broad-Spectrum Pathogen Protection in a Dicot

Plants are continuously infected by various pathogens throughout their lifecycle. Previous studies have reported that the expression of Class III acyl-CoA-binding proteins (ACBPs) such as the Arabidopsis ACBP3 and rice ACBP5 were induced by pathogen infection. Transgenic Arabidopsis AtACBP3-overexpressors (AtACBP3-OEs) displayed enhanced protection against the bacterial biotroph, Pseudomonas syringae, although they became susceptible to the fungal necrotroph Botrytis cinerea. A Class III ACBP from a monocot, rice (Oryza sativa) OsACBP5 was overexpressed in the dicot Arabidopsis. The resultant transgenic Arabidopsis lines conferred resistance not only to the bacterial biotroph P. syringae but to fungal necrotrophs (Rhizoctonia solani, B. cinerea, Alternaria brassicicola) and a hemibiotroph (Colletotrichum siamense). Changes in protein expression in R. solani-infected Arabidopsis OsACBP5-overexpressors (OsACBP5-OEs) were demonstrated using proteomic analysis. Biotic stress-related proteins including cell wall-related proteins such as FASCILIN-LIKE ARABINOGALACTAN-PROTEIN10, LEUCINE-RICH REPEAT EXTENSIN-LIKE PROTEINS, XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE PROTEIN4, and PECTINESTERASE INHIBITOR18; proteins associated with glucosinolate degradation including GDSL-LIKE LIPASE23, EPITHIOSPECIFIER MODIFIER1, MYROSINASE1, MYROSINASE2, and NITRILASE1; as well as a protein involved in jasmonate biosynthesis, ALLENE OXIDE CYCLASE2, were induced in OsACBP5-OEs upon R. solani infection. These results indicated that upregulation of these proteins in OsACBP5-OEs conferred protection against various plant pathogens.

Characterization of Alternaria Species Associated with Leaf Spot Disease of Armoracia rusticana in Serbia

Leaf spot diseases caused by Alternaria species have been reported worldwide in plants in the Brassicaceae family. However, there is little information on Alternaria species causing diseases in horseradish. In the present study, 89 Alternaria spp. isolates from Armoracia rusticana, sampled from nine districts in Serbia, were characterized based on their morphology, physiology, and molecular markers. Morphological characterization and molecular analyses based on ITS, GAPDH, Alt a 1, and PM-ATP sequences identified three distinct species associated with the disease: Alternaria brassicae, A. brassicicola, and A. alternata. For all species, growth and sporulation rates at 0, 5, 10, 15, 20, 25, 30, 35, and 40°C showed a quadratic response, with A. alternata having the widest temperature optimum (20 to 30°C) while A. brassicicola had higher optimum temperatures (20 to 25°C) than A. brassicae (15 to 20°C). To gain a better understanding of the pathogenicity of these species, the influence of leaf age, host susceptibility, and ability to infect artificially wounded and nonwounded leaves were tested. The pathogenicity test identified A. brassicicola and A. brassicae as the main causal agents of horseradish leaf spot disease. Results indicated that young and intact leaves of horseradish and cabbage were less susceptible to infection and also suggested the potential for cross-infection between these two hosts. Haplotype networks showed haplotype uniformity for A. brassicae, two haplotype groups of A. brassicicola, and eight haplotype groups of A. alternata in Serbia and suggest the possible association of some haplotypes with the geographic area. This study is the first to investigate Alternaria leaf spot disease on A. rusticana in Serbia and is the first record of A. brassicicola on horseradish in this country.

Phyllosphere Colonization by a Soil Streptomyces sp. Promotes Plant Defense Responses Against Fungal Infection

Streptomycetes are soil-dwelling, filamentous actinobacteria and represent a prominent bacterial clade inside the plant root microbiota. The ability of streptomycetes to produce a broad spectrum of antifungal metabolites suggests that these bacteria could be used to manage plant diseases. Here, we describe the identification of a soil Streptomyces strain named AgN23 which strongly activates a large array of defense responses when applied on Arabidopsis thaliana leaves. AgN23 increased the biosynthesis of salicylic acid, leading to the development of salicylic acid induction deficient 2 (SID2)-dependent necrotic lesions. Size exclusion fractionation of plant elicitors secreted by AgN23 showed that these signals are tethered into high molecular weight complexes. AgN23 mycelium was able to colonize the leaf surface, leading to plant resistance against Alternaria brassicicola infection in wild-type Arabidopsis plants. AgN23-induced resistance was found partially compromised in salicylate, jasmonate, and ethylene mutants. Our data show that Streptomyces soil bacteria can develop at the surface of plant leaves to induce defense responses and protection against foliar fungal pathogens, extending their potential use to manage plant diseases.

Baseline Sensitivity and Action Mechanism of Propamidine Against Alternaria brassicicola, the Causal Agent of Dark Leaf Spot on Cabbage

In the current study, a total of 53 isolates of Alternaria brassicicola collected from Shaanxi Province of China were characterized for their sensitivity to propamidine. The EC₅₀ (50% effective concentration) values for propamidine inhibiting mycelial growth and spore germination ranged from 0.515 to 3.247 µg/ml and 0.393 to 2.982 µg/ml, with average EC₅₀ values of 1.327 ± 0.198 µg/ml and 1.106 ± 0.113 µg/ml, respectively. In greenhouse experiments, propamidine at 100 µg/ml provided >90% efficacy against dark leaf spot on cabbage, which was higher than the efficacy obtained by azoxystrobin at the same concentration. After treatment with propamidine, fungal growth distortions were observed in the form of excess mycelial branching, thickened cell walls, decreased cell membrane permeability, and increased chitin content. Interestingly, colony color faded after treatment with propamidine compared with that of the untreated parental isolates. Importantly, the expressions of melanin biosynthesis-associated genes Amr1, Scd1, Brn1, and Brn2 were downregulated at different levels. The obtained baseline sensitivity and control efficacy data suggested that propamidine inhibited not only growth of A. brassicicola but also melanin biosynthesis, which could reduce the biocompatibility of A. brassicicola in the field. These biological characteristics encourage further investigation of the mechanism of action of propamidine against A. brassicicola.

Enhancement of ABA Sensitivity Through Conditional Expression of the ARF10 Gene in Brassica juncea Reveals Fertile Plants with Tolerance Against Alternaria brassicicola

Concomitant increase of auxin-responsive factors ARF16 and ARF17, along with enhanced expression of ARF10 in resistant Sinapis alba compared with that in susceptible Brassica juncea upon challenge with Alternaria brassicicola, revealed that abscisic acid (ABA)-auxin crosstalk is a critical factor for resistance response. Here, we induced the ABA response through conditional expression of ARF10 in B. juncea using the A. brassicicola-inducible GH3.3 promoter. Induced ABA sensitivity caused by conditional expression of ARF10 in transgenic B. juncea resulted in tolerance against A. brassicicola and led to enhanced expression of several ABA-responsive genes without affecting the auxin biosynthetic gene expression. Compared with ABI3 and ABI4, ABI5 showed maximum upregulation in the most tolerant transgenic lines upon pathogen challenge. Moreover, elevated expression of ARF10 by different means revealed a direct correlation between ARF10 expression and the induction of ABI5 protein in B. juncea. Through in vitro DNA-protein experiments and chromosome immunoprecipitation using the ARF10 antibody, we demonstrated that ARF10 interacts with the auxin-responsive elements of the ABI5 promoter. This suggests that ARF10 may function as a modulator of ABI5 to induce ABA sensitivity and mediate the resistance response against A. brassicicola.

Responses to Hydric Stress in the Seed-Borne Necrotrophic Fungus Alternaria brassicicola

Alternaria brassicicola is a necrotrophic fungus causing black spot disease and is an economically important seed-borne pathogen of cultivated brassicas. Seed transmission is a crucial component of its parasitic cycle as it promotes long-term survival and dispersal. Recent studies, conducted with the Arabidopsis thaliana/A. brassicicola pathosystem, showed that the level of susceptibility of the fungus to water stress strongly influenced its seed transmission ability. In this study, we gained further insights into the mechanisms involved in the seed infection process by analyzing the transcriptomic and metabolomic responses of germinated spores of A. brassicicola exposed to water stress. Then, the repertoire of putative hydrophilins, a group of proteins that are assumed to be involved in cellular dehydration tolerance, was established in A. brassicicola based on the expression data and additional structural and biochemical criteria. Phenotyping of single deletion mutants deficient for fungal hydrophilin-like proteins showed that they were affected in their transmission to A. thaliana seeds, although their aggressiveness on host vegetative tissues remained intact.

An insight into the mechanism of antifungal activity of biogenic nanoparticles than their chemical counterparts

Herein, we describe the enhanced antifungal activity of silver nanoparticles biosynthesized by cell free filtrate of Trichoderma viride (MTCC 5661) in comparison to chemically synthesized silver nanoparticles (CSNP) of similar shape and size. Biosynthesized silver nanoparticles (BSNP) enhanced the reduction in dry weight by 20 and 48.8% of fungal pathogens Fusarium oxysporum and Alternaria brassicicola respectively in comparison to their chemical counterparts (CSNP). Nitroblue tetrazolium and Propidium iodide staining demonstrated the higher generation of superoxide radicals lead to higher death in BSNP treated fungus in comparison to CSNP. Scanning electron microscopy of A. brassicicola revealed the osmotic imbalance and membrane disintegrity to be major cause for fungal cell death after treatment with BSNP. To gain an insight into the mechanistic aspect of enhanced fungal cell death after treatment of BSNP in comparison to CSNP, stress responses and real time PCR analysis was carried out with A. brassicicola. It revealed that generation of ROS, downregulation of antioxidant machinery and oxidative enzymes, disruption of osmotic balance and cellular integrity, and loss of virulence are the mechanisms employed by BSNP which establishes them as superior antifungal agent than their chemical counterparts. With increasing drug resistance and ubiquitous presence of fungal pathogens in plant kingdom, BSNP bears the candidature for new generation of antifungal agent.

The MAP Kinase Kinase Gene AbSte7 Regulates Multiple Aspects of Alternaria brassicicola Pathogenesis

Mitogen-activated protein kinase (MAPK) cascades in fungi are ubiquitously conserved signaling pathways that regulate stress responses, vegetative growth, pathogenicity, and many other developmental processes. Previously, we reported that the AbSte7 gene, which encodes a mitogen-activated protein kinase kinase (MAPKK) in Alternaria brassicicola, plays a central role in pathogenicity against host cabbage plants. In this research, we further characterized the role of AbSte7 in the pathogenicity of this fungus using ΔAbSte7 mutants. Disruption of the AbSte7 gene of A. brassicicola reduced accumulation of metabolites toxic to the host plant in liquid culture media. The ΔAbSte7 mutants could not efficiently detoxify cruciferous phytoalexin brassinin, possibly due to reduced expression of the brassinin hydrolase gene involved in detoxifying brassinin. Disruption of the AbSte7 gene also severely impaired fungal detoxification of reactive oxygen species. AbSte7 gene disruption reduced the enzymatic activity of cell wall-degrading enzymes, including cellulase, β-glucosidase, pectin methylesterase, polymethyl-galacturonase, and polygalacturonic acid transeliminase, during host plant infection. Altogether, the data strongly suggest the MAPKK gene AbSte7 plays a pivotal role in A. brassicicola during host infection by regulating multiple steps, and thus increasing pathogenicity and inhibiting host defenses.

Beneficial and Pathogenic Arabidopsis Root-Interacting Fungi Differently Affect Auxin Levels and Responsive Genes During Early Infection

Auxin (indole-3-acetic acid, IAA) is an important phytohormone involved in root growth and development. Root-interacting beneficial and pathogenic fungi utilize auxin and its target genes to manipulate the performance of their hosts for their own needs. In order to follow and visualize auxin effects in fungi-colonized Arabidopsis roots, we used the dual auxin reporter construct DR5::EGFP-DR5v2::tdTomato and fluorescence microscopy as well as LC-MS-based phytohormone analyses. We demonstrate that the beneficial endophytic fungi Piriformospora indica and Mortierella hyalina produce and accumulate IAA in their mycelia, in contrast to the phytopathogenic biotrophic fungus Verticillium dahliae and the necrotrophic fungus Alternaria brassicicola. Within 3 h after exposure of Arabidopsis roots to the pathogens, the signals of the auxin-responsive reporter genes disappeared. When exposed to P. indica, significantly higher auxin levels and stimulated expression of auxin-responsive reporter genes were detected both in lateral root primordia and the root elongation zone within 1 day. Elevated auxin levels were also present in the M. hyalina/Arabidopsis root interaction, but no downstream effects on auxin-responsive reporter genes were observed. However, the jasmonate level was strongly increased in the colonized roots. We propose that the lack of stimulated root growth upon infection with M. hyalina is not caused by the absence of auxin, but an inhibitory effect mediated by high jasmonate content.

Ascorbic Acid and Glucosinolate Levels in New Czech Cabbage Cultivars: Effect of Production System and Fungal Infection

Nutritional value and disease-preventive effects of cabbage are well-known. Levels of the antioxidant compounds ascorbic acid (AA) and glucosinolates (GSL) in new Czech cabbage cultivars were determined in the context of different production systems. The contents of AA and GSLs in cabbage biomass were determined by HPLC. Individual GSLs were identified according to their exact masses with sinigrin used as the external standard. Artificial infection with A. brassicicola generally raised the AA levels. The major GSLs (≥10 mg kg⁻¹) were glucobrassicin, sinigrin, and glucoiberin. Indole and aliphatic GSLs were present, but no aromatic ones were detected. Ecological growth conditions and the artificial fungal infection increased the total content of GSLs and, also, of the methoxylated indole GSLs. Sulforaphane, iberin, indole-3-carbinol, and ascorbigen resulting from the hydrolysis of GSLs were found in both cultivars. The amounts and profiles of GSLs present in the two Czech cultivars demonstrated their good nutritional value. The decomposition products sulforaphane, iberin, indole-3-carbinol, and ascorbigen detected improve its health-promoting qualities and represent a suitable component of the human diet.

Defense and signalling metabolites of the crucifer Erucastrum canariense: Synchronized abiotic induction of phytoalexins and galacto-oxylipins

Erucastrum canariense Webb & Berthel. (Brassicaceae) is a wild crucifer that grows in rocky soils, in salt and water stressed habitats, namely in the Canary Islands and similar environments. Abiotic stress induced by copper chloride triggered formation of a phytoalexin and galacto-oxylipins in E. canariense, whereas wounding induced galacto-oxylipins but not phytoalexins. Analysis of the metabolite profiles of leaves of E. canariense followed by isolation and structure determination afforded the phytoalexin erucalexin, the phytoanticipin indolyl-3-acetonitrile, the galacto-oxylipins arabidopsides A, C, and D, and the oxylipin 12-oxophytodienoic acid. In addition, arabidopsides A and D were also identified in extracts of leaves of Nasturtium officinale R. Br.

Biocontrol Potential of an Endophytic Streptomyces sp. Strain MBCN152-1 against Alternaria brassicicola on Cabbage Plug Seedlings

In the present study, 77 strains of endophytic actinomycetes isolated from cabbage were screened in order to assess their biocontrol potential against Alternaria brassicicola on cabbage seedlings. In the first and second screening trials, cabbage seedlings pretreated with mycelial suspensions of each isolate were spray-inoculated with A. brassicicola. Strain MBCN152-1, which exhibited the best protection in screening trials and had no adverse effects on seedling growth, was selected for the greenhouse trial. In the greenhouse trial, cabbage seedlings, which had been grown in plug trays filled with soil mix containing spores of MBCN152-1 (1×108 spores g-1 of soil mix), were spray-inoculated with A. brassicicola and grown in greenhouse conditions. MBCN152-1 reduced disease incidence and significantly increased the number of viable seedlings. The efficacy of MBCN152-1 against damping-off caused by seed-borne A. brassicicola was then evaluated. Cabbage seeds, artificially infested with A. brassicicola, were sown in soil mix containing MBCN152-1 spores. The disease was completely suppressed when infested seeds were sown in a soil mix blended with MBCN152-1 at 1.5×107 spores g-1 of soil mix. These results strongly suggest that MBCN152-1 has the potential to control A. brassicicola on cabbage plug seedlings. MBCN152-1 was identified as a Streptomyces humidus-related species based on 16S rDNA sequencing. Scanning electron microscopy showed that the hyphae of MBCN152-1 multiplied on the surface of the seedlings and penetrated their epidermal cells. In conclusion, strain MBCN152-1 is a promising biocontrol agent against A. brassicicola on cabbage plug seedlings.

Biocontrol Potential of Streptomyces hydrogenans Strain DH16 toward Alternaria brassicicola to Control Damping Off and Black Leaf Spot of Raphanus sativus

Biocontrol agents and their bioactive metabolites provide one of the best alternatives to decrease the use of chemical pesticides. In light of this, the present investigation reports the biocontrol potential of Streptomyces hydrogenans DH16 and its metabolites towards Alternaria brassicicola, causal agent of black leaf spot and damping off of seedlings of crucifers. In vitro antibiosis of strain against pathogen revealed complete suppression of mycelial growth of pathogen, grown in potato dextrose broth supplemented with culture supernatant (20% v/v) of S. hydrogenans DH16. Microscopic examination of the fungal growth showed severe morphological abnormalities in the mycelium caused by antifungal metabolites. In vivo studies showed the efficacy of streptomycete cells and culture supernatant as seed dressings to control damping off of Raphanus sativus seedlings. Treatment of pathogen infested seeds with culture supernatant (10%) and streptomycete cells significantly improved seed germination (75-80%) and vigor index (1167-1538). Furthermore, potential of cells and culture supernatant as foliar treatment to control black leaf spot was also evaluated. Clearly visible symptoms of disease were observed in the control plants with 66.81% disease incidence and retarded growth of root system. However, disease incidence reduced to 6.78 and 1.47% in plants treated with antagonist and its metabolites, respectively. Additionally, treatment of seeds and plants with streptomycete stimulated various growth traits of plants over uninoculated control plants in the absence of pathogen challenge. These results indicate that S. hydrogenans and its culture metabolites can be developed as biofungicides as seed dressings to control seed borne pathogens, and as sprays to control black leaf spot of crucifers.

The biosynthesis of brassicicolin A in the phytopathogen Alternaria brassicicola

Alternaria brassicicola (Schwein.) Wiltshire is a phytopathogenic fungus that together with A. brassicae causes Alternaria black spot disease in Brassica species. Brassicicolin A is the major host-selective phytotoxin produced in cultures of A. brassicicola. Biosynthetic studies to establish the metabolic precursors of brassicicolin A were carried out with isotopically labeled compounds. Incorporation of D-[¹³C₆]glucose, L-[¹⁵N]valine, or L-[²H₈]valine into brassicicolin A was established using ¹H, ¹³C, ¹⁵N NMR and INADEQUATE spectroscopy and HPLC-ESI-MS spectrometry. Based on analyses of the spectroscopic data, the labeling patterns of brassicicolin A isolated from cultures incubated with the labeled precursors are found to be consistent with both the glycolytic and the valine pathways. That is, the carbons of mannitol and acetyl units and the isocyanide carbon atoms are derived from D-[¹³C₆]glucose whereas the hydroxyisopentanoyl and isocyanoisopentanoyl units are derived from L-valine, including the nitrogen atoms of both isocyanide groups.

Biotransformation of rutabaga phytoalexins by the fungus Alternaria brassicicola: Unveiling the first hybrid metabolite derived from a phytoalexin and a fungal polyketide

The biotransformations of the rutabaga phytoalexins rutalexin, brassicanate A, isalexin and rapalexin A by the plant pathogenic fungus Alternaria brassicicola are reported. While the biotransformations of rutalexin, brassicanate A, and isalexin are fast, rapalexin A is resistant to fungal transformation. Unexpectedly, biotransformation of rutalexin yields a hybrid metabolite named rutapyrone, derived from rutalexin metabolism and phomapyrone G, a fungal metabolite produced by A. brassicicola. These fungal transformations are detoxification reactions likely carried out by different enzymes. The discovery of rapalexin A resistance to detoxification suggests that this phytoalexin in combination with additional phytoalexins could protect crucifers against this pathogen. Phytoalexins resistant to degradation by A. brassicicola are expected to provide the producing plants with higher disease resistance levels.

Zinc triggers signaling mechanisms and defense responses promoting resistance to Alternaria brassicicola in Arabidopsis thaliana

According to the elemental defense hypothesis the accumulation of trace elements by plants may substitute for organic defenses, while the joint effects hypothesis proposes that trace elements and organic defenses can have additive or synergistic effects against pathogens or herbivores. To evaluate these hypotheses the response of the pathosystem Alternaria brassicicola-Arabidopsis thaliana to control (2μM) and surplus (12μM) Zn was evaluated using the camalexin deficient mutant pad3-1 and mtp1-1, a mutant with impaired Zn vacuolar storage, along with the corresponding wildtypes. In vitro, a 50% inhibition of fungal growth was achieved by 440μM Zn. A. thaliana leaves could accumulate equivalent concentrations without harm. In fact, surplus Zn enhanced the resistance of A. thaliana to fungal attack in Columbia (Col-0), Wassilewskija (WS), and mtp1-1. However, surplus Zn was unable to protect pad3-1 demonstrating that Zn cannot substitute for camalexin, the main organic defense in A. thaliana. High, non phytotoxic leaf Zn concentrations enhanced the resistance to A. brassicicola of A. thaliana genotypes able to produce camalexin. This was mainly due to Zn-induced enhancement of the JA/ETH signaling pathway leading to enhanced PAD3 expression. These results support the joint effects hypothesis and highlight the importance of adequate Zn supply for reinforced pathogen resistance.

AbSte7, a MAPKK Gene of Alternaria brassicicola, is Involved in Conidiation, Salt/Oxidative Stress, and Pathogenicity

Alternaria brassicicola (Schwein.) invades Brassicaceae and causes black spot disease, significantly lowering productivity. Mitogen-activated protein kinases (MAPKs) and their upstream kinases, including MAPK kinases (MAPKKs) and MAPKK kinases (MAPKKK), comprise one of the most important signaling pathways determining the pathogenicity of diverse plant pathogens. The AbSte7 gene in the genome of A. brassicicola was predicted to be a homolog of yeast Ste7, a MAPKK; therefore, the function was characterized by generating null mutant strains with a gene replacement method. AbSte7 replacement mutants (RMs) had a slower growth rate and altered colony morphology compared with the wild-type strain. Disruption of the AbSte7 gene resulted in defects in conidiation and melanin accumulation. AbSte7 was also involved in the resistance pathways in salt and oxidative stress, working to negatively regulate salt tolerance and positively regulate oxidative stress. Pathogenicity assays revealed that AbSte7 RMs could not infect intact cabbage leaves, but only formed very small lesions in wounded leaves, whereas typical lesions appeared on both intact and wounded leaves inoculated with the wild-type strain. As the first studied MAPKK in A. brassicicola, these data strongly suggest that the AbSte7 gene is an essential element for the growth, development, and pathogenicity of A. brassicicola.

BRI1-EMS-suppressor 1 gain-of-function mutant shows higher susceptibility to necrotrophic fungal infection

Brassinosteroids (BRs) are plant-specific steroids that are involved in plant growth and defense responses. However, the exact roles of BR in plant defense are unclear. We used the bes1-D gain-of-function mutant to define the underlying relationship between plant growth and defense through BR signaling and innate immunity. In bes1-D, further downstream component BES1 transcription factor is stabilized, leading to the activation of BR signaling. Previous reports on BES1 target genes showed that approximately 10% are related to biotic stress responses. Therefore, the bes1-D PTI responses were examined. The bes1-D mutant was specifically susceptible to Alternaria brassicicola, a necrotrophic fungus, which successfully produced spore, resulting in considerable cell death. However, it was not affected by a biotrophic pathogen, Pseudomonas syringae pv. tomato (Pst) DC3000. Instead of a ROS burst, a representative initial PTI responses, higher ROS accumulation was sustained in bes1-D than in the wild type plant. PDF1.2 expression was not induced in response to fungal pathogen infection in bes1-D. These results suggest that BES1 is also involved in JA-related defense responses, especially in response to necrotrophic pathogens.

Simplified Assays for Evaluation of Resistance to Alternaria brassicicola and Turnip Mosaic Virus

Studying the natural defense mechanisms developed by model plants such as Arabidopsis is an important approach towards the improvement of crop species. The availability of mutants as well as the relative easiness to silence any gene in Arabidopsis provides an invaluable source of genotypes that can be used to discover new elements involved in the defense response. Here we describe simple and reliable methods to evaluate susceptibility/resistance to the pathogenic fungus Alternaria brassicicola and the viral pathogen Turnip mosaic virus.

Expanding the nasturlexin family: Nasturlexins C and D and their sulfoxides are phytoalexins of the crucifers Barbarea vulgaris and B. verna

The metabolites produced in leaves of the crucifers winter cress (Barbarea vulgaris) and upland cress (Barbarea verna) abiotically elicited were investigated and their chemical structures were elucidated by analyses of spectroscopic data and confirmed by syntheses. Nasturlexins C and D and their sulfoxides are cruciferous phytoalexins displaying antifungal activity against the crucifer pathogens Alternaria brassicicola, Leptosphaeria maculans and Sclerotinia sclerotiorum. The biosynthesis of these metabolites is proposed based on pathways of cruciferous indolyl phytoalexins. This work indicates that B. vulgaris and B. verna have great potential as sources of defense pathways transferable to agriculturally important crops within the Brassica species.

Trichoderma volatiles effecting Arabidopsis: from inhibition to protection against phytopathogenic fungi

Trichoderma species are present in many ecosystems and some strains have the ability to reduce the severity of plant diseases by activating various defense pathways via specific biologically active signaling molecules. Hence we investigated the effects of low molecular weight volatile compounds of Trichoderma asperellum IsmT5 on Arabidopsis thaliana. During co-cultivation of T. asperellum IsmT5 without physical contact to A. thaliana we observed smaller but vital and robust plants. The exposed plants exhibit increased trichome numbers, accumulation of defense-related compounds such as H2O2, anthocyanin, camalexin, and increased expression of defense-related genes. We conclude that A. thaliana perceives the Trichoderma volatiles as stress compounds and subsequently initiates multilayered adaptations including activation of signaling cascades to withstand this environmental influence. The prominent headspace volatile of T. asperellum IsmT5 was identified to be 6-pentyl-α-pyrone (6PP), which was solely applied to A. thaliana to verify the growth and defense reactions. Most noticeable is that A. thaliana preexposed to 6PP showed significantly reduced symptoms when challenged with Botrytis cinerea and Alternaria brassicicola, indicating that defense-activated plants subsequently became more resistant to pathogen attack. Together, these results support that products that are based on Trichoderma volatiles have the potential being a useful biocontrol agent in agriculture.

Glucosinolate-derived isothiocyanates impact mitochondrial function in fungal cells and elicit an oxidative stress response necessary for growth recovery

Glucosinolates are brassicaceous secondary metabolites that have long been considered as chemical shields against pathogen invasion. Isothiocyanates (ITCs), are glucosinolate-breakdown products that have negative effects on the growth of various fungal species. We explored the mechanism by which ITCs could cause fungal cell death using Alternaria brassicicola, a specialist Brassica pathogens, as model organism. Exposure of the fungus to ICTs led to a decreased oxygen consumption rate, intracellular accumulation of reactive oxygen species (ROS) and mitochondrial-membrane depolarization. We also found that two major regulators of the response to oxidative stress, i.e., the MAP kinase AbHog1 and the transcription factor AbAP1, were activated in the presence of ICTs. Once activated by ICT-derived ROS, AbAP1 was found to promote the expression of different oxidative-response genes. This response might play a significant role in the protection of the fungus against ICTs as mutants deficient in AbHog1 or AbAP1 were found to be hypersensitive to these metabolites. Moreover, the loss of these genes was accompanied by a significant decrease in aggressiveness on Brassica. We suggest that the robust protection response against ICT-derived oxidative stress might be a key adaptation mechanism for successful infection of host plants by Brassicaceae-specialist necrotrophs like A. brassicicola.

Non-indolyl cruciferous phytoalexins: Nasturlexins and tridentatols, a striking convergent evolution of defenses in terrestrial plants and marine animals?

Highly specialized chemical defense pathways are a particularly noteworthy metabolic characteristic of sessile organisms, whether terrestrial or marine, providing protection against pests and diseases. For this reason, knowledge of the metabolites involved in these processes is crucial to producing ecologically fit crops. Toward this end, the elicited chemical defenses of the crucifer watercress (Nasturtium officinale R. Br.), i.e. phytoalexins, were investigated and are reported. Almost three decades after publication of cruciferous phytoalexins derived from (S)-Trp, phytoalexins derived from other aromatic amino acids were isolated; their chemical structures were determined by analyses of their spectroscopic data and confirmed by synthesis. Nasturlexin A, nasturlexin B, and tridentatol C are hitherto unknown phenyl containing cruciferous phytoalexins produced by watercress under abiotic stress; tridentatol C is also produced by a marine animal (Tridentata marginata), where it functions in chemical defense against predators. The biosynthesis of these metabolites in both a terrestrial plant and a marine animal suggests a convergent evolution of unique metabolic pathways recruited for defense.

Chitin Nanofiber Elucidates the Elicitor Activity of Polymeric Chitin in Plants

Chitin, an N-acetyl-D-glucosamine polymer, is a component of fungal cell walls and a microbe/pathogen-associated molecular pattern that elicits plant defense responses. As polymeric chitin is difficult to handle due to its insolubility in water, many studies on chitin-induced immune responses have used water-soluble low-molecular weight chitin instead. Thus, it is unclear if polymeric chitin can induce resistance. Here, we examined the elicitor activity of chitin nanofiber (CNF) of submicron thickness prepared from polymeric chitin. CNF showed a high dispersing ability in water and induced both reactive oxygen species (ROS) production and chitin-induced defense-related gene expression in Arabidopsis thaliana seedlings. The Arabidopsis chitin elicitor receptor kinase 1 (Atcerk1) mutant, which is impaired in chitin perception, also failed to respond to CNF. CNF exposure triggered ROS generation in suspension-cultured cells from Oryza sativa. Furthermore, pre-treatment of Arabidopsis leaves with CNF effectively reduced pathogen infection by both the fungus Alternaria brassicicola and the bacterium Pseudomonas syringae pv. tomato DC3000. These results demonstrate that CNF has elicitor activity and will help define the role of polymeric chitin in plant immune responses.

Characterization of a fungicidal substance produced by Eupenicillium brefeldianum isolated from soil for plant disease control and its significance in nature

BACKGROUND

A fungus identified as Eupenicillium brefeldianum was isolated from soil amended with vegetable tissues.

RESULT

When grown in liquid medium prepared from the same vegetable tissues, E. brefeldianum produced a substance capable of preventing disease development of leaf spots of mustard cabbage caused by Alternaria brassicicola and inhibiting the germination of A. brassicicola conidia. The inhibitory substance was fungicidal and was very stable under high temperature and extreme pH. It was soluble in polar solvents but not soluble in non-polar solvents, and did not have charges on its molecule. This is the first discovery of the production of a fungicidal substance by this fungus.

CONCLUSION

Results from this study suggest the possession of a strong competitive saprophytic ability by E. brefeldianum, which in turn may explain the widespread occurrence of this fungus in soils.

Impact of the UPR on the virulence of the plant fungal pathogen A. brassicicola

The fungal genus Alternaria contains many destructive plant pathogens, including Alternaria brassicicola, which causes black spot disease on a wide range of Brassicaceae plants and which is routinely used as a model necrotrophic pathogen in studies with Arabidopsis thaliana. During host infection, many fungal proteins that are critical for disease progression are processed in the endoplasmic reticulum (ER)/Golgi system and secreted in planta. The unfolded protein response (UPR) is an essential part of ER protein quality control that ensures efficient maturation of secreted and membrane-bound proteins in eukaryotes. This review highlights the importance of the UPR signaling pathway with respect to the ability of A. brassicicola to efficiently accomplish key steps of its pathogenic life cycle. Understanding the pathogenicity mechanisms that fungi uses during infection is crucial for the development of new antifungal therapies. Therefore the UPR pathway has emerged as a promising drug target for plant disease control.

Salicylic acid-mediated establishment of the compatibility between Alternaria brassicicola and Brassica juncea is mitigated by abscisic acid in Sinapis alba

This work addresses the changes in the phytohormonal signature in the recognition of the necrotrophic fungal pathogen Alternaria brassicicola by susceptible Brassica juncea and resistant Sinapis alba. Although B. juncea, S. alba and Arabidopsis all belong to the same family, Brassicaceae, the phytohormonal response of susceptible B. juncea towards this pathogen is unique because the latter two species express non-host resistance. The differential expression of the PR1 gene and the increased level of salicylic acid (SA) indicated that an SA-mediated biotrophic mode of defence response was triggered in B. juncea upon challenge with the pathogen. Compared to B. juncea, resistant S. alba initiated enhanced abscisic acid (ABA) and jasmonic acid (JA) responses following challenge with this pathogen, as revealed by monitoring the expression of ABA-related genes along with the concentration of ABA and JA. Furthermore, these results were verified by the exogenous application of ABA on B. juncea leaves prior to challenge with A. brassicicola, which resulted in a delayed disease progression, followed by the inhibition of the pathogen-mediated increase in SA response and enhanced JA levels. Therefore, it seems that A. brassicicola is steering the defence response towards a biotrophic mode by mounting an SA response in susceptible B. juncea, whereas the enhanced ABA response of S. alba not only counteracts the SA response but also restores the necrotrophic mode of resistance by enhancing JA biosynthesis.

Presence of LYM2 dependent but CERK1 independent disease resistance in Arabidopsis

Plants have the ability to detect invading fungi through the perception of chitin fragments released from the fungal cell walls. Plant chitin receptor consists of two types of plasma membrane proteins, CEBiP and CERK1. However, the contribution of these proteins to chitin signaling is different between Arabidopsis and rice. In Arabidopsis, it seems CERK1 receptor kinase is enough for both ligand perception and signaling, whereas both CEBiP and OsCERK1 are required for chitin signaling in rice. Here we report that Arabidopsis CEBiP homolog, LYM2, is not involved in chitin signaling but contributes to resistance against a fungal pathogen, Alternaria brassicicola, indicating the presence of a novel disease resistance mechanism in Arabidopsis.

Fungal-specific transcription factor AbPf2 activates pathogenicity in Alternaria brassicicola

Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. To identify molecular determinants of pathogenicity, we created non-pathogenic mutants of a transcription factor-encoding gene, AbPf2. The frequency and timing of germination and appressorium formation on host plants were similar between the non-pathogenic ∆abpf2 mutants and wild-type A. brassicicola. The mutants were also similar in vitro to wild-type A. brassicicola in terms of vegetative growth, conidium production, and responses to a phytoalexin, reactive oxygen species and osmolites. The hyphae of the mutants grew slowly but did not cause disease symptoms on the surface of host plants. Transcripts of the AbPf2 gene increased exponentially soon after wild-type conidia contacted their host plants . A small amount of AbPf2 protein, as monitored using GFP fusions, was present in young, mature conidia. The protein level decreased during saprophytic growth, but increased and was located primarily in fungal nuclei during pathogenesis. Levels of the proteins and transcripts sharply decreased following colonization of host tissues beyond the initial infection site. When expression of the transcription factor was induced in the wild-type during early pathogenesis, 106 fungal genes were also induced in the wild-type but not in the ∆abpf2 mutants. Notably, 33 of the 106 genes encoded secreted proteins, including eight putative effector proteins. Plants inoculated with ∆abpf2 mutants expressed higher levels of genes associated with photosynthesis, the pentose phosphate pathway and primary metabolism, but lower levels of defense-related genes. Our results suggest that AbPf2 is an important regulator of pathogenesis, but does not affect other cellular processes in A. brassicicola.

Role of mannitol metabolism in the pathogenicity of the necrotrophic fungus Alternaria brassicicola

In this study, the physiological functions of fungal mannitol metabolism in the pathogenicity and protection against environmental stresses were investigated in the necrotrophic fungus Alternaria brassicicola. Mannitol metabolism was examined during infection of Brassica oleracea leaves by sequential HPLC quantification of the major soluble carbohydrates and expression analysis of genes encoding two proteins of mannitol metabolism, i.e., a mannitol dehydrogenase (AbMdh), and a mannitol-1-phosphate dehydrogenase (AbMpd). Knockout mutants deficient for AbMdh or AbMpd and a double mutant lacking both enzyme activities were constructed. Their capacity to cope with various oxidative and drought stresses and their pathogenic behavior were evaluated. Metabolic and gene expression profiling indicated an increase in mannitol production during plant infection. Depending on the mutants, distinct pathogenic processes, such as leaf and silique colonization, sporulation, survival on seeds, were impaired by comparison to the wild-type. This pathogenic alteration could be partly explained by the differential susceptibilities of mutants to oxidative and drought stresses. These results highlight the importance of mannitol metabolism with respect to the ability of A. brassicicola to efficiently accomplish key steps of its pathogenic life cycle.

Selenium accumulation protects Brassica juncea from invertebrate herbivory and fungal infection

•  Certain plant species hyperaccumulate selenium (Se) up to 0.6% of their dry weight. It is not known whether Se hyperaccumulation offers the plants any advantage. In this study the hypothesis was tested that Se can protect plants from invertebrate herbivory or fungal infection. •  Indian mustard (Brassica juncea) plants grown with or without Se were subjected to herbivory by caterpillars (Pieris rapae) and snails (Mesodon ferrissi), or to fungal infection by a root/stem pathogen (Fusarium sp.) and a leaf pathogen (Alternaria brassicicola). •  When given a choice between leaves with or without Se (0.1% Se of leaf d. wt), the caterpillars strongly preferred leaves without Se (P < 0.01), while the snails preferred leaves containing Se (P < 0.015). When consumed, the Se leaves were lethal to the caterpillars. The snails showed no toxicity symptoms, even though their tissue Se concentrations were comparable with the caterpillars. Se-containing plants were less susceptible to infection by both fungi. •  In conclusion, Se was shown to protect Indian mustard plants from fungal infection and from herbivory by caterpillars, but not by snails.