Botrytis cinerea methyl isocitrate lyase mediates oxidative stress tolerance and programmed cell death by modulating cellular succinate levels

Grapevine gray mold disease: infection, defense and management

Grapevine (Vitis vinifera L.,) is among the world's leading fruit crops. The production of grapes is severely affected by many diseases including gray mold, caused by the necrotrophic fungus Botrytis cinerea. Although all Vitis species can be hosts for B. cinerea, V. vinifera are particularly susceptible. Accordingly, this disease poses a significant threat to the grape industry and causes substantial economic losses. Development of resistant V. vinifera cultivars has progressed from incidental selection by farmers, to targeted selection through the use of statistics and experimental design, to the employment of genetic and genomic data. Emerging technologies such as marker-assisted selection and genetic engineering have facilitated the development of cultivars that possess resistance to B. cinerea. A promising method involves using the CRISPR/Cas9 system to induce targeted mutagenesis and develop genetically modified non-transgenic crops. Hence, scientists are now engaged in the active pursuit of identifying genes associated with susceptibility and resistance. This review focuses on the known mechanisms of interaction between the B. cinerea pathogen and its grapevine host. It also explores innate immune systems that have evolved in V. vinifera, with the objective of facilitating the rapid development of resistant grapevine cultivars.

The Botrytis cinerea transglycosylase BcCrh4 is a cell death-inducing protein with cell death-promoting and -suppressing domains

Botrytis cinerea is a necrotrophic fungal plant pathogen that causes grey mould and rot diseases in many crops. Here, we show that the B. cinerea BcCrh4 transglycosylase is secreted during plant infection and induces plant cell death and pattern-triggered immunity (PTI), fulfilling the characteristics of a cell death-inducing protein (CDIP). The CDIP activity of BcCrh4 is independent of the transglycosylase enzymatic activity, it takes place in the apoplast and does not involve the receptor-like kinases BAK1 and SOBIR1. During saprophytic growth, BcCrh4 is localized in the endoplasmic reticulum and in vacuoles, but during plant infection, it accumulates in infection cushions (ICs) and is then secreted to the apoplast. Two domains within the BcCrh4 protein determine the CDIP activities: a 20aa domain at the N' end activates intense cell death and PTI, while a stretch of 52aa in the middle of the protein induces a weaker response and suppresses the activity of the 20aa N' domain. Deletion of bccrh4 affected fungal development and IC formation in particular, resulting in reduced virulence. Collectively, our findings demonstrate that BcCrh4 is required for fungal development and pathogenicity, and hint at a dual mechanism that balances the virulence activity of this, and potentially other CDIPs.

Propionate poses antivirulence activity against Botrytis cinerea via regulating its metabolism, infection cushion development and overall pathogenic factors

Botrytis cinerea is a devastating pathogen causing gray mold in fruits and vegetables if not properly managed. Although the mechanisms remain unclear, we previously revealed that the safe food additive calcium propionate (CP) could suppress gray mold development on grapes. The present study reports that sub-lethal dose of CP (0.2 % w/v) could allow growth with substantial reprograming the genome-wide transcripts of B. cinerea. Upon CP treatment, the genes related to fungal methylcitrate cycle (responsible for catabolizing propionate) were upregulated. Meanwhile, CP treatment broadly downregulated the transcript levels of the virulence factors. Further comparative analysis of multiple transcriptomes confirmed that the CP treatment largely suppressed the expression of genes related to development and function of infection cushion. Collectively, these findings indicate that CP can not only reduce fungal growth, but also abrogate fungal virulence factors. Thus, CP has significant potential for the control of gray mold in fruit crops.

Botrytis cinerea Transcription Factor BcXyr1 Regulates (Hemi-)Cellulase Production and Fungal Virulence

Botrytis cinerea is an agriculturally notorious plant-pathogenic fungus with a broad host range. During plant colonization, B. cinerea secretes a wide range of plant-cell-wall-degrading enzymes (PCWDEs) that help in macerating the plant tissue, but their role in pathogenicity has been unclear. Here, we report on the identification of a transcription factor, BcXyr1, that regulates the production of (hemi-)cellulases and is necessary for fungal virulence. Deletion of the bcxyr1 gene led to impaired spore germination and reduced fungal virulence and reactive oxygen species (ROS) production in planta. Secreted proteins collected from the bcxyr1 deletion strain displayed a weaker cell-death-inducing effect than the wild-type secretome when infiltrated to Nicotiana benthamiana leaves. Transcriptome sequencing (RNA-seq) analysis revealed 41 genes with reduced expression in the Δbcxyr1 mutant compared with those in the wild-type strain, of which half encode secreted proteins that are particularly enriched in carbohydrate-active enzyme (CAZyme)-encoding genes. Among them, we identified a novel putative expansin-like protein that was necessary for fungal virulence, supporting the involvement of BcXyr1 in the regulation of extracellular virulence factors. IMPORTANCE PCWDEs are considered important components of the virulence arsenal of necrotrophic plant pathogens. However, despite intensive research, the role of PCWDEs in the pathogenicity of necrotrophic phytopathogenic fungi remains ambiguous. Here, we demonstrate that the transcription factor BcXyr1 regulates the expression of a specific set of secreted PCWDE-encoding genes and that it is essential for fungal virulence. Furthermore, we identified a BcXyr1-regulated expansin-like gene that is required for fungal virulence. Our findings provide strong evidence for the importance of PCWDEs in the pathogenicity of B. cinerea and highlight specific PCWDEs that might be more important than others.

Synergy of cystamine and pyraclostrobin against Fusarium graminearum involves membrane permeability mitigation and autophagy enhancement

The application of fungicide mixture is one of the most important measures to extend the service life of highly selective fungicides. Pyraclostrobin (PYR), which has been extensively used to control plant diseases by inhibiting mitochondrial respiration of pathogenic fungi, is at a high risk of resistance development. In this study, the potential of PYR alone or in combination with cystamine, an inhibitor of microbial transglutaminase, to suppress Fusarium graminearum was tested in vitro and in vivo. A synergistic effect of PYR/CYS mixture was observed both in vitro and when applied to etiolated wheat coleoptile. The control effect of PYR/CYS mixture on F. graminearum was better than that of PYR alone, which was reflected by the increased protection effect. The discrepancies of membrane permeability and the redox-physiological state were observed between PYR and PYR/CYS treatments, suggesting that an increased PYR availability in F. graminearum mycelia could be related with the observed synergistic action. Moreover, a synergistic profile was observed between PYR and CYS in regard of massive autophagosomes in mycelia, indicating that enhanced autophagy could be involved in the mode of action of PYR/CYS mixture. The differential content of mitochondrial metabolites between PYR and PYR/CYS treatments also provided evidence for CYS contribution to the fungicidal action of PYR/CYS mixture. The results provide insight into the synergistic mechanism of action of PYR/CYS mixture and an effective way to enhance the efficiency of PYR to combat F. graminearum.

2E,4E-Decadienoic Acid, a Novel Anti-Oomycete Agent from Coculture of Bacillus subtilis and Trichoderma asperellum

Phytophthora nicotianae is an oomycete pathogen of global significance threatening many important crops. It is mainly controlled by chemosynthetic fungicides, which endangers ecosystem and human health; thus, there is an urgent need to explore alternatives for these fungicides. In this study, a new anti-oomycete aliphatic compound, 2E,4E-decadienoic acid (DDA), was obtained through coculture of Bacillus subtilis Tpb55 and Trichoderma asperellum HG1. Both in vitro and in vivo tests showed that DDA had a strong inhibitory effect against P. nicotianae. In addition, rhizosphere microbiome analysis showed that DDA reduced the relative abundance of Oomycota in rhizosphere soil. Transcriptome sequencing (RNA-Seq) analysis revealed that treatment of P. nicotianae with DDA resulted in significant downregulation of antioxidant activity and energy metabolism, including antioxidant enzymes and ATP generation, and upregulation of membrane-destabilizing activity, such as phospholipid synthesis and degradation. The metabolomic analysis results implied that the pathways influenced by DDA were mainly related to carbohydrate metabolism, energy metabolism, and the cell membrane. The biophysical tests further indicated that DDA produced oxidative stress on P. nicotianae, inhibited antioxidant enzyme and ATPase activity, and increased cell membrane permeability. Overall, DDA exerts inhibitory activity by acting on multiple targets in P. nicotianae, especially on the cell membrane and mitochondria, and can therefore serve as a novel environment-friendly agent for controlling crop oomycete disease. IMPORTANCE P. nicotianae is an oomycete pathogen that is destructive to crops. Although some oomycete inhibitors have been used during crop production, most are harmful to the ecology and lead to pathogen resistance. Alternatively, medium-chain fatty acids have been reported to exhibit antimicrobial activity in the medical field in previous studies; however, their potential as biocontrol agents has rarely been evaluated. Our in vivo and in vitro analyses revealed that the medium-chain fatty acid 2E,4E-decadienoic acid (DDA) displayed specific inhibitory activity against oomycetes. Further analysis indicated that DDA may acted on multiple targets in P. nicotianae, especially on the cell membrane and mitochondria. Our findings highlight the potential of DDA in controlling oomycete diseases. In conclusion, these results provide insights regarding the future use of green and environment-friendly anti-oomycete natural products for the prevention and control of crop oomycete diseases.

Extracellular Fragmented Self-DNA Is Involved in Plant Responses to Biotic Stress

A growing body of evidence indicates that extracellular fragmented self-DNA (eDNA), by acting as a signaling molecule, triggers inhibitory effects on conspecific plants and functions as a damage-associated molecular pattern (DAMP). To evaluate early and late events in DAMP-dependent responses to eDNA, we extracted, fragmented, and applied the tomato (Solanum lycopersicum) eDNA to tomato leaves. Non-sonicated, intact self-DNA (intact DNA) was used as control. Early event analyses included the evaluation of plasma transmembrane potentials (Vm), cytosolic calcium variations (Ca²⁺ _{cy t}), the activity and subcellular localization of both voltage-gated and ligand-gated rectified K⁺ channels, and the reactive oxygen species (ROS) subcellular localization and quantification. Late events included RNA-Seq transcriptomic analysis and qPCR validation of gene expression of tomato leaves exposed to tomato eDNA. Application of eDNA induced a concentration-dependent Vm depolarization which was correlated to an increase in (Ca²⁺)_cyt; this event was associated to the opening of K⁺ channels, with particular action on ligand-gated rectified K⁺ channels. Both eDNA-dependent (Ca²⁺)_cyt and K⁺ increases were correlated to ROS production. In contrast, application of intact DNA produced no effects. The plant response to eDNA was the modulation of the expression of genes involved in plant-biotic interactions including pathogenesis-related proteins (PRPs), calcium-dependent protein kinases (CPK1), heat shock transcription factors (Hsf), heat shock proteins (Hsp), receptor-like kinases (RLKs), and ethylene-responsive factors (ERFs). Several genes involved in calcium signaling, ROS scavenging and ion homeostasis were also modulated by application of eDNA. Shared elements among the transcriptional response to eDNA and to biotic stress indicate that eDNA might be a common DAMP that triggers plant responses to pathogens and herbivores, particularly to those that intensive plant cell disruption or cell death. Our results suggest the intriguing hypothesis that some of the plant reactions to pathogens and herbivores might be due to DNA degradation, especially when associated to the plant cell disruption. Fragmented DNA would then become an important and powerful elicitor able to trigger early and late responses to biotic stress.