SANT proteins modulate gene expression by coordinating histone H3KAc and Khib levels and regulate plant heat tolerance.

Histone post-translational modifications (PTMs), such as acetylation and recently identified lysine 2-hydroxyisobutyrylation (Khib), act as active epigenomic marks in plants. SANT domain-containing proteins SANT1, SANT2, SANT3, and SANT4 (SANT1/2/3/4), derived from PIF/Harbinger transposases, form a complex with HISTONE DEACETYLASE 6 (HDA6) to regulate gene expression via histone deacetylation. However, whether SANT1/2/3/4 coordinates different types of PTMs to regulate transcription and mediate responses to specific stresses in plants remains unclear. Here, in addition to modulating histone deacetylation, we found that SANT1/2/3/4 proteins acted like HDA6 or HDA9 in regulating the removal of histone Khib in Arabidopsis (Arabidopsis thaliana). Histone H3 lysine acetylation (H3KAc) and histone Khib were coordinated by SANT1/2/3/4 to regulate gene expression, with H3KAc playing a predominant role and Khib acting complementarily to H3KAc. SANT1/2/3/4 mutation significantly increased the expression of heat-inducible genes with concurrent change of H3KAc levels under normal and heat stress conditions, resulting in enhanced thermotolerance. This study revealed the critical roles of Harbinger transposon-derived SANT domain-containing proteins in transcriptional regulation by coordinating different types of histone PTMs and in the regulation of plant thermotolerance by mediating histone acetylation modification.

A fungal CFEM-containing effector targets NPR1 regulator NIMIN2 to suppress plant immunity.

Colletotrichum fructicola causes a broad range of plant diseases worldwide and secretes many candidate proteinous effectors during infection, but it remains largely unknown regarding their effects in conquering plant immunity. Here, we characterized a novel effector CfEC12 that is required for the virulence of C. fructicola. CfEC12 contains a CFEM domain and is highly expressed during the early stage of host infection. Overexpression of CfEC12 suppressed BAX-triggered cell death, callose deposition and ROS burst in Nicotiana benthamiana. CfEC12 interacted with apple MdNIMIN2, a NIM1-interacting (NIMIN) protein that putatively modulates NPR1 activity in response to SA signal. Transient expression and transgenic analyses showed that MdNIMIN2 was required for apple resistance to C. fructicola infection and rescued the defence reduction in NbNIMIN2-silenced N. benthamiana, supporting a positive role in plant immunity. CfEC12 and MdNPR1 interacted with a common region of MdNIMIN2, indicating that CfEC12 suppresses the interaction between MdNIMIN2 and MdNPR1 by competitive target binding. In sum, we identified a fungal effector that targets the plant salicylic acid defence pathway to promote fungal infection.

A fungal effector suppresses plant immunity by manipulating DAHPS-mediated metabolic flux in chloroplasts.

Plant secondary metabolism represents an important and ancient form of defense against pathogens. Phytopathogens secrete effectors to suppress plant defenses and promote infection. However, it is largely unknown, how fungal effectors directly manipulate plant secondary metabolism. Here, we characterized a fungal defense-suppressing effector CfEC28 from Colletotrichum fructicola. Gene deletion assays showed that ∆CfEC28-mutants differentiated appressoria normally on plant surface but were almost nonpathogenic due to increased number of plant papilla accumulation at attempted penetration sites. CfEC28 interacted with a family of chloroplast-localized 3-deoxy-d-arabinose-heptulonic acid-7-phosphate synthases (DAHPSs) in apple. CfEC28 inhibited the enzymatic activity of an apple DAHPS (MdDAHPS1) and suppressed DAHPS-mediated secondary metabolite accumulation through blocking the manganese ion binding region of DAHPS. Dramatically, transgene analysis revealed that overexpression of MdDAHPS1 provided apple with a complete resistance to C. fructicola. We showed that a novel effector CfEC28 can be delivered into plant chloroplasts and contributes to the full virulence of C. fructicola by targeting the DAHPS to disrupt the pathway linking the metabolism of primary carbohydrates with the biosynthesis of aromatic defense compounds. Our study provides important insights for understanding plant-microbe interactions and a valuable gene for improving plant disease resistance.

Fusarium graminearum effector FgEC1 targets wheat TaGF14b protein to suppress TaRBOHD-mediated ROS production and promote infection.

Fusarium head blight (FHB), caused by Fusarium graminearum, is a devastating disease of wheat globally. However, the molecular mechanisms underlying the interactions between F. graminearum and wheat remain unclear. Here, we identified a secreted effector protein, FgEC1, that is induced during wheat infection and is required for F. graminearum virulence. FgEC1 suppressed flg22- and chitin-induced callose deposition and reactive oxygen species (ROS) burst in Nicotiana benthamiana. FgEC1 directly interacts with TaGF14b, which is upregulated in wheat heads during F. graminearum infection. Overexpression of TaGF14b increases FHB resistance in wheat without compromising yield. TaGF14b interacts with NADPH oxidase respiratory burst oxidase homolog D (TaRBOHD) and protects it against degradation by the 26S proteasome. FgEC1 inhibited the interaction of TaGF14b with TaRBOHD and promoted TaRBOHD degradation, thereby reducing TaRBOHD-mediated ROS production. Our findings reveal a novel pathogenic mechanism in which a fungal pathogen acts via an effector to reduce TaRBOHD-mediated ROS production.

Transcriptomic analysis reveals candidate genes regulating development and host interactions of Colletotrichum fructicola.

BACKGROUND: Colletotrichum is a fungal genus in Ascomycota that contain many plant pathogens. Among all Colletotrichum genomes that have been sequenced, C. fructicola contains the largest number of candidate virulence factors, such as plant cell wall degrading enzymes, secondary metabolite (SM) biosynthetic enzymes, secreted proteinases, and small secreted proteins. Systematic analysis of the expressional patterns of these factors would be an important step toward identifying key virulence determinants. RESULTS: In this study, we obtained and compared the global transcriptome profiles of four types of infection-related structures: conidia, appressoria, infected apple leaves, and cellophane infectious hyphae (bulbous hyphae spreading inside cellophane) of C. fructicola. We also compared the expression changes of candidate virulence factors among these structures in a systematic manner. A total of 3189 genes were differentially expressed in at least one pairwise comparison. Genes showing in planta-specific expressional upregulations were enriched with small secreted proteins (SSPs), cytochrome P450s, carbohydrate-active enzymes (CAZYs) and secondary metabolite (SM) synthetases, and included homologs of several known candidate effectors and one SM gene cluster specific to the Colletotrichum genus. In conidia, tens of genes functioning in triacylglycerol biosynthesis showed coordinately expressional upregulation, supporting the viewpoint that C. fructicola builds up lipid droplets as energy reserves. Several phosphate starvation responsive genes were coordinately up-regulated during early plant colonization, indicating a phosphate-limited in planta environment immediately faced by biotrophic infectious hyphae. CONCLUSION: This study systematically analyzes the expression patterns of candidate virulence genes, and reveals biological activities related to the development of several infection-related structures of C. fructicola. Our findings lay a foundation for further dissecting infection mechanisms in Colletotrichum and identifying disease control targets.

Genome comparisons reveal accessory genes crucial for the evolution of apple Glomerella leaf spot pathogenicity in Colletotrichum fungi.

Apple Glomerella leaf spot (GLS) is an emerging fungal disease caused by Colletotrichum fructicola and other Colletotrichum species. These species are polyphyletic and it is currently unknown how these pathogens convergently evolved to infect apple. We generated chromosome-level genome assemblies of a GLS-adapted isolate and a non-adapted isolate in C. fructicola using long-read sequencing. Additionally, we resequenced 17 C. fructicola and C. aenigma isolates varying in GLS pathogenicity using short-read sequencing. Genome comparisons revealed a conserved bipartite genome architecture involving minichromosomes (accessory chromosomes) shared by C. fructicola and other closely related species within the C. gloeosporioides species complex. Moreover, two repeat-rich genomic regions (1.61 Mb in total) were specifically conserved among GLS-pathogenic isolates in C. fructicola and C. aenigma. Single-gene deletion of 10 accessory genes within the GLS-specific regions of C. fructicola identified three that were essential for GLS pathogenicity. These genes encoded a putative non-ribosomal peptide synthetase, a flavin-binding monooxygenase and a small protein with unknown function. These results highlight the crucial role accessory genes play in the evolution of Colletotrichum pathogenicity and imply the significance of an unidentified secondary metabolite in GLS pathogenesis.

A novel effector CfEC92 of Colletotrichum fructicola contributes to glomerella leaf spot virulence by suppressing plant defences at the early infection phase.

The ascomycete fungus Colletotrichum fructicola causes diseases on a broad range of plant species. On susceptible cultivars of apple, it induces severe early defoliation and fruit spots, named glomerella leaf spot (GLS), but the mechanisms of pathogenicity have remained elusive. Phytopathogens exhibit small secreted effectors to advance host infection by manipulating host immune reactions. We report the identification and characterization of CfEC92, an effector required for C. fructicola virulence. CfEC92 is a Colletotrichum-specific small secreted protein that suppresses BAX-triggered cell death in Nicotiana benthamiana. Accumulation of the gene transcript was barely detectable in conidia or vegetative hyphae, but was highly up-regulated in appressoria formed during early apple leaf infection. Gene deletion mutants were not affected in vegetative growth, appressorium formation, or appressorium-mediated cellophane penetration. However, the mutants were significantly reduced in virulence toward apple leaves and fruits. Microscopic examination indicated that infection by the deletion mutants elicited elevated deposition of papillae at the penetration sites, and formation of infection vesicles and primary hyphae was retarded. Signal peptide activity, subcellular localization, and cell death-suppressive activity (without signal peptide) assays suggest that CfEC92 could be secreted and perform virulence functions inside plant cells. RNA sequencing and quantitative reverse transcription PCR results confirmed that the deletion mutants triggered elevated host defence reactions. Our results strongly support the interpretation that CfEC92 contributes to C. fructicola virulence as a plant immunity suppressor at the early infection phase.