The Fungal-Specific Transcription Factor VpFSTF1 Is Required for Virulence in Valsa pyri

Two Polyketide Synthase Genes, VpPKS10 and VpPKS33, Regulated by VpLaeA Are Essential to the Virulence of Valsa pyri

Valsa pyri, the causal agent of pear canker disease, typically induces cankers on the bark of infected trees and even leads to tree mortality. Secondary metabolites produced by pathogenic fungi play a crucial role in the pathogenic process. In this study, secondary metabolic regulator VpLaeA was identified in V. pyri. VpLaeA was found to strongly affect the pathogenicity, fruiting body formation, and toxicity of secondary metabolites of V. pyri. Additionally, VpLaeA was found to be required for the response of V. pyri to some abiotic stresses. Transcriptome data analysis revealed that many of differentially expressed genes were involved in the secondary metabolite biosynthesis. Among them, about one third of secondary metabolite biosynthesis core genes were regulated by VpLaeA at different periods. Seven differentially expressed secondary metabolite biosynthesis core genes (VpPKS9, VpPKS10, VpPKS33, VpNRPS6, VpNRPS7, VpNRPS16, and VpNRPS17) were selected for knockout. Two modular polyketide synthase genes (VpPKS10 and VpPKS33) that were closely related to the virulence of V. pyri from the above seven genes were identified. Notably, VpPKS10 and VpPKS33 also affected the production of fruiting body of V. pyri but did not participate in the resistance of V. pyri to abiotic stresses. Overall, this study demonstrates the multifaceted biological functions of VpLaeA in V. pyri and identifies two toxicity-associated polyketide synthase genes in Valsa species fungi for the first time.

Screening of pathogenicity-deficient Penicillium italicum mutants established by Agrobacterium tumefaciens-mediated transformation

Blue mold, caused by Penicillium italicum, is one of the main postharvest diseases of citrus fruits during storage and marketing. The pathogenic mechanism remains largely unclear. To explore the potential pathogenesis-related genes of this pathogen, a T-DNA insertion library of P. italicum PI5 was established via Agrobacterium tumefaciens-mediated transformation (ATMT). The system yielded 200-250 transformants per million conidia, and the transformants were genetically stable after five generations of successive subcultures on hygromycin-free media. 2700 transformants were obtained to generate a T-DNA insertion library of P. italicum. Only a few of the 200 randomly selected mutants exhibited significantly weakened virulence on citrus fruits, with two mutants displaying attenuated sporulation. The T-DNA in the two mutants existed as a single copy. Moreover, the mutant genes PiBla (PITC_048370) and PiFTF1 (PITC_077280) identified may be involved in conidia production by regulating expressions of the key regulatory components for conidiogenesis. These results demonstrated that the ATMT system is useful to obtain mutants of P. italicum for further investigation of the molecular mechanisms of pathogenicity and the obtained two pathogenesis-related genes might be novel loci associated with pathogenesis and conidia production.

Chromosome-level genome provides new insight into the overwintering process of Korla pear (Pyrus sinkiangensis Yu)

Korla pear has a unique taste and aroma and is a breeding parent of numerous pear varieties. It is susceptible to Valsa mali var. pyri, which invades bark wounded by freezing injury. Its genetic relationships have not been fully defined and could offer insight into the mechanism for freezing tolerance and disease resistance. We generated a high-quality, chromosome-level genome assembly for Korla pear via the Illumina and PacBio circular consensus sequencing (CCS) platforms and high-throughput chromosome conformation capture (Hi-C). The Korla pear genome is ~ 496.63 Mb, and 99.18% of it is assembled to 17 chromosomes. Collinearity and phylogenetic analyses indicated that Korla might be derived from Pyrus pyrifolia and that it diverged ~ 3.9-4.6 Mya. During domestication, seven late embryogenesis abundant (LEA), two dehydrin (DHN), and 54 disease resistance genes were lost from Korla pear compared with P. betulifolia. Moreover, 21 LEA and 31 disease resistance genes were common to the Korla pear and P. betulifolia genomes but were upregulated under overwintering only in P. betulifolia because key cis elements were missing in Korla pear. Gene deletion and downregulation during domestication reduced freezing tolerance and disease resistance in Korla pear. These results could facilitate the breeding of novel pear varieties with high biotic and abiotic stress resistance.

MaAzaR Influences Virulence of Metarhizium acridum against Locusta migratoria manilensis by Affecting Cuticle Penetration

The entomopathogenic fungus (EPF) Metarhizium acridum is a typical filamentous fungus and has been used to control migratory locusts (Locusta migratoria manilensis). This study examines the impact of the Zn(II)2Cys6 transcription factor, MaAzaR, in the virulence of M. acridum. Disruption of MaAzaR (ΔMaAzaR) diminished the fungus's ability to penetrate the insect cuticle, thereby decreasing its virulence. The median lethal time (LT50) for the ΔMaAzaR strain increased by approximately 1.5 d compared to the wild-type (WT) strain when topically inoculated, simulating natural infection conditions. ΔMaAzaR compromises the formation, turgor pressure, and secretion of extracellular hydrolytic enzymes in appressoria. However, the growth ability of ΔMaAzaR within the hemolymph is not impaired; in fact, it grows better than the WT strain. Moreover, RNA-sequencing (RNA-Seq) analysis of ΔMaAzaR and WT strains grown for 20 h on locust hindwings revealed 87 upregulated and 37 downregulated differentially expressed genes (DEGs) in the mutant strain. Pathogen-host interaction database (PHI) analysis showed that about 40% of the total DEGs were associated with virulence, suggesting that MaAzaR is a crucial transcription factor that directly regulates the expression of downstream genes. This study identifies a new transcription factor involved in EPF cuticle penetration, providing theoretical support and genetic resources for the developing highly virulent strains.

The critical roles of the Zn2Cys6 transcription factor Fp487 in the development and virulence of Fusarium pseudograminearum: A potential target for Fusarium crown rot control

Fusarium crown rot (FCR) caused by Fusarium pseudograminearum poses a significant threat to wheat production in the Huang-Huai-Hai region of China. However, the pathogenic mechanism of F. pseudograminearum is still poorly understood. Zn2Cys6 transcription factors, which are exclusive to fungi, play pivotal roles in regulating fungal development, drug resistance, pathogenicity, and secondary metabolism. In this study, we present the functional characterization of a Zn2Cys6 transcription factor F. pseudograminearum, designated Fp487. In F. pseudograminearum, Fp487 is shown to be required for mycelial growth through gene knockout and phenotypic analyses. Compared with wild-type CF14047, the ∆Fp487 mutant displayed a slight reduction in growth rate but a significant decrease in conidiogenesis, pathogenicity and 3-acetyl-deoxynivalenol (3AcDON) production. Moreover, the mutant exhibited heightened sensitivity to oxidative and cytomembrane stress. Furthermore, we synthesized dsRNA from the Fp487 gene in vitro, resulting in a reduction in the growth rate of F. pseudograminearum and its virulence on barley leaves through spray-induced gene silencing (SIGS). Notably, this study makes the first instance of inducing the expression of abundant dsRNA from F. pseudograminearum by engineering the Escherichia coli strain HT115 (DE3) and utilizing the SIGS technique to evaluate the virulence effect of dsRNA on F. pseudograminearum. In conclusion, our findings revealed the crucial role of Fp487 in regulating pathogenicity, stress responses, DON production, and conidiogenesis in F. pseudograminearum. Furthermore, Fp487 is a potential RNAi-based target for FCR control.

Biocontrol activity and potential mechanism of volatile organic compounds from Aspergillus niger strain La2 against pear Valsa canker

BACKGROUND

Valsa canker caused by Valsa pyri is one of the most destructive diseases of pear, leading to severe yield and economic losses. Volatile organic compounds (VOCs) from endophytes have important roles in the regulation of plant disease. In this study, we investigated the biocontrol activity of the endophytic fungus Aspergillus niger strain La2 and its antagonistic VOCs against pear Valsa canker.

RESULTS

Strain La2 exhibited an obvious inhibitory effect against V. pyri. A colonization assay suggested that strain La2 could complete its life cycle on pear twigs. The symptoms of pear Valsa canker were weakened on detached pear twigs after treatment with strain La2. In addition, VOCs from strain La2 also significantly suppressed mycelial growth in V. pyri. Based on the results of headspace solid-phase microextraction/gas chromatography-mass spectrometry analysis, six possible VOCs produced by strain La2 were detected, of which 2,4-di-tert-butylphenol and 4-methyl-1-pentanol were the main antagonistic VOCs in terms of their effect on pear Valsa canker in vitro and in vivo. Further results showed that 4-methyl-1-pentanol could destroy the V. pyri hyphal structure and cell membrane integrity. Importantly, the activities of pear defense-related enzymes (polyphenol oxidase, phenylalanine ammonia lyase and superoxide dismutase) were enhanced after 4-methyl-1-pentanol treatment in pear twigs, suggesting that 4-methyl-1-pentanol might induce a plant disease resistance response.

CONCLUSION

Aspergillus niger strain La2 and its VOCs 2,4-di-tert-butylphenol and 4-methyl-1-pentanol have potential as novel biocontrol agents of pear Valsa canker. © 2024 Society of Chemical Industry.

Decrypting biocontrol functions and application modes by genomes data of three Trichoderma Strains/Species

Trichoderma is an excellent biocontrol agent, but most Trichoderma genomes remained at the scaffold level, which greatly limits the research of biocontrol mechanism. Here, we reported the chromosome-level genome of Trichoderma harzianum CGMCC20739 (Tha739), T. asperellum CGMCC11653 (Tas653) and T. atroviride CGMCC40488 (Tat488), they were assembled into 7 chromosomes, genome size were 40 Mb (10,611 genes), 37.3 Mb (10,102 genes) and 36.3 Mb (9,896 genes), respectively. The positive selected genes of three strains were associated to response to stimulus, signaling transduction, immune system and localization. Furthermore, the number of transcription factors in Tha739, Tas653 and Tat488 strains had significant difference, which may contribute to the differential biocontrol function and stress tolerance. The genes related to signal transduction and gene clusters related to antimicrobial compounds in Tha739 were more than those in Tas653 and Tat488, which showed Tha739 may keenly sense other fungi and quickly secret antimicrobial compounds to inhibit other fungi. Tha739 also contained more genes associated to detoxification, antioxidant and nutrition utilization, indicating it had higher stress-tolerance to hostile environments. And the substrate for synthesizing IAA in Tha739 was mainly 3-indole acetonitrile and indole acetaldehyde, but in Tat488, it was indole-3-acetamide, moreover, Tha739 secreted more phosphatase and phytase and was more related to soil phosphorus metabolism, Tat488 secreted more urease and was more related to soil nitrogen metabolism. These candidate genes related to biocontrol function and stress-tolerance laid foundations for construction of functional strains. All above proved the difference in biocontrol function of Tha739, Tas653 and Tat488 strains, however, the defects in individual strains could be compensated for through Trichoderma-biome during the commercial application process of biocontrol Trichoderma strains.

Exploration of the Biocontrol Activity of Bacillus atrophaeus Strain HF1 against Pear Valsa Canker Caused by Valsa pyri

Valsa pyri-induced pear Valsa canker is among the most prevalent diseases to impact pear quality and yields. Biocontrol strategies to control plant disease represent an attractive alternative to the application of fungicides. In this study, the potential utility of Bacillus atrophaeus strain HF1 was assessed as a biocontrol agent against pear Valsa canker. Strain HF1 suppressed V. pyri mycelium growth by 61.20% and induced the development of malformed hyphae. Both culture filtrate and volatile organic compounds (VOCs) derived from strain HF1 were able to antagonize V. pyri growth. Treatment with strain HF1-derived culture filtrate or VOCs also induced the destruction of hyphal cell membranes. Headspace mixtures prepared from strain HF1 were analyzed, leading to the identification of 27 potential VOCs. Of the thirteen pure chemicals tested, iberverin, hexanoic acid, and 2-methylvaleraldehyde exhibited the strongest antifungal effects on V. pyri, with respective EC50 values of 0.30, 6.65, and 74.07 μL L-1. Fumigation treatment of pear twigs with each of these three compounds was also sufficient to prevent the development of pear Valsa canker. As such, these results demonstrate that B. atrophaeus strain HF1 and the volatile compounds iberverin, hexanoic acid, and 2-methylvaleraldehyde exhibit promise as novel candidate biocontrol agents against pear Valsa canker.

Antifungal Compound from the Predatory Bacterium Lysobacter enzymogenes Inhibits a Plant Pathogenic Fungus by Targeting the AAA ATPase VpVeb1

Heat-stable antifungal factor (HSAF) isolated from Lysobacter enzymogenes is considered a potential biocontrol agent. However, the target of HSAF in phytopathogenic fungi remains unclear. In this study, we investigated the target of HSAF in Valsa pyri that causes fatal pear Valsa canker. Thirty-one HSAF-binding proteins were captured and identified by surface plasmon resonance (SPR) and high-performance liquid chromatography-mass spectrometry (LC-MS/MS), and 11 deletion mutants were obtained. Among these mutants, only ΔVpVEB1 showed decreased sensitivity to HSAF. Additionally, ΔVpVEB1 exhibited significantly reduced virulence in V. pyri. Molecular docking and SPR results revealed that HSAF bound to threonine 569 and glycine 570 of VpVeb1, which are crucial for AAA ATPase activity. Another study showed that HSAF could decrease the ATPase activity of VpVeb1, leading to the reduced virulence of V. pyri. Taken together, this study first identified the potential target of HSAF in fungi. These findings will help us better understand the model of action of HSAF to fungi.

A Putative Zn(II)2Cys6-Type Transcription Factor FpUme18 Is Required for Development, Conidiation, Cell Wall Integrity, Endocytosis and Full Virulence in Fusarium pseudograminearum

Fusarium pseudograminearum is one of the major fungal pathogens that cause Fusarium crown rot (FCR) worldwide and can lead to a substantially reduced grain yield and quality. Transcription factors play an important role in regulating growth and pathogenicity in plant pathogens. In this study, we identified a putative Zn(II)2Cys6 fungal-type domain-containing transcription factor and named it FpUme18. The expression of FpUME18 was induced during the infection of wheat by F. pseudograminearum. The ΔFpume18 deletion mutant showed defects in growth, conidial production, and conidial germination. In the responses to the cell wall, salt and oxidative stresses, the ΔFpume18 mutant inhibited the rate of mycelial growth at a higher rate compared with the wild type. The staining of conidia and mycelia with lipophilic dye FM4-64 revealed a delay in endocytosis when FpUME18 was deleted. FpUME18 also positively regulated the expression of phospholipid-related synthesis genes. The deletion of FpUME18 attenuated the pathogenicity of wheat coleoptiles. FpUME18 also participated in the production of the DON toxin by regulating the expression of TRI genes. Collectively, FpUme18 is required for vegetative growth, conidiation, stress response, endocytosis, and full virulence in F. pseudograminearum.

Loss of Flagella-Related Genes Enables a Nonflagellated, Fungal-Predating Bacterium To Strengthen the Synthesis of an Antifungal Weapon

Loss of flagellar genes causes a nonmotile phenotype. The genus Lysobacter consists of numerous environmentally ubiquitous, nonflagellated bacteria, including Lysobacter enzymogenes, an antifungal bacterium that is beneficial to plants. L. enzymogenes still has many flagellar genes on its genome, although this bacterium does not engage in flagella-driven motility. Here, we report that loss of certain flagellar genes allows L. enzymogenes to strengthen its evolutionarily gained capacity in fungal killing. To clarify why this bacterium loses flagellar genes during the evolutionary process, we cloned several representative flagellar genes from Xanthomonas oryzae, a flagellated, phylogenetically related species of Lysobacter, and introduced them individually into L. enzymogenes to mimic genomic reacquisition of lost flagellar genes. Heterogeneous expression of the three X. oryzae flagellar structural genes (Xo-motA, Xo-motB, Xo-fliE) and one flagellar regulatory gene (Xo-fleQ) remarkably weakened the bacterial capacity to kill fungal pathogens by impairing the synthesis of an antifungal weapon, known as the heat-stable antifungal factor (HSAF). We further investigated the underlying mechanism by selecting Xo-FleQ as the representative because it is a master transcription factor responsible for flagellar gene expression. Xo-FleQ inhibited the transcription of operon genes responsible for HSAF synthesis via direct binding of Xo-FleQ to the promoter region, thereby decreasing HSAF biosynthesis by L. enzymogenes. These observations suggest a possible genome and function coevolution event, in which an antifungal bacterium deletes certain flagellar genes in order to enhance its ability to kill fungi. IMPORTANCE It is generally recognized that flagellar genes are commonly responsible for the flagella-driven bacterial motility. Thus, finding nonflagellated bacteria partially or fully lost flagellar genes is not a surprise. However, the present study provides new insights into this common idea. We found that loss of either certain flagellar structural or regulatory genes (such as motA, motB, fliE, and fleQ) allows a nonflagellated, antifungal bacterium (L. enzymogenes) to stimulate its fungal-killing capacity, outlining a genome-function coevolution event, where an antifungal bacterium "smartly" designed its genome to "delete" crucial flagellar genes to coordinate flagellar loss and fungal predation. This unusual finding might trigger bacteriologists to reconsider previously ignored functions of the lost flagellar genes in any nonflagellated, pathogenic, or beneficial bacteria.

The Transcription Factor VpxlnR Is Required for the Growth, Development, and Virulence of the Fungal Pathogen Valsa pyri

Pears (Pyrus sp.) are widely cultivated in China, and their yield accounts for more than 60% of global pear production. The fungal pathogen Valsa pyri is a major causal agent of pear canker disease, which results in enormous losses of pear production in northern China. In this study, we characterized a Zn₂Cys₆ transcription factor that contains one GAL4 domain and a fungal-trans domain, which are present in VpxlnR. The vpxlnR gene expression was upregulated in the invasion stage of V. pyri. To investigate its functions, we constructed gene deletion mutants and complementary strains. We observed that the growth of the vpxlnR mutants was reduced on potato dextrose agar (PDA), Czapek plus glucose or sucrose compared with that of the wild-type strain. Additionally, vpxlnR mutants exhibited loss of function in fruiting body formation. Moreover, vpxlnR mutants were more susceptible to hydrogen peroxide (H₂O₂) and salicylic acid (SA) and were reduced in their virulence at the early infection stage. According to a previous study, VpxlnR-interacting motifs containing NRHKGNCCGM were searched in the V. pyri genome, and we obtained 354 target genes, of which 148 genes had Clusters of Orthologous Groups (COG) terms. PHI-BLAST was used to identify virulence-related genes, and we found 28 hits. Furthermore, eight genes from the 28 PHI-BLAST hits were further assessed by yeast one-hybrid (Y1H) assays, and five target genes, salicylate hydroxylase (VP1G_09520), serine/threonine-protein kinase (VP1G_03128), alpha-xylosidase (VP1G_06369), G-protein beta subunit (VP1G_02856), and acid phosphatase (VP1G_03782), could interact with VpxlnR in vivo. Their transcript levels were reduced in one or two vpxlnR mutants. Taken together, these findings imply that VpxlnR is a key regulator of growth, development, stress, and virulence through controlling genes involved in signaling pathways and extracellular enzyme activities in V. pyri. The motifs interacting with VpxlnR also provide new insights into the molecular mechanism of xlnR proteins.

The zinc finger protein StMR1 affects the pathogenicity and melanin synthesis of Setosphaeria turcica and directly regulates the expression of DHN melanin synthesis pathway genes

The infection and colonization of pathogenic fungi are often regulated by transcription factors. In our previous study, the zinc finger protein-encoding gene StMR1 was found to be highly expressed during the infection process of Setosphaeria turcica, the pathogen causing northern corn leaf blight. Evolutionary tree analysis showed that this gene was associated with regulatory factors of melanin synthesis. However, the regulatory mechanism of melanin synthesis and its effect on pathogenicity remain unclear. In this study, the function of StMR1 was analyzed by gene knockout. When the expression level of StMR1 in the mutants was significantly reduced, the colony color became lighter, the mycelia were curved and transparent, and the mutant showed a significant loss of pathogenicity. In addition, compared with wild-type, the accumulation of melanin decreased significantly in △Stmr1. RNA-seq analysis revealed 1,981 differentially expressed genes between the wild-type and knockout mutant, among which 39 genes were involved in melanin metabolism. qPCR revealed that the expression levels of 6 key genes in the melanin synthesis pathway were significantly reduced. ChIP-PCR and yeast one-hybrid assays confirmed that StMR1 directly binds to the promoters of St3HNR, St4HNR, StPKS, and StLAC2 in the DHN melanin synthesis pathway and regulates gene expression. The C2H2-type zinc fingers and Zn(Ⅱ)2Cys6 binuclear cluster in StMR1 was important for the binding to targets.

Biological Control of Pear Valsa Canker Caused by Valsa pyri Using Penicillium citrinum

Valsa canker caused by Valsa pyri is one of the most destructive diseases of commercial pear. For the present analysis, 29 different endophytic fungal strains were isolated from the branches of a healthy pear tree. In dual culture assays, strain ZZ1 exhibited robust antifungal activity against all tested pathogens including Valsa pyri. Microscopic analyses suggested that following co-culture with ZZ1, the hyphae of V. pyri were ragged, thin, and ruptured. ZZ1 also induced significant decreases in lesion length and disease incidence on detached pear branches inoculated with V. pyri. ZZ1 isolate-derived culture filtrates also exhibited antifungal activity against V. pyri, decreasing mycelial growth and conidium germination and inhibiting V. pyri-associated lesion development on pear branches. These results suggest that the ZZ1 isolate has the potential for use as a biological control agent against V. pyri. The strain was further identified as Penicillium citrinum based on its morphological characteristics and molecular analyses. Overall, these data highlight a potentially valuable new biocontrol resource for combating pear Valsa canker.

Antifungal Mechanism of Dipicolinic Acid and Its Efficacy for the Biocontrol of Pear Valsa Canker

Valsa pyri is a fatal canker pathogen that causes significant reduction of crop yield in pear orchards. V. pyri invades the trunk phloem, and is difficult to control by chemical treatment. In this work, it was found for the first time that Bacillus subtilis-produced dipicolinic acid (DPA) exhibits antifungal activity against different canker pathogens, including Alteraria alternata, Botryosphaeria dothidea, Rhizoctonia solani, and V. pyri. Growth inhibition of V. pyri was observed at less than 5 mM concentration (pH = 5.6). DPA showed the highest antifungal activity at acidic pH values and in the presence of bivalent metals, such as zinc(II), cobalt(II), and copper(II). Measurement of mRNA expression levels and scanning electron microscope (SEM) observations revealed that DPA causes V. pyri apoptosis via inhibition of chitin biosynthesis and subsequent cell lysis. Interestingly, DPA showed high stability in the pear bark and was able to cross the pear tree bark into the phloem, protecting the internal phases of the pear trunk. In preventive applications, DPA reduced the canker symptoms of V. pyri on Cuigan pear trees by 90%. Taken together, an efficient strategy for the management of V. pyri-caused canker disease was developed using a novel antifungal agent, DPA, with strong antifungal activity and particular diffusion properties.