PdSlt2 Penicillium digitatum mitogen-activated-protein kinase controls sporulation and virulence during citrus fruit infection

Inhibitory Properties of Cinnamon Bark Oil against Postharvest Pathogen Penicillium digitatum In Vitro

Penicillium digitatum is a major postharvest pathogen that threatens the global citrus fruit industry and causes great economic losses annually. In the present study, inhibitory properties of cinnamon bark oil (CBO) against P. digitatum in vitro were investigated. Results indicated that 0.03% CBO could efficiently inhibit the spore germination, germ tube elongation, mycelial growth, colonial expansion and conidial accumulation of P. digitatum. The results of fluorescein diacetate (FDA) and MitoTraker Orange (MTO) staining also proved the suppression effects of CBO against P. digitatum. Meanwhile, CBO could inhibit green mold rots induced by P. digitatum in citrus fruit when the working concentration of CBO exceeded 0.06%. In addition, the expressions of 12 genes critical for the growth and virulence of P. digitatum were also significantly regulated under CBO stress. Through a transcriptomic analysis, a total of 1802 common differentially expressed genes (DEGs) were detected in P. digitatum after 4 h and 8 h of CBO treatment. Most of the DEG products were associated with carbohydrate, amino acid and lipid metabolism. They directly or indirectly led to the disturbance of the membrane and the generation of reactive oxygen species (ROS). Our results may deepen the understanding of antifungal properties of CBO against P. digitatum and provide the theoretical foundation to uncover the antifungal mechanism of CBO at the molecular level.

Discovery and Transcriptional Profiling of Penicillium digitatum Genes That Could Promote Fungal Virulence during Citrus Fruit Infection

Green mold caused by Penicillium digitatum (Pers.:Fr.) Sacc is the most prevalent postharvest rot concerning citrus fruits. Using the subtractive suppression hybridization (SSH) technique, different P. digitatum genes have been identified that could be involved in virulence during citrus infection in the early stages, a crucial moment that determines whether the infection progresses or not. To this end, a comparison of two P. digitatum strains with high and low virulence has been carried out. We conducted a study on the gene expression profile of the most relevant genes. The results indicate the importance of transcription and regulation processes as well as enzymes involved in the degradation of the plant cell wall. The most represented expressed sequence tag (EST) was identified as PDIP_11000, associated with the FluG domain, which is putatively involved in the activation of conidiation. It is also worth noting that PDIP_02280 encodes a pectin methyl esterase, a cell wall remodeling protein with a high expression level in the most virulent fungal strains, which is notably induced during citrus infection. Furthermore, within the group with the greatest representation and showing significant induction in the early stages of infection, regulatory proteins (PDIP_68700, PDIP_76160) and a chaperone (PDIP_38040) stand out. To a lesser extent, but not less relevant, it is worth distinguishing different regulatory proteins and transcription factors, such as PDIP_00580, PDIP_49640 and PDIP_78930.

Ultrastructural observation and transcriptome analysis provide insights into mechanisms of Penicillium expansum invading apple wounds

Penicillium expansum is a pathogen causing enormous postharvest losses of fruits, especially apples. In this study, we first investigated the morphological changes of P. expansum within apple wounds during infectious process by microscopic observation. We found that conidia swelled and secreted potential hydrophobin in 4 h, germinated in 8 h, and finally formed conidiophores in 36 h, a critical control time point to prevent the second contamination of spores. We then compared the transcript accumulation of P. expansum in apple tissues and liquid culture at 12 h. In total, 3168 and 1318 up-regulated and down-regulated genes were identified. Among them, genes regarding the biosynthesis of substances such as ergosterol, organic acid, cell wall degrading enzymes, and patulin were induced in expression. Pathways were activated, including autophagy, the mitogen-activated protein kinase, and pectin degradation. Our findings provide insights into the lifestyle and the mechanisms of P. expansum invading apple fruits.

GAR-transferase contributes to purine synthesis and mitochondrion function to maintain fungal development and full virulence of Penicillium digitatum

Penicillium digitatum is one of the most critical phytopathogens during the citrus postharvest period. However, the molecular mechanism of pathogenesis remains to be further explored. Purine is a multiple functional substance in organisms. To verify the role of the de novo purine biosynthesis (DNPB) pathway in P. digitatum, we investigated the third gene Pdgart, glycinamide ribonucleotide (GAR)-transferase, of this pathway in this study. The deletion mutant ΔPdgart was generated in the principle of homologous recombination via Agrobacterium tumefaciens-mediated transformation (ATMT). The phenotypic assay indicated that the ΔPdgart mutant displayed severe defects in hyphae growth, conidiation and germination, which can be rescued by the addition of exogenous ATP and AMP. Compared with wild-type strain N1, the ATP level of strain ΔPdgart was detected to be sharply declined during conidial germination, and this was resulted from the damage to purine synthesis and aerobic respiration. The pathogenicity assay suggested that mutant ΔPdgart infected citrus fruit but attenuated disease, which was owing to its reduced production of organic acids and activities of cell wall degradation enzymes. Additionally, the ΔPdgart mutant showed altered sensitivity to stress agents and fungicides. Taken together, the present study provides insights into the essential functions of Pdgart, and paves the way for further study and novel fungicide development.

Elicitation of Fruit Fungi Infection and Its Protective Response to Improve the Postharvest Quality of Fruits

Fruit diseases brought on by fungus infestation leads to postharvest losses of fresh fruit. Approximately 30% of harvested fruits do not reach consumers’ plates due to postharvest losses. Fungal pathogens play a substantial part in those losses, as they cause the majority of fruit rots and consumer complaints. Understanding fungal pathogenic processes and control measures is crucial for developing disease prevention and treatment strategies. In this review, we covered the presented pathogen entry, environmental conditions for pathogenesis, fruit’s response to pathogen attack, molecular mechanisms by which fungi infect fruits in the postharvest phase, production of mycotoxin, virulence factors, fungal genes involved in pathogenesis, and recent strategies for protecting fruit from fungal attack. Then, in order to investigate new avenues for ensuring fruit production, existing fungal management strategies were then assessed based on their mechanisms for altering the infection process. The goal of this review is to bridge the knowledge gap between the mechanisms of fungal disease progression and numerous disease control strategies being developed for fruit farming.

Unveiling the Role Displayed by Penicillium digitatum PdMut3 Transcription Factor in Pathogen-Fruit Interaction

Zn2Cys6 transcription factors are unique to fungi and are involved in different regulatory functions. In this study, we have identified the Penicillium digitatumPdMut3 gene, which encodes a putative Zn (II) 2Cys6 DNA-binding protein. Elimination of PdMut3 in Pd1 strain caused increased virulence during citrus infection. The transcription of the PdMut3 gene showed a higher expression rate during fungal growth and less transcription during fruit infection. Furthermore, the deletion of the gene in the wild-type isolate of P. digitatum did not produce any modification of the sensitivity to different fungicides, indicating that the gene is not associated with resistance to fungicides. In contrast, PdMut3 null mutants showed a reduction in growth in minimal media, which was associated with severe alterations in conidiophore development and morphological alterations of the hyphae. Mutants showed greater sensitivity to compounds that interfere with the cell wall and an invasive growth block. Thus, PdMut3 might have an indirect role in fungi virulence through metabolism and peroxisomes development.

Molecular Mechanisms Underlying Fungicide Resistance in Citrus Postharvest Green Mold

The necrotrophic fungus Penicillium digitatum (Pd) is responsible for the green mold disease that occurs during postharvest of citrus and causes enormous economic losses around the world. Fungicides remain the main method used to control postharvest green mold in citrus fruit storage despite numerous occurrences of resistance to them. Hence, it is necessary to find new and more effective strategies to control this type of disease. This involves delving into the molecular mechanisms underlying the appearance of resistance to fungicides during the plant–pathogen interaction. Although mechanisms involved in resistance to fungicides have been studied for many years, there have now been great advances in the molecular aspects that drive fungicide resistance, which facilitates the design of new means to control green mold. A wide review allows the mechanisms underlying fungicide resistance in Pd to be unveiled, taking into account not only the chemical nature of the compounds and their target of action but also the general mechanism that could contribute to resistance to others compounds to generate what we call multidrug resistance (MDR) phenotypes. In this context, fungal transporters seem to play a relevant role, and their mode of action may be controlled along with other processes of interest, such as oxidative stress and fungal pathogenicity. Thus, the mechanisms for acquisition of resistance to fungicides seem to be part of a complex framework involving aspects of response to stress and processes of fungal virulence.

Mining RNA-Seq Data to Depict How Penicillium digitatum Shapes Its Transcriptome in Response to Nanoemulsion

Penicillium digitatum is the most severe pathogen that infects citrus fruits during storage. It can cause fruit rot and bring significant economic losses. The continuous use of fungicides has resulted in the emergence of drug-resistant strains. Consequently, there is a need to develop naturally and efficiently antifungal fungicides. Natural antimicrobial agents such as clove oil, cinnamon oil, and thyme oil can be extracted from different plant parts. They exhibited broad-spectrum antimicrobial properties and have great potential in the food industry. Here, we exploit a novel cinnamaldehyde (CA), eugenol (EUG), or carvacrol (CAR) combination antifungal therapy and formulate it into nanoemulsion form to overcome lower solubility and instability of essential oil. In this study, the antifungal activity evaluation and transcriptional profile of Penicillium digitatum exposed to compound nanoemulsion were evaluated. Results showed that compound nanoemulsion had a striking inhibitory effect on P. digitatum in a dose-dependent manner. According to RNA-seq analysis, there were 2,169 differentially expressed genes (DEGs) between control and nanoemulsion-treated samples, including 1,028 downregulated and 1,141 upregulated genes. Gene Ontology (GO) analysis indicated that the DEGs were mainly involved in intracellular organelle parts of cell component: cellular respiration, proton transmembrane transport of biological process, and guanyl nucleotide-binding molecular function. KEGG analysis revealed that metabolic pathway, biosynthesis of secondary metabolites, and glyoxylate and dicarboxylate metabolism were the most highly enriched pathways for these DEGs. Taken together, we can conclude the promising antifungal activity of nanoemulsion with multiple action sites against P. digitatum. These outcomes would deepen our knowledge of the inhibitory mechanism from molecular aspects and exploit naturally, efficiently, and harmlessly antifungal agents in the citrus postharvest industry.

The completed genome sequence of the pathogenic ascomycete fungus Penicillium digitatum

P. digitatum, the causative agent of green mold, is one of the most destructive pathogens in the citrus industry. To facilitate basal researches on this important plant pathogen, here we report a finished genome sequence for P. digitatum strain PDW03 using a combination of Illumina, PacBio, and Hi-C sequencing technologies. The assembly comprised 6 chromosomes from telomere to telomere and encodes approximately 9000 proteins. Genomic re-analyses identified 302 Carbohydrate-active enzymes, 420 secreted proteins, and 39 secondary metabolite (SM) gene clusters. Furthermore, we found 10 fragmentary SM clusters in the P. digitatum PDW03 genome. Pangenome analysis based on 5 P. digitatum genomes available showed that conserved orthogroups account for ~68% of the species pangenome. Taken together, this fully completed P. digitatum genome will provide an optimum resource for further researches to investigate the driving forces of fungal host switch and effectors functioning in plant-pathogen interaction.

Penicilliumdigitatum MFS transporters can display different roles during pathogen-fruit interaction

Major facilitator superfamily (MFS) comprises a large family of fungal transporters. In this work four Penicillium digitatum MFS transporters named PdMFS2-5 were identified and functionally characterized through gene elimination and gene overexpression with aim of unveil the similarities and differences among members of the same family during pathogen-fruit interaction. Fungal mutants in which each of the MFS transporters were individually deleted, displayed a clear effect on their infective capacity during citrus fruit infection especially in two of them. In contrast, the observed effect on fungicide sensitivity limits PdMFS2 and PdMFS3 as transporters underlying fungicide resistance. Moreover, overexpression transformants confirmed P. digitatum MFS transporters function and PdMFS2 and PdMFS3 were able to confer fungicide resistance to P. digitatum strains originally fungicide sensitive. Gene transcription rate depended on each MFS transporter being PdMFS4 the one with higher gene expression. Transcriptional profiling was similar regardless the P. digitatum strain. The gene expression analysis showed an increase of PdMFSs transcription in all overexpression transformants, particularly in Pd27 strain. Expression analysis carried out during P. digitatum-citrus fruit interaction confirmed the contribution of all PdMFSs, excepting PdMFS5, in fungal virulence. These results indicate that MFS fungal transporters might be part of different processes and can replace other genes functions giving them a very high degree of versatility.

Significance of 195 bp-enhancer of PdCYP51B in the acquisition of Penicillium digitatum DMI resistance and increase of fungal virulence

Two sterol 14α-demethylase genes from Penicillium digitatum, PdCYP51A and PdCYP51B, were evaluated and revealed that 95% of Imazalil (IMZ)-resistant isolates carried a 195-bp insertion in the PdCYP51B promoter. We functionally characterized both sterol 14α-demethylases by overexpression. Molecular analysis of overexpression mutants showed that the introduction of PdCYP51B insertion is more stable than the five-tandem repeat PdCYP51A sequence previously described that confers DMI fungicide resistance. The both enhancers can coexist in P. digitatum isolates that initially contained the 195-bp PdCYP51B insertion but the introduction of 195-bp PdCYP51B enhancer promoted the loss of the five-tandem repeat of PdCYP51A. The incorporation of 195-bp PdCYP51B resulted in an increase of DMI fungicide resistance in mutants from already resistant isolates and confers resistance to DMIs in mutants from sensitive isolates. Transcription evaluation of the both genes showed noticeable induction in all overexpression mutants, except for those coming from the five-tandem repeat PdCYP51A sequence, whereas PdCYP51A expression dropped dramatically. Only PdCYP51B exhibited up-regulation during citrus infection compared to axenic growth, and the role of PdCYP51B in fungal virulence was further reinforced since strains with low virulence showed increased infectivity in overexpression mutants. This study suggested the predominant role of the PdCYP51B enhancer in the acquisition of DMI resistance and fungal virulence, by replacing homologues genes with same putative function.

Citrus Postharvest Green Mold: Recent Advances in Fungal Pathogenicity and Fruit Resistance

As the major postharvest disease of citrus fruit, postharvest green mold is caused by the necrotrophic fungus Penicillium digitatum (Pd), which leads to huge economic losses worldwide. Fungicides are still the main method currently used to control postharvest green mold in citrus fruit storage. Investigating molecular mechanisms of plant-pathogen interactions, including pathogenicity and plant resistance, is crucial for developing novel and safer strategies for effectively controlling plant diseases. Despite fruit-pathogen interactions remaining relatively unexplored compared with well-studied leaf-pathogen interactions, progress has occurred in the citrus fruit-Pd interaction in recent years, mainly due to their genome sequencing and establishment or optimization of their genetic transformation systems. Recent advances in Pd pathogenicity on citrus fruit and fruit resistance against Pd infection are summarized in this review.

Transcriptome analysis of fungicide-responsive gene expression profiles in two Penicillium italicum strains with different response to the sterol demethylation inhibitor (DMI) fungicide prochloraz

Background

Penicillium italicum (blue mold) is one of citrus pathogens causing undesirable citrus fruit decay even at strictly-controlled low temperatures (< 10 °C) during shipping and storage. P. italicum isolates with considerably high resistance to sterol demethylation inhibitor (DMI) fungicides have emerged; however, mechanism(s) underlying such DMI-resistance remains unclear. In contrast to available elucidation on anti-DMI mechanism for P. digitatum (green mold), how P. italicum DMI-resistance develops has not yet been clarified.

Results

The present study prepared RNA-sequencing (RNA-seq) libraries for two P. italicum strains (highly resistant (Pi-R) versus highly sensitive (Pi-S) to DMI fungicides), with and without prochloraz treatment, to identify prochloraz-responsive genes facilitating DMI-resistance. After 6 h prochloraz-treatment, comparative transcriptome profiling showed more differentially expressed genes (DEGs) in Pi-R than Pi-S. Functional enrichments identified 15 DEGs in the prochloraz-induced Pi-R transcriptome, simultaneously up-regulated in P. italicum resistance. These included ATP-binding cassette (ABC) transporter-encoding genes, major facilitator superfamily (MFS) transporter-encoding genes, ergosterol (ERG) anabolism component genes ERG2, ERG6 and EGR11 (CYP51A), mitogen-activated protein kinase (MAPK) signaling-inducer genes Mkk1 and Hog1, and Ca²⁺/calmodulin-dependent kinase (CaMK) signaling-inducer genes CaMK1 and CaMK2. Fragments Per Kilobase per Million mapped reads (FPKM) analysis of Pi-R transcrtiptome showed that prochloraz induced mRNA increase of additional 4 unigenes, including the other two ERG11 isoforms CYP51B and CYP51C and the remaining kinase-encoding genes (i.e., Bck1 and Slt2) required for Slt2-MAPK signaling. The expression patterns of all the 19 prochloraz-responsive genes, obtained in our RNA-seq data sets, have been validated by quantitative real-time PCR (qRT-PCR). These lines of evidence in together draw a general portrait of anti-DMI mechanisms for P. italicum species. Intriguingly, some strategies adopted by the present Pi-R were not observed in the previously documented prochloraz-resistant P. digitatum transcrtiptomes. These included simultaneous induction of all major EGR11 isoforms (CYP51A/B/C), over-expression of ERG2 and ERG6 to modulate ergosterol anabolism, and concurrent mobilization of Slt2-MAPK and CaMK signaling processes to overcome fungicide-induced stresses.

Conclusions

The present findings provided transcriptomic evidence on P. italicum DMI-resistance mechanisms and revealed some diversity in anti-DMI strategies between P. italicum and P. digitatum species, contributing to our knowledge on P. italicum DMI-resistance mechanisms.

PdMFS1 Transporter Contributes to Penicilliun digitatum Fungicide Resistance and Fungal Virulence during Citrus Fruit Infection

A new Penicillium digitatum major facilitator superfamily (MFS) transporter (PdMFS1) was identified and functionally characterized in order to shed more light on the mechanisms underlying fungicide resistance. PdMFS1 can play an important role in the intensification of resistance to fungicides normally used in P. digitatum postharvest treatments. In the PdMFS1 disrupted mutants, a slight effect in response to chemical fungicides was observed, but fungicide sensitivity was highly affected in the overexpression mutants which became resistant to wide range of chemical fungicides. Moreover, P. digitatum knock-out mutants exhibited a lower rate of fungal virulence when infected oranges were stored at 20 °C. Disease symptoms were higher in the PdMFS1 overexpression mutants coming from the low-virulent P. digitatum parental strain. In addition, the gene expression analysis showed an induction of PdMFS1 transcription in all overexpression mutants regardless from which progenitor came from, and four-time intensification of the parental wild type strain during citrus infection reinforcing PdMFS1 role in fungal virulence. The P. digitatum MFS transporter PdMFS1 contributes not only to the acquisition of wide range of fungicide resistance but also in fungal virulence during citrus infection.