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.

Characterizing the Palm Pathogenic Thielaviopsis Species from Florida

Thielaviopsis paradoxa sensu lato is a soilborne fungal pathogen that causes Thielaviopsis trunk rot and heart rot in palms. The loss of structural integrity resulting from trunk rot can cause the palm trunk to collapse suddenly and poses a serious threat to life and property. Even though rudimentary knowledge about the Thielaviopsis infection process in palms is available, nothing is known about the T. paradoxa species complex in the US. The aim of this study was to characterize T. paradoxa s. lat. isolates collected from diseased palms grown in Florida. Multi-locus phylogeny using three genes, ITS, β-tubulin, and tef1-α, revealed that the isolates separate into two distinct clades with high bootstrap support. The majority of the isolates clustered with the species T. ethacetica, while two isolates formed a separate clade, distinct from T. musarum, and might represent an undescribed Thielaviopsis species. One representative isolate from each clade, when grown on three distinct media and at four different temperatures, showed differences in gross colony morphology, as well as growth rates. The T. ethacetica isolate TP5448 and the Thielaviopsis sp. isolate PLM300 grew better at opposite ends of the temperature spectrum tested in this study, i.e., 35 °C and 10 °C, respectively. In pathogenicity assays on whole plants, the T. ethacetica isolate proved to be more aggressive than Thielaviopsis sp. isolate PLM300, as it produced larger lesions when inoculated on wounded leaflets. An unequal distribution was observed for the mating-type locus of T. ethacetica, as 12 isolates carried the MAT1-1-1 allele, while the status for four isolates remained undefined. Variation in mycelial growth in response to different fungicides was also observed between the two clades. These results demonstrate the existence of two Thielaviopsis clades that can infect palms in Florida and underscore the need for targeted sampling to help uncover the diversity of Thielaviopsis species across palm-growing regions in the US.

The Snf5-Hsf1 transcription module synergistically regulates stress responses and pathogenicity by maintaining ROS homeostasis in Sclerotinia sclerotiorum

The SWI/SNF complex is guided to the promoters of designated genes by its co-operator to activate transcription in a timely and appropriate manner to govern development, pathogenesis, and stress responses in fungi. Nevertheless, knowledge of the complexes and their co-operator in phytopathogenic fungi is still fragmented. We demonstrate that the heat shock transcription factor SsHsf1 guides the SWI/SNF complex to promoters of heat shock protein (hsp) genes and antioxidant enzyme genes using biochemistry and pharmacology. This is accomplished through direct interaction with the complex subunit SsSnf5 under heat shock and oxidative stress. This results in the activation of their transcription and mediates histone displacement to maintain reactive oxygen species (ROS) homeostasis. Genetic results demonstrate that the transcription module formed by SsSnf5 and SsHsf1 is responsible for regulating morphogenesis, stress tolerance, and pathogenicity in Sclerotinia sclerotiorum, especially by directly activating the transcription of hsp genes and antioxidant enzyme genes counteracting plant-derived ROS. Furthermore, we show that stress-induced phosphorylation of SsSnf5 is necessary for the formation of the transcription module. This study establishes that the SWI/SNF complex and its co-operator cooperatively regulate the transcription of hsp genes and antioxidant enzyme genes to respond to host and environmental stress in the devastating phytopathogenic fungi.

Two Putative Pheromone Receptors, but Not Their Cognate Pheromones, Regulate Female Fertility in the Atypical Mating Fungus Colletotrichum fructicola

Colletotrichum fungi are a group of damaging phytopathogens with atypical mating type loci (harboring only MAT1-2-1 but not MAT1-1-1) and complex sexual behaviors. Sex pheromones and their cognate G-protein-coupled receptors are conserved regulators of fungal mating. These genes, however, lose function frequently among Colletotrichum species, indicating a possibility that pheromone signaling is dispensable for Colletotrichum sexual reproduction. We have identified two putative pheromone-receptor pairs (PPG1:PRE2, PPG2:PRE1) in C. fructicola, a species that exhibits plus-to-minus mating type switching and plus-minus-mediated mating line development. Here, we report the generation and characterization of gene-deletion mutants for all four genes in both plus and minus strain backgrounds. Single-gene deletion of pre1 or pre2 had no effect on sexual development, whereas their double deletion caused self-sterility in both the plus and minus strains. Moreover, double deletion of pre1 and pre2 caused female sterility in plus-minus outcrossing. Double deletion of pre1 and pre2, however, did not inhibit perithecial differentiation or plus-minus-mediated enhancement of perithecial differentiation. Contrary to the results with pre1 and pre2, double deletion of ppg1 and ppg2 had no effect on sexual compatibility, development, or fecundity. We concluded that pre1 and pre2 coordinately regulate C. fructicola mating by recognizing novel signal molecule(s) distinct from canonical Ascomycota pheromones. The contrasting importance between pheromone receptors and their cognate pheromones highlights the complicated nature of sex regulation in Colletotrichum fungi.

Cross-Talk and Multiple Control of Target of Rapamycin (TOR) in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic phytopathogenic fungus that cross-talks with its hosts for control of cell-death pathways for colonization. Target of rapamycin (TOR) is a central regulator that controls cell growth, intracellular metabolism, and stress responses in a variety of eukaryotes, but little is known about TOR signaling in S. sclerotiorum. In this study, we identified a conserved TOR signaling pathway and characterized SsTOR as a critical component of this pathway. Hyphal growth of S. sclerotiorum was retarded by silencing SsTOR, moreover, sclerotia and compound appressoria formation were severely disrupted. Notably, pathogenicity assays of strains shows that the virulence of the SsTOR-silenced strains were dramatically decreased. SsTOR was determined to participate in cell wall integrity (CWI) by regulating the phosphorylation level of SsSmk3, a core MAP kinase in the CWI pathway. Importantly, the inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. Taken together, our results suggest that SsTOR is a global regulator controlling cell growth, stress responses, cell wall integrity, autophagy, and virulence of S. sclerotiorum. IMPORTANCE TOR is a conserved protein kinase that regulates cell growth and metabolism in response to growth factors and nutrient abundance. Here, we used gene silencing to characterize SsTOR, which is a critical component of TOR signaling pathway. SsTOR-silenced strains have limited mycelium growth, and the virulence of the SsTOR-silenced strains was decreased. Phosphorylation analysis indicated that SsTOR influenced CWI by regulating the phosphorylation level of SsSmk3. Autophagy is essential to preserve cellular homeostasis in response to cellular and environmental stresses. Inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. These findings further indicated that SsTOR is a global regulator of the growth, development, and pathogenicity of S. sclerotiorum in multiple ways.

The C2H2 Transcription Factor SsZFH1 Regulates the Size, Number, and Development of Apothecia in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a notorious phytopathogenic Ascomycota fungus with a host range of >600 plant species worldwide. This homothallic Leotiomycetes species reproduces sexually through a multicellular apothecium that produces and releases ascospores. These ascospores serve as the primary inoculum source for disease initiation in the majority of S. sclerotiorum disease cycles. The regulation of apothecium development for this pathogen and other apothecium-producing fungi remains largely unknown. Here, we report that a C2H2 transcription factor, SsZFH1 (zinc finger homologous protein), is necessary for the proper development and maturation of sclerotia and apothecia in S. sclerotiorum and is required for the normal growth rate of hyphae. Furthermore, ΔSszfh1 strains exhibit decreased H₂O₂ accumulation in hyphae, increased melanin deposition, and enhanced tolerance to H₂O₂ in the process of vegetative growth and sclerotia formation. Infection assays on common bean leaves, with thin cuticles, and soybean and tomato leaves, with thick cuticles, suggest that the deletion of Sszfh1 slows the mycelial growth rate, which in turn affects the expansion of leaf lesions. Collectively, our results provide novel insights into a major fungal factor mediating maturation of apothecia with additional effects on hyphae and sclerotia development.

Genome sequence of Monilinia vaccinii-corymbosi sheds light on mummy berry disease infection of blueberry and mating type

Mummy berry disease, caused by the fungal pathogen Monilinia vaccinii-corymbosi (Mvc), is one of the most economically important diseases of blueberries in North America. Mvc is capable of inducing two separate blighting stages during its life cycle. Infected fruits are rendered mummified and unmarketable. Genomic data for this pathogen is lacking, but could be useful in understanding the reproductive biology of Mvc and the mechanisms it deploys to facilitate host infection. In this study, PacBio sequencing and Hi-C interaction data were utilized to create a chromosome-scale reference genome for Mvc. The genome comprises nine chromosomes with a total length of 30 Mb, an N50 length of 4.06 Mb, and an average 413X sequence coverage. A total of 9399 gene models were predicted and annotated, and BUSCO analysis revealed that 98% of 1,438 searched conserved eukaryotic genes were present in the predicted gene set. Potential effectors were identified, and the mating-type (MAT) locus was characterized. Biotrophic effectors allow the pathogen to avoid recognition by the host plant and evade or mitigate host defense responses during the early stages of fruit infection. Following locule colonization, necrotizing effectors promote the mummification of host tissues. Potential biotrophic effectors utilized by Mvc include chorismate mutase for reducing host salicylate and necrotrophic effectors include necrosis-inducing proteins and hydrolytic enzymes for macerating host tissue. The MAT locus sequences indicate the potential for homothallism in the reference genome, but a deletion allele of the MAT locus, characterized in a second isolate, indicates heterothallism. Further research is needed to verify the roles of individual effectors in virulence and to determine the role of the MAT locus in outcrossing and population genotypic diversity.

Efficient genome editing using endogenous U6 snRNA promoter-driven CRISPR/Cas9 sgRNA in Sclerotinia sclerotiorum

We previously reported on a CRISPR-Cas9 genome editing system for the necrotrophic fungal plant pathogen Sclerotinia sclerotiorum. This system (the TrpC-sgRNA system), based on an RNA polymerase II (RNA Pol II) promoter (TrpC) to drive sgRNA transcription in vivo, was successful in creating gene insertion mutants. However, relatively low efficiency targeted gene editing hampered the application of this method for functional genomic research in S. sclerotiorum. To further optimize the CRISPR-Cas9 system, a plasmid-free Cas9 protein/sgRNA ribonucleoprotein (RNP)-mediated system (the RNP system) and a plasmid-based RNA polymerase III promoter (U6)-driven sgRNA transcription system (the U6-sgRNA system) were established and evaluated. The previously characterized oxaloacetate acetylhydrolase (Ssoah1) locus and a new locus encoding polyketide synthase12 (Sspks12) were targeted in this study to create loss-of-function mutants. The RNP system, similar to the TrpC-sgRNA system we previously reported, creates mutations at the Ssoah1 gene locus with comparable efficiency. However, neither system successfully generated mutations at the Sspks12 gene locus. The U6-sgRNA system exhibited a significantly higher efficiency of genemutation at both loci. This technology provides a simple and efficient strategy for targeted gene mutation and thereby will accelerating the pace of research of pathogenicity and development in this economically important plant pathogen.

Molecular Dissection of Perithecial Mating Line Development in Colletotrichum fructicola, a Species with a Nontypical Mating System Featuring Plus-to-Minus Switch and Plus-Minus-Mediated Sexual Enhancement

The genetic regulation of Colletotrichum (Glomerella) sexual reproduction does not strictly adhere to the Ascomycota paradigm and remains poorly understood. Morphologically different but sexually compatible strain types, termed plus and minus, have been recognized, but the biological and molecular distinctions between these strain types remain elusive. In this study, we characterized the sexual behaviors of a pair of plus and minus strains of C. fructicola with the aid of live-cell nucleus-localized fluorescent protein labeling, gene expression, and gene mutation analyses. We confirmed a genetically stable plus-to-minus switching phenomenon and demonstrated the presence of both cross-fertilized and self-fertilized perithecia within the mating line (perithecia cluster at the line of colony contact) between plus and minus strains. We demonstrated that pheromone signaling genes (a-factor-like and α-factor-like pheromones and their corresponding GPCR receptors) were differently expressed between vegetative hyphae of the two strains. Moreover, deletion of pmk1 (a FUS/KSS1 mitogen-activate protein kinase) in the minus strain severely limited mating line formation, whereas deletion of a GPCR (FGSG_05239 homolog) and two histone modification factors (hos2, snt2) in the minus strain did not affect mating line development but altered the ratio between cross-fertilization and self-fertilization within the mating line. We propose a model in which mating line formation in C. fructicola involves enhanced protoperithecium differentiation and enhanced perithecium maturation of the minus strain mediated by both cross-fertilization and diffusive effectors. This study provides insights into mechanisms underlying the mysterious phenomenon of plus-minus-mediated sexual enhancement being unique to Colletotrichum fungi. IMPORTANCE Plus-minus regulation of Colletotrichum sexual differentiation was reported in the early 1900s. Both plus and minus strains produce fertile perithecia in a homothallic but inefficient manner. However, when the two strain types encounter each other, efficient differentiation of fertile perithecia is triggered. The plus strain, by itself, can also generate minus ascospore progeny at high frequency. This nontypical mating system facilitates sexual reproduction and is Colletotrichum specific; the underlying molecular mechanisms, however, remain elusive. The current study revisits this longstanding mystery using C. fructicola as an experimental system. The presence of both cross-fertilized and self-fertilized perithecia within the mating line was directly evidenced by live-cell imaging with fluorescent markers. Based on further gene expression and gene mutation analysis, a model explaining mating line development (plus-minus-mediated sexual enhancement) is proposed. Data reported here have the potential to allow us to better understand Colletotrichum mating and filamentous ascomycete sexual regulation.

Transcriptional profile of oil palm pathogen, Ganoderma boninense, reveals activation of lignin degradation machinery and possible evasion of host immune response

BACKGROUND

The white-rot fungi in the genus Ganoderma interact with both living and dead angiosperm tree hosts. Two Ganoderma species, a North American taxon, G. zonatum and an Asian taxon, G. boninense, have primarily been found associated with live palm hosts. During the host plant colonization process, a massive transcriptional reorganization helps the fungus evade the host immune response and utilize plant cell wall polysaccharides.

RESULTS

A publicly available transcriptome of G. boninense - oil palm interaction was surveyed to profile transcripts that were differentially expressed in planta. Ten percent of the G. boninense transcript loci had altered expression as it colonized oil palm plants one-month post inoculation. Carbohydrate active enzymes (CAZymes), particularly those with a role in lignin degradation, and auxiliary enzymes that facilitate lignin modification, like cytochrome P450s and haloacid dehalogenases, were up-regulated in planta. Several lineage specific proteins and secreted proteins that lack known functional domains were also up-regulated in planta, but their role in the interaction could not be established. A slowdown in G. boninense respiration during the interaction can be inferred from the down-regulation of proteins involved in electron transport chain and mitochondrial biogenesis. Additionally, pathogenicity related genes and chitin degradation machinery were down-regulated during the interaction indicating G. boninense may be evading detection by the host immune system.

CONCLUSIONS

This analysis offers an overview of the dynamic processes at play in G. boninense - oil palm interaction and provides a framework to investigate biology of Ganoderma fungi across plantations and landscape.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.

Genomic and transcriptomic insights into Raffaelea lauricola pathogenesis

BACKGROUND

Laurel wilt caused by Raffaelea lauricola is a lethal vascular disease of North American members of the Lauraceae plant family. This fungus and its primary ambrosia beetle vector Xyleborus glabratus originated from Asia; however, there is no report of laurel wilt causing widespread mortality on native Lauraceae trees in Asia. To gain insight into why R. lauricola is a tree-killing plant pathogen in North America, we generated and compared high quality draft genome assemblies of R. lauricola and its closely related non-pathogenic species R. aguacate.

RESULTS

Relative to R. aguacate, the R. lauricola genome uniquely encodes several small-secreted proteins that are associated with virulence in other pathogens and is enriched in secondary metabolite biosynthetic clusters, particularly polyketide synthase (PKS), non-ribosomal peptide synthetase (NRPS) and PKS-NRPS anchored gene clusters. The two species also exhibit significant differences in secreted proteins including CAZymes that are associated with polysaccharide binding including the chitin binding CBM50 (LysM) domain. Transcriptomic comparisons of inoculated redbay trees and in vitro-grown fungal cultures further revealed a number of secreted protein genes, secondary metabolite clusters and alternative sulfur uptake and assimilation pathways that are coordinately up-regulated during infection.

CONCLUSIONS

Through these comparative analyses we have identified potential adaptations of R. lauricola that may enable it to colonize and cause disease on susceptible hosts. How these adaptations have interacted with co-evolved hosts in Asia, where little to no disease occurs, and non-co-evolved hosts in North America, where lethal wilt occurs, requires additional functional analysis of genes and pathways.

Highly Contiguous Genome Resource of Colletotrichum fructicola Generated Using Long-Read Sequencing

Colletotrichum fructicola is a plant-pathogenic fungus with a broad host range. It causes significant losses to important crops, including apple, pear, strawberry, and other Rosaceae and non-Rosaceae species. To date, two short read-based C. fructicola genomes are publicly available, but both are fragmented. In this study, we re-sequenced the genome of C. fructicola using nanopore long-read technology and refined the assembly with Hi-C map data. The resulting high-quality assembly is an important resource for further comparative and experimental studies with C. fructicola.

Mechanisms of Broad Host Range Necrotrophic Pathogenesis in Sclerotinia sclerotiorum

Among necrotrophic fungi, Sclerotinia sclerotiorum is remarkable for its extremely broad host range and for its aggressive host tissue colonization. With full genome sequencing, transcriptomic analyses and the increasing pace of functional gene characterization, the factors underlying the basis of this broad host range necrotrophic pathogenesis are now being elucidated at a greater pace. Among these, genes have been characterized that are required for infection via compound appressoria in addition to genes associated with colonization that regulate oxalic acid (OA) production and OA catabolism. Moreover, virulence-related secretory proteins have been identified, among which are candidates for manipulating host activities apoplastically and cytoplasmically. Coupled with these mechanistic studies, cytological observations of the colonization process have blurred the heretofore clear-cut biotroph versus necrotroph boundary. In this review, we reexamine the cytology of S. sclerotiorum infection and put more recent molecular and genomic data into the context of this cytology. We propose a two-phase infection model in which the pathogen first evades, counteracts and subverts host basal defense reactions prior to killing and degrading host cells. Spatially, the pathogen may achieve this via the production of compatibility factors/effectors in compound appressoria, bulbous subcuticular hyphae, and primary invasive hyphae. By examining the nuances of this interaction, we hope to illuminate new classes of factors as targets to improve our understanding of broad host range necrotrophic pathogens and provide the basis for understanding corresponding host resistance.

The GATA-type IVb zinc-finger transcription factor SsNsd1 regulates asexual-sexual development and appressoria formation in Sclerotinia sclerotiorum

The sclerotium, a multicellular structure composed of the compact aggregation of vegetative hyphae, is critical for the long-term survival and sexual reproduction of the plant-pathogenic fungus Sclerotinia sclerotiorum. The development and carpogenic germination of sclerotia are regulated by integrating signals from both environmental and endogenous processes. Here, we report the regulatory functions of the S. sclerotiorum GATA-type IVb zinc-finger transcription factor SsNsd1 in these processes. SsNsd1 is orthologous to the Aspergillus nidulans NsdD (never in sexual development) and the Neurospora crassa SUB-1 (submerged protoperithecia-1) proteins. Ssnsd1 gene transcript accumulation remains relatively low, but variable, during vegetative mycelial growth and multicellular development. Ssnsd1 deletion mutants (Δnsd1-KOs) produce phialides and phialospores (spermatia) excessively in vegetative hyphae and promiscuously within the interior medulla of sclerotia. In contrast, phialospore development occurs only on the sclerotium surface in the wild-type. Loss of SsNsd1 function affects sclerotium structural integrity and disrupts ascogonia formation during conditioning for carpogenic germination. As a consequence, apothecium development is abolished. The Ssnsd1 deletion mutants are also defective in the transition from hyphae to compound appressorium formation, resulting in a loss of pathogenicity on unwounded hosts. In sum, our results demonstrate that SsNsd1 functions in a regulatory role similar to its ascomycete orthologues in regulating sexual and asexual development. Further, SsNsd1 appears to have evolved as a regulator of pre-penetration infectious development required for the successful infection of its many hosts.

Introduction of Large Sequence Inserts by CRISPR-Cas9 To Create Pathogenicity Mutants in the Multinucleate Filamentous Pathogen Sclerotinia sclerotiorum

The necrotrophic fungal plant pathogen Sclerotinia sclerotiorum is responsible for substantial global crop losses annually resulting in localized food insecurity and loss of livelihood. Understanding the basis of this broad-host-range and aggressive pathogenicity is hampered by the quantitative nature of both host resistance and pathogen virulence. To improve this understanding, methods for efficient functional gene characterization that build upon the existing complete S. sclerotiorum genome sequence are needed. Here, we report on the development of a clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (CRISPR-Cas9)-mediated strategy for creating gene disruption mutants and the application of this technique for exploring roles of known and hypothesized virulence factors. A key finding of this research is that transformation with a circular plasmid encoding Cas9, target single guide RNA (sgRNA), and a selectable marker resulted in a high frequency of targeted, insertional gene mutation. We observed that 100% of the mutants integrated large rearranged segments of the transforming plasmid at the target site facilitated by the nonhomologous end joining (NHEJ) repair pathway. This result was confirmed in multiple target sites within the same gene in three independent wild-type isolates of S. sclerotiorum and in a second independent gene. Targeting the previously characterized Ssoah1 gene allowed us to confirm the loss-of-function nature of the CRISPR-Cas9-mediated mutants and explore new aspects of the mutant phenotype. Applying this technology to create mutations in a second previously uncharacterized gene allowed us to determine the requirement for melanin accumulation in infection structure development and function.IMPORTANCE Fungi that cause plant diseases by rotting or blighting host tissue with limited specificity remain among the most difficult to control. This is largely due to the quantitative nature of host resistance and a limited understanding of fungal pathogenicity. A mechanistic understanding of pathogenicity requires the ability to manipulate candidate virulence genes to test hypotheses regarding their roles in disease development. Sclerotinia sclerotiorum is among the most notorious of these so-called broad-host-range necrotrophic plant pathogens. The work described here provides a new method for rapidly constructing gene disruption vectors to create gene mutations with high efficiency compared with existing methods. Applying this method to characterize gene functions in S. sclerotiorum, we confirm the requirement for oxalic acid production as a virulence factor in multiple isolates of the fungus and demonstrate that melanin accumulation is not required for infection. Using this approach, the pace of functional gene characterization and the understanding of pathogenicity and related disease resistance will increase.

Pathogenic adaptations of Colletotrichum fungi revealed by genome wide gene family evolutionary analyses

The fungal genus Colletotrichum contains hemibiotrophic phytopathogens being highly variable in host and tissue specificities. We sequenced a C. fructicola genome (1104–7) derived from an isolate of apple in China and compared it with the reference genome (Nara_gc5) derived from an isolate of strawberry in Japan. Mauve alignment and BlastN search identified 0.62 Mb lineage-specific (LS) genomic regions in 1104–7 with a length criterion of 10 kb. Genes located within LS regions evolved more dynamically, and a strongly elevated proportion of genes were closely related to non-Colletotrichum sequences. Two LS regions, containing nine genes in total, showed features of fungus-to-fungus horizontal transfer supported by both gene order collinearity and gene phylogeny patterns. We further compared the gene content variations among 13 Colletotrichum and 11 non-Colletotrichum genomes by gene function annotation, OrthoMCL grouping and CAFE analysis. The results provided a global evolutionary picture of Colletotrichum gene families, and identified a number of strong duplication/loss events at key phylogenetic nodes, such as the contraction of the detoxification-related RTA1 family in the monocot-specializing graminicola complex and the expansions of several ammonia production-related families in the fruit-infecting gloeosporioides complex. We have also identified the acquirement of a RbsD/FucU fucose transporter from bacterium by the Colletotrichum ancestor. In sum, this study summarized the pathogenic evolutionary features of Colletotrichum fungi at multiple taxonomic levels and highlights the concept that the pathogenic successes of Colletotrichum fungi require shared as well as lineage-specific virulence factors.

An atypical forkhead-containing transcription factor SsFKH1 is involved in sclerotial formation and is essential for pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic plant pathogen with a worldwide distribution. The sclerotia of S. sclerotiorum are pigmented multicellular structures formed from the aggregation of vegetative hyphae. These survival structures play a central role in the life and infection cycles of this pathogen. Here, we characterized an atypical forkhead (FKH)-box-containing protein, SsFKH1, involved in sclerotial development and virulence. To investigate the role of SsFkh1 in S. sclerotiorum, the partial sequence of SsFkh1 was cloned and RNA interference (RNAi)-based gene silencing was employed to alter the expression of SsFkh1. RNA-silenced mutants with significantly reduced SsFkh1 RNA levels exhibited slow hyphal growth and sclerotial developmental defects. In addition, the expression levels of a set of putative melanin biosynthesis-related laccase genes and a polyketide synthase-encoding gene were significantly down-regulated in silenced strains. Disease assays demonstrated that pathogenicity in RNAi-silenced strains was significantly compromised with the development of a smaller infection lesion on tomato leaves. Collectively, the results suggest that SsFkh1 is involved in hyphal growth, virulence and sclerotial formation in S. sclerotiorum.

A Global Perspective on the Population Structure and Reproductive System of Phyllosticta citricarpa

The citrus pathogen Phyllosticta citricarpa was first described 117 years ago in Australia; subsequently, from the summer rainfall citrus-growing regions in China, Africa, and South America; and, recently, the United States. Limited information is available on the pathogen's population structure, mode of reproduction, and introduction pathways, which were investigated by genotyping 383 isolates representing 12 populations from South Africa, the United States, Australia, China, and Brazil. Populations were genotyped using seven published and eight newly developed polymorphic simple-sequence repeat markers. The Chinese and Australian populations had the highest genetic diversities, whereas populations from Brazil, the United States, and South Africa exhibited characteristics of founder populations. The U.S. population was clonal. Based on principal coordinate and minimum spanning network analyses, the Chinese populations were distinct from the other populations. Population differentiation and clustering analyses revealed high connectivity and possibly linked introduction pathways between South Africa, Australia, and Brazil. With the exception of the clonal U.S. populations that only contained one mating type, all the other populations contained both mating types in a ratio that did not deviate significantly from 1:1. Although most populations exhibited sexual reproduction, linkage disequilibrium analyses indicated that asexual reproduction is important in the pathogen's life cycle.

Sexual Reproduction in the Citrus Black Spot Pathogen, Phyllosticta citricarpa

Citrus black spot (Phyllosticta citricarpa) causes fruit blemishes and premature fruit drop, resulting in significant economic losses in citrus growing areas with summer rainfall across the globe. The mating type locus of P. citricarpa has recently been characterized, revealing the heterothallic nature of this pathogen. However, insight into the occurrence of mating and the impact of completing the sexual cycle of P. citricarpa was lacking. To investigate the occurrence and impact of sexual reproduction, we developed a method to reliably, and for the first time, produce ascospores of P. citricarpa on culture media. To demonstrate meiosis during the mating process, we identified recombinant genotypes through multilocus genotyping of single ascospores. Because the process of fertilization was not well understood, we experimentally determined that fertilization of P. citricarpa occurs via spermatization. Our results demonstrate that P. citricarpa is heterothallic and requires isolates of different MAT idiomorphs to be in direct physical contact, or for spermatia to fulfill their role as male elements to fertilize the receptive organs, in order to initiate the mating process. The impact of mating on the epidemiology of citrus black spot in the field is discussed.

Comparing Avocado, Swamp Bay, and Camphortree as Hosts of Raffaelea lauricola Using a Green Fluorescent Protein (GFP)-Labeled Strain of the Pathogen

Raffaelea lauricola, a fungal symbiont of the ambrosia beetle Xyleborus glabratus, causes laurel wilt in members of the Lauraceae plant family. North American species in the family, such as avocado (Persea americana) and swamp bay (P. palustris), are particularly susceptible to laurel wilt, whereas the Asian camphortree (Cinnamomum camphora) is relatively tolerant. To determine whether susceptibility is related to pathogen colonization, a green fluorescent protein-labeled strain of R. lauricola was generated and used to inoculate avocado, swamp bay, and camphortree. Trees were harvested 3, 10, and 30 days after inoculation (DAI), and disease severity was rated on a 1-to-10 scale. By 30 DAI, avocado and swamp bay developed significantly more severe disease than camphortree (mean severities of 6.8 and 5.5 versus 1.6, P < 0.003). The extent of xylem colonization was recorded as the percentage of lumena that were colonized by the pathogen. More xylem was colonized in avocado than camphortree (0.9% versus 0.1%, P < 0.03) but colonization in swamp bay (0.4%) did not differ significantly from either host. Although there were significant correlations between xylem colonization and laurel wilt severity in avocado (r = 0.74), swamp bay (r = 0.82), and camphortree (r = 0.87), even severely affected trees of all species were scarcely colonized by the pathogen.

Mating Type and Simple Sequence Repeat Markers Indicate a Clonal Population of Phyllosticta citricarpa in Florida

Phyllosticta citricarpa, the citrus black spot pathogen, was first identified in Florida in March 2010. Subsequently, this pathogen has become established in Florida but can be easily confused with the endemic nonpathogenic citrus endophyte P. capitalensis. In this study, the mating-type (MAT) loci of P. citricarpa and P. capitalensis were identified via draft genome sequencing and were characterized at the structural and sequence levels. P. citricarpa was determined to have an idiomorphic, heterothallic MAT locus structure, whereas P. capitalensis was found to have a single MAT locus consistent with a homothallic mating system. A survey of P. citricarpa isolates from Florida revealed that only the MAT1-2 idiomorph existed in the Floridian population. In contrast, isolates collected from Australia exhibited a 1:1 ratio of MAT1-1 and MAT1-2 isolates. Development and analysis of simple sequence repeat markers revealed a single multilocus genotype (MLG) in the Floridian population (n = 70) and 11 MLG within the Australian population (n = 24). These results indicate that isolates of P. citricarpa from Florida are likely descendent from a single clonal lineage and are reproducing asexually. The disease management focus in Florida will need to be concentrated on the production and dispersal of pycnidiospores.

Characterization of MAT gene functions in the life cycle of Sclerotinia sclerotiorum reveals a lineage-specific MAT gene functioning in apothecium morphogenesis

Sclerotinia sclerotiorum (Lib.) de Bary is a phytopathogenic fungus that relies on the completion of the sexual cycle to initiate aerial infections. The sexual cycle produces apothecia required for inoculum dispersal. In this study, insight into the regulation of apothecial multicellular development was pursued through functional characterization of mating-type genes. These genes are hypothesized to encode master regulatory proteins required for aspects of sexual development ranging from fertilization through fertile fruiting body development. Experimentally, loss-of-function mutants were created for the conserved core mating-type genes (MAT1-1-1, and MAT1-2-1), and the lineage-specific genes found only in S. sclerotiorum and closely related fungi (MAT1-1-5, and MAT1-2-4). The MAT1-1-1, MAT1-1-5, and MAT1-2-1 mutants are able to form ascogonia but are blocked in all aspects of apothecium development. These mutants also exhibit defects in secondary sexual characters including lower numbers of spermatia. The MAT1-2-4 mutants are delayed in carpogenic germination accompanied with altered disc morphogenesis and ascospore production. They too produce lower numbers of spermatia. All four MAT gene mutants showed alterations in the expression of putative pheromone precursor (Ppg-1) and pheromone receptor (PreA, PreB) genes. Our findings support the involvement of MAT genes in sexual fertility, gene regulation, meiosis, and morphogenesis in S. sclerotiorum.

Fungal oxalate decarboxylase activity contributes to Sclerotinia sclerotiorum early infection by affecting both compound appressoria development and function

Sclerotinia sclerotiorum pathogenesis requires the accumulation of high levels of oxalic acid (OA). To better understand the factors affecting OA accumulation, two putative oxalate decarboxylase (OxDC) genes (Ss-odc1 and Ss-odc2) were characterized. Ss-odc1 transcripts exhibited significant accumulation in vegetative hyphae, apothecia, early stages of compound appressorium development and during plant colonization. Ss-odc2 transcripts, in contrast, accumulated significantly only during mid to late stages of compound appressorium development. Neither gene was induced by low pH or exogenous OA in vegetative hyphae. A loss-of-function mutant for Ss-odc1 (Δss-odc1) showed wild-type growth, morphogenesis and virulence, and was not characterized further. Δss-odc2 mutants hyperaccumulated OA in vitro, were less efficient at compound appressorium differentiation and exhibited a virulence defect which could be fully bypassed by wounding the host plant prior to inoculation. All Δss-odc2 phenotypes were restored to the wild-type by ectopic complementation. An S. sclerotiorum strain overexpressing Ss-odc2 exhibited strong OxDC, but no oxalate oxidase activity. Increasing inoculum nutrient levels increased compound appressorium development, but not penetration efficiency, of Δss-odc2 mutants. Together, these results demonstrate differing roles for S. sclerotiorum OxDCs, with Odc2 playing a significant role in host infection related to compound appressorium formation and function.

The Arabidopsis Mediator Complex Subunit16 Is a Key Component of Basal Resistance against the Necrotrophic Fungal Pathogen Sclerotinia sclerotiorum

Although Sclerotinia sclerotiorum is a devastating necrotrophic fungal plant pathogen in agriculture, the virulence mechanisms utilized by S. sclerotiorum and the host defense mechanisms against this pathogen have not been fully understood. Here, we report that the Arabidopsis (Arabidopsis thaliana) Mediator complex subunit MED16 is a key component of basal resistance against S. sclerotiorum. Mutants of MED16 are markedly more susceptible to S. sclerotiorum than mutants of 13 other Mediator subunits, and med16 has a much stronger effect on S. sclerotiorum-induced transcriptome changes compared with med8, a mutation not altering susceptibility to S. sclerotiorum. Interestingly, med16 is also more susceptible to S. sclerotiorum than coronatine-insensitive1-1 (coi1-1), which is the most susceptible mutant reported so far. Although the jasmonic acid (JA)/ethylene (ET) defense pathway marker gene PLANT DEFENSIN1.2 (PDF1.2) cannot be induced in either med16 or coi1-1, basal transcript levels of PDF1.2 in med16 are significantly lower than in coi1-1. Furthermore, ET-induced suppression of JA-activated wound responses is compromised in med16, suggesting a role for MED16 in JA-ET cross talk. Additionally, MED16 is required for the recruitment of RNA polymerase II to PDF1.2 and OCTADECANOID-RESPONSIVE ARABIDOPSIS ETHYLENE/ETHYLENE-RESPONSIVE FACTOR59 (ORA59), two target genes of both JA/ET-mediated and the transcription factor WRKY33-activated defense pathways. Finally, MED16 is physically associated with WRKY33 in yeast and in planta, and WRKY33-activated transcription of PDF1.2 and ORA59 as well as resistance to S. sclerotiorum depends on MED16. Taken together, these results indicate that MED16 regulates resistance to S. sclerotiorum by governing both JA/ET-mediated and WRKY33-activated defense signaling in Arabidopsis.

Oxaloacetate acetylhydrolase gene mutants of Sclerotinia sclerotiorum do not accumulate oxalic acid, but do produce limited lesions on host plants

The oxaloacetate acetylhydrolase (OAH, EC 3.7.1.1)-encoding gene Ss-oah1 was cloned and functionally characterized from Sclerotinia sclerotiorum. Ss-oah1 transcript accumulation mirrored oxalic acid (OA) accumulation with neutral pH induction dependent on the pH-responsive transcriptional regulator Ss-Pac1. Unlike previously characterized ultraviolet (UV)-induced oxalate-deficient mutants ('A' mutants) which retain the capacity to accumulate OA, gene deletion Δss-oah1 mutants did not accumulate OA in culture or during plant infection. This defect in OA accumulation was fully restored on reintroduction of the wild-type (WT) Ss-oah1 gene. The Δss-oah1 mutants were also deficient in compound appressorium and sclerotium development and exhibited a severe radial growth defect on medium buffered at neutral pH. On a variety of plant hosts, the Δss-oah1 mutants established very restricted lesions in which the infectious hyphae gradually lost viability. Cytological comparisons of WT and Δss-oah1 infections revealed low and no OA accumulation, respectively, in subcuticular hyphae. Both WT and mutant hyphae exhibited a transient association with viable host epidermal cells at the infection front. In summary, our experimental data establish a critical requirement for OAH activity in S. sclerotiorum OA biogenesis and pathogenesis, but also suggest that factors independent of OA contribute to the establishment of primary lesions.

Peroxysomal carnitine acetyl transferase influences host colonization capacity in Sclerotinia sclerotiorum

In lower eukaryotes, the glyoxylate cycle allows cells to utilize two-carbon compounds when simple sugars are not available. In filamentous fungi, glyoxylate metabolism is coupled with β-oxidation of fatty acids, and both are localized to ubiquitous eukaryotic organelles called peroxisomes. Acetyl coenzyme A (acetyl-CoA) produced during β-oxidation is transported via the cytosol into mitochondria for further metabolism. A peroxisomal-specific pathway for acetyl-CoA transport requiring peroxisomal carnitine acetyl transferase (CAT) activity has been identified in Magnaporthe grisea peroxisomes. Here, we report that a Sclerotinia sclerotiorum ortholog of the M. grisea peroxisomal CAT-encoding gene Pth2 (herein designated Ss-pth2) is required for virulence-associated host colonization. Null (ss-pth2) mutants, obtained by in vitro transposon mutagenesis, failed to utilize fatty acids, acetate, or glycerol as sole carbon sources for growth. Gene expression analysis of these mutants showed altered levels of transcript accumulation for glyoxylate cycle enzymes. Ss-pth2 disruption also affected sclerotial, apothecial, and appressorial development and morphology, as well as oxalic acid accumulation when cultured with acetate or oleic acid as sole carbon nutrient sources. Although mutants were able to penetrate and initially colonize host tissue, subsequent colonization was impaired. Genetic complementation with the wild-type Ss-pth2 restored wild-type virulence phenotypes. These findings suggest an essential role in S. sclerotiorum for the peroxisomal metabolic pathways for oxalic acid synthesis and host colonization.

Lifestyle transitions in plant pathogenic Colletotrichum fungi deciphered by genome and transcriptome analyses

Colletotrichum species are fungal pathogens that devastate crop plants worldwide. Host infection involves the differentiation of specialized cell types that are associated with penetration, growth inside living host cells (biotrophy) and tissue destruction (necrotrophy). We report here genome and transcriptome analyses of Colletotrichum higginsianum infecting Arabidopsis thaliana and Colletotrichum graminicola infecting maize. Comparative genomics showed that both fungi have large sets of pathogenicity-related genes, but families of genes encoding secreted effectors, pectin-degrading enzymes, secondary metabolism enzymes, transporters and peptidases are expanded in C. higginsianum. Genome-wide expression profiling revealed that these genes are transcribed in successive waves that are linked to pathogenic transitions: effectors and secondary metabolism enzymes are induced before penetration and during biotrophy, whereas most hydrolases and transporters are upregulated later, at the switch to necrotrophy. Our findings show that preinvasion perception of plant-derived signals substantially reprograms fungal gene expression and indicate previously unknown functions for particular fungal cell types.

pH modulation differs during sunflower cotyledon colonization by the two closely related necrotrophic fungi Botrytis cinerea and Sclerotinia sclerotiorum

During pathogenesis on sunflower cotyledons, Botrytis cinerea and Sclerotinia sclerotiorum show a striking resemblance in symptom development. Based on pH change profiles, the colonization process of both fungi can be divided into two stages. The first stage is associated with a pH decrease, resulting from an accumulation of citric and succinic acids. The second stage is correlated with a pH increase, resulting from an accumulation of ammonia. In this article, we also report that oxalic acid is produced at the late stage of the colonization process and that ammonia accumulation is concomitant with a decrease in free amino acids in decaying tissues. Sclerotinia sclerotiorum produces eight-fold more oxalic acid and two-fold less ammonia than B. cinerea. Consequently, during sunflower cotyledon colonization by B. cinerea, pH dynamics differ significantly from those of S. sclerotiorum. In vitro assays support the in planta results and show that decreases in pH are linked to glucose consumption. At different stages of the colonization process, expression profiles of genes encoding secreted proteases were investigated. This analysis highlights that the expression levels of the B. cinerea protease genes are higher than those of S. sclerotiorum. This work suggests that the overt similarities of S. sclerotiorum and B. cinerea symptom development have probably masked our recognition of the dynamic and potentially different metabolic pathways active during host colonization by these two necrotrophic fungi.

Evidence for morphological, vegetative, genetic, and mating-type diversity in Sclerotinia homoeocarpa

Morphology, vegetative compatibility groups, and molecular characteristics were compared among 47 isolates of the dollar spot pathogen Sclerotinia homoeocarpa. Isolates were collected from cool- and warm-season turfgrasses in Florida and the northern United States. Mycelial pigment accumulation, substratal stromata formation, and symptom development were used to separate the collection into two distinct morphological types: a common-type (C-type) and a Floridian-type (F-type). Phylogenetic relationships estimated from ITS sequences supported the morphological typing. Identification and characterization of the S. homoeocarpa mating-type locus revealed an idiomorphic organization for both C- and F-types with nearly equal frequencies of each mating types present in both groups. These findings suggest heterothallic control of mating and indicate potential for outcrossing in both groups. Dollar spot disease of turfgrass in Florida is caused by two distinct morphological types of S. homoeocarpa which may be cryptic species. These findings could have implications for disease management.

Sclerotinia sclerotiorum γ-glutamyl transpeptidase (Ss-Ggt1) is required for regulating glutathione accumulation and development of sclerotia and compound appressoria

Transcripts encoding Sclerotinia sclerotiorum γ-glutamyl transpeptidase (Ss-Ggt1) were found to accumulate specifically during sclerotium, apothecium, and compound appressorium development in S. sclerotiorum. To determine the requirement of this protein in these developmental processes, gene deletion mutants of Ss-ggt1 were generated and five independent homokaryotic ΔSs-ggt1 mutants were characterized. All deletion mutants overproduced sclerotial initials that were arrested in further development or eventually produced sclerotia with aberrant rind layers. During incubation for carpogenic germination, these sclerotia decayed and failed to produce apothecia. Total glutathione accumulation was approximately 10-fold higher and H(2)O(2) hyperaccumulated in ΔSs-ggt1 sclerotia compared with the wild type. Production of compound appressoria was also negatively affected. On host plants, these mutants exhibited a defect in infection efficiency and a delay in initial symptom development unless the host tissue was wounded prior to inoculation. These results suggest that Ss-Ggt1 is the primary enzyme involved in glutathione recycling during these key developmental stages of the S. sclerotiorum life cycle but Ss-Ggt1 is not required for host colonization and symptom development. The accumulation of oxidized glutathione is hypothesized to negatively impact these developmental processes by disrupting the dynamic redox environment associated with multicellular development.

Genomic analysis of the necrotrophic fungal pathogens Sclerotinia sclerotiorum and Botrytis cinerea

Sclerotinia sclerotiorum and Botrytis cinerea are closely related necrotrophic plant pathogenic fungi notable for their wide host ranges and environmental persistence. These attributes have made these species models for understanding the complexity of necrotrophic, broad host-range pathogenicity. Despite their similarities, the two species differ in mating behaviour and the ability to produce asexual spores. We have sequenced the genomes of one strain of S. sclerotiorum and two strains of B. cinerea. The comparative analysis of these genomes relative to one another and to other sequenced fungal genomes is provided here. Their 38-39 Mb genomes include 11,860-14,270 predicted genes, which share 83% amino acid identity on average between the two species. We have mapped the S. sclerotiorum assembly to 16 chromosomes and found large-scale co-linearity with the B. cinerea genomes. Seven percent of the S. sclerotiorum genome comprises transposable elements compared to <1% of B. cinerea. The arsenal of genes associated with necrotrophic processes is similar between the species, including genes involved in plant cell wall degradation and oxalic acid production. Analysis of secondary metabolism gene clusters revealed an expansion in number and diversity of B. cinerea-specific secondary metabolites relative to S. sclerotiorum. The potential diversity in secondary metabolism might be involved in adaptation to specific ecological niches. Comparative genome analysis revealed the basis of differing sexual mating compatibility systems between S. sclerotiorum and B. cinerea. The organization of the mating-type loci differs, and their structures provide evidence for the evolution of heterothallism from homothallism. These data shed light on the evolutionary and mechanistic bases of the genetically complex traits of necrotrophic pathogenicity and sexual mating. This resource should facilitate the functional studies designed to better understand what makes these fungi such successful and persistent pathogens of agronomic crops.

The development-specific protein (Ssp1) from Sclerotinia sclerotiorum is encoded by a novel gene expressed exclusively in sclerotium tissues

The gene encoding a development-specific protein (Ssp1) was identified; it previously was described as the major protein present in mature sclerotia of Sclerotinia sclerotiorum. To determine the developmental specificity of ssp1 gene expression in relation to protein accumulation we examined transcript and protein accumulation during various growth and development stages of the lifecycle. We found that ssp1 transcript accumulated exclusively within developing sclerotium tissue and not in any other examined stage of growth or development. In contrast high levels of Sspl protein were detectable by western blot and tandem mass spectrometry analyses in all stages of sclerotium as well as apothecium development. Immunolocalization further indicated that Ssp1 protein bodies were depleted from the sclerotium tissue surrounding the site of apothecium germination, but by this method Sspl was not detected in the apothecium. Together these findings suggest that Sspl is not metabolized during carpogenic germination, instead it is translocated from the sclerotium to the apothecium in an antigenically novel form. Outside the Sclerotiniaceae ssp1 homologs were found only from the sclerotium-forming Aspergillus species A. flavus and A. oryzae. Further studies concerning the regulation and function of this gene and its occurrence in other species have the potential to inform our understanding of sclerotium development and the evolution of sclerotia and other forms of fungal stroma.

An activating mutation of the Sclerotinia sclerotiorum pac1 gene increases oxalic acid production at low pH but decreases virulence

SUMMARY The production of oxalic acid by Sclerotinia sclerotiorum is regulated by the ambient pH environment. This regulation and that of a few investigated pH-responsive genes is mediated in part by the zinc finger transcription factor encoded by pac1, an orthologue of the Aspergillus nidulans pacC gene. We manipulated the pac1 sequence by site-directed mutagenesis to create a dominant activating pac1(c) mutation and introduced this allele into a pac1 loss-of-function (Deltapac1) strain. Consistent with a constitutive activation of Pac1 function, oxalic acid accumulation in recovered Deltapac1-pac1(c) strains was largely independent of ambient pH. Likewise, all three Deltapac1-pac1(c) strains accumulated detectable pac1 transcripts in a pH 3 environment; however, accumulation of pac1 transcripts remained alkaline-inducible, but much reduced relative to wild-type in two of the three Deltapac1-pac1(c) strains. Surprisingly, the accumulation of pg1 and acp1 transcripts, normally favoured by low pH conditions, were up-regulated across the range of ambient pH conditions examined (pH 3.4-7.2). Accumulation of neutral pH-expressed endopolygalacturonase-6 (pg6) transcripts, however, did not differ from wild-type. In pathogenicity assays using Arabidopsis and detached tomato leaflets, Deltapac1-pac1(c) strains were reduced in virulence despite the ability to accumulate oxalic acid independent of the prevailing ambient pH environment. These results support the hypothesis that appropriate gene regulation in response to ambient pH is important for full S. sclerotiorum virulence independent of oxalic acid accumulation.

Deletion of the adenylate cyclase (sac1) gene affects multiple developmental pathways and pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum, a broad host range plant pathogen, produces pigmented, multihyphal sclerotia that are capable of long-term survival. Under favorable conditions, sclerotia carpogenically germinate to give rise to apothecia and forcibly discharged ascospores which serve as the primary source of inoculum in the disease cycle. The molecular regulator(s) of sclerotial development in filamentous fungi are largely unknown; however, pharmacological data has revealed that cyclic AMP (cAMP) negatively regulates sclerotial biogenesis in S. sclerotiorum. Based on this observation, we analyzed the role of cAMP by deleting the single copy adenylate cyclase (AC) sac1 gene from S. sclerotiorum. In culture, cyclic AMP levels in the knock-out (KO1) strain were greatly reduced compared to wild type, the hyphal branching pattern was altered, microconidia (spermatia) were more abundant, and aberrant sclerotia were produced in a concentric pattern. The KO1 strain was pathogenic on mechanically wounded tissues; however, virulence was severely attenuated. The pathogenicity defect on unwounded leaves is attributed to the absence of infection cushions and the attenuated virulence on wounded leaves correlates with the slow growth rate observed in culture. This study presents the first description of an adenylate cyclase mutant that affects both pathogenicity and sclerotial development in a broad host range necrotroph.

A gene with domains related to transcription regulation is required for pathogenicity in Colletotrichum acutatum causing Key lime anthracnose

SUMMARY Colletotrichum acutatum causes Key lime anthracnose (KLA) and postbloom fruit drop (PFD) of citrus. We utilized restriction enzyme-mediated integration (REMI) mutagenesis to produce six non-pathogenic mutants from a KLA isolate after screening 1064 transformants on detached Key lime leaves. Subsequently, a gene designated KLAP1 (Key Lime Anthracnose Pathogenicity) was identified from one of the mutants and was demonstrated genetically to be required for pathogenicity to Key lime leaves. The predicted polypeptide encoded by KLAP1 contains a cAMP and cGMP-dependent protein kinase phosphorylation site, and two RGD (Arg-Gly-Asp) cell attachment sequences, a bipartite nuclear targeting sequence, a fungal G-protein alpha subunit signature, a putative metal-binding zinc finger (Cys(2)His(2)) and a putative HMG-I/Y ('high mobility group' non-histone chromatin protein encoding genes) DNA-binding domain (A+T hook), suggesting that KLAP1 may function as a transcription activator in C. acutatum. Sequences homologous to KLAP1 were detected in most C. acutatum isolates examined, and similarity was found in several classes of fungi, animals, plants and bacteria, indicating that KLAP1 is a putative, uncharacterized, conserved transcription activator in fungi. Targeted gene disruption of KLAP1 yielded mutants that were blocked in the penetration stage and were completely defective in pathogenicity on Key lime leaves, but remained pathogenic to flower petals. Complementation of a klap1-null mutant with a full-length KLAP1 gene clone restored complete ability to incite lesions on Key lime. The results indicate that KLAP1 is an important pathogenicity factor in C. acutatum.

The Sclerotinia sclerotiorum pac1 gene is required for sclerotial development and virulence

The synergistic activities of oxalic acid and endopolygalacturonases are thought to be essential for full virulence of Sclerotinia sclerotiorum and other oxalate-producing plant pathogens. Both oxalic acid production and endopolygalacturonase activity are regulated by ambient pH. Since many gene products with pH-sensitive activities are regulated by the PacC transcription factor in Aspergillus nidulans, we functionally characterized a pacC gene homolog, pac1, from S. sclerotiorum. Mutants with loss-of-function alleles of the pac1 locus were created by targeted gene replacement. In vitro mycelial growth of these pac1 mutants was normal at acidic pH, but growth was inhibited as culture medium pH was increased. Development and maturation of sclerotia in culture was also aberrant in these pac1 replacement mutants. Although oxalic acid production remained alkaline pH-responsive, the kinetics and magnitude of oxalate accumulation were dramatically altered. Additionally, maximal accumulation of endopolygalacturonase gene transcripts (pg1) was shifted to higher ambient pH. Virulence in loss-of-function pac1 mutants was dramatically reduced in infection assays with tomato and Arabidopsis. Based on these results, pac1 appears to be necessary for the appropriate regulation of physiological processes important for pathogenesis and development of S. sclerotiorum.

pH signaling in Sclerotinia sclerotiorum: identification of a pacC/RIM1 homolog

Sclerotinia sclerotiorum acidifies its ambient environment by producing oxalic acid. This production of oxalic acid during plant infection has been implicated as a primary determinant of pathogenicity in this and other phytopathogenic fungi. We found that ambient pH conditions affect multiple processes in S. sclerotiorum. Exposure to increasing alkaline ambient pH increased the oxalic acid accumulation independent of carbon source, sclerotial development was favored by acidic ambient pH conditions but inhibited by neutral ambient pH, and transcripts encoding the endopolygalacturonase gene pg1 accumulated maximally under acidic culture conditions. We cloned a putative transcription factor-encoding gene, pac1, that may participate in a molecular signaling pathway for regulating gene expression in response to ambient pH. The three zinc finger domains of the predicted Pac1 protein are similar in sequence and organization to the zinc finger domains of the A. nidulans pH-responsive transcription factor PacC. The promoter of pac1 contains eight PacC consensus binding sites, suggesting that this gene, like its homologs, is autoregulated. Consistent with this suggestion, the accumulation of pac1 transcripts paralleled increases in ambient pH. Pac1 was determined to be a functional homolog of PacC by complementation of an A. nidulans pacC-null strain with pac1. Our results suggest that ambient pH is a regulatory cue for processes linked to pathogenicity, development, and virulence and that these processes may be under the molecular regulation of a conserved pH-dependent signaling pathway analogous to that in the nonpathogenic fungus A. nidulans.

Minimizing the impact of community violence on child witnesses

Child witnesses respond to violent events in two stages: an immediate reaction to the trauma followed by a response to the trauma and grief. The child's stage of development, circumstances surrounding the incident, and reactions of trusted adults affect responses. Secondary prevention measures during the first stage focus on protection and advocacy, while second stage interventions help the child acknowledge and tolerate the realities of the violent event. Child witnesses are at risk for posttraumatic stress disorder and other long-term social, emotional, and developmental problems. Individual characteristics, early life experiences, and protective factors in the environment contribute to children's resilience and ability to survive and grow into healthy adults.

Molecular characterization of the afl-1 locus in Aspergillus flavus

An unusual mutation at the afl-1 locus, affecting aflatoxin biosynthesis in Aspergillus flavus 649, was investigated. The inability of strain 649 to produce aflatoxin was found to be the result of a large (greater than 60 kb) deletion that included a cluster of aflatoxin biosynthesis genes. Diploids formed by parasexual crosses between strain 649 and the aflatoxigenic strain 86 did not produce aflatoxin, indicating the dominant nature of the afl-1 mutation in strain 649. In metabolite feeding experiments, the diploids did not convert three intermediates in the aflatoxin pathway to aflatoxin. Northern (RNA blot) analysis of the diploids grown in medium conducive for aflatoxin production indicated that the aflatoxin pathway genes nor1, ver1, and omt1 were not expressed; however, there was low-level expression of the regulatory gene aflR. Pulsed-field electrophoresis gels indicated a larger (6 Mb) chromosome in strain 649 than the apparently homologous (4.9 Mb) chromosome in strain 86. The larger chromosome in strain 649 suggests that a rearrangement occurred in addition to the deletion. From these data, we proposed that a trans-sensing mechanism in diploids is responsible for the dominant phenotype associated with the afl-1 locus in strain 649. Such a mechanism is known in Drosophila melanogaster but has not been described for fungi.

Family-centered care at home for families with children who are technology dependent

The philosophy of family-centered care recognizes parents as equal in partnerships with professionals. An adapted nursing process can provide a useful mechanism for members of this partnership to create and maintain a system that promotes an optimal quality of life at home for children who are technology dependent and their families. A family-centered assessment requires the nurse to facilitate the family's self-assessment and promote a family systems perspective. The family and the nurse together develop a plan of care, with the family establishing priorities. The nurse assumes joint responsibility with the family in implementing the plan of care, and promotes the principle of normalization and the use of informal options whenever possible. Evaluation is formal, informal, and ongoing and includes the family and nurse's evaluation of both outcome and process.

Nurses as gangbusters: a response to gang violence in America

A growing number of children in America are joining gangs. Once a youth becomes associated with a gang, violence may be inevitable. Previously considered only a criminal justice problem, violence is now regarded as a preventable public health problem. Health care professionals have been challenged to design effective anti-violence strategies as part of health care reform. Nurses with knowledge about gangs can make a significant contribution to addressing this challenge.

A clinical therapeutic trial of cyclosporine in myasthenia gravis

We randomly assigned 39 patients with steroid-dependent generalized myasthenia gravis to treatment with cyclosporine (5 mg/kg per body weight in divided doses) or placebo. Duration of treatment was 6 months. Patients were evaluated monthly. Primary measures of efficacy were quantified strength testing, antihuman acetylcholine receptor antibody titer, and dosage of corticosteroid medication. At the end of the study, patients in the cyclosporine group had significantly greater improvement in strength (p = 0.004) and a reduction in antireceptor antibody titer (p = 0.01). Percentage reduction of steroid medication was greater in the cyclosporine group, although the difference was not statistically significant (p = 0.12). There were no treatment failures, and there was one drug failure in the cyclosporine group. In the placebo group, there were three treatment failures. No significant nephrotoxicity was noted at this dosage during the first 6 months. During the subsequent 18 months of open-label therapy, continued reduction in steroid dosage occurred. Cumulative side effects, however, caused 35% of patients to discontinue the medication; 10% did so secondary to slowly progressive nephrotoxicity.