Expression of the Avirulence gene Avr9 of the fungal tomato pathogen Cladosporium fulvum is regulated by the global nitrogen response factor NRF1

Balancing act: The dynamic relationship between nutrient availability and plant defence

Plants depend heavily on soil nutrients for growth, development and defence. Nutrient availability is crucial not only for sustaining vital biochemical processes but also for mounting effective defences against a diverse array of pathogens. Macronutrients such as nitrogen, phosphorus and potassium significantly influence plant defence mechanisms by providing essential building blocks for the synthesis of defence compounds, immune signalling and physiological responses like stomatal regulation. Micronutrients like zinc, copper and iron are essential for balancing reactive oxygen species and other reactive compounds in plant immune responses. Although substantial circumstantial evidence links nutrient availability to plant defence, the molecular mechanisms underlying this process have only recently started to be understood. This review focuses on summarizing recent advances in understanding the molecular mechanisms by which nitrogen, phosphorus and iron interact with plant defence mechanisms and explores the potential for engineering nutritional immunity in crops to enhance their resilience against pathogens.

Plant mineral nutrition and disease resistance: A significant linkage for sustainable crop protection

Complete and balanced nutrition has always been the first line of plant defense due to the direct involvement of mineral elements in plant protection. Mineral elements affect plant health directly by modulating the activity of redox enzymes or improving the plant vigor indirectly by altering root exudates, and changing microflora population dynamics, rhizosphere soil nutrient content, pH fluctuation, lignin deposition, and phytoalexin biosynthesis. Nitrogen (N) is one of the most important macronutrients having a significant impact on the host-pathogen axis. N negatively affects the plant's physical defense along with the production of antimicrobial compounds, but it significantly alleviates defense-related enzyme levels that can eventually assist in systemic resistance. Potassium (K) is an essential plant nutrient, when it is present in adequate concentration, it can certainly increase the plant's polyphenolic concentrations, which play a critical role in the defense mechanism. Although no distinguished role of phosphorus (P) is observed in plant disease resistance, a high P content may increase the plant's susceptibility toward the invader. Manganese (Mn) is one of the most important micronutrients, which have a vital effect on photosynthesis, lignin biosynthesis, and other plant metabolic functions. Zinc (Zn) is a part of enzymes that are involved in auxin synthesis, infectivity, phytotoxin, and mycotoxin production in pathogenic microorganisms. Similarly, many other nutrients also have variable effects on enhancing or decreasing the host susceptibility toward disease onset and progression, thereby making integrative plant nutrition an indispensable component of sustainable agriculture. However, there are still many factors influencing the triple interaction of host-pathogen-mineral elements, which are not yet unraveled. Thereby, the present review has summarized the recent progress regarding the use of macro- and micronutrients in sustainable agriculture and their role in plant disease resistance.

An Integrated Omics Approach Uncovers the Novel Effector Ecp20-2 Required for Full Virulence of Cladosporium fulvum on Tomato

The fungus Cladosporium fulvum causes the leaf mould in tomatoes. During the colonization of the host, it secretes plenty of effector proteins into the plant apoplast to suppress the plant’s immune system. Here, we characterized and functionally analyzed the Ecp20-2 gene of C. fulvum using combined omics approaches. RNA-sequencing of susceptible tomato plants inoculated with C. fulvum race 0WU showed strongly induced expression of the Ecp20-2 gene. Strong upregulation of expression of the Ecp20-2 gene was confirmed by qPCR, and levels were comparable to those of other known effectors of C. fulvum. The Ecp20-2 gene encodes a small secreted protein of 149 amino acids with a predicted signal peptide of 17 amino acids. Mass spectrometry of apoplastic fluids from infected tomato leaves revealed the presence of several peptides originating from the Ecp20-2 protein, indicating that the protein is secreted and likely functions in the apoplast. In the genome of C. fulvum, Ecp20-2 is surrounded by various repetitive elements, but no allelic variation was detected in the coding region of Ecp20-2 among 120 C. fulvum isolates collected in Japan. Δecp20-2 deletion mutants of strain 0WU of C. fulvum showed decreased virulence, supporting that Ecp20-2 is an effector required for full virulence of the fungus. Virulence assays confirmed a significant reduction of fungal biomass in plants inoculated with Δecp20-2 mutants compared to those inoculated with wild-type, Δecp20-2-complemented mutants, and ectopic transformants. Sequence similarity analysis showed the presence of Ecp20-2 homologs in the genomes of several Dothideomycete fungi. The Ecp20-2 protein shows the best 3D homology with the PevD1 effector of Verticillium dahliae, which interacts with and inhibits the activity of the pathogenesis-related protein PR5, which is involved in the immunity of several host plants.

Multi-Omic Investigation of Low-Nitrogen Conditional Resistance to Clubroot Reveals Brassica napus Genes Involved in Nitrate Assimilation

Nitrogen fertilization has been reported to influence the development of clubroot, a root disease of Brassicaceae species, caused by the obligate protist Plasmodiophora brassicae. Our previous works highlighted that low-nitrogen fertilization induced a strong reduction of clubroot symptoms in some oilseed rape genotypes. To further understand the underlying mechanisms, the response to P. brassicae infection was investigated in two genotypes "Yudal" and HD018 harboring sharply contrasted nitrogen-driven modulation of resistance toward P. brassicae. Targeted hormone and metabolic profiling, as well as RNA-seq analysis, were performed in inoculated and non-inoculated roots at 14 and 27 days post-inoculation, under high and low-nitrogen conditions. Clubroot infection triggered a large increase of SA concentration and an induction of the SA gene markers expression whatever the genotype and nitrogen conditions. Overall, metabolic profiles suggested that N-driven induction of resistance was independent of SA signaling, soluble carbohydrate and amino acid concentrations. Low-nitrogen-driven resistance in "Yudal" was associated with the transcriptional regulation of a small set of genes, among which the induction of NRT2- and NR-encoding genes. Altogether, our results indicate a possible role of nitrate transporters and auxin signaling in the crosstalk between plant nutrition and partial resistance to pathogens.

Unravelling the Roles of Nitrogen Nutrition in Plant Disease Defences

Nitrogen (N) is one of the most important elements that has a central impact on plant growth and yield. N is also widely involved in plant stress responses, but its roles in host-pathogen interactions are complex as each affects the other. In this review, we summarize the relationship between N nutrition and plant disease and stress its importance for both host and pathogen. From the perspective of the pathogen, we describe how N can affect the pathogen’s infection strategy, whether necrotrophic or biotrophic. N can influence the deployment of virulence factors such as type III secretion systems in bacterial pathogen or contribute nutrients such as gamma-aminobutyric acid to the invader. Considering the host, the association between N nutrition and plant defence is considered in terms of physical, biochemical and genetic mechanisms. Generally, N has negative effects on physical defences and the production of anti-microbial phytoalexins but positive effects on defence-related enzymes and proteins to affect local defence as well as systemic resistance. N nutrition can also influence defence via amino acid metabolism and hormone production to affect downstream defence-related gene expression via transcriptional regulation and nitric oxide (NO) production, which represents a direct link with N. Although the critical role of N nutrition in plant defences is stressed in this review, further work is urgently needed to provide a comprehensive understanding of how opposing virulence and defence mechanisms are influenced by interacting networks.

An NMRA-Like Protein Regulates Gene Expression in Phytophthora capsici to Drive the Infection Cycle on Tomato

Phytophthora spp. cause devastating disease epidemics on important crop plants and pose a grave threat to global crop production. Critically, Phytophthora pathogens represent a distinct evolutionary lineage in which pathogenicity has been acquired independently. Therefore, there is an urgent need to understand and disrupt the processes that drive infection if we aspire to defeat oomycete pathogens in the field. One area that has received little attention thus far in this respect is the regulation of Phytophthora gene expression during infection. Here, we characterize PcNMRAL1 (Phyca11_505845), a homolog of the Aspergillus nidulans nitrogen metabolite repression regulator NMRA and demonstrate a role for this protein in progression of the Phytophthora capsici infection cycle. PcNmrAL1 is coexpressed with the biotrophic marker gene PcHmp1 (haustorial membrane protein 1) and, when overexpressed, extends the biotrophic infection stage. Microarray analyses revealed that PcNmrAL1 overexpression in P. capsici leads to large-scale transcriptional changes during infection and in vitro. Importantly, detailed analysis reveals that PcNmrAL1 overexpression induces biotrophy-associated genes while repressing those associated with necrotrophy. In addition to factors controlling transcription, translation, and nitrogen metabolism, PcNMRAL1 helps regulate the expression of a considerable effector repertoire in P. capsici. Our data suggests that PcNMRAL1 is a transcriptional regulator that mediates the biotrophy to necrotrophy transition. PcNMRAL1 represents a novel factor that may drive the Phytophthora disease cycle on crops. This study provides the first insight into mechanisms that regulate infection-related processes in Phytophthora spp. and provides a platform for further studies aimed at disabling pathogenesis and preventing crop losses.

Comparative proteomic analysis between nitrogen supplemented and starved conditions in Magnaporthe oryzae

BACKGROUND

Fungi are constantly exposed to nitrogen limiting environments, and thus the efficient regulation of nitrogen metabolism is essential for their survival, growth, development and pathogenicity. To understand how the rice blast pathogen Magnaporthe oryzae copes with limited nitrogen availability, a global proteome analysis under nitrogen supplemented and nitrogen starved conditions was completed.

METHODS

M. oryzae strain 70-15 was cultivated in liquid minimal media and transferred to media with nitrate or without a nitrogen source. Proteins were isolated and subjected to unfractionated gel-free based liquid chromatography-tandem mass spectrometry (LC-MS/MS). The subcellular localization and function of the identified proteins were predicted using bioinformatics tools.

RESULTS

A total of 5498 M. oryzae proteins were identified. Comparative analysis of protein expression showed 363 proteins and 266 proteins significantly induced or uniquely expressed under nitrogen starved or nitrogen supplemented conditions, respectively. A functional analysis of differentially expressed proteins revealed that during nitrogen starvation nitrogen catabolite repression, melanin biosynthesis, protein degradation and protein translation pathways underwent extensive alterations. In addition, nitrogen starvation induced accumulation of various extracellular proteins including small extracellular proteins consistent with observations of a link between nitrogen starvation and the development of pathogenicity in M. oryzae.

CONCLUSION

The results from this study provide a comprehensive understanding of fungal responses to nitrogen availability.

Fungal Plant Pathogenesis Mediated by Effectors

The interactions between fungi and plants encompass a spectrum of ecologies ranging from saprotrophy (growth on dead plant material) through pathogenesis (growth of the fungus accompanied by disease on the plant) to symbiosis (growth of the fungus with growth enhancement of the plant). We consider pathogenesis in this article and the key roles played by a range of pathogen-encoded molecules that have collectively become known as effectors.

Regulation of proteinaceous effector expression in phytopathogenic fungi

Effectors are molecules used by microbial pathogens to facilitate infection via effector-triggered susceptibility or tissue necrosis in their host. Much research has been focussed on the identification and elucidating the function of fungal effectors during plant pathogenesis. By comparison, knowledge of how phytopathogenic fungi regulate the expression of effector genes has been lagging. Several recent studies have illustrated the role of various transcription factors, chromosome-based control, effector epistasis, and mobilisation of endosomes within the fungal hyphae in regulating effector expression and virulence on the host plant. Improved knowledge of effector regulation is likely to assist in improving novel crop protection strategies.

Impact of biotic and abiotic factors on the expression of fungal effector-encoding genes in axenic growth conditions

In phytopathogenic fungi, the expression of hundreds of small secreted protein (SSP)-encoding genes is induced upon primary infection of plants while no or a low level of expression is observed during vegetative growth. In some species such as Leptosphaeria maculans, this coordinated in-planta upregulation of SSP-encoding genes expression relies on an epigenetic control but the signals triggering gene expression in-planta are unknown. In the present study, biotic and abiotic factors that may relieve suppression of SSP-encoding gene expression during axenic growth of L. maculans were investigated. Some abiotic factors (temperature, pH) could have a limited effect on SSP gene expression. In contrast, two types of cellular stresses induced by antibiotics (cycloheximide, phleomycin) activated strongly the transcription of SSP genes. A transcriptomic analysis to cycloheximide exposure revealed that biological processes such as ribosome biosynthesis and rRNA processing were induced whereas important metabolic pathways such as glycogen and nitrogen metabolism, glycolysis and tricarboxylic acid cycle activity were down-regulated. A quantitatively different expression of SSP-encoding genes compared to plant infection was also detected. Interestingly, the same physico-chemical parameters as those identified here for L. maculans effectors were identified to regulate positively or negatively the expression of bacterial effectors. This suggests that apoplastic phytopathogens may react to similar physiological parameters for regulation of their effector genes.

Mutation of AREA affects growth, sporulation, nitrogen regulation, and pathogenicity in Colletotrichum gloeosporioides

The GATA transcription factor AreA is a global nitrogen regulator that restricts the utilization of complex and poor nitrogen sources in the presence of good nitrogen sources in microorganisms. In this study, we report the biological function of an AreA homolog (the CgareA gene) in the fruit postharvest pathogen Colletotrichum gloeosporioides. Targeted gene deletion mutants of areA exhibited significant reductions in vegetative growth, increases in conidia production, and slight decreases in conidial germination rates. Quantitative RT-PCR (qRT-PCR) analysis revealed that the expression of AreA was highly induced under nitrogen-limiting conditions. Moreover, compared to wild-type and complemented strains, nitrogen metabolism-related genes were misregulated in ΔareA mutant strains. Pathogenicity assays indicated that the virulence of ΔareA mutant strains were affected by the nitrogen content, but not the carbon content, of fruit hosts. Taken together, our results indicate that CgareA plays a critical role in fungal development, conidia production, regulation of nitrogen metabolism and virulence in Colletotrichum gloeosporioides.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.

Cladosporium fulvum Effectors: Weapons in the Arms Race with Tomato

In this review, I recount my personal history. My drive to study host-pathogen interactions was to find alternatives for agrochemicals, which was triggered after reading the book "Silent Spring" by Rachel Carson. I reflect on my research at the Laboratory of Phytopathology at Wageningen University, where I have worked for my entire career on the interaction between Cladosporium fulvum and tomato, and related gene-for-gene pathosystems. I describe different methods used to identify and sequence avirulence (Avr) genes from the pathogen and resistance (R) genes from the host. The major genes involved in classical gene-for-gene interactions have now been identified, and breeders can produce plants with multiple R genes providing durable and environmentally safe protection against pathogens. In some cases, this might require the use of genetically modified plants when R genes cannot be introduced by classical breeding.

Reversible Oxidation of a Conserved Methionine in the Nuclear Export Sequence Determines Subcellular Distribution and Activity of the Fungal Nitrate Regulator NirA

The assimilation of nitrate, a most important soil nitrogen source, is tightly regulated in microorganisms and plants. In Aspergillus nidulans, during the transcriptional activation process of nitrate assimilatory genes, the interaction between the pathway-specific transcription factor NirA and the exportin KapK/CRM1 is disrupted, and this leads to rapid nuclear accumulation and transcriptional activity of NirA. In this work by mass spectrometry, we found that in the absence of nitrate, when NirA is inactive and predominantly cytosolic, methionine 169 in the nuclear export sequence (NES) is oxidized to methionine sulfoxide (Metox169). This oxidation depends on FmoB, a flavin-containing monooxygenase which in vitro uses methionine and cysteine, but not glutathione, as oxidation substrates. The function of FmoB cannot be replaced by alternative Fmo proteins present in A. nidulans. Exposure of A. nidulans cells to nitrate led to rapid reduction of NirA-Metox169 to Met169; this reduction being independent from thioredoxin and classical methionine sulfoxide reductases. Replacement of Met169 by isoleucine, a sterically similar but not oxidizable residue, led to partial loss of NirA activity and insensitivity to FmoB-mediated nuclear export. In contrast, replacement of Met169 by alanine transformed the protein into a permanently nuclear and active transcription factor. Co-immunoprecipitation analysis of NirA-KapK interactions and subcellular localization studies of NirA mutants lacking different parts of the protein provided evidence that Met169 oxidation leads to a change in NirA conformation. Based on these results we propose that in the presence of nitrate the activation domain is exposed, but the NES is masked by a central portion of the protein (termed nitrate responsive domain, NiRD), thus restricting active NirA molecules to the nucleus. In the absence of nitrate, Met169 in the NES is oxidized by an FmoB-dependent process leading to loss of protection by the NiRD, NES exposure, and relocation of the inactive NirA to the cytosol.

Expression of Five Endopolygalacturonase Genes and Demonstration that MfPG1 Overexpression Diminishes Virulence in the Brown Rot Pathogen Monilinia fructicola

Monilinia fructicola is a devastating pathogen on stone fruits, causing blossom blight and fruit rot. Little is known about pathogenic mechanisms in M. fructicola and related Monilinia species. In this study, five endopolygalacturonase (endo-PG) genes were cloned and functionally characterized in M. fructicola. Quantitative reverse-transcriptase PCR (qRT-PCR) revealed that the five MfPG genes are differentially expressed during pathogenesis and in culture under various pH regimes and carbon and nitrogen sources. MfPG1 encodes the major endo-PG and is expressed to significantly higher levels compared to the other four MfPGs in culture and in planta. MfPG1 function during pathogenesis was evaluated by examining the disease phenotypes and gene expression patterns in M. fructicola MfPG1-overexpressing strains and in strains carrying the β-glucuronidase (GUS) reporter gene fused with MfPG1 (MfPG1-GUS). The MFPG1-GUS reporter was expressed in situ in conidia and hyphae following inoculation of flower petals, and qRT-PCR analysis confirmed MfPG1 expression during pathogenesis. MfPG1-overexpressing strains produced smaller lesions and higher levels of reactive oxygen species (ROS) on the petals of peach and rose flowers than the wild-type strain, suggesting that MfPG1 affecting fungal virulence might be in part resulted from the increase of ROS in the Prunus–M. fructicola interactions.

Transcriptome sequencing uncovers the Avr5 avirulence gene of the tomato leaf mold pathogen Cladosporium fulvum

The Cf-5 gene of tomato confers resistance to strains of the fungal pathogen Cladosporium fulvum carrying the avirulence gene Avr5. Although Cf-5 has been cloned, Avr5 has remained elusive. We report the cloning of Avr5 using a combined bioinformatic and transcriptome sequencing approach. RNA-Seq was performed on the sequenced race 0 strain (0WU; carrying Avr5), as well as a race 5 strain (IPO 1979; lacking a functional Avr5 gene) during infection of susceptible tomato. Forty-four in planta-induced C. fulvum candidate effector (CfCE) genes of 0WU were identified that putatively encode a secreted, small cysteine-rich protein. An expressed transcript sequence comparison between strains revealed two polymorphic CfCE genes in IPO 1979. One of these conferred avirulence to IPO 1979 on Cf-5 tomato following complementation with the corresponding 0WU allele, confirming identification of Avr5. Complementation also led to increased fungal biomass during infection of susceptible tomato, signifying a role for Avr5 in virulence. Seven of eight race 5 strains investigated escape Cf-5-mediated resistance through deletion of the Avr5 gene. Avr5 is heavily flanked by repetitive elements, suggesting that repeat instability, in combination with Cf-5-mediated selection pressure, has led to the emergence of race 5 strains deleted for the Avr5 gene.

Mechanisms of nutrient acquisition and utilization during fungal infections of leaves

Foliar fungal pathogens challenge global food security, but how they optimize growth and development during infection is understudied. Despite adopting several lifestyles to facilitate nutrient acquisition from colonized cells, little is known about the genetic underpinnings governing pathogen adaption to host-derived nutrients. Homologs of common global and pathway-specific gene regulatory elements are likely to be involved, but their contribution to pathogenicity, and how they are connected to broader genetic networks, is largely unspecified. Here, we focus on carbon and nitrogen metabolism in foliar pathogens and consider what is known, and what is not known, about fungal exploitation of host nutrient and ask how common metabolic regulators have been co-opted to the plant-pathogenic lifestyle as well as how nutrients are utilized to drive infection.

Glutamate Metabolism in Plant Disease and Defense: Friend or Foe?

Plant glutamate metabolism (GM) plays a pivotal role in amino acid metabolism and orchestrates crucial metabolic functions, with key roles in plant defense against pathogens. These functions concern three major areas: nitrogen transportation via the glutamine synthetase and glutamine-oxoglutarate aminotransferase cycle, cellular redox regulation, and tricarboxylic acid cycle-dependent energy reprogramming. During interactions with pathogens, the host GM is markedly altered, leading to either a metabolic state, termed “endurance”, in which cell viability is maintained, or to an opposite metabolic state, termed “evasion”, in which the process of cell death is facilitated. It seems that endurance-natured modulations result in resistance to necrotrophic pathogens and susceptibility to biotrophs, whereas evasion-related reconfigurations lead to resistance to biotrophic pathogens but stimulate the infection by necrotrophs. Pathogens, however, have evolved strategies such as toxin secretion, hemibiotrophy, and selective amino acid utilization to exploit the plant GM to their own benefit. Collectively, alterations in the host GM in response to different pathogenic scenarios appear to function in two opposing ways, either backing the ongoing defense strategy to ultimately shape an efficient resistance response or being exploited by the pathogen to promote and facilitate infection.

The Ustilago maydis Nit2 homolog regulates nitrogen utilization and is required for efficient induction of filamentous growth

Nitrogen catabolite repression (NCR) is a regulatory strategy found in microorganisms that restricts the utilization of complex and unfavored nitrogen sources in the presence of favored nitrogen sources. In fungi, this concept has been best studied in yeasts and filamentous ascomycetes, where the GATA transcription factors Gln3p and Gat1p (in yeasts) and Nit2/AreA (in ascomycetes) constitute the main positive regulators of NCR. The reason why functional Nit2 homologs of some phytopathogenic fungi are required for full virulence in their hosts has remained elusive. We have identified the Nit2 homolog in the basidiomycetous phytopathogen Ustilago maydis and show that it is a major, but not the exclusive, positive regulator of nitrogen utilization. By transcriptome analysis of sporidia grown on artificial media devoid of favored nitrogen sources, we show that only a subset of nitrogen-responsive genes are regulated by Nit2, including the Gal4-like transcription factor Ton1 (a target of Nit2). Ustilagic acid biosynthesis is not under the control of Nit2, while nitrogen starvation-induced filamentous growth is largely dependent on functional Nit2. nit2 deletion mutants show the delayed initiation of filamentous growth on maize leaves and exhibit strongly compromised virulence, demonstrating that Nit2 is required to efficiently initiate the pathogenicity program of U. maydis.

A nitrogen response pathway regulates virulence functions in Fusarium oxysporum via the protein kinase TOR and the bZIP protein MeaB

During infection, fungal pathogens activate virulence mechanisms, such as host adhesion, penetration and invasive growth. In the vascular wilt fungus Fusarium oxysporum, the mitogen-activated protein kinase Fmk1 is required for plant infection and controls processes such as cellophane penetration, vegetative hyphal fusion, or root adhesion. Here, we show that these virulence-related functions are repressed by the preferred nitrogen source ammonium and restored by treatment with l-methionine sulfoximine or rapamycin, two specific inhibitors of Gln synthetase and the protein kinase TOR, respectively. Deletion of the bZIP protein MeaB also resulted in nitrogen source-independent activation of virulence mechanisms. Activation of these functions did not require the global nitrogen regulator AreA, suggesting that MeaB-mediated repression of virulence functions does not act through inhibition of AreA. Tomato plants (Solanum lycopersicum) supplied with ammonium rather than nitrate showed a significant reduction in vascular wilt symptoms when infected with the wild type but not with the DeltameaB strain. Nitrogen source also affected invasive growth in the rice blast fungus Magnaporthe oryzae and the wheat head blight pathogen Fusarium graminearum. We propose that a conserved nitrogen-responsive pathway might operate via TOR and MeaB to control virulence in plant pathogenic fungi.

Fungal effector proteins: past, present and future

The pioneering research of Harold Flor on flax and the flax rust fungus culminated in his gene-for-gene hypothesis. It took nearly 50 years before the first fungal avirulence (Avr) gene in support of his hypothesis was cloned. Initially, fungal Avr genes were identified by reverse genetics and map-based cloning from model organisms, but, currently, the availability of many sequenced fungal genomes allows their cloning from additional fungi by a combination of comparative and functional genomics. It is believed that most Avr genes encode effectors that facilitate virulence by suppressing pathogen-associated molecular pattern-triggered immunity and induce effector-triggered immunity in plants containing cognate resistance proteins. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either on the plasma membrane or inside the plant cell. Indirect recognition of an effector (also known as the guard model) implies that the virulence target of an effector in the host (the guardee) is guarded by the resistance protein (the guard) that senses manipulation of the guardee, leading to activation of effector-triggered immunity. In this article, we review the literature on fungal effectors and some pathogen-associated molecular patterns, including those of some fungi for which no gene-for-gene relationship has been established.

Fungal effector proteins

It is accepted that most fungal avirulence genes encode virulence factors that are called effectors. Most fungal effectors are secreted, cysteine-rich proteins, and a role in virulence has been shown for a few of them, including Avr2 and Avr4 of Cladosporium fulvum, which inhibit plant cysteine proteases and protect chitin in fungal cell walls against plant chitinases, respectively. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either inside the plant cell or on plasma membranes. Several secreted effectors function inside the host cell, but the uptake mechanism is not yet known. Variation observed among fungal effectors shows two types of selection that appear to relate to whether they interact directly or indirectly with their cognate resistance proteins. Direct interactions seem to favor point mutations in effector genes, leading to amino acid substitutions, whereas indirect interactions seem to favor jettison of effector genes.

Cross-species hybridization with Fusarium verticillioides microarrays reveals new insights into Fusarium fujikuroi nitrogen regulation and the role of AreA and NMR

In filamentous fungi, the GATA-type transcription factor AreA plays a major role in the transcriptional activation of genes needed to utilize poor nitrogen sources. In Fusarium fujikuroi, AreA also controls genes involved in the biosynthesis of gibberellins, a family of diterpenoid plant hormones. To identify more genes responding to nitrogen limitation or sufficiency in an AreA-dependent or -independent manner, we examined changes in gene expression of F. fujikuroi wild-type and DeltaareA strains by use of a Fusarium verticillioides microarray representing approximately 9,300 genes. Analysis of the array data revealed sets of genes significantly down- and upregulated in the areA mutant under both N starvation and N-sufficient conditions. Among the downregulated genes are those involved in nitrogen metabolism, e.g., those encoding glutamine synthetase and nitrogen permeases, but also those involved in secondary metabolism. Besides AreA-dependent genes, we found an even larger set of genes responding to N starvation and N-sufficient conditions in an AreA-independent manner. To study the impact of NMR on AreA activity, we examined the expression of several AreA target genes in the wild type and in areA and nmr deletion and overexpression mutants. We show that NMR interacts with AreA as expected but affects gene expression only in early growth stages. This is the first report on genome-wide expression studies examining the influence of AreA on nitrogen-responsive gene expression in a genome-wide manner in filamentous fungi.

Multi-factor regulation of pectate lyase secretion by Colletotrichum gloeosporioides pathogenic on avocado fruits

Tissue alkalinization during Colletotrichum gloeosporioides attack enhances the expression of PELB, which encodes pectate lyase (PL), and PL secretion, which is considered essential for full virulence. We studied the regulation of PL secretion by manipulation of C. gloeosporioides PELB. PELB was down-regulated by knocking out PAC1, which encodes the PacC transcription factor that regulates gene products with pH-sensitive activities. We functionally characterized a PACC gene homologue, PAC1, from C. gloeosporioides wild-type (WT) Cg-14 and two independent deletion strains, Deltapac1(372)and Deltapac1(761). Loss-of-function PAC1 mutants showed 85% reduction of PELB transcript expression, delayed PL secretion and dramatically reduced virulence, as detected in infection assays with avocado fruits. In contrast, PELB was up-regulated in the presence of carbon sources such as glucose. When glucose was used as a carbon source in the medium for the WT strain and the Deltapac1 mutant at pH 6.0, PELB transcript expression and PL secretion were activated. Other sugars, such as sucrose and fructose (but not galactose), also activated PELB expression. These results suggest that the pH-regulated response is only part of a multi-factor regulation of PELB, and that sugars are also needed to promote the transition from quiescent to active necrotrophic development by the pathogen.

The novel Cladosporium fulvum lysin motif effector Ecp6 is a virulence factor with orthologues in other fungal species

During tomato leaf colonization, the biotrophic fungus Cladosporium fulvum secretes several effector proteins into the apoplast. Eight effectors have previously been characterized and show no significant homology to each other or to other fungal genes. To discover novel C. fulvum effectors that might play a role in virulence, we utilized two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) to visualize proteins secreted during C. fulvum-tomato interactions. Three novel C. fulvum proteins were identified: CfPhiA, Ecp6 and Ecp7. CfPhiA shows homology to proteins found on fungal sporogenous cells called phialides. Ecp6 contains lysin motifs (LysM domains) that are recognized as carbohydrate-binding modules. Ecp7 encodes a small, cysteine-rich protein with no homology to known proteins. Heterologous expression of Ecp6 significantly increased the virulence of the vascular pathogen Fusarium oxysporum on tomato. Furthermore, by RNA interference (RNAi)-mediated gene silencing we demonstrate that Ecp6 is instrumental for C. fulvum virulence on tomato. Hardly any allelic variation was observed in the Ecp6 coding region of a worldwide collection of C. fulvum strains. Although none of the C. fulvum effectors identified so far have obvious orthologues in other organisms, conserved Ecp6 orthologues were identified in various fungal species. Homology-based modelling suggests that the LysM domains of C. fulvum Ecp6 may be involved in chitin binding.

Fow2, a Zn(II)2Cys6-type transcription regulator, controls plant infection of the vascular wilt fungus Fusarium oxysporum

The filamentous fungus Fusarium oxysporum is a soil-borne parasite that causes vascular wilts in a wide variety of crops by directly penetrating roots and colonizing the vascular tissue. In previous work, we generated the non-pathogenic mutant B137 of the melon wilt pathogen F. oxysporum f. sp. melonis by using restriction enzyme-mediated integration (REMI) mutagenesis. Molecular characterization of B137 revealed that this mutant has a single-copy plasmid insertion in a gene, designated FOW2, which encodes a putative transcription regulator belonging to the Zn(II)2Cys6 family. The REMI mutant B137 and other FOW2-targeted mutants completely lost pathogenicity, but were not impaired in vegetative growth and conidiation in cultures. Microscopic observation of infection behaviours of green fluorescent protein (GFP)-marked wild-type and mutant strains revealed that the mutants were defective in their abilities to invade roots and colonize plant tissues. FOW2 is conserved in F. oxysporum pathogens that infect different plants. The FOW2-targeted mutants of the tomato wilt pathogen F. oxysporum f. sp. lycopersici also lost pathogenicity. Nuclear localization of Fow2 was verified using strains expressing Fow2-GFP and GFP-Fow2 fusion proteins. These data strongly suggest that FOW2 encodes a transcription regulator controlling the plant infection capability of F. oxysporum pathogens.

Heterochromatin-like regions as ecological niches for avirulence genes in the Leptosphaeria maculans genome: map-based cloning of AvrLm6

Map-based cloning of avirulence genes of the AvrLml-2-6 cluster was recently undertaken in Leptosphaeria maculans and led to the identification of AvrLm1. The ensuing chromosome walk toward AvrLm6 resulted in the delineation of a 562-kb bacterial artificial chromosome (BAC) clone contig in an avirulent isolate. Following sequencing of the contig and sequence comparison with a virulent isolate, four AvrLm6 candidate genes were identified. Complementation of the virulent isolate with the four candidates was performed and one gene was found to fully restore the avirulent phenotype on Rlm6 oilseed rape genotypes. AvrLm6 was found to be located in the same genome context as AvrLml, because it is a solo gene surrounded by 85 and 48 kb of degenerated repeats on its 5' and 3' sides, respectively. AvrLm6 is an orphan gene encoding a small, potentially secreted, cysteine-rich protein. Comparison of AvrLm1 and AvrLm6 expressions by quantitative reverse-transcription polymerase chain reaction revealed that both genes are highly overexpressed during primary leaf infection. Using RNA interference, decreasing expression of AvrLm6 was shown to result in virulence toward Rlm6 genotypes whenever the expression was reduced by more than 60% compared with the wild-type isolate.

The plant nitrogen mobilization promoted by Colletotrichum lindemuthianum in Phaseolus leaves depends on fungus pathogenicity

Nitrogen plays an essential role in the nutrient relationship between plants and pathogens. Some studies report that the nitrogen-mobilizing plant metabolism that occurs during abiotic and biotic stress could be a 'slash-and-burn' defence strategy. In order to study nitrogen recycling and mobilization in host plants during pathogen attack and invasion, the Colletotrichum lindemuthianum/Phaseolus vulgaris interaction was used as a model. C. lindemuthianum is a hemibiotroph that causes anthracnose disease on P. vulgaris. Non-pathogenic mutants and the pathogenic wild-type strain were used to compare their effects on plant metabolism. The deleterious effects of infection were monitored by measuring changes in chlorophyll, protein, and amino acid concentrations. It was shown that amino acid composition changed depending on the plant-fungus interaction and that glutamine accumulated mainly in the leaves infected by the pathogenic strain. Glutamine accumulation correlated with the accumulation of cytosolic glutamine synthetase (GS1 alpha) mRNA. The most striking result was that the GS1 alpha gene was induced in all the fungus-infected leaves, independent of the strain used for inoculation, and that GS1 alpha expression paralleled the PAL3 and CHS defence gene expression. It is concluded that a role of GS1 alpha in plant defence has to be considered.

The global nitrogen regulator, FNR1, regulates fungal nutrition-genes and fitness during Fusarium oxysporum pathogenesis

SUMMARY Fusarium oxysporum is a soil-borne pathogen that infects plants through the roots and uses the vascular system for host ingress. Specialized for this route of infection, F. oxysporum is able to adapt to the scarce nutrient environment in the xylem vessels. Here we report the cloning of the F. oxysporum global nitrogen regulator, Fnr1, and show that it is one of the determinants for fungal fitness during in planta growth. The Fnr1 gene has a single conserved GATA-type zinc finger domain and is 96% and 48% identical to AREA-GF from Gibberella fujikuroi, and NIT2 from Neurospora crassa, respectively. Fnr1 cDNA, expressed under a constitutive promoter, was able to complement functionally an N. crassa nit-2(RIP) mutant, restoring the ability of the mutant to utilize nitrate. Fnr1 disruption mutants showed high tolerance to chlorate and reduced ability to utilize several secondary nitrogen sources such as amino acids, hypoxanthine and uric acid, whereas growth on favourable nitrogen sources was not affected. Fnr1 disruption also abolished in vitro expression of nutrition genes, normally induced during the early phase of infection. In an infection assay on tomato seedlings, infection rate of disruption mutants was significantly delayed in comparison with the parental strain. Our results indicate that FNR1 mediates adaptation to nitrogen-poor conditions in planta through the regulation of secondary nitrogen acquisition, and as such acts as a determinant for fungal fitness during infection.

A defect in nir1, a nirA-like transcription factor, confers morphological abnormalities and loss of pathogenicity in Colletotrichum acutatum

SUMMARY A non-pathogenic mutant of Colletotrichum acutatum, designated Ca5, exhibited epiphytic hyphal growth and did not cause lesions on strawberry plants but grew necrotrophically when inoculated directly onto wounded stolons. In the absence of an external nitrogen source, the mutant exhibited extended germ-tube growth prior to appressorium formation. The deduced product of the impaired gene (nir1) is similar to NirA, an Aspergillus nidulans transcriptional regulator of nitrogen metabolism. Inoculation of leaves with wild-type or Ca5 conidia in the presence of a preferred nitrogen source resulted in massive epiphytic hyphal production, appressorium formation and rapid symptom development. Expression of C. acutatum wild-type nitrate reductase (nit1) and glutamine synthetase (gln1) was induced by nitrate but only nit1 expression was repressed in a rich medium. nit1 transcription increased during the appressorium-production stage, indicating that nitrogen starvation constitutes a cue for the regulation of appressorium development. The presence of nit1 transcript during various phases of infection is indicative of partial nitrogen starvation in planta. cAMP-dependent protein kinase A (PKA) was determined to be a negative regulator of immediate post-germination appressoria formation in the wild-type. As inhibition of PKA activity in the nir1 mutant did not affect appressoria formation, we suggest that NIR1 acts either in parallel or downstream of the PKA pathway. Our results show that nir1 is a pathogenicity determinant and a regulator of pre-infection development under nitrogen-starvation conditions and that nitrogen availability is a significant factor in the pre-penetration phase.

Gene expression profiling of the nitrogen starvation stress response in the mycorrhizal ascomycete Tuber borchii

The focus of this work is on the nitrogen starvation stress responses operating in a plant symbiotic fungus. A cDNA array profiling analysis was conducted on N-limited mycelia of the mycorrhizal ascomycete Tuber borchii. Fifty-one unique transcripts, out of 2062 redundant arrayed cDNAs, were differentially expressed by at least 1.5-fold in response to N deprivation. Only two N assimilation components-a nitrate transporter and a high-affinity ammonium transporter-were found among differentially expressed genes. All the other N status responsive genes code for as yet unidentified hypothetical proteins or components not directly involved in N assimilation or metabolism, especially carbohydrate binding proteins and oligosaccharide as well as lipid modifying enzymes. A subset of cDNA array data were confirmed and extended by Northern blot analysis, which showed that most of the latter components respond not only to nitrogen, but also to carbon source depletion.

Global gene expression during nitrogen starvation in the rice blast fungus, Magnaporthe grisea

Efficient regulation of nitrogen metabolism likely plays a role in the ability of fungi to exploit ecological niches. To learn about regulation of nitrogen metabolism in the rice blast pathogen Magnaporthe grisea, we undertook a genome-wide analysis of gene expression under nitrogen-limiting conditions. Five hundred and twenty genes showed increased transcript levels at 12 and 48 h after shifting the fungus to media lacking nitrate as a nitrogen source. Thirty-nine of these genes have putative functions in amino acid metabolism and uptake, and include the global nitrogen regulator in M. grisea, NUT1. Evaluation of seven nitrogen starvation-induced genes revealed that all were expressed during rice infection. Targeted gene replacement on one such gene, the vacuolar serine protease, SPM1, resulted in decreased sporulation and appressorial development as well as a greatly attenuated ability to cause disease. Data are discussed in the context of nitrogen metabolism under starvation conditions, as well as conditions potentially encountered during invasive growth in planta.

Nitrogen controls in planta expression of Cladosporium fulvum Avr9 but no other effector genes

SUMMARY During growth on its host tomato, the apoplast-colonizing fungal pathogen Cladosporium fulvum secretes several effector proteins. The expression of the Avr9 gene encoding one of these effector proteins has previously been shown to be strongly induced in vitro during nitrogen deprivation. This led to the hypothesis that expression of additional effector genes in C. fulvum could be triggered by nitrogen starvation conditions that are encountered in the host. We now show that expression of most effectors is not affected by varying levels of nitrogen supplementation in vitro. In addition, we demonstrate that the nitrogen response regulator Nrf1 only regulates Avr9 expression during infection of the host, whereas none of the other known effectors is significantly controlled by this transcription factor in planta. Deletion of Nrf1, but not of Avr9, significantly reduces C. fulvum virulence. Therefore, it is concluded that Nrf1 controls, in addition to Avr9, unidentified effector genes that are required for full virulence of C. fulvum.

Small proteins of plant-pathogenic fungi secreted during host colonization

Small proteins secreted by plant pathogenic fungi in their hosts have been implicated in disease symptom development as well as in R-gene mediated disease resistance. Characteristically, this class of proteins shows very limited phylogenetic distribution, possibly due to accelerated evolution stimulated by plant-pathogen arms races. Partly due to lack of clues from primary sequences, insight into the biochemical functions or molecular targets of these proteins has been slow to emerge. However, for some proteins important progress has recently been made in this direction. Expression of the genes for small secreted proteins is in many cases specifically induced after infection, which should help to advance our still very limited understanding of how plant pathogens recognize and respond to the host environment.

Nitrogen-responsive genes are differentially regulated in planta during Fusarium oxyspsorum f. sp. lycopersici infection

SUMMARY Nitrogen is an essential growth component whose availability will limit microbial spread, and as such it serves as a key control point in dictating an organism's adaptation to various environments. Little is known about fungal nutrition in planta. To enhance our understanding of this process we examined the transcriptional adaptation of Fusarium oxysporum f. sp. lycopersici, the causal agent for vascular wilt in tomato, during nutritional stress and plant colonization. Using RT-PCR and microarray technology we compared fungal gene expression in planta to axenic nitrogen starvation culture. Several expressed sequence tags, representing at least four genes, were identified that are concomitantly induced during nitrogen starvation and in planta growth. Three of these genes show similarity to a general amino acid permease, a peptide transporter and an uricase, all functioning in organic nitrogen acquisition. We further show that these genes represent a distinguishable subset of the nitrogen-responsive transcripts that respond to amino acids commonly available in the plant. Our results indicate that nitrogen starvation partially mimics in planta growth conditions, and further suggest that minute levels of organic nitrogen sources dictate the final outcome of fungal gene expression in planta. The nature of the identified transcripts suggests modes of nutrient uptake and survival for Fusarium during colonization.

Cladosporium fulvum (syn. Passalora fulva), a highly specialized plant pathogen as a model for functional studies on plant pathogenic Mycosphaerellaceae

SUMMARY Taxonomy: Cladosporium fulvum is an asexual fungus for which no sexual stage is currently known. Molecular data, however, support C. fulvum as a member of the Mycosphaerellaceae, clustering with other taxa having Mycosphaerella teleomorphs. C. fulvum has recently been placed in the anamorph genus Passalora as P. fulva. Its taxonomic disposition is supported by its DNA phylogeny, as well as the distinct scars on its conidial hila, which are typical of Passalora, and unlike Cladosporium s.s., which has teleomorphs that reside in Davidiella, and not Mycosphaerella. Host range and disease symptoms: The presently known sole host of C. fulvum is tomato (members of the genusLycopersicon). C. fulvum is mainly a foliar pathogen. Disease symptoms are most obvious on the abaxial side of the leaf and include patches of white mould that turn brown upon sporulation. Due to stomatal clogging, curling of leaves and wilting can occur, leading to defoliation. C. fulvum as a model pathogen: The interaction between C. fulvum and tomato is governed by a gene-for-gene relationship. A total of eight Avr and Ecp genes, and for four of these also the corresponding plant Cf genes, have been cloned. Obtaining conclusive evidence for gene-for-gene relationships is complicated by the poor availability of genetic tools for most Mycosphaerellaceae-plant interactions. Newly developed tools, including Agrobacterium-mediated transformation and RNAi, added to the genome sequence of its host tomato, which will be available within a few years, render C. fulvum attractive as a model species for plant pathogenic Mycosphaerellaceae.

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Identification of cis-active elements in Ustilago maydis mig2 promoters conferring high-level activity during pathogenic growth in maize

The Ustilago maydis mig2 cluster comprises five highly homologous genes that display a pronounced plant-specific expression profile. A 350-bp mig2-5 promoter fragment contained all elements sufficient to confer differential promoter activity. Mutational analysis of this region, fused to the green fluorescent protein reporter gene, allowed dissecting core promoter elements required for high-level promoter activity from elements conferring inducible expression in planta. In particular, the presence of several 5'-CCA-3' motifs within a short stretch of the mig2-5 promoter was decisive for inducible promoter activity. On this basis, we reconstituted an artificial promoter whose inducible activity specifically relied on multiple CCA motifs. In addition, we identified a novel mig2 homologous gene, mig2-6, that is not part of the mig2 cluster, but displayed the strongest differential expression profile among mig2 genes. The deletion of all six mig2 genes did not compromise the ability to induce tumor formation in infected maize plants. Comparative sequence analysis including the mig2-6 promoter revealed an over-representation of the consensus motif 5'-MNMNWNCCAMM-3'. We discuss putative transcriptional activators involved in mig2 regulation.

Cladosporium fulvum circumvents the second functional resistance gene homologue at the Cf-4 locus (Hcr9-4E ) by secretion of a stable avr4E isoform

Introgression of resistance trait Cf-4 from wild tomato species into tomato cultivar MoneyMaker (MM-Cf0) has resulted in the near-isogenic line MM-Cf4 that confers resistance to the fungal tomato pathogen Cladosporium fulvum. At the Cf-4 locus, five homologues of Cladosporium resistance gene Cf-9 (Hcr9s) are present. While Hcr9-4D represents the functional Cf-4 resistance gene matching Avr4, Hcr9-4E confers resistance towards C. fulvum by mediating recognition of the novel avirulence determinant Avr4E. Here, we report the isolation of the Avr4E gene, which encodes a cysteine-rich protein of 101 amino acids that is secreted by C. fulvum during colonization of the apoplastic space of tomato leaves. By complementation we show that Avr4E confers avirulence to strains of C. fulvum that are normally virulent on Hcr9-4E-transgenic plants, indicating that Avr4E is a genuine, race-specific avirulence determinant. Strains of C. fulvum evade Hcr9-4E-mediated resistance either by a deletion of the Avr4E gene or by production of a stable Avr4E mutant protein that carries two amino acid substitutions, Phe(82)Leu and Met(93)Thr. Moreover, we demonstrate by site-directed mutagenesis that the single amino acid substitution Phe(82)Leu in Avr4E is sufficient to evade Hcr9-4E-mediated resistance.

Nitrogen metabolism and virulence of Candida albicans require the GATA-type transcriptional activator encoded by GAT1

Nitrogen acquisition and metabolism is central to microbial growth. A conserved family of zinc-finger containing transcriptional regulators known as GATA-factors ensures efficient utilization of available nitrogen sources by fungi. GATA factors activate expression of nitrogen catabolic pathways when preferred nitrogen sources are absent or limiting, a phenomenon known as nitrogen catabolite repression. GAT1 of Candida albicans encodes a GATA-factor homologous to the AREA protein of Aspergillus nidulans and related transcription factors involved in nitrogen regulation. Two observations implicated GAT1 in nitrogen regulation. The growth of mutants lacking GAT1 was reduced when isoleucine, tyrosine or tryptophan were the sole source of nitrogen. Secondly, when cultured on a secondary nitrogen source, gat1Delta mutants were unable to activate expression of GAP1, UGA4 or DAL5, which were shown to be nitrogen regulated in C. albicans. This regulatory defect did not prevent filamentation of gat1Delta mutants in nitrogen repressing or non-repressing conditions, demonstrating that nitrogen catabolite repression does not influence dimorphism. The mutants were, however, highly attenuated in a murine model of disseminated candidiasis. Attenuation was not associated with any diminution of growth in serum or ability to utilize serum amino acids. The results indicate an important role for nitrogen regulation in the virulence of C. albicans.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

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.

Intraspecific comparative genomics to identify avirulence genes from Phytophthora

Members of the oomycete genus Phytophthora cause some of the most devastating plant diseases in the world and are arguably the most destructive pathogens of dicot plants. Phytophthora research has entered the genomics era. Current genomic resources include expressed sequence tags from a variety of developmental and infection stages, as well as sequences of selected regions of Phytophthora genomes. Genomics promise to impact upon our understanding of the molecular basis of infection by Phytophthora, for example, by facilitating the isolation of genes encoding effector molecules with a role in virulence and avirulence. Based on prevalent models of plant-pathogen coevolution, some of these effectors, notably those with avirulence functions, are predicted to exhibit significant sequence variation within populations of the pathogen. This and other features were used to identify candidate avirulence genes from sequence databases. Here, we describe a strategy that combines data mining with intraspecific comparative genomics and functional analyses for the identification of novel avirulence genes from Phytophthora. This approach provides a rapid and efficient alternative to classical positional cloning strategies for identifying avirulence genes that match known resistance genes. In addition, this approach has the potential to uncover 'orphan' avirulence genes for which corresponding resistance genes have not previously been characterized.

Promoter analysis of the avirulence gene Avr9 of the fungal tomato pathogen Cladosporium fulvum in the model filamentous fungus Aspergillus nidulans

The promoter of avirulence gene Avr9 of the fungal tomato pathogen Cladosporium fulvum contains 12 sequences within a region of 0.6 kb that are reminiscent of the binding sequences of the GATA-type regulator involved in nitrogen utilisation of the filamentous fungi Aspergillus nidulans and Neurospora crassa. Mutational analysis of this 0.6-kb promoter region, fused to the beta-glucuronidase reporter gene, revealed that two regions, each containing two TAGATA boxes in inverted orientation and overlapping by two base pairs, are important for induction of Avr9 promoter activity in A. nidulans. Each overlapping TAGATA box differentially affected Avr9 promoter activity when shifted apart by nucleotide insertions. The other regions, which do not contain two overlapping TAGATA boxes have no, or only a limited, contribution to the inducibility of promoter activity.