Fluorescence-Based Assays to Analyse Phosphatidylinositol 5-Phosphate in Autophagy.

Autophagosome formation is stimulated by VPS34-dependent PI(3)P formation and by alternative VPS34-independent pathways. We recently described that PI(5)P regulates autophagosome biogenesis and rescues autophagy in VPS34-inactivated cells, suggesting that PI(5)P contributes to canonical autophagy. Our analysis revealed a hitherto unknown functional interplay between PIKfyve and PIPK type II in controlling PI(5)P levels in the context of autophagy. Among phosphoinositides, visualization of PI(5)P in intact cells has remained difficult. While PI(5)P has been implicated in signaling pathways, chromatin organization, bacterial invasion, and cytoskeletal remodeling, our study is the first report showing PI(5)P localization on autophagosomes and early autophagosomal structures when autophagy is induced by nutrient deprivation (amino acids or glucose starvation). We provided a detailed analysis of PI(5)P distribution by the use of super-resolution structured illuminated microscopy. Here, we present a set of tools for detection of PI(5)P during autophagy by confocal microscopy, live-cell imaging, and super-resolution microscopy.

Striate palmoplantar keratoderma arising from desmoplakin and desmoglein 1 mutations is associated with contrasting perturbations of desmosomes and the keratin filament network.

BACKGROUND: Several hereditary human diseases are now known to be caused by distinct mutations in genes encoding various desmosome components. Although the effects of some of these mutant genes have been analysed by targeted disruption experiments in mouse models, little is known about the cell and tissue changes in affected human patients. OBJECTIVES: To investigate the effects of heterozygous nonsense mutations in desmoplakin (Dp) and desmoglein (Dsg) 1 which cause the autosomal dominant disorder striate palmoplantar keratoderma (SPPK), focusing on changes in desmosome structure and composition and the associated keratin intermediate filament (KIF) network in palm skin, and in cultured keratinocytes generated from the same site. METHODS: We analysed palm and nonpalm skin sections from four SPPK patients with Dp mutations and one patient with a Dsg1 mutation with respect to tissue and subcellular morphologies, and correlated the in vivo and in vitro findings. RESULTS: Using electron microscopy, we found abnormalities of desmosomes and cell-cell adhesion in the suprabasal layers in the epidermis from patients with both Dsg1- and Dp-associated SPPK. These changes were more advanced in skin from patients with Dp mutations. Both Dp and Dsg1 mutations were accompanied by significantly reduced numbers of desmosomes in the suprabasal layers, while decreased desmosome size was evident only in Dsg1-associated SPPK. Confocal microscopy analysis showed marked differences in the expression of keratins and of desmosome components, both between the two types of SPPK, and between SPPK and normal skin. The expression of keratins K5, K14 and K10 was reduced in Dsg1-associated SPPK skin, whereas perinuclear aggregation of keratin filaments was more evident in Dp-associated SPPK. In both types of SPPK upregulation of K16 was pronounced and involucrin labelling was abnormal. CONCLUSIONS: Mutations in Dp and Dsg1 genes causing SPPK may be associated with perturbations in epidermal differentiation accompanied by a marked disruption of several components of the epidermal scaffold including desmosomes and the KIF network.

Human placental amnion is a novel substrate for detecting autoantibodies in autoimmune bullous diseases by immunoblotting.

BACKGROUND: Identification of antigens by immunoblotting techniques, using epidermal and dermal extracts, is regarded as essential for making a definitive diagnosis in autoimmune bullous diseases (AIBDs). These procedures involve epidermal-dermal separation for subsequent protein extraction, which may result in partial loss of some antigenic polypeptides and changes in the conformational epitopes targeted by autoantibodies in AIBDs. It may therefore be necessary to use different substrates for consistent results. Objectives To evaluate the usefulness of human placental amnion extract as a substrate for immunoblotting in the diagnosis of AIBDs. METHODS: We checked the structural components of the desmosomes and basement membrane zone (BMZ) of amnion by electron microscopy. Using immunofluorescence and immunoblotting techniques, we tested the amnion immunoreactivity with antibodies to desmosomal and BMZ proteins, and with sera from 76 patients with AIBDs including pemphigus vulgaris, pemphigus foliaceus, bullous pemphigoid (BP), pemphigoid gestationis, linear IgA bullous dermatosis, epidermolysis bullosa acquisita, paraneoplastic pemphigus and mucous membrane pemphigoid. RESULTS: The desmosomes and BMZ of the amnion tissue were ultrastructurally similar to those in skin. Antigen mapping confirmed that amnion contains all the proteins that were recognized by a panel of monoclonal and polyclonal antibodies. Immunoblotting showed that the antibodies clearly detected bands corresponding to desmogleins 1 and 3, desmocollins 1 and 2, desmoplakins 1 and 2, three subunits (alpha3, beta3 and gamma2) of laminin 5, BP antigens 1 and 2, the 97-kDa LAD antigen and type VII collagen. In addition, most of the patient sera (82%) reacted exclusively with their respective antigens. CONCLUSIONS: Harvesting proteins from amnion does not require epidermal-dermal separation, and a sufficient yield of desmosomal and hemidesmosomal proteins can be obtained. Therefore, amnion may be a more reliable source of substrate than skin samples for immunoblot analysis of AIBDs.

Psoriasis bullosa acquisita.

We report a 51-year-old man with a 20-year history of chronic plaque psoriasis who developed an autoimmune subepidermal blistering eruption that had clinical features of bullous pemphigoid, erythema multiforme and epidermolysis bullosa acquisita. Investigations revealed a 1 : 400 titre circulating and in vivo bound IgG autoantibody that mapped to the dermal side of 1 m NaCl-split skin and localized to the lower lamina lucida/upper lamina densa on immunogold electron microscopy. Immunoblotting, using dermal extracts, showed serum binding to antigens of approximately 200- and approximately 260 kDa. Indirect immunofluorescence microscopy, using the patient's serum on archival skin sections taken from selected individuals with different forms of inherited epidermolysis bullosa as substrate, showed normal basement membrane labelling on all samples apart from recessive dystrophic epidermolysis bullosa skin (with inherent mutations in the type VII collagen gene): in these cases there was a complete absence of immunostaining. Clinically, the patient responded rapidly to combination treatment with intravenous immunoglobulin and oral corticosteroids, dapsone and mycophenolate mofetil. Autoimmune subepidermal blistering has been reported in other patients with psoriasis, although no specific target antigen has ever been determined. Our study provides preliminary evidence that, for this patient at least, the autoantibody may be targeted against a skin component closely associated with type VII collagen (the epidermolysis bullosa acquisita antigen). Therefore, we propose the term 'psoriasis bullosa acquisita' for this and possibly other patients with similar skin eruptions.

Pemphigoid nodularis (non-bullous): a clinicopathological study of five cases.

Pemphigoid nodularis is a rare clinical variant of pemphigoid characterized by overlapping clinical features of both prurigo nodularis lesions and pemphigoid-like blisters. In pemphigoid nodularis, the immunopathological findings are identical to those of bullous pemphigoid (BP). We describe five patients who presented with the typical clinical phenotype of prurigo nodularis, who were found to have circulating and tissue-bound antibasement membrane zone autoantibodies. By immunoelectron microscopy and Western immunoblotting studies, the circulating antibodies were shown to target the hemidesmosome and specifically the BP antigens 1 and 2 (BP180 and BP230). In contrast to the majority of reported cases, none of these patients has ever developed blisters. The role of antibasement membrane zone antibodies in the development of the eruption, or the role of the eruption in the development and persistence of autoantibodies, is not clear. These cases demonstrate that the presence of these antibodies is not sufficient for the development of blisters.

The transition of pemphigus vulgaris into pemphigus foliaceus: a reflection of changing desmoglein 1 and 3 autoantibody levels in pemphigus vulgaris.

The transition of pemphigus vulgaris (PV) into pemphigus foliaceus (PF) is rare and the immunological changes underlying this event are not well understood. We report a 44-year-old woman who presented with oral and cutaneous erosions typical of PV. Over a 9-year period, the clinical features evolved into those of PF. To examine whether quantitative changes in desmoglein (Dsg) antibodies were associated with this transition, Dsg1 and Dsg3 antibody levels were measured by enzyme-linked immunosorbent assay in 82 sequential serum samples collected over this period. At presentation, when the phenotype was PV with oral and cutaneous erosions, antibodies to both Dsg1 and Dsg3 were detected. The disappearance of oral involvement was associated with a decline in Dsg3 antibodies, which are now undetectable, while the development of more severe skin involvement was associated with rising Dsg1 antibody levels. These data strongly suggest that the change in clinical features is a reflection of qualitative and quantitative changes in antibody profile. It is not known whether the transition to PF is permanent or whether disease relapses in the future may be associated with the re-emergence of Dsg3 antibodies, oral ulceration and a PV phenotype.

Site-directed mutagenesis and phylogenetic comparisons of the Escherichia coli Tus protein: DNA-protein interactions alone can not account for Tus activity.

The Tus protein of Escherichia coli is capable of arresting DNA replication in an orientation-dependent manner when bound to specific sequences in the bacterial chromosome called Ter sites. Arrest of DNA replication has been postulated to occur either by a barrier mechanism, where Tus acts as a physical block to replication fork progression, or through protein-protein interactions between Tus and some component of the replication fork. A previous mutational analysis of Tus suggested that the amino acids in the L1 loop might play a role in replication arrest. Site-directed mutagenesis of amino acids in the L1 loop and other amino acid residues on the "non-permissive" face of Tus was performed to identify residues that affected Tus function. One mutant, E47Q, gave results that are inconsistent with the barrier model, showing a greater affinity for the Ter site (with a t 1/2 of 348 min versus 150 min for wild-type Tus) but a reduced ability to arrest DNA replication in vivo. In addition to the site-directed mutagenesis studies, the tus genes of Salmonella, Klebsiella, and Yersinia were sequenced and the proteins expressed in E. coli to assess their ability to arrest DNA replication. The results presented here support a role for protein-protein interactions in Tus function, and suggest that residues E47 and E49 participate in replication fork arrest.

The severity of cutaneous and oral pemphigus is related to desmoglein 1 and 3 antibody levels.

BACKGROUND: Pemphigus vulgaris (PV) and foliaceus (PF) are characterized by antibodies to the desmosomal proteins desmoglein 3 (Dsg3) and desmoglein 1 (Dsg1), respectively. Past studies using indirect immunofluorescence (IIF) as a measure of pemphigus antibody levels have failed to demonstrate consistently a relationship between disease severity and IIF titres. However, IIF is not able to measure separately Dsg1 and 3 antibodies, unlike enzyme-linked immunosorbent assays (ELISA), which utilize recombinant proteins. OBJECTIVES: To compare independently Dsg1 and 3 antibody levels with the severity of both cutaneous and oral involvement in PV and PF. Patients and methods Four hundred and twenty-four serum samples were analysed from 80 subjects with PV and 24 with PF. IgG antibodies to Dsg1 and 3 were measured by ELISA. For every sample analysed, the associated severity of skin and oral disease were graded from 0 to 3; quiescent, mild, moderate and severe. RESULTS: A relationship between Dsg1 antibodies and skin severity was demonstrated such that a 10-unit increase in Dsg1 ELISA value was associated with a 34% chance of having a higher severity score [95% confidence interval (CI), 25-45%, P < 0.0005]. This was observed in both PV and PF. Oral severity was associated with Dsg3 antibody levels and a 10-unit increase in the Dsg3 ELISA value was associated with a 25% chance of a higher oral severity score (CI 17-33%, P < 0.0005). We were unable to demonstrate a relationship between Dsg1 antibodies and oral severity, even after adjusting for the effect of Dsg3 antibodies. Similarly, there was no relationship between Dsg3 antibodies and skin severity. CONCLUSIONS: This study suggests that the clinical phenotype of pemphigus, in particular the balance of skin and oral disease, is determined principally by the quantities of Dsg1 and 3 autoantibodies, respectively.

A keratin 14 'knockout' mutation in recessive epidermolysis bullosa simplex resulting in less severe disease.

Epidermolysis bullosa simplex (EBS) is a blistering skin disease caused in most cases by mis-sense mutations in genes encoding the basal epidermal keratin (K) 5 and K14. The inheritance is usually autosomal dominant and the mutant keratin proteins appear to exert a dominant negative effect on the keratin intermediate filament cytoskeleton in basal keratinocytes. We report a child with a homozygous K14 mutation resulting in the complete absence of K14 protein in the epidermis; remarkably, he only had mild to moderate disease. Electron microscopy of a skin biopsy showed a marked reduction in numbers of keratin intermediate filaments in the basal keratinocytes. Immunofluorescence microscopy using monoclonal antibody LL001 against K14 showed no staining, suggesting a functional knockout of K14. Sequence analysis of genomic DNA revealed a homozygous mutation in codon 31 of K14 that resulted in a premature stop codon further downstream in exon 1. The child's mother, who is unaffected by the disease, is heterozygous for the mutation. The consanguineous father was unaffected and unavailable for testing. The resulting mRNA is predicted to encode a protein of 116 amino acids, of which the first 30 are identical to the normal K14 sequence, and the remaining 86 residues are mis-sense sequence. Four previously reported cases of autosomal recessive EBS with functional knockout of K14 were severely affected by blistering, in contrast to our patient in whom the predicted protein has only the first 30 amino acids of K14 and is therefore the closest to a true knockout of K14 protein yet identified.

A study of desmoglein 1 autoantibodies in pemphigus vulgaris: racial differences in frequency and the association with a more severe phenotype.

BACKGROUND: Pemphigus vulgaris (PV) is characterized by pathogenic autoantibodies to desmoglein (Dsg) 3, but additional antibodies to Dsg1, the pemphigus foliaceus antigen, are detectable in some cases. OBJECTIVES: To investigate the clinical significance of the presence of both Dsg 1 and 3 antibodies. METHODS: In 79 subjects with PV, enzyme-linked immunosorbent assays were used to detect IgG autoantibodies reactive with the ectodomain of Dsg1 and Dsg3. RESULTS: There was a clear association between the clinical phenotype and the Dsg antibody profile. All subjects had Dsg3 autoantibodies and 61% had coexisting Dsg1 antibodies (Dsg3+/Dsg1+). PV limited entirely to the mucosal surfaces was seen only in Dsg3+/Dsg1- patients, while additional Dsg1 antibodies (Dsg3+/Dsg1+) predicted cutaneous in addition to mucosal involvement. Although minor cutaneous involvement was observed in most Dsg3+/Dsg1- patients, severe cutaneous involvement was seen only in Dsg3+/Dsg1+ patients. Dsg1 antibodies were detectable early in the course of disease and their appearance did not relate to the use of systemic therapy. The proportion of Dsg1+ patients was higher in those of Indian origin compared with white northern Europeans (P < 0.05). CONCLUSIONS: These data suggest that the presence of Dsg1 antibodies is predictive of a potentially more severe disease and that genetic factors may determine the Dsg antibody profile.

Diagnosis of pemphigus by ELISA: a critical evaluation of two ELISAs for the detection of antibodies to the major pemphigus antigens, desmoglein 1 and 3.

Pemphigus vulgaris (PV) and pemphigus foliaceus (PF) are characterized by autoantibodies to the desmosomal glycoproteins desmoglein 3 (Dsg 3) and Dsg 1 (Dsg 1), respectively. In this study, two enzyme-linked immunosorbent assays (ELISA) which detect IgG autoantibodies to Dsg 1 and Dsg 3 have been evaluated. A total of 317 normal and disease controls, 82 patients with PV and 25 with PF were studied. The Dsg 3 ELISA was positive in all 34 patients with untreated PV and the Dsg 1 ELISA was positive in all 10 with untreated PF. When patients undergoing treatment were included, the sensitivities fell to 95% and 92%, respectively, but still compared favourably to the sensitivity of indirect immunofluorescence which was 79% in PV and 84% in PF. All PF sera were negative in the Dsg 3 ELISA and the specificity of both assays was 98% or greater. Large numbers of samples could be analysed simultaneously over a relatively short time period. The Dsg 1 and Dsg 3 ELISAs also provided objective, quantitative, reproducible data which allowed differentiation of PV from PF and in view of these advantages, they are likely to become a routine technique in diagnostic laboratories.

The use of two substrates to improve the sensitivity of indirect immunofluorescence in the diagnosis of pemphigus.

The aim of this study was to re-evaluate indirect immunofluorescence (IIF) comparing two substrates, normal human skin (HS) and monkey oesophagus (MO) using serum from 29 pemphigus patients classified according to the presence of serum autoantibodies to either desmoglein (Dsg) 1 or Dsg3 detected by enzyme-linked immunosorbent assay (ELISA). Overall, the sensitivity of IIF was 83% on HS and 90% on MO. When data from both substrates were combined, the sensitivity increased to 100%. When sera from pemphigus foliaceus (PF) patients were studied, which contained Dsg1 antibodies only, the sensitivity of IIF was greatest on HS and titres were on average 4.8 doubling dilutions higher than on MO. In contrast, when sera containing autoantibodies only to Dsg3 from pemphigus vulgaris (PV) patients was studied, the sensitivity was greatest on MO and titres were on average 4.4 doubling dilutions higher than on HS. There was a significant correlation between Dsg1 antibody levels and IIF titres on HS and between Dsg3 antibody levels and IIF titres on MO. The investigation of immunobullous disorders in the future is likely to move towards antigen-specific techniques such as the Dsg ELISAs used in this study. However, in laboratories which currently rely on IIF for detecting pemphigus autoantibodies, the data presented in this study strongly suggest that two substrates should be used for IIF screening: one rich in Dsg1, such as HS, and the other rich in Dsg3, such as MO. This combination of substrates should not only increase the sensitivity of detecting pemphigus antibodies, but will aid in the differentiation of PV from PF. It is also possible that the data might be more useful for disease monitoring.

Michel's medium: a potential alternative to cryoprotection for tissue transport in the investigation of genetic skin disease.

Michel's medium is now well established as a transport medium to maintain tissue-fixed immunoreactants prior to direct immunofluorescence and immunoelectron microscopy. This is a practical alternative to cryofixation prior to transportation when sending skin biopsies for the investigation of cutaneous immunobullous disease. In this study we have demonstrated preservation of the cutaneous basement membrane zone proteins in skin biopsies stored in Michel's medium for periods of up to 28 days - proving that Michel's medium can be used as a transport medium when sending skin biopsies for immunohistochemical studies to determine the structural and molecular deficiencies in genodermatoses such as inherited forms of epidermolysis bullosa.

Striate palmoplantar keratoderma resulting from desmoplakin haploinsufficiency.

Recently, the first example of a human mutation in the gene encoding the desmosomal plaque protein, desmoplakin, has been described in a patient with autosomal dominant striate palmoplantar kerato-derma. We now report a further case of a desmoplakin mutation in a proband with striate palmoplantar keratoderma that also results in a null allele and haploinsufficiency. The mutation was a heterozygous G > A transition at the donor + 1 site of intron 7 of the desmoplakin gene (939 + 1 G > A; Genbank M77830). The aberrant splicing leads to retention of the entire intron 7, which contains a premature termination codon within the N-terminal domain of the peptide. Because the mutant null allele could not be identified on cDNA sequencing, we determined by polymerase chain reaction the exon-intron organization of the desmoplakin gene to facilitate analysis of genomic DNA. The gene spans approximately 45 kb of chromosome 6 and comprises 24 exons ranging in size from 51 bp to 3922 bp. We have also characterized fully the 3'UTR of the desmoplakin cDNA. This study demonstrates the relevance of haploinsufficiency for desmoplakin in the pathogenesis of this genodermatosis. Assessment of family members bearing the mutant allele also emphasizes the significance of an individual's age and exposure to skin trauma in manifesting full phenotypic expression of the disorder.

Homozygous nonsense mutation in helix 2 of K14 causes severe recessive epidermolysis bullosa simplex.

We have studied a consanguineous family containing two children with severe, generalized epidermolysis bullosa simplex (EBS). Electron microscopy of skin biopsies from the affected individuals showed that basal keratinocytes were devoid of tonofilament bundles, although some single intermediate filament were visible. Genetic linkage analysis with the microsatellite probe D12S96 excluded the type II keratin gene cluster in this family. However, homozygosity by descent was observed with the polymorphic probes KRT9, KRT10 Ava II, and D17S1787 in both affected children, consistent with a recessive defect in a type I keratin. Immunoreactivity to keratin K5 and K15 was normal, but monoclonal antibodies LL001 and RCK107 against K14 showed no staining, suggesting a deficiency of K14 in these individuals. mRNA extracted from biopsy material was amplified by RT-PCR to obtain full-length K14 cDNA. Direct automated sequencing identified a homozygous nonsense mutation, W305X. A Hinf I restriction enzyme site is created by this nucleotide transition, which was used to confirm the presence of the mutation in this kindred and exclude it from 100 normal chromosomes. This is the fourth kindred with severe recessive EBS for whom a mutation has been found in the K14 gene. In this instance, the premature termination codon is the farthest downstream of the reported cases, occurring in the helix 2 domain and so giving a much longer translation product. Nevertheless, the heterozygous carriers are unaffected by the disease and display no epidermal fragility. We postulate that translation of the potentially dominant-negative truncated K14 might be down-regulated due to instability of the mutant mRNA, as observed in previous cases with similar mutations.

sfi-independent filamentation in Escherichia coli Is lexA dependent and requires DNA damage for induction.

In Escherichia coli, damage to DNA induces the expression of a set of genes known collectively as the SOS response. Part of the SOS response includes genes that repair DNA damage, but another part of the response coordinates DNA replication and septation to prevent untimely cell division. The classic SOS gene product that inhibits cell division is SfiA (or SulA), which binds to FtsZ and prevents septum formation until the DNA damage has been repaired. However, another pathway acts to coordinate DNA replication and cell division when sfiA, or the sfi-dependent pathway, is inoperative. Until recently, little was known of this alternative pathway, which is called the sfi-independent pathway. We report here that sfi-independent filamentation is suppressed by lexA(Ind-) mutations, suggesting that derepression of the LexA regulon is necessary for sfi-independent induction. However, expression of LexA-controlled genes is not sufficient; DNA damage is also required to induce this secondary pathway of cell division inhibition. Furthermore, we postulate that loss of the common regulatory circuitry of the sfi-dependent and sfi-independent pathways by recA or lexA mutants uncouples cell division and DNA replication.

Pathotypes of Cochliobolus sativus on Barley in North Dakota.

Spot blotch, caused by Cochliobolus sativus, has become an important disease on two-rowed barley cultivars in North Dakota. Information on the variability of virulence in C. sativus is essential for the development of spot blotch-resistant barley germ plasm. To establish the number and distribution of C. sativus virulence types in North Dakota, 22 isolates from the state were evaluated for their virulence on three differential barley genotypes: ND 5883, Bowman, and ND B112. Eleven additional isolates from Minnesota, South Dakota, Nebraska, Colorado, Japan, and China were included for comparison. Three pathotypes were identified from the thirty-three isolates tested based on the infection responses of the three host genotypes. These pathotypes were designated 0, 1, and 2 according to the coded triplet system of nomenclature. In North Dakota, pathotype 0 (exhibits low virulence on all three differentials) comprised 65%, pathotype 2 (exhibits high virulence on Bowman and low virulence on ND 5883 and ND B112) comprised 25%, and pathotype 1 (exhibits high virulence on ND 5883 and low virulence on Bowman and ND B112) comprised 10% of the total number of barley isolates examined. Within North Dakota, pathotype 1 was found in the east-central and northeastern region only, whereas pathotypes 2 and 0 were distributed throughout the state where samples were taken. Isolates with the pathotype 1 virulence pattern were identified in Colorado, Nebraska, Japan, and China. The remaining isolates collected outside North Dakota were pathotype 0. The information obtained from this study will facilitate the deployment of effective resistance to C. sativus in barley.

Genetics of Virulence in Cochliobolus sativus and Resistance in Barley.

ABSTRACT Spot blotch, caused by Cochliobolus sativus, is one of the most common foliar diseases of barley in the upper midwest region of the United States. To examine the genetics of host-specific virulence in C. sativus, a cross was made between isolate ND90Pr (which exhibits high virulence on barley genotype Bowman and low virulence on genotype ND 5883) and ND93-1 (which exhibits low virulence on both genotypes). Ascospore progeny segregated 48:55 for low virulence/high virulence on Bowman, indicating the presence of a single virulence gene in isolate ND90Pr. To complement the study of host-specific virulence in the pathogen, an experiment also was conducted on the genetics of specific resistance in the host. Progeny from a Bowman/ND 5883 cross were evaluated for their infection responses (IRs) to isolate ND90Pr at the seedling stage. The F(2) population segregated 1:3 for low IRs (resistant)/high IRs (susceptible), indicating the presence of a single resistance gene in genotype ND 5883. This result was confirmed in the F(3) generation, as a 1:2:1 ratio was found for homozygous resistant, segregating, and homozygous susceptible families, respectively. The data from this study demonstrate that both virulence in the pathogen and resistance in the host are under monogenic control in this specific host genotype/fungal isolate combination.