Overcoming Host Resistance Research Articles

Novel Plasmopara viticola isolate surpasses grapevine Rpv3.1 and Rpv3.2 resistance but not Rpv12

AbstractGrape downy mildew, caused by the oomycete Plasmopara viticola, is a major threat to Vitis vinifera grape production. Several breeding programs rely on resistance factors (Rpvs) from American and Asian Vitis species to confer resistance to P. viticola. Although the use of these crossing hybrids offers a promising strategy for disease control, variants capable of overcoming host resistance are emerging. A thorough characterization of these isolates is needed to define new strategies for breeding programs. The investigation of varying aggressiveness of two P. viticola pathotypes was investigated. One pathotype is unable to overcome Rpv3.1 based resistance and another pathotype was isolated from a grapevine carrying the Rpv3.1 locus. Their aggressiveness was evaluated on five grapevine genotypes harboring distinct combinations of Rpv loci. Macroscopic and microscopic development of the pathogen was characterised throughout its colonization of grapevine leaves. Our findings demonstrate that the P. viticola isolate NW-10/16 is not only capable of overcoming Rpv3-based resistance but also exhibited a faster growth, increased sporulation, and a denser mycelial colonization within the plant mesophyll when compared with the avRpv3 + isolate in susceptible plants. The identification and characterization of new, more aggressive, P. viticola pathotypes is crucial to promote durable management of the resistant grapevine hybrids and to mitigate the threat of downy mildew.

Analysis of five near-complete genome assemblies of the tomato pathogen Cladosporium fulvum uncovers additional accessory chromosomes and structural variations induced by transposable elements effecting the loss of avirulence genes

BackgroundFungal plant pathogens have dynamic genomes that allow them to rapidly adapt to adverse conditions and overcome host resistance. One way by which this dynamic genome plasticity is expressed is through effector gene loss, which enables plant pathogens to overcome recognition by cognate resistance genes in the host. However, the exact nature of these loses remains elusive in many fungi. This includes the tomato pathogen Cladosporium fulvum, which is the first fungal plant pathogen from which avirulence (Avr) genes were ever cloned and in which loss of Avr genes is often reported as a means of overcoming recognition by cognate tomato Cf resistance genes. A recent near-complete reference genome assembly of C. fulvum isolate Race 5 revealed a compartmentalized genome architecture and the presence of an accessory chromosome, thereby creating a basis for studying genome plasticity in fungal plant pathogens and its impact on avirulence genes.ResultsHere, we obtained near-complete genome assemblies of four additional C. fulvum isolates. The genome assemblies had similar sizes (66.96 to 67.78 Mb), number of predicted genes (14,895 to 14,981), and estimated completeness (98.8 to 98.9%). Comparative analysis that included the genome of isolate Race 5 revealed high levels of synteny and colinearity, which extended to the density and distribution of repetitive elements and of repeat-induced point (RIP) mutations across homologous chromosomes. Nonetheless, structural variations, likely mediated by transposable elements and effecting the deletion of the avirulence genes Avr4E, Avr5, and Avr9, were also identified. The isolates further shared a core set of 13 chromosomes, but two accessory chromosomes were identified as well. Accessory chromosomes were significantly smaller in size, and one carried pseudogenized copies of two effector genes. Whole-genome alignments further revealed genomic islands of near-zero nucleotide diversity interspersed with islands of high nucleotide diversity that co-localized with repeat-rich regions. These regions were likely generated by RIP, which generally asymmetrically affected the genome of C. fulvum.ConclusionsOur results reveal new evolutionary aspects of the C. fulvum genome and provide new insights on the importance of genomic structural variations in overcoming host resistance in fungal plant pathogens.

Molecular Mechanism of Overcoming Host Resistance by the Target of Rapamycin Gene in Leptographium qinlingensis.

Leptographium qinlingensis is a fungal symbiont of the Chinese white pine beetle (Dendroctonus armandi) and a pathogen of the Chinese white pine (Pinus armandii) that must overcome the terpenoid oleoresin defenses of host trees to invade and colonize. L. qinlingensis responds to monoterpene flow with abundant mechanisms that include the decomposing and use of these compounds as a nitrogen source. Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in both plants and animals through integration of nutrients, energies, hormones, growth factors and environmental inputs to control proliferation, growth and metabolism in diverse multicellular organisms. In this study, in order to explore the relationship between TOR gene and carbon sources, nitrogen sources, host nutrients and host volatiles (monoterpenoids) in L. qinlingensis, we set up eight carbon source treatments, ten nitrogen source treatments, two host nutrients and six monoterpenoids (5%, 10% and 20%) treatments, and prepared different media conditions. By measuring the biomass and growth rate of mycelium, the results revealed that, on the whole, the response of L. qinlingensis to nitrogen sources was better than carbon sources, and the fungus grew well in maltose (carbon source), (NH4)2C2O4 (inorganic nitrogen source), asparagine (organic nitrogen source) and P. armandii (host nutrient) versus other treatments. Then, by analyzing the relationship between TOR expression and different nutrients, the data showed that: (i) TOR expression exhibited negative regulation in response to carbon sources and host nutrition. (ii) The treatments of nitrogen sources and terpenoids had positively regulatory effects on TOR gene; moreover, the fungus was most sensitive to β-pinene and 3-carene. In conclusion, our findings reveal that TOR in L. qinlingensis plays a key role in the utilization of host volatiles as nutrient intake, overcoming the physical and chemical host resistances and successful colonization.

New Pathotype Nomenclature for Better Characterisation the Virulence and Diversity of Blumeria graminis f.sp. avenae Populations

Fungal cereal pathogens, including Blumeria graminis f.sp. avenae, have the ability to adapt to specific conditions, which in turn leads to overcoming host resistance. An important aspect is the standardized way of characterizing the races and pathotypes of the pathogen. In the presented work, for the first time it was proposed to use a unified letter code that allows describing the pathotypes of B. graminis f.sp. avenae. The set of 14 oat genotypes were used as a differential set. This set included genotypes having so far described powdery mildew resistance genes Pm1–Pm11, and two genotypes (A. sterilis and A. strigosa) with effective sources of resistance to Bga. Based on the analysis of 160 Bga isolates collected in 2016–2019 from 4 locations in Poland, the most numerous was the TBBB pathotype, represented by 30% of the tested isolates. It was present in all analyzed populations. Subsequently, 8.1% and 6.3% of the isolates represented the TBCB and RBBB pathotypes, respectively.

Dynamics of Verticillium dahliae race 1 population under managed agricultural ecosystems

BackgroundPlant pathogens and their hosts undergo adaptive changes in managed agricultural ecosystems, by overcoming host resistance, but the underlying genetic adaptations are difficult to determine in natural settings. Verticillium dahliae is a fungal pathogen that causes Verticillium wilt on many economically important crops including lettuce. We assessed the dynamics of changes in the V. dahliae genome under selection in a long-term field experiment.ResultsIn this study, a field was fumigated before the Verticillium dahliae race 1 strain (VdLs.16) was introduced. A derivative 145-strain population was collected over a 6-year period from this field in which a seggregating population of lettuce derived from Vr1/vr1 parents were evaluated. We de novo sequenced the parental genome of VdLs.16 strain and resequenced the derivative strains to analyze the genetic variations that accumulate over time in the field cropped with lettuce. Population genomics analyses identified 2769 single-nucleotide polymorphisms (SNPs) and 750 insertion/deletions (In-Dels) in the 145 isolates compared with the parental genome. Sequence divergence was identified in the coding sequence regions of 378 genes and in the putative promoter regions of 604 genes. Five-hundred and nine SNPs/In-Dels were identified as fixed. The SNPs and In-Dels were significantly enriched in the transposon-rich, gene-sparse regions, and in those genes with functional roles in signaling and transcriptional regulation.ConclusionsUnder the managed ecosystem continuously cropped to lettuce, the local adaptation of V. dahliae evolves at a whole genome scale to accumulate SNPs/In-Dels nonrandomly in hypervariable regions that encode components of signal transduction and transcriptional regulation.

English

Late blight is highly variable adapting to new fungicides and overcoming host resistance. The objective of the study was to determine efficacy of Trichoderma asperellum against Phytophthora infestans and its compatibility with fungicides. T. asperellum at 33% (3 × 106), 66% (7 × 106) and 100% (1 × 107 CFU/mL; g/L),   Ridomil® (Metalaxyl 4% + Mancozeb 64%) and Mistress 72® (Cynamoxil 4% + Mancozeb 64%) were plated alongside P. infestans in vitro and detached leaflets assay. Results indicated that Ridomil® and Mistress 72® completely inhibited mycelial growth of P. infestans in vitro and in detached leaves. The 33% T. asperellum concentration had the lowest inhibitory power (38.0%) while 66% (91.10%) and 100% (91.30%) T. asperellum concentrations were not significantly different (P=0.05). Lesion sizes were not significantly different in 66% T. asperellum (1.91 cm2) and 100% (1.89 cm2) concentration while 33% concentration (3.5 cm2) and untreated (3.55 cm2) did not differ significantly. Ridomil® and Mistress 72® had no significant effect on T. asperellum mycelial growth. The results suggest that T. asperellum at 66% was effective in managing late blight. Results further indicate that T. asperellum could be used in combination with fungicides for effective and economical option.   Key words: Trichoderma asperellum, Phytophthora infestans, detached leaf assay, in vitro.

Evaluation of Soybean Germplasm for Resistance to Multiple Nematode Species: Heterodera glycines, Meloidogyne incognita, and Rotylenchulus reniformis

Plant‐parasitic nematode species are one of the greatest threats to cause serious yield loss throughout soybean production regions of the United States. Soybean [Glycine max (L.) Merr.] cultivars of different maturity groups carry known resistance to soybean cyst nematode (Heterodera glycines, SCN) at rgh1 and Rhg4 loci. Continuous use of these genes caused SCN population shifts and overcoming host resistance over time. Lack of genetic diversity created a bottleneck in development of new cultivars containing novel SCN resistance genes. Additional sources of resistance to southern root‐knot nematode (Meloidogyne incognita, SRKN) and reniform nematode (Rotylenchlus reniformis, RN) could result in supplemental protection of soybean yield. This research focused on determining known and new sources of SCN resistance in 1220 soybean accessions divided into two groups: (i) early‐maturity groups (000‐II) screened with two SCN populations, and (ii) medium‐maturity groups (III‐V) screened with six SCN populations. Based on the initial screening results, a subset of 76 accessions was selected for further phenotyping with SCN, SRKN, and RN populations and supplemented by genotyping using molecular markers. The results revealed that 56 lines were resistant to two nematode species in various combinations, and 12 lines were resistant to all three nematode species. Moreover, potential new sources of resistance‐carrying genes other than rhg1 and Rhg4 loci were pinpointed. Interestingly, RN resistance highly correlated with rhg1 locus suggesting a common gene or tight linkage. Identified resistant soybean accessions provide promising materials for discovery of novel genes to support cultivar development.

Genotyping of Plasmodiophora brassicae reveals the presence of distinct populations

BackgroundPlasmodiophora brassicae is a soilborne pathogen of the family Brassicaceae and the causal agent of clubroot disease. In Canada, P. brassicae is now one of the most important constraints to canola (Brassica napus) production, and is managed mainly by the deployment of resistant cultivars. In recent years, however, new strains of the pathogen have emerged that are capable of overcoming host resistance, posing new challenges for disease management. Despite its economic significance, molecular studies of P. brassicae are rare, mainly because this microorganism cannot be cultured outside of its host.ResultsRestriction site-associated DNA sequencing (RADseq) was used to examine the genetic diversity within P. brassicae single-spore and field isolates collected from across Canada. The isolates included individuals that were either capable or incapable of causing disease on clubroot resistant canola cultivars. Over 8750 variants were identified through RADseq. Population analysis indicated that most isolates belonged to one of two distinct populations, corresponding with the ability of isolates to cause disease on resistant cultivars. Within each population, there were low levels of genetic diversity. One thousand and fifty of the genetic variants that distinguished the two populations were nonsynonymous, altering the coding sequences of genes.ConclusionThe application of RADseq revealed two distinct populations of P. brassicae in Canada, suggesting multiple introductions of the pathogen into the country. The genetic variation found here will be important for future research and monitoring of the pathogen.

Sound-Triggered Production of Antiaggregation Pheromone Limits Overcrowding ofDendroctonus valensAttacking Pine Trees

For insects that aggregate on host plants, both attraction and antiaggregation among conspecifics can be important mechanisms for overcoming host resistance and avoiding overcrowding, respectively. These mechanisms can involve multiple sensory modalities, such as sound and pheromones. We explored how acoustic and chemical signals are integrated by the bark beetle Dendroctonus valens to limit aggregation in China. In its native North American range, this insect conducts nonlethal attacks on weakened trees at very low densities, but in its introduced zone in China, it uses mixtures of host tree compounds and the pheromone component frontalin to mass attack healthy trees. We found that exo-brevicomin was produced by both female and male D. valens, and that this pheromone functioned as an antiaggregating signal. Moreover, beetles feeding in pairs or in masses were more likely than were beetles feeding alone to produce exo-brevicomin, suggesting a potential role of sound by neighboring beetles in stimulating exo-brevicomin production. Sound playback showed that an agreement sound was produced by both sexes when exposed to the aggregation pheromone frontalin and attracts males, and an aggressive sound was produced only by males behaving territorially. These signals triggered the release of exo-brevicomin by both females and males, indicating an interplay of chemical and sonic communication. This study demonstrates that the bark beetle D. valens uses sounds to regulate the production of an antiaggregation pheromone, which may provide new approaches to pest management of this invasive species.

Mutations That Determine Resistance Breaking in a Plant RNA Virus Have Pleiotropic Effects on Its Fitness That Depend on the Host Environment and on the Type, Single or Mixed, of Infection.

Overcoming host resistance in gene-for-gene host-virus interactions is an important instance of host range expansion, which can be hindered by across-host fitness trade-offs. Trade-offs are generated by negative effects of host range mutations on the virus fitness in the original host, i.e., by antagonistic pleiotropy. It has been reported that different mutations in Pepper mild mottle virus (PMMoV) coat protein result in overcoming L-gene resistance in pepper. To analyze if resistance-breaking mutations in PMMoV result in antagonistic pleiotropy, all reported mutations determining the overcoming of L(3) and L(4) alleles were introduced in biologically active cDNA clones. Then, the parental and mutant virus genotypes were assayed in susceptible pepper genotypes with an L(+), L(1), or L(2) allele, in single and in mixed infections. Resistance-breaking mutations had pleiotropic effects on the virus fitness that, according to the specific mutation, the host genotype, and the type of infection, single or mixed with other virus genotypes, were antagonistic or positive. Thus, resistance-breaking mutations can generate fitness trade-offs both across hosts and across types of infection, and the frequency of host range mutants will depend on the genetic structure of the host population and on the frequency of mixed infections by different virus genotypes. Also, resistance-breaking mutations variously affected virulence, which may further influence the evolution of host range expansion. A major cause of virus emergence is host range expansion, which may be hindered by across-host fitness trade-offs caused by negative pleiotropy of host range mutations. An important instance of host range expansion is overcoming host resistance in gene-for-gene plant-virus interactions. We analyze here if mutations in the coat protein of Pepper mild mottle virus determining L-gene resistance-breaking in pepper have associated fitness penalties in susceptible host genotypes. Results show that pleiotropic effects of resistance-breaking mutations on virus fitness depend on the specific mutation, the susceptible host genotype, and the type of infection, single or mixed, with other virus genotypes. Accordingly, resistance-breaking mutations can have negative, positive, or no pleiotropic effects on virus fitness. These results underscore the complexity of host range expansion evolution and, specifically, the difficulty of predicting the overcoming of resistance factors in crops.

Development and use of microsatellite markers to study diversity, reproduction and population genetic structure of the cereal pathogen Ramularia collo-cygni

Ramularia collo-cygni (Rcc) is a major pathogen of barley that causes economically serious yield losses. Disease epidemics during the growing season are mainly propagated by asexual air-borne spores of Rcc, but it is thought that Rcc undergoes sexual reproduction during its life cycle and may also disperse by means of sexual ascospores. To obtain population genetic information from which to infer the extent of sexual reproduction and local genotype dispersal in Rcc, and by implication the pathogen’s ability to adapt to fungicides and resistant cultivars, we developed ten polymorphic microsatellite markers, for which primers are presented. We used these markers to analyse the population genetic structure of this cereal pathogen in two geographically distant populations from the Czech Republic (n=30) and the United Kingdom (n=60) that had been sampled in a spatially explicit manner. Genetic diversity at the microsatellite loci was substantial, Ht=0.392 and Ht=0.411 in the Czech and UK populations respectively, and the populations were moderately differentiated at these loci (Θ=0.111, P<0.01). In both populations the multilocus genotypic diversity was very high (one clonal pair per population, resulting in >96% unique genotypes in each of the populations) and there was a lack of linkage disequilibrium among loci, strongly suggesting that sexual reproduction is an important component of the life cycle of Rcc. In an analysis of spatial genetic structure, kinship coefficients in all distance classes were very low (−0.0533 to 0.0142 in the Czech and −0.0268 to 0.0042 in the Scottish population) and non-significant (P>0.05) indicating lack of subpopulation structuring at the field scale and implying extensive dissemination of spores. These results suggest that Rcc possesses a high evolutionary potential for developing resistance to fungicides and overcoming host resistance genes, and argue for the development of an integrated disease management system that does not rely solely on fungicide applications.

The Pheromone Frontalin and its Dual Function in the Invasive Bark Beetle Dendroctonus valens

The red turpentine beetle, Dendroctonus valens LeConte, is one of the most destructive invasive forest pests in China, having killed more than 6 million pines since its first outbreak in 1999. Little is known about D. valens pheromone biology and no aggregation pheromone has yet been identified. Analysis by gas chromatograph/mass spectrometer of volatiles collected from live beetles in China showed that female beetles produce frontalin and males do not. Olfactory assays in the laboratory showed that males were attracted to frontalin at a wide range of concentrations, whereas females were attracted to it at a narrow range of concentrations. In field trials, 3-carene, a monoterpene kairomone from a pine tree selected to host the beetles attracted both sexes, and when frontalin was added, the total number of beetles captured increased by almost 200%. However, increasing concentrations of frontalin significantly decreased the percentage of female beetles trapped. These results suggest a new role of frontalin as an aggregation pheromone in addition to a female-produced sex pheromone, which was previously shown in a North American population. The dual functions of the pheromone frontalin produced by D. valens females, as well as its ecological significance for overcoming host resistance, are discussed.

Pharmacological Immunosuppression Reduces But Does Not Eliminate the Need for Total-Body Irradiation in Nonmyeloablative Conditioning Regimens for Hematopoietic Cell Transplantation

In the dog leukocyte antigen (DLA)-identical hematopoietic cell transplantation (HCT) model, stable marrow engraftment can be achieved with total-body irradiation (TBI) of 200 cGy when used in combination with postgrafting immunosuppression. The TBI dose can be reduced to 100 cGy without compromising engraftment rates if granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood mononuclear cells (G-PBMC) are infused with the marrow. T cell-depleting the G-PBMC product abrogates this effect. These results were interpreted to suggest that the additional T cells provided with G-PBMC facilitated engraftment by overcoming host resistance. We therefore hypothesized that the TBI dose may be further reduced to 50 cGy by augmenting immunosupression either by (1) tolerizing or killing recipient T cells, or (2) enhancing the graft-versus-host (GVH) activity of donor T cells. To test the first hypothesis, recipient T cells were activated before HCT by repetitive donor-specific PBMC infusions followed by administration of methotrexate (MTX) (n = 5), CTLA4-Ig (n = 4), denileukin diftitox (Ontak; n = 4), CTLA4-Ig + MTX (n = 8), or 5c8 antibody (anti-CD154) + MTX (n = 3). To test the second hypothesis, recipient dendritic cells were expanded in vivo by infusion of Flt3 ligand given either pre-HCT (n = 4) or pre- and post-HCT (n = 5) to augment GVH reactions. Although all dogs showed initial allogeneic engraftment, sustained engraftment was seen in only 6of 42 dogs (14% of all dogs treated in 9 experimental groups). Hence, unless more innovative pharmacotherapy can be developed that more forcefully shifts the immunologic balance in favor of the donor, noncytotoxic immunosuppressive drug therapy as the sole component of HCT preparative regimens may not suffice to ensure sustained engraftment.

A single amino acid change in HC-Pro of soybean mosaic virus alters symptom expression in a soybean cultivar carrying Rsv1 and Rsv3

It is generally believed that infidelity of RNA virus replication combined with R-gene-driven selection is one of the major evolutionary forces in overcoming host resistance. In this study, we utilized an avirulent soybean mosaic virus (SMV) mutant to examine the possibility of emergence of mutant viruses capable of overcoming R-gene-mediated resistance during serial passages. Interestingly, we found that the emerged progeny virus induced severe rugosity and local necrotic lesions in Jinpumkong-2 (Rsv1 + Rsv3) plants, while SMV-G7H provoked a lethal systemic hypersensitive response. Genome sequence analysis of the emerged progeny virus revealed that the mutation in CI that had caused SMV-G7H to lose its virulence was restored to the original sequence, and a single amino acid was newly introduced into HC-Pro, which means that the symptom alteration was due to this single amino acid mutation in HC-Pro. Our results suggest that SMV HC-Pro functions as a symptom determinant in the SMV-soybean pathosystem.

23 INVITED T regs in cancer

In the dog leukocyte antigen (DLA)-identical hematopoietic cell transplantation (HCT) model, stable marrow engraftment can be achieved with total-body irradiation (TBI) of 200 cGy when used in combination with postgrafting immunosuppression. The TBI dose can be reduced to 100 cGy without compromising engraftment rates if granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood mononuclear cells (G-PBMC) are infused with the marrow. T cell-depleting the G-PBMC product abrogates this effect. These results were interpreted to suggest that the additional T cells provided with G-PBMC facilitated engraftment by overcoming host resistance. We therefore hypothesized that the TBI dose may be further reduced to 50 cGy by augmenting immunosupression either by (1) tolerizing or killing recipient T cells, or (2) enhancing the graft-versus-host (GVH) activity of donor T cells. To test the first hypothesis, recipient T cells were activated before HCT by repetitive donor-specific PBMC infusions followed by administration of methotrexate (MTX) (n = 5), CTLA4-Ig (n = 4), denileukin diftitox (Ontak; n = 4), CTLA4-Ig + MTX (n = 8), or 5c8 antibody (anti-CD154) + MTX (n = 3). To test the second hypothesis, recipient dendritic cells were expanded in vivo by infusion of Flt3 ligand given either pre-HCT (n = 4) or pre- and post-HCT (n = 5) to augment GVH reactions. Although all dogs showed initial allogeneic engraftment, sustained engraftment was seen in only 6 of 42 dogs (14% of all dogs treated in 9 experimental groups). Hence, unless more innovative pharmacotherapy can be developed that more forcefully shifts the immunologic balance in favor of the donor, noncytotoxic immunosuppressive drug therapy as the sole component of HCT preparative regimens may not suffice to ensure sustained engraftment.

Promotion of Stem Cell-Derived New T Cell Generation by CD62L− Memory T Cells Requires Alloantigen Recognition.

We recently demonstrated that non-GVHD-inducing CD62L− memory T cells promote T cell regeneration from hematopoietic stem/progenitor cells in the C57BL/6 (H2b) to BALB/c (H2d) bone marrow transplantation model (BLOOD 2004,103:1534). In this model, 1×107 T cell depleted bone marrow from C57BL/6, CD45.2, Thy1.2 mice and 1×106 CD62L− T cells from C57BL/6, CD45.1, Thy1.1 mice are used and the recipient mice are lethally irradiated. This animal model allows us to differentiate the T cells of different origin by flow cytometry. In the current study, we further investigated how CD62L− T cells promote new T cell generation and whether there is any functional relevance using the same model. Previous data suggest that the promoting effect of CD62L− T cells on stem cell-derived new T cell generation is associated with the facilitation of engraftment because host radioresisdant T cells were depleted in CD62L− T cell recipients but not in the T cell-depleted bone marrow control mice. To determine whether CD62L− T cells promote new T cell generation by overcoming T cell-mediated host resistance, we performed the experiment using SCID mice as recipients. SCID mice lack both T and B cells. In consistent with the previous results, serial peripheral blood T cell counts following bone marrow transplantation demonstrated that the promoting effect of CD62L− T cells were dramatically decreased in the SCID recipients, indicating that CD62L− T cell promote new T cell regeneration by overcoming host resistance. To examine whether CD62L− T cells overcome host resistance by “veto effect”, we performed the experiment using CD62L− T cells from (BALB/cxC57BL/6)F1 mice. T cells from F1 mice do not recognize the parental recipient as non-self and can not initiate classical T cell response against the parental antigens. It was previously reported by other investigators that T cells from F1 mice preserve “veto effect”. However, no promoting effect was observed in the recipients of CD62L− T cells from the F1 mice, demonstrating that “veto effect” is not involved. Rather, alloantigen recognition by donor CD62L− T cells is required for the promoting effect. In agreement with the hypothesis that CD62L− T cells promote new T cell generation by enhancing engraftment through overcoming host resistance, both white cell and platelet recovery in peripheral blood was accelerated in CD62L− T cell recipients comparing with that in the control T cell-depleted bone marrow recipients. Finally, we sought to determine whether these findings have any functional relevance in vivo. We evaluated the functional immune recovery by measuring the ability to inhibit the growth of a recipient-type leukemia/lymphoma cell line called BCL1 cells injected intravenously into the transplantation recipients on day +7. While six out of eight T cell-depleted bone marrow control mice developed tumor and died within 30 days after bone marrow transplantation, none of the CD62L− T cell recipients (n=12) developed tumor and all survived more than 60 days after transplantation. As expected, none of the CD62L− T cell recipients developed graft-versus-host disease. Our data indicate that CD62L− T cells promote new T cell generation by enhancing engraftment through overcoming host resistance. This promoting effect requires alloantigen recognition and is not mediated by “veto effect”. Our current focus is trying to determine how allogeneic CD62L− T cells overcome host resistance without causing graft-versus-host disease.

Topology: a novel method to describe branching patterns in Peronospora viciae colonies

Topology provides a novel means to describe branching patterns and has not been applied to fungal colonies previously. For any branched structure, various topologies are possible, and these lie between two extremes, a herringbone pattern (main axis with primary laterals) and a dichotomous pattern (highly branched system). We applied topological methods to colonies of Peronospora viciae 48 h after inoculation of Pisum sativum leaves. The methods are based on two simulations, one developed for channel networks such as found in river systems and another for biological systems. Although not a true herringbone form, the Peronospora viciae colonies have a strong herringbone element within their growth pattern. All 25 colonies analysed fell into the random distribution according to the confidence limits calculated from simulations for biological systems. These confidence limits, however, represent the percentile distribution of all simulated networks, and only those structures with a perfect herringbone or dichotomous topology fall outside the range. The tendency of P. viciae colonies towards herringbone growth is reflected by the topological indices for altitude and external pathlength (a(obs)/E(a) and pe(obs)/E(pe), where a = altitude, pe = external pathlength, obs = observed for the P. viciae colonies and E = expected values for random growth), and the slope of the regression analysis for a(obs) and pe(obs). We consider this trend as significant because it was consistent for all but one of the colonies, and implies that growth can be envisaged as an intermediate between random and herringbone topology. It is proposed that initial herringbone growth may reflect a strategy that is aimed at overcoming host resistance, achieving rapid colonisation of infected tissue and maximising the potential for nutrient acquisition. This topology would also increase the likelihood of finding a compatible mating type for reproduction between heterothallic isolates.

Pectin, a second inducer for laccase production by Botrytis cinerea

Pectin acts as a second inducer of extracellular laccase formation by Botrytis cinerea, in the presence of a phenolic substance as a first inducer, but pectin alone fails to induce enzyme formation. The possible advantages of this mechanism for the fungus during the process of infection and overcoming host resistance are discussed.

Parasite-Host Population Systems and Genetic Stability

Parasite populations (houseflies) were exposed to and isolated with each of six simulated host-plant types with the following six chemicals as toxicants (resistant factors) in the plant types: citric acid, copper sulfate, magnesium nitrate, sodium chloride, ammonium phosphate, and potassium hydroxide. When the fly population was exposed to a single host-toxicant type, the fly evolved rapidly (seven to 10 generations) to overcome the single host toxic ant. However, when the fly population was exposed to all six simulated hogfishes types in one multi cell system, the fly population did not evolve and overcome any of the host-toxic ant types. Relative to the genetic feedback mechanism, coevolution, and stability among parasite and host, the evidence suggests that genetic stability (preventing the parasite from overcoming host resistance) is possible in parasite-host population systems with the appropriate combination of (1) suitable genetic diversity (resistant characters) in the host population, (2) ample gene flow between parasite colonies (if colonies occur in the parasite-host population system), and (3) suitable selection coefficients on the parasite population resulting from its host population

Inoculum Potential and the Fusarial Wilt of Cotton

THE importance of inoculum potential of a pathogen in overcoming host resistance and causing infection is now well recognized1. This is particularly so with root-infecting fungi which have to overcome not only host resistance but also competition from soil saprophytes. As pointed out by Sanford2, “The importance to disease incidence of serious competition between the pathogen and saprophytes associated in general soil habitat or in the rhizosphere seems to be undetermined”. A fungus like Fusarium oxy-sporum f. vasinfectum (Atk.) Snyd. and Hans., having a high degree of competitive saprophytic ability3, can be expected to build up sufficient inoculum in soil by colonizing saprophytic substrates. Whether such a change in its inoculum, potential will cause any alteration in its infective capacity is not understood. A study of infective behaviour of the fungus with changes in inoculum potential was therefore undertaken by us.