Showing papers on "Plant disease resistance published in 1995"

A Receptor Kinase-Like Protein Encoded by the Rice Disease Resistance Gene, Xa21

Abstract: The rice Xa21 gene, which confers resistance to Xanthomonas oryzae pv. oryzae race 6, was isolated by positional cloning. Fifty transgenic rice plants carrying the cloned Xa21 gene display high levels of resistance to the pathogen. The sequence of the predicted protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response. Characterization of Xa21 should facilitate understanding of plant disease resistance and lead to engineered resistance in rice.

A receptor kinase-like protein encoded by the rice disease resistance gene, Xa21. - eScholarship

Abstract: The rice Xa21 gene, which confers resistance to Xanthomonas oryzae pv. oryzae race 6, was isolated by positional cloning. Fifty transgenic rice plants carrying the cloned Xa21 gene display high levels of resistance to the pathogen. The sequence of the predicted protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response. Characterization of Xa21 should facilitate understanding of plant disease resistance and lead to engineered resistance in rice.

Wheat Rusts: An Atlas of Resistance Genes

Abstract: 1: Wheat Rusts and the Genetic Bases on Disease Resistance. Introduction. Biology of Pathogen and Host. Host:Pathogen Genetics. Methodologies in Wheat Rust Disease. A Coordinated Strategy for Rust Control. Conclusion. Guide to Gene Descriptions and Illustrations. 2: The Genes for Resistance to Leaf Rust in Wheat and Triticale. Catalogued Leaf Rust Resistance Genes. Temporarily Designated and Miscellaneous Leaf Rust Resistance Genes. 3: The Genes for Resistance to Stem Rust in Wheat and Triticale. Catalogued Leaf Rust Resistance Genes. Temporarily Designated and Miscellaneous Stem Rust Resistance Genes. 4: The Genes for Resistance to Stripe Rust in Wheat and Triticale. Catalogued Stripe Rust Resistance Genes. Temporarily Designated and Miscellaneous Stripe Rust Resistance Genes. References. Appendix I: Species Names Used in the Text, Genomic Formulae and Synonyms. Appendix II: Pathotypes and Accessions (Cultures) Used in Photography.

Molecular genetics of plant disease resistance

Abstract: Plant breeders have used disease resistance genes (R genes) to control plant disease since the turn of the century. Molecular cloning of R genes that enable plants to resist a diverse range of pathogens has revealed that the proteins encoded by these genes have several features in common. These findings suggest that plants may have evolved common signal transduction mechanisms for the expression of resistance to a wide range of unrelated pathogens. Characterization of the molecular signals involved in pathogen recognition and of the molecular events that specify the expression of resistance may lead to novel strategies for plant disease control.

Structure of the Arabidopsis RPM1 gene enabling dual specificity disease resistance

Abstract: Plants can recognize pathogens through the action of disease resistance (R) genes, which confer resistance to pathogens expressing unique corresponding avirulence (avr) genes. The molecular basis of this gene-for-gene specificity is unknown. The Arabidopsis thaliana RPM1 gene enables dual specificity to pathogens expressing either of two unrelated Pseudomonas syringae avr genes. Despite this function, RPM1 encodes a protein sharing molecular features with recently described single-specificity R genes. Surprisingly, RPM1 is lacking from naturally occurring, disease-susceptible Arabidopsis accessions.

Arabidopsis signal transduction mutant defective in chemically and biologically induced disease resistance.

Abstract: Plants possess multiple resistance mechanisms that guard against pathogen attack. Among these are inducible systems such as systemic acquired resistance (SAR). SAR is activated by pathogen exposure and leads to an increase in salicylic acid (SA), high-level expression of SAR-related genes, and resistance to a spectrum of pathogens. To identify components of the signal transduction pathways regulating SAR, a mutant screen was developed that uses 2,6-dichloroisonicotinic acid as an activator of SAR gene expression and pathogen resistance, followed by assays for resistance to the fungal pathogen Peronospora parasitica. Mutants from this screen were subsequently examined to assess their defense responses. We describe here a recessive mutation that causes a phenotype of insensitivity to chemical and biological inducers of SAR genes and resistance. These data indicate the existence of a common signaling pathway that couples these diverse stimuli to induction of SAR genes and resistance. Because of its non-inducible immunity phenotype, we call this mutant nim1. Although nim1 plants fail to respond to SA, they retain the ability to accumulate wild-type levels of SA, a probable endogenous signal for SAR. Further, the ability of nim1 plants to support growth of normally incompatible races of a fungal pathogen indicates a role for this pathway in expression of genetically determined resistance, consistent with earlier findings for transgenic plants engineered to break down SA. These results suggest that the wild-type NIM1 gene product functions in a pathway regulating acquired resistance, at a position downstream of SA accumulation and upstream of SAR gene induction and expression of resistance.

Types and components of resistance to Fusarium head blight of wheat

Abstract: Resistance of wheat to Fusarium head blight caused by Fusarium graminearum and F. culmorum was identified in natural epidemics in 1985 and 1987 as well after artificial inoculations (1983–1988 and 1984–1987). Out of 25 genotypes tested, five were identified with no significant difference in head blight scores, but differing significantly in yield after artificial inoculation, i.e. tolerance differences were detected at different resistance levels. Some genotypes that were similar in yield or head blight scores differed in seed infection severity. Genotypes with awns were more susceptible to head blight when tested under natural epidemic condition in the field; but this trait did not influence head blight severity in artificial inoculations. Dwarf genotypes were more severely infected by head blight than tall genotypes under natural conditions, but genotypes of different plant height classes were similarly susceptible after artificial inoculations. In the early generations of a breeding programme resistance measured by visual evaluation of artificial inoculation is the most important way to screen. If selection of dwarf and awned genotypes cannot be avoided, the higher susceptibility caused by awns and dwarfness under natural epidemic conditions can be decreased by a higher level of physiological resistance, as variability in physiological resistance is available. In later generations, traits like percentage of seed infection or tolerance can be identified by additionally measuring yield reduction. Stability of disease reaction appears to be connected with resistance level, the most resistant genotypes are the most stable, and the most susceptible ones tend to have more unstable reactions in different epidemic conditions.

Host Plant Resistance to Insects

Abstract: Crop plant and insect diversity secondary plant metabolites for insect resistance insect-plant interactions host plant selection mechanisms of resistance factors affecting expression of resistance screening for insect resistance plant resistance and insect pest management genetics of resistance to insects breeding for resistance to insects.

The L6 gene for flax rust resistance is related to the Arabidopsis bacterial resistance gene RPS2 and the tobacco viral resistance gene N.

Abstract: The L6 rust resistance gene from flax was cloned after tagging with the maize transposable element Activator. The gene is predicted to encode two products of 1294 and 705 amino acids that result from alternatively spliced transcripts. The longer product is similar to the products of two other plant disease resistance genes, the tobacco mosaic virus resistance gene N of tobacco and the bacterial resistance gene RPS2 of Arabidopsis. The similarity involves the presence of a nucleotide (ATP/GTP) binding site and several other amino acid motifs of unknown function in the N-terminal half of the polypeptides and a leucine-rich region in the C-terminal half. The truncated product of L6, which lacks most of the leucine-rich C-terminal region, is similar to the truncated product that is predicted from an alternative transcript of the N gene. The L6, N, and RPS2 genes, which control resistance to three widely different pathogen types, are the foundation of a class of plant disease resistance genes that can be referred to as nucleotide binding site/leucine-rich repeat resistance genes.

NDR1, a locus of Arabidopsis thaliana that is required for disease resistance to both a bacterial and a fungal pathogen.

Abstract: We have employed Arabidopsis thaliana as a model host plant to genetically dissect the molecular pathways leading to disease resistance. A. thaliana accession Col-0 is susceptible to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 but resistant in a race-specific manner to DC3000 carrying any one of the cloned avirulence genes avrB, avrRpm1, avrRpt2, and avrPph3. Fast-neutron-mutagenized Col-0 M2 seed was screened to identify mutants susceptible to DC3000(avrB). Disease assays and analysis of in planta bacterial growth identified one mutant, ndr1-1 (nonrace-specific disease resistance), that was susceptible to DC3000 expressing any one of the four avirulence genes tested. Interestingly, a hypersensitive-like response was still induced by several of the strains. The ndr1-1 mutation also rendered the plant susceptible to several avirulent isolates of the fungal pathogen Peronospora parasitica. Genetic analysis of ndr1-1 demonstrated that the mutation segregated as a single recessive locus, located on chromosome III. Characterization of the ndr1-1 mutation suggests that a common step exists in pathways of resistance to two unrelated pathogens.

Disease resistance conferred by expression of a gene encoding H2O2-generating glucose oxidase in transgenic potato plants.

Abstract: Plant defense responses to pathogen infection involve the production of active oxygen species, including hydrogen peroxide (H2O2). We obtained transgenic potato plants expressing a fungal gene encoding glucose oxidase, which generates H2O2 when glucose is oxidized. H2O2 levels were elevated in both leaf and tuber tissues of these plants. Transgenic potato tubers exhibited strong resistance to a bacterial soft rot disease caused by Erwinia carotovora subsp carotovora, and disease resistance was sustained under both aerobic and anaerobic conditions of bacterial infection. This resistance to soft rot was apparently mediated by elevated levels of H2O2, because the resistance could be counteracted by exogenously added H2O2-degrading catalase. The transgenic plants with increased levels of H2O2 also exhibited enhanced resistance to potato late blight caused by Phytophthora infestans. The development of lesions resulting from infection by P. infestans was significantly delayed in leaves of these plants. Thus, the expression of an active oxygen species-generating enzyme in transgenic plants represents a novel approach for engineering broad-spectrum disease resistance in plants.

Systemic acquired resistance in Arabidopsis requires salicylic acid but not ethylene

Abstract: Systemic acquired resistance (SAR) is an inducible plant response to infection by a necrotizing pathogen. In the induced plant, SAR provides broad-spectrum protection against not only the inducing pathogen, but also against other, unrelated pathogens. Both salicylic acid (SA) and SAR-gene expression have been implicated as playing important roles in the initiation and maintenance of SAR. Here, we describe the characterization of transgenic Arabidopsis plants that express the bacterial nahG gene encoding salicylate hydroxylase, an enzyme that can metabolize SA. Strong, constitutive expression of this gene prevents pathogen-induced accumulation of SA and the activation of SAR by exogenous SA. We show that SAR in Arabidopsis can be induced by inoculation with Pseudomonas syringe pv. tomato against infection by a challenge inoculation with Peronospora parasitica. This response is abolished in transgenic, nahG-expressing Arabidopsis, but not in ethylene-insensitive mutants. These experiments support the critical role of SA in SAR and show that ethylene sensitivity is not required for SAR induction. The NahG Arabidopsis plants will be important for future studies aimed at understanding the role of SA in plant disease resistance mechanisms.

Molecular mechanisms of defense by rhizobacteria against root disease.

Abstract: Genetic resistance in plants to root diseases is rare, and agriculture depends instead on practices such as crop rotation and soil fumigation to control these diseases "Induced suppression" is a natural phenomenon whereby a soil due to microbiological changes converts from conducive to suppressive to a soilborne pathogen during prolonged monoculture of the susceptible host Our studies have focused on the wheat root disease "take-all," caused by the fungus Gaeumannomyces graminis var tritici, and the role of bacteria in the wheat rhizosphere (rhizobacteria) in a well-documented induced suppression (take-all decline) that occurs in response to the disease and continued monoculture of wheat The results summarized herein show that antibiotic production plays a significant role in both plant defense by and ecological competence of rhizobacteria Production of phenazine and phloroglucinol antibiotics, as examples, account for most of the natural defense provided by fluorescent Pseudomonas strains isolated from among the diversity of rhizobacteria associated with take-all decline There appear to be at least three levels of regulation of genes for antibiotic biosynthesis: environmental sensing, global regulation that ties antibiotic production to cellular metabolism, and regulatory loci linked to genes for pathway enzymes Plant defense by rhizobacteria producing antibiotics on roots and as cohabitants with pathogens in infected tissues is analogous to defense by the plant's production of phytoalexins, even to the extent that an enzyme of the same chalcone/stilbene synthase family used to produce phytoalexins is used to produce 2,4-diacetylphloroglucinol The defense strategy favored by selection pressure imposed on plants by soilborne pathogens may well be the ability of plants to support and respond to rhizosphere microorganisms antagonistic to these pathogens

Homology‐dependent resistance: transgenic virus resistance in plants related to homology‐dependent gene silencing

Abstract: Tobacco plants transformed with the RNA polymerase (RdRp) gene of potato virus X (PVX) that are extremely resistant to infection by potato virus X have previously been described. The PVX-resistant plants accumulated the RdRp protein at a lower level than fully susceptible plants transformed with the same RdRp construct. In this paper the difference between the PVX-resistant and susceptible transformed plants is investigated and it is demonstrated that there are three associated phenotypes of the RdRp transgene that vary in parallel between transformed lines. These phenotypes are: accumulation of the transgenic RdRp RNA at a low level; strain-specific resistance to PVX; and the ability of the transgene to trans-inactivate homologous transgenes. This gene-silencing potential of the transgenes conferring PVX resistance was illustrated by analysis of progeny from a cross between a transformant that was extremely resistant to PVX and a second PVX-susceptible transformant. In other transformants, in which the resistance was less extreme, the same three phenotypes were associated but in a transgene dosage-dependent manner. The same association of strain-specific resistance and low-level accumulation of the transgenic RdRp RNA was observed with plants that were transformed with mutant or wild-type versions of the RdRp gene. Strain-specific resistance was also produced in plants transformed with untranslatable versions of the RdRp transgene. Based on these data it is proposed that homology-dependent gene silencing and transgenic resistance to PVX may be due to the same RNA-based mechanism. An undefined genomic feature is proposed to account for the variation in the resistance and trans-inactivation phenotypes of different transformants. It is further proposed that this genome feature influences a cytoplasmic mechanism that degrades RNA with sequence homology to the silencing transgene.

Development and molecular cytogenetic analysis of wheat-Haynaldia villosa 6VS/6AL translocation lines specifying resistance to powdery mildew.

Abstract: Several Triticum aestivum L.-Haynaldia villosa disomic 6VS/6AL translocation lines with powdery mildew resistance were developed from the hybridization between common wheat cultivar Yangmai 5 and alien substitution line 6V(6A). Mitotic and meiotic C-banding analysis, aneuploid analysis with double ditelosomic stocks, in situ hybridization, as well as the phenotypic assessment of powdery mildew resistance, were used to characterize these lines. The same translocated chromosome, with breakpoints near the centromere, appears to be present in all the lines, despite variation among the lines in their morphology and agronomic characteristics. The resistance gene, conferred by H. villosa and designated as Pm21, is a new and promising source of powdery mildew resistance in wheat breeding.

Field Evaluation of Transgenic Squash Containing Single or Multiple Virus Coat Protein Gene Constructs for Resistance to Cucumber Mosaic Virus, Watermelon Mosaic Virus 2, and Zucchini Yellow Mosaic Virus

Abstract: Transgenic inbred squash lines containing various combinations of the cucumber mosaic virus (CMV), watermelon mosaic virus 2 (WMV 2) or zucchini yellow mosaic virus (ZYMV) coat protein (CP) genes were produced using Agrobacterium-mediated transformation. Progeny from lines transformed with single or multiple CP gene constructs were tested for virus resistance under field conditions, and exhibited varying levels of resistance to infection by CMV, WMV 2 or ZYMV. Most transgenic lines remained nonsymptomatic throughout the growing seasons and produced marketable fruits, while other lines showed a delay in the onset of symptoms and/or a reduction in symptom severity. A few lines failed to display any level of resistance. Depending on the CP gene used, 40 to 95% of the transgenic lines containing single CP constructs of either CMV, WMV 2 or ZYMV were resistant to the virus from which the CP gene was derived. Transgenic lines transformed with a double CP construct containing the CP genes from CMV and WMV 2, designated CW, displayed high level of resistance to CMV and WMV 2. A transgenic line, designated ZW-20, which contained the CP genes from ZYMV and WMV 2 displayed excellent resistance to ZYMV and WMV 2 in that most of the plants showed complete resistance. A few plants developed localized chlorotic dots or blotches, yet fruits remained asymptomatic. Southern blot analysis revealed that the CP inserts of some resistant plants of line ZW-20 were no longer linked to the neomycin phosphotransferase II (NPT II) gene. This loss of linkage allowed the marker gene to be separated from the virus resistance trait by Mendelian segregation. Further analysis of these plants showed that they contained multiple WMV 2 inserts which were designated B and H, the latter consisting of two hybridization signals. Analysis of inoculated plants showed that plants with the H inserts were symptomless or developed only chlorotic dots, while those without the H insert developed more prominent chlorotic blotches. In addition to lines with resistance to two viruses, a line with resistance to three viruses was also identified. Transgenic line CZW-3, transformed with the triple CP construct containing the CMV, WMV 2 and ZYMV CP genes, exhibited resistance to all three viruses. These two transgenic inbred lines, ZW-20 and CZW-3, have allowed for the development of commercial squash hybrids with multiple virus resistance.

Characterization of quantitative trait loci (QTLs) in cultivated rice contributing to field resistance to sheath blight (Rhizoctonia solani)

Abstract: Sheath blight, caused by Rhizoctonia solani, is one of the most important diseases of rice. Despite extensive searches of the rice germ plasm, the major gene(s) which give complete resistance to the fungus have not been identified. However, there is much variation in quantitatively inherited resistance to R. solani, and this type of resistance can offer adequate protection against the pathogen under field conditions. Using 255 F4 bulked populations from a cross between the susceptible variety ‘Lemont’ and the resistant variety ‘Teqing’, 2 years of field disease evaluation and 113 well-distributed RFLP markers, we identified six quantitative trait loci (QTLs) contributing to resistance to R. solani. These QTLs are located on 6 of the 12 rice chromosomes and collectively explain approximately 60% of the genotypic variation or 47% of the phenotypic variation in the ‘Lemont’x‘Teqing’ cross. One of these resistance QTLs (QSbr4a), which accounted for 6% of the genotypic variation in resistance to R. solani, appeared to be independent of associated morphological traits. The remaining five putative resistance loci (QSbr2a, QSbr3a, QSbr8a, QSbr9a and QSbr12a) all mapped to chromosomal regions also associated with increased plant height, three of which were also associated with QTLs causing later heading. This was consistent with the observation that heading date and plant height accounted for 47% of the genotypic variation in resistance to R. solani in this population. There were also weak associations between resistance to R. solani and leaf width, which were likely due to linkage with a QTL for this trait rather than to a physiological relationship.

Induced resistance to disease in plants.

Abstract: Induced resistance in Legumes B.J. Deverall, E.K. Dann. Induced resistance in the Solanaceae O.L. Ozeretkovskaya. Induced resistance in Cucurbits P. Hammerschmidt, P. Yang-Cashman. Induced resistance in Monocots U. Steiner, F. Schonbeck. Molecular regulation of systemic induced resistance B.A. Stermer. Thoughts on the role and evolution of induced resistance in natural ecosystems, and its relationship to other types of plant defenses against disease M.C. Heath. Practical application and implementation of induced resistance S. Tuzun, J. Kloepper. Induced systemic resistance -- An overview J. Kuc. Index.

Tagging and combining bacterial blight resistance genes in rice using RAPD and RFLP markers

Abstract: Four genes of rice,Oryza sativa L., conditioning resistance to the bacterial blight pathogenXanthomonas oryzae pv.oryzae (X. o. pv.oryzae), were tagged by restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers. No recombinants were observed betweenxa-5 and RFLP marker lociRZ390, RG556 orRG207 on chromosome 5.Xa-3 andXa-4 were linked to RFLP locusXNpb181 at the top of chromosome 11, at distances of 2.3 cM and 1.7 cM, respectively. The nearest marker toXa-10, also located on chromosome 11, was the RAPD locusO07 2000 at a distance of 5.3 cM. From this study, the conventional map [19, 28] and two RFLP linkage maps of chromosome 11 [14, 26] were partially integrated. Using the RFLP and RAPD markers linked to the resistance genes, we selected rice lines homozygous for pairs of resistance genes,Xa-4 +xa-5 andXa-4 +Xa-10. Lines carryingXa-4 +xa-5 andXa-4 +Xa-10 were evaluated for reaction to eight strains of the bacterial blight pathogen, representing eight pathotypes and three genetic lineages. As expected, the lines carrying pairs of genes were resistant to more of the isolates than their single-gene parental lines. Lines carryingXa-4 +xa-5 were more resistant to isolates of race 4 than were either of the parental lines (‘quantitative complementation’). No such effects were seen forXa-4 +Xa-10. Thus, combinations of resistance genes provide broader spectra of resistance through both ordinary gene action expected and quantitative complementation.

Evaluation of foliar resistance to Uncinula necator in Chinese wild Vitis species.

Abstract: Resistance to grape powdery mildew ( Uncinula necator BURR.) of 13 known Vitis species and 5 unclassified grapes native to China was evaluated. 88 clones were tested with natural infection and a subset of 34 were artificially inoculated during the years of 1991-1992. 68 clones showed resistance to U. necator . In general, we found clones of V. bryoniifolia, V. davidii , and V. piasezkii resistant to U. necator . However, intraspecific variations were observed. Results from this study indicate that Chinese native wild grapes may provide valuable germplasm resources for powdery mildew resistance.

Resistance of Transgenic Hybrid Squash ZW-20 Expressing the Coat Protein Genes of Zucchini Yellow Mosaic Virus and Watermelon Mosaic Virus 2 to Mixed Infections by Both Potyviruses

Abstract: The resistance of three transgenic yellow crookneck squash lines expressing the zucchini yellow mosaic virus (ZYMV) and/or watermelon mosaic virus 2 (WMV 2) coat protein (CP) genes was investigated under field conditions. Resistance was evaluated under high disease pressure achieved by mechanical inoculations of ZYMV and WMV 2 and natural challenge inoculations by aphid vectors. Assessment of infection rates was based on visual monitoring of symptom development, enzyme-linked immunosorbent assays (ELISA), infectivity assays, and analysis of virus occurrence in fruits. Horticultural performance was evaluated by counting the number of mature fruits per plant and estimating their fresh fruit weight The transgenic line ZW-20B expressing both the ZYMV and WMV 2 CP genes showed excellent resistance in that none of the plants developed severe foliar symptoms, although localized chlorotic dots or blotches appeared on some leaves. In contrast, the two transgenic lines expressing only single CP genes, either the ZYMV or WMV 2 CP gene, developed severe symptoms by the end of the trial period, as did the nontransformed control lines. Differences between transgenic squash lines were even more dramatic in fruit quality: All transgenic ZW-20B fruits were symptomless, while all fruits from the two transgenic lines with single CP genes showed symptoms, and thus were unmarketable. ZW-20B is a hybrid of commercial quality, and its high level of resistance to ZYMV and WMV 2 should enable the production of yellow summer squash in areas where both potyviruses are prevalent This transgenic yellow crookneck squash line is especially valuable in light of the difficulties of obtaining similar resistance by conventional breeding strategies.

The use of cell culture for subchromosomal introgressions of barley yellow dwarf virus resistance from Thinopyrum intermedium to wheat

Abstract: Barley yellow dwarf virus (BYDV) resistance has been transferred to wheat from a group 7 chromosome of Thinopyrum (Agropyron) intermedium. The source of the resistance gene was the L1 disomic addit...

Intergeneric transfer and functional expression of the tomato disease resistance gene Pto.

Abstract: Plant disease resistance loci have been used successfully in breeding programs to transfer traits from resistant germplasm to susceptible plant cultivars. The molecular cloning of plant disease resistance genes now permits the transfer of such traits across species boundaries by genetic transformation of recipient hosts. The tomato disease resistance gene Pto confers resistance to strains of the bacterial pathogen Pseudomonas syringae pv tomato expressing the avirulence gene avrPto. Transformation of Nicotiana benthamiana with Pto results in specific resistance to P. s. pv tabaci strains carrying avrPto. The resistant phenotype is manifested by a strong inhibition of bacterial growth and the ability to exhibit a hypersensitive response. Resistance cosegregates with the Pto gene in transgene selfings and testcrosses. Our results demonstrate the conservation of disease resistance functions across genus boundaries and suggest that the utility of host-specific resistance genes can be extended by intergeneric transfer.

Mapping loci controlling Brassica napus resistance to Leptosphaeria maculans under different screening conditions.

Abstract: Leptosphaeria maculans, the causal agent of blackleg of crucifers, is a major threat to rapeseed (Brassica napus) production throughout the world. Genes controlling blackleg resistance in B. napus were mapped using an F 1 -derived doubled haploid (DH) population of 105 lines and 138 restriction fragment length polymorphism (RFLP) markers. The host-pathogen interaction phenotype was assessed qualitatively and by several quantitative measurements using different environments and plant developmental stages. A single major locus controlling blackleg resistance (LEM1) was mapped to linkage group 6 based on qualitative scores of the interaction phenotype on inoculated cotyledons. This resistance locus was also identified by interval mapping using quantitative measurements of the interaction phenotype on cotyledon- and stem-inoculated plants. Four other genomic regions were significantly associated with quantitative measurements of resistance on cotyledon and stem, among them a marker locus interval in linkage group 17 that included a pathogenesis related gene (PR2). Two genomic regions associated with resistance in field-evaluated plants were different from those identified in cotyledon- and stem-evaluated plants. The use of different environments and plant developmental stages for mapping disease resistance loci is discussed

Genetic analysis of resistance to scab in spring wheat cultivar Frontana

Abstract: The Brazilian spring wheat (Triticum aestivum) cv. Frontana is resistant to scab (caused by Fusarium graminearum) in Mexico and various other countries. The number of genes involved in resistance to this disease was estimated. This was done by evaluating random inbred F 6 lines and their parents for scab resistance. The lines were derived from crosses of Frontana with susceptible, or moderately susceptible, cvs. Inia 66, Opata 85, and Pavon 76. These evaluations took place in the field with a mixture of Mexican isolates of F. graminearum. Spikes were inoculated by placing a tiny tuft of cotton soaked with the inoculum in the middle spikelet close to the anthers just prior to anthesis. Glassine bags were placed over the inoculated spikes. These spikes were harvested 45 days after inoculation. Scab severity was determined by recording the number of infected and healthy spikelets. Analyses of variance showed significant differences in scab severities of the F 6 lines. The distribution of F 6 lines was continuous in each cross. The narrow-sense heritability estimates for the crosses evaluated during 1991 and 1993 were 0.66 and 0.93, respectively. Both quantitative and qualitative models were applied to estimate the number of segregating genes. The resistance of Frontana is controlled by the additive interaction of a minimum of three minor genes. Transgressive segregants were identified in each cross, indicating that the susceptible (or moderately susceptible) parents also carry one (or two) minor genes. The combinations of these genes with the genes in Frontana have given F 6 lines with significantly better scab resistance than that of Frontana

Random amplified polymorphic DNA markers tightly linked to a gene for resistance to white pine blister rust in sugar pine.

Abstract: We have genetically mapped a gene for resistance to white pine blister rust (Cronartium ribicola Fisch.) in sugar pine (Pinus lambertiana Dougl.) by using an approach which relies on three factors: (i) the ability to assay for genetic markers in the haploid stage of the host's life cycle, using megagametophyte seed tissue; (ii) a simple and clearly defined pathosystem; and (iii) the use of random amplified polymorphic DNA (RAPD) markers that can be quickly and efficiently evaluated. Resistance to white pine blister rust in sugar pine is known to be controlled by a single dominant gene (R). Maternal segregation of R and dominant RAPD markers were scored simultaneously following collection of megagametophytes for DNA assays and seedling inoculation with C. ribicola. Bulked samples of haploid megagametophyte DNA from resistant and susceptible offspring of segregating full-sib and half-sib families were used to evaluate 800 random decanucleotide primers. Ten loci linked with the gene for resistance to white pine blister rust were identified and segregation data were obtained from five families. Six of the linked markers were within 5 centimorgans of the gene, and one marker was 0.9 centimorgan from R. These and other markers derived by this approach may provide starting points for map-based cloning of this important gene.

Disomic Thinopyrum intermedium addition lines in wheat with barley yellow dwarf virus resistance and with rust resistances.

Abstract: Zhong 5 is a partial amphiploid (2n = 56) between Triticum aestivum (2n = 42) and Thinopyrum intermedium (2n = 42) carrying all the chromosomes of wheat and seven pairs of chromosomes from Th. intermedium. Following further backcrossing to wheat, six independent stable 2n = 44 lines were obtained representing 4 disomic chromosome addition lines. One chromosome confers barley yellow dwarf virus (BYDV) resistance, whereas two other chromosomes carry leaf and stem rust resistance; one of the latter also confers stripe rust resistance. Using RFLP and isozyme markers we have shown that the extra chromosome in the Zhong 5-derived BYDV resistant disomic addition lines (Z1, Z2, or Z6) belongs to the homoeologous group 2. It therefore carries a different locus to the BYDV resistant group 7 addition, L1, described previously. The leaf, stem, and stripe rust resistant line (Z4) carries an added group 7 chromosome. The line Z3 has neither BYDV nor rust resistance, is not a group 2 or group 7 addition, and is probably a group 1 addition. The line Z5 is leaf and stem rust resistant, is not stripe rust resistant, and its homoeology remains unknown.

Plasmodiophora brassicae : aspects of pathogenesis and resistance in Brassica oleracea

Abstract: Clubroot is one of the most damaging diseases inBrassica oleracea crops world-wide. The pathogenicity ofPlasmodiophora brassicae is highly variable between as well as within field populations. Several sources of resistance to clubroot have been identified inB. oleracea. Generally, resistance tends to inherit partly as a recessive, partly as an additive trait, and appears to be controlled by few major genes. Progress in the understanding of the inheritance of resistance is being made through the use of single-spore isolates of the pathogen, and the use of molecular markers for resistance genes.