Monogenic and epistatic resistance to bean rust infection in common bean
In order to determine the relationship between resistance genes and hydrolytic enzyme induction, the interaction between two Phaseolus vulgaris cultivars (Sierra and Olathe) and two Uromyces appendiculatus races (47 and 55) were analyzed. Resistance of Sierra to race 55 was expressed as hypersensitive flecking, whereas the resistance of Olathe to race 47 was not associated with macroscopic symptoms and was classed as immunity. The Sierra/race 47 and Olathe/race 55 interactions were compatible. An analysis of a F2 population and F3 families demonstrated that a monogenic factor controlled resistance to race 47 while two genes controlled the interaction with race 55. At one locus determining resistance to race 58 resistance is dominant to susceptibility, and at the second locus hypersensitive resistance is dominant to immunity. Both alleles at the second locus are hypostatic to the susceptibility allele at the first locus. The induction of the hydrolytic enzymes β-1,3-glucanase and chitinase was monitored for the four interactions over a ten-day period. For the compatible interactions, the hydrolytic enzyme activities increased slightly over time. This pattern was also noted for the Olathe/race 55 immune-resistance interaction. In contrast, the β-1,3-glucanase and chitinase activity for the Sierra/race 55 hypersensitive-resistance interaction was significantly greater from day 2 through day 10. These results suggest the possibility that induction of hydrolytic enzyme activity may be associated with hypersensitive, but not immune resistance in the bean/bean rust interaction.
Available from: Bret Cooper
- "The large number of resistance (R) genes for bean rust may correlate with the high pathogen population diversity; with 90 different races identified . The locus Ur-3 confers resistance to 44 out of the 89 U. appendiculatus races present in the USA [5,6]. Besides the Ur-3 locus, a number of other R genes were identified in bean; such as locus Ur-4 for race 49, locus Ur-11 epistatic to Ur-4 for race 67 or locus Ur-13 mapped to the linkage group B8 [7,8]. "
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ABSTRACT: Phaseolus vulgaris (common bean) is the second most important legume crop in the world after soybean. Consequently, yield losses due to fungal infection, like Uromyces appendiculatus (bean rust), have strong consequences. Several resistant genes were identified that confer resistance to bean rust infection. However, the downstream genes and mechanisms involved in bean resistance to infection are poorly characterized.
A subtractive bean cDNA library composed of 10,581 unisequences was constructed and enriched in sequences regulated by either bean rust race 41, a virulent strain, or race 49, an avirulent strain on cultivar Early Gallatin carrying the resistance gene Ur-4. The construction of this library allowed the identification of 6,202 new bean ESTs, significantly adding to the available sequences for this plant. Regulation of selected bean genes in response to bean rust infection was confirmed by qRT-PCR. Plant gene expression was similar for both race 41 and 49 during the first 48 hours of the infection process but varied significantly at the later time points (72-96 hours after inoculation) mainly due to the presence of the Avr4 gene in the race 49 leading to a hypersensitive response in the bean plants. A biphasic pattern of gene expression was observed for several genes regulated in response to fungal infection.
The enrichment of the public database with over 6,000 bean ESTs significantly adds to the genomic resources available for this important crop plant. The analysis of these genes in response to bean rust infection provides a foundation for further studies of the mechanism of fungal disease resistance. The expression pattern of 90 bean genes upon rust infection shares several features with other legumes infected by biotrophic fungi. This finding suggests that the P. vulgaris-U. appendiculatus pathosystem could serve as a model to explore legume-rust interaction.
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ABSTRACT: Race-specific resistance to the bean rust pathogen (Uromyces appendiculatus) is provided by a number of loci in common bean (Phaseolus vulgaris). The Ur-3 locus controls hypersensitive resistance (HR) to 44 of the 89 races curated in the United States. To better understand resistance mediated by this locus, we developed new genetic material for analysis. We developed a population of mutagenized seed of cv. Sierra (genotype = Ur-3 ur-4 ur-6) that was screened with a bean rust race that is normally incompatible (HR response) on Ur-3 genotypes. We discovered two mutants of common bean, crg and ur3-delta3, in which uredinia formed on leaves (a compatible interaction) following infection. The F1 generation from a cross of these two mutants expressed the HR response, and the F2 generation segregated in a ratio of 9:7 (HR/uredinia formation). Therefore, the two genes are unlinked. Further genetic analysis determined that the mutation in ur3-delta3 was in the Ur-3 locus, and the mutation in crg was in a newly discovered gene given the symbol Crg (Complements resistance gene). Each mutation was inherited in a recessive manner. Unlike ur3-delta3, crg expressed reduced compatibility to bean rust races 49 and 47 that are normally fully compatible on genotypes, such as Sierra, that are homozygous recessive at the Ur-4 and Ur-6 loci. This suggests a gene mutated in crg is normally a positive compatibility factor for the bean-bean rust interaction. Polymerase chain reaction analysis of crg with primers to common bean resistance gene analogs (RGA) that contain a nucleotide-binding site sequence similar to those found in a number of plant disease resistance genes revealed that crg is missing the SB1 RGA, but not the linked SB3 and SB5 RGAs. Genetic analyses revealed that Crg cosegregates with the SB1 RGA. These results demonstrate that Crg is located near a RGA cluster in the common bean genome.
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ABSTRACT: Chitinase enzymes are induced in many plants in response to pathogen challenge and other stress stimuli. Chitinase induction
in flax leaves in response to inoculation with flax rust, caused by Melampsora lini (Pers.) Lev., was investigated in a line of flax (Forge) that has four resistance genes, viz. L6, M, N and P2. Four avirulent rust strains, each of which interacts with just one of the resistance genes in Forge, as well as a strain
that is virulent on Forge, were used. Thus chitinase levels associated with resistance reactions triggered by the L6, M, N or P2 genes, and by a susceptible reaction, have been compared in the same host genotype. A marked increase in chitinase activity
in inoculated leaves was observed with all four resistance reactions, with the increase occurring earlier with the P2 resistance reaction compared with the L6, M, and N reactions. A moderate increase in chitinase activity was also observed in systemic (new-growth) leaves of flax plants inoculated
with strains interacting with the M, N or P2 genes. Leaves inoculated with a virulent strain of rust also had increased chitinase activity but the increase was much less
than that found in leaves inoculated with the avirulent strains.
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