|Samac, Deborah - Debby|
Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2007
Publication Date: 1/1/2008
Citation: Ameline-Torregrosal, C., Cazaux, M., Danesh, D., Chardon, F., Cannon, S.B., Esquerre-Tugaye, M., Dumas, B., Young, N.D., Samac, D.A., Huguet, T., Jacquet, C. 2008. Genetic dissection of resistance to anthracnose and powdery mildew in Medicago truncatula. Molecular Plant-Microbe Interactions. 21:61-69. Interpretive Summary: Plant diseases are a major limitation to crop production worldwide. Despite the economic need to find novel disease resistance genes in legume crop plants such as soybean, pea, alfalfa, and red clover, only a few resistance genes have been isolated in this plant family. To overcome the roadblocks in crop legume improvement, a plant related to alfalfa, barrel medic, has been developed as a model plant for legume biology. In previous work, barrel medic lines were selected in order to analyze disease resistance mechanisms against anthracnose and powdery mildew, two major diseases of crop legumes. Resistance was expressed for both diseases as a localized hypersensitive response in which defensive compounds were generated, indicating that a single gene triggered the resistance response. The two parental lines, a susceptible and resistant, selected for each disease were crossed and used to develop gene mapping populations. These populations were used to locate a region on chromosome 4 containing the primary genes for resistance to anthracnose and powdery mildew, and two regions on chromosome 5 also contributing resistance to powdery mildew. The region on chromosome 4 consists of at least 25 potential disease resistance genes. All but one of these was found to be active in at least one stage of plant growth or in response to pathogens. One gene was found to be expressed uniquely in response to the anthracnose pathogen. Identifying major disease resistance genes and DNA markers associated with these genes will enable plant breeders to rapidly identify superior disease resistant plants and transfer the resistance into productive cultivated varieties, reducing reliance on fungicides and increasing plant productivity.
Technical Abstract: Medicago truncatula was used to characterize resistance to anthracnose and powdery mildew, respectively caused by Colletotrichum trifolii and Erysiphe pisi. Two isolates of E. pisi (Ep-p from pea and Ep-a from alfalfa) and two races of C. trifolii (races 1 and 2) were used in this study. The A17 genotype was resistant and displayed a hypersensitive response after inoculation with either pathogen, while lines F83005.5 and DZA315.16 were susceptible to anthracnose and powdery mildew, respectively. To identify the genetic determinants underlying resistance in A17, two F7 recombinant inbred line (RIL) populations, LR4 (A17 x DZA315.16) and LR5 (A17 x F83005.5), were phenotyped with E. pisi isolates and C. trifolii races, respectively. Intermediate phenotypes as well as transgressive segregation were observed, thereby indicating that resistance in RIL populations might be considered as a quantitative trait. The use of a consensus genetic map revealed that: i) resistance to anthracnose is mainly governed by a single major locus to both races, located on the upper part of chromosome 4; ii) resistance to powdery mildew involves three distinct loci (one on chromosome 4 for Ep-p and two on chromosome 5 for Ep-a); and iii) the main genetic determinants responsible for resistance to C. trifolii and Ep-p are located in overlapping loci. In silico analysis in this genome region identified the presence of several clusters of NBS-LRR genes. Many of these genes have atypical resistance gene analog structures and displayed differential expression patterns in distinct stress-related cDNA libraries.