|Samac, Deborah - Debby|
Submitted to: Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2007
Publication Date: 4/16/2007
Citation: Foster-Hartnett, D., Danesh, D., Penuela, S., Sharopova, N., Endre, G., VandenBosch, K., Young, N.D., Samac, D.A. 2007. Molecular and cytological responses of Medicago truncatula to Erysiphe pisi. Molecular Plant Pathology. 8(3):307-319. Interpretive Summary: The barrel medic, a plant closely related to alfalfa, is resistant to several diseases that cause important economic damage to crop plants. By screening a large collection of diverse types of barrel medic, plants with resistance to the fungus causing powdery mildew of pea and alfalfa was identified. Microscopic studies of the fungus interacting with the resistant plants showed that the plants responded to fungal attack by rapidly producing compounds that inhibit fungal growth and with plant cell death at the location of penetration by the fungus. As a consequence, the pathogen cannot continue to grow or complete its life cycles and the plants remain undamaged. This response, along with other previous results, indicates that there is a single gene responsible for triggering the resistance response, although many genes are required for achieving resistance. Using microarrays of more than 6,000 barrel medic genes, the genes turned on and off in resistant and susceptible plants in response to the pathogens were identified. A set of genes activated in both resistant and susceptible plants were found that constitute a basal defense response. A different set of genes was activated in just the resistant plants, many of which are involved in forming antifungal compounds. In addition, new genes involved in resistance were identified, some of which are unique to legumes, the plants in the pea and bean family. In plants resistant to powdery mildew, many genes are turned off in response to the pathogen. An examination of the DNA surrounding genes activated and repressed in response to the pathogens found previously characterized sets of sequences, as well as novel sequences, that interact with proteins controlling gene activation. This study identified mechanisms used by barrel medic to resist disease, a number of the genes involved in the process, and the DNA sequences controlling activation of a set of these genes. This information can be used to develop improved crop plants with resistance to powdery mildew and reduce the need for fungicides to control disease.
Technical Abstract: Powdery mildew is an economically important disease in a number of crop legumes; however, little is known about resistance to the disease in these species. To gain a better understanding of the genetics of resistance and plant responses to powdery mildew in legumes, we developed a pathosystem with Medicago truncatula and Erysiphe pisi. Screening accessions of M. truncatula identified genotypes that are highly susceptible, moderately resistant, and highly resistant to the fungus. In the highly resistant genotype fungal growth was arrested after appressorium development with no colony formation, while in the moderately resistant genotype a small number of colonies formed. Both resistant and moderately resistant genotypes produced hydrogen peroxide and fluorescent compounds at pathogen penetration sites, consistent with a hypersensitive response (HR), although the response was delayed in the moderately resistant genotype. Very little hydrogen peroxide or fluorescence was detected in the susceptible accession. Microarray analysis of E. pisi-induced early transcriptional changes detected 55 genes associated with the basal defense response that were similarly regulated in all three genotypes. These included pathogenesis-related genes and other genes involved in defense, signal transduction, senescence, cell wall metabolism, and abiotic stress. Genes associated with the HR response included flavonoid pathway genes, and others involved in transport, transcription regulation, and signal transduction. A total of 34 potentially novel unknown genes, including two legume-specific genes, were identified in both the basal response and HR categories. Potential binding sites for two defense-related transcription regulators, Myb and Whirly, were identified in promoter regions of induced genes, and four novel motifs were found in promoter regions of genes repressed in the resistant interaction.