Abstract
Little is known about the nature of effective defense mechanisms in legumes to pathogens of remotely related plant species. Some rust species are among pathogens with broad host range causing dramatic losses in various crop plants. To understand and compare the different host and nonhost resistance (NHR) responses of legume species against rusts, we characterized the reaction of the model legume Medicago truncatula to one appropriate (Uromyces striatus) and two inappropriate (U. viciae-fabae and U. lupinicolus) rusts. We found that similar pre and post-haustorial mechanisms of resistance appear to be operative in M. truncatula against appropriate and inappropriate rust fungus. The appropriate U. striatus germinated better on M. truncatula accessions then the inappropriate U. viciae-fabae and U. lupinicolus, but once germinated, germ tubes of the three rusts had a similar level of success in finding stomata and forming an appressoria over a stoma. However, responses to different inappropriate rust species also showed some specificity, suggesting a combination of non-specific and specific responses underlying this legume NHR to rust fungi. Further genetic and expression analysis studies will contribute to the development of the necessary molecular tools to use the present information on host and NHR mechanisms to breed for broad-spectrum resistance to rust in legume species.
Highlights
Rusts are a large group of obligate biotrophic basidiomycete fungi that can cause dramatic losses in various crop plants
Percentage of spore germination on M. truncatula accessions was higher for the appropriate rust U. striatus (74.8%) than for the inappropriate U. viciae-fabae (44.3%) and U. lupinicolus (49.9%)
Significant, genotypic differences were detected among M. truncatula accessions for the percentage of U. striatus and U. viciae-fabae germination, but not for U. lupinicolus (Table 1)
Summary
Rusts are a large group of obligate biotrophic basidiomycete fungi that can cause dramatic losses in various crop plants. Selection for resistance to rust infection was based on highly specific, clearly recognized complete resistance, which is usually controlled by single genes (Ayliffe et al, 2008). This form of resistance provided, in most of the cases, only transient protection due to the evolution of virulent fungal isolates that negated breeders’ efforts and lead to spectacular “boom and bust” cycles. This has raised a major concern on durability of resistance and its implications for resistance breeding. The single-gene resistance most commonly used in rust resistance breeding is typically due to a post-haustorial defense mechanism, in which the plant cell collapses after the rust fungus started to form a haustorium in the cell resulting in hypersensitivity
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