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Different Resistance Mechanisms of Medicago truncatula Ecotypes Against the Rust Fungus Uromyces striatus

A pathosystem consisting of the model plant Medicago truncatula and the rust fungus Uromyces striatus was characterized. From a collection of 113 mostly European accessions of M. truncatula, the vast majority were found to be susceptible to U. striatus, whereas 5 accessions showed strong resistance reactions. Stomatal surface characteristics, even if partly occluded, did not interfere with the ability of U. striatus germ tubes to infect. After penetration, the resistant ecotypes reacted with various degrees of cell death during different stages of haustorial establishment. Whereas four ecotypes showed a typical hypersensitive reaction by developing necrotic lesions, one ecotype (F11.008) exhibited a prehaustorial type of defense without hypersensitive response. This ecotype may be used as a source of nonhost-type of resistance against U. striatus.