Positional Cloning and Characterization of RCT1, an Anthracnose Resistance Gene in Medicago

Alfalfa (Medicago sativa L.) is the most important and widely grown forage legume worldwide. In the US, alfalfa ranks fourth in acreage planted and dollar value (behind com, soybeans, and wheat). The losses of alfalfa production due to disease are estimated to exceed $ 1 billion annually. An improved understanding of the mechanisms underlying host resistance will facilitate the development of resistant cultivars and thus positively impact improvement in and sustainability of US agriculture. Unfortunately, cultivated alfalfa has an intractable genetic system due mainly to its tetrasomic inheritance and cross-pollinating nature. Consequently, the inheritance of most agronomically important traits in alfalfa is poorly understood. However, alfalfa is closely related to the model legume Medicago truncatula and both species share many common pathogens. Thus, alfalfa should be the immediate beneficiary of genomic studies in M truncatula. Our long-term goal is to take advantage of M. truncatula as a model system to map and clone the counterparts of agronomically important genes in alfalfa. The objective of this research is to use M truncatula as a surrogate genome to characterize the host resistance against Colletotrichum trifolii, the causal agent of anthracnose on alfalfa. Anthracnose is one of the most destructive diseases of alfalfa in the US, causing up to 25-30% losses in forage yield as well as losses in plant stand and vigor. The resistance genes identified in M. truncatula will provide new tools for the improvement of cultivated alfalfa, either by means of transgenic approaches or by providing molecular landmarks for marker assisted selection in alfalfa. As a prelude to this work, we have recently established a new pathosystem using M. truncatula and C. trifolii. Preliminary data have shown that the resistance of M. truncatula against C. trifolii race 1 is controlled by a single dominant gene termed RCTl (Resistance to Colletotrichum lrifolii race 1) associated with a hypersensitive response. The specific objectives of the proposed research are: (1) Genetic mapping of RCTJ in M. truncatula; (2) Map-based cloning of RCTl; and (3) Transcript profiling of M. truncatula genes involved in RCTl-mediated signaling pathway by using oligomicroarrays.