Introduction

MN-1008 is a non-nodulating (Nod") mutant of alfalfa (Medicago sativa L.) isolated and described by Peterson and Barnes (1981). The mutant plants showed neither root hair deformation, cortical cell division nor calcium spiking after inoculation with Sinorhizobium meliloti or Nod factor (NF) treatment (Dudley, Long 1989; Ehrhardt et al. 1996; Endre et al. 1996). Variation in membrane depolarization activity was demonstrated by Felle et al. (1996) using individual plantlets from the MN-1008 seed population. In addition MN-1008 was resistant not only to Rhizobium infection but to vesicular-arbuscular mycorrhiza colonization as well (Brudbury et al. 1991). Spontaneous nodulation in the absence of Rhizobium (referred to as Nar phenotype, Truchet et al. 1989) is one of the unique features of alfalfa. Interestingly the non-nodulation alfalfa mutant, MN-1008, did not lose this character and was susceptible to spontaneous nodulation (Caetano-Annoles et al. 1993). In accordance with this phenomenon early nodulins (e.g. ENOD2 and ENOD40) could be induced by cytokinin (Hirsch et al. 1997) implying that in this mutant the extrinsic signal perception was abolished but the capacity of nodule initiation and development, as inner genetic program of the plant, was intact. As a consequence, the identification and isolation of the gene conditioning the non-nodulation phenotype in mutant MN1008 would shed light on a key function involved in the initiation of the signal transduction pathway leading to nodule formation. To achieve this goal the map-based cloning strategy was adapted for the tetraploid alfalfa.

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