Isolation from alfalfa of resistance gene analogues containing nucleotide binding sites.

Cloned resistance (R) genes from a broad range of plant species are known to share similarities in DNA sequence and structural motifs. Degenerate oligonucleotide primers designed from conserved regions of the nucleotide binding site (NBS), common to many R genes, were used to amplify the NBS regions from genomic DNA from alfalfa ( Medicago sativa L). Sequence comparisons of the amplified fragments indicated that at least 18 families of NBS-containing R genes are present in alfalfa. Comparisons to R genes from other species suggested a polyphyletic origin of these gene families. Using the same degenerate primers, PCR analysis of cDNA prepared from a plant not challenged with a pest or pathogen revealed that many of the NBS-containing gene families were transcribed actively. Amplification of NBS regions from other Medicago species showed the presence of some NBS-containing genes not present in alfalfa. These results indicate that the NBS-containing R genes comprise a large gene family in Medicago, at least some of which are transcribed in healthy plants, and that different Medicago species carry unique NBS genes.