THE ROOT-KNOT NEMATODE RESISTANCE GENE MI FROM TOMATO IS A MEMBER OF THE LEUCINE-ZIPPER, NUCLEOTIDE-BINDING, LEUCINE-RICH REPEAT FAMILY OF PLANT GENES

The Mi locus of tomato confers resistance to root knot nematodes. Toma to DNA spanning the locus was isolated as bacterial artificial chromos ome clones, and 52 kb of contiguous DNA was sequenced. Three open read ing frames were identified with similarity to cloned plant disease res istance genes. Two of them, Mi-1.1 and Mi-1.2, appear to be intact gen es; the third is a pseudogene. A 4-kb mRNA hybridizing with these gene s is present in tomato roots. Complementation studies using cloned cop ies of Mi-1.1 and Mi-1.2 indicated that Mi-1.2, but not Mi-1.1, is suf ficient to confer resistance to a susceptible tomato line with the pro geny of transformants segregating for resistance. The cloned gene most similar to Mi-1.2 is Prf, a tomato gene required for resistance to Ps eudomonas syringae. Prf and Mi-1.2 share several structural motifs, in cluding a nucleotide binding site and a leucine-rich repeat region, th at are characteristic of a family of plant proteins, including several that are required for resistance against viruses, bacteria, fungi, an d now, nematodes.