Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: 35pg1
MAP-BASED CLONING OF DISEASE RESISTANCE GENES FROM TOMATO
(LYCOPERSICON ESCULENTUM).
Martin W. Ganal, Gregory Martin, and Steven. D. Tanksley
Department of Plant Breeding and Biometry, Cornell University,
252 Emerson Hall, Ithaca, NY 14853, USA.
Resistance genes against plant pathogens are of immense
value for agriculture and the study of plant-pathogen
interactions. Unfortunately, nothing is known about their gene
products, mode of action, and to date they have not been
accessible by the standard cloning techniques. We have used the
cultivated tomato (Lycopersicon esculentum) as a model system to
establish a cloning system for such genes solely based on their
position on a genetic map (map-based cloning or positional
cloning). We present here the development of relevant
technologies and results from the cloning of two such genes. The
Mi gene confers resistance to many different strains of root knot
nematodes (Meloidogyne,) and the Pto gene is responsible for
resistance against Pseudomonas syrinae pv. tomato. Initially, a
highly saturated RFLP map of tomato with more than 1000 RFLP
markers has been used to map both genes in large populations.
Further markers close to these genes have then been identified
using the RAPD technique together with nearly isogenic lines.
Once the gene was localized to a specific interval or region,
physical mapping by pulsed field gel electrophoresis was used to
determine the physical size and construct a detailed physical map
of the region. Markers showing perfect cosegregation with the
genes were used to isolate DNA clones from a library constructed
in yeast artificial chromosomes (YACs). In both cases, we were
successful in the isolation of large YAC clones encompassing the
entire region containing the genes. The YAC clones were also
used to identify transcripts from the regions out of CDNA
libraries. Currently, both cDNA clones and genomic clones
from the respective regions are being transferred into
susceptible tomato plants via Agrobacterium based Ti vectors in
an effort to transfer and identify the genes responsible for
resistance to these two pathogens.
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