Rapid identification of markers linked to a Pseudomonas resistance gene in tomato by using random primers and near-isogenic lines.

An approach to isolating DNA sequences that are linked to important plant genes is described. The strategy is based upon a recent modification of the polymerase chain reaction in which synthetic primers are used to amplify random sequences from genomic DNA. This technique, used in conjunction with near-isogenic lines (which differ only by the presence or absence of the target gene and a small region of surrounding DNA), leads to the rapid identification of sequences linked to the gene of interest. The feasibility of this method has been demonstrated by analyzing a pair of tomato near-isogenic lines that differ for a region on chromosome 5 that contains a gene (Pto) conferring resistance to Pseudomonas syringae pv. tomato. One hundred forty-four random primers were screened on these lines, and seven amplified products were identified that were present in one but not the other line. Of four products that were further investigated, three were confirmed by segregation analysis to be tightly linked to the Pto gene. Linked sequences identified by this method are useful for detecting the presence of the target gene in plant populations (e.g., in plant breeding) and, if very tightly linked, as starting points for a chromosome walk to isolate the gene. Since near-isogenic lines are a typical product of plant breeding and classical genetic studies, this method is applicable to a wide variety of species.