The development of sequence tagged site marker sets for disease resistance gene analogs in maize (Zea mays L.)

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Molecular analyses of plant disease-resistance genes have now shown that the majority of encoded proteins contain both nucleotide-binding site (NBS) and leucine-rich repeat (LRR) regions (Takken and Joosten, 2000). Although there is a high level of genetic variation among these disease-resistance proteins, the NBS region is highly conserved. Additionally, primers that are based on these conserved NBS motifs have been extensively used for the isolation of NBS-related sequences by polymerase chain reaction (PCR) and designated as resistance gene analogs (RGA; Collins et al ., 1998; Leister et al ., 1998; Seah et al ., 1998; Mago et al ., 1999; Collins et al ., 2001). Importantly, the genomic organization of RGA in several plant species indicates that many are linked to known disease resistance loci, and are clustered at those loci (Kanazin et al . 1996; Shen et al . 1998). These characteristics suggest that molecular analyses of RGA would potentially be very useful for both the molecular breeding and isolation of disease-resistance genes in plants. In maize ( Zea mays L), Collins et al . (1998) have cloned several RGA by PCR and linked them to known disease-resistance loci. They have also identified 11 classes of maize RGA, based upon both sequencing analysis and hybridization profiles. Although some disease-resistance loci in the maize genome have been shown to localize with a few RGA, most of these loci have not yet been examined for such colocalization. The sequencing of the Arabidopsis and rice genomes has now revealed that there is diversity among the NBS-related sequences that are found in both monocotyledonous and dicotyledonous plants. The estimated number of NBS-related genes is approximately 200 in the Arabidopsis genome and is predicted to be greater in the rice genome (Meyers et al ., 1999). In comparison, the number of maize RGA previously reported by Collins et al . (1998) is still thought to represent only a small subset of the total number of NBS-related sequences. In the present study, we have isolated additional RGA clones from the maize genome by large-scale sequencing and developed sequenced tag site (STS) marker sets from these RGA to facilitate the molecular breeding of disease-resistance in maize.