No abstract

The majority of plant disease resistance proteins identified to date belong to a limited number of structural classes, of which those containing nucleotide-binding site (NBS) motifs are the most common. This study provides a detailed analysis of the NBS-encoding genes of the fifth sequenced angiosperm, Carica papaya. Despite having a significantly larger genome than Arabidopsis thaliana, papaya has fewer NBS genes. Nevertheless, papaya maintains genes belonging to both Toll/interleukin-1 receptor (TIR) and non-TIR subclasses. Papaya's NBS gene family shares most similarity with Vitis vinifera homologs, but seven non-TIR members with distinct motif sequence represent a novel subgroup. Transcript splice variants and adjacent genes encoding resistance-associated proteins may provide functional compensation for the apparent scarcity of NBS class resistance genes. Looking forward, the papaya NBS gene family is uniquely small in size but structurally diverse, making it suitable for functional studies aimed at a broader understanding of plant resistance genes.

containing avrXa1 (Yoshimura et al., 1998). Unlike Xa21, Xa1 is a single copy gene and pathogen and Markers were generated that are linked to the rice bacterial blight wound inducible. Xa1 is predicted to code for a cytoresistance gene Xa7. Amplified restriction fragment length polymorphism (AFLP) analysis of a segregating, near-isogenic F 3 population plasmic protein with nucleotide binding sites (NBS) and of IR24 ϫ IRBB7 revealed one polymorphic fragment, M1, which a LRR domain (Yoshimura et al., 1998) and, therefore, was mapped to position 107.3 centimorgans (cM) on the Rice Genome a member of the large NBS-LRR class of resistance Research Program (RGP) map. Sequence comparisons of resistant genes (Hulbert et al., 2001). The two most recent and susceptible lines near M1 were used to develop additional mark-R-genes cloned from rice, Pib and Pi-ta, confer resisers. A sequence tagged site (STS) named M2 was mapped proximal tance to the fungal pathogen Magnaporthe grisea (Heto M1 and farther from Xa7, indicating that Xa7 lies distal to M1. bert) Barr and also contain NBS-LRR motifs (Wang On the distal side of M1, two simple sequence repeats (SSRs), M3 et al., 1999; Bryan et al., 2000). and M4, were mapped 0.5 and 1.8 cM, respectively, from Xa7. The Xa7 is a dominant resistance gene directed against pattern of recombinants was consistent with the order of M1-Xa7-Xoo and was originally identified in rice cultivar DV85, M3-M4, and the map distances indicated that Xa7 is located in the region corresponding to the ends of the physically mapped Clemson International Rice Research Institute accession number University Genomics Institute (CUGI) bacterial artificial chromo-8839 (Sidhu et al., 1978). The gene was transferred to some (BAC) contigs 96 and 143. A complex repeat was identified in cultivar IR24, and the near-isogenic line IRBB7, with the DNA sequence from rice (Oryza sativa L.) cultivars 93-11 and Xa7, was created by recurrent backcrossing (Ogawa et Nipponbare that matched the end of contig 96 and a previously al., 1991). Xa7 is an example of a R-gene that is directed mapped expressed sequence tag (EST) marker (C52865S). Amplificaagainst an avirulence gene family (Hopkins et al., 1992) tion of the repeat and flanking sequences revealed the presence andand is a potential source of broad resistance to bacterial absence of the repeat in IR24 and IRBB7, respectively. No recombiblight of rice (Vera Cruz et al., 2000). The corresponding nants were identified between Xa7 and the polymorphic repeat, which avirulence gene to Xa7, avrXa7, has been cloned and was named M5, in 277 F 3 susceptible progeny of the IR24 ϫ IRBB7 is a member of the avrBs3 gene family (Bonas et al.,