Rice Genetics IV - IRRI books - International Rice Research Institute

et al 2000). Sequence analysis of the predicted proteins reveals that resistance genesof diverse origin and pathogen specificity share similar structural motifs, indicativeof the common surveillance strategy adopted by plant species to detect invading pathogens.Based on the similarity in predicted protein structures, the cloned resistancegenes can be grouped into four main classes. The first class includes the tomato genePto, which confers resistance to Pseudomonas syringae pv. tomato containing theavirulence gene avrPto (Martin et al 1993). Pto encodes a serine/threonine kinasethat interacts with the avrPto gene product of the pathogen (Scofield et al 1996, Tanget al 1996). Genes with resistance against diverse pathogens from Arabidopsis (RPS2,RPP5, and Rpm1), potato (Rx), rice (Xa1, Pib, and Pita), tobacco (N), flax (L6 andM), and tomato (Prf, Bs2, Sw5, Mi, and I2) constitute the second class of plant resistancegenes. All of them contain a nucleotide-binding site (NBS) and leucine-richrepeats (LRR) (see review by Ellis et al 2000). The Cf9, Cf2, and Cf4 genes, whichmediate resistance to the fungal pathogen Cladosporium fulvum in tomato (Jones et al1994, Dixon et al 1996, Thomas et al 1997), form a third class of resistance genes.These Cf genes encode putative membrane-anchored proteins with the LRR motif inthe extracellular domain and a short C-terminal tail in the intracellular domain. Thefourth class of disease resistance genes is represented by a single member, Xa21, arice gene conferring resistance to Xanthomonas oryzae pv. oryzae (Song et al 1995).The fifth class is represented by the Xa21 gene family member, Xa21D, which encodesa presumed extracellular LRR lacking a transmembrane and kinase domains(see below).Cloning and characterizing the bacterialblight resistance gene Xa21Bacterial blight, caused by the bacterial pathogen Xanthomonas oryzae pv. oryzae, orXoo, is a severe disease of rice that causes significant yield losses annually. Geneticstudies on the inheritance of genes with resistance to bacterial blight began aboutthree decades ago in Japan and at the International Rice Research Institute (Mew1987). Major genes from different sources of resistant donors have been identified.As of now, 24 bacterial blight resistance genes have been found on different chromosomalloci (Kinoshita 1995, Lin et al 1998). Some of these have been used widely inrice breeding programs and some of them are targets of cloning efforts in severallaboratories. Thus far, only two resistance genes, Xa21 and Xa1, have been isolatedusing a map-based cloning strategy (Song et al 1995, Yoshimura et al 1998). Xa1encoded a 1,802-amino acid protein with high similarity to the deduced polypeptidedomains of other resistance genes (e.g., RPS2, RPM1, N, and L6) and has two motifsof NBS and an LRR domain. In contrast to the constitutive expression seen in allcloned resistance genes, Xa1 expression is induced upon inoculation with Xoo or bywounding.Through wide hybridization, Xa21 was transferred from the wild species Oryzalongistaminata to the susceptible rice cultivar IR24, generating the near-isogenic lineIRBB21 (Ikeda et al 1990, Khush et al 1990). In the test for disease resistance spec-336 Guo-Liang Wang et al