A “defeated” rice resistance gene acts as a QTL against a virulent strain of Xanthomonas oryzae pv. oryzae

Abstract

The genetic components responsible for qualitative and quantitative resistance of rice plants to three strains (CR4, CXO8, and CR6) of Xanthomonas oryzae pv. oryzae (Xoo) were investigated using a set of 315 recombinant inbred lines (RILs) from the cross Lemont (japonica) x Teqing (indica) and a complete linkage map with 182 well distributed RFLP markers. We mapped a major gene (Xa4) and ten quantitative trait loci (QTLs) which were largely responsible for segregation of the resistance phenotype in the RILs. The Teqing allele at the Xa4 locus, Xa4T, acted as a dominant resistance gene against CR4 and CXO8. The breakdown of Xa4T-associated resistance mediated by the mutant allele at the avrXa4 locus in the virulent strain CR6 results from significant changes in both gene action (lose of dominance) and the magnitude of gene effect (approximately 50% reduction). Nevertheless, Xa4T still acted as a recessive QTL with a significant residual effect against CR6. The mutant alleles at the avrXa4 locus in CXO8 and CR6 that lead to a reduction in effect, or "breakdown", of Xa4T were apparently accompanied by corresponding penalties for their fitness. The quantitative component of resistance to Xoo in the RILs was largely due to a number of resistance QTLs. Most resistance QTLs mapped to genomic locations where major resistance genes and/or QTLs for resistance to Xoo, blast and sheath blight were identified in the same cross. Most QTLs showed consistent levels of resistance against all three Xoo strains. Our results suggest that a high level of durable resistance to Xoo may be achieved by the cumulative effects of multiple QTLs, including the residual effects of "defeated" major resistance genes.

Multiple functional polymorphisms in a single disease resistance gene in rice enhance durable resistance to blast

“…A weak allele of an R gene is considered to be a component of plant defense 10,12,33 , yet direct evidence that links NBS-LRRs and QTLmediated resistance is limited. On account of the small gene effects, isolating genes associated with QTL-mediated resistance is timeconsuming and more difficult than isolating genes for race-specific resistance.…”

Genetic mapping of resistance to bacterial blight disease in cassava (Manihot esculenta Crantz)

“…At least 18 major genes have been described, and one, Xa21, a broad-spectrum gene for bacterial-blight resistance in rice, has rectytly been cloned (Kinoshita 1991;McCouch et al 1991;Ronald et al 1992;Song et al 1996;Yoshimura et al 1996). However, ten quantitative trait loci have also been detected against three Xoo strains (Li et al 1999). In the common bean, 18 markers ' located in different genomic regions and explaining 1629% of the phenotypic variance, account for resistance to Xanthomonas campestris pv.…”

Expressional and Biochemical Characterization of Rice Disease Resistance Gene Xa3/Xa26 Family

“…Teqing carries an R gene, Xa4, for Xoo resistance (Li et al 1999). The Xa4 locus is tightly linked to Xa3/ Xa26 ).…”

“…LRR receptor proteins often form homo-or hetero-dimers in signal transduction (Torii 2004). To provide more information on the expressional and biochemical characteristics of the Xa3/Xa26 family, we examined the expression patterns of Xa3/Xa26 family members in the indica rice variety Teqing, which carries the bacterial blight resistance gene Xa4 with a chromosomal location tightly linked to Xa3/Xa26 (Li et al 1999;Sun et al 2003). We also examined the expression patterns of the japonica rice variety Nipponbare, which carries at least one uncharacterized blast resistance gene (S. Wang et al unpubl.…”