Identification of five new blast resistance genes in the highly blast-resistant rice variety IR64 using a QTL mapping strategy.

Rice progenies used for the construction of genetic maps permit exhaustive identification and characterization of resistance genes present in their parental cultivars. We inoculated a rice progeny derived from the cross IR64 x Azucena with different Magnaporthe grisea isolates that showed differential responses on the parental cultivars. By QTL mapping, nine unlinked loci conferring resistance to each isolate were identified and named Pi-24( t) to Pi-32( t). They could correspond to nine specific resistance genes. Five of these resistance loci (RLs) were mapped at chromosomal locations where no resistance gene was previously reported, defining new resistance genes. Using degenerate primers of the NBS (nucleotide binding site) motif found in many resistance genes, two resistance gene analogues (RGAs) IR86 and IR14 were identified and mapped closely to two blast RLs (resistance identified in this study, i.e. Pi-29(t) and Pi-30(t) respectively). These two RLs may correspond to the Pi-11 and Pi-a blast resistance genes previously identified. Moreover, the ir86 and ir14 genes have been identified "in silico" on the indica rice cultivar 93-11, recently sequenced by Chinese researchers. Both genes encodes NBS-LRR-like proteins that are characteristics of plant-disease resistance genes.

Genetic analysis of resistance to blast in the Vietnamese rice cultivar 'Chiembac'.

The resistance to rice blast disease in the Vietnamese traditional rice cultivar 'Chiembac' was studied. The blast resistance spectrum in 'Chiembac' and 15 rice differentials carrying different known resistance genes was identified using 25 Pyricularia grisea isolates derived from 15 AFLP lineages from the North, Center and South of Vietnam. None of the differential lines carrying a single resistance gene could effectively control all tested Vietnamese blast isolates. 'Chiembac' showed a different resistance pattern compared to that of the differential lines. A cross between 'Chiembac' and 'CR203', an improved rice cultivar, was made and the F2 population was used for characterization and mapping of the resistance genes in 'Chiembac'. Genetic analysis showed that the resistance against two representative isolates from two predominant lineages, VT7 and 12, in 'Chiembac' was controlled by the single dominant genes Pi-VT7 and Pi-I2. The resistance gene Pi-VT7 was closely linked to Pi-I2 and was mapped to chromosome 12 using the framework mapping population 'IR64' x 'Azucena' of 124 double haploid progenies. The resistance to the Vietnamese blast isolate VT7 in 'IR64' was also studied. The latter was controlled by one locus with major effect located on chromosome 12 and mapped closely to the AFLP marker NIN080, which was also tightly linked to the resistance gene Pi-VT7 in 'Chiembac'. Thus, the resistance locus Pi-VT7 and the resistance locus in 'IR64' probably belong to a cluster of resistance genes.

The inheritance of host plant resistance and its effect on the relative infection efficiency of Magnaporthe grisea in rice cultivars.

The inheritance of host plant resistance and its effect on the relative infection efficiency for leaf blast was studied in the crosses 'IR36'/'CO39' (partially resistant × highly susceptible) and 'IR36'/'IR64' (both partially resistant). On the natural scale, gene action appeared multiplicative. After log transformation, additive effects described most of the genetic variation in the cross 'IR36'/'CO39', while additive and dominance effects were about equal in magnitude in the cross 'IR36'/'IR64'. Dominance was towards increased resistance. No transgressive segregation occurred in the cross 'IR36'/'CO39'. The number of genes that reduce lesion number was estimated to be zero in 'CO39' and five or more in 'IR36'. The cross 'IR36'/'IR64' showed transgressive segregation in both directions, and 'IR36' and 'IR64' each contain at least one gene that is not present in the other cultivar. The heritabilities (narrow sense) in the F2 were low (range 0.06-0.16), while narrow sense heritabilities based on F3 lines were much higher (range 0.41-0.68). Lesion numbers in F3 lines were reasonably correlated with those in F5 progenies derived from the same F2 plant (r was±0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F3 lines.