Genetic control of broad-spectrum resistance to turnip mosaic virus in Brassica rapa (Chinese cabbage).

The Brassica rapa line RLR22 was resistant to eight diverse turnip mosaic virus (TuMV) isolates. A B. rapa genetic map based on 213 marker loci segregating in 120 first back-cross (B(1)) individuals was established and aligned with the B. rapa genome reference map using some of the RFLP probes. B(1) individuals were self-pollinated to produce B(1)S(1) families. The existence of two loci controlling resistance to TuMV isolate CDN 1 was established from contrasting patterns of segregation for resistance and susceptibility in the B(1)S(1) families. The first gene, recessive TuMV resistance 01 (retr01), had a recessive allele for resistance, was located on the upper portion of chromosome R4 and was epistatic to the second gene. The second gene, Conditional TuMV resistance 01 (ConTR01), possessed a dominant allele for resistance and was located on the upper portion of chromosome R8. These genes also controlled resistance to TuMV isolate CZE 1 and might be sufficient to explain the broad-spectrum resistance of RLR22. The dominant resistance gene, ConTR01, was coincident with one of the three eukaryotic initiation factor 4E (eIF4E) loci of B. rapa and possibly one of the loci of eIF(iso)4E. The recessive resistance gene retr01 was apparently coincident with one of the three loci of eIF(iso)4E in the A genome of Brassica napus and therefore, by inference, in the B. rapa genome. This suggested a mode of action for the resistance that is based on denying the viral RNA access to the translation initiation complex of the plant host. The gene retr01 is the first reported example of a recessive resistance gene mapped in a Brassica species.

The Vh8 locus of a new gene-for-gene interaction between Venturia inaequalis and the wild apple Malus sieversii is closely linked to the Vh2 locus in Malus pumila R12740-7A.

The wild apple (Malus sieversii) is a large-fruited species from Central Asia, which is used as a source of scab resistance in cultivar breeding. Phytopathological tests with races of Venturia inaequalis were performed to differentiate scab-resistance genes in Malus as well as an avirulence gene in the pathogen. A novel gene-for-gene interaction between V. inaequalis and Malus was identified. The locus of the scab-resistance gene Vh8 is linked with, or possibly allelic to, that of the Vh2 gene in Malus pumila Russian apple R12740-7A, at the lower end of linkage group 2 of Malus. Race 8 isolate NZ188B.2 is compatible with Vh8, suggesting the loss or modification of the complementary AvrVh8 gene, while isolate 1639 overcomes both Vh2 and Vh8, but is incompatible with at least one other gene not detected by any of the other race isolates tested. Our research is the first to differentiate scab-resistance genes in a putative gene cluster in apple with the aid of races of V. inaequalis.