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Identification of a Hidden Resistance Gene in Tetraploid Wheat Using Laboratory Strains of Pyricularia oryzae Produced by Backcrossing

All authors: Graduate School of Agricultural Sciences, Kobe University, Kobe 657-8501, Japan; first author: Crop Protection Cluster, University of the Philippines Los Baños, College, Laguna, 4031, Philippines; and sixth author: RIKEN Yokohama Institute, Tsurumi-ku, Yokohama, 230-0045 Japan.

In the process (BC₃F₁ generation) of backcrossing an Avena isolate of Pyricularia oryzae with a Triticum isolate, color mutants with white mycelia were obtained. These white mutants lacked virulence on all (31/31) hexaploid and most (28/32) tetraploid wheat lines tested. In a BC₄F₁ population, white and black cultures segregated in a 1:1 ratio, suggesting that the mutant phenotype is controlled by a single gene. Furthermore, the mycelial color was perfectly linked with avirulence in the BC₄F₁ population; white cultures were all avirulent on common wheat (Triticum aestivum) ‘Norin 4’ (N4) whereas black cultures were all virulent. White cultures in the BC₃F₁ and BC₄F₁ generations were also avirulent on tetraploid wheat (T. dicoccoides) accession ‘KU109’ (Tat4), which was susceptible to all cultures derived from the parental wild isolates through the BC₂F₁ generation. A cross between Tat4 and a susceptible tetraploid (T. paleocolchicum) accession ‘KU196’ (Tat14) produced resistant and susceptible F₂ seedlings in a 3:1 ratio against the white cultures. In the F₃ generation homozygous resistant/segregating/homozygous susceptible lines segregated in a 1:2:1 ratio. These results suggest that the resistance of Tat4 to the white cultures is controlled by a single major gene. This gene, tentatively designated as RmgTd(t), is considered to be a hidden resistance gene because it was not detected with the Br58, F₁, BC₁F₁, or BC₂F₁ cultures. Cytological analysis revealed that the moderate resistance controlled by RmgTd(t) was associated with a hypersensitive reaction of mesophyll cells.