Identification of a hidden resistance gene in tetraploid wheat using laboratory strains of Pyricularia oryzae produced by backcrossing.

ABSTRACT

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 11 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 31 ratio against the white cultures. In the F3 generation homozygous resistant/segregating/homozygous susceptible lines segregated in a 121 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.