Marker-assisted development and evaluation of monogenic lines of rice cv. Kaohsiung 145 carrying blast resistance genes.

Rice blast is a serious threat to global rice production. Large-scale and long-term cultivation of rice varieties with a single blast resistance gene usually leads to breakdown of resistance. To effectively control rice blast in Taiwan, marker-assisted backcrossing was conducted to develop monogenic lines carrying different blast resistance genes in the genetic background of an elite japonica rice cultivar, 'Kaohsiung 145' (KH145). Eleven International Rice Research Institute (IRRI)-bred blast-resistant lines (IRBLs) showing broad-spectrum resistance to local Pyricularia oryzae isolates were used as resistance donors. Sequencing analysis revealed that the recurrent parent, KH145, does not carry known resistance alleles at the target Pi2/9, Pik, Pita, and Ptr loci. For each IRBL x KH145 cross, we screened 21-370 (average of 108) plants per generation from the BC1F1 to BC3F1/BC4F1 generation. A total of 1499 BC3F2/BC4F2 lines carrying homozygous resistance alleles were selected and self-crossed for 4-6 successive generations. The derived lines were also evaluated for background genotype using genotyping by sequencing, for blast resistance under artificial inoculation and natural infection conditions, and for agronomic performance in multiple field trials. In Chiayi and Taitung blast nurseries in 2018-2020, Pi2, Pi9, and Ptr conferred high resistance, Pi20 and Pik-h moderate resistance, and Pi1, Pi7, Pik-p, and Pik susceptibility to leaf blast; only Pi2, Pi9, and Ptr conferred effective resistance against panicle blast. The monogenic lines showed similar agronomic traits, yield, and grain quality as KH145, suggesting the potential of growing a mixture of lines to achieve durable resistance in the field.