Gene selection, interaction and pyramiding for blast resistance in rice

With the breakdown of several major genes, combination of genes known as gene pyramiding was recommended. Recent reports, however, indicated interaction of the genes, thus increased number of genes is not necessarily equated to better resistance. Four major genes for blast resistance (P1-q1, Pi-q5, Pi-q6 and Pi-b2) were tagged from a population of recombinant inbred lines. Using the molecular markers as tag for each gene, lines with 0, 1, 2, 3, and 4 genes were evaluated at the blast nursery using three parameters: area under disease progress curve (AUDPC), percent diseased leaf area (percent DLA) and Standard Evaluation System for blast (SES rating). Results showed both direct gene effect and gene x gene interaction. Pi-q6 significantly affected AUDPC and SES rating while Pi-q1 contributed significant variation for AUDPC and percent DLA. Pi-q5 affected all parameters but not Pi-b2. In most cases, Pi-q6 interacted with other major genes but not Pi-b2 gene. Regression analyses revealed that the best fitting combinations has only three genes. These were Pi-q1, Pi-q5 and Pi-q6 for AUDPC and percent DLA, and Pi-q5, Pi-q6 and Pi-b2 for SES rating. These results suggest that gene selection and their interaction have to be considered before gene pyramiding