Integration of Bp MADS 4 on various linkage groups improves the utilization of the rapid cycle breeding system in apple

Summary Rapid cycle breeding in apple is a new approach for the rapid introgression of agronomically relevant traits (e.g. disease resistances) from wild apple species into domestic apple cultivars ( Malus  ×  domestica Borkh.). This technique drastically shortens the long‐lasting juvenile phase of apple. The utilization of early‐flowering apple lines overexpressing the Bp MADS 4 gene of the European silver birch ( Betula pendula Roth.) in hybridization resulted in one breeding cycle per year. Aiming for the selection of non‐transgenic null segregants at the end of the breeding process, the flower‐inducing transgene and the gene of interest (e.g. resistance gene) that will be introgressed by hybridization need to be located on different chromosomes. To improve the flexibility of the existing approach in apple, this study was focused on the development and characterization of eleven additional Bp MADS 4 overexpressing lines of four different apple cultivars. In nine lines, the flowering gene was mapped to different linkage groups. The differences in introgressed T‐ DNA sequences and plant genome deletions post‐transformation highlighted the unique molecular character of each line. However, transgenic lines demonstrated no significant differences in flower organ development and pollen functionality compared with non‐transgenic plants. Hybridization studies using pollen from the fire blight‐resistant wild species accession Malus fusca MAL 0045 and the apple scab‐resistant cultivar ‘Regia’ indicated that Bp MADS 4 introgression had no significant effect on the breeding value of each transgenic line.