Simplified Biotron Breeding System (sBBS): An Efficient Rapid Generation Advancement System for Rice Breeding

In recent years, rapid generation advancement techniques have been used also for developing experimental lines, such as recombinant inbred lines (RILs), backcrossed inbred lines (BILs), chromosome segment substitution lines (CSSLs) and isogenic cultivars, and contributed the progress in rice genomics research. A new method, the biotron breeding system (BBS), offers a way to rapidly produce advanced generations in rice. This approach uses a growth chamber (biotron) to control CO₂ levels, accompanied by removal of tillers and embryo rescue for the reducing seed maturation period. However, tiller removal and embryo rescue for the individuals in each generation are not practical for large populations. To devise a rapid, simple, and effective system for producing advanced generations, we investigated the influences of CO₂ application, tiller removal, and root restriction on the number of days to flowering (NDF) in a growth chamber. Increased CO₂ significantly decreased NDF, but tiller removal and root restriction had little effect on NDF and drastically reduced the seed yield. Based on these findings, we proposed a simplified version of BBS (sBBS) that eliminates the need for tiller removal and embryo rescue. Using sBBS, the interval between generations of ‘Nipponbare’ could be reduced to less than 3 months, without requiring onerous manipulations of the plants. To demonstrate the practical utility of sBBS, we used it to develop isogenic lines using ‘Akidawara’ and ‘Oborozuki’ as the recipient and donor parents, respectively. Using this system, we were able to perform four cycles of crossing per year.