Mapping quantitative trait loci controlling root length in rice seedlings grown with low or sufficient NH⁺₄ supply using backcross recombinant lines derived from a cross between Oryza sativa L. and Oryza glaberrima Steud

To understand genetic adaptation of rice to environmental nitrogen status, this study mapped and verified the quantitative trait locus/loci for root elongation in response to exogenous NH⁺₄ concentration. Significant inhibition of root elongation by increasing NH⁺₄ concentration in hydroponic culture was observed in seedlings of Oryza sativa cv Taichung 65 (japonica variety) but not in O. glaberrima line IRGC 104038. A total of 161 backcross recombinant lines between these species were employed to map the quantitative trait locus/loci controlling root length of seedlings grown hydroponically with a low (5 μM) or sufficient NH⁺₄ (500 μM) supply. Five quantitative trait locus/loci were detected: two on chromosome 1 and three singles on chromosomes 2, 3, and 6. The quantitative trait locus/loci qRL1.1 was significant under sufficient NH⁺₄ supply. Progeny analysis using heterogeneous inbred-type family nearly isogenic lines (HIF-NILs) for the candidate region of qRL1.1 revealed that seminal roots of IRGC 104038-fixed genotypes were significantly longer (12.4 or 11.6%) than those of Taichung 65-fixed genotypes when grown with 500 μM NH⁺₄, but not without or with 5 μM NH⁺₄. qRL1.1 was confirmed and delimited within a 4-Mbp-long region on the long-arm region of chromosome 1. Comparative mapping with genes for NH⁺₄ uptake and nitrogen metabolism showed the apparent location of the aspartate aminotransferase gene (OsAAT2) within the candidate region of qRL1.1. Together, the results suggest that qRL1.1 is an adaptive quantitative trait locus/loci for rice root elongation in response to sufficient supply of external NH⁺₄ and OsAAT2 may be a candidate gene responsible for qRL1.1.