Analysis of QTL for Grain Size in a Rice Chromosome Segment Substitution Line Z1392 with Long Grains and Fine Mapping of qGL-6

BACKGROUND: Grain size affects not only rice yield but is also an important element in quality of appearance. However, the mechanism for inheritance of grain size is unclear. RESULTS: A rice chromosome segment substitution line Z1392, which harbors three substitution segments and produces grains of increased length, was identified. The three chromosome segments were located on chromosomes 1, 5, and 6, and the average length of the substitution segment was 3.17 Mb. Cytological analysis indicates that the predominant cause of increased grain length in Z1392 could be cell expansion in the glumes. Seven quantitative trait loci (QTLs) for grain size related traits were identified using the secondary F(2) population produced by Nipponbare/Z1392. The inheritance of grain length in Z1392 was mainly controlled by two major QTLs, qGL-5 and qGL-6. qGL-6 was localized on a 1.26 Mb region on chromosome 6, and OsARF19 may be its candidate gene. Based on QTL mapping, three single-segment substitution lines (S1, S2, and S3) and two double-segment substitution lines (D1 and D2) were selected, and the mapping accuracy for qGL-5 and qGL-6 was further verified using three single-segment substitution lines. Analysis of QTL additive and epistatic effects revealed that the additive effect of alleles qGL-5 and qGL-6 from ‘Xihui 18’ was estimated to increase grain length of Z1392 by 0.22 and 0.15 mm, respectively. In addition, a positive epistatic interaction between qGL-5 and qGL-6 was detected, which indicates that the pyramiding of qGL-5 and qGL-6 for grain length produces a novel genotype with longer grains. CONCLUSIONS: Inheritance of grain length in the triple-segment substitution line Z1392 is mainly controlled by two major QTLs, qGL-5 and qGL-6. qGL-6 was found to be located in a 1.26 Mb region on chromosome 6, and OsARF19 may be its candidate gene. A positive epistatic interaction between qGL-5 and qGL-6 results in longer grains. The present results can be used to facilitate cloning of the qGL-5 and qGL-6 genes and contribute to improvement of grain yield in rice.