Genetic Dissection of Tiller Number qTN4 in Rice

Tiller number (TN) is an important trait that contributes to yield in rice (Oryza sativa L.). A set of 76 chromosome segment substitution lines (CSSLs) derived from the super-hybrid cross between Zhonghui 9308 (ZH9308) and Xieqingzao B (XQZB) was used to map quantitative trait loci (QTL) controlling tiller number (TN). A total of four QTLs were detected in Fuyang, Zhejiang Province (30.15 degrees N, 120 degrees E). Two QTLs were detected in Lingshui, Hainan Province (18.5 degrees N, 110 degrees E) in our previous study. To further map the QTL on chromosome 4, namely qTN4, the line CSSL29 with a lower tiller number was selected to cross with ZH9308 to develop the secondary F-2 population. In the F-2:3 population, the qTN4 was validated and subsequently narrowed down to a 4.08 Mb region. What is more, combined phenotype with genotype, qTN4 was dissected into two QTLs, qTN4.1 and qTN4.2, in the F-4:5 population. The qTN4.1 and qTN4.2 explained 34.31% and 32.05% of the phenotypic variance, with an additive effect of 1.47 and 1.38, respectively. Finally, the qTN4.1 and qTN4.2 were fine-mapped into a 193.55 Kb and 175.12 Kb intervals on chromosome 4, respectively. Based on genotype and phenotype, four near-isogenic lines (NILs) were selected in the mapping populations. Compared with NILCSSL29, tiller number (TN), grain setting rate, grain length (GL), the ratio of grain length to width (LWR) and grain yield per plant of NILZH9308, NIL-qTN4.1(ZH9308) and NIL-qTN4.2(ZH9308) were increased, and the heading date of these three lines were earlier than that of NILCSSL29. Interestingly, among the candidate genes of qTN4.1 and qTN4.2, except for LOC_Os04g23550, none of the other genes has been cloned, indicating the existence of a novel gene-controlling tiller number. These results lay a foundation for the analysis of QTL controlling tiller number in ZH9308 and provide a theoretical basis for the application of ZH9308 in super-hybrid breeding.