Genotype Analysis Of Heading Date In Rice Cultivars And Fine Mapping Of The Inhibitor For Photoperiod Sensitivity

Growth duration or heading date is an important agronomic trait, which is responsible for the regional and seasonal adaptation of rice varieties(Vergara BS, Chang TT 1985; Cai et al 1987, 1988). Growth duration in dee can be divided into two phases: basic vegetative phase and photoperiod sensitive phase. Several major genes for heading date have been identified in rice, including Ef-1 controlling basic vegetative growth(Yamagata et al. 1986; Okumoto et al. 1992; Tanisaka et al. 1992; Ichitani et al. 1997, 1998; Inoue et al. 1998), E₁—E₃ and Se-1 controlling photoperiod sensitivity(Vergara BS, Chang TT. 1985; Ichitani et al. 1997, 1998; Luo et al. 2002). Among these genes, the two major photoperiod sensitive loci E₁ and Se-1 have been proven to be the most common alleles controlling heading date (Yamagata et al. 1986, Ichitani et al. 1997, 1998). In addition, Ef-1 can accelerate floral initiation under both short and long days and partially counteract the effects of photoperiod sensitivity gene under long days(Nishida et al. 2002; Xu et al. 2006). The recessive inhibitor of photoperiod sensitivity i-Se-1 confers early heading even under long days due to complete inhibition of Se-1(Ohshima et al. 1993, 1994; Luo et al. 2002). However, the information of heading date has seldom been applied in rice breeding.In the present study, some genetic stocks and materials, including heading date tester lines such as EG0～EG7, ER, LR and Taichung 65(T65)～T65m, four heading date QTL near-isogenic lines were used for genetic analysis of genotype of heading date in early, middle and late Indica and Japonica cultivars from Middle-lower regions of the Yangtze river, Southwest and South China regions. Stable QTLs for heading date in rice were detected spanning the whole rice genome using six RIL populations, and partly near-isogenic lines were constructed according to mapping results.HD QTL were first mapped in a recombinant inbred line(RIL) and a chromosome segment substitution line(CSSL) population, both derived from the cross Asominori(japonica)×IR24(indica). Their effect on photopedod sensitivity(PS) was then assessed across three contrasting environments. An inhibitor dth-8 in the indica variety IR24for PS accelerating flowering of japonica×indica hybrids was fine mapped by backcross populations of AIS61 and Asominori and physically localized to a DNA fragment of known length. Discussing and exploring distribution laws of genotype for rice heading date and designing some cultivars or combinations harboring moderate growth duration. 1. Genotypes of Heading Date of early Indica rice in ChinaAll early indica rice eultivars in the Yangtze region and south China carried dominant early heading gene Ef-1 and recessive inhibitors hd2. A part of cultivars carried no PS gene, while the remaining carried either E₁ or Se-1^t. These early indica rice cultivars exhibited no or low PS, short basic vegetative phase and short growth duration.2. Genotypes of Heading Date of middle indica rice in Yangtze regionTo make clear the major genes or genotypes for heading date in rice, major rice cultivars planted in different ecologic regions of China were genotyped for heading date using heading date tester lines such as EG0～EG7, ER, LR and Taichung65(T65～T65m). Our results showed that most of the late indica rice cultivars in this region carried dominant Ef-1, together with both E₁ and Se-1or only one of them, exhibiting strong PS and long basic vegetative phase.3. Genotypes of Heading Date of late Indica rice in ChinaMiddle indica rice cultivars in this region had complex genotypes for heading date and could be classified into two types: one bearing either E₁ or Se-1 and Ef-1, exhibited a certain degree of PS and a short basic vegetative phase; the other carried photoperiod insensitivity gene e₁ or Se-1^e and recessive late heading gene ef-1, showing low PS and long basic vegetative phase.4. Stable expression of QTL for rice heading dateStable QTLs controlling heading date in rice was identified using four sets of recombinant inbred lines(RILs) population derived from the cross Nipponbare and Kasalath, Kilyang23 and Akihikari, Koshihikari and Guichao2, Asominori and IR24 in multi-year, qDTH-2, qDTH-3, qDTH-6, qDTH-7 boring photoperiod sensitivity and qDTH-8, qDTH-10 harboring character of early heading were detected repeatedly, these QTLs were validated further by chromosome segment substitution lines derived from Asominori and IR24. Some near-isogenic lines were constructed according to these major QTL.5. Fine mapping of a photoperiod sensitivity inhibitor gene in riceFlowering time is critical for the proper adaptation of rice cultivars to specific growing area and cropping season. We reported the detection of four QTL controlling heading date, using a set of both recombinant inbred and chromosome segment substitution lines, derived from a cross between Asominori (a highly photoperiod sensitive japonica variety) and IR24(an insensitive indica variety). The effect of heterozygosity of the QTL was also investigated, and the genotype with respect to heading date genes was determined via a near-isogenic line analysis, qDTH-8 is stably expressed both in Asominori and in mixed Asominori/IR24 genetic background across multiple environments. The qDTH-8 allele from IR24 accelerates flowering. Backcross progeny testing allowed qDTH-8 to be defined as a single Mendelian factor, designated dth-8. High-resolution genetic and physical maps were constructed to fine map dth-8, exploiting newly developed molecular markers designed from the appropriate region of the rice genome sequence, along with a large segregating population. As a result, the location of dth-8 was narrowed to a 12kbp segment on chromosome 8 delineated by the microsatellite loci RM8-8 and ind8-47. We demonstrated that the presence of the recessive allele at this locus significantly advances the heading date of japonica×indica hybrids, by inhibiting the expression of the photosensitivity gene E₁.6. Exploring for design breeding of heading date in rice.With the sequencing of rice genome and the functional validation of genes controlling important agronomic traits, breeding by design can be achieved through a combination of transferring gene, marker technology and traditional approaches. Based on the precise genetic mapping of genes affecting heading date and our multiple year data on heading date of rice, rice breeding for moderate heading date by design was carried out in this study, aiming to avoid transgressive late heading of indica-japonica hybrids and blindness in selecting combinations of hybrid rice, as well as to breed eultivars with moderate growth duration in different habitats. Results of field experiments fully confirmed the efficiency of breeding for heading date by molecular design. Meanwhile, the problems existing in breeding by design and the feasibility of breeding by molecular design in some crops or some traits were discussed.