Quantitative triat loci (QTLs) for yield and related traits in rice were mapped based on RFLP maps from two indica/indica F2 populations, Tesanai 2/CB and Waiyin 2/CB. In Tesanai 2/CB, 14 intervals carrying QTLs for eight traits were detected, including 3 for grain weight per plant (GWT), 2 for number of panicles per plant (NP), 2 for number of grains per panicle (NG), 1 for total number of spikelets per panicle (TNS), 1 for spikelet fertility (SF), 3 for 1000-grain weight (TGWT), 1 for spikelet density (SD), and 1 for number of first branches per main panicle. The 3 QTLs for GWT were located on chromosomes 1, 2, and 4, with 1 in each chromosome. The additive effect of the single locus ranged from 2.0 g to 9.1 g. A major gene (np4) for NP was detected on chromosome 4 within the interval of RG143-RG214, about 4cM for RG143, and this locus explained 26.1% of the observed phenotypic variance for NP. The paternal allele of this locus was responsible for reduced panicles per plant (3 panicles per plant). In another population, Waiyin 2/CB, 12 intervals containing QTLs for six of the above-mentioned traits were detected, including 3 for GWT, 2 for each of NP, TNS, TGWT and SD, 1 for SF. Three QTLs for GWT were located on chromosome 1, 4, and 5, respectively. The additive effect of the single locus for GWT ranged from 6.7 g to 8.8 g, while the dominance effect was 1.7-11.5 g. QTL mapping in two populations with a common male parent is compared and discussed.

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Grain size is the major determinant of grain weight, and a trait having an important role in grain quality. It is controlled by several major quantitative trait loci (QTL) and many minor QTL. Identification of QTL for grain size is important for understanding the genetic and molecular network regulating grain size in rice. Following previous identification of QTL for grain weight and size using an F 2:3 family derived from the indica rice cross Teqing/IRBB52, one QTL, qTGW5 having significant effects on grain length and weight was targeted for validation, dissection and fine-mapping. Firstly, the effect of qTGW5 was validated using two near-isogenic line (NIL)-F 2 populations. Then, qTGW5 was dissected into two closely linked QTL using four NIL populations. One of them, qGL5.1, having significant effects on grain length, width and weight, was located within an 1896.4-kb region. The other one, qGL5.2 controlling grain length, was further delimited into a 68.8-kb region using seven NIL-F 2 populations. Six annotated genes were found in the qGL5.2 region, of which five showed nucleotide polymorphisms between the two parental lines. Additionally, three of the six annotated genes showed significant expression differences between NIL Teqing and NIL IRBB52 in young panicles using qRT-PCR. The results will facilitate cloning the minor QTL and understanding the genetic architecture for grain size in rice.

Wild species of Oryza is an important resource of the genes for resistance to brown planthopper (BPH). In this study, two populations were developed from an interspecific cross between Dongxiang wild rice (Oryza rufipogon Griff.) and cultivated rice Xieqingzao B (Oryza sativa L.) and used to determine quantitative trait loci (QTLs) for BPH resistance. A BC 1 F 5 population derived from Xieqingzao B//Xieqingzao B/Dongxiang wild rice was infested with BPH collected from the paddy field. Two QTLs were detected, of which qBph2 was located in the interval RM29-RG157 on chromosome 2 and qBph7 in the interval RM11-RM234 on chromosome 7, and the Dongxiang wild rice allele decreased seedling mortality by 22.2% and 43.7%, respectively. Validation of the two QTLs was followed by testing a BC 3 F 3 population with BPH biotypes I, II, and III, respectively. QTL analysis showed that qBph2 confers resistance to biotypes I and II, while qBph7 to biotypes I and III. The two QTLs have a great potential in the improvement of BPH resistance of rice varieties.

Grain weight and size, mostly determined by grain length, width and thickness, are crucial traits affecting grain quality and yield in rice. A quantitative trait locus controlling grain length and width in rice, qGS1-35.2, was previously fine-mapped in a 57.7-kb region on the long arm of chromosome 1. In this study, OsPUB3, a gene encoding a U-box E3 ubiquitin ligase, was validated as the causal gene for qGS1-35.2. The effects were confirmed firstly by using CRISPR/Cas9-based mutagenesis and then through transgenic complementation of a Cas9-free knock-out (KO) mutant. Two homozygous KO lines were produced, each having a 1-bp insertion in OsPUB3 which caused frameshift mutation and premature termination. Compared with the recipient and a transgenic-negative control, both mutants showed significant decreases in grain weight and size. In transgenic complementation populations derived from four independent T0 plants, grain weight of transgenic-positive plants was significantly higher than transgenic-negative plants, coming with increased grain length and a less significant decrease in grain width. Based on data documented in RiceVarMap V2.0, eight haplotypes were classified according to six single-nucleotide polymorphisms (SNPs) found in the OsPUB3 coding region of 4695 rice accessions. Significant differences on grain size traits were detected between the three major haplotypes, Hap1, Hap2 and Hap3 that jointly occupy 98.6% of the accessions. Hap3 having the largest grain weight and grain length but intermediate grain width exhibits a potential for simultaneously improving grain yield and quality. In another set of 257 indica rice cultivars tested in our study, Hap1 and Hap2 remained to be the two largest groups. Their differences on grain weight and size were significant in the background of non-functional gse5, but non-significant in the background of functional GSE5, indicating a genetic interaction between OsPUB3 and GSE5. Cloning of OsPUB3 provides a new gene resource for investigating the regulation of grain weight and size.

Background: Most agronomical traits of crops are complex traits controlled by several major quantitative trait locus (QTL) and many minor QTL. Grain size determines grain weight and influences rice appearance quality. Identification of minor QTL is important for understanding the genetic and molecular network regulating grain size in rice. Following previous identification of QTL for grain weight and size using populations derived from the Teqing/IRBB52 indica rice cross, one QTL, qTGW5/qGL5 having significant effects on grain weight and length, was targeted for validation, dissection and fine-mapping.Result: Firstly, the effect of qTGW5/qGL5 was validated using two near isogenic line (NIL) F2 populations. Then, qTGW5/qGL5 was dissected into two closely linked QTL for grain size using four sets of NILs with sequential segregating regions. One of them, qTGW5 with the IRBB52 alleles increased grain weight, length and width with the same allelic direction, was located within an 1896.4-kb region flanked by RM18865 and Fi25273. The other one, qGL5 controlling grain length, was further delimited into a 68.8-kb region using seven NIL-F2 populations. Six annotated genes were found in the qGL5 region, of which five showed nucleotide polymorphisms between the two parental lines. In three of the six annotated genes, significant expression differences were detected between qGL5-NILs.Conclusions: Two closely-linked QTL having small effects for grain size in rice were separated using NIL-derived populations. One of them, qGL5 was fine-mapped into a 68.8-kb region containing six annotated genes. Our work lays a foundation for cloning minor QTL for grain size and offers potential targets for marker-assisted breeding in rice.