Dissecting the genetic basis of heterosis in elite super-hybrid rice.

Y900 is one of the top hybrid rice (Oryza sativa) varieties, with its yield exceeding 15 t·hm-2. To dissect the mechanism of heterosis, we sequenced the male parent line R900 and female parent line Y58S using long-read and Hi-C technology. High-quality reference genomes of 396.41 Mb and 398.24 Mb were obtained for R900 and Y58S, respectively. Genome-wide variations between the parents were systematically identified, including 1,367,758 single-nucleotide polymorphisms, 299,149 insertions/deletions, and 4,757 structural variations. The level of variation between Y58S and R900 was the lowest among the comparisons of Y58S with other rice genomes. More than 75% of genes exhibited variation between the two parents. Compared with other two-line hybrids sharing the same female parent, the portion of Geng/japonica (GJ)-type genetic components from different male parents increased with yield increasing in their corresponding hybrids. Transcriptome analysis revealed that the partial dominance effect was the main genetic effect that constituted the heterosis of Y900. In the hybrid, both alleles from the two parents were expressed, and their expression patterns were dynamically regulated in different tissues. The cis-regulation was dominant for young panicle tissues, while trans-regulation was more common in leaf tissues. Overdominance was surprisingly prevalent in stems and more likely regulated by the trans-regulation mechanism. Additionally, R900 contained many excellent GJ haplotypes, such as NARROW LEAF1, Oryza sativa SQUAMOSA PROMOTER BINDING PROTEIN-LIKE13, and Grain number, plant height, and heading date8, making it a good complement to Y58S. The fine-tuned mechanism of heterosis involves genome-wide variation, GJ introgression, key functional genes, and dynamic gene/allele expression and regulation pattern changes in different tissues and growth stages.

ABCG Transporters in the Adaptation of Rice to Salt Stresses.

The ATP-binding cassette (ABC) proteins are a diverse family of transmembrane transporter proteins widely identified in various organisms. The ABCG transporters belong to the G subfamily of the ABC transporter family. Rarely research on ABCG transporters involved in salt tolerance of rice was found. In this study, the evolutionary relationships, conserved motifs, intra- and inter-species homologous genes, and cis-acting elements of ABCG subfamily members were analyzed, and the expression changes of these genes under salt stress at 0 h, 3 h, and 24 h were detected. Based on these results, the candidate gene OsABCG7, which is induced by salt stress, was selected for further studies. Yeast experiments confirmed that the OsABCG7 gene might be involved in the regulation of salt tolerance. The abcg7 mutant showed a higher degree of leaf wilting and a lower survival rate, exhibiting a salt-sensitive phenotype. Systematic analysis of this family in rice helps design effective functional analysis strategies and provides data support for understanding the role of ABCG transporters under salt stress.

Novel Salinity-Tolerant Third-Generation Hybrid Rice Developed via CRISPR/Cas9-Mediated Gene Editing.

Climate change has caused high salinity in many fields, particularly in the mud flats in coastal regions. The resulting salinity has become one of the most significant abiotic stresses affecting the world's rice crop productivity. Developing elite cultivars with novel salinity-tolerance traits is regarded as the most cost-effective and environmentally friendly approach for utilizing saline-alkali land. To develop a highly efficient green strategy and create novel rice germplasms for salt-tolerant rice breeding, this study aimed to improve rice salinity tolerance by combining targeted CRISPR/Cas9-mediated editing of the OsRR22 gene with heterosis utilization. The novel alleles of the genic male-sterility (GMS) and elite restorer line (733Srr22-T1447-1 and HZrr22-T1349-3) produced 110 and 1 bp deletions at the third exon of OsRR22 and conferred a high level of salinity tolerance. Homozygous transgene-free progeny were identified via segregation in the T2 generation, with osrr22 showing similar agronomic performance to wild-type (733S and HZ). Furthermore, these two osrr22 lines were used to develop a new promising third-generation hybrid rice line with novel salinity tolerance. Overall, the results demonstrate that combining CRISPR/Cas9 targeted gene editing with the "third-generation hybrid rice system" approach allows for the efficient development of novel hybrid rice varieties that exhibit a high level of salinity tolerance, thereby ensuring improved cultivar stability and enhanced rice productivity.

Genetic analysis for rice grain quality traits in the YVB stable variant line using RAD-seq.

The future of rice breeding will likely be built on the basis of the further utilization of heterosis between elite cultivars and genetic resources from distant subspecies of rice. Previous studies have proved that exogenous genomic DNA transformation methods can be used to transfer genetic information from distant relatives (donor) into cultivated rice (recipient). However, the mechanism underlying this form of genetic transfer is poorly characterized, and the genes that cause the phenotypic changes in these variants are typically difficult to identify. This study examined YVB, a stable variant line with greatly improved grain quality traits that was derived from an indica variety (V20B) by transferring genomic DNA of O.minuta through the "spike-stalk injection method (SIM)". We used restriction-site associated DNA sequencing technology (RAD-seq) to evaluate a population of BC1F5 backcross lines (YVB × V20B); the RAD-seq data were used to construct a genetic linkage map with high-density SNPs for use in association analysis exploring genotype-phenotype relationships at the whole-genome level. A total of 17 quantitative trait loci (QTLs) for rice quality traits were mapped to chromosomes 3, 5, 6, 8, and 9. 8 major QTLs controlling different phenotypic variations were mapped to the same region of chromosome 5. This region contained the GS5 gene for grain weight and the qSW5/GW5 gene for grain width. This study provides new resources and insights into the molecular mechanisms of grain trait phenotypic variation and the transmission of genetic information via the introduction of genomic DNA to a distantly related crop relative species.

Deep re-sequencing of a widely used maintainer line of hybrid rice for discovery of DNA polymorphisms and evaluation of genetic diversity.

Genetic diversity within parental lines of hybrid rice is the foundation of heterosis utilization and yield improvement. Previous studies have suggested that genetic diversity was narrow in cytoplasmic male sterile (CMS/A line) and restorer lines (R line) for Three-line hybrid rice. However, the genetic diversity within maintainer lines (B line), especially at a genome-wide scale, remains largely unknown. In the present study, we performed deep re-sequencing of the elite maintainer line V20B (Oryza sativa L. ssp. indica). We then compared the V20B sequence with the 93-11 (Oryza sativa L. ssp. indica) genome sequence. 112.1 × 106 paired-end reads (PE reads) were generated with approximately 30-fold sequencing depth. The V20B PE reads uniquely covered 87.6 % of the 93-11 genome sequence. Overall, a total of 660,778 single-nucleotide polymorphism (SNPs) and 266,301 insertions and deletions (InDels) were identified, yielding an average of 2.1 SNPs/kb and 0.8 InDels/kb. Genome-wide distribution of the SNPs and InDels was non-random, and variation-rich and variation-poor regions were identified in all chromosomes. A total of 20,562 non-synonymous SNPs spanning 8,854 genes were annotated. Our results identified DNA polymorphisms at the genome-wide scale and uncovered the high level of genetic diversity between V20B and 93-11. Our results proved that next-generation sequencing technologies can be powerful tools to study genome-wide DNA polymorphisms, to query genetic diversity, and to enable molecular improvement efforts with Three-line hybrid rice. Further, our results also indicated that 93-11 could be used as core germplasm for the improvement of wild-abortive CMS lines and the maintainer lines.

Improvement of the Rice "Easy-to-Shatter" Trait via CRISPR/Cas9-Mediated Mutagenesis of the qSH1 Gene.

"Easy-to-shatter" trait is a major cause of rice crop yield losses, emphasizing the economic value of developing elite rice cultivars with reduced seed shattering capable of achieving higher yields. In the present study, we describe the development of new indica rice lines that exhibit lower rates of seed shattering following the targeted CRISPR/Cas9-mediated editing of the qSH1 gene. We were able to identify qSH1 mutant T0 transgenic plants, with transgene-free homozygous mutants being obtained via segregation in the T1 generation. We then utilized two T2 transgene-free homozygous lines in order to assess the degree of seed shattering and major agronomic traits of these mutant lines and of wild-type rice plants (HR1128-WT). This approach revealed that qsh1 homozygous mutant lines exhibited significantly reduced seed shattering relative to HR1128-WT without any significant changes in other analyzed agronomic traits. We then used these mutant lines to develop new promising hybrid rice lines with intermediate seed shattering. Overall our results reveal that combining targeted gene editing via CRISPR/Cas9 with heterosis utilization approach can allow for the efficient development of novel promising hybrid rice cultivars that exhibit a intermediate of seed shattering, thereby ensuring better stability and improved rice yields.

The Cds.71 on TMS5 May Act as a Mutation Hotspot to Originate a TGMS Trait in Indica Rice Cultivars.

The gene tms5, which controls thermo-sensitive genic male sterility (TGMS), has been widely used in two-line hybrid rice breeding in China. The tms5 lines have two sources, namely, AnnongS-1 (AnS) and Zhu1S (ZhS) and, interestingly, are commonly subject to an alteration at cds.71. However, whether cds.71 acts as a mutation hotspot is unknown. Herein, another tms5 mutant named T98S (induced from T98B by irradiation) was used to explore this. First, the gene of tms(t) responsible for T98S was fine-mapped on chromosome 2 based on an F2 group of T98S/R893. In T98S, the candidate gene TMS5 (LOC_Os02g12290.1) mutated at cds.71 with a transversion from cytosine (C) to adenine (A), as also observed in AnS and ZhS. Moreover, the entire coding sequence of TMS5 from T98B converted T98S from sterile to fertile by Agrobacterium tumefaciens-mediated transformation, confirming that T98S is controlled by tms5. Next, detection on nearly 40,000 single nucleotide polymorphisms (SNPs) on Rice 56K SNP Array revealed T98S was 99.99% similar to T98B but only 72.84% and 77.47% similar to AnS and ZhS, respectively, demonstrating that T98S originated from T98B rather than from existing tms5 lines. Furthermore, the cds.70 was found to exist as a T/G haplotype, and it was T rather than G that helped to induce a TGMS trait. The T frequency was 67.52% in indica rice but decreased to 1.75% in japonica rice in 2,644 cultivars tested, which partly explains why tms5 mutants were mostly found in indica lines. Our findings provide evidence that cds.71 may act as a mutation hotspot and clues for breeding TGMS lines in a more efficient way.

Cyclic Digestion and Ligation-Mediated PCR Used for Flanking Sequence Walking.

Ligation-mediated PCR (LM-PCR) is a classical method for isolating flanking sequences; however, it has a common limitation of reduced success rate owing to the circularization or multimerization of target restriction fragments including the known sequence. To address this limitation, we developed a novel LM-PCR method, termed Cyclic Digestion and Ligation-Mediated PCR (CDL-PCR). The novelty of this approach involves the design of new adapters that cannot be digested after being ligated with the restriction fragment, and cyclic digestion and ligation may be manipulated to block the circularization or multimerization of the target restriction fragments. Moreover, to improve the generality and flexibility of CDL-PCR, an adapter precursor sequence was designed, which could be digested to prepare 12 different adapters at low cost. Using this method, the flanking sequences of T-DNA insertions were obtained from transgenic rice and Arabidopsis thaliana. The experimental results demonstrated that CDL-PCR is an efficient and flexible method for identifying the flanking sequences in transgenic rice and Arabidopsis thaliana.

Resequencing of 1,143 indica rice accessions reveals important genetic variations and different heterosis patterns.

Obtaining genetic variation information from indica rice hybrid parents and identification of loci associated with heterosis are important for hybrid rice breeding. Here, we resequence 1,143 indica accessions mostly selected from the parents of superior hybrid rice cultivars of China, identify genetic variations, and perform kinship analysis. We find different hybrid rice crossing patterns between 3- and 2-line superior hybrid lines. By calculating frequencies of parental variation differences (FPVDs), a more direct approach for studying rice heterosis, we identify loci that are linked to heterosis, which include 98 in superior 3-line hybrids and 36 in superior 2-line hybrids. As a proof of concept, we find two accessions harboring a deletion in OsNramp5, a previously reported gene functioning in cadmium absorption, which can be used to mitigate rice grain cadmium levels through hybrid breeding. Resource of indica rice genetic variation reported in this study will be valuable to geneticists and breeders.

Conversion of a rice CMS maintainer into a photo- or thermo-sensitive genetic male sterile line.

A maintainer line of 3-line hybrid rice commonly presents a certain genetic distance to a 2-line restorer line, but in many cases, 2-line restorer lines present defects upon recovery of the object cytoplasmic male sterile (CMS) line of the maintainer line, which impedes the utilization of their heterosis. Here, we report a strategy and an example of converting a maintainer into a photoperiod/temperature-sensitive genic male sterile (P/TGMS) line with an almost identical genetic background, thus maximizing the heterosis. Firstly, through treatment of maintainer line T98B with 60CO-γ irradiation, we identified the TGMS line T98S, which is sterile at higher temperatures and fertile at lower temperatures. Secondly, the T98S line was proven to be identical to T98B with regard to genetic background via an examination of 48 parental polymorphous SSR markers and exhibited excellent blossom traits similar to those of T98B, with an extensive forenoon flowering rate of 75.92% and a high exertion rate of 64.59%. Thirdly, in a combination test, three out of six hybrids from T98S crossed with 2-line restorer lines showed a yield increase of 6.70-15.69% for 2 consecutive years. These results demonstrated that the strategy can generate a new P/TGMS line with strong general combining ability (converted from a maintainer line), thus helping to increase the genetic diversity of male sterile heterotic groups.

In Vitro Seamless Stack Enzymatic Assembly of DNA Molecules Based on a Strategy Involving Splicing of Restriction Sites.

The standard binary enzymatic assembly, which operates by inserting one DNA fragment into a plasmid, has a higher assembly success rate than the polynary enzymatic assembly, which inserts two or more fragments into the plasmid. However, it often leaves a nucleotide scar at the junction site. When a large DNA molecule is assembled stepwise into a backbone plasmid in a random piecewise manner, the scars will damage the structure of the original DNA sequence in the final assembled plasmids. Here, we propose an in vitro Seamless Stack Enzymatic Assembly (SSEA) method, a novel binary enzymatic assembly method involving a seamless strategy of splicing restriction sites via a stepwise process of multiple enzymatic reactions that does not leave nucleotide scars at the junction sites. We have demonstrated the success and versatility of this method through the assembly of 1) a 4.98 kb DNA molecule in the 5' → 3' direction using BamHI to generate the sticky end of the assembly entrance, 2) a 7.09 kb DNA molecule in the 3' → 5' direction using SmaI to generate the blunt end of the assembly entrance, and 3) an 11.88 kb DNA molecule by changing the assembly entrance.

QTL analysis and dissection of panicle components in rice using advanced backcross populations derived from Oryza Sativa cultivars HR1128 and 'Nipponbare'.

Panicle traits are among the most important agronomic characters which directly relate to yield in rice. Grain number (GN), panicle length (PL), primary branch number (PBN), and secondary branch number (SBN) are the major components of rice panicle structure, and are all controlled by quantitative trait loci (QTLs). In our research, four advanced backcross overlapping populations (BIL152, BIL196a, BIL196b, and BIL196b-156) carrying introgressed segments from chromosome 6 were derived from an indica/japonica cross that used the super-hybrid rice restorer line HR1128 and the international sequenced japonica cultivar 'Nipponbare' as the donor and recurrent parents, respectively. The four panicle traits, GN, PL, PBN, and SBN, were evaluated for QTL effects using the inclusive composite interval mapping (ICIM) method in populations over two years at two sites. Results showed that a total of twelve QTLs for GN, PL, PBN, and SBN were detected on chromosome 6. Based on marker loci physical positions, the QTLs were found to be tightly linked to three important chromosomal intervals described as RM7213 to RM19962, RM20000 to RM20210, and RM412 to RM20595. Three QTLs identified in this study, PL6-5, PBN6-1, and PBN6-2, were found to be novel compared with previous studies. A major QTL (PL6-5) for panicle length was detected in all four populations at two locations, and its position was narrowed down to a 1.3Mb region on chromosome 6. Near isogenic lines (NILs) carrying PL6-5 will be developed for fine mapping of the QTL, and our results will provide referable information for gene excavation of panicle components in rice.