Source-Sink Relationship in Intersubspecific Hybrid Rice

Three indica restorer lines （Mianhui 725, Shuhui 527, Shuhui 881）, an American rice variety Lemont and a javanica rice variety Xiangdali were crossed with japonica Kitaake, and five F1 hybrids were obtained to study the photosynthetic and agronomic traits. The data on photosynthetic characteristics indicated that the net photosynthetic rate （Pn） of the five F1 hybrids was significantly higher than that of their parents （or one of them） under high photosynthetic flux density （PFD）; while the overall performance of hybrids was better than their respective parents in apparent quantum yield （AQY）, carboxylation efficiency （CE） and CO2 compensation point （CCP）. Moreover, the photosynthetic performance of the five F1 were different due to the variation in heredity and the typical indica-japonica hybrids, Mianhui 725/Kitaake and Shuhui 527/Kitaake, were better than the others on this aspect. The agronomic traits revealed that the five F1 exhibited different heterosis, with Shuhui 881/Kitaake the largest sink followed by Mianhui 725/Kitaake, Shuhui 527/Kitaake, Lemont/Kitaake and Xiangdali/Kitaake. The production potential of indica-japonica hybrids was higher than that of the other two hybrids, which was consistent with the performance of Pn. However, the superior trait of indica-japonica hybrids on sink size has not been fully turned into high yield because of abnormal seed setting. Therefore, attention should be paid to the proper genome coordination and appropriate genetic distance so as to achieve super high yielding.

Breeding lines with neutral genes to improve fertility of intersubspecific hybrid rice

Since the sterility-neutral allele S~n has been incorporatedinto indica or japonica varieties, many intersubspecifichybrids have been released commercially. These hybridsshowed high heterosis, but some of them exhibited unstableseed setting rate under low temperature. When the hybridsflowered at low temperature, the fertility of female gamete

Fixation of hybrid sterility genes and favorable alleles of key yield-related genes with dominance contribute to the high yield of the Yongyou series of intersubspecific hybrid rice

In rice,the Yongyou series of Xian-Geng intersubspecific hybrids have excellent production performance,as shown by their extremely high yield.However,the mechanisms underlying the success of these rice hybrids are unclear.In this study,three F2 populations are generated from three Yongyou hybrids to determine the genetic basis of the extremely high yield of intersubspecific hybrids.Genome constitution analysis reveals that the female and male parental lines belong to the Geng and Xian subspecies,respectively,although introgression of 20%of the Xian ancestry and 14%of the Geng ancestry are observed.Twenty-five percent of the hybrid genomes carries homozygous Xian or Geng fragments,which harbors hybrid sterility genes such as Sd,Sc,f5,and q S12 and favorable alleles of key yield-related genes,including NAL1,Ghd7,and Ghd8.None of the parents carries the S5+killer of the S5 killer-protector system.Compatible allele combinations of hybrid sterility genes ensure the fertility of these intersubspecific hybrids and overcome the bottleneck in applying intersubspecific hybrids.Additive effects of favorable alleles of yield-related genes fixed in both parents enhances midparent values.Many QTLs for yield and its key component spikelets per panicle shows dominance and the net positive dominant effects lead to heterosis.These factors result in an extremely high yield of the hybrids.These findings will aid in the development of new intersubspecific rice hybrids with diverse genetic backgrounds.

Practice and Thought on Developing Hybrid Rice for Super High Yield by Exploiting Inter-subspecific Heterosis

Since the breakthrough of grain yield owing to the development of dwarf rice and three-line system hybrid rice, rice breeding for high yield hardly had showed significant progress in the next successive two decades. It was considered that utilizing heterosis between subspecific varieties (Oryza sativa L.) would be an effective approach to increase yield further. During 1987-1993, an indica-japonica hybrid Yayou 2 yielded as high as 10.5 t/ha; however, it failed to be commercialized because of seed purity problem due to non-uniform emasculation by chemical agent in seed production, and sensitivity of seed setting in Ft plants to environmental conditions. In the past decade, two inter-subspeific hybrids, Liangyoupeijiu (Peiai 64S/9311, javanica/indica) and Liangyou E32 (Peiai 64S/E32, javanica/ laponica); both of them exhibited grain yield higher than 10.5 t/ha, and were widely judged as the pioneers of super hybrid rice. Liangyoupeijiu has been successfully popularized over 4 million hectare in wide climatic areas, while Liangyou E32 made a yield record and offered a model of plant ideotype for super hybrid rice. It was considered that in combination with plant ideotype, active physiological functions, and wide-range adaptability to ecological conditions, exploitation of indica-japonica heterosis would be the key approach for super hybrid rice breeding.

Studies on Seed Setting Stability in Associated with Temperature for a Two-Line Hybrid Rice,Liangyoupeijiu

When being planted in wide areas in southern China (23°23' - 33°23' N, 98°35' - 120°19' E, sea level above 2.7 - 1285 m), Liangyoupeijiu (LYPJ), a two-line hybrid rice combination, showed a seed setting rate of 75.2 - 77.2%, which was lower by 4.3 - 7.5 percent point than that of an indica hybrid rice Shanyou 63 (CK), with similar values of grain yield and coefficients of variation to CK. Sowing during 5-25th of May in Nanjing (32°3' N, 118°48' E), China, LYPJ headed before 4th September, and gave a seed setting rate of 75 - 90%, and grain yield over 1 kg/m2. If the sowing date was delayed to 14 - 15th, June, its heading date would be as late as 17th - 21st September, and seed setting rate would be declined by 10 - 15% in comparison with that on a suitable sowing date. When flowering took place at an average daily temperature range of 13.7 - 28.5℃ for five days, the spikelet fertility (SF) would be increased by 1.9- 10.7%, for each increment of 1℃. The suitable (SF≥90%) and safe (SF≥75%) temperatures for flowering stage were indicated to be 26.5℃ and 22.9℃, respectively. To get a high and stable seed set in cultivating LYPJ, it was recommended that LYPJ would be planted in the areas south of 34癗, and the favorable average daily temperature during flowering stage should be at 26 - 28℃.

Genomic insights on the contribution of introgressions from Xian/Indica to the genetic improvement of Geng/Japonica rice cultivars

Hybridization between Xian/indica(XI)and Geng/japonica(GJ)rice combined with utilization of plant ideotypes has greatly contributed to yield improvements in modern GJ rice in China over the past 50 years.To explore the genomic basis of improved yield and disease resistance in GJ rice,we conducted a large-scale genomic landscape analysis of 816 elite GJ cultivars representing multiple eras of germplasm from China.We detected consistently increasing introgressions from three XI subpopulations into GJ cultivars since the 1980s and found that the XI genome introgressions significantly increased the grain number per panicle(GN)and decreased the panicle number per plant.This contributed to the improvement of plant type during modern breeding,changing multi-tiller plants tomoderate tiller plants with a large panicle size and increasing the blast resistance.Notably,we found that key gene haplotypes controlling plant architecture,yield components,and pest and disease resistance,including IPA1,SMG1,DEP3,Pib,Pi-d2,and Bph3,were introduced from XI rice by introgression.By GWAS analysis,we detected a GN-related gene Gnd5,which had been consistently introgressed from XI into GJ cultivars since the 1980s.Gnd5 is a GRAS transcription factor gene,and Gnd5 knockout mutants showed a significant reduction in GN.The estimated genetic effects of genes varied among different breeding locations,which explained the distinct introgression levels of XI gene haplotypes,including Gnd5,DEP3,etc.,to these GJ breeding pedigrees.These findings reveal the genomic contributions of introgressions from XI to the trait improvements of GJ rice cultivars and provide new insights for future rice genomic breeding.

Identification and classification of subspecies of Asian cultivated rice and their hybrids

Having reviewed the major classification systems proposed by various scholars across the world, it is found that \%indica\% and \%japonica\% under \%O.sativa\% L. are two major directions thoroughly differentiated from the Asian cultivated rice, forming the framework of the classification structure. A system with \%indica\% and \%japonica\% as the only two subspecies is therefore reiterated.There are various ways to determine the indica_japonica identity of hybrid rice, but the "combined morphological trait index"(CMT index) method is more efficient and easier to handle, although the isozyme analysis, molecular marker analysis and grain quality assay methods are also feasible.