Genetic Analysis Of Heterosis And Panicle Length Of Neo-tetraploid Rice Line 25-26

Autotetraploid rice is generated from diploid rice through colchicine.Compared with diploid rice,tetraploid rice has twice the number of chromosomes,and probability of multiple allelic and non-allelic interaction increases,which makes F₁ hybrid having a great biological advantage.However,due to the low fertility of autotetraploid rice,it is difficult to use it directly in production.Through years of research,our laboratory has developed new tetraploid rice germplasm with normal seed set rate,which is obviously different from autotetraploid rice,so it is named as"neo-tetraploid rice".Preliminary research showed that the F₁ hybrid of neo-tetraploid rice and autotetraploid rice has obvious advantages and has the prospect of commercial utilization.In order to evaluate its utilization value,a long panicle type neo-teraploid rice line 25-26 and a short panicle neo-tetraploid rice line H1 was used to prepare a F₁ hybrid combination to conduct the following studies:（1）Systematic evaluation of the superior performance of F₁ hybrid;（2）Analyze the differentially expressed genes involved in heterosis by transcriptome,and initially reveal its molecular genetic basis;（3）Preliminary genetic analysis of its panicle length.The main findings are as follows:（1）Evaluation of hybrid vigor of new tetraploid rice.By investigating the main agronomic traits of the parents and F₁ hybrid,and analyzing the heterosis performance of each trait,the rezsults showed that important traits including total grain number,yield per plant,and seed setting rate have obvious advantages.Through systematic investigation of plant traits such as plant height,flag leaf width,panicle length,total grain number,grain width,average grain projected area,thousand-grain weight,and seed setting rate of parents and their hybrid F₁,the average value of F₁ hybrid traits was higher than that of parents,9 traits were significantly higher than the parents.Mean while,these 9 traits displayed mid-parent advantages,and the highest in the yield per plant（124.82%）,followed by the total number of grains（43.37%）,seed setting rate（33.08%）,plant height（25.53%）,thousand grain weight（17.06%）,panicle length（11.96%）,flag leaf width（4.49%）,flag leaf length（3.10%）,yield per plant（4.49%）,and the lowest for grain width（1.77%）.In addition to the length of the flag leaf,the other eight traits have super-parental advantages,and the highest was the yield per plant（104.34%）,followed by the total number of grains（25.81%）,thousand grain weight（14.65%）,seed setting rate（11.76%）,plant height（11.43%）,panicle length（1.42%）,flag leaf width（1.35%）,and the lowest was for grain width（0.92%）.The above results indicated that the main agronomic traits of the new tetraploid rice hybrids have obvious heterosis,especially the yield-related traits are more obvious and have application value.（2）Molecular genetic analysis of neo-teraploid rice heterosis.Using eight different tissues before and after flowering of parent and F₁ hybrid,we studied the differentially expressed genes of F₁ hybrid by transcriptome sequencing,and found significant differential expressions related to major functional genes and yield-related QTLs,and a complex molecular genetic mechanism was found associated wiht heterosis.RNA-seq studies showed that F₁ hybrid compared with the parents exhibited a total of52236 differential genes in 8 different tissues（pre-and post-flowering flag leaf,leaf sheath,spikelets,secondary branches）.A total of 11,850 differentially expressed genes(DEG_HPU)were expressed.Through GO enrichment analysis,it was found that DEG_HPU is mainly enriched in the processes of transcription and macromolecular metabolism.KEGG enrichment analysis found that DEG_HPU is mainly enriched in plant hormone signal transduction and biosynthesis of secondary metabolites.Compared with known QTLs,109QTLs related to rice yield were found.A total of 685 known cloned genes were detected in DEG_HPU,including short grain gene（SG1）,grain weight gene（Os SPL16）,semi-dwarf gene（sd1）,gibberellin 2 oxidase gene（Os GA2ox6）,short panicle gene（SP1）,multiple tiller dwarf genes（D3）and MADS box gene families（Os MDP1,Os MADS7,Os MADS8,Os MADS1）,etc.The above results indicated that the heterosis of the neo-tetraploid rice is complex,and the differential genes are the main source of heterosis.（3）Genetic analysis of neo-teraploid rice panicle length.Through analysis of the transcriptome at the young panicle stage,1662 differentially expressed genes（DEGs）between hybrid and parents were detected,of which 298 were specifically expressed between hybrids and parents(DEG_HPU).Of these,seven were specifically exprssed in panicle,including OsMADS58,psbS1,CYP734A2,KRP1,OsRDCP1and WOX4 and WOX11.The differential expressions of these genes have an effect on the regulation of meristems and hormones,which may affect the development of panicle length.In summary,it is believed that the neo-teraploid rice hybrids have high heterosis for many agronomic traits,especially the yield-related traits.The molecular genetic mechanism of heterosis is complex and involves differential expressions of a series of genes.Long panicles are controlled by dominant genes and can be used in heterosis.The research results provide useful germplasm resources for tetraploid rice breeding,especially for heterosis applications,and also provide a basis for further analysis of molecular mechanism of heterosis.