Identification Of QSS-5^N22 And Fine Mapping Of QSS-9^Kasalath For Seed Storability In Rice ?Oryza Sativa L.?

Seed storability in rice is an important trait for both ecological and agronomic aspects due to its correlation with high seed germination and seedling vigor.During warehousing period,seeds begin to deteriorate,lose vigor,and as a result,become more sensitive to stresses during germination,and ultimately die.Strong storability reduces the risk of rapid seed deterioration,which is a serious problem for rice production in tropical Asia.Due to the economic development and increasing labor costs,direct seeding rice cultivation becomes more and more popular in many Asian countries because of the low production costs and easy operation.A high seed vigor and germanition ability after stored for long time are necessary conditions to promoting direct seeding rice cultivation.So the breeding and mechanism research of seed storability graudually become a hot spot in rice.Although many quantitative trait loci(QTL)data reported previously enhanced our understanding of the genetic basis of seed storability,its molecular mechanism remains unknown.Moreover,because of lack of the markers tightly linked to QTLs for seed storability,there is less of value for marker-assisted selection in breeding rice varieties with better seed storability.Screeding major and stable QTLs for seed storability and finding markers tightly linked to these QTLs have great significance for rice breeding and study on seed storability mechanism.The results of this study divided into two aspects were as follows:1.Genetic dissection of seed storability using two different populations with a same parent rice cultivar N22After stored at room temperature(<30?)and 40-60%relative humidity(RH)for 42 months,N22 still displayed>90%germination capability.Two segregating populations with N22(indica)as a common parent,viz.a set of 122 backcross-inbred lines(BILs)derived from the backcross Nanjing35(japonica)/N22//Nanjing35 and another population comprising 189 recombinant inbred lines(RILs)from the cross of USSR5(japonica)and N22,were studied to detect QTL controlling seed storability.Germination percentage(GP)was used to evaluate seed storability after aging treated under three different conditions,viz.natural,artificial and combined aging treatments.A total of eleven QTLs were identified on chromosomes 1,2,3,5,6 and 9,respectively.Among them,qSSn-1 and qSSn-9 were common in the two populations and the former was a new QTL for seed storability,which was divided into two QTLs in RILs,viz.qSSn-1 and qSSn-1-2.In contrast,seven QTLs(qSSnj-2-1,qSSn-2-2,qSSn-2-3,qSSn-3,qSSn-5,qSSn-6-1 and qSSn-6-2)and qSSn-2-4 were detected only in BILs and RIL,respectively.The N22-derived alleles increased the seed storability at all the loci except qSSnj-2-1.We also investigated the pyramiding effect of QTLs using five selected lines from BILs and verified qSSn-5 with a near-isogenic line(NIL).These results provide an opportunity for pyramiding or map-based cloning major QTLs for seed storability in rice.2.Fine mapping of qSSk-9,a major and stable quantitative trait locus,for seed storability in rice(Oryza sativa L.)(1)Under natural aging for 32 months,a set of 98 BILs derived from the backcross Nipponabre(japonica)/Kasalath(indica)//Nipponbare was used to identify QTL for seed storability.As shown as results,a total of three QTLs were detected on chromosome 4,7 and 9,respectively.Among them,a major QTL,qSSk-9,explained the highest phenotypic variation of 45.5%with the LOD value of 14.6,while the phenotypic variation explain of qSSk-4 and qSSk-7 were 7.1%and 8.5%,with LOD values of 2.7 and 3.6,respectively.The Kasalath-derived alleles increased the seed storability at all the loci.The existence of the QTL qSSk-9 was confirmed using Kasalath chromosome segment substitution lines(CSSLs)in a Nipponbare genetic background.(2)This study presents data on fine mapping of the chromosomal location of qSSk-9 based on previous results.Firstly,the effect of Kasalath allele of qSSk-9 was validated using two chromosome segment substitution lines,SL226 and SL36,which harboured the target QTL from Kasalath in the genetic background of Koshihikari and Nipponbare respectively,under different aging treatments in different environments.Subsequently,an F2 population from a crosss between Koshihikari and SL226 and another F2 population derived from Nipponbare/SL36,were used for fine mapping of qSSk-9.Simultaneously,thirty recombinants selected from F2 plants derived from Nipponbare/SL36 were planted in two environments,viz.Hainan and Nanjing,to produce seed for F3 progeny testing.Four sub-near isogenic lines(sub-NILs)that represented different recombination breakpoints across the qSSk-9 region were developed from F3 progeny and used to confirm chromosomal location of qSSk-9.As shown as results,germination rates of SL226 and SL36 harbored the Kasalath allele of qSSk-9 were significantly higher than those of Koshihikari and Nipponbare respectively,after artificial aging or natural aging treatments in different years and environments.The F2 populations showed a continuous distribution in germination rates after artificial aging or natural aging treamnets.Windows QTL Cartographer 2.5 software packages were used for identification of QTL for seed storability in F2 populations.A major QTL,qSSk-9,was found and located between markers Yl and L9-13 on chromosome 9,with the LOD values of 30.9-35.6 and explained the phenotypic variation of44.0-52.9%,respectively.Based on the F3 progeny testing and confirmation by four sub-NILs,finally,the qSSk-9 locus was located between the Indel markers Y10 and Y13,which delimit a region of 147 kb in the Nipponbare genome.These results were provide a springboard for map-based cloning of qSSk-9 and possibilities for breeding rice varieties with strong seed storability.