QTL Mapping Of Important Agronomic Traits In Rice Chromosome Fragment Substitution Lines Z8 And Z519 And Selection Of Secondary Substitution Lines

Plant height and grain shape are important agronomic traits in rice.These traits associate tightly with yield performance of rice varieties,and are controlled by multiple genes,exhibiting typical quantitative features.Chromosomal segment substitution line（CSSL）presents as potential material for the dissection of quantitative traits,which can significantly decrease the enfluences of genetic background to the target traits,simplifying the methological difficulties,improving the dissection presicision and reliability.In the present study,two rice CSSLs of Z8 and Z519 were constructed via the integretation of back-crossing,self-pollination,and maker assisted selection（MAS）.Both of Z8 and Z519 were screened from the descdents with Nipponbare as receptor.Z8’s donor was restorer line R225 with increased grain length and width,while Z519’s donor was restorer line Xihui18 with significantly decreased plant height.Based on these two rice CSSLs of Z8 and Z519,we carried out QTL mapping of important agronomic traits,and breeding of secondary substitution lines.The main results are as follows:1.QTL mapping of important agronomic traits and breeding of secondary substitution lines via Z81.1 Molecular and phenotypic characterization of Z8Z8 possessed increased grain length and width,derived from CSSL descdents with Nipponbare as receptor and restorer line R225 as donor.Molecular characterization results showed that Z8 carried 14 chromosomal segments from R225,which located on the chromosomes of 1-3（Chr1-Chr3）,Chr5-Chr9,and Chr11-Chr11.Among these chromosomes,Chr2,Chr7-Chr9 each carried 2 substitution segments with length ranging from 1.55 Mb to 23.35 Mb,averaging 8.41 Mb.Results from phenotyping showed that 9 traits,i.e.grian length,grain width,ratio of grain length to width,plant height,no.of primary and secondary panicle branches,1000-grain weight,grain density and total number of grains per panicle increased by 8.78 cm,2.93,and 8.13,11.79 mm,1.73 mm,0.23,3.09 g,32.48 and 36.95 in Z8,significantly or extremely significantly higher than those of receptor Nipponbare.While non-significant differences were obserbed for both both the no.of filled grains per panicle and yield performance between Z8 and Nipponbasre.1.2 Glume cytological analysis of Z8Scanning electron microscopy（SEM）results showed that compared with Nipponbare,the length and width of grain hull epidermal cells from Z8 increased by34.14μm and 5.2μm,respectively（P<0.01）,while the number of cells per unit area decreased by 3.4（P<0.01）.These SEM results indicated that the increased grain length and width of Z8 may be caused by the longitudinal and lateral expansion of the glume cells.1.3 QTL mapping of important agronomic traits via Z8Through F₂segregation population constructed by Nipponbare and Z8,a total of 33QTL were detected for 10 important agronomic traits,including grain length（GL）and grain width（GW）.Among 6 detected QTL fro GL,q GL7-1,q GL2-1,and q GL2-2exhibited positive effects to increase grain length by 0.06 mm,0.05 mm,and 0.08 mm,respectively,while the other 3 QTL,i.e.q GL1,q GL9 and q GL11 possessed negative effects to decrease grain length by 0.04 mm,0.04 mm,and 0.06 mm,seperately,with corresponding contribution rates（CRs）of 14.4%,7.83%,4.44%,3.53%,3.48%and8.53%.Besides,another 6 QTL were detected to control GW,among which q GW7-1,q GW8-2,and q GW8-3 presented positive effects to increase grain width by 0.02 mm,0.02 mm,and 0.01 mm,respectively,while q GW2-4,q GW6,and q GW9 showed negative effects ot decrease grain width by 0.03 mm,0.03 mm,and 0.02 mm,respectively,with corresponding contribution rates（CRs）of 4.7%,3.41%,3.43%,8.32%,6.81%and 4.8%.1.4 Breeding of secondary substitution lines via Z8Based on the results of QTL mapping,serials of secondary substitution lines were screened via MAS based on Z8.According to the no.of carried chromosomal segments,these secondary substitution lines possessed 8 different types,including 2double-segment substitution lines（D1-D2）,4 three-segment substitution lines（T1-T4）,3 four-segment substitution lines（FS1-FS3）,5 five-segment substitution lines（FSL1-FSL5）,3 six-segment substitution lines（SS1-SS3）,3 seven-segment substitution lines（SSL1-SSL3）,2 nine-segment substitution lines lines（NS1-NS2）and 1twelve-segment substitution line（TSL1）were selected by MAS method.Breeding of these secondary substitution lines provide potential materials and informative references for further research on the genetic mechanism of the target traits.2.QTL mapping of important agronomic traits and breeding of secondary substitution lines via Z5192.1 Molecular and phenotypic characterization of Z519Z519 was derived from CSSL descdents with Nipponbare as receptor and restorer line Xihui18 as donor,possessing decreased plant height.Molecular characterization results showed that Z519 carried 3 chromosomal segments from the Xihui18,locating on Chr6 and Chr12,respectively.Between these two chromosomes,Chr12 carried 2substitution segments with length of 0.23 Mb and 2.5 Mb,averaging 1.71 Mb.Results from phenotyping showed that 9 traits,i.e.plant height,panicle length,the no.of primary and secondary branches,filled grains per panicle,total grains per panicle,grain width,yield per plant,correspondingly decreased by 28.06 cm,3.44 cm,1.66,14.62,50.92,61.43,0.15 mm,15.66 g and 24.4,stistically lower thant those of Nipponbare（P<0.01）.While no significant differences were obserebved among 4 traits of no.of effective panicles,grain length,seed setting rate,and 1000-grain weight between Z519and Nipponbare.2.2 QTL mapping of important agronomic traits via Z519Through the F₂segregation population constructed by Nipponbare and Z519,a total of 7 QTL were detected for 6 important agronomic traits,including 2 for plant height（PH,q PH6 and q PH12）,and 1 for each of the other 5 traits,i.e.panicle length（q PL6）,no.of primary（q NPB6）and secondary（q SPB6）branches,grains per panicle（q GPP6）,and total no.of grains per panicle（q SPP6）.The contribution rate of the 7QTL ranged from 11.07%to 44.94%.2.3 Breeding of secondary substitution lines via Z519Based on the results of QTL mapping,4 secondary substitution lines were screened via MAS based on Z519,including 2 single-segment substitution lines（S1-S2）,and 2double-segment substitution lines（D1-D2）.The construction of these secondary substitution lines effectively simplified the genetic mechanism of quantitative traits,purify the genetic background,and provide ideal materials for further studies on intergenic additive effects,epistatic effects and fine mapping.