Genetic Basis Of Chilling Tolerance At The Germination And Seedling Stages In Maize

Maize is very sensitive to low temperature at its early stages.Chilling stress at the early stage will affect seed germination,decrease plant height,inhibit the growth and development of root,reduce leaf area,retard photosynthesis,even cause seedlings death,and greatly limit its distribution and yield potential.In this study,QTL mapping for chilling tolerance on the germination and seedling stages was conducted with an F2:3 population including 207 families derived from a cross between a chilling sensitive inbred,“K932”,and a chilling tolerance inbred,“Mei C” in field and growth chambers.The main results are listed as follows.1.Cold tolerance in the F2:3 population was identified by relative germination ratio(RGR),relative seedling ratio(RSR),leaf green degree(LGD),leaf rolling degree(LRD),ratio of root-to-shoot(RRS),water content in the shoot and leaves(WCS),and soluble sugar content(SSC)in field and growth chambers.Cold tolerance of the chilling tolerance inbred,Mei C was better than that of the chilling sensitive inbred,K932 in the two stages,and the difference between the two parents was significant for the trait of SSC.In addition,the F2:3 families demonstrated much greater variation for all the traits,and they were fit or closely fit to normal distribution.The RGR was significantly and positively correlated with the RSR at the germination stage(r = 0.39).Moreover,the correlations between LGD and LRD1,LRD and WCS were significant at the seedling stage,and the correlation coefficients were 0.50 and0.29 respectively.In general,correlation between the traits at the germination stage and that at the seedling stage is not strong,this implied genetic basis of chilling tolerance between the two stages is different.2.An molecular linkage map was constructed with 152 pairs of polymorphic SSR markers.The linkage map covered the 10 maize chromosomes,and the total genetic distance of 10 linkage groups reached 1958.2cM with an average distance of 12.88 cM between adjective markers.A total of 14 QTLs for chilling tolerance were detected at the two stages.The number of QTLs for chilling tolerance at the germination and seedling stage was 5 and 14,respectively.These QTL were distributed onchromosomes 1,2,3,7,8 and 10,and individual QTL could explain 5.75% to 25.29%phenotypic variation.Three QTLs,umc1774-phi265454,umc1495-umc1717 and umc1983-bnlg1022,were identified at the two stages.The locus with larger effects,umc1774-phi265454,was found to be associated with multiple traits,indicating that this QTL was related to chilling tolerance at the two stages.In addition,this was confirmed by adding more SSR markers in this region.However,for most of the loci,the QTLs were only related to chilling tolerance at the germination or seedling stage,respectively,and this result reflected the differences in the genetic basis of chilling tolerance at the two stages.Analysis of the gene actions of QTLs showed that most QTLs(11/14)showed partial dominance or overdominance,suggesting that heterosis predominates in the early chilling tolerance of maize3.The expression of ten candidate genes predicted by the significant loci revealed by a previous GWAS study for chilling tolerance at seedling stage was conducted in the two parents and their hybrid at chilling stress condition.In general,the exprssion of these candidate genes were induced to different degrees by chilling stress.Most of the genes were apparently induced by chilling stress in the hybrid,whereas not induced in the cold-sensitive parent,K932,and apparently induced in the cold-tolerant individuals,Mei C for only several genes.This result further demonstrates the importance of heterosis on chilling tolerance in maize seedlings.In a word,chilling tolerance of maize at early stage is a complex quantitative trait and controlled by polygenes.And the genetic basis of chilling tolerance at different growth stages is different,though heterosis plays an important role in chilling tolerance at both the two stages.The molecular markers tightly associated with chilling tolerance identified in this study could provide useful information for fine-mapping and cloning the main QTLs and facilitate to Marker assisted selection(MAS)in maize chilling tolerance breeding.