| Maize is an important feed,industrial raw material and food crop.Chlorophyll content is one of the most important physiological traits as it is closely related to leaf photosynthetic efficiency and crop yield potential.Most cloned genes controlling chlorophyll content are from Arabidopsis and rice.However,genetic basis of chlorophyll content in maize is still unclear.In this study,the genetic basis of the chlorophyll content in maize was elucidated using an association mapping population(n=538)and two recombinant inbred line populations(RIL335 and RIL958).The main results are as follows:1.Phenotypic analysis The chlorophyll content of maize three leaves in the association mapping population(AMP)and two recombinant inbred line populations was investigated,which followed normal distribution,indicating that it is typical quantitative trait.Significant positive correlation(P<0.01)for the chlorophyll content among three ear leaves under different genetic backgrounds was observed;Analysis of variance(ANOVA)for the chlorophyll content in the AMP showed that environmental effects,genotype effects,genotype and environmental interaction effects were significant(P<0.05),which indicated that significant genetic variation in chlorophyll content.Broad-sense heritability(H2)of maize three leaves is greater than 0.6,H2 for above the uppermost ear leaf,uppermost ear leaf and below the uppermost ear leaf is 0.66,0.66 and 0.67,respectively.2.Genome-wide association studies Genome-wide association studies(GWAS)with three models(Q,K,Q+K)for chlorophyll content in maize three ear leaves at five environments(Hebi,Sanya,Changsha,Yuanyang,Yongcheng)were carried out using 558 629 single nucleotide polymorphisms(SNPs),and K model has the greatest success in reducing false positive than other two models.Based on the result of K model,a total of 18 loci involving in 29 significantly SNP-traits associations were detected(P ? 3.99×10-6),and 76 candidate genes were found.Each locus can explain 4.53%-6.71% phenotypic variation.Moreover,two loci were found to be co-localized in two environments or leaves.And the 76 genes,85.5%(65/76)of the candidate genes have express QTLs(e QTLs)and 11.8%(9/76)of them were significantly associated with the corresponding phenotype(P < 0.05),indicating that these nine genes may affect phenotypic variation by regulating their expression.3.Linkage mappingSeventeen QTLs in the two RIL populations were identified.In detail,9 and 8 loci were identified in RIL335 and RIL958,respectively.Among them,3,4 and 2 QTLs were identified in the above the uppermost ear leaf,uppermost ear leaf and below the uppermost ear leaf in RIL335,each QTL could explain 6.56%-17.71% of phenotypic variation,with average of 11.71%.Besides,4,3,and 1 QTLs were identified in the above the uppermost ear leaf,uppermost ear leaf and below the uppermost ear leaf in RIL958,each QTL could explained phenotypic variation ranged from 6.40% to 10.73%,with a mean of 8.52%.4.Co-localized QTL of association and linkage analysis Two QTLs identified in the two RIL populations were also found in GWAS,indicating that the two QTLs can exist stably in different genetic backgrounds.Two genes(GRMZM2G168858 and GRMZM2G371345)within the two co-located loci may be participated in chlorophyll synthesis and degradation pathways.In details,GRMZM2G168858 encodes NADPH-cytochrome reductase 2,which is an important reductase of cytochrome P450 family and participates in the catabolism of chlorophyll in leaves.GRMZM2G371345 encodes flavonol 3-O-glucosyltransferase that as a substrate for UDP-glucose involved in chlorophyll synthesis and degradation pathways.In addition,a gene GRMZM2G091837(Zm SGR1)identified in RIL335 has been cloned and proved to be involved in the catabolism of chlorophyll and its over-expression can delay leaf senescence in maize.This study provided more information in understanding the genetic basis of chlorophyll content in maize.The loci or candidate genes identified would be helpful in ideotype-based maize breeding with high photosynthetic efficiency. |
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