Genetic Architecture Of Maize Leaf Number And Target QTL Cloning

The number of leaves and their spatial distributions on plants are critical factors determining plant architecture in maize.The total leaf number(TLN)of a maize plant consists of two components,the number of leaves above the primary(uppermost)ear(LA)and the number of leaves below the primary ear(LB).Previous studies have shown that an optimal proportion of LA and LB is critical for enhancing photosynthetic efficiency and improving plant population structure.Leaf number is correlated with other traits related to plant architecture such as plant height and ear height.What's more,leaf number is also correlated with flowering time,a trait that is key to local environmental adaptation.However,the genetic control of leaf number in maize has been less extensively studied.Studying the genetic basis and molecular mechanism of maize leaf number are important for breeding varieties with ideal plant architecture and high yield.In this study,a large set of 866 maize-teosinte BC2S3 RILs were used to comprehensively dissect the genetic architecture of leaf number and their relationship to flowering time.Four traits including TLN,LA,LB and DTA were investigated for QTL mapping.Two QTL for leaf number were further fine mapped using NILs.The major results are as follows:(1)A total of 73 QTLs were detected for the traits of leaf number and flowering time.Their genetic architecture appeared to be different:LA and DTA were controlled by a major QTL plus many small-effect QTLs,while LB and TLN were controlled by many small-effect QTLs.The maize alleles at most QTLs for LA tend to increase the number of leaves above the primary ear.However,the maize alleles at most QTLs for LB tend to decrease the number of leaves below the primary ear.The results indicated that LA and LB may be under differentially directional selection during maize domestication and improvement.What's more,only three QTLs were simultaneously detected for LA and LB,suggesting that LA and LB might be under relatively independent genetic control.(2)15 out of 38 nonoverlapping regions in the maize genome exhibited pleiotropic effects for leaf number and DTA,suggesting that leaf number and flowering time are commonly regulated at a modest level.The effect of the cloned flowering time genes dlf1,ZCN8 and ZmCCT were further validated by NIL analysis,with significant effect for LB and TLN but not for LA.(3)qLA1-1,a major-effect QTL for LA,explained 21.5%of the variation.To fine-map qLA1-1,we analyzed 1200 F2 plants generated from a HIF and delimited qLA1-1 to region with about 20-Mb severe recombination suppression.Comparing with the NAM population,we found the severe recombination suppression was from teosinte.(4)qB6-1,a QTL for LB on chromosome 6,explained 4.6%of the variation.Through 800 F2 plants derived from a HIF,we narrowed the causal region to a 40 Kb intergenic region and considered the downstream WRKY transcription factor as candidate gene.Expression analysis showed that the WRKY gene showed significantly differential expression between NILs.WRKY::Mu phenotypic analysis showed that the mutation increased LB and 100 kernel weight and delayed flowering time,while it had no effect on LA.The results demonstrated that the WRKY gene is a positive regulatory factor of LB.