Genetic Analyses Of Flowering Time Of Lettuce

Lettuce(Lactuca sativa)is the most important green leafy vegetable in Compositae family.With the growing demands for lettuce,the lettuce industry requires year-round production systems to fit the balance between supplies and demands.However,the 'early bolting' phenomenon of lettuce is the main challenge for this goal.Flowering is the most crucial turning point for plants to finish the transition from vegetative to reproductive growth.This trait not only determines the growths and repro-ductions of plants,but also influences the qualities and yields of the agricultural products.Thus,the mechanism underlying flowering time has been investigated extensively.How-ever,genetics and molecular mechanisms on flowering time in lettuce remain poorly un-derstood,though it is one of the critical traits for lettuce.In this study,we identified three loci that regulate the flowering time of lettuce using forward genetics.The main results are as follows:1)A new approach by combining BSR-seq and eQTL analysis,was developed to study a flowering time locus and identify candidate gene.Using this method,we identi-fied LsPHYC as the candidate gene for a flowering time locus.A frameshift mutation caused the loss-of-function of the LsPHYC gene,and led to later flowering in lettuce.2)The second locus controlling flowering time in lettuce was genetically analyzed using map-based cloing method,and the LsPHYB gene was identified as the candidate gene.A nonsense mutation abolished the function of the gene and led to late flowering.We also discovered that LsPHYB genetically interacted with LsPHYC,In general,LsPHYB played a leading role in regulating both flowering time and accumulations of chlorophylls;LsPHYB showed contradict function on flowering under different LsPHYC background.In contrast to the function of delaying flowering of the PHYB and PHYC in Arabidopsis,these two phytochromes in lettuce promote flowering.Expression analyses and qRT-PCR analysis suggested that LsCCT is likely to be involved in this new regula-tion network.3)We finished the primary map-based cloning of the third flowering time gene using a segregating population generated from a wild lettuce and a stem lettuce,and AGL24 was predicted to be the candidate gene.An insertion in the 2092 bp region of AGL24 was detected in the late flowering material,which may downregulate the expression of this gene and finally lead to the late flowering phenotype.The three loci identified from this study will provide a theoretical basis for the culti-vation of bolting-resistant lettuce,which may help the lettuce industry to partially solve the 'early bolting' problem.