Abstract
Key messageA population of lettuce that segregated for photoperiod sensitivity was planted under long-day and short-day conditions. Genetic mapping revealed two distinct sets of QTLs controlling daylength-independent and photoperiod-sensitive flowering time.The molecular mechanism of flowering time regulation in lettuce is of interest to both geneticists and breeders because of the extensive impact of this trait on agricultural production. Lettuce is a facultative long-day plant which changes in flowering time in response to photoperiod. Variations exist in both flowering time and the degree of photoperiod sensitivity among accessions of wild (Lactuca serriola) and cultivated (L. sativa) lettuce. An F6 population of 236 recombinant inbred lines (RILs) was previously developed from a cross between a late-flowering, photoperiod-sensitive L. serriola accession and an early-flowering, photoperiod-insensitive L. sativa accession. This population was planted under long-day (LD) and short-day (SD) conditions in a total of four field and screenhouse trials; the developmental phenotype was scored weekly in each trial. Using genotyping-by-sequencing (GBS) data of the RILs, quantitative trait loci (QTL) mapping revealed five flowering time QTLs that together explained more than 20% of the variation in flowering time under LD conditions. Using two independent statistical models to extract the photoperiod sensitivity phenotype from the LD and SD flowering time data, we identified an additional five QTLs that together explained more than 30% of the variation in photoperiod sensitivity in the population. Orthology and sequence analysis of genes within the nine QTLs revealed potential functional equivalents in the lettuce genome to the key regulators of flowering time and photoperiodism, FD and CONSTANS, respectively, in Arabidopsis.
Highlights
Flowering time is a complex phenotype affected by both the genetic makeup of the plant and environmental conditions (Srikanth and Schmid 2011)
Arabidopsis thaliana is a facultative long-day (LD) plant, for which longer photoperiods accelerate flowering in an incremental fashion, while shorter photoperiods delay flowering but do not completely suppress it (Mouradov et al 2002; Fornara et al 2010)
A lettuce recombinant inbred lines (RILs) population that segregated for both daylength-independent flowering time and photoperiod sensitivity was grown under multiple daylength conditions
Summary
Flowering time is a complex phenotype affected by both the genetic makeup of the plant and environmental conditions (Srikanth and Schmid 2011). Environmental factors documented to influence flowering time include vernalization (Johanson et al 2000; Gendall et al 2001; Searle et al 2006), ambient temperature (Balasubramanian et al 2006; Reeves et al 2007), daylength The control of floral initiation by daylength is termed photoperiodism. The current model for the molecular mechanism of the photoperiodic control of flowering time in Arabidopsis centers around a zinc finger transcription factor, CONSTANS (CO). Arabidopsis alters flowering time as a result of differential accumulation of CO in its leaf tissues under different daylengths. A multi-layer regulatory network controls the quantity of CO by regulating the transcription level of the CO gene (Park et al 1999; An et al 2004; Imaizumi et al 2005) and by coordinating ubiquitin-mediated post-translational
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