A Composite Analysis of Flowering Time Regulation in Lettuce

Abstract

Plants undergo profound physiological changes when transitioning from vegetative to reproductive growth. These changes affect crop production, as in the case of leafy vegetables. Lettuce is one of the most valuable leafy vegetable crops in the world. Past genetic studies have identified multiple quantitative trait loci (QTLs) that affect the timing of the floral transition in lettuce. Extensive functional molecular studies in the model organism Arabidopsis provide the opportunity to transfer knowledge to lettuce to explore the mechanisms through which genetic variations translate into changes in flowering time. In this review, we integrated results from past genetic and molecular studies for flowering time in lettuce with orthology and functional inference from Arabidopsis. This summarizes the basis for all known genetic variation underlying the phenotypic diversity of flowering time in lettuce and how the genetics of flowering time in lettuce projects onto the established pathways controlling flowering time in plants. This comprehensive overview reveals patterns across experiments as well as areas in need of further study. Our review also represents a resource for developing cultivars with delayed flowering time.

Keywords: circadian clock; crop breeding; flowering time; genetic mapping; genome-wide association study; lettuce; quantitative trait loci (QTL) mapping.

FIGURE 1

(A) Physical location of 35 bolting time (green) and 32 flowering time (yellow) consensus quantitative trait loci (QTLs) in lettuce. (B) Physical location of 167 QTLs reported in a total of 56 field and greenhouse experiments. Each track represents one experiment. Earlier reported experiments are in the outer tracks. Bolting time QTLs are indicated by green color blocks and flowering time QTLs by orange blocks. Saturation of the color blocks indicates percent phenotypic variance explained by the QTLs. The physical locations of 12 QTLs from three experiments (Summer 2003 from Lavelle, 2009 and both experiments from Hartman et al., 2013b) could not be located on the genome because amplified fragment length polymorphism (AFLP) markers were used. (C) Location of lettuce orthologs of genes with flowering time function in Arabidopsis. Flowering time orthologs within known QTLs are highlighted in fuchsia. (D) Gene density of the lettuce genome. (E) Flowering time orthologs within the same orthogroup are connected. Darker shade connections indicate larger orthogroups.

FIGURE 2

A simplified schematic of the core flowering time regulatory network in Arabidopsis. Colors of the nodes represent current information on orthology of these genes in the lettuce genome. Please refer to Srikanth and Schmid (2011); Bouché et al. (2015), and Wils and Kaufmann (2017) for examples of detailed depictions of these complex pathways. (CO: Constans; FLD: Flowering locus D; FRI: Frigida; FT: Flowering locus T; GA3OX1: Gibberellin 3-beta-dioxygenase 1; LFY: Leafy; PHYA: Phytochrome A; PHYB: Phytochrome B; SOC1: Suppressor of CONSTANS overexpression 1; SPL: squamosa promoter binding protein like; SVP: Short vegetative phase.)