UC Davis

Lettuce (Lactuca sativa) is one of the most popular leafy vegetables in the United States. A popular leafy vegetable harvested in its vegetative stage, cultivated lettuce has been selected for delayed flowering and insensitivity to photoperiod, both intentionally and unintentionally. In the past decades, the advancement in functional genetics in model organisms and the development of marker technologies have allowed fast and accurate mapping of quantitative trait loci (QTL) that control flowering time in lettuce. These past endeavors have been summarized in Chapter Two of this dissertation. In Chapters Three and Four of this dissertation, I mapped QTLs for two distinct and understudied components of flowering time in lettuce: daily floral opening time and photoperiod sensitivity. Chapter Three reported the discovery of two QTLs, qDFO2.1 and 8.1, that controlled daily floral opening time in lettuce. These are the first genetic loci reported to regulate floral opening. The study in Chapter Four revealed five QTLs on Chromosomes 1, 2 and 4 that explained photoperiod sensitivity in lettuce; these QTLs were distinct from loci associated with long-day flowering-time in lettuce. Chapter Four also identified a candidate gene as the functional ortholog of Arabidopsis CONSTANS in lettuce. These results supported the complex, networked architecture of the flowering time regulatory pathways in lettuce. Chapter Five highlighted the importance of microRNAs (miRNAs) in floral development. This chapter provided updated, tissue-specific miRNA and miRNA target annotations for lettuce. The diversity of DNA sequence variations at miRNA regions within the genus Lactuca was also characterized. The differential expression pattern of miRNAs between vegetative and floral tissues supported a model where archetypal miRNAs, miRNA156 and miRNA172, and their regulatory functions were conserved between Arabidopsis and lettuce. Chapter Six of the dissertation proposed promising future experiments to extend the novel methodologies developed in this dissertation and to further elucidate the molecular regulatory network for flowering time in lettuce. Overall, this dissertation found new angles to provide knowledge in the realms of genetics and genomics of flowering time regulation in lettuce. Results from these studies can be used to effectively accelerate vegetable breeding efforts.