Abstract
To improve several agronomic traits in crisphead lettuce (Lactuca sativa L.) under high-temperature growth conditions, we investigated the correlation among those traits in multiple cultivars and performed genetic mapping of their causal genes. In a field cultivation test of Empire type (serrated leaf) and Salinas type (wavy leaf) cultivars, Empire type cultivars showed increased tipburn susceptibility and late bolting compared with Salinas type cultivars. We attempted genetic mapping of leaf shape and bolting time by ddRAD-seq using the F2 population derived from a cross between 'VI185' (Empire type) and 'ShinanoGreen' (Salinas type). These analyses suggested that both traits are controlled by a single locus in LG5. Genotyping of 51 commercial lettuce cultivars with a tightly linked marker (LG5_v8_252.743Mbp) at this locus showed an association between its genotype and the serrated leaf phenotype. By further fine mapping and transcriptome analysis, a gene encoding putative CIN-like TCP transcription factor was determined to be a candidate gene at this locus and was designated as LsTCP4. An insertion of retrotransposable element was found in the allele of 'VI185', and its transcript level in the leaves was lower than that in 'ShinanoGreen'. Because shapes of leaf epidermal cells in 'VI185' were similar to those in the TCP family mutant of Arabidopsis thaliana, the leaf shape phenotype was likely caused by reduced expression of LsTCP4. Furthermore, because it is known that the TCP family protein also controls flowering time via interaction with FT in A. thaliana, it was highly possible that LsTCP4 gave pleiotropic effects on both leaf shape and bolting time in lettuce.