Natural variation and genetic analyses of seed and seedling traits in lettuce: Discovery, confirmation and proposed role for a quantitative trait locus in regulating seed dormancy at high temperature

Genetic analyses of natural variation can provide insight into the regulation of important agronomic or physiological traits of plants. A survey of the natural variation present in four species of Lactuca identified species-specific differences in seed weight, oil content, fatty-acid composition, and germination in response to light and temperature. A QTL analysis was performed in a recombinant inbred line (RIL) population derived from L. sativa cv. Salinas x L. serriola UC96US23. QTL were identified for seed oil content, seedling growth and quality, and germination in response to light and temperature. The Htg6.1 QTL described ∼55% of phenotypic variation for germination at high temperature (35°C) in multiple environments. The failure of seeds to germinate when imbibed at warm temperatures (thermodormancy) results in yield losses in major lettuce growing areas, thus making the alleviation of thermodormancy a valuable breeding goal.;Marker-assisted backcrossing/selection was utilized to introgress the Htg6.1 QTL-allele originating from UC96US23 into the Salinas background and develop near-isogenic lines (NILs). The QTL increased the mean upper temperature limit (UTL) of seed germination in NILs by an average of 2.3°C, confirming its role as a significant source of germination thermotolerance in lettuce. Thirty-four candidate genes for germination/dormancy and seed oil biosynthesis/metabolism were mapped. Genes in the ABA, GA and ethylene biosynthetic or response pathways were collocated with QTL for germination in the absence of light and germination at 25°C. A gene critical for ABA biosynthesis, 9-cis-epoxycarotenoid dioxygenase (LsNCED4), collocated with Htg6.1. Expression of LsNCED4 was significantly lower in thermotolerant NILs homozygous for the UC96US23 allele at Htg6.1 compared to expression in thermosensitive NILs homozygous for the Salinas Htg6.1 allele.;In Salinas and UC96US23, gene expression patterns contrasted at germination-permissive and -inhibitory temperatures. Increased ABA content, sensitivity, and ABA-related gene expression in Salinas seeds was associated with the inability to germinate at 33°C and with corresponding repression of genes in the GA and ethylene biosynthetic pathways. These results suggest that when the UC96US23 allele of LsNCED4 at Htg6.1 is present, imbibition at high temperature does not increase ABA synthesis, allowing expression of GA and ethylene biosynthetic genes and promotion of germination.