Lettuce (Lactuca sativa L.) is a winter annual or biennial crop and a popular leafy vegetable. Its growth, development, flowering and subsequent seed germination are regulated not only by its genotype but also by the environment in which it is produced. The interaction of a genotype with its environment determines whether the crop is going to be successful or not under a given condition. Ecologically, climate change influences reproductive fitness traits such as flowering and seed germination. Higher temperatures due to global warming may induce early bolting or may prevent seed germination, both of which are undesirable traits for agriculture. Inhibition of seed germination due to high temperature results in poor crop stands, and early bolting results in unharvested lettuce crops. Also, some lettuce varieties retain darkness-induced seed germination inhibition and dormancy-related traits. Mitigation of light and temperature sensitivity of lettuce seed germination requires physiological treatments (priming) that add to production costs, and if the temperature is too hot during production, crops may bolt early during their development, reducing yields.
Lettuce seeds are positively photoblastic in nature, meaning they require a period of light during imbibition to germinate. Positive photoblastism results in the inhibition of seed germination in the absence of light, known as skotoinhibition. Many lettuce cultivars retain this property and cv. Grand Rapids ( Lactuca sativa L.) is a classic example that was used in the initial demonstration of photo-reversibility of seed germination. While seeds of cultivar Salinas (L. sativa L.) can germinate in the dark, an accession (US96UC23) of its progenitor species (L. serriola) retains the positive photoblastism trait. Seed priming coupled with brief exposure to light during imbibition can overcome the skotoinhibition. Alternatively, the skotoinhibition trait could be eliminated through marker-assisted breeding if the genetic loci controlling the trait were known. However, very little is known about the underlying genetics of the skotoinhibition trait. Therefore, we undertook a genetic mapping project to understand the genetics underlying positive photoblastism with the intent that trait-linked marker information can be used to genetically overcome skotoinhibition and provide information about the molecular mechanism of photoblastism. A recombinant inbred line (RIL) mapping population derived from cv. Salinas and US96UC23 was used to identify the underlying quantitative trait loci (QTL) for lettuce seed light sensitivity. A significant dark germination quantitative trait locus (QTL) (qDKG7.1) was identified on chromosome 7 that explained one-third of the total phenotypic variance associated with seed germination in the dark. Multiple genes known to affect seed germination and dormancy were differentially expressed in skototolerant versus skotoinhibited seeds, suggesting that photoblastism is under genetic control. Fine mapping and molecular studies revealed that light-sensitive regulation of the gene encoding GIBBERELLIN 2-OXIDASE 2 (GA2ox2) was associated with natural variation of the skotoinhibition trait in lettuce seed. We further found that single-nucleotide polymorphisms (SNPs) in the promotor region of LsGA2ox2 were conserved within photoblastic phenotype groups of lettuce genotypes, further supporting LsGA2ox2 as the candidate gene for qDKG7.1.
Inhibition of lettuce seed germination at warm temperatures, known as thermoinhibition, is an undesirable trait for lettuce production, especially in hotter climates such as the Imperial Valley of southern California and Colorado River Valley of southwestern Arizona where lettuce seed is planted in late summer or early fall. Seeds remain inhibited due to high soil temperature and result in poor crop stands. Also, seeds produced in different environments often have different temperature sensitivities for germination, affecting the reliability of stand establishment under diverse conditions. While much has been learned about the genetics of thermoinhibition, very little is known about the underlying genetics of how the environment during seed production influences subsequent seed germination traits. Seeds of a RIL mapping population derived from a thermotolerant lettuce (L. sativa) line (PI251246; PI) and a thermosensitive lettuce cultivar (Salinas; Sal) were produced in five locations to study how maternal environment affects subsequent germination traits. A major high-temperature germination QTL (qHTG9.1) responsible for thermotolerance in this mapping population was consistently detected across environments while minor QTLs were environment-specific. A significant QTL for the genotype by maternal environment interaction (GxE) effect was identified on chromosome nine that co-located qHTG9.1. This QTL for thermotolerance was shown previously to be associated with the temperature-sensitive regulation of ETHYLENE RESPONSE FACTOR 1 (ERF1) . Thus, the maternal environment during seed development apparently acts through this same gene to affect subsequent thermosensitivity of germination. Multiple genes related to dormancy and germination are differentially expressed during seed development and the following imbibition in relation to the maternal environment during seed development. (Abstract shortened by ProQuest.)
|Advisor:||Bradford, Kent J.|
|Commitee:||Famula, Thomas R., Michelmore, Richard W.|
|School:||University of California, Davis|
|Department:||Horticulture and Agronomy|
|School Location:||United States -- California|
|Source:||DAI-B 79/01(E), Dissertation Abstracts International|
|Subjects:||Genetics, Horticulture, Plant sciences|
|Keywords:||GA2ox2, GxE interaction, Maternal environmental effect, PHYC, Phenotypic plasticity, Photoblastism|
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