Mapping and identification of genetic loci affecting earliness of bolting and flowering in lettuce

Authors: ROSENTAL, LEAH - Ben Gurion University Of Negev; STILL, DAVID - California Polytechnic State University; YOU, YOUNGSOOK - California Polytechnic State University; Hayes, Ryan; Simko, Ivan

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2021
Publication Date: 9/15/2021
Citation: Rosental, L., Still, D.W., You, Y., Hayes, R.J., Simko, I. 2021. Mapping and identification of genetic loci affecting earliness of bolting and flowering in lettuce. Theoretical and Applied Genetics. 134:3319-3337. https://doi.org/10.1007/s00122-021-03898-9.
DOI: https://doi.org/10.1007/s00122-021-03898-9

Interpretive Summary: Lettuce is grown for its leaf rosettes harvested close to the end of the plant’s vegetative stage. Bolting, defined as stem elongation, marks the transition in the plant life cycle from a vegetative to a flowering stage. Premature bolting may prevent the crop from being harvested thus resulting in economic losses. The transition from a vegetative to reproductive stage is a complex process that involves many genetic and environmental factors. In this study the effects of photoperiod and ambient temperature on the regulation of bolting and flowering was studied. Plants were grown in four combinations of short and long days (8 and 16 hours light, respectively) and low and high temperature (20°C and 35°C, respectively). A major locus for bolting and flowering time was found on chromosome 7 and two candidate genes were identified. In short days and high temperature conditions the locus on chromosome 7 had no effect on plant development. Instead, several small-effect loci on chromosomes 2, 3, 6, 8 and 9 were found to regulate the transition to flowering stage. Polymorphic loci were used to design a genetic marker assay which may be used to identify the late bolting allele.

Technical Abstract: Lettuce is grown for its leaf rosettes harvested close to the end of the plant’s vegetative stage. Bolting, defined as stem elongation, marks the transition in the plant life cycle from a vegetative to a flowering stage. Premature bolting may prevent the crop from being harvested thus resulting in economic losses. The transition from a vegetative to reproductive stage is a complex process that involves many genetic and environmental factors. In this study the effects of photoperiod and ambient temperature on the regulation of bolting and flowering was studied by utilizing a lettuce mapping population to identify quantitative trait loci (QTL) and by gene expression analyses of genotypes with contrasting bolting phenotypes. A recombinant inbred line (RIL) population, derived from a cross between PI251246 (an early bolting phenotype) and cv. Salinas (late bolting phenotype), was grown in four combinations of short and long days (8 and 16 hours light, respectively) and low and high temperature (20°C and 35°C, respectively). QTL models revealed both genetic (G) and environmental (E) effects, and GxE interactions. A major QTL for bolting and flowering time was found on chromosome 7 (qFLT7.2, top LOD = 18.8) and two candidate genes were identified by fine mapping, homology and gene expression studies. In short days and high temperature conditions qFLT7.2 had no effect on plant development, while several small-effect loci on chromosomes 2, 3, 6, 8 and 9 were found to regulate the transition to flowering stage. Of these, the QTL on chromosome 2, qBFr2.1, co-located with the lettuce Flowering Locus T (LsFT) gene. The gene coding sequence and promoter region revealed polymorphisms between the two tested genotypes which may explain the difference in gene expression. Polymorphic loci were used to design a genetic marker assay which may be used to identify the late bolting allele originating from PI251246.