Resolving a century-old enigma: potato 'Bolters' originate from instability of the StCDF1.3 allele

Potato bolters are caused by excision of a transposon from the StCDF1.3 allele, resulting in a somatic mutant with late maturity. Somatic mutations during vegetative propagation can lead to novel genotypes, known as sports. In cultivated potato (Solanum tuberosum), a recurring sport type, called 'Bolters', is characterized by vigorous haulms and prolonged flowering. Bolters emerge spontaneously during potato cultivation. While deviating phenotypes are typically rogued during clonal propagation, certain bolters have been selected as sub-clonal strains. Their delayed maturity results in a longer growing season and higher yield, in particular when cultivated under short daylengths. Despite their prevalence and agronomical benefits, the genetic basis of bolters has remained unresolved 160 years after their first description in the literature. We investigated whether allelic variation at the StCDF1 locus, a central regulator of potato life cycle, underlies the bolter phenotype. We describe 34 bolters from eight cultivars. Bolters are isogenic with their parent varieties and carried new StCDF1 alleles. These arose from excision events of the Class II TIR transposon disrupting the StCDF1.3 allele conferring early maturity. Among the newly formed alleles, we predominantly identified StCDF1.2 variants, characterized by a 7-nucleotide insertion and associated with a mild effect on early maturity. We also found novel variants, including StCDF1.7, with a 6-nucleotide in-frame insertion, which appears to confer an even milder shortening of the life cycle. Based on this knowledge, we propose that selecting bolters represents a promising breeding strategy to expand the cultivation range of elite varieties and to enhance allelic diversity at a key regulatory locus.