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Background  Mutation breeding is an extraordinary tool in plant breeding to increase the genetic variability, where mutations in anthocyanin biosynthesis are targets to generate distinctive phenotypes in ornamental species. In poinsettia, ionizing radiation is routinely applied in breeding programs to obtaining a range of colours, with nearly all pink and white varieties being obtained after γ- or X-ray mutagenesis of red varieties. In the present study we performed a thorough characterization of a potential mutagenesis target gene as the main responsible for the ‘ white paradox ’ in poinsettia.    Results  We identified a  GST  gene in poinsettia ( Bract1 ) as an essential factor for the expression of anthocyanin-based red colouration of bracts, which presents a high phylogenetic similarity to known anthocyanin-related GSTs. Red poinsettia varieties and white mutants generated from these varieties by X-ray were analysed for polymorphisms related to the ‘ white paradox ’ in the species. A 4 bp mutation in a short repeat within the coding region of  Bract1  is most likely responsible for the appearance of white phenotypes upon irradiation treatment. The polymorphism between wild-type and mutant alleles co-segregates with the phenotype in progeny from heterozygous red and white parents. Moreover, overexpression of  Bract1  wild-type allele in Arabidopsis  tt19  mutants restored the anthocyanin phenotype, while the  Bract1  mutated allele showed to be non-functional.    Conclusions  The identified repeat seems to be highly unstable, since mutated plants can be easily detected among fewer than 200 shoots derived from 10 mutated plants. Our data indicate that particular short repeat sequences, similar to microsatellite sequences or so-called dynamic mutations, might be hot spots for genetic variability. Moreover, the identification of the  Bract1  mutation fills a gap on the understanding on the molecular mechanism of colour formation in poinsettia.    Supplementary Information  The online version contains supplementary material available at 10.1186/s12864-021-07527-z.

A highly mutable GST is essential for bract colouration in Euphorbia pulcherrima Willd. Ex Klotsch