Double flower is associated with defects in a Phantastica-like gene in Catharanthus roseus

Floral morphological traits, such as double-flowering, are important aspects of the commercial value of ornamental plants. Commercially available double-flower Madagascar periwinkle (Catharanthus roseus), cv. 'Sunny Princess', exhibits a unique morphology in which normal floral organs, including pistils and stamens, are retained, while additional petals develops at the base of the corolla. The aim of this study was to elucidate the molecular basis of double-flowering in C. roseus. Quantitative trait locus (QTL) analysis of F₂ populations derived from crosses between single- and double-flower plants revealed that the PHANTASTICA-like (PHAL) gene is associated with the trait. Two PHAN homologs were identified in C. roseus (CrPHAN and PHAN-like (PHAL)). Double-flower plants consistently harbored a frameshift mutation immediately downstream of the PHAL initiation codon, likely resulting in a loss of function. Additionally, a nonsense mutation in PHAL was identified in double-flower cultivars with different genetic backgrounds, as further evidence of the role of this gene. These findings demonstrate that the loss of PHAL function is associated with double flowering in C. roseus. The results provide new insights into the genetic regulation of floral morphology in ornamental cultivars.