Molecular and genetic regulation of petal number variation in plants

Floral forms with an increased number of petals, also known as double flower (DF) with great agronomic and economic values, have been selected and conserved in many domesticated plants, particularly in ornamentals. The molecular and genetic mechanisms that control this trait are therefore a hot topic, not only for scientists, but also for breeders. In this review, we summarize the current knowledge on the gene regulatory networks of flower initiation and development and known mutations that lead to variation of petal number in many species. Besides the well-accepted miR172/AP2-like module, for which many questions remain unanswered, we also discuss the current knowledge of other pathways in which mutations also lead to extra-petals formation, such as those involved in meristem development, hormones signaling, epigenetic regulations, and responses to environmental signals. We discuss how the concept of "natural mutants" and the recent advances in genomics and genome editing makes it possible to explore the molecular mechanisms underlying the DF formation, and how such knowledge could contribute to the future breeding and selection of this trait in more crops.