B Class Floral Homeotic Genes are Involved in the Petal Identity and Flower Meristem Determinations in Chrysanthemum morifolium

Chrysanthemum morifolium, an ornamental crop with diverse forms of inflorescence, is a good model for studying flower development in Asteraceae. However, the genetic background is complex and the mechanisms of regulating flower development are still unclear. Here, we identified two natural mutant lines of chrysanthemum and named them M1 and M2 according to the severity of the phenotype. Both lines showed defects in petal identity, and the petals of the M1 line had a mild phenotype: partially loss of petal identity and conversion of petals into green, leaf-like organs. The M2 line had severe phenotypes: in addition to severe petal defects, secondary inflorescences were produced in the capitulum to replace the normal ray and disc florets, which indicated a transformation of a flower meristem into an inflorescence meristem. Transcriptome sequencing of WT and M2 inflorescences was performed and found altered expression of floral organ development A, B and E class genes, where B and E class genes were significantly down-regulated. qRT-PCR analysis in both M1 and M2 lines revealed that the expression of three chrysanthemum class B genes CmAP3.1, CmAP3.2 and CmPI, was negatively correlated with phenotypic severity. This suggests that class B genes in chrysanthemum not only have conserved functions in determining petal identity but also were involved in the determinacy of the flower meristem. This study provides insights into the functions of class B genes in flower development, and is informative for dissecting the molecular mechanisms of flower development in chrysanthemum.