Transcriptomic analysis reveals the formation mechanism of anemone-type flower in chrysanthemum.

Background: The ray and disc florets on the chrysanthemum capitulum are morphologically diverse and have remarkably abundant variant types, resulting in a rich variety of flower types. An anemone shape with pigmented and elongated disk florets is an important trait in flower shape breeding of chrysanthemums. The regulatory mechanism of their anemone-type disc floret formation was not clear, thus limiting the directional breeding of chrysanthemum flower types. In this study, we used morphological observation, transcriptomic analysis, and gene expression to investigate the morphogenetic processes and regulatory mechanisms of anemone-type chrysanthemum.

Result: Scanning electron microscopy (SEM) observation showed that morphological differences between non-anemone-type disc florets and anemone-type disc florets occurred mainly during the petal elongation period. The anemone-type disc florets elongated rapidly in the later stages of development. Longitudinal paraffin section analysis revealed that the anemone-type disc florets were formed by a great number of cells in the middle layer of the petals with vigorous division. We investigated the differentially expressed genes (DEGs) using ray and disc florets of two chrysanthemum cultivars, 082 and 068, for RNA-Seq and their expression patterns of non-anemone-type and anemone-type disc florets. The result suggested that the CYCLOIDEA2 (CYC2s), MADS-box genes, and phytohormone signal-related genes appeared significantly different in both types of disc florets and might have important effects on the formation of anemone-type disc florets. In addition, it is noteworthy that the auxin and jasmonate signaling pathways might play a vital role in developing anemone-type disc florets.

Conclusions: Based on our findings, we propose a regulatory network for forming non-anemone-type and anemone-type disc florets. The results of this study lead the way to further clarify the mechanism of the anemone-type chrysanthemum formation and lay the foundation for the directive breeding of chrysanthemum petal types.