Abstract
Nature often creates complex structures by rearranging pre-existing units. One such example is the flower head (capitulum) in daisies, where a group of flowers (florets) and phyllaries (modified bracts) are arranged to superficially mimic a single flower. The capitulum is a key taxonomical innovation that defines the daisy family (Asteraceae), the largest flowering plant group. However, patterning mechanisms underlying its structure remain elusive. Here, we show that auxin, a plant hormone, provides a developmental patterning cue for the capitulum. During capitulum development, a temporal auxin gradient occurs, regulating the successive and centripetal formation of distinct florets and phyllaries. Disruption of the endogenous auxin gradient led to homeotic conversions of florets and phyllaries in the capitulum. Furthermore, auxin regulates floral meristem identity genes, such as Matricaria inodora RAY2 and M inodora LEAFY, which determine floret and phyllary identity. This study reveals the mechanism of capitulum patterning and highlights how common developmental tools, such as hormone gradients, have independently evolved in plants and animals.
Highlights
Nature often creates complex structures by rearranging pre-existing units
A M. inodora capitulum consists of green phyllaries, white ray florets, and yellow disc florets (Fig. 1, A–C)
The M. inodora capitulum meristem successively generates phyllaries (Fig. 1D, stage 1), ray (Fig. 1E, stage 2), and disc florets (Fig. 1F, stages 3 and 4), which is followed by rapid petal elongation of the ray florets during stages 5 and 6 (Fig. 1, G and H)
Summary
A M. inodora capitulum consists of green phyllaries, white ray florets, and yellow disc florets (Fig. 1, A–C). These three structures are asynchronously formed in the developing capitulum. The M. inodora capitulum meristem successively generates phyllaries (Fig. 1D, stage 1), ray (Fig. 1E, stage 2), and disc florets (Fig. 1F, stages 3 and 4), which is followed by rapid petal elongation of the ray florets during stages 5 and 6 (Fig. 1, G and H). Consists of a pool of fast-dividing undifferentiated cells in the center of the meristem dome with phyllaries and florets forming in the peripheral zone. The capitulum forms the spiral of incipient phyllary primordia (Fig. 1D, Pi, stage-1 capitulum), followed by the spiral of incipient ray (Fig. 1E, Ri, stage 2), and disc (Fig. 1F, Di, stage 3) floret primordia consecutively
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