Phyllotactic patterning of gerbera flower heads

Significance

Flower heads, such as those of gerbera or sunflower, comprise hundreds of bracts and florets. They are organized into conspicuous spiral patterns with intriguing mathematical properties: For example, the numbers of left- and right-winding spirals are typically consecutive Fibonacci numbers. We examined auxin reporter lines of gerbera and combined diverse microscopy and modeling techniques to understand how these patterns develop. The critical process is the early patterning of bracts, which emerge in a specific order and guide the placement of the subsequent bracts and florets. This process, controlled by expansion and contraction of the organogenetic zone during head growth, is different from the extensively studied phyllotactic patterning in the model plants Arabidopsis and tomato.

Abstract

Phyllotaxis, the distribution of organs such as leaves and flowers on their support, is a key attribute of plant architecture. The geometric regularity of phyllotaxis has attracted multidisciplinary interest for centuries, resulting in an understanding of the patterns in the model plants Arabidopsis and tomato down to the molecular level. Nevertheless, the iconic example of phyllotaxis, the arrangement of individual florets into spirals in the heads of the daisy family of plants (Asteraceae), has not been fully explained. We integrate experimental data and computational models to explain phyllotaxis in Gerbera hybrida. We show that phyllotactic patterning in gerbera is governed by changes in the size of the morphogenetically active zone coordinated with the growth of the head. The dynamics of these changes divides the patterning process into three phases: the development of an approximately circular pattern with a Fibonacci number of primordia near the head rim, its gradual transition to a zigzag pattern, and the development of a spiral pattern that fills the head on the template of this zigzag pattern. Fibonacci spiral numbers arise due to the intercalary insertion and lateral displacement of incipient primordia in the first phase. Our results demonstrate the essential role of the growth and active zone dynamics in the patterning of flower heads.