The Romanesco cauliflower variety has an unusual shape: It is made up of numerous small pyramids, which in turn consist of pyramids. How this happens, researchers have now found out with the help of a combination of plant biology and mathematical modeling. According to this, the gene regulation during the development of the flowers is so disturbed that the buds do not turn into flowers, but into stems, which form further flower attachments, which in turn become stems. This creates a fractal structure in which each component is composed of small repetitions of itself.
Nature is rich in geometric shapes. The coils of snail shells correspond to the Fibonacci sequence, in which each number corresponds to the sum of its two predecessors. Snow crystals consist of tiny branches that repeat themselves in different dimensions and thus form fractals. Many flowers are also structured according to similar patterns and form Fibonacci spirals or fractals.
Stems instead of flowers
The Romanesco cauliflower has a particularly striking geometric structure: its components form both fractals and Fibonacci spirals. A team led by Eugenio Azpeitia from the University of Lyon in France has now analyzed how this form comes about. “The cauliflower florets are the inflorescence of the plant,” explain the researchers. “The repetitive, self-similar structure arises because the flower precursors do not grow into flowers, but instead create additional inflorescences.” This phenomenon can be seen in both the classic cauliflower and the Romanesco variant. However, it was not yet clear what the underlying molecular and genetic processes are.
Azpeitia and his colleagues carried out various experiments, analyzes and calculations to uncover the riddle: Using classic cauliflower and romanesco, they investigated which genes are active in the development of the florets and how they influence each other. In computer simulations, they simulated the processes and varied individual factors in order to find out how this changes the result. To further research the role of the genes involved, they also modified the model plant thale cress so that it also formed cauliflower-like structures.
Regulatory networks out of balance
The result: when a flower develops normally, a precisely coordinated network of genes is active, which activate, inhibit or reinforce each other at the appropriate time. In this way, they ensure that an undifferentiated precursor tissue first develops buds and then flowers. In the case of cauliflower, on the other hand, this network is out of step. As a result, genes that are actually responsible for the development of the flower are down-regulated, while another gene that stops the flowering process is activated more intensively.
This leads to the fact that the precursor tissue has in its genetic “memory” that it should become flowers, but instead develops into ever larger stems, which again produce flower precursors, which in turn become stems. This process takes place in both classic cauliflower and romanesco and could also be reconstructed in thale cress if the researchers influenced the corresponding genes.
Product of domestication
But why does classic cauliflower form broad florets, while romanesco forms tapering, pyramidal structures? In the computer model, the researchers showed: In Romanesco, the new stems form new inflorescences that become stems after an ever shorter period of time. As a result, the new structures are each smaller than their predecessors. With classic cauliflower, on the other hand, the reproduction rate is constant.
“The conical shapes that appear in Romanesco spirals in all scales represent an additional geometric variation that was achieved through domestication,” the researchers write. “These results show how fractal patterns can be created by growth and development networks that change the dynamics of plant tissues.”
Source: Eugenio Azpeitia (University of Lyon, France) et al., Science, doi: 10.1126 / science.abg5999