The veil is lifted on the fractal shape of cauliflower and romanesco 🥬

By Christophe Godin and Francois Parcy, Inrae

Through the process of domestication, humans have selected plants best suited to their needs, for example, those with larger fruits or seeds that remain attached to the spike. It was known that these plants were genetically slightly different from wild plants, but only in recent years have we begun to identify the genetic origins of these differences and understand how they result in changes in shape, size, or color.


Cabbages (Brassica oleracea) represent a spectacular example of domestication. Starting from wild plants producing a few leaves and then a flowering stem similar to that of rapeseed, domestication has led to a wide variety of vegetables with contrasting appearances such as green cabbage, kale, kohlrabi, Brussels sprouts, and cabbages where the flowering stem, called an inflorescence, has transformed into broccoli, cauliflower, or even the fascinatingly fractal-shaped Romanesco cauliflower.


The latter features spirals made of small conical pyramids called florets, each resembling the overall conical shape of the whole vegetable. Each of these florets is also composed of spiral arrangements of even smaller conical florets, and so on. This property, where the same geometric pattern appears at every scale within a shape (known as self-similarity), gives Romanesco cauliflower its remarkable "fractal" character.

How have genetic changes accumulated over the centuries so profoundly modified the growth of stems and flowers, creating such complex and perfect fractal shapes?

A genetic battle between stems and flowers

How have genetic changes accumulated over the centuries so profoundly modified the growth of stems and flowers, creating such complex and perfect fractal shapes?

Our international consortium tackled this mystery using Arabidopsis thaliana, a well-studied laboratory weed and relative of the cabbage. Our findings were recently featured on the cover of Science magazine. In Arabidopsis, it was observed in the 1990s that two mutations, i.e., genetic disturbances, are sufficient to transform flowers into miniature cabbages! This suggested that the number of essential mutations required to alter the structure of an ancestral plant into an edible cauliflower might actually be relatively limited.


How can two mutations lead to such a dramatic change in shape? To understand this, we must delve into the way plants grow.

The above-ground part of a plant grows from a bud contained in the seed, which produces a stem and new buds distributed along this stem. These, in turn, produce other stems and new buds, and so on. These buds can either grow immediately or remain dormant. When they grow, they can form leafy stems or flowers.


The outcome depends on the result of a struggle within each new bud between "stem" genes and "flower" genes competing to define the bud's identity. This struggle involves a complex network of alliances and interactions that is difficult to unravel.

To better understand this intricate web and its impact on plant growth, we developed an approach combining biological experimentation, mathematical modeling, and simulated 3D plant development. Step by step, this analysis identified the genetic mechanism responsible for the cauliflower's shape and clarified how it influences plant growth.

In short, it's a territorial war. In a normal plant, when flower buds are developed, an initial gene, the “floral architect,” becomes active in the buds. However, to form the flower, it must summon other floral genes for support to prevent the invasion of the bud by stem genes. The bud then definitively acquires the "flower" identity.

Infinite stem production creates fractals

In Arabidopsis cauliflower, the floral architect calls for reinforcements, but none arrive—and for good reason: those very genes have been inactivated by the two mutations! As a result, the bud, after initially entering a floral state, is taken over by the activity of stem genes, which then impose their identity.

But these buds are not entirely normal stems! Our team showed that their brief transition into a flower state irreversibly alters them, enabling them—unlike normal stems—to start growing immediately (without dormancy as in the normal plant), without leaves, and to multiply quickly, almost infinitely.

These modified stem buds generate new flower buds, which fail to produce flowers and instead revert to stem buds, attempting again to produce new flower buds, to no avail, and so on. The cauliflower, therefore, results from a true chain reaction triggered by the brief transition of the bud into the "flower" state, leading to a dense aggregation of stems on stems on stems... forming the cauliflower's spiraling fractal structure.



Edible cauliflower and Romanesco are formed through a mechanism similar to that of Arabidopsis, though they are larger and more compact. But why does Romanesco have such a strikingly fractal appearance? In fact, cauliflower is already fractal. It features florets that are self-similar across scales and organized in spirals. However, this is not as apparent because the overall structure is flattened, and the individual florets are not well-defined.

In Romanesco, the pyramidal shape of each floret creates a pronounced relief, highlighting its fractal nature. Researchers have shown through numerical simulations and experiments on Arabidopsis cauliflower that this property likely comes from the fact that within Romanesco buds, new buds are produced at an increasing rate as each bud grows, whereas in cauliflower, this rate remains constant.

This feature is sufficient to accelerate stem growth for each floret and give them a pyramidal shape. The fractal structure of Romanesco is essentially a "relief" version of the fractal structure of cauliflower.


Our study provides a deep understanding of how gene activity combines with growth to shape flowering plants. It reveals the ability of stem buds to multiply to an extreme degree—a capacity usually concealed in nature by various mechanisms: buds either produce flowers or stems that elongate before generating new buds, creating less compact structures than cabbages; on a stem, growth priorities typically follow a hierarchy, with lateral buds often remaining dormant as long as the main bud grows; finally, the flowering mechanism itself ends a bud's activity by turning it into a flower.

All these mechanisms, which normally limit stem proliferation in most plants, are simultaneously inactivated in cauliflower, allowing it to produce these repetitive and compact stem structures.


These findings on the model plant Arabidopsis open new avenues in research and agronomy. In research, for example, they can guide our quest to identify the genetic changes during domestication that are ultimately responsible for the unique forms of cauliflowers and Romanesco. In agronomy, they provide a valuable framework to consider new advancements in domestication.

Once all the mutations responsible for the cauliflower's shape are identified, it may become possible to domesticate wild green cabbages that offer agricultural advantages (such as better resistance to diseases or higher temperatures) and have them produce cauliflowers or Romanesco. This approach, called de novo domestication, aims to accelerate the domestication process (using, for instance, genome editing techniques) that took our ancestors several millennia to achieve.