Yes, multi-flowering roses have undergone a genetic mutation

• Almost all roses grown today bloom again until fall, according to our partner The Conversation.

• But contrary to what one might think given the omnipresence of these ascending roses, it is not a quality common to the Rosa genus but rather an anomaly due to a mutation.

• This analysis was conducted by Fabrice Foucher, geneticist-genomician of the rose, and Cristiana Oghina-Pavie, lecturer in contemporary history.

In the hedges, in the gardens, each spring marks the return of the roses in bloom.

For single-flowered wild roses (such as

) and old garden varieties, flowering stops in early summer.

However, almost all roses grown today bloom again until autumn and sometimes until the first frosts.

According to the expression of the gardeners, they “go up” in bloom.

Contrary to what one might think given the omnipresence of these repeating roses in our gardens, this is not a common quality in the genus

, but rather an anomaly, due to a mutation.

How did this exception become the norm in the selection of ornamental roses and with what consequences on cultivated diversity?

​Loss of a floral repressor

First of all, it is necessary to understand why the rosebushes go up.

In wild roses or old cultivated roses, after spring flowering, the plant accumulates a molecule that blocks flowering.

It is a floral repressor, the production of which prevents the rosebush from flowering again before the following spring.

In remontant roses, the first flowering is not followed by the production of this repressor.

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This absence is due to a genetic mutation within the gene encoding the repressor.

The researchers were thus able to identify two mutations, leading to an absence of accumulation of the repressor and therefore responsible for the rise in flowering.

One of the mutations is explained by the insertion of a sequence (a retrotransposon) in the gene.

By analyzing this insertion, the scientists showed that this mutation is not present in roses cultivated in Europe before the end of the 18th century, all single-flowering.

The mutation is quite recent and is thought to have appeared in a wild Chinese species,

var.

spontaneous.

The planetary journey of a mutation

Between this Asian origin of the mutation and its widespread presence in all garden roses grown today, a few chapters in the history of ornamental roses are missing.

Some of these chapters are related to the planetary circulation of plants.

The first remontant roses were imported from China to England at the end of the 18th century, from where they spread to other European countries, first in the collections of amateurs, then in nurseries and gardens.

Among them, a rose bush now called “Old Blush”, from Chinese nurseries.

In the 19th century, other remontant roses, resulting from a cross between

and other species, were imported into France from the United States (roses of the horticultural group of Noisette) and the Island of Réunion (the rosebushes of Île Bourbon).

Asian introductions continued with the so-called Tea roses, endowed with a particular smell, in the 1820s. All were integrated into the horticultural selection which took on unprecedented growth and had no common measure with that of other cultivated plants.

​Business and passion

If roses have been cultivated since Antiquity and their beauty constantly praised, it was not until the 19th century that gardeners really took an interest in their diversity.

Nurserymen and amateurs are engaged in a frantic race for novelty, in search of ever more floriferous plants, more original colors, larger flowers, shrubs adapted to the most diverse ornamental uses.

The introduction of re-blooming has certainly helped to make roses the most intensively bred, marketed and cultivated garden plant.

Breeders seek to combine the attributes of old European roses (flowers with a large number of petals, intense colour, characteristic "rose" fragrance, hardiness) with the rise of roses of Asian origin, a quality allowing roses to be his garden throughout the summer.

In addition, repeat-flowering varieties lend themselves to forcing, that is to say growing in a heated greenhouse which, by processes whose technicality continues to increase, provides flowers throughout the winter.

Endowed with a great variability by sowing or by spontaneous mutations, as well as a relative ease of identical multiplication by grafting or cuttings, roses are becoming the flagship product of commercial horticulture.

​10,000 new varieties in the 19th century

They nevertheless remain an object of attachment, of the patient attention of amateur horticulturists or merchants which sometimes borders on devouring passion.

International exchange networks, collections, specialized journals, competitions and rose exhibitions enliven these circles of connoisseurs.

This “rosomania” resulted in around 10,000 varieties obtained during the 19th century.

Among them, some become real stars.

An international plebiscite was organized in 1878 by the

and the winner was the variety “La France”, obtained in 1867 by the Lyonnais Guillot.

Its large, fragrant flowers of a "light pink colour, silver inside and lilac on the outside" with "substantial, spoon-shaped and very long petals", the hardiness and the rise in flowering make it stand out as the incarnation of an ideal of beauty.

Lovers still consider “France” today as the first modern rose, to signify that it is the first of the horticultural group of so-called Hybrid Tea roses.

Most of the varieties obtained since that date, used both for garden cultivation and for the production of cut flowers, belong to this group.

The consequences of an ornamental choice

From the general observation of gardeners, roses are extraordinarily diverse.

All you have to do is browse through a collection, such as La Roseraie départementale du Val-de-Marne in L'Haÿ-les-Roses or the Roseraie Loubert in Rosiers sur Loire to be convinced of this, faced with the spectacle of thousands of varieties whose morphology is always different.

Researchers are wondering about the genetic substrate of this apparent diversity.

How is the choice of breeders and consumers reflected in the historical construction of genetic diversity?

To answer these questions, the researchers found in the collections around 1400 varieties prior to 1900, preserved since the date of their obtention by vegetative propagation.

Grafting and cuttings are, in fact, cloning processes.

Submitting these plants to genetic analysis makes it possible to compare them, measure their degree of kinship, but also to follow the progression of the traits valued in the selection and in particular the increase in flowering.

During the 19th century, the version of the gene encoding the floral repressor is present in more and more selected varieties and, moreover, the number of remontant varieties is increasingly important.

Thus, the progressive selection of the mutated allele would have led to an intensification of the ascent.

From a European to Asian genetic background

These results were confronted with tens of thousands of pages of historical sources, to understand the reasoning of breeders when they chose this or that variety, what made it commercially successful, how they were named, classified and described, which have been retained or lost.

They have established that the varieties selected during the 19th century are genetically closer and closer to Asian roses.

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In other words, the preference of growers for the rise in flowering had such a force of attraction that it led, around 1860, to a shift from a European genetic background to an Asian genetic background.

This change remained permanent and has since been accentuated.

Even the roses whose flower looks like an old rose, selected since the end of the 20th century, are remontant.

They therefore contain part of the Asian genome.

This interdisciplinary research reconstructs the entanglement of plant genetics and human history that has shaped the diversity of this familiar plant in our gardens.

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Declaration of interests

• Science

• Video

• The Conversation

• Flowers

• Genetic

• Garden