How plants choose to leave or flower

Make a flower or make a leaf? A funny dilemma for plants. But important confusion. It is thanks to their flowers, in particular, that most existing plants can reproduce. however, “Flowering is an energy-intensive process”, explains Christian Fankhauser, who directs the Center for Integrative Genomics at the University of Lausanne (Switzerland). So the plant should be carefully selected for the moment when it will display its delicate corolla.

It is also a confusion, because it is from the same “house” of plant cells that one fine day a bud will emerge, and a leaf will emerge or a flower will open. This focus is the “apical meristem”, a small spherical mass that covers the top of the shoot.

How does the fate of these unspecialized vegetative cells (“stem cells”) play out? We know that day length (“photoperiod”) and the rhythm of the seasons are critical factors. But how do they work and how can plants detect these signals? The molecular clockwork operation that triggers flowering, this little miracle, has been known for more than twenty years. Those who grow leaves are, surprisingly, shrouded in mystery.

The same meticulous process has been recreated by the American team in the journal Science From 9 February. where it appears “If leaf growth, like flower blooming, depends on photoperiod, the two processes are controlled by different systems”Sums up Christian Fankhauser.

Let us first observe the blooming of the flower. Beneath the fragile enchantment, a small army of shadows is at work. Every day, the plant produces a protein called “constance” under the action of genes, according to the circadian rhythm. CO

. This protein, which accumulates in the leaves throughout the day, will stimulate flowering…if the season is right. Leaves, in addition, are equipped with two low-intensity light sensors: photoreceptors sensitive to red light (“phytochromes”) and others sensitive to blue light (“cryptochromes”), which allow them to assess the length of the day.

The green miracle

Consider a plant species that flowers in spring. In autumn, when the days become shorter, the plant cannot find enough light in the evening. The constance protein, which is very unstable, is then degraded by specific enzymes: flowering is inhibited. But in spring, when the days are longer, leaves are full of these proteins at dusk, when they detect less light. Their photoreceptors, activated, block the enzymes that destroy the constance until… and the floral gear is set in motion. Preserved, constance induces the production of another protein, florigen FT (flowering locus T), which then migrates, via sap, to the meristem. There, it reprograms the genes of undifferentiated cells… thus awakening the flower bud. To complete the picture, let us add that to flower in spring, some plants also need to be exposed to the cold of winter – an epigenetic phenomenon called “vernalization”.

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