Flowering Signal Florigen

The question asks about the genetic basis of the mobile signal called florigen, which plays a crucial role in controlling when a plant flowers. Florigen is transported throughout the plant, specifically from the leaves where it is produced to the shoot apical meristem where it induces the transition from vegetative growth to reproductive growth (flowering).

Let's look at the options provided:

• Flowering locus C (FLC): This gene is known to be a major repressor of flowering in *Arabidopsis*. High levels of FLC protein prevent flowering, while its repression or degradation promotes flowering. It does not encode the mobile florigen signal itself.
• Flowering locus D (FD): FD is a transcription factor that acts in the shoot apical meristem. It interacts with the florigen protein (encoded by FT) to activate downstream genes that trigger flowering. While essential for flowering, FD is not the mobile florigen signal.
• Flowering locus T (FT): This gene encodes a small protein that is synthesized primarily in the leaves in response to signals like day length. The FT protein is the mobile signal that travels through the phloem to the shoot apical meristem. It is widely accepted that the FT protein acts as florigen in many plant species, including *Arabidopsis*.
• CONSTANS (CO): CO is a transcription factor that regulates the expression of the *FT* gene in leaves, particularly in response to light conditions. It controls when *FT* is transcribed but does not encode the mobile florigen protein itself.

Based on extensive research, the mobile signal florigen that controls the flowering status of plants is encoded by the Flowering locus T (FT) gene.