life11e_ch37

Florigen is a small protein

The characterization of florigen was made possible by genetic and molecular studies of the model plant Arabidopsis. Three genes are involved in the signaling response for flowering (Figure 37.11).

Figure 37.11 Florigen and Its Molecular Biology Florigen is a protein (FT) made in the phloem companion cells. It travels in the sieve elements from the leaf to the bud meristem. There, florigen combines with another protein to stimulate transcription of genes that initiate flower formation.

FT (FLOWERING LOCUS T) codes for florigen. FT is a small (20 kilodalton [kDa]) protein that can travel through plasmodesmata. FT is synthesized in the phloem companion cells of the leaf and then diffuses into the adjacent sieve elements. It then is carried through the phloem to the apical meristem. If the FT gene is coupled to an active promoter and expressed at high levels in the shoot meristem, flowering is induced even in the absence of an appropriate photoperiodic stimulus.
CO (CONSTANS) codes for the transcription factor that activates the synthesis of FT. As described above, CO expression follows a circadian rhythm, and stabilization of the CO protein by photoreceptors allows it to function. Like FT, CO is expressed in leaf companion cells. If CO is experimentally overexpressed in the leaf, flowering is induced. However, if CO is overexpressed in the apical meristem, flowering is not induced, indicating that CO functions in the leaf.

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FD (FLOWERING LOCUS D) encodes a protein that binds to FT protein when it arrives in the apical meristem. The FD protein is a transcription factor that, when bound to FT, activates promoters for meristem identity genes, such as APETALA1 (see Figure 37.11). The expression of FD primes meristem cells to change from a vegetative fate to a reproductive fate once FT arrives.

Before FT was isolated, grafting experiments indicated that many different plant species could be induced to flower by the same chemical signal. Results of molecular experiments confirmed that the FT gene is involved in photoperiod signaling in many species:

Transgenic plants (e.g., tobacco and tomato) that express the Arabidopsis FT gene at high levels flower regardless of day length.
Transgenic Arabidopsis plants that express FT homologs from other plants (e.g., rice and tomato) flower regardless of day length.

How is this molecular system related to the photoperiodic stimulus? This has been described in the long-day plant Arabidopsis. Recall that the stimulus appears to be phytochrome, which exists in two forms, P_r and P_fr. At night, the P_fr form gradually is converted back to P_r. The P_r stimulates the breakdown of CO protein in proteasomes. So in the morning and during the day, CO protein levels go down. By the end of the day, P_r levels go down, allowing CO to accumulate. So the key to flowering is a high level of the transcription factor CO, and this in turn is related to a low level of P_r. Now, think of a short night (long day): there is not a lot of dark time for all of the P_fr to be converted back to P_r, and a long day also causes more conversion of P_r to P_fr. The low level of P_r results in less breakdown of CO protein, and the high level of CO results in the transcription of genes for flowering, such as FT. To summarize:

Low P_r → more CO → more transcription of FT → flowering

We have considered the photoperiodic regulation of flowering, from photoreceptors in the leaf to florigen that travels from the induced leaf to the sites of flower formation. In some plants, however, flowering is induced by other stimuli. These additional stimuli can function with photoperiodism or independently of it.