Question

Consider four observations about the FT gene and flowering transition in the shoot apical meristem (SAM) of Arabidopsis thaliana:
(i) The FT promoter is active in leaves alone.
(ii) The ft null mutation causes delayed flowering of the SAM.
(iii) Expressing a recombinant FT protein fused to a nuclear localization signal (NLS) under the endogenous promoter does not rescue the delayed-flowering phenotype of the ft null mutant.
(iv) Downregulation of FT transcript in the SAM by RNAi in a wild-type background does not alter flowering.
Which conclusion best explains these observations?

• A. FT protein resident in leaves causes flowering transition of the SAM.
• B. FT transcript moves from leaves to the meristem and promotes flowering.
• C. FT protein moves from leaves to the SAM and promotes flowering.
• D. Both FT transcript and FT protein are required in the SAM to promote flowering.

Hint:

Florigen in {Arabidopsis} is FT {protein}, produced in leaves and transported via phloem to the SAM; immobilizing FT (e.g., with an NLS) blocks rescue because movement is essential.

Answer 1

The Correct Option is C

1) Site of expression.
Observation (i) shows the FT gene is transcribed in leaves, not in the SAM $⇒$ any signal reaching the SAM must originate in leaves.
2) Requirement of FT function.
Observation (ii) indicates FT function is necessary for timely flowering $⇒$ loss of FT delays flowering.
3) Mobility requirement points to the protein, not mRNA.
Observation (iv) shows reducing FT mRNA in the SAM does not affect flowering $⇒$ the transcript need not be present (or move) to the SAM; rule out option (B).
4) Nuclear-trapped FT fails to rescue.
Observation (iii) says FT–NLS expressed from its native (leaf) promoter does not rescue the mutant $⇒$ FT must traffic from leaves through the phloem and act non–cell-autonomously; trapping it in the nucleus prevents movement and function.
5) Conclusion.
Together, these show that the mobile signal (florigen) is the FT protein synthesized in leaves that moves to the SAM to promote flowering $⇒$ option (C).