A Taxicab For Florigen? – How the Flowering Signal Moves from Leaf to Shoot Apex.

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A Flowering Uber?

Batman has the Batmobile.

Dr. Who has the Tardis.

Is it possible that the protein that triggers flowering in plants also has its own special transport vehicle?

A paper from Prof. Hao Yu’s lab at National University of Singapore published in the 6 June 2016 issue of Nature Plants appears to support this idea. (Please see Ref. 1 below.)

Before I tell you about this report, let me remind you that:

  • 1 In many plants, flowering may be regulated by photoperiod, that is, the relative lengths of day and night, over a 24-hr period.
  • 2 This photoperiod is sensed in the leaves of the plant, mediated by the photoreceptor phytochrome.
  • 3 When the plant experiences a flower-inducing photoperiod, the leaves produce a substance long-known as “florigen”, whose true identity was discovered only about 10 years ago (see Ref. 2 below), namely, FT protein.
  • 4 The FT proteins are then transported via the phloem to the shoot apical meristems (SAMs).
  • 5 At the SAMs, the FT proteins interact with specific transcription factors called FD proteins to activate floral identity genes, thus inducing flowering.

    Today’s paper (Ref. 1) has to do with step 4 above, namely, how FT proteins are transported from leaves to SAMs.

    Briefly, Yang Zhu and co-investigators present several pieces of evidence that are consistent with the idea that proteins, with the unwieldy name of “SODIUM POTASSIUM ROOT DEFECTIVE 1”, abbreviated NaKR1, mediate the phloem transport of FT proteins from leaves to SAMs.

    First, let me tell you that the NaKR1 protein has previously been shown to travel in the phloem and to be involved in the long-distance phloem transport of sucrose and also, perhaps, mineral nutrients such as sodium (Na+) and potassium (K+). And, interestingly, it was also previously shown than NaKR1 affects flowering time under certain conditions.

    Knowing all this, Zhu and co-workers (Ref. 1) found that the expression of the genes coding for FT and NaKR1 were similarly affected by photoperiod and by flowering-related genetic factors, and that the relative levels and cellular locations of both FT and NaKR1 coincided with each other.

    Moreover, “We provide evidence that NaKR1 interacts in vivo with FT, and promotes flowering by regulating long-distance movement of FT from leaves to the shoot apex through the phloem stream. Our results suggest that NaKR1 is a hitherto unknown regulator specifically required for photoperiodic control of long-distance delivery of florigenic signals.” (From Ref. 1 below)

    Although the evidence supports the idea that NaKR1 is important in long-distance phloem transport of the flowering signal (at least in Arabidopsis), it’s unclear exactly how it does so.

    References

    1. Zhu, Y., L. Liu, L. Shen & H. Yu (2016) “NaKR1 regulates long-distance movement of FLOWERING LOCUS T in Arabidopsis.” Nature Plants, Vol. 2, Article number: 16075;
    doi:10.1038/nplants.2016.75. (Abstract)

    2. Corbesier, L., et al. (2007) “FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis.” Science, Vol. 316, pp. 1030-1033; DOI: 10.1126/science.1141752 (Abstract).

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