Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 186

Proteins have been shown to move through the phloem from source to sink tissues,
butCOprotein itself is unlikely to be the long-distance signal. The SUC2 promoter
is specific to the companion cells of the phloem in source leaves, and expression
of GUS enzyme from this promoter produced staining specifically in the vascular
tissue of these leaves (Truernit et al. , 1996). In contrast, expression of GFP from the
same promoter produced fluorescence both in source and sink leaves, indicating that
GFP can be downloaded from the companion cells into the phloem sieve elements
and transported to sink leaves. However, CO protein is approximately 20 kDa larger
than GFP and expression of CO:GFP from the SUC2 promoter complemented the
co mutation, but GFP fluorescence was detected only in the vascular tissue and not
in the meristem or leaf epidermal cells (An et al. , 2004). This, together with the
observation that CO does not promote flowering when expressed in the meristem,
suggests that CO protein acts in the phloem to promote flowering and is not trans-
ported to other cells.
The mechanism by which CO acts to promote flowering in the phloem involves
the FT gene. FT mRNA abundance was increased in the phloem of SUC2::CO
plants, and the ft mutation strongly suppressed the early flowering of SUC2::CO (An
et al. , 2004). Furthermore, expression of FT in the phloem from the SUC2 promoter
complemented the co mutation. However, in contrast to CO, FT promoted flowering
when expressed in the meristem and the epidermal layer, as well as the phloem (An
et al. , 2004). No data are available on the movement of the FT protein between
cells. However, FT is a small protein of 23 kD (Kardailsky et al. , 1999; Kobayashi
et al. , 1999) and is, therefore, smaller than GFP, suggesting that it may be able to
move freely between cells. Also, the observation that FT can promote flowering
when expressed in the meristem is consistent with the idea that the protein could
move from the phloem to the meristem where it acts to promote flower development.
However, these data could also be explained if FT can act in almost any cell type to
trigger the synthesis of a small molecule that induces flowering and is able to move
freely between cells.
The biochemical function of FT is unknown. It is a member of a small protein
family in Arabidopsis and shares homology with characterized proteins in other
species. These proteins are referred to as CETS, after CENTRORADIALIS (CEN) of
Antirrhinum , TERMINAL FLOWER 1 (TFL1) of Arabidopsis and SELF PRUNING
(SP) of tomato (Bradley et al. , 1997; Pnueli et al. , 1998; Kardailsky et al. , 1999;
Kobayashi et al. , 1999). CETS proteins share homology to RAF kinase inhibitor
proteins of mammals (Kardailsky et al. , 1999; Pnueli et al. , 2001), and the crystal
structure of CEN is similar to that of RAF kinase inhibitors (Banfield & Brady,
2000). In the yeast two-hybrid system, SP interacted with a NIMA-like kinase, bZIP
transcription factors and a 14-3-3 protein, which led to the suggestion that CETS
proteins act as adapters in a variety of signaling pathways (Pnueli et al. , 2001).
How these interactions relate to the role of FT in promoting flowering is unknown.
Combining ft mutations with mutations affecting flower development identified a
strong genetic interaction between ft and mutations in the floral organ identity gene
LEAFY ( LFY ) (Ruiz-Garcia et al. , 1997). The ft lfy double mutant failed to produce