Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 187

any mature floral organs, and resembled a lfy apetala1 ( ap1 ) double mutant. This
suggested that the role of FT in promoting flowering may involve the activation
of AP1 , which is expressed exclusively at the meristem in floral primordia. This
suggests that in SUC2::FT plants, FT might activate a long-distance signal in the
phloem that leads to AP1 activation in the meristem or that FT protein might move
to the meristem where it activates AP1 .Analternative possibility is that in wild-type
plants FT mRNA is also expressed in the meristem, although so far this has not been
detected.
7.2.3.3 Identifying the floral stimulus: a perspective from Arabidopsis
molecular genetics
Analysis of the spatial regulation of components of the photoperiod pathway of
Arabidopsis showed that a long-distance, graft-transmissible signal similar to the
floral stimulus acts within this pathway (Takada & Goto, 2003; An et al. , 2004).
CO acts cell autonomously in the phloem to activate FT expression, and therefore
the signal does not seem to comprise the CO protein itself, but rather is regulated by
CO. In turn, FT is still activated by CO, and promotes flowering when expressed in
the phloem, but also does so when expressed in other tissues, including the meris-
tem. The small size of the FT protein suggests that it may move from the phloem
to the meristem and directly trigger changes in gene expression. Symplastic down-
loading of proteins from the sieve elements into the sink tissues of the apex through
plasmodesmata has been proposed (Ruiz-Medrano et al. , 2001; see also Chapter 5),
suggesting that FT may move directly by this mechanism into apical cells and induce
flowering. Furthermore, the size exclusion limit or selectivity of plasmodesmata that
allow downloading from the sieve elements into the meristem have been shown to
change around the time of flowering (Gisel et al. , 2002), suggesting that this might
be an important regulatory step. If FT does move from the companion cells of the
leaf to the meristem, nothing is known of the mechanisms underlying its export from
the leaf and import to the crucial cells of the meristem.
An alternative possibility is that FT acts in the phloem to induce the expression of
enzymes that generate small molecules that are transported to the apex, or of small
RNAs that induce flowering. Such small molecules might be metabolites or growth
regulators, such as sucrose or cytokinin, as suggested from physiological studies.
Alternatively, RNA molecules can be transmitted through the phloem (see Chap-
ter 3). Strikingly, small RNAs that induce gene silencing have been demonstrated to
cross graft junctions (Palauqui et al. , 1997). Furthermore, endogenous microRNAs
that decrease the expression of transcription factors related to APETALA2 were
recently shown to promote flowering of Arabidopsis (Aukerman & Sakai, 2003).
Although these particular microRNAs appear to be expressed at the shoot apex,
other microRNAs expressed in the leaf might be transported to the apex where they
influence flowering-time. In support of the biological significance of long-distance
transport of RNA, the mRNA of the mouse ears mutant gene was recently shown to
cross graft junctions in tomato plants (Kim et al. , 2001). The mouse ears mutation
is dominant and caused by a complex fusion of a gene encoding an enzyme in the