Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 193

In contrast, LFY mRNA is expressed in all layers of the flower primordium and
within their domains of expression the floral organ identity genes are also expressed
in all layers, therefore there is no obvious requirement for protein movement. One
possibility is that the induction of transcription of floral meristem identity and floral
organ identity genes does not occur reliably within their domain of expression and
that short-distance transcription factor movement ensures that gene activity occurs
reliably in every cell.

7.4 Perspectives

Application of molecular genetic approaches in Arabidopsis is likely to lead to the
identification of the long-distance signal involved in leaf-apex signaling during floral
induction, whereas the existence of short-distance signaling between cell layers
in the floral meristem has been established and shown to involve movement of
particular transcription factors. However, there are also processes of short-distance
signaling between cell layers of the flower that have not yet been described and
appear not to involve movement of the primary floral organ identity transcription
factors (Efremova et al. , 2001). Furthermore, the mechanisms by which these signals
move between cells are unknown. Plasmodesmata appear to play important roles
both in the movement of transcription factors in the floral primordium and in the
downloading of the long-distance floral induction signal from the phloem companion
cells into the sieve elements. Explaining how selectivity and size exclusion limits
of plasmodesmata are determined, and identification of the structural components
of plasmodesmata, will be essential in understanding the mechanisms of signaling
between cells during flowering.
In addition, there are likely to be relationships between the long-distance signals
that induce flowering and later processes during floral development that have not
been described. Normally, the long-distance induction of flowering from the leaf
is assumed to be involved in the initial induction of flowering and to set in train a
series of events that results in floral development. However, there is evidence in some
species and particular environmental conditions that the long-distance signal has a
continued role in the later events of floral development. This is most evident in the
process of floral reversion. For example, if Arabidopsis plants are grown under short
days, exposed to several long days and then returned to short days, they are induced
to flower by the long days but subsequently revert to vegetative growth (Laibach,
1951; Martinez-Zapater et al. , 1994). This phenomenon is more pronounced in
Impatiens . Floral reversion in Impatiens can cause floral organs within a developing
flower to revert to vegetative structures, suggesting that the floral inductive signal is
required to maintain expression of floral organ identity genes (Pouteau et al. , 1997).
The two processes described here of long-distance signaling to induce flowering
and signaling within the developing flower may therefore be interrelated at levels
which are not yet described, and of which we have no mechanistic information.