Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 191

connect neighboring cells, and consist of an outer membrane derived from plasma
membrane and a segment of the endoplasmic reticulum that is continuous between
the two cells (Lucas et al. , 2001; see Chapter 5). Plasmodesmata are either made at
the cell plate during cytokinesis (primary plasmodesmata) or are formed between
existing cells (secondary plasmodesmata) (Lucas et al. , 2001; Wu et al. , 2002). The
types of molecule that move through plasmodesmata appear to be tightly regulated
by the developmental stage of the plant and the environmental signals to which it
is exposed (Ding et al. , 1992; Oparka et al. , 1999; Kim et al. , 2003). Also, the
size exclusion limit of plasmodesmata, which defines the size of the molecules that
can move through particular plasmodesmata, appears to change depending on the
cell type in which they are present and the stage in development at which they are
studied. Evidence that KN moves through plasmodesmata is indirect and is based on
the observation that expression of the KN protein increases the size exclusion limit
of plasmodesmata, allowing the KN mRNA to move to neighboring cells (Lucas
et al. , 1995). Also, RNAs and proteins encoded by other genes have been shown
to move between plant cells through plasmodesmata (Lucas et al. , 2001; Wu et al. ,
2002; see also Chapter 5).
DEF was the first protein involved in flower development that was shown to move
between cells (Perbal et al. , 1996). In chimeric Antirrhinum plants that expressed
the DEF gene only in the L2 and L3 layers of petals, the mRNA was detected
specifically in these layers, whilst the DEF protein was present in all three layers.
This observation demonstrated that the DEF protein moves from inner layers into the
L1 and probably explains the wild-type L1 phenotype shown by plants expressing
DEF mRNA only in the L2 and L3. In contrast, in sectored plants expressing the
DEF mRNA only in the L1, DEF protein was detected in the L1, but not the L2
or L3 layers (Perbal et al. , 1996). This indicated that the DEF protein could not
move through plasmodesmata from the L1 to underlying layers, consistent with
the cell autonomy of the complementation of the def mutant phenotype. Taken
together, these data indicate that trafficking of the DEF MADS box transcription
factor occurs between layers but that this movement is polar; it occurs from the
L2 or L3 layers to the L1, but not from the L1 to inner layers. Movement of DEF
from the L2 to the L1 is likely to occur through secondary plasmodesmata, since
primary plasmodesmata, which are formed during cell division, cannot be formed
between cells that are not clonally related. However, DEF is also unlikely to be
trafficked through primary plasmodesmata, since small, cell autonomous revertant
sectors were previously described on Antirrhinum def mutant petals (Carpenter &
Coen, 1990).
Trafficking of the LFY transcription factor was tested in transgenic plants in
which expression of LFY mRNA from the epidermal ML1 promoter complemented
the lfy mutation (Sessions et al. , 2000). Although in these plants LFY mRNA was
detected only in the L1 layer, the protein was detected in all layers. LFY protein
is therefore able to traffick from the L1 to inner layers. Movement of this protein
was tested more extensively using fusions with GFP (Wu et al. , 2003). Expression
of several LFY-GFP fusion proteins from the ML1 promoter complemented the lfy