Agriculture Reference
In-Depth Information
mutation, as shown for the wild-type LFY protein. GFP fluorescence was detected
strongly in the L1 but also up to four cell layers deeper into the meristem. A gradient
of fluorescence was detected from the L1 into the deeper layers. Previously, proteins
were proposed to move through plasmodesmata either using a specific mechanism
that utilizes signals within the protein or through nontargeted movement that in-
volves only diffusion (Crawford & Zambryski, 1999). The gradient of fluorescence
observed with LFY-GFP fusion proteins suggests that the movement may be non-
targeted because proteins that move through a targeted mechanism, such as viral
movement proteins, are able to move further and do not show a gradient in abundance
in deeper layers. LFY-GFP also did not appear to move laterally within a layer, but
moved effectively between layers, suggesting that movement might be restricted to
secondary plasmodesmata and not occur between primary plasmodesmata.
During flower development, not all transcription factors move between cells.
Expression of
AP1
mRNA from the
ML1
promoter did not rescue the
ap1
mutant
phenotype in underlying layers, and AP1:GFP fusion proteins remained in the L1
cells when expressed from the
ML1
promoter (Wu
et al.
, 2003). If LFY protein
moves passively between cells through secondary plasmodesmata, then why does
AP1, which is a smaller protein than LFY, not move between cells? Several possi-
bilities have been suggested (Wu
et al.
, 2003). Efficient nuclear localization of tran-
scription factors may reduce the possibilities for movement, and there is evidence
that AP1:GFP is more efficiently targeted to the nucleus than LFY:GFP fusions.
Alternatively, the formation of higher order complexes, similar to those formed by
AP1 and other MADS box transcription factors during floral development, might
effectively increase the size of the protein and thereby restrict its movement by
diffusion.
Short-range signaling between plant cells during floral development may not
only involve trafficking of organ and meristem identity proteins. Expression of DEF
in the L1 of
Antirrhinum
partially restores the petal identity of underlying cells, but
the protein itself apparently does not move to the L2 (Efremova
et al.
, 2001). This
suggests that DEF activates signaling processes in the L1 that influence the identity
of the subepidermal cells, but the nature of these signals is not known. These signals
may be even more effective at promoting petal and stamen identity in
Arabidopsis
since epidermal expression of DEF in an
ap3
mutant almost completely rescued the
effect of the
ap3
mutation on petal and stamen identity, but also in this species the
AP3 protein was not able to move between cell layers.
How important is transcription factor movement between cells in the development
of a wild-type flower? During root development, the
SHORTROOT
(
SHR
) mRNA,
which encodes a transcription factor of the GRAS family, is expressed only in the
stele, but is required for the development of the adjacent endodermis (Helariutta
et al.
, 2000; see also Chapter 8). Analysis of the SHR protein showed that it is
present both in the stele and endodermis, indicating that the protein must move
from the stele to the endodermis, where it is required for the development of this
cell layer and for the activation of the
SCARECROW
gene (Nakajima
et al.
, 2001).
