Agriculture Reference
In-Depth Information
macromolecules such as transcription factors, plant-defence-related proteins and
RNA (Mezitt & Lucas, 1996; Ghoshroy
et al.
, 1997; McLean
et al.
, 1997; Haywood
et al.
, 2002). Intercellular protein trafficking through plasmodesmata is now thought
to be an important means to regulate plant developmental processes, physiological
functions and plant defence reactions (Ding, 1998).
Studies into the mechanisms by which plant viruses move from cell to cell within
host tissue provided initial evidence that plasmodesmata mediate the trafficking of
macromolecules (Deom
et al.
, 1992; Lucas & Gilbertston, 1994; Carrington
et al.
,
1996; Ding, 1997). Some viruses, such as Cowpea mosaic virus, move through
plasmodesmata as intact virions, causing permanent modification to plasmodes-
mal structure (Hull, 1992; Storms
et al.
, 1995). Other viruses, typified by TMV,
cause transient alterations to plasmodesmata in order to traffic the viral genome
as a ribonucleoprotein complex (Deom
et al.
, 1992; Citovsky & Zambryski, 1993;
McLean
et al.
, 1993; Lucas & Gilberston, 1994). Many viruses have been found
to encode one or more non-structural proteins, termed movement proteins (MPs),
that interact with plasmodesmata to facilitate viral spread between cells (reviewed
in Wolf
et al.
, 1989; Atabekov & Taliansky, 1990; Robards & Lucas, 1990; Deom
et al.
, 1992; Lucas & Gilbertson, 1994; Carrington
et al.
, 1996; Gilbertson & Lucas,
1996; Nelson & van Bel, 1998). The most studied viral MP is the 30-kDa MP of
TMV, which has a plasmodesmal targeting signal, increases the plasmodesmal SEL,
mediates its own transport into neighbouring cells and potentiates the cell-to-cell
movement of the virus in the form of complex with the viral genomic RNA (for
recent reviews see Haywood
et al.
, 2002; Roberts & Oparka, 2003).
Based on the understanding gained from studying viral MPs, a considerable
amount of evidence has accumulated to support the concept of cell-to-cell traf-
ficking of plant gene products (Gilberston & Lucas, 1996; Ding, 1997; Ghoshroy
et al.
, 1997; Zambryski & Crawford, 2000; Haywood
et al.
, 2002). Endogenous pro-
teins that move between plant cells have been termed non-cell autonomous proteins
(NCAPs; Lee
et al.
, 2003). Although recent evidence has shown that some tran-
scription factors may pass through meristematic cells by diffusion (Wu
et al.
, 2003),
many of the NCAPs identified to date appear selectively to increase the SEL of plas-
modesmata (recently reviewed by Oparka, 2004). The first NCAP identified was
the transcription factor
KNOTTED
1 (KN1; Jackson
et al.
, 1994). The KN1 protein
was found to traffic between cells in the leaf and SAM by increasing plasmodes-
mal SEL, and was capable of trafficking its own RNA between tobacco mesophyll
cells (Kim
et al.
, 2002b). Kragler
et al.
(1998) have since shown that KN1 requires
the activity of a chaperone to unfold the protein, and a plasmodesmal receptor to
allow selective movement from cell to cell. Studies performed on a number of other
plant transcription factors involved in flower (
FLORICAULA
- Carpenter & Coen,
1990, 1995;
DEFICIENS
- Perbal
et al.
, 1996;
LEAFY
- Sessions
et al.
, 2000),
leaf (
GNARLEY1
-Foster
et al.
, 1999) and root (
WEREWOLF
- Lee & Schiefel-
bein, 1999;
CAPRICE
- Dolan & Costa, 2001;
SHORT ROOT
- Nakajima
et al.
,
2001) development have shown that these endogenous proteins have the capacity to
interact with and move through plasmodesmata (Haywood
et al.
, 2002). Work by
