Agriculture Reference
In-Depth Information
Nakajima
et al.
(2001) has highlighted the biological significance of the movement
of a transcription factor beyond its site of synthesis. Experiments performed on the
short-root (SHR)
Arabidopsis
mutant provided evidence that SHR protein acts as
an NCAP, moving from its site of synthesis in the stele into adjacent cells of the
endodermis to convey the positional information necessary for determining the fate
of endodermal cells.
A number of endogenous proteins have also been found to traffic through the
specialised plasmodesmata that connect sieve elements and companion cells, subse-
quently trafficking long distances within the phloem before being unloaded into sink
tissues. Evidence that sieve element proteins may 'gate' mesophyll plasmodesmata
has been demonstrated by microinjection experiments involving phloem proteins
(PP2 - Balachandran
et al.
, 1997; RPP13-1 - Ishiwatari
et al.
, 1998; CmPP16 -
Xoconostle-Cazares
et al.
, 1999; CmPP36 - Xoconostle-Cazares
et al.
, 2000). In
addition to macromolecules, plant RNAs have also been found to traffic into and
through the phloem pathway (see Chapter 3 for a full review). The CmPP16 protein
identified from phloem sap of
Curcurbita maxima
allows the transport of both sense
and antisense RNA in the phloem, and has been shown to move from cell to cell in
tobacco mesophyll cells and bind its own RNA (Xoconostle-Cazares
et al.
, 1999).
Grafting experiments have provided other examples of specific mRNA trafficking.
Forexample, RNA transcripts encoding a mutated homeodomain protein from the
mouse ears (
me
) mutant of tomato have been found to move from a rootstock via the
phloem into the SAM of a grafted scion and to induce a change in leaf developmen-
tal patterns (Kim
et al.
, 2001). Post-transcriptional gene silencing signals, which
result in the sequence-specific degradation of targeted mRNA, are also thought to
move through plasmodesmata, and several studies have provided evidence that short
RNA species can enter the phloem pathway and subsequently unload in sink tis-
sues (Voinnet
et al.
, 1998; Hamilton & Baulcombe, 1999; for in depth review see
Chapter 3). These studies illustrate the role of macromolecular and RNA trafficking
in plant development, supporting the concept that plants routinely use endogenous
proteins and mRNAs as intercellular signals that can traffic through plasmodesmata.
Readers are referred to a series of comprehensive reviews (Ding, 1997, 1998; Lucas
et al.
, 2001; Ruiz-Medrano
et al.
, 2001; Ueki & Citovsky, 2001; Haywood
et al.
,
2002; Heinlein, 2002; Lindsey
et al.
, 2002; Wu
et al.
, 2002; Roberts & Oparka, 2003;
Oparka, 2004) for further information on symplastic trafficking of proteins and RNA.
5.3.1
Passive transport and the basal SEL
Transport through plasmodesmata is described as passive/non-selective when
molecules below the basal SEL traffic without inducing changes in plasmodesmata
structure or modifying the basal SEL (Schulz, 1999). Initial studies of basal SEL
gave rise to the consensus that only small molecules ranging between 850 and 900
Da move freely through plasmodesmata (Tucker, 1982; Goodwin, 1983; Erwee &
Goodwin, 1985; Terry & Robards, 1987; Burnell, 1988). However, many exceptions
were found to this rule. Kikuyama
et al.
(1982) were the first to report the passive
movement of a 45-kDa protein labelled with FITC in the green alga
Nitella
. Studies
