Agriculture Reference
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Figure 5.4 Non-selective movement of GFP in sink epidermal cells of Arabidopsis . Asterisk indicates
the initial bombarded cell expressing GFP from the CaMV 35s promoter. Bar: 20 mm.
in other two-dimensional cell-file systems such as stamen hairs of Setcreasea pur-
purea (Yang et al. , 1995) and leaf trichomes of tobacco (Waigmann & Zambryski,
1995) revealed that the SEL of plasmodesmata can be much greater than 1 kDa.
Additionally, Wang and Fisher (1994) demonstrated that dextrans ( M r of 10 kDa)
applied to the post-phloem crease of the wheat grain were capable of moving from
cell to cell, and Kempers and van Bel (1997) showed that dextrans ( M r of 10 kDa)
could move through plasmodesmata between sieve elements and companion cells in
the phloem of Vicia faba . The 27-kDa GFP has also been shown to move passively in
specific tissues (see Fig. 5.4; Imlau et al. , 1999; Oparka et al. , 1999). GFP has been
shown to traffic through plasmodesmata from companion cells to sieve elements
and migrate from the phloem into sink tissues such as the seed coat, root tips and
sink-leaf mesophyll cells (Imlau et al. , 1999). In sink-leaf tissue of tobacco, where
plasmodesmata are predominately simple in structure, Oparka et al. (1999) found
that GFP-fusion proteins with an M r up to 50 kDa could move freely, revealing a high
basal SEL for plasmodesmata in sink tissue. However, in mature source-leaf tissues
of tobacco, where plasmodesmata are branched, movement of GFP was restricted,
indicating that plasmodesmata in source tissue have a low SEL (Oparka et al. , 1999).
GFP can move passively between cells of Arabidopsis leaf and stem epidermis, but
not in the epidermis of either tomato or cucumber (Itaya et al. , 2000). Itaya et al.
(2000) also found that GFP could move from the epidermis into the cortex of cucum-
ber hypocotyls, but movement from the epidermis into the mesophyll in cotyledons
was restricted. Studies using both fluorescent probes, such as dextrans, and GFP
have shown that the basal SEL of plasmodesmata, and therefore passive trafficking
of molecules through plasmodesmata can vary depending on plant species, between
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