Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 100

transporters, SWEET sucrose efflux transporters, AtSWEET11 and AtSWEET12,
required for phloem unloading was described (Chen et al. 2012 ).
The mechanisms of sugar exit from the root cells and their retrieval into the
cells, however, are far from elucidation. According to recent data, sugar exit from
root cells into the apoplast may be mediated by the same transporters that provide
for their entry. Use of FRET nanosensors shows the presence of very rapid
exchange on membranes of the external cell layers of root tips. Sucrose accu-
mulation occurred for 10 s; it was completely reversible during 10-180 s (Chau-
dhuri et al. 2008 ). It was suggested that one and the same transporter is responsible
for both fluxes, into the cell and from the cell. At least one SUT can move sucrose
in either direction based on prevailing conditions (Carpaneto et al. 2005 ). More-
over, it is not excluded that at modest changes in the structure of transmembrane
transporter SUT, transferring sucrose against the gradient of its concentration, it
can be transformed into SUF operating along the gradient; with its help, sucrose
escapes the cell passively (Reinders et al. 2002 ; Kuhn et al. 2003 ; Ayre 2011 ).
In the root tip, the main way for sucrose exit from the phloem is symplastic one;
its transmembrane transfer does not play a great role in this process. However,
some proportion of sucrose coming to the root evidently leaches into the apoplast
and should be absorbed by adjoining cells via transmembrane mechanisms. In
experiments with the absorption of exogenous 14 C-sucrose by 1-cm segments of
the maize seedling root tips, the inhibitor of H + /sucrose SUT symporters, PCMBS,
which can not penetrate through the membrane, inhibited absorption by 30 %. This
inhibition depended weakly on PCMBS concentration in the range of 0.5-5 mM. It
seems likely that this 30 % inhibition is a maximal one and only 30 % of trans-
membrane absorption is determined by symporters; a ''passive'' translocator of
SUF type that may participate in the uptake of other part of sucrose. A protonophor
CCCP exerted much stronger inhibition, evidently affecting intracellular metab-
olization of absorbed sucrose. SA affected sucrose absorption from the solution:
5 9 10 -4 and 10 -4 M SA suppressed sucrose uptake, whereas 10 -6 M SA weakly

but significantly stimulated it (Table 3 ).

3.4 Sugar Transport Along the Phloem (Long-Distance
Transport)

Sucrose synthesized in the mesophyll cell cytoplasm moves to the phloem of
vascular bundles. The main pathway for its movement between the mesophyll cells
and parenchymal cells of vascular bundle is a symplastic one. During this
movement, the regulatory mechanisms described in the Sect. 3.2 operate. Into the
phloem (sieve element/companion cell), sucrose can penetrate differently (Gam-
alei 2004 ). Different plant species used varied loading ways (Rennie and Turgeon
2009 ; Slewinski and Braun 2010 ; Liesche and Schulz 2012 ): (1) symplastic
loading, when plasmodesmal routes are not interrupted and sucrose is transported