Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 99

Table 3 14 C-sucrose absorption by apical segments of maize seedling roots
Treatment Uptake (lmoles/g fr.wt 10 tips) % from MES
MES, pH 5.5 48.8 ± 1.2 100
SA 0.5 mM 20.0 ± 3.8 40.9
SA 0.1 mM 35.1 ± 3.3 71.9
SA 0.01 mM 39.0 ± 3.0 80.0
SA 0.001 mM 55.2 ± 2.4 113.1
PCMBS 0.5 mM 34.8 ± 1.5 71.4
PCMBS 1 mM 33.7 ± 4.3 69.0
PCMBS 5 mM 32.3 ± 3.2 66.2
CCCP 0.01 mM 19.0 ± 0.8 38.9
Notes 1-cm root tips were incubated for 1 h in 1 mM 14 C-sucrose dissolved in 15 mM MES, pH
5.5. Inhibitors and SA were added to the incubation medium
10 -10 M stimulated activity and 10 -4 -10 -5 M inhibited it (Ladyzhenskaya and
Korablyova 2011 ). Leaf spraying with SA increased the content of H + -ATPase
protein, i.e., enzyme activation occurred on transcriptional and translational level
(Liu et al. 2009a ). SA action on membrane functional activity may be related to
stimulation of ROS generation, which will modify membrane structure by mem-
brane lipid peroxidation (Kawano et al. 1998 ).
It was established that transmembrane sucrose transport occurs in symport with
protons by protein transporters. The genes encoding Suc/H + symport-Suc trans-
porters (SUT) are known as Suc carriers (SUC) (see reviews Lalonde et al. 2004 ;
Sauer 2007 ; Slewinski and Braun 2010 ; Kuhn and Grof 2010 ). Transporters were
detected along the entire route of sucrose transport: at sucrose entry into the
conducting phloem complex, in phloem cells involved in long-distance transport
and in cells participating in phloem unloading (Truernit and Sauer 1995 ; Kuhn
et al. 2003 ). It was shown that SUT expression depends on light and some hor-
mones (ethylene, gibberellin, ABA) (see references in Ayre 2011 ). It is quite
unclear how SA affects transporter expression and functioning.
At high sucrose concentration in the external solution, its absorption occurs along
the gradient of concentration and the rate of absorption (i.e. facilitated diffusion) is
linear. Passive transport occurs with the help of recently found proteins Suc-facil-
itators (SUF) which are member of the class of sucrose transporters SUT. SUF
proteins are poorly characterized. They participate predominantly in the passive exit
from the cell independent of energy supply and pH gradient. They are not inhibited
by para-chloromercuribenzene sulfonic acid (PCMBS) (Zhou et al. 2007 ). It is
assumed that SUF proteins are involved, in sucrose escape into the apoplast. Such a
process occurs along the entire phloem route in leaf petiole, stem, and roots. Efflux to
the apoplast determines sucrose exit from the mesophyll cells before their uptake
into the sieve elements and companion cell during apoplastic phloem loading and
exit from the phloem in sinks at apoplastic unloading. Passive transport turned out to
be not completely independent of energy supply because it is inhibited at anoxia
(Anderson 1983 ). Recently, one more subfamily of plasma membrane-localized