Transport of Salicylic Acid and Related Compounds - Salicylic Acid: Plant Growth and Development - page 45

Due to its physicochemical properties (Table 1 and see below), SAG diffusion
through phospholipidic layers must be very low and this suggests that another
mechanism is involved in SAG compartmentation within the cell. In fact, two
active mechanisms control SAG accumulation in the vacuole (Fig. 1 ). In soybean
cells, the vacuolar uptake of the conjugate occurs through an ATP-binding cassette
transporter mechanism (Dean and Mills 2004 ) whereas in tobacco cells it occurs
through an H + -antiport mechanism energized by the proton gradient resulting from
the activity of the tonoplast proton pumps (Dean et al. 2005 ).
Free endogenous SA can enter the neighboring cells either via the plasmo-
desmata (symplastic pathway) or via the cell wall (apoplastic pathway) (Fig. 1 ). In
the latter case, the hormone must cross the plasma membrane of the donor cell and
then the plasma membrane of the receiving cell.
It is well known that the physicochemical properties of SA make it well suitable
for rapid diffusion through the plasma membrane of animal and plant cells, more
especially under its undissociated form (Table 1 ). From the apoplastic compart-
ment (pH 4.5-5.5), SA can accumulate in the cytosol (pH ^ 7.3) under its anionic
form via the ion trap mechanism (Yalpani et al. 1991 ). Nevertheless, a slow SA
efflux also occurs (Chen et al. 2001 ). SA at 20 or 200 lM added to tobacco cells in
suspension is rapidly taken up (within 5 min). With time (5 h) 50 and 85 % of the
absorbed SA are excreted from the cells treated with 20 and 200 lM respectively.
SA excretion is significantly inhibited by EGTA and the inhibition can be reversed
by Ca 2+ addition in the 200 lM but not in the 20 lM treatment. Similarly,
cycloheximide blocks SA excretion only in the 200 lM treatment. According the
authors, this may suggest, among other hypotheses, a possible involvement of an
inducible SA efflux carrier under the latter experimental conditions in addition to an
efflux carrier constitutively present (Chen et al. 2001 ; Kawano et al. 2004 ) (Fig. 1 ).
3 Long Distance Transport of SA and Related Compounds
3.1 Phloem Transport
3.1.1 Phloem Loading Strategies
The phloem is a central actor in plant growth and development, allocating organic
nutrients, ions, water, hormones and other signals from the leaves to the sink
organs. Analyses of phloem sap indicate that sugars, potassium and amino acids
are the main osmotic components (Dinant et al. 2010 and references therein).
There are two main phloem loading mechanisms (Van Bel 1993 ; Turgeon 2010 ).
The first one called apoplastic loading provides the driving force for nutrient
transport by generating turgor pressure in the sieve-tubes. In many herbaceous
species, phloem loading involves an apoplastic step. Sucrose enters the cell wall
space in the vicinity of the companion cell-sieve element complex and the
mechanism of its accumulation in this complex is a cotransport with protons