Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 102

sieve tubes, non-functioning, for example, in the winter or those, which do not
cope with too rapid assimilate flow, are temporarily excluded from the path of
assimilate movement. Callose mechanic rigidity makes plugging the conductive
pathways the important way to protect plant against the spread of pathogens (Van
Bel 2003a , b ).
Callose deposition on the sieve plates is reversible: callose disappears when the
damaging factor is no more. Maize mutant deficient in sucrose export (sxd1) is
characterized by enhanced callose deposition in vascular tissues of source leaves,
between bundle sheath and vascular parenchyma cells (Botha and Cross 2000 ). It
might be that just callose causes retardation of assimilate loading into the phloem
of such mutants. In another mutant with disturbed glucan synthase-like7, one of
the callose synthase isoforms, callose content is reduced only on sieve plates. This
mutation did not affect general plant phenotype but disturbed flowering stem
growth and reduced the size of all flower parts. Since assimilate movement sup-
pression induced a decrease in sugar content in the flowering axis, it was assumed
that growth retardation was a consequence of carbohydrate starvation. Thus, a
regulated callose deposition on the sieve plates is a process necessary for normal
phloem transport regulation (Barratt et al. 2011 ).
There is a lot that is common in the regulation of sieve plate pore and plas-
modesma conductivity. Thus, the remnants of destroyed plastids and filamentous P
proteins, natural components of sieve elements, can accumulate in pores and
interfere with the assimilate flow in the phloem. A dense network of endoplasmic
reticulum can also inhibit phloem flow (Knoblauch and van Bel 1998 ). Since the
rate of accumulation of such cytoplasmic components depends on the rate of flow,
therefore the rate of phloem transport may also affect the conductivity of sieve
plate pores. However, the significance of this physical pore blockage has been
questioned (Froelich et al. 2011 ). The causes and effects on phloem transport of
other protein structures of sieve elements, forisomes, are also doubtful. Only
recently, when phloem flow was simulated artificially via ''sieve elements,'' the
confirmation of such forisome function was obtained. It was shown that, in the
presence of Ca 2+ , lens-like forisomes swelled so strong that clog ''sieve pores''
(Hafke et al. 2009 ; Knoblauch et al. 2012 ).
Besides considerations related to the analogy between the transport through
plasmodesmata and the sieve pores, we have no information on the participation of
SA in the mechanism of substance long-distance transport.
3.5 Effect of Salicylic Acid on the Electric Potential
of the Membranes
Since SA affects cell membrane permeability and ion fluxes across them (review
by Krasavina 2007 ), it may not be surprising to correlate it with the observed
effects on changes of the membrane potential (MP). As early as in 1974, Glass and