Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 86

the contents of polysaccharides and starch (Maria et al. 2000 ). In this case, the
ratio between soluble sugars and starch changed. According to Dong et al. ( 2011 ),
treatment of arabidopsis plant with SA resulted in an increase in the contents of
glucose, fructose, raffinose, and stachyose in both leaves and roots but the contents
of sucrose and starch in the leaves decreased. The hydrolysis of these compounds
was possibly a cause for soluble sugar accumulation in leaves. Activation of
invertase and amylase after SA treatments is argued as a reason for this assumption
(Kaveh et al. 2004 ; Bernard et al. 2012 ). An increase in monosaccharides is
important for plant growth not only to increase the turgor pressure in the cells but
the metabolization of glucose and fructose is used as a source of energy and
structural elements. In addition, hexoses participate in plant defense responses via
activation of PR-genes (Herbers et al. 1996 ).
3 Sugar Transport in Plants
Plant growth activation may be a consequence of directed assimilate transport to
growing apices. Sugar distribution over the plant is dependent on many complexly
regulated processes. Primarily, sugar transport through membranes of intracellular
organelles has an important significance. Initially, trioses produced in photosyn-
thetic reactions are transported through the chloroplast envelope into the cyto-
plasm where sucrose is synthesized. The regulation of this early step of transport is
poorly studied. There are few data on SA-induced changes in the chloroplast
membrane permeability (Uzunova and Popova 2000 ; Maslenkova et al. 2009 ).
Further carbohydrate transport is carried out mainly in the form of sucrose. The
need for the delivery of sugar to each cell of a large multicellular organism
requires the development of a variety of ways and mechanisms of transport. There
are two main categories of this process: (1) the intercellular transport on short
distances, so-called short-distance transport and (2) long-distance transport, i.e.,
assimilate delivery to distant organs.
3.1 SA Impact on Sucrose Transport to the Root
Various mechanisms and various ways of regulation are involved in cell-to-cell
and long-distance transport, and they are still far from being fully disclosed. Short-
distance transport (cell-to-cell transport) functions in sites of sucrose distribution
predominantly between parenchymal cells: between mesophyll cells, between
parenchymal cells of vascular bundles and phloem conducting complex (sieve
elements plus companion cells), and between the cells in the site of phloem
unloading, including those in the root apex.
Along the entire pathway of transport, the exchange between cells occurs. Such
an exchange can take place through intercellular bridges—plasmodesmata—or