Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 101

passively along the gradient created in the mesophyll; (2) apoplastic loading, when
sucrose exits into the apoplast and is absorbed actively across the membranes of
specific companion cells (intermediate cells); (3) the mechanism of Suc polymer
trapping, when in companion cells oligosaccharides are synthesized from sucrose
and this synthesis affects the osmotic potential of these cells. The oligosaccharides
and sugar alcohols are transported along the phloem together with sucrose. After
loading into the phloem, sugars move by bulk flow along the gradient of the
hydrostatic pressure generated on the both ends of sieve tubes (Srivastava et al.
2009 ; Turgeon and Medville 2011 ).
Long-distance transport occurs along the vascular system, sieve tubes con-
sisting of sieve elements separated by sieve plates. Transport along sieve elements
is metabolically and energetically provided by companion cells connected with
sieve elements by specific, branched plasmodesmata with opened pores from the
side of the sieve element. Sieve element and companion cells represent a complex
for long-distance transport.
It is believed since long that the phloem transports assimilates over long dis-
tances in isolation from the surrounding tissues (Kempers et al. 1998 ). However,
exchange with flanking tissues is inevitable because sugars are required for the
development of these tissues and to perform the function of nutrient storing.
Therefore, along the entire pathway of phloem transport, sucrose exits from the
transport channel into the apoplast may be retrieve into the phloem cells with the
help of transporter AtSUC2 (Srivastava et al. 2008 ). According to Minchin et al.
(Minchin and Thorpe 1987 ), the output from the transport phloem is about 6 % of
transported sugars, and return loading in the phloem is about 3 %.
After reaching the terminal sink tissues, sucrose abandons sieve elements and is
distributed between the cells. The way of exit from the conducting complex differs
in different tissues. In growing sinks, both exit and further distribution occurs in
symplast (Ruan and Patrick 1995 ; Stadler et al. 2005a , b ). In the organs accu-
mulating compounds and at the sites of contacts between daughter and mother
tissues, symplastic pathway is interrupted by apoplastic one (Patrick 1997 ; Lal-
onde et al. 2003 ; Zhang et al. 2006 , 2007 ).
Sieve elements are connected with each other by sieve plates performed with
numerous pores; the endoplasmic reticulum and cytoplasmic strands pass through
these pores. Pore field of sieve plates manifests enhanced permeability. This is the
main pathway providing for mass transport to the consuming sinks. The rate of
phloem transport is determined by the length of the channel, density of pores on
the sieve plate and their diameter (Thompson 2006 ).
Sieve pores are functional analogues of plasmodesmata. Therefore, many fea-
tures of regulation of pore permeability are similar to those of plasmodesma
conductivity regulation. This is well known characteristic of callose to clog sieve
pores. There is a correlation between callose deposition and assimilate transport in
plants. Thus, when sieve elements are damaged, callose deposition on sieve plates
is activated (Eschrich 1965 ; Esau and Thorsch 1985 ; Cronshaw and Esau 1968 ;
Evert and Derr 1964 ; McNairn and Currier 1968 ). Callose deposition is detected as
early as within 20 min, after injury (Mullendore et al. 2010 ). In this way, damaged