Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 93

Respiration. Assimilates flow to sinks depending on the sink strength, which is
determined among other factors by the intensity of metabolism in the tissue. Since
most part of sucrose coming to the root is used in respiration, this process can
characterize the degree of sucrose metabolization. Measurement of CO 2 evolution
by root segments showed that a significant inhibition of respiration rate occurred
only under the influence of 1 mM SA; the lower concentrations were inefficient.
Therefore, the effects of most commonly used SA concentrations are not evidently
determined by changes in the rate of metabolism (Table 2 ).
Callose. Callose deposition is well known and is widely applied way for under-
standing plasmodesma permeability. Callose is a b-glucan polysaccharide, which
is deposited in the neck region of plasmodesmata, narrowing their holes and
retarding transport. Callose can accumulate around the plasmodesma and function
as a sphincter, pressing the hole. It is known since long that callose deposition is a
rapid and sensitive plant response to every external stress factor (Currier and
Webster 1964 ; Shimoura and Dijkstra 1975 ). Callose appears very rapidly, with in
minutes after stimulation (Furch et al. 2010 ; Radford et al. 1998 ). Mechanical
pressure, ultra sound, cooling, or heating, osmotic stress, and also cell damage
induce callose deposition (Currier and Webster 1964 ; McNairn and Currier 1968 ;
Samardakiewicz et al. 2012 ). Callose plays an important role during pathogenesis,
isolating the infected region and hindering pathogen spreading (Van Bel 2003a , b ).
At normal cell development and functioning, callose is required during cell
division, during the formation of cell plate or sieve pores, at protonema differ-
entiation, seed germination, pollen formation and maturation.
The level of callose is determined by the balance between its synthesis and
degradation. The synthesis of callose is catalyzed by callose synthase (ß-1,3-
glucan synthase), the transmembrane protein localized in plasmalemma. Callose
synthase transfer glucose from uridine diphosphate glucose to oligosaccharide
chain on the outer side of the plasmalemma (Simpson et al. 2009 ; Zavaliev et al.
2011 ). Callose degradation occurs with the involvement of ß-1,3-glucanase
localized on plasmodesmata (Levy et al. 2007 ). The intracellular content of callose
can be changed by detergents, polycations, agents interacting with phospholipids
(polymyxin B, phospholipase), i.e., by affecting membrane fluidity and perme-
ability (Köhle et al. 1985 ). The effects on callose synthesis and breakdown are one
of the ways of SA action on the plasmodesma capacity for transport.
Tobacco plant leaves or epidermis strip treatment with SA resulted in the
appearance of points of callose fluorescence in the cell walls (Krasavina et al.
2002 ). Callose deposition can be noted as early as with in 1 h, after treatment with
SA; the reaction is reversible, and fluorescence disappeared after SA removal.
The cause for callose accumulation under the influence of SA treatment may be
its action on activities of both enzymes determining callose accumulation in the cell,
callose synthase and ß-1,3-glucanase. Arabidopsis plant treatment with SA activated
expression of AtGSL5 as early as within 2 h; this gene encodes a protein homologous
to the catalytic subunit of ß-1,3-glucan synthase (Østergaard et al. 2002 ). Such
activation was transient and in 16 h the content of mRNA reduced to the initial level