ISSN 1021-4437, Russian Journal of Plant Physiology, 2017, Vol. 64, No. 4, pp. 616–624. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © Yu.L. Moshchenskaya, N.A. Galibina, L.V. Topchieva, L.L. Novitskaya, 2017, published in Fiziologiya Rastenii, 2017, Vol. 64, No. 4, pp. 301–310.
Expression of Genes Encoding Sucrose Synthase Isoforms
during Anomalous Xylogenesis in Karelian Birch
Yu. L. Moshchenskaya
, N. A. Galibina
*, L. V. Topchieva
, and L. L. Novitskaya
Institute of Forestry, Karelian Research Center, Russian Academy of Sciences, Petrozavodsk, Karelia, 185910 Russia
Institute of Biology, Karelian Research Center, Russian Academy of Sciences, Petrozavodsk, Karelia, 185910 Russia
Received May 20, 2016
Abstract⎯Activity of sucrose synthase (SS) and the transcript levels for SS-encoding genes SUS1, SUS2, and
SUS3 were assayed during xylogenesis in two varieties of silver birch: common birch (Betula pendula Roth var.
pendula) and Karelian birch (B. pendula var. carelica (Mercklin)). The xylem specimens of Karelian birch
were sampled from the trunks with the “normal” wood structure (unpatterned trees) and from the trunks of
patterned trees in stem regions exhibiting patterned and unpatterned wood. In the period of high cambial
activity, the SS activity in common silver birch was elevated due to expression of SuSy1 isoform (SUS1 gene).
The UDP-glucose produced in these trees was largely used for cellulose synthesis in cell walls of conducting
vessels and fibers that are the main wood constituents in these plants. In Karelian birch, the patterned wood
with abundant parenchymal cells was formed on the background of a substantial decrease in expression of
SUS1 and SUS2 genes, inhibition of SS activity, and lowering the cellulose content per unit tissue weight.
Within the same trees of Karelian birch, the unpatterned wood regions containing numerous conducting ves-
sels and fibers were formed on the background of high expression of SUS1 and SUS2 genes, elevated SS activ-
ity, and substantial accumulation of cellulose and starch. The unpatterned plants of Karelian birch occupied
intermediate position in terms of SS activity and cellulose content between the common birch and Karelian
birch trees with the patterned wood. The results shed new light on the role of SS in the induction of structural
anomalies in the wood of Karelian birch.
Keywords: Betula pendula var. carelica, patterned wood, sucrose decomposition, expression of
SUS1/SUS2/SUS3, starch, cellulose
Karelian birch Betula pendula Roth var. carelica
(Mercklin) is a variety of common silver birch and is
known for its structurally anomalous patterned wood.
It represents a valuable material for studying the
mechanisms of xylogenesis in woody plants. Xylogen-
esis is based on the incorporation of carbon into poly-
meric components of the xylem cell walls (cellulose,
hemicelluloses, pectins, and lignin). Sucrose is the
main carbon source for biosynthesis of polysaccha-
rides. The use of sucrose in metabolism of sink tissues
is only possible after preliminary splitting of sucrose by
invertase or sucrose synthase. In woody plants,
sucrose synthase (SS) is the main enzyme responsible
for the sink strength of growing stem tissues during dif-
ferentiation of xylem derivatives from the cambium .
It catalyzes the reversible conversion of sucrose in the
presence of UDP into fructose and UDP-glucose, the
latter serves as a substrate for synthesis of glucan
chains of cellulose. The membrane-bound form of SS
is a component of the cellulose synthase complex,
where it directly supplies UDP-glucose for cellulose
synthesis [2, 3].
Our studies demonstrated an enhanced SS activity
in the area of xylem formation in common silver
birch (B. pendula var. pendula) having a “normal”
(species-specific) wood structure . This indicates
the use of large amounts of sucrose for synthesis of
cell-wall components in the xylem, which, in turn,
ensures a high sink capacity of the tissue and promotes
an intense outflow of the disaccharide from the
phloem conduits. In Karelian birch trees having trunks
with the patterned wood, the SS activity during xylo-
genesis was much lower than in common birch, while
sucrose concentrations in the xylem formation zone
and in the phloem conduits were higher than in com-
mon birch trees . The excess sucrose is released into
the apoplast, which promotes the activity of cell wall
invertase (apoplast invertase) in the xylem and the
phloem . The products of invertase-mediated
decomposition of sucrose are glucose and fructose.
Abbreviations: PCR—polymerase chain reaction; RQ—relative
quantity of gene expression; SS—sucrose synthase.