ISSN 1021-4437, Russian Journal of Plant Physiology, 2009, Vol. 56, No. 4, pp. 462–469. © Pleiades Publishing, Ltd., 2009.
Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 509–517.
Carbohydrates are major components of the eco-
nomically important sink organs, and their content con-
stitutes a major determinant for yield and quality of
plants, including peach [1, 2]. Peach fruit mainly con-
tains soluble carbohydrates such as sucrose, glucose,
and fructose. In the early stage of peach fruit develop-
ment, ﬂesh cells mainly contain a vast amount of fruc-
tose and glucose, while sucrose content is very low.
Sucrose accumulates predominantly during peach fruit
development, accounting for about 80% of the total
sugar at harvest, and it is the primary carbon source
translocated from leaf tissues to the growing fruit .
This text was submitted by the authors in English.
Sucrose might also help to protect plants from environ-
mental stresses, such as cold and drought . In recent
years, it has been recognized that sucrose also acts as a
signal compound transferring information about exter-
nal environment between different parts of the plant .
Sucrose synthase (SuSy, UDP-D-Glc:D-Fru 2-
glucosyltransferase; EC 184.108.40.206) catalyzes sucrose
degradation and synthesis in the reactions: sucrose +
UDP fructose + UDP-glucose. In sink organs,
SuSy displays typically the predominant sucrolytic
activity . SuSy activation is associated with the rapid
accumulation of sucrose in peach fruit, while the activ-
ity of acid invertase and sucrose phosphate synthase,
which are also related to sucrose metabolism, could
hardly be detected . It was concluded that SuSy is
closely related to sucrose accumulation as the key
enzyme of sucrose metabolism . The signiﬁcance of
SuSy for growth processes has been reported for sev-
eral tissues, for example in fruits [8–10]. A positive
association between SuSy activity and sink size has
Effects of Low Light on Phloem Ultrastructure
and Subcellular Localization of Sucrose Synthase
X. Q. Wang, W. D. Huang, and J. C. Zhan
College of Food Science and Nutritional Engineering, China Agricultural University, Beijing 100083, P.R. China;
Received September 1, 2007
—Using electron microscopy, the ultrastructure of phloem unloading zone was examined in the
Ait. fruit. Our study showed that, in the SE/CC (sieve element/companion cell)
complexes, CC developing under low light had a thin cytoplasm layer with few mitochondria and numerous
small vacuoles, and not clearly seen nuclei. The cytoplasm vacuolation indicated that the cytoskeleton was
destroyed at low light. The effects of low light on CC development suggest that unloading evidently linked to
the low accumulation of soluble sugars by fruit. At the young fruit stage, ﬂesh parenchyma around the phloem
tissue had no starch grains in the plastids in fruit developing under low light. This is a further indication that
less photoassimilates was translocated from source leaves to fruit sinks under low light during the young fruit
developmental stage. The activity of sucrose synthase (SuSy), the key enzyme of sucrose metabolism in fruit,
increased dramatically during fruit maturation. The highest SuSy activity during the rapid fruit growth phase
suggests that sink strength could be correlated with the SuSy activity. The high SuSy activity under normal light
possibly indicates that fruit had a capacity to utilize sucrose irrespective of their site of phloem unloading.
Immunogold electron microscopy showed that SuSy was localized mainly in the vacuole of ﬂesh parenchyma cells.
The vacuole-localized SuSy can hydrolyze sucrose imported from the phloem, which may explain the apparent cor-
relation between SuSy activity and phloem unloading. The double sieve element (SE/SE) complexes occurred in a
greater number and had thicker cell walls under normal light intensity than under low light intensity. These data dem-
onstrate clearly that low light decreased SuSy activity in the control of phloem unloading.
Key words: Prunus persica var. nectarina - immunogold electron microscopy localization - fruit - sucrose syn-
thase - low light
: CC-companion cells; FP—ﬂesh parenchyma cells;
NCW—nacreous cell wall; P—plastids; PCMBS—parachloromer-
curibenzene sulfonic acid; PD—plasmodesma; PP—phloem paren-
chyma cell; SE—sieve element; SuSy—sucrose synthase; TEM—
transmission electron microscopy; V—vacuole.