ISHS Acta Horticulturae 587: International Symposium on Asian Pears, Commemorating the 100th Anniversary of Nijisseiki Pear
THE MECHANISMS OF SUCROSE ACCUMULATION IN JAPANESE PEAR (PYRUS PYRIFOLIA NAKAI) FRUIT |
| Authors: | K. Tanase, K. Shiratake, S. Yamaki |
| Keywords: | Pyrus pyrifolia, fruit quality, Japanese pear, sugar metabolism, sucrose synthase |
| Abstract:
Sucrose accumulation is one of the important factors that determine fruit enlargement and quality. It is influenced by many accumulated ingredients, especially, by the amount and kind of accumulated sugars. The profiles of sugar accumulation have been investigated in many fruits. In Japanese pear fruit, sucrose content differs among cultivars. Sucrose content in some fruits, which is very low in young fruit, increases with maturation. This increase is accompanied with a rise in both sucrose synthase (SS; EC 2.4.1.13) and sucrose phosphate synthase (SPS, EC 2.4.1.14) activities. Therefore, sucrose deposited in the cells is usually re-synthesized by SS and SPS. In Japanese pear fruit, SS activity is high at the young stage of development, decreases in the middle stage and then increases again at the mature stage. This enzyme which catalyzes the reversible reaction (sucrose + UDP ⇔ fructose + UDP-glucose), is the key enzyme of sucrose metabolism and has two or more isozymes. Anion exchange chromatography of the extract from Japanese pear fruit showed two peaks of SS. The first peak, which was eluted at low KCl concentration, was defined as SSI, while the second peak, which was eluted at high KCl concentration, was defined as SSII. SSI appears mainly in young and immature fruits and participates in the supply of substrate for starch and cell wall polysaccharides, while SS II appears with fruit maturation and participates in sucrose synthesis. Furthermore, the kinetic parameters of these SSs that were purified were determined and found to be clearly different. The kinetic parameters of SSI were very similar to those of other plants and the reaction inclined toward sucrose cleavage, while the reaction of SSII from kinetic parameters inclined toward sucrose synthesis. This is unique to SS in Japanese pear fruit. Modulation of kinetic parameters of SS was caused by protein phosphorylation, that is, SSI is a phosphorylated type and SSII is a non-phosphorylated type. Therefore, the physiological roles of these SS isoforms may be controlled by phosphorylation throughout the development of Japanese pear fruit. | |
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