Change of Sucrose Synthase Activity in Developing Endosperm of Rice Cultivars

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Abstract
Sucrose synthase (SS, EC 2.4.1.13) in rice (Oryza sativa L.) endosperm acts as the first step in the sucrose‐starch conversion pathway and is thought to have some relation to photosynthate accumulation and grain development. This study investigated genotypic differences in the change of SS activity in developing endosperm using four rice cultivars with different grain sizes and grain‐filling rates. The difference in SS activity between grain positions within a panicle was also examined. In addition to SS activity, activities of acid invertase (AI, EC 3.2.1.26) and UDP‐glucose pyrophosphorylase (UDPGP, 2.7.7.9) were observed in the same samples used for SS activity assay. In every cultivar and grain position, SS activity showed a maximum value at the middle of the linearly increasing period of grain dry weight, and decreased thereafter. Mean SS activity per gram fresh weight across sampling times and grain positions was not significantly different among cultivars. A significant difference among cultivars was detected in SS activity per grain. As for the difference between grain positions, the activities both per gram fresh weight and per grain were significantly higher in grains on primary branches showing a higher grain‐filling rate. The changes of AI and UDPGP activities did not correspond with the change of grain dry weight. These results suggest that SS activity in developing rice endosperm plays a role in the regulatory system of grain development, but does not have any relation to genetic variations in grain size and grain‐filling rate.

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