We considered that, if the coupled PPase reaction should take place, there would be no discrimination of the two hexosyl residues in starch synthesis because the intermediate, G1P, should be in equilibrium with other hexose phosphates in the metabolic pool of starch synthesis. Nevertheless, we isolated UDPG, ADPG and hexose phosphates from the radiotracer-fed grains, analyzed their tracer contents, and obtained results as shown in Table 2. These results indicated that the glucose moieties of the two sugar nucleotides were derived mainly from the glucose moiety while all sugar monophosphates, including G1P, were from the fructose moiety of sucrose when sucrose was fed. The only plausible explanation was that the sugar nucleotide derived directly from a SuS catalyzed reaction was incorporated into starch. By considering the earlier finding that ADP plus sucrose was a better combination of substrates than UDP plus sucrose in the reconstituted starch synthesizing system, we proposed that, in the growing rice seed, SuS catalyzed reaction would directly provide at least a part of ADPG needed for starch synthesis.

Besides our demonstration of the importance of SuS in the starch synthesis in rice seeds, other types of work also provided certain evidence in supporting the view. One such work was to correlate the enzyme activities and the status of grain filling on the same rice plant. Flowering of spikelets takes place from the top down to the basal one; and the basal spikelet reaches anthesis a week after the top spikelet on the same branch and achieves a poorer grain filling. A measurement of sucrose translocation showed that the basal spikelet did not receive less sucrose than the top one. However, the activities of SuS and IT were higher and lower, respectively, in the endosperm cells of the top spikelet than those of the basal spikelet. Based on these comparative data, it was concluded that the level of SuS activity was positively correlated with the degree of grain filling, or the biosynthesis of starch (9).


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