Difference and molecular mechanism of soluble sugar metabolism and quality of different rice panicle in <italic>japonica</italic> rice

Abstract

<p id="C3">This study investigated the mechanism of starch and rice quality differences on different parts of spikelet in <italic>japonica</italic> rice. According to the internodes of the primary stem on main panicle, rice grains on the panicles were divided into top, middle, and the bottom spikelets. The dynamic changes of soluble carbohydrate and starch, starch biosynthesis related enzyme activity, gene expression on different parts of panicles at different filling stage and the starch content, chalkiness degree after harvest of six varieties were measured. The results showed that the amylose content of the five varieties was in an order as top &gt; middle &gt; bottom except Xiushui 134. The chalkiness degree of Jia 58 and Zhongjia 8 was higher than the other varieties, and the chalkiness degree of rice grains on the middle part of panicles was lower than that on the bottom part, and the rice grains on the top part had the highest chalkiness degree. Rice grains of high chalkiness degree had significant difference in starch granules size and shape. We found that the shape of starch granules was semi-ellipsoid or other spherical curved in rice grains with high chalkiness degree and mostly regular polyhedra in rice grains with low chalkiness degree. The contents of sucrose, glucose, and fructose in grains of different varieties and parts were different during grain filling period, but the dynamic changes of sucrose content were first decreased and then slightly increased, while glucose and fructose were basically increased first and then decreased. The contents of invertase and AGPase were significantly different among rice grains on top, middle, and bottom parts of panicles. However, the relative expression levels of AGPase were in an order as top &gt; middle &gt; bottom during the whole filling stage. The relative expression levels of invertase were in an order as top &lt; middle &lt; bottom during the early filling stage, and the rule was completely the opposite at late filling stage. The expression levels of <italic>OsCIN2 </italic>and<italic> OsCIN5</italic> encoding sucrose invertase genes were in an order as middle &gt; bottom &gt; top, and the expression levels of <italic>OsAGPL1</italic>,<italic> OsAGPL2</italic>, and <italic>OsAGPS1a</italic> encoding AGPase genes were in an order as top &gt; middle ≈ bottom. It concluded that the amylose content and chalkiness degree of grains in different panicles had significant difference and also affected by varieties. The content of soluble carbohydrate, starch biosynthesis related enzyme activity, and the gene expression of encoding related enzymes had an effect on starch biosynthesis. The genes of <italic>OsCIN2, OsCIN5, OsAGPL1</italic>,<italic> OsAGPL2</italic>, and<italic> OsAGPS1a</italic> played an important role in the difference of starch content and chalkiness degree in different panicles.