Difference in AGPase subunits could be associated with starch accumulation in grains between two wheat cultivars

The rates of starch accumulation, the activities of ADP-glucose pyrophosphorylase (AGPase, the key enzyme of starch synthesis), and the transcript levels of its four subunits were measured and compared between two large-spike winter wheat cultivars (Yujiao 2 and Lankao Aizao 8) during the periods of grain filling. During the first 10 days of grain filling periods, there were no differences in the rates of starch accumulation and activities of AGPase between the two wheat cultivars. In subsequent filling periods, however, the rates of starch accumulation and activities of AGPase in grains of cv. Yujiao 2 were significantly higher than those in cv. Lankao Aizao 8. cDNA sequences for four subunits of AGPase (cytosolic small subunit, SSU I; plastidial small subunit, SSU II; cytosolic large subunit, LSU I and plastidial large subunit, LSU II) were amplified, and their transcript levels during the whole grain filling were measured using semi-quantitative RT-PCR. No difference in transcript levels of SSU I, SSU II and LSU II was found between the two wheat cultivars during the whole grain filling period. However, the transcript levels of LSU I showed an obvious difference since 15 days of anthesis between two wheat cultivars, and LSU I transcripts were higher in cv. Yujiao 2 than those in cv. Lankao Aizao 8. This was clearly consistent with those in the starch accumulation rates and AGPase activities, implying that LSU I transcript levels could be related to the differences in the activities of AGPase and rates of starch accumulation.