Activity and expression of adenosine diphosphate glucose pyrophosphorylase in developing rice grains: Varietal differences and implications on grain filling

ADP-glucose pyrophosphorylase (AGPase) is a key enzyme in starch biosynthesis in plants. The activity, expression and gene sequence of AGPase were studied in grains of five rice cultivars representing different origins to gain insight into their variations in relation to starch accumulation. The varieties were Jaya, GEB24 (indica), BSI 115 (japonica), Mahsuri (indica-japonica derivative) and CoRH2 (hybrid). The grain weight increased in all of them until 20 dpa, although at varying rates. It was relatively steady in Mahsuri, in others there was a steep increase from 10 to 20 days post-anthesis (dpa). The total AGPase activity increased steadily from 5 to 20 dpa in Jaya and GEB24, but dropped after 15 dpa in BSI 115 and CoRH2 while in Mahsuri, it maintained the 10 dpa low level until 20 dpa. The AGPase cDNAs did not show any sequence differences in the five varieties. The transcripts of both sub units of AGPase accumulated comparably during the initial phase of grain development suggesting their coordinated expression and dropped after 15 dpa in all the varieties. On the other hand, AGPase protein level remained unchanged after reaching the maximum in CoRH2 and continued to increase up to 20 dpa in the other four varieties. Thus the drop in the enzyme activity cannot be attributed to changes in the transcript or protein amounts per se. We infer that the drop in activity, its varying levels and varying duration in the varieties studied is likely due to differential stability of the active enzyme protein during grain development and that the extended duration of enzyme activity may contribute to increased grain weight.