Positional variations of rice protein compositions accumulation within a panicle during the grain filling.

Grain protein is a critical quality attribute of rice that influences consumer preferences. However, the spatial variation in protein accumulation within a rice panicle remains poorly understood. This study investigated the dynamics of protein accumulation, including protein components and protein synthesis-related enzymes and genes, among grains located at the top, middle, and bottom primary rachises of a rice panicle during the grain filling. The results revealed significant variations in protein compositions across different rachis positions. The contents of albumin, globulin, prolamin, glutelin, and total protein contents exhibited fluctuations during grain filling. Notably, the grain position had a significant effect on glutelin content, with grains at the bottom primary rachis consistently having higher glutelin level than those at the top and middle rachises, except 17 days after flowering (DAF). A similar trend was observed for total protein content. Grains at the bottom rachis demonstrated dominance in the rate of protein accumulation, initiating rapid accumulation 2.0 d later and 2.2 d earlier than grains at the top and middle rachises, respectively. Furthermore, the duration of active protein accumulation was 1.9 d and 3.4 d shorter for grains at the bottom rachis compared to those at the top and middle rachises, respectively. This phenomenon was attributed to alterations in enzymatic activities. Specifically, the activities of glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate pyruvate transaminase (GPT), and glutamic oxalo-acetic transaminase (GOT) in grains located at the basal rachis exhibited a marked increase from 8 DAF to 17 DAF. These activities were significantly elevated compared to those observed in grains at the top and middle rachis, although they experienced a subsequent sharp decline. The glutelin content and enzymatic activities demonstrated a strong correlation, either positive or negative, at 11 DAF and 20 DAF. These findings suggest that the positional changes of grain protein were closely associated with nitrogen assimilation and glutelin accumulation during the rice grain filling process.