Imbalanced starch and protein accumulation during grain filling under salt stress leads to increased protein components in rice grains

Context

Soil salinization has emerged as a limiting factor that perturbs the metabolism of protein and starch in rice grains. However, the regulatory mechanism underlying salt stress-mediated synthesis and accumulation of these components remain poorly understood.

Method

A two-year field experiment was conducted using four rice genotypes (Youxianghongdao, Tiandaohong2, Suixiangheinuo, Taiheixiang9) subjected to three saline irrigation treatments: fresh water irrigation (CK) and mixtures of seawater and freshwater i.e., 3.9 dS m⁻¹ (T1) and 7.5 dS m⁻¹ (T2). Key parameters, including nitrogen metabolism related enzymes, nitrogen uptake, sink capacity, grain filling characteristics, protein and starch and its components were measured.

Results

Salt stress significantly enhanced protein component contents, with T1 and T2 treatments increasing albumin (41.26–44.67 %; 77.82–81.93 %), globulin (42.42–45.03 %; 54.69–58.61 %), prolamin (26.27–28.54 %; 58.17–58.28 %), and glutelin (31.45–33.99 %; 42.35–44.36 %) compared to CK. This enhancement resulted from two synergistic mechanisms: 1) direct promotion of protein components accumulation, and 2) indirect concentration effects due to the changes of grain weight and starch accumulation. Salt stress increased grain nitrogen supply (GNS) as the decrease of sink capacity exceeded that of post-anthesis nitrogen accumulation in panicle (PNAP). Concurrently, starch accumulation was inhibited through reduced leaf area index (LAI), chlorophyll content (as indicated by SPAD), grain filling rate and shortened grain-filling duration, resulting in a decrease in grain weight and concentration of protein components in smaller-sized grains.

Conclusions

The salinity-induced increase of protein components content in rice grain arises from both enhanced protein accumulation and concentration effects in smaller-size grains, mediated through altered nitrogen partitioning and suppressed starch biosynthesis during grain filling.